COLD-ROLLED STEEL SHEET

Abstract

Provided is a cold-rolled steel sheet having a chemical composition consisting of, in mass %, C: 0.0005 to 0.0070%, Si: 0.01 to 1.50%, Mn: 0.05 to 1.50%, P: 0.001 to 0.150%, S: 0.0005 to 0.0100%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, Ti: 0.010 to 0.150%, Sn: 0.020 to 0.080%, an optional element, and the balance: Fe and an impurity, wherein formulas [Ti-48/32×S-48/14×N-48/12×C≥−0.010] and [5.0≤11Si+33Mn+21Mo+17(Cr+Cu+Ni)−30Al≤50.0] are satisfied, rL is 1.50 or more, rC is 1.50 or more, rD is 1.50 or more, mr is 1.70 or more, and Δr falls within a range of −0.40 to 0.40.

Description

TECHNICAL FIELD

The present disclosure relates to a cold-rolled steel sheet.

BACKGROUND ART

Automobile parts are produced by press forming of a steel sheet as a raw material, for example. Recently, those parts have come to have more diverse and complicated shapes. Therefore, the cold-rolled steel sheet used as a raw material is required to have an excellent formability, in particular, an excellent deep drawability. To meet such needs, for example, Patent Documents 1 and 2 disclose a cold-rolled steel sheet improved in deep drawability.

LIST OF PRIOR ART DOCUMENTS

Patent Document

• Patent Document 1: JP2006-219737A
• Patent Document 2: JP2009-270191A

SUMMARY OF INVENTION

Technical Problem

In the field of automobile parts described above, in recent years, raw materials have been required to be further improved in corrosion resistance from the viewpoint of the use environment or the like. However, the cold-rolled steel sheets disclosed in Patent Documents 1 and 2 are not sufficiently examined in terms of corrosion resistance. No cold-rolled steel sheet is reported which contains a certain amount of Sn in the chemical composition of the steel sheet in order to improve corrosion resistance and at the same time has excellent and less variable deep drawability.

In view of these circumstances, an objective of the present disclosure is to solve the problems described above and provide a cold-rolled steel sheet that contains a certain amount or more of Sn in order to improve the corrosion resistance and at the same time is improved in deep drawability.

Solution to Problem

The gist of the present disclosure, which has been made to solve the problem described above, is the cold-rolled steel sheet described below.

(1) A cold-rolled steel sheet having a chemical composition consisting of, in mass %,

• • C: 0.0005 to 0.0070%,
  • Si: 0.01 to 1.50%,
  • Mn: 0.05 to 1.50%,
  • P: 0.001 to 0.150%,
  • S: 0.0005 to 0.0100%,
  • Al: 0.005 to 0.100%,
  • N: 0.0010 to 0.0200%,
  • Ti: 0.010 to 0.150%,
  • Sn: 0.020 to 0.080%,
  • Nb: 0 to 0.060%,
  • B: 0 to 0.0030%,
  • Mo: 0 to 0.50%,
  • Cr: 0 to 3.00%,
  • W: 0 to 3.00%,
  • Cu: 0 to 3.00%,
  • Ni: 0 to 3.00%,
  • Ca: 0 to 0.1000%,
  • REM: 0 to 0.100%,
  • V: 0 to 0.10%,
  • As: 0 to 0.10%,
  • Sb: 0 to 0.10%,
  • Pb: 0 to 0.10%,
  • Bi: 0 to 0.10%, and
  • the balance: Fe and an impurity, wherein
  • the following formulas (i) and (ii) are satisfied,
  • rL, which is an r value in a rolling direction, is 1.50 or more,
  • rC, which is an r value in a direction perpendicular to the rolling direction, is 1.50 or more,
  • rD, which is an r value in a direction at 45° with respect to the rolling direction, is 1.50 or more,
  • mr, which is defined by the following formula (iii), is 1.70 or more, and
  • Δr, which is defined by the following formula (iv), falls within a range of −0.40 to 0.40,

$\begin{array}{cc}\mathrm{Ti}-48/32\times S-48/14\times N-48/12\times C\ge -0.01& \left(i\right)\end{array}$ $\begin{array}{cc}5.\le 11\mathrm{Si}+33\mathrm{Mn}+21\mathrm{Mo}+17\left(\mathrm{Cr}+\mathrm{Cu}+\mathrm{Ni}\right)-30\mathrm{Al}\le 50.& \left(\mathrm{ii}\right)\end{array}$ $\begin{array}{cc}\mathrm{mr}=\left(\mathrm{rL}+2\mathrm{rD}+\mathrm{rC}\right)/4& \left(\mathrm{iii}\right)\end{array}$ $\begin{array}{cc}\Delta r=\left(\mathrm{rL}+\mathrm{rC}-2\mathrm{rD}\right)/2& \left(\mathrm{iv}\right)\end{array}$

• • where each symbol of element in the formulas (i) and (ii) denotes a content (in mass %) of the element contained in the steel, which is 0% if the element is not contained.

(2) The cold-rolled steel sheet according to (1) described above, wherein a random intensity ratio (A), which is a random intensity ratio in a {100}<001> orientation, is 1.8 or less,

• • a random intensity ratio (B), which is a random intensity ratio in a {111}<112> orientation, is 4.0 or more,
  • a random intensity ratio (C), which is a random intensity ratio in a {111}<110> orientation, is 4.0 or more, and
  • a ratio between the random intensity ratio (B) and the random intensity ratio (C) satisfies the following formula (v):
  • 0.80≤random intensity ratio (B)/random intensity ratio (C)≤1.20 . . . (v).

(3) The cold-rolled steel sheet according to (1) or (2) described above, wherein the chemical composition contains, in mass %, one or more selected from:

• • Nb: 0.001 to 0.060%,
  • B: 0.0001 to 0.0030%,
  • Mo: 0.01 to 0.50%,
  • Cr: 0.01 to 3.00%,
  • W: 0.01 to 3.00%,
  • Cu: 0.01 to 3.00%,
  • Ni: 0.01 to 3.00%,
  • Ca: 0.0001 to 0.1000%,
  • REM: 0.001 to 0.100%,
  • V: 0.01 to 0.10%,
  • As: 0.01 to 0.10%,
  • Sb: 0.01 to 0.10%,
  • Pb: 0.01 to 0.10%, and
  • Bi: 0.01 to 0.10%.

Advantageous Effects of Invention

According to the present disclosure, a cold-rolled steel sheet can be provided which contains a certain amount or more of Sn in order to improve the corrosion resistance and at the same time is improved in deep drawability.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a diagram showing primary orientations on ODF for a cross section of φ2=45°.

DESCRIPTION OF EMBODIMENTS

In order to provide a cold-rolled steel sheet that contains a certain amount or more of Sn in order to improve the corrosion resistance and at the same time has an excellent deep drawability, the inventor has made various investigations and made the following findings (a) to (c).

(a) Sn is a relatively inexpensive element and is an element commonly used to improve the corrosion resistance. In the chemical composition of the steel sheet, a content of Sn of 0.020% or more is particularly effective for improving corrosion resistance.

(b) In order to improve the deep drawability, it is necessary to improve the r value, which is an index of formability, and reduce the in-plane anisotropy. The “in-plane anisotropy” means a difference in ease of plastic deformation due to a difference in direction in the same plane when plastic deformation occurs. The higher the in-plane anisotropy, the more difficult it is to achieve uniform formation and to form a complicated shape. As a result, the yield is more likely to decrease. As the content of Sn increases, the absolute value of the r value decreases, and the in-plane anisotropy increases. The inventor has found that, when the content of Sn is 0.020% or more, this tendency is particularly marked and the deep drawability decreases.

(c) The inventor investigated how the content of Sn of 0.020% or more adversely affects the deep drawability. Then, the inventor estimated that a transition band, which becomes a nucleation site for recrystallization, is formed in a grain with the {100}<001> orientation and grains with the {100}<001> orientation and orientations close thereto are likely to develop during annealing. Then, from the viewpoint of improving the hardenability and reducing grains with the {100}<001> orientation at the end of hot rolling, the inventor found that it is effective to control various production conditions of hot rolling (specifically, the reduction at 950° C. or less, the rolling shape factor for the last pass and the rolling shape factor for the pass immediately preceding the last pass, and the time from the end of hot rolling to the start of cooling, or the like), for example.

In addition, the inventor estimated that the in-plane anisotropy can be improved by improving the r value, developing grains with the {111}<112> orientation and grains with {111}<110> orientation and controlling the ratio between the grains with these two orientations. Then, the inventor found that it is also effective to control the cold rolling ratio and the annealing temperature of the annealing subsequent to the cold rolling. Based on these findings, the inventor has clarified that the decrease of the r value and the increase of the in-plane anisotropy are reduced by such structure control.

An embodiment of the present disclosure is based on the findings described above. In the following explanation, the requirements of the embodiment will be described in detail.

1. Chemical Composition

Limitation reasons for elements are as follows. Note that in the following description, the unit “%” of the content means “mass %”.

C: 0.0005 to 0.0070%

C has an effect of improving the strength. In addition, a certain content of C has an effect of improving the r value and reducing the in-plane anisotropy. Therefore, the content of C is 0.0005% or more. The content of C is preferably 0.0007% or more, 0.0010% or more, or 0.0015% or more. However, if C is excessively contained, the r value decreases. Therefore, the content of C is 0.0070% or less. The content of C is preferably less than 0.0050%, 0.0040% or less, or 0.0030% or less.

Si: 0.01 to 1.50%

Si is a deoxidizing element. Si is also an element that is improved in strength by solid-solution strengthening. Therefore, the content of Si is 0.01% or more. The content of Si is preferably 0.03% or more, 0.05% or more, or 0.10% or more. However, if the content of Si is more than 1.50%, the press formability or the ease of chemical treatment decreases. Therefore, the content of Si is 1.50% or less. When hot-dip galvanizing is used, the content of Si is preferably 1.20% or less, 1.00% or less, 0.80% or less, 0.65% or less, 0.50% or less, or 0.35% or less, since a problem, such as a decrease of the plating adhesiveness or a decrease of the productivity due to a delay of an alloying reaction, occurs.

Mn: 0.05 to 1.50%

Mn is a solid-solution strengthening element and has an effect of improving the strength. Therefore, a certain amount of Mn is contained according to the application. In addition, a content of Mn of less than 0.05% is not particularly advantageous in terms of property and leads to an increase of the steel making cost, so that the content of Mn is 0.05% or more. From this point of view, the content of Mn is preferably 0.10% or more, 0.20% or more, or 0.30% or more. However, if Mn is excessively contained, the strength excessively increases, so that the r value decreases, and the adhesiveness of the galvanizing is inhibited. Therefore, the content of Mn is preferably 1.50% or less, 1.20% or less, 1.00% or less, or 0.90% or less.

P: 0.001 to 0.150%

Although P is an impurity contained in the steel, P can increase the strength at low cost, as with Si. In addition, P has an effect of refining a hot-rolled structure and improving the deep drawability. Therefore, the content of P is 0.001% or more. The content of P is preferably 0.005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. However, if P is excessively contained, the fatigue strength after spot welding deteriorates or the yield strength excessively increases, thereby causing a surface shape defect during press forming. Furthermore, the alloying reaction extremely slows down during continuous hot-dip galvanizing, and the productivity decreases. In addition, the secondary workability also decreases. Therefore, the content of P is 0.150% or less. The content of P is preferably 0.100% or less, 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

S: 0.0005 to 0.0100%

S is an impurity contained in the steel. If the content of S is more than 0.0100%, a hot crack and a decrease of the deep drawability are more likely to occur. Therefore, the content of S is 0.0100% or less. The content of S is preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. Although the content of S is preferably reduced as far as possible, if the content of S is excessively reduced, the production cost increases. Therefore, the content of S is 0.0005% or more. The content of S is preferably 0.0010% or more.

Al: 0.005 to 0.100%

Al is an element used as a deoxidation adjuster. In addition, Al markedly increases the transformation point and therefore has an effect of adjusting the transformation point. Therefore, the content of Al is 0.005% or more. The content of Al is preferably 0.010% or more, 0.0015% or more, or 0.0020% or more. However, if Al is excessively contained, the effect described above is saturated, and a nitride or the like may be formed to decrease the r value. Therefore, the content of Al is 0.100% or less. The content of Al is preferably 0.080% or less, 0.0060% or less, or 0.0050% or less.

N: 0.0010 to 0.0200%

N is an impurity contained in the steel, and a content of N of less than 0.0010% leads to an increase of the steel making cost. Therefore, the content of N is 0.0010% or more. The content of N is preferably 0.0015% or more, 0.0020% or more, or 0.0025% or more. However, if N is excessively contained, a nitride is excessively formed, and the r value decreases. Therefore, the content of N is 0.0200% or less. The content of N is preferably 0.0150% or less, 0.0100% or less, or 0.0080% or less.

Ti: 0.010 to 0.150%

Ti has an effect of improving the r value by forming a carbide, reducing dissolved C and refining grains of the hot rolled steel sheet. Therefore, the content of Ti is 0.010% or more. The content of Ti is preferably 0.020% or more or 0.025% or more. Note that with the steel sheet according to this embodiment, the content of Ti is reduced in order to avoid increasing the production cost. In addition, if Ti is excessively contained, the r value decreases. Therefore, the content of Ti is 0.150% or less. The content of Ti is preferably 0.100% or less, 0.080% or less, less than 0.070%, or 0.050% or less.

Sn: 0.020 to 0.080%

Sn has an effect of improving the corrosion resistance. Therefore, the content of Sn is 0.020% or more. Since the higher the content of Sn, the higher the corrosion resistance is, the content of Sn is preferably 0.025% or more and more preferably 0.030% or more. However, Sn is an element that decreases the r value and increases the in-plane anisotropy. Therefore, if Sn is excessively contained, the properties described above markedly deteriorate even if the production conditions and the like are controlled. Therefore, the content of Sn is 0.080% or less. The content of Sn is preferably 0.070% or less, 0.060% or less, or 0.050% or less.

In addition to the elements described above, one or more selected from Nb, B, Mo, Cr, W, Cu, Ni, Ca, REM, V, As, Sb, Pb and Bi may be contained in the ranges described below. Limitation reasons for the elements will be described. These elements are not necessarily contained, and the lower limit of the content of these elements is 0%.

Nb: 0 to 0.060%

Nb has an effect of refining grains of the hot-rolled steel sheet and improving the r value. Therefore, Nb can be contained as required. However, if Nb is excessively contained, recrystallization is markedly inhibited. Therefore, the content of Nb is 0.060% or less. The content ofNb is more preferably 0.040% or less or 0.020% or less. On the other hand, in order to achieve the effect described above, the content of Nb is preferably 0.001% or more and more preferably 0.005% or more.

B: 0 to 0.0030%

B has an effect of strengthening the grain boundary. B also has an effect of improving the strength of the steel sheet. Therefore, B can be contained as required. However, if B is excessively contained, the strength excessively increases, the r value decreases, and as a result, the deep drawability also decreases. Therefore, the content of B is 0.0030% or less. The content of B is preferably 0.0020% or less or 0.0015% or less. On the other hand, in order to achieve the effect described above, the content of B is preferably 0.0001% or more and more preferably 0.0002% or more.

Mo: 0 to 0.50%

With the steel sheet according to this embodiment, the decrease of the r value caused by containing Sn needs to be reduced. It is considered that if a grain with the {100}<001> orientation remains in the phase of the hot-rolled steel sheet, formation of a deformation structure referred to as a transition band is promoted, and the decrease of the r value described above occurs. Thus, grains with orientations other than the {100}<001> orientation, such as the {100}<011> orientation, the {211}<011> orientation and the {332}<113> orientation, need to be developed in the phase of the hot-rolled steel sheet.

Mo reduces formation of the transition band by increasing the hardenability, developing a transformation texture in the hot-rolled steel sheet and inhibiting development of grains with the {100}<001> orientation. As a result, Mo has an effect of improving the r value. Therefore, Mo can be contained as required. However, if Mo is excessively contained, the r value decreases, and the deep drawability decreases. Therefore, the content of Mo is 0.50% or less. The content of Mo is preferably 0.30% or less or 0.20% or less. On the other hand, in order to achieve the effect described above, the content of Mo is preferably 0.01% or more.

Cr: 0 to 3.00%

As with Mo, Cr has an effect of improving the r value by increasing the hardenability and developing a transformation texture in the hot-rolled steel sheet. Therefore, Cr can be contained as required. However, if Cr is excessively contained, the production cost increases. Therefore, the content of Cr is 3.00% or less. The content of Cris preferably 2.50% or less, 2.00% or less, 1.50% or less, or 1.00% or less. On the other hand, in order to achieve the effect described above, the content of Cr is preferably 0.01% or more and more preferably 0.03% or more.

W: 0 to 3.00%

W has an effect of improving the strength. Therefore, W can be contained as required. However, if W is excessively contained, the production cost increases. Therefore, the content of W is 3.00% or less. The content of W is preferably 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.50% or less, 0.10% or less, or 0.05% or less. On the other hand, in order to achieve the effect described above, the content of W is more preferably 0.01% or more.

Cu: 0 to 3.00%

As with Mo, Cu has an effect of improving the r value by increasing the hardenability and developing a transformation texture in the hot-rolled steel sheet. Therefore, Cu can be contained as required. However, if Cu is excessively contained, the production cost increases. Therefore, the content of Cu is 3.00% or less. The content of Cu is preferably 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.50% or less, or 0.30% or less. On the other hand, in order to achieve the effect described above, the content of Cu is preferably 0.01% or more.

Ni: 0 to 3.00%

As with Mo, Ni has an effect of improving the r value by increasing the hardenability and developing a transformation texture in the hot-rolled steel sheet. Therefore, Ni can be contained as required. However, if Ni is excessively contained, the production cost increases. Therefore, the content of Ni is 3.0% or less. The content of Ni is preferably 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.50% or less, or 0.25% or less. On the other hand, in order to achieve the effect described above, the content of Ni is preferably 0.01% or more.

Ca: 0 to 0.1000%

Ca is effective as a deoxidizing element and has an effect of controlling the form of sulfide. Therefore, Ca can be contained as required. However, if Ca is excessively contained, the production cost increases. Therefore, the content of Ca is 0.1000% or less. The content of Ca is preferably 0.0500% or less, 0.0300% or less, 0.0150% or less, 0.0075% or less, or 0.0030% or less. On the other hand, in order to achieve the effect described above, the content of Ca is preferably 0.0001% or more and more preferably 0.0005% or more.

REM: 0 to 0.100%

REM has an effect of improving the strength. Therefore, REM can be contained as required. However, if REM is excessively contained, the effect is saturated, and the production cost increases. Therefore, the content of REM is 0.100% or less. The content of REM is preferably 0.080% or less, 0.060% or less, 0.040% or less, 0.020% or less, 0.010% or less, or 0.005% or less. On the other hand, in order to achieve the effect described above, the content of REM is preferably 0.001% or more.

REM refers to a total of 17 elements, Sc, Y and lanthanoid, and the content of REM described above means the total content of these elements. Industrially, REM is commonly added in the form of misch metal.

V: 0 to 0.10%

V has an effect of refining grains of the hot-rolled steel sheet. Therefore, V can be contained as required. However, if V is excessively contained, recrystallization is excessively inhibited. Therefore, the content of V is 0.10% or less. The content of V is preferably 0.08% or less, 0.06% or less, or 0.04% or less. On the other hand, in order to achieve the effect described above, the content of V is preferably 0.01% or more.

• • As: 0 to 0.10%
  • Sb: 0 to 0.10%
  • Pb: 0 to 0.10%
  • Bi: 0 to 0.10%

As, Sb, Pb and Bi are elements that may be mixed as impurities. Although these elements are preferably reduced as far as possible, these elements may be included in the steel, depending on the raw material. Therefore, the content of As is 0.10% or less. The content of Sb is 0.10% or less. The content of Pb is 0.10% or less. The content of Bi is 0.10% or less. These elements are preferably reduced as far as possible, and the total content of these elements is preferably 0.40% or less. The lower limit of the content of these elements may be 0.01%.

In the chemical composition according to this embodiment, the balance consists of Fe and impurities. The “impurity” here means a constituent that is mixed because of the raw material, such as ore or scrap, or various factors of the production process when steel sheets are industrially produced and is allowable to an extent the impurity has no adverse effect in this embodiment.

Formula (i)

The chemical composition described above satisfies the following formula (i).

$\begin{array}{cc}\mathrm{Ti}-48/32\times S-48/14\times N-48/12\times C\ge -0.01& \left(i\right)\end{array}$

In this formula (i), each symbol of element denotes the content (in mass %) of the element contained in the steel, and the value is 0 if the element is not contained.

The value of the left side of the formula (i) is a value obtained by subtracting the amount of Ti precipitated in the forms of Ti sulfide, Ti nitride and Ti carbide from the content of Ti, and serves as an index of the amount of dissolved Ti. By controlling the amount of dissolved Ti to fall within an appropriate range, the amount of dissolved C in the hot-rolled steel sheet can be reduced, and the r value can be improved. Therefore, with the steel sheet according to this embodiment, the effect described above can be achieved while reducing Ti as far as possible. Therefore, the value of the left side of the formula (i) is −0.010 (in mass %, and the same holds true for the description below in which units are not indicated) or more, preferably 0.010 or more, and more preferably 0.013 or more. Note that the upper limit value of the value of the left side of the formula (i) is not particularly limited. However, when the value of the left side of the formula (i) is calculated from the range of the content of each element of the steel sheet according to this embodiment, the upper limit value is 0.143. Note that the formula (i) is experimentally derived.

Formula (ii)

The chemical composition described above satisfies the following formula (ii).

$\begin{array}{cc}5.\le 11\mathrm{Si}+33\mathrm{Mn}+21\mathrm{Mo}+17\left(\mathrm{Cr}+\mathrm{Cu}+\mathrm{Ni}\right)-30\mathrm{Al}\le 50.& \left(\mathrm{ii}\right)\end{array}$

In this formula (ii), each symbol of element denotes the content (in mass %) of the element contained in the steel, and the value is 0 if the element is not contained.

The formula (ii) is an index of the hardenability after hot rolling and is experimentally derived. When the value of the middle of the formula (ii) is less than 5.0 (in mass %, and the same holds true for the description below in which units are not indicated), a sufficient hardenability cannot be ensured, and no aggregate structure develops in the hot-rolled steel sheet. As a result, it is considered that grains with the {100}<001> orientation remain in the hot-rolled steel sheet, the decrease of the r value due to Sn cannot be inhibited, and the in-plane anisotropy increases. Therefore, the value of the middle of the formula (ii) is 5.0 or more. The value of the middle of the formula (ii) is preferably 7.0 or more, and more preferably 9.0 or more. On the other hand, if the value of the middle of the formula (ii) is more than 50.0, the effect of reducing grains with the {100}<001> orientation is saturated, and recovery and recrystallization are markedly delayed. As a result, the r value decreases. Therefore, the value of the middle of the formula (ii) is 50.0 or less. The value of the middle of the formula (ii) is preferably 40.0 or less.

2. r value
2-1. r value in each direction

With the steel sheet according to this embodiment, rL, which is an r value in a rolling direction, rC, which is an r value in a direction perpendicular to the rolling direction, and rD, which is an r value in a direction at 45° with respect to the rolling direction, fall within the ranges described below.

Specifically, rL, which is the r value in the rolling direction, is 1.50 or more. If rL is less than 1.50, an excellent deep drawability cannot be achieved. rL is preferably 1.60 or more, and more preferably 1.70 or more. Note that although the upper limit of rL is not particularly limited, the upper limit is typically on the order of 3.00.

Similarly, rC, which is the r value in a direction perpendicular to the rolling direction, is 1.50 or more. If rC is less than 1.50, an excellent deep drawability cannot be achieved. rC is preferably 1.60 or more, and more preferably 1.70 or more. Note that although the upper limit of rC is not particularly limited, the upper limit is typically on the order of 3.00.

rD, which is the r value in a direction at 45° with respect to the rolling direction, is also 1.50 or more. If rD is less than 1.50, an excellent deep drawability cannot be achieved. rD is preferably 1.60 or more, and more preferably 1.70 or more. Note that although the upper limit of rD is not particularly limited, the upper limit is typically on the order of 3.00.

Note that the r values (rL, rD, rC) described above are measured by performing a tensile test in accordance with JIS Z 2254: 2021, 3.1. Note that in the test, a tensile elongation (total elongation) of 15.0±1.0% is applied to cause a strain, and the length between gauge marks is measured after the strain is removed. However, in accordance with the remark 3 of item 3.1 of JIS Z 2254: 2021, the r value is calculated according to the formula (2) of the remark 3 using the measurement result of the length described above (instead of thickness measurement). In the tensile test, three test specimens are used for each direction, and a mean value thereof is regarded as the r value in the direction.

Three types of test specimens, specifically, a test specimen whose longitudinal direction coincides with the rolling direction, a test specimen whose longitudinal direction coincides with a direction perpendicular to the rolling direction, and a test specimen whose longitudinal direction coincides with a direction at 45° with respect to the rolling direction, are prepared. The shape of the test specimens conforms to the No. 5 test coupon of JIS Z 2241: 2022, and the gauge length of the parallel part is 20 mm. In addition, in order that the test specimens are not affected by the mechanical processing during extraction thereof, a predetermined number of test specimens are extracted from a middle part of the steel sheet in the width direction that exhibits mean characteristic values, at positions at predetermined distances.

To measure the r values, the rolling direction needs to be determined. For example, when the steel sheet is in the shape of a coil, the longitudinal direction (coiling direction) of the coil is the rolling direction. On the other hand, in the case of a cut sheet, the rolling direction is determined in the procedure described below. First, when a streak pattern is recognized on a surface of a cut sheet (steel sheet), the direction parallel to the streak pattern is the rolling direction. On the other hand, when no streak pattern is recognized, a measurement based on the electron back scattering diffraction (EBSD) is performed on the assumption that a cross section of the cut sheet in the direction of the sheet thickness is a cross section perpendicular to the width direction during rolling, and an orientation distribution function (ODF) is created for a cross section of φ2=45°. This procedure of measurement and analysis is the same as the procedure for the random intensity ratio described later.

First, whether or not an edge of the cut sheet is parallel or perpendicular to the rolling direction is checked. When the edge of the cut sheet is not parallel or perpendicular to the rolling direction, the sum of random intensity ratios for Φ=0 to 45° other than φ1=0° or φ1=900 is more than a sum (A) of random intensity ratios for φ1=0° and Φ=0 to 45° or a sum (B) of random intensity ratios for φ1=90° and Φ=0 to 45°. In this case, the measurement of the random intensity ratio is performed while shifting the direction of extraction of the cross section stepwise to search for a direction in which the sum of random intensity ratio for φ1=0° or φ1=90° is maximized, and the direction in which the sum is maximized is a direction parallel or perpendicular to the rolling direction. In this way, a cross section parallel or perpendicular to the rolling direction can be determined.

In the cross section parallel or perpendicular to the rolling direction, the random intensity ratio (A) and the random intensity ratio (B) are compared. If (A)>(B), the cross section for which the measurement is performed is a cross section perpendicular to the rolling width direction, and if (A)<(B), the cross section for which the measurement is performed is a cross section perpendicular to the rolling direction. In this way, the rolling direction can also be determined by determining the cross section. Note that the sum of random intensity ratio can be determined by calculating the random intensity ratio every five degrees of Φ, such as for Φ=0°, 5°, 100 and so on, and summing the random intensity ratios.

2-2. Mean r Value

With the steel sheet according to this embodiment, mr (mean r value) defined by the following formula (iii) is 1.70 or more.

$\begin{array}{cc}\mathrm{mr}=\left(\mathrm{rL}+2\mathrm{rD}+\mathrm{rC}\right)/4& \left(\mathrm{iii}\right)\end{array}$

Note that each symbol in the formula (iii) is defined as follows, as described above.

• • rL: r value in the rolling direction
  • rD: r value in the direction at 45° with respect to the rolling direction
  • rC: r value in the direction perpendicular to the rolling direction

mr is referred to also as a mean plastic strain ratio. The greater the value of mr, the more excellent the deep drawability is, and if mr is less than 1.70, an excellent deep drawability cannot be achieved. Therefore, mr is 1.70 or more, and more preferably 1.80 or more. Note that although the upper limit of mr is not particularly limited, mr is typically on the order of 3.00.

2-3. Δr

With the steel sheet according to this embodiment, Ar, which is an index of the in-plane anisotropy and defined by the following formula (iv), falls within a range of −0.40 to 0.40.

$\begin{array}{cc}\Delta r=\left(\mathrm{rL}+\mathrm{rC}-2\mathrm{rD}\right)/2& \left(\mathrm{iv}\right)\end{array}$

Note that each symbol in the formula (iv) is defined as follows, as described above.

• • rL: r value in the rolling direction
  • rD: r value in the direction at 45° with respect to the rolling direction
  • rC: r value in the direction perpendicular to the rolling direction

Δr is preferably as close to 0 as possible, since the in-plane anisotropy decreases. With the steel sheet according to this embodiment, if Δr is less than −0.40, the in-plane anisotropy is high, and an excellent deep drawability cannot be achieved. Therefore, Ar is −0.40 or more, and is preferably −0.35 or more, −0.30 or more, or −0.25 or more. On the other hand, even if Δr is more than 0.40, the in-plane anisotropy is high, and an excellent deep drawability cannot be achieved. Therefore, Δr is 0.40 or less, and is preferably 0.35 or less, 0.30 or less, or 0.25 or less. That is, |Δr|, which is the absolute value of Δr, is 0.40 or less. |Δr| is more preferably 0.35 or less, 0.30 or less, or 0.25 or less.

3. Random Intensity Ratio

With the steel sheet according to this embodiment, as described above, grains with the {100}<001> orientation that is a cause of the decrease of the r value are desirably reduced. Therefore, the random intensity ratio (A), which is the random intensity ratio in the {100}<001> orientation, is preferably 1.8 or less, and more preferably 1.6 or less or 1.5 or less.

Furthermore, grains with γ-fiber orientations that have an effect of improving the r value, that is, grains with the {111}orientation on the sheet surface are desirably developed within an appropriate range. Therefore, the random intensity ratio (B), which is a random intensity ratio in the {111}<112> orientation is preferably 4.0 or more, and more preferably 4.5 or more or 5.0 or more. Similarly, a random intensity ratio (C), which is a random intensity ratio in the {111}<110> orientation, is preferably 4.0 or more, and more preferably 4.5 or more or 5.0 or more.

On the other hand, if γ-fiber grains are excessively developed, the in-plane anisotropy increases. Among the grains with γ-fiber orientations, grains with the {111}<112> orientation are likely to preferentially grow during annealing. Therefore, in order to make grains grow while keeping the r value and the in-plane anisotropy in balance, not only grains with the {111}<112> orientation but also grains with the other {111}orientations need to be made to grow. Therefore, the ratio between the random intensity ratio (B) and the random intensity ratio (C), or more specifically, the ratio of the random intensity ratio (B) to the random intensity ratio (B) preferably satisfies the following formula (v).

0.80≤random intensity ratio (B)/random intensity ratio (C)≤1.20 . . . (v)

If the ratio between the random intensity ratio (B) and the random intensity ratio (C), which is the value of the middle of the formula (v), is less than 0.80, the growth of grains with the γ-fiber orientations is not well-balanced, and the r value is more likely to decrease. Therefore, the value of the middle of the formula (v) is preferably 0.80 or more, and more preferably 0.85 or more or 0.90 or more.

Similarly, if the value of the middle of the formula (v) is more than 1.20, the growth of grains with the γ-fiber orientations is not well-balanced, and grains with the {111}<112> orientation are excessively developed in the structure. As a result, the in-plane anisotropy increases. Therefore, the value of the middle of the formula (v) is preferably 1.20 or less, and more preferably 1.15 or less or 1.10 or less.

The random intensity ratios (the random intensity ratio (A), the random intensity ratio (B) and the random intensity ratio (C)) are measured in the procedure described below. Specifically, the random intensity ratio is calculated based on the degree of integration of each crystal orientation on ODF for a cross section of φ2=45° created based on crystal orientation data obtained by EBSD measurement. Note that although the measurement is performed for a cross section perpendicular to the rolling width direction, ODF is determined by converting the crystal orientation data obtained for the cross section into data for a direction perpendicular to the sheet surface.

ODF is used for indicating orientations of a crystal structure with low symmetry and therefore is commonly represented by φ1=0 to 360°, Φ=0 to 180° and φ2=0 to 360°, and each orientation is indicated by (hkl) [uvw]. However, with the steel sheet according to this embodiment, a bcc crystal structure with high symmetry is used, and therefore, Φ and φ2 are represented in a range of 0 to 90°.

In addition, the range of φ1 varies with whether the symmetry of the deformation is considered in the calculation. With the steel sheet according to this embodiment, the symmetry of the rolling deformation is considered, and pl is represented as φ1=0 to 90°. FIG. 1 shows primary orientations on ODF for a cross section of φ2=45°. In this drawing, (111) [1-10] and (111) [0-11] are equivalent, and both mean the same as {111}<110>, which is a representation considering the symmetry. Therefore, the random intensity ratios for these orientations are the same value, and with the steel sheet according to this embodiment, the value for φ1=0° and Φ=55° is used as the random intensity ratio for {111}<110>. Similarly, the value for φ1=45° and Φ=0° is used for {100}<001>, and the value for φ1=90° and Φ=55° is used for {111}<112>.

Note that, in ODF, for example, the {100}<001> orientation indicates an orientation represented by φ1=45° and Φ=0° in the cross section of φ2=45° in a strict sense. However, since a measurement error may occur due to the mechanical processing of the test specimen or setting of the specimen, a maximum value of the random intensity ratios in the ranges of orientations of φ1=40 to 50° and Φ=0 to 5° is used as a representative of the random intensity ratio for the orientation. That is, the random intensity ratio is measured by allowing an error of ±5° about each of axes of φ1 and Φ. Similarly, for the other orientations, the random intensity ratio is measured by allowing an error of ±5 about each of axes of φ1 and Φ.

When creating ODF, using OIM Analysis, which is analysis software available from TSL Solutions Co., Ltd, analysis is performed under the following conditions.

• • Calculation Method: Harmonic Series Expansion
  • Series Rank[L]: 16
  • Gaussian Half-Width[degrees]: 5
  • Sample Symmetry: Orthotropic (Rolled sheet)

Note that, in the measurement based on EBSD, the test specimen can be created by polishing the steel sheet by mechanical polishing, chemical polishing or the like and polishing the cross section thereof perpendicular to the sheet width direction to a mirror finish by buffing or the like and then removing any strain by electrolytic polishing or chemical polishing. The measurement range is overall thickness in the sheet thickness direction and 1 mm in the rolling direction. Although the measurement interval is not particularly limited, the measurement interval is preferably adjusted so that the number of measurement points is ten thousand or more.

Although structure analysis was performed in various methods for the steel sheet according to this embodiment (the cold-rolled steel sheet of the invention) and steel sheets that do not satisfy the target values (the r value for each direction, the mr value and the Ar value) of deep drawing described above, structural characteristics of the steel sheet according to this embodiment was not able to be clarified. However, it was confirmed that the steel sheet according to this embodiment at least satisfies the requirements of the random intensity ratio described above, although some of the steel sheets that satisfy the requirements of the random intensity ratio do not satisfy the target values of deep drawing.

4. Tensile Strength

With the steel sheet according to this embodiment, the tensile strength (referred to also as TS) preferably falls within a range of 270 to 450 MPa. If TS is less than 270 MPa, the desired strength cannot be achieved, and if TS is more than 450 MPa, the deep drawability and formability decrease.

5. Plating Layer

The steel sheet according to this embodiment may have a plating layer on the surface thereof as required. The plating layer is preferably an electroplating layer, a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer. Note that the electroplating layer is a plating layer formed by electroplating. The hot-dip galvanizing layer is a plating layer formed by immersing a steel sheet in zinc molten at high temperature. The alloyed hot-dip galvanizing layer is a plating layer formed by subjecting a steel sheet subjected to hot-dip galvanizing to heat treatment, thereby generating iron-zinc alloy.

6. Production Method

A preferred production method for the steel sheet according to this embodiment will be described. The steel sheet according to this embodiment can be stably produced in the production method described below.

The steel having the chemical composition described above is molten and cast in a conventional method to produce a slab. The slab may be a continuous cast slab, one produced by a thin slab caster or the like. Furthermore, the slab may be produced in a process in which hot rolling is performed immediately after casting, such as continuous casting and direct rolling (CC-DR).

The produced slab is hot-rolled to form a hot-rolled steel sheet. Although the heating temperature of the slab during hot rolling is not particularly limited, the heating temperature commonly falls within a range of 1150 to 1350° C. If the heating temperature of the slab is less than 1150° C., it is difficult to adequately dissolve the carbide or the like, and if the heating temperature of the slab is more than 1350° C., grains are coarsened. In addition, the energy required for the production increases, and the production cost increases.

In the hot rolling, a frictional force between the rolling roll and the steel sheet causes a shear deformation in an outer layer of the hot-rolled steel sheet, and grains with different crystal orientations than those of grains in a middle layer in the sheet thickness develop. Such crystal orientations include the {100}<001> orientation that is a cause of formation of the transition band. Therefore, the shear-deformed layer is preferably minimized.

Typically, the hot rolling includes two rolling steps, coarse rolling and finish rolling performed after the coarse rolling. In each rolling step, a plurality of rolling mills having a pair of rolling rolls is provided in sequence. The steel sheet is rolled by passing through the rolling rolls of the plurality of rolling mills. Here, passing a steel sheet through a pair of rolls of one rolling mill for rolling is referred to as a pass. In the coarse rolling or finish rolling, a desired sheet thickness is achieved through a plurality of passes.

The higher the rolling shape factors in the last pass in the finish rolling (referred to also as the last pass, hereinafter) and the pass preceding the last pass (referred to also as the preceding pass, hereinafter), the more likely to develop the shear-deformed layer described above is. Therefore, the rolling shape factor in the last pass of the finish rolling is 3.3 or less, and the rolling shape factor in the preceding pass is 3.6 or less. Note that the rolling shape factor is defined by the following formula (a). Each symbol in the formula (a) is defined as follows.

$\begin{array}{cc}\Gamma =\mathrm{ld}/\mathrm{hm}& \left(a\right)\end{array}$

Each symbol in the formula (a) is defined as follows.

• • Γ: rolling shape factor
  • ld: projected length of arc of contact
  • hm: mean sheet thickness

ld and hm are calculated according to the following formulas (b) and (c).

$\begin{array}{cc}\mathrm{ld}=\surd \left(R\times \left(H-h\right)\right)& \left(b\right)\end{array}$ $\begin{array}{cc}\mathrm{hm}=\left(H+2h\right)/3& \left(b\right)\end{array}$

Each symbol in the formula (b) is defined as follows.

• • R: radius of roll
  • H: entry-side sheet thickness (the sheet thickness before the sheet enters the rolling mill of a pass)
  • h: exit-side sheet thickness (the sheet thickness when the sheet exits the rolling mill of a pass)

A finish rolling end temperature (referred to as FT, hereinafter) falls within a range of 880 to 950° C. If FT is less than 880° C., the hot rolling produces an outer layer in the a zone. As a result, grains with the {110}orientation that increases the anisotropy of the r value may remain after annealing after cold rolling. This makes it difficult to decrease the in-plane anisotropy. Besides, if the various r values decrease, mr may decrease. Therefore, FT is 880° C. or more. On the other hand, if FT is more than 950° C., the grains may be coarsened, and the various r value and mr may decrease. Therefore, FT is 950° C. or less.

In the hot rolling, the reduction in the temperature range of 980° C. or less falls within a range of 35 to 55%. If the reduction in the temperature range of 980° C. or less is less than 35%, the various r values are likely to decrease. As a result, mr may decrease. Therefore, the reduction in the temperature range of 980° C. or less is 35% or more. On the other hand, if the reduction in the temperature range of 980° C. or less is more than 55%, mr decreases to less than 1.70. Therefore, the reduction in the temperature range of 980° C. or less is 55% or less.

After the hot rolling, the hot-rolled steel sheet is cooled. The cooling is started within 0.5 s after completion of the hot rolling. If the time until the cooling is started after the hot rolling is more than 0.5 s, the r values are generally likely to decrease, and as a result, mr is less than 1.70. Therefore, the time until the cooling is started after the hot rolling is 0.5 s or less.

After the hot rolling, the hot-rolled steel sheet is cooled at a mean cooling rate of 20° C./s or more. If the mean cooling rate is less than 20° C./s, the transformation texture cannot be adequately developed in the hot-rolled steel sheet. As a result, grains with the {100}<001> orientation that is a cause of formation of the transition band develop, and the r value decreases. In addition, among grains with the γ-fiber orientations, grains with the {111}<112> orientation are excessively generated, and the in-plane anisotropy increases. Therefore, the mean cooling rate is 20° C./s or more.

After the cooling described above, the steel sheet is coiled in such a manner that a coiling temperature T_e satisfies the following formula (a).

$\begin{array}{cc}580\le T_c\left(°C.\right)\le T+575& \left(a\right)\end{array}$

T in the formula (a) is calculated according to the following formula (b).

$\begin{array}{cc}T=11\mathrm{Si}+33\mathrm{Mn}+21\mathrm{Mo}+17\left(\mathrm{Cr}+\mathrm{Cu}+\mathrm{Ni}\right)-30\mathrm{Al}& \left(b\right)\end{array}$

In this formula (b), each symbol of element denotes the content (in mass %) of the element contained in the steel, and the value is 0 if the element is not contained.

If the coiling temperature T_e is less than 580° C., precipitation of Ti carbide does not occur during coiling, so that dissolved C remains in the hot-rolled steel sheet, and the r value decreases. As a result, mr may decrease. Therefore, the coiling temperature T_e is 580° C. or more. On the other hand, even if the coiling temperature T_e is more than T+575° C., the transformation texture does not adequately develop, grains with the {100}<001> orientation that is a cause of formation of the transition band develop, and the r value decreases. Therefore, the coiling temperature T_e is T+575° C. or less. Note that pickling may be performed after coiling as required.

Cold rolling is then performed to form a cold-rolled steel sheet. The rolling ratio of the cold rolling (referred to as a cold rolling ratio, hereinafter) falls within a range of 60 to 90%. If the cold rolling ratio is less than 60%, grains with the {111}orientation that improves the r value do not adequately develop, and the mean r value decreases. Therefore, the cold rolling ratio is 60% or more. On the other hand, if the cold rolling ratio is more than 90%, grains with the {111}<112> orientation excessively develop, and the in-plane anisotropy increases. Therefore, the cold rolling ratio is 90% or less.

The produced cold-rolled steel sheet is annealed. The annealing temperature is 680° C. or more. If the annealing temperature is less than 680° C., recovery and recrystallization do not adequately occur, and the r value is not improved. In addition, the in-plane anisotropy increases. Therefore, the annealing temperature is 680° C. or more.

The duration for which the annealing temperature is held, that is, the annealing duration, is not particularly limited. However, from the viewpoints of completing recovery and recrystallization and of the production cost, for example, the annealing duration typically falls within a range of 1 s to 10 min. In the annealing, the annealing temperature may be held for the annealing duration. After the annealing, pickling may be performed as required.

Note that the produced steel sheet may be immersed in a galvanizing bath to produce a galvanized steel sheet having a galvanizing layer on the surface. The galvanized steel sheet may be subjected to an alloying heat treatment for alloying to produce an alloyed galvanized steel sheet. Alternatively, the produced steel sheet may be subjected to electroplating to produce an electroplated steel sheet. The conditions of the plating can be those of conventional methods.

For example, in the hot-dip galvanizing or alloying hot-dip galvanizing, the composition of the plating may contain Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni or the like as required. In addition, the alloying heat treatment is often performed in a range of 450 to 600° C., and the duration of the alloying heat treatment is often 10 s or more.

In the following, the steel sheet according to the present disclosure will be more specifically described with reference to examples. However, this embodiment is not limited to these examples.

EXAMPLE

Steels having chemical compositions described in Table 1 were molten to produce slabs. The produced slabs were hot-rolled and coiled and then cold-rolled and annealed (for an annealing duration of 60 s) under conditions described in Table 2 to produce cold-rolled steel sheets.

TABLE 1
Steel	Chemical composition (in mass %, balance: Fc and impurities)
No.	C	Si	Mn	P	S	Al	N	Ti	Sn	Nb	B	Mo	Cr
A	0.0025	0.01	0.13	0.009	0.0050	0.028	0.0025	0.032	0.025	—	—	—	0.90
B	0.0007	0.01	0.10	0.010	0.0041	0.031	0.0018	0.025	0.032	—	—	—	0.25
C	0.0032	0.01	0.20	0.011	0.0020	0.035	0.0042	0.068	0.028	—	—	—	0.05
D	0.0023	0.03	0.18	0.021	0.0024	0.043	0.0025	0.034	0.072	—	—	0.47	0.40
E	0.0042	0.01	0.10	0.025	0.0051	0.009	0.0062	0.068	0.036	0.017	0.0009	—	0.04
F	0.0020	0.02	0.10	0.010	0.0010	0.030	0.0032	0.023	0.023	0.015	—	—	0.11
G	0.0018	1.42	0.65	0.010	0.0021	0.039	0.0023	0.041	0.042	—	—	0.05	0.03
H	0.0020	0.02	0.12	0.010	0.0040	0.028	0.0042	0.058	0.046	—	—	—	0.09
I	0.0023	0.02	0.92	0.020	0.0020	0.028	0.0038	0.039	0.058	0.010	—	—	0.10
J	0.0020	0.05	1.21	0.030	0.0030	0.027	0.0020	0.029	0.020	—	0.0018	—	0.05
K	0.0033	0.12	0.80	0.050	0.0029	0.028	0.0110	0.070	0.067	0.012	0.0007	—	0.12
L	0.0032	0.35	0.10	0.016	0.0009	0.027	0.0031	0.038	0.052	0.053	—	—	0.16
M	0.0068	0.02	0.20	0.011	0.0050	0.030	0.0175	0.145	0.027	0.015	—	—	0.25
N	0.0021	0.02	0.82	0.009	0.0040	0.095	0.0021	0.042	0.022	—	—	—	0.10
O	0.0027	0.50	0.95	0.089	0.0060	0.042	0.0038	0.038	0.035	—	—	—	—
P	0.0028	0.01	0.12	0.010	0.0040	0.033	0.0098	0.089	0.120	0.023	—	0.12	0.53
Q	0.0023	0.01	1.50	0.012	0.0015	0.026	0.0065	0.078	0.023	—	—	0.30	0.76
R	0.0038	0.02	0.80	0.010	0.0020	0.044	0.0059	0.009	0.034	—	—	—	0.06
S	0.0028	0.01	1.32	0.042	0.0027	0.042	0.0055	0.180	0.057	—	—	—	0.03
T	0.0034	0.01	0.09	0.050	0.0020	0.042	0.0025	0.042	0.023	0.023	—	—	0.01
U	0.0023	0.01	0.15	0.011	0.0040	0.036	0.0027	0.056	0.015	—	—	—	0.15
											Left
											side	Middle
											value of	value of
Steel	Chemical composition (in mass %, balance: Fc and impurities)	formula	formula
No.	W	Cu	Ni	Ca	REM	V	As	Sb	Pb	Bi	(i)	(ii)
A	—	0.05	0.04	—	—	—	—	—	—	—	0.006	20.4
B	—	0.07	0.05	—	—	0.07	—	—	—	—	0.010	8.8
C	0.02	0.60	0.40	—	—	—	—	—	—	—	0.038	23.5
D	—	0.18	0.10	—	—	—	—	—	—	0.01	0.013	26.4
E	—	0.18	0.12	—	—	—	—	—	—	—	0.022	8.9
F	—	0.18	0.10	0.0006	—	—	—	—	—	—	0.002	9.3
G	—	0.06	0.05	—	—	—	—	—	0.02	—	0.023	39.3
H	—	0.15	0.08	—	—	—	0.02	0.01	—	—	0.030	8.8
I	—	0.05	0.07	—	—	—	—	—	—	—	0.014	33.5
J	—	0.15	0.09	—	0.003	—	—	—	—	—	0.010	44.6
K	—	0.06	0.03	0.0080	—	—	—	—	—	—	0.015	30.5
L	0.03	0.03	0.02	—	0.082	—	—	—	—	—	0.013	9.9
M	—	0.07	0.05	—	—	—	—	—	—	—	0.050	12.2
N	—	0.19	0.13	—	—	—	—	—	—	0.01	0.020	31.6
O	—	—	—	—	—	—	—	—	—	—	0.005	35.6
P	—	0.09	0.05	—	—	—	—	—	—	—	0.038	16.9
Q	—	0.37	0.22	—	—	—	—	0.02	—	—	0.044	78.1
R	—	0.07	0.04	—	—	—	—	—	—	—	−0.029	28.1
S	—	0.03	0.02	—	—	0.030	—	—	—	—	0.146	43.7
T	—	0.02	0.02	—	—	—	—	—	0.01	—	0.017	2.7
U	—	0.25	0.13	—	—	—	—	—	—	—	0.032	12.9
Underlines indicate that the values do not meet the chemical composition according to the present embodiment.

TABLE 2
										Cold
		Reduction	Shape factor		Cooling	Cooling		Coiling	rolling	Annealing
Production	Steel	at 950° C.	Preceding	Last	FT	start time	rate	575 + T	temperature	ratio	temperature
No.	No.	or less (%)	pass	pass	(° C.)	(s)	(° C./s)	(° C.)	(° C.)	(%)	(° C.)
1	A	43	2.6	2.4	910	0.5	28	595.4	580	83	800
2		39	2.6	2.4	890	1.5	25	595.4	580	80	810
3	B	37	2.7	2.5	935	0.3	23	583.8	580	75	880
4		38	3.7	3.4	926	0.3	25	583.8	580	82	820
5	C	42	2.8	2.6	920	0.3	34	598.5	580	83	800
6		36	2.9	2.6	944	0.3	10	598.5	580	83	790
7	D	41	3.6	3.3	894	0.3	29	601.4	590	80	830
8		49	2.7	2.4	850	0.4	32	601.4	580	83	800
9	E	35	2.8	2.5	940	0.4	26	583.9	580	82	840
10		47	2.9	2.5	923	0.4	39	583.9	650	84	780
11	F	50	3.0	2.8	935	0.4	24	584.3	580	76	820
12		52	3.9	2.8	926	0.4	29	584.3	580	82	780
13		36	3.2	2.9	945	0.4	27	584.3	500	81	820
14	G	54	3.0	2.7	890	0.1	30	614.3	600	79	800
15		43	3.0	2.7	925	0.1	37	614.3	610	55	820
16	H	44	2.8	2.7	930	0.1	38	583.8	580	82	820
17		46	3.4	3.1	938	0.1	22	583.8	580	92	860
18	I	47	2.9	2.6	912	0.2	33	608.5	600	85	850
19		59	3.4	3.2	885	0.2	30	608.5	600	80	820
20	J	44	3.4	2.7	906	0.3	23	619.6	610	80	790
21		40	3.3	3.3	928	0.3	32	619.6	590	74	650
22	K	45	3.3	2.8	925	0.2	39	605.5	590	80	790
23		39	3.5	3.5	920	0.2	30	605.5	600	80	820
24	L	40	2.8	2.4	926	0.1	27	584.9	580	76	820
25		24	2.5	2.2	926	0.1	30	584.9	580	80	820
26	M	40	2.9	2.7	910	0.1	22	587.2	580	82	840
27	N	38	2.9	2.7	940	0.3	20	606.6	590	82	840
28		35	3.5	3.3	960	0.4	50	606.6	600	80	800
29	O	40	2.9	2.7	900	0.2	30	610.6	600	80	820
30	P *	45	3.4	3.0	890	0.3	27	591.9	580	74	800
31	Q *	39	3.2	3.0	932	0.3	21	653.1	620	78	840
32	R *	38	3.0	2.9	920	0.3	36	603.1	580	78	780
33	S *	42	3.5	3.0	910	0.3	25	618.7	600	80	810
34	T *	46	3.4	3.1	899	0.3	24	577.7	570	78	800
35	U *	36	2.9	2.6	906	0.3	26	587.9	580	80	800
36		40	3.8	3.0	900	0.2	25	587.9	580	80	820
* indicates that the steel does not meet the chemical composition according to the present embodiment.
Underlines indicate that the values do not meet preferred production conditions according to the present embodiment.

For the produced steel sheets and plated steel sheets, the various random intensity ratios (the random intensity ratio (A), the random intensity ratio (B) and the random intensity ratio (C)) were measured, and the ratio between the random intensity ratio (B) and the random intensity ratio (C) was calculated. Furthermore, for the produced steel sheets, the tensile strength and the various r values (rL, rC and rD) were measured by a tensile test, and the mean r value and Δr were calculated from the various r values. In the following, the measurement method for the various random intensity ratios and the method of the tensile test will be described.

(Random Intensity Ratio)

The various random intensity ratios were measured in the procedure described below. Specifically, the random intensity ratios were calculated based on the degree of integration of each crystal orientation on ODF for a cross section of φ2=45° created based on crystal orientation data obtained by EBSD measurement.

ODF is used for indicating orientations of a crystal structure with low symmetry and therefore is commonly represented by φ1=0 to 360°, Φ=0 to 180° and φ2=0 to 360°, and each orientation is indicated by (hkl) [uvw]. However, with the steel sheet according to this embodiment, a bcc crystal structure with high symmetry is used, and therefore, Φ and φ2 are represented in a range of 0 to 90°.

In addition, the range of pl varies with whether the symmetry of the deformation is considered in the calculation. With the steel sheet according to this embodiment, the symmetry of the rolling deformation is considered, and pl is represented as φ1=0 to 90°. FIG. 1 shows primary orientations on ODF for a cross section of φ2=45°. In this drawing, (111) [1-10] and (111) [0-11] are equivalent, and both mean the same as {111}<110>, which is a representation considering the symmetry. Therefore, the random intensity ratios for these orientations are the same value, and with the steel sheet according to this embodiment, the value for φ1=0° and Φ=55° is used as the random intensity ratio for {111}<110>. Similarly, the value for φ1=45° and Φ=0° is used for {100}<001>, and the value for φ1=90° and Φ=55° is used for {111}<112>.

Note that, in ODF, for example, the {100}<001> orientation indicates an orientation represented by φ1=45° and Φ=0° in the cross section of φ2=45° in a strict sense. However, since a measurement error may occur due to the mechanical processing of the test specimen or setting of the specimen, a maximum value of the random intensity ratios in the ranges of orientations of φ1=40 to 50° and Φ=0 to 5° is used as a representative of the random intensity ratio for the orientation. That is, the random intensity ratio is measured by allowing an error of ±5° about each of axes of φ1 and Φ. Similarly, for the other orientations, the random intensity ratio is measured by allowing an error of ±5 about each of axes of φ1 and Φ.

When creating ODF, using OIM Analysis, which is analysis software available from TSL Solutions Co., Ltd, analysis is performed under the following conditions.

• • Calculation Method: Harmonic Series Expansion
  • Series Rank[L]: 16
  • Gaussian Half-Width[degrees]: 5
  • Sample Symmetry: Orthotropic (Rolled sheet)

Note that, in the measurement based on EBSD, the test specimen was created by polishing the steel sheet by mechanical polishing, chemical polishing or the like and polishing the cross section thereof perpendicular to the sheet width direction to a mirror finish by buffing or the like and then removing any strain by electrolytic polishing or chemical polishing. The measurement range was overall thickness in the sheet thickness direction and 1 mm in the rolling direction. The number of measurement points was adjusted to be ten thousand or more.

(Tensile Test)

For the produced steel sheets, a tensile test was performed to measure the various r values. Each r value was measured by performing the tensile test in accordance with JIS Z 2254: 2021, 3.1. Note that in the test, a tensile elongation (total elongation) of 15.0±1.0% was applied to cause a strain, and the length between gauge marks was measured after the strain was removed. In accordance with the remark 3 of JIS Z 2254: 2021, 3.1, the r value was calculated according to the formula (2) of the remark 3 using the measurement result of the length described above. In the tensile test, three test specimens were used for each direction, and a mean value thereof was regarded as the r value in the direction.

Three types of test specimens, specifically, a test specimen whose longitudinal direction coincides with the rolling direction, a test specimen whose longitudinal direction coincides with a direction perpendicular to the rolling direction, and a test specimen whose longitudinal direction coincides with a direction at 45° with respect to the rolling direction, were prepared. The shape of the test specimens conformed to the No. 5 test coupon of JIS Z 2241: 2022, and the gauge length of the parallel part was 20 mm. In addition, in order that the test specimens were not affected by the mechanical processing during extraction thereof, a predetermined number of test specimens were extracted from a middle part of the steel sheet in the width direction that exhibited mean characteristic values, at positions at predetermined distances.

In addition, a No. 5 test coupon of JIS Z 2241: 2022 that was different from the test specimens described above was fabricated in such a manner that the longitudinal direction thereof coincided with the rolling direction, the tensile strength (TS) of the test specimen was measured by performing the tensile test on the test specimen until rupture occurred. Table 3 below shows the result.

TABLE 3
			Random intensity ratio
			{100}<001>	{111}<112>	{111}<110>
			random	random	random
Production	Steel	TS	intensity	intensity	intensity		r value
No.	No.	MPa	ratio (A)	ratio (B)	ratio (C)	(B)/(C)	L	D	C	mr	Δr	Remarks
1	A	293	1.0	10.8	9.1	1.18	2.20	2.05	2.26	2.14	0.18	Inventive example
2		295	2.0	11.1	8.5	1.30	2.18	1.88	2.43	2.09	0.43 *	Comparative example
3	B	270	0.6	10.0	9.0	1.11	2.18	2.02	2.31	2.13	0.23	Inventive example
4		283	2.6	9.9	6.9	1.42	2.12	1.76	2.29	1.98	0.45 *	Comparative example
5	C	342	1.1	10.9	9.5	1.14	2.25	2.03	2.30	2.15	0.25	Inventive example
6		345	2.1	6.9	5.0	1.38	1.75	1.46 *	2.01	1.67 *	0.42 *	Comparative example
7	D	320	0.8	6.2	5.6	1.10	1.96	1.57	1.90	1.75	0.36	Inventive example
8		328	2.2	5.5	5.6	0.98	1.73	1.40 *	1.80	1.58 *	0.37	Comparative example
9	E	326	1.2	10.5	9.2	1.15	2.21	2.02	2.16	2.10	0.17	Inventive example
10		330	2.1	7.2	10.6	0.68	1.87	1.48 *	1.90	1.68 *	0.41 *	Comparative example
11	F	295	1.3	9.9	9.4	1.06	2.21	1.99	2.26	2.11	0.25	Inventive example
12		300	2.5	9.8	7.3	1.35	2.08	1.72	2.28	1.95	0.46 *	Comparative example
13		295	1.2	5.1	4.6	1.11	1.92	1.49 *	1.87	1.69 *	0.41 *	Comparative example
14	G	444	1.3	8.4	9.3	0.90	2.19	1.84	1.98	1.96	0.25	Inventive example
15		440	1.1	3.9	3.7	1.05	1.60	1.35 *	1.66	1.49 *	0.28	Comparative example
16	H	305	0.3	10.0	9.0	1.11	2.13	2.03	2.29	2.12	0.18	Inventive example
17		289	0.0	13.0	8.9	1.46	2.23	1.82	2.50	2.09	0.55 *	Comparative example
18	I	325	1.5	7.5	7.2	1.04	1.90	1.72	2.06	1.85	0.26	Inventive example
19		333	1.5	9.7	7.2	1.35	2.13	1.73	2.22	1.95	0.45 *	Comparative example
20	J	369	0.5	10.0	9.2	1.09	2.10	2.03	2.44	2.15	0.24	Inventive example
21		421	0.3	4.8	4.8	1.00	1.23 *	1.96	1.79	1.73	−0.45 *	Comparative example
22	K	401	0.6	7.8	8.1	0.96	1.95	1.73	2.07	1.87	0.28	Inventive example
23		400	2.4	9.2	7.4	1.24	1.99	1.70	2.25	1.91	0.42 *	Comparative example
24	L	332	1.4	9.2	8.3	1.11	2.06	1.85	2.24	2.00	0.30	Inventive example
25		334	2.1	10.0	7.5	1.33	2.21	1.81	2.37	2.05	0.48 *	Comparative example
26	M	349	1.1	10.6	9.9	1.08	2.23	2.00	2.30	2.13	0.27	Inventive example
27	N	328	0.3	9.8	9.9	0.98	2.13	1.95	2.22	2.06	0.23	Inventive example
28		335	2.1	6.0	4.8	1.24	1.80	1.44 *	1.89	1.64 *	0.41 *	Comparative example
29	O	442	0.5	9.0	8.0	1.13	1.96	1.87	2.11	1.95	0.17	Inventive example
30	P *	341	2.7	3.9	3.5	1.10	1.68	1.33 *	1.87	1.55 *	0.45 *	Comparative example
31	Q *	389	1.3	3.8	3.6	1.06	1.15 *	1.86	1.73	1.65 *	−0.42 *
32	R *	336	1.5	3.3	3.1	1.06	1.55	1.49 *	1.79	1.58 *	0.18
33	S *	385	1.7	3.2	4.8	0.67	1.43 *	1.73	1.23 *	1.53 *	−0.40
34	T *	320	3.1	6.9	7.2	0.96	1.73	1.48 *	1.99	1.67 *	0.38
35	U *	299	0.1	11.5	11.7	0.98	2.05	1.88	2.18	2.00	0.23
36		287	0.0	11.2	10.7	1.05	2.05	1.86	2.15	1.98	0.24
* indicates that the steel does not meet requirements of the present embodiment.

In the production Nos. 1, 3, 5, 7, 9, 11, 14, 16, 18, 20, 22, 24, 26 and 27 that satisfy the requirements of this embodiment, an excellent deep drawability was exhibited. On the other hand, in the production Nos. 2, 4, 6, 8, 10, 12, 13, 15, 17, 19, 21, 23, 25, 28 and 29 that did not satisfy the requirements of this embodiment, at least one or more of the various r values, the mean r value and Δr decreased, and an excellent deep drawability was not exhibited.

Claims

1. A cold-rolled steel sheet having a chemical composition consisting of, in mass %, Ti - 48 / 32 × S - 48 / 14 × N - 48 / 12 × C ≥ - 0.01 ( i ) 5. ≤ 11 ⁢ Si + 33 ⁢ Mn + 21 ⁢ Mo + 17 ⁢ ( Cr + Cu + Ni ) - 30 ⁢ Al ≤ 50. ( ii ) mr = ( rL + 2 ⁢ rD + rC ) / 4 ( iii ) Δ ⁢ r = ( rL + rC - 2 ⁢ rD ) / 2 ( iv )

C: 0.0005 to 0.0070%,

Si: 0.01 to 1.50%,

Mn: 0.05 to 1.50%,

P: 0.001 to 0.150%,

S: 0.0005 to 0.0100%,

Al: 0.005 to 0.100%,

N: 0.0010 to 0.0200%,

Ti: 0.010 to 0.150%,

Sn: 0.020 to 0.080%,

Nb: 0 to 0.060%,

B: 0 to 0.0030%,

Mo: 0 to 0.50%,

Cr: 0 to 3.00%,

W: 0 to 3.00%,

Cu: 0 to 3.00%,

Ni: 0 to 3.00%,

Ca: 0 to 0.1000%,

REM: 0 to 0.100%,

V: 0 to 0.10%,

As: 0 to 0.10%,

Sb: 0 to 0.10%,

Pb: 0 to 0.10%,

Bi: 0 to 0.10%, and

the balance: Fe and an impurity, wherein

the following formulas (i) and (ii) are satisfied,

rL, which is an r value in a rolling direction, is 1.50 or more,

rC, which is an r value in a direction perpendicular to the rolling direction, is 1.50 or more,

rD, which is an r value in a direction at 45° with respect to the rolling direction, is 1.50 or more,

mr, which is defined by the following formula (iii), is 1.70 or more, and

Δr, which is defined by the following formula (iv), falls within a range of −0.40 to 0.40,

where each symbol of element in the formulas (i) and (ii) denotes a content (in mass %) of the element contained in the steel, which is 0% if the element is not contained.

2. The cold-rolled steel sheet according to claim 1, wherein a random intensity ratio (A), which is a random intensity ratio in a {100}<001> orientation, is 1.8 or less,

a random intensity ratio (B), which is a random intensity ratio in a {111}<112> orientation, is 4.0 or more,

a random intensity ratio (C), which is a random intensity ratio in a {111}<110> orientation, is 4.0 or more, and

a ratio between the random intensity ratio (B) and the random intensity ratio (C) satisfies the following formula (v):

0.80≤random intensity ratio (B)/random intensity ratio (C)≤1.20... (v).

3. The cold-rolled steel sheet according to claim 1, wherein the chemical composition contains, in mass %, one or more selected from:

Nb: 0.001 to 0.060%,

B: 0.0001 to 0.0030%,

Mo: 0.01 to 0.50%,

Cr: 0.01 to 3.00%,

W: 0.01 to 3.00%,

Cu: 0.01 to 3.00%,

Ni: 0.01 to 3.00%,

Ca: 0.0001 to 0.1000%,

REM: 0.001 to 0.100%,

V: 0.01 to 0.10%,

As: 0.01 to 0.10%,

Sb: 0.01 to 0.10%,

Pb: 0.01 to 0.10%, and

Bi: 0.01 to 0.10%.

4. The cold-rolled steel sheet according to claim 2, wherein the chemical composition contains, in mass %, one or more selected from:

Nb: 0.001 to 0.060%,

B: 0.0001 to 0.0030%,

Mo: 0.01 to 0.50%,

Cr: 0.01 to 3.00%,

W: 0.01 to 3.00%,

Cu: 0.01 to 3.00%,

Ni: 0.01 to 3.00%,

Ca: 0.0001 to 0.1000%,

REM: 0.001 to 0.100%,

V: 0.01 to 0.10%,

As: 0.01 to 0.10%,

Sb: 0.01 to 0.10%,

Pb: 0.01 to 0.10%, and

Bi: 0.01 to 0.10%.

5. A cold-rolled steel sheet having a chemical composition comprising, in mass %, Ti - 48 / 32 × S - 48 / 14 × N - 48 / 12 × C ≥ - 0.01 ( i ) 5. ≤ 11 ⁢ Si + 33 ⁢ Mn + 21 ⁢ Mo + 17 ⁢ ( Cr + Cu + Ni ) - 30 ⁢ Al ≤ 50. ( ii ) mr = ( rL + 2 ⁢ rD + rC ) / 4 ( iii ) Δ ⁢ r = ( rL + rC - 2 ⁢ rD ) / 2 ( iv )

C: 0.0005 to 0.0070%,

Si: 0.01 to 1.50%,

Mn: 0.05 to 1.50%,

P: 0.001 to 0.150%,

S: 0.0005 to 0.0100%,

Al: 0.005 to 0.100%,

N: 0.0010 to 0.0200%,

Ti: 0.010 to 0.150%,

Sn: 0.020 to 0.080%,

Nb: 0 to 0.060%,

B: 0 to 0.0030%,

Mo: 0 to 0.50%,

Cr: 0 to 3.00%,

W: 0 to 3.00%,

Cu: 0 to 3.00%,

Ni: 0 to 3.00%,

Ca: 0 to 0.1000%,

REM: 0 to 0.100%,

V: 0 to 0.10%,

As: 0 to 0.10%,

Sb: 0 to 0.10%,

Pb: 0 to 0.10%,

Bi: 0 to 0.10%, and

the balance: Fe and an impurity, wherein

the following formulas (i) and (ii) are satisfied,

rL, which is an r value in a rolling direction, is 1.50 or more,

rC, which is an r value in a direction perpendicular to the rolling direction, is 1.50 or more,

rD, which is an r value in a direction at 45° with respect to the rolling direction, is 1.50 or more,

mr, which is defined by the following formula (iii), is 1.70 or more, and

Δr, which is defined by the following formula (iv), falls within a range of −0.40 to 0.40,

where each symbol of element in the formulas (i) and (ii) denotes a content (in mass %) of the element contained in the steel, which is 0% if the element is not contained.