HIGH STRENGTH HOT ROLLED STEEL SHEET

Abstract

There is provided a high strength hot rolled steel sheet including a predetermined chemical composition. A structure includes, by area ratio, 80% or more of polygonal ferrite, a total of 5% or less of martensite and austenite, and a total of 5% or less of pearlite and cementite, and a remainder is at least one selected from bainitic ferrite and bainite. When a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller. A grain of the polygonal ferrite contains 5×107 pieces/mm2 or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, the aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3. The tensile strength is 540 MPa or higher.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot rolled steel sheet, and particularly relates to a high strength hot rolled steel sheet which has excellent hole expansibility and is suitable for chassis components and the like of automobiles formed into various forms through pressing or the like.

RELATED ART

Hot rolled steel sheets are manufactured at relatively low cost and are widely used for various types of industrial equipment including automobiles. Recently, from a viewpoint on the restriction on carbon dioxide emission entailing the measures against global warming, fuel efficiency of automobiles has been required to be improved. Moreover, for the purpose of reducing weight and ensuring collision safety of vehicle bodies, high strength hot rolled steel sheets have been widely applied to automobile components.

Needless to mention, steel sheets for automobile components have to satisfy not only the strength but also various types of workability such as press formability and weldability required at the time of forming the components. For example, when chassis components are press-formed, the frequency of use of stretch flange forming and burring forming is extremely high. Therefore, high strength hot rolled steel sheets for the chassis components are required to have excellent hole expansibility. In addition, in the chassis components, from a viewpoint on ensuring safety, many components are required to avoid plastic deformation even in a case where a large load is applied. Therefore, steel sheets for the chassis components are required to have a high yield ratio.

Generally, in high strength hot rolled steel sheets, in order to achieve both the high yield ratio and the excellent hole expansibility, it is examined that the structure is uniformly strengthened by controlling the steel structure to be a single phase structure containing any one of ferrite, bainitic ferrite, bainite, and the like, through solid solution strengthening of Mn, Si, and the like, and/or carbide of Ti, Nb, V, and the like or precipitation strengthening due to Cu.

For example, Patent Document 1 discloses a technology that relates to a high strength hot rolled steel sheet having excellent hole expansibility, in which Ti carbide including Mo is dispersed in a substantially single phase structure of ferrite in a uniform and fine manner. However, in the technology of Patent Document 1, it is essential to add Mo which is a very expensive alloying element. Therefore, from an economic viewpoint, the configuration is not suitable for mass production.

Patent Document 2 discloses a technology in which elongation and stretch flangeability of a high strength hot rolled steel sheet are improved by appropriately controlling cooling of Ti-added steel containing predetermined amounts of Mn and Si during a period from hot rolling to coiling such that a structure having ferrite and bainite is achieved, and causing TiC to be finely precipitated. However, in Patent Document 2, there is no consideration for the yield ratio which is one of characteristics necessary for a hot rolled steel sheet applied to chassis components. In addition, even though bainite has a low yield ratio compared to ferrite after precipitation strengthening, the technology of Patent Document 2 allows bainite to be included up to 50%, and it is analogized that a high yield ratio cannot be maintained. Moreover, the definition of ferrite defined in Patent Document 2 is unclear, and it is assumed that the ferrite includes so-called bainitic ferrite or pseudo-polygonal ferrite which is not polygonal ferrite. As the reason, in Patent Document 2, a temperature range of 720° C. or lower at which polygonal ferrite is not sufficiently formed is also allowed as a first cooling stop temperature. Bainitic ferrite and pseudo-polygonal ferrite have a structure indicating a yield ratio lower than that of polygonal ferrite.

Patent Document 3 discloses a Ti-added high strength hot rolled steel sheet of which toughness and hole expansibility are improved by reducing the Mn content and controlling the percentage of C which is precipitated as cementite. However, in the hot rolled steel sheet of Patent Document 3, in a case of postulating application to chassis components, for example, a high yield ratio of 75% or more is not obtained in high strength steel of 540 MPa or higher.

In addition, Patent Document 4 discloses a technology that relates to a high strength hot rolled steel sheet having excellent hole expansibility, in which corsening of TiC is suppressed by reducing the Mn content and the Si content and adding certain amounts of Ti and B. However, since B has an effect of suppressing the recrystallization of austenite from being recrystallized, in a case of being subjected to multiple addition together with Ti having a similar effect, a rolling load during hot rolling increases remarkably, resulting in an increase of a load to a hot rolling mill. Therefore, there is concern that the technology of Patent Document 4 causes operational trouble. In addition, since the strength of a final product considerably varies when the B content fluctuates only by several ppm, steel essentially containing B is not suitable for mass production.

Patent Document 5 discloses a high strength hot rolled steel sheet which has a high yield ratio and excellent hole expansibility and is obtained by cooling steel containing large amounts of Si, Mn, and Ti under an appropriate cooling condition and causing the structure to be a single phase structure of granular bainitic ferrite. However, in the technology of Patent Document 5, in order to obtain the granular bainitic ferrite structure, large amounts of Si and Mn need to be contained, thereby leading to a problem of an increase in alloying cost.

PRIOR ART DOCUMENT

[Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-322540

[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2007-009322

[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H10-287949

[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2012-026032

[Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2004-307919

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The present invention has been made in consideration of the current circumstances described above. An object of the present invention is to provide a high strength hot rolled steel sheet having a high yield ratio and excellent hole expansibility. The high strength in the present invention indicates that tensile strength (TS) is 540 MPa or higher.

Means for Solving the Problem

Ti is relatively inexpensive and exhibits remarkable precipitation strengthening with a minute amount of Ti content. In order to achieve excellent hole expansibility, the inventors have examined structures of hot rolled steel sheets on the premise that polygonal ferrite is employed as a main constituent. Moreover, in order to improve the strength of the structure having excellent hole expansibility and containing polygonal ferrite as a main constituent, the inventors have examined utilization of precipitation strengthening of Ti. The inventors have also intensively examined a technique of improving the hole expansibility in a Ti-containing high strength hot rolled steel sheet in which Ti precipitates are precipitated in polygonal ferrite as a main constituent of the structures. As a result, the following knowledge has been acquired.

The inventors have measured micro-hardness of each ferrite grain in steel having polygonal ferrite as a main constituent of the structure. As a result, it has been found that the hardness significantly varies depending on each of the measured grains. Furthermore, it has been found that hole expansibility can be remarkably improved by reducing unevenness in the hardness of ferrite grains.

In addition, the inventors have observed the intragranular state of polygonal ferrite of a sample having inferior hole expansibility, using a transmission electron microscope. As a result, it has been found that a large amount of Ti-based anisometric carbide stretched in a particular orientation of the ferrite is precipitated and this exerts an adverse influence on the hole expansibility. In the related art, there have been few reports on the shape of Ti carbide affecting hole expansibility, and the mechanism of the shape of Ti-based carbide affecting hole expansibility is obscure. However, compared to Ti-based isometric carbide, Ti-based anisometric carbide is highly consistent with matrix phase ferrite, and it is estimated that considerable consistency distortion is accumulated around the Ti-based anisometric carbide. Therefore, it is estimated that this consistency distortion incites cracks to be propagated during hole expanding resulting in deterioration of the hole expansibility.

The present invention has been made based on the knowledge described above. The gist is as follows.

(1) According to an aspect of the present invention, there is provided a hot rolled steel sheet including, as a chemical composition, by mass %, C: 0.010% to 0.200%, Si: 0.001% to 2.50%, Mn: 0.001% to 1.50%, P: 0.050% or less, S: 0.010% or less, N: 0.0070% or less, Al: 0.001% to 0.50%, Ti: 0.050% to 0.30%, V: 0% to 0.50%, Nb: 0% to 0.090%, Cr: 0% to 0.50%, Ni: 0% to 0.50%, Cu: 0% to 0.50%, Mo: 0% to 0.50%, B: 0% to 0.0050%, Ca: 0% to 0.01%, Mg: 0% to 0.01%, Bi: 0% to 0.01%, and a remainder of Fe and impurities. The structure includes, by area ratio, 80% or more of a polygonal ferrite, a total of 5% or less of a martensite and an austenite, and a total of 5% or less of a pearlite and a cementite, and the remainder is at least one selected from a bainitic ferrite and a bainite. When a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller. A grain of the polygonal ferrite contains 5×10⁷ pieces/mm² or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, the aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3. The tensile strength is 540 MPa or higher.

(2) In the hot rolled steel sheet according to (1) the chemical composition may include, by mass %, at least one selected from the group consisting of V: 0.010% to 0.50%, Nb: 0.001% to 0.090%, Cr: 0.001% to 0.50%, Ni: 0.001% to 0.50%, Cu: 0.001% to 0.50%, Mo: 0.001% to 0.50%, and B: 0.0001% to 0.0050%.

(3) In the hot rolled steel sheet according to (1) or (2), the chemical composition may include, by mass %, at least one selected from the group consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.

(4) The hot rolled steel sheet according to any one of (1) to (3) may further include a hot-dip galvanized layer on a surface.

Effects of the Invention

According to the aspect of the present invention, it is possible to inexpensively manufacture a high strength hot rolled steel sheet having a high yield ratio and excellent hole expansibility. In addition, the steel sheet according to the aspect of the present invention also has excellent hole expansibility even during stretch flanges forming frequently employed for automobile components, particularly chassis components and the like. Therefore, the steel sheet particularly contributes to reducing weight and ensuring collision safety of vehicle bodies in automobile fields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of a treatment pattern of hot rolling.

FIG. 2 is a schematic view showing an example of a heat treatment pattern in a galvannealing line employed in Example 2.

FIG. 3A is a view showing an example of micro-hardness distribution of polygonal ferrite measured in Example 1.

FIG. 3B is a view showing another example of micro-hardness distribution of polygonal ferrite measured in Example 1.

EMBODIMENT OF THE INVENTION

Hereinafter, a high strength hot rolled steel sheet according to an embodiment of the present invention (hereinafter, will be sometimes referred to as a hot rolled steel sheet according to the present embodiment) will be described in detail.

There is provided a hot rolled steel sheet according to the present embodiment.

(a) The hot rolled steel sheet according to the present embodiment includes, as a chemical composition, by mass %, C: 0.010% to 0.200%, Si: 0.001% to 2.50%, Mn: 0.001% to 1.50%, P: 0.050% or less, S: 0.010% or less, N: 0.0070% or less, Al: 0.001% to 0.50%, and Ti: 0.050% to 0.30%. Furthermore, as necessary, the hot rolled steel sheet includes at least one selected from the group consisting of V: 0.50% or less, Nb: 0.090% or less, Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, B: 0.0050% or less, Ca: 0.01% or less, Mg: 0.01% or less, and Bi: 0.01% or less; and a remainder of Fe and impurities.

(b) A structure includes, by area ratio, 80% or more of polygonal ferrite, a total of 5% or less of martensite and austenite, and a total of 5% or less of pearlite and cementite, and the remainder is at least one selected from bainitic ferrite and bainite.

(c) When a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller.

(d) A grain of the polygonal ferrite contains 5×10⁷ pieces/mm² or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, an aspect ratio which is a ratio of the length of a long side to the length of a short side is less than 3.

(e) Tensile strength is 540 MPa or higher.

<Chemical Composition of Steel Sheet>

First, the reasons for limiting the chemical composition of the hot rolled steel sheet according to the present embodiment will be described. Hereinafter, all the percentage signs “%” regulating the chemical composition indicate “mass %”.

[C: 0.010% to 0.200%]

C is an element essential to high-strengthening of a steel sheet performed through precipitation strengthening or solid solution strengthening. In order to achieve this effect, the C content is set to 0.010% or more, is preferably set to 0.020% or more, and is more preferably set to 0.040% or more. Meanwhile, when there is an excessive amount of C, forming of polygonal ferrite is suppressed and cementite is likely to be formed. In addition, the hardness difference among the grains of polygonal ferrite tends to increase. As a result, hole expansibility deteriorates. In addition, weldability also deteriorates remarkably. Therefore, the C content is set to 0.200% or less, is preferably set to 0.130% or less, and is more preferably set to 0.110% or less.

[Si: 0.001% to 2.50%]

Si is a solid solution strengthening element and is an element effective in high-strengthening of a steel sheet. In order to achieve this effect, the Si content is set to 0.001% or more, is preferably set to 0.01% or more, and is more preferably set to 0.04% or more. Meanwhile, if there is an excessive amount of Si, island-shaped scale is generated, and surface quality deteriorates. Therefore, the Si content is set to 2.50% or less, is preferably set to 1.30% or less, and is more preferably set to 0.80% or less.

[Mn: 0.001% to 1.50%]

Mn is an element effective in improving strength of a steel sheet. In addition, Mn is an element which fixes S in steel as MnS and suppresses hot embrittlement caused by a solid solution S. In order to achieve these effects, the Mn content is set to 0.001% or more, is preferably set to 0.10% or more, and is more preferably set to 0.45% or more. Meanwhile, when there is an excessive amount of Mn, ferritic transformation from austenite is delayed, so that it becomes difficult to obtain 80 area % or more of polygonal ferrite, and hole expansibility deteriorates. Therefore, the Mn content is set to 1.50% or less, is preferably set to 1.00% or less, and is more preferably set to 0.80% or less.

[P: 0.050% or Less]

P is an element contained as impurities, which cause weldability and toughness of a steel sheet to deteriorate. Therefore, it is preferable to have a small amount of P. However, in a case where the P content exceeds 0.050%, the influence described above becomes prominent. Accordingly, as a range in which deterioration of weldability and toughness is not prominent, the P content is set to 0.050% or less, is preferably set to 0.020% or less, and is more preferably set to 0.010% or less.

[S: 0.010% or Less]

S is an element contained as impurities, forming MnS in steel and causing hole expansibility of a steel sheet to deteriorate. Therefore, it is preferable to have a small amount of S. However, in a case where the S content exceeds 0.010%, the influence described above becomes prominent. Accordingly, as a range in which deterioration of hole expansibility is not prominent, the S content is set to 0.010% or less, is preferably set to 0.0050% or less, and is more preferably set to 0.0020% or less.

[N: 0.0070% or Less]

N is an element contained as impurities, forming coarse nitride in steel and causing hole expansibility of a steel sheet to deteriorate remarkably. Therefore, it is preferable to have a small amount of N. However, in a case where the N content exceeds 0.0070%, the influence described above becomes prominent. Accordingly, as a range in which deterioration of hole expansibility is not prominent, the N content is set to 0.0070% or less and is preferably set to 0.0050% or less.

[Al: 0.001% to 0.50%]

Al is an element effective in deoxidation of steel. In order to achieve this effect, the Al content is set to 0.001% or more. Meanwhile, although the Al content exceeds 0.50%, not only the effect is saturated but also a cost increase is caused. Therefore, the Al content is set to 0.50% or less, is preferably set to 0.20% or less, and is more preferably set to 0.10% or less.

[Ti: 0.050% to 0.30%]

Ti is an element forming carbide in steel and inducing uniform precipitation strengthening of ferrite. In addition, Ti is also an element having effects of reducing the amount of a solid solution C by being precipitated as TiC and inhibiting precipitation of cementite which causes deterioration of hole expansibility. Therefore, in the hot rolled steel sheet according to the present embodiment, Ti is a particularly important element. When the Ti content is less than 0.050%, the effect is not sufficient. Accordingly, the Ti content is set to 0.050% or more, is preferably set to 0.100% or more, and is more preferably set to 0.130% or more. Meanwhile, if the Ti content exceeds 0.30%, toughness deteriorates remarkably and an unnecessary cost increase is caused. Therefore, the Ti content is set to 0.30% or less, is preferably set to 0.25% or less, and is more preferably set to 0.20% or less.

Basically, the hot rolled steel sheet according to the present embodiment contains the chemical composition described above and the remainder of Fe and impurities. However, in order to improve strength and hole expansibility, in place of a part of Fe, within the range described below, the hot rolled steel sheet may further include at least one selected from the group consisting of V, Nb, Cr, Ni, Cu, Mo, B, Ca, Mg, and Bi. However, since these elements are not necessarily contained, their lower limits are 0%. Here, impurities denote components incorporated due to raw materials such as ores and scraps, or other factors when steel is industrially manufactured.

[V: 0.010% to 0.50%]

Similar to Ti, V is an element forming carbide in steel. In addition, V is an element of which the solubility product in austenite is greater than that of Ti and which is effective in high-strengthening of a steel sheet. Therefore, although it is expensive compared to Ti, V may be contained as necessary. When the V content is less than 0.010%, the effect described above cannot be sufficiently obtained. Accordingly, in a case of obtaining the effect described above, the V content is set to 0.010% or more, is preferably set to 0.070% or more, and is more preferably set to 0.140% or more. Meanwhile, when there is an excessive amount of V, a cost rise is caused. Therefore, even in a case where V is contained, the V content is set to 0.50% or less.

[Nb: 0.001% to 0.090%]

Similar to Ti, Nb is an element which forms carbide in steel and is effective in high-strengthening of a steel sheet. Therefore, although it is expensive compared to Ti, Nb may be contained as necessary. When the Nb content is less than 0.001%, the effect described above cannot be sufficiently obtained. Accordingly, in a case of obtaining the effect described above, the Nb content is set to 0.001% or more. Meanwhile, when there is an excessive amount of Nb, plastic anisotropy of a steel sheet increases, and hole expansibility deteriorates. Therefore, even in a case where Nb is contained, the Nb content is set to 0.090% or less.

[Cr: 0.001% to 0.50%]

[Ni: 0.001% to 0.50%]

[Cu: 0.001% to 0.50%]

[Mo: 0.001% to 0.50%]

[B: 0.0001% to 0.0050%]

All of Cr, Ni, Cu, Mo, and B are elements effective in high-strengthening of a steel sheet. Therefore, as necessary, the elements may be contained independently, or two or more thereof may be contained multiply. In order to achieve the effect described above, there is a need to include Cr: 0.001% or more, Ni: 0.001% or more, Cu: 0.001% or more, Mo: 0.001% or more, and B: 0.0001% or more. Meanwhile, similar to Mn, these elements delay ferritic transformation after hot rolling. Therefore, when there are excessive amounts of the elements, it becomes difficult to obtain, by area ratio, 80% or more of polygonal ferrite in the structure of a hot rolled steel sheet, and hole expansibility of a hot rolled steel sheet deteriorates. Therefore, even in a case where the elements are contained, the amounts thereof are set to Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, and B: 0.0050% or less respectively; and are preferably set to Cr: 0.20% or less, Ni: 0.20% or less, Cu: 0.20% or less, Mo: 0.09% or less, and B: 0.0040% or less, respectively.

[Ca: 0.0001% to 0.01%]

[Mg: 0.0001% to 0.01%]

[Bi: 0.0001% to 0.01%]

Ca and Mg are elements contributing to fine dispersion of inclusions in steel. Bi is an element mitigating micro-segregation of substitutional type alloying elements such as Mn and Si in steel. All of the elements contribute to improvement of hole expansibility of a steel sheet. Therefore, as necessary, the elements may be contained independently, or two or more thereof may be contained multiply. In order to achieve the effect described above, each of the elements needs to be contained 0.0001% or more. Meanwhile, when there are excessive amounts of these elements, ductility deteriorates. Therefore, even in a case where the elements are contained, the amounts of the elements are set to 0.01% or less.

<Structure of Hot Rolled Steel Sheet>

Next, the reasons for limiting the structure of the hot rolled steel sheet according to the present embodiment will be described.

[Area Ratio of Polygonal Ferrite: 80% or More]

Polygonal ferrite has a structure effective in improving hole expansibility. In order to ensure hole expansibility, the area ratio of polygonal ferrite is set to 80% or more, is preferably set to 90% or more, and is more preferably set to 95% or more. The area ratio of polygonal ferrite may be 100%. That is, the hot rolled steel sheet according to the present embodiment may be constituted of single phase polygonal ferrite.

[Total Area Ratio of Martensite and Austenite: 5% or Less]

If the area ratio of martensite and austenite exceeds 5% in total, hole expansibility deteriorates remarkably. Therefore, the total area ratio of the martensite and austenite is set to 5% or less and is preferably set to 2% or less. In addition, the total area ratio may be 0% (that is, none of martensite and austenite are contained). In addition, the austenite mentioned herein is so-called residual austenite.

[Total Area Ratio of Pearlite and Cementite: 5% or Less]

If the area ratio of pearlite and cementite exceeds 5% in total, hole expansibility deteriorates remarkably. Therefore, the total area ratio of the pearlite and cementite is set to 5% or less, is preferably set to 3% or less, and is more preferably set to 1% or less. In addition, the total area ratio may be 0% (that is, none of the pearlite and cementite are contained).

[Structure of Remainder]

The structure of the remainder other than those described above includes at least one selected from bainitic ferrite and bainite. However, in a case where the total area ratio of the structure described above is 100%, none of bainitic ferrite and bainite are included.

After a structure of a sample cut out from a hot rolled steel sheet is revealed through etching, the structure described above can be identified from a photograph of the structure.

Polygonal ferrite formed by a diffusion mechanism has no internal structures in grains, and its grain boundary is linear or forms an arc. Meanwhile, bainitic ferrite and bainite have an internal structure, have an acicular intergranular shape, and have a structure distinctly different from that of polygonal ferrite. Therefore, polygonal ferrite, bainite, and bainitic ferrite can be determined based on the intergranular shape and the presence or absence of the internal structure from a photograph of the structure obtained by using an optical microscope after etching performed with nital. In a case where the internal structure is not distinctly revealed and a structure having an acicular intergranular shape (pseudo-polygonal ferrite) is present, it is counted as bainitic ferrite.

In addition, since cementite and pearlite are etched in black, their structures can be distinctly discriminated.

In addition, an image analysis is performed with respect to a photograph of the structure obtained by means of an optical microscope employing a Le Pera-etched sample, so that the total area ratio of residual austenite and martensite can be calculated.

In the present embodiment, a structure of a steel sheet is observed at ¼ position of the depth in a sheet thickness, in which a representative structure of the steel sheet is shown.

[Standard Deviation σHV of Micro-hardness of 50 Arbitrary Pieces of Polygonal Ferrite Present within Range of ±100 μm from Central Plane in Sheet Thickness Direction: 30 or Smaller]

As described above, hole expansibility of a hot rolled steel sheet can be remarkably improved by reducing unevenness in hardness of ferrite grains. Specifically, when a hardness (micro-hardness) of 50 arbitrary pieces of polygonal ferrite present within a range of ±100 μm from a central plane (a face which includes a central portion of the sheet thickness of a steel sheet and is perpendicularly orthogonal to the sheet thickness direction) in the sheet thickness direction is measured, and when a standard deviation of the micro-hardness is the σHV, excellent hole expansibility can be obtained by setting the σHV to 30 or smaller. Therefore, the σHV is set to 30 or smaller. Since the standard deviation is preferred to be small, the lower limit of the σHV is zero.

A specific method of measuring the σHV will be described below. As a sample for measuring hardness, a steel sheet of which a cross section in a rolling direction is subjected to mirror polishing and in which chemical polishing is performed using colloidal silica in order to remove a worked layer on a surface layer and then the grain boundary is revealed through nital-etching is used. The micro-hardness is measured using a micro-hardness measuring apparatus (brand name: FISCHERSCOPE HM 2000 XYp) by pushing a pyramidal Vickers indenter having an apex angle of 136° into a grain such that its indentation does not overlap the grain boundary of ferrite with respect to randomly selected 50 pieces of polygonal ferrite (grains) which are present within a range of ±100 μm from the central plane in the sheet thickness direction. The indentation load is set to 20 N. The standard deviation σHV of the micro-hardness is obtained from the 50 pieces of obtained data.

[Ti-Containing Carbide Present in Grain of Polygonal Ferrite: 5×10⁷ Pieces/mm² or More]

[Aspect Ratio of Long Side/Short Side in 50% or More of Ti-containing Carbide Present in Grain of Polygonal Ferrite, Less than 3]

In the hot rolled steel sheet according to the present embodiment, 5×10⁷ pieces/mm² or more of Ti-containing carbide are included in a grain of polygonal ferrite. When there are 5×10⁷ pieces/mm² or less, precipitation strengthening is insufficient, thereby resulting in strength deficiency. Meanwhile, there is no need to regulate the upper limit. Generally, when the number is within the component range described above, the number does not exceed 1×10¹¹ pieces/mm².

In addition, among the pieces of Ti-containing carbide present in a grain of polygonal ferrite, when 50% or more of the carbide, by the number percentage, has the ratio of the length of the short side to the length of the long side (aspect ratio expressed as long side/short side) less than 3, excellent hole expansibility can be obtained. It is preferable to include ⅔ or more of Ti-containing carbide having the aspect ratio of long side/short side less than 3 among the Ti-containing carbide present in a grain of polygonal ferrite. The percentage of the Ti-containing carbide having the aspect ratio less than 3 may be 100%.

The percentage of the Ti-containing carbide having the aspect ratio less than 3 is obtained by setting orientation of an electron beam to be parallel to <001> of matrix phase ferrite and obtaining carbide having the aspect ratio of long side/short side less than 3 with respect to the total number of pieces of observed Ti-containing carbide when 100 or more pieces of Ti-containing carbide are observed using a transmission electron microscope (magnification: 200,000-fold).

In the present embodiment, the Ti-containing carbide is carbide containing Ti, and the Ti-containing carbide may further contain at least one of V and Nb. That is, the Ti-containing carbide also includes a state where carbide has a crystal structure (NaCl structure) of Ti-containing carbide and some locations of Ti are substituted with V or Nb.

[Hot-Dip Galvanized Layer]

The hot rolled steel sheet according to the present embodiment may have a known hot-dip galvanized layer on its surface. The hot-dip galvanized layer may be a galvannealed layer which is alloyed. In a case where a steel sheet has a hot-dip galvanized layer, rust is restrained from being generated, and the corrosion resistance of the hot rolled steel sheet is improved.

<Mechanical Characteristics of Steel Sheet>

[Tensile Strength (TS): 540 MPa or Higher]

[Ratio (Yield Ratio) of Tensile Strength (TS) and 0.2% Proof Stress (YS): 75% or More]

[Product (TS·λ) of Tensile Strength (TS) and Hole Expanding Rate (2) Regulated by JFST 1001: 50,000 MPa·% or Higher]

In order to satisfy strict performance required in recent high strength hot rolled steel sheets for automobiles, as their mechanical characteristics, it is preferable that tensile strength TS is 540 MPa or higher, the ratio (yield ratio (YR)) of the tensile strength TS and 0.2% proof stress YS is 75% or more, and the product (TSλ) of the tensile strength TS and a hole expanding rate λ regulated by JFST 1001 is 50,000 MPa·% or higher. The hot rolled steel sheet according to the present embodiment aims to be provided with all the high tensile strength, the high yield ratio, and the high balance between the tensile strength and the hole expansibility (TS·λ) by controlling the chemical composition and the structure.

The tensile strength is preferably set to 590 MPa or higher. In addition, if the tensile strength exceeds 1,180 MPa, fatigue properties of a weld portion deteriorate. Accordingly, it is preferable to be 1,180 MPa or lower.

Next, a preferable manufacturing method for obtaining the hot rolled steel sheet according to the present embodiment will be described. The hot rolled steel sheet according to the present embodiment can be stably manufactured in accordance with a manufacturing method including the following processes (A) to (D), and it is preferable.

(A) A slab obtained from molten steel having the chemical composition within the range described above is heated to approximately 1,200° C.

(B) The heated slab is subjected to rough rolling such that the cumulative rolling reduction within a range from 1,050° C. to 1,150° C. becomes 50% or larger.

(C) The steel sheet after rough rolling is subjected to finish rolling such that the cumulative rolling reduction at 1,050° C. or lower ranges from 20% to 80%, the rolling reduction of the last pass ranges from 15% to 35%, and the temperature of the last pass (finishing temperature) becomes 930° C. or higher.

(D) Thereafter, with respect to the hot rolled steel sheet, i) as primary cooling, cooling is performed under the condition that the average cooling rate within the temperature range from a finish rolling last pass temperature to MT (720° C.≤MT≤830° C.) becomes 30° C./s or faster. Thereafter, ii) as secondary cooling, cooling is performed for t seconds which is regulated by t(sec)=5·(Mn)² under the condition that the average cooling rate within the temperature range from MT to Tx (720° C.≤Tx<MT) (here, (Mn) is the Mn content by unit mass %) becomes 10° C./s or slower. Subsequently, iii) as third cooling, cooling is performed under the condition that the average cooling rate within the temperature range from Tx to CT (450° C.≤CT≤650° C.) which is a secondary cooling end temperature becomes 30° C./s or faster. Then, after being cooled to CT, the hot rolled steel sheet is coiled.

Hereinafter, the reasons will be described.

<Heating Process>

In the heating process, a slab having a chemical composition as described above is heated to approximately 1,200° C. From viewpoints on affecting the precipitation density of Ti-containing carbide in a grain of polygonal ferrite, and the solid solution states of carbide forming elements such as Ti, Nb, and V; and restraining coarse carbide from being formed, in order to obtain desired performance, it is preferable that the heating temperature is within the temperature range from 1,150° C. to 1,250° C.

<Rough Rolling Process>

The heated slab becomes a hot rolled steel sheet via the hot rolling process including the rough rolling process and the finish rolling process. When the hot rolled steel sheet according to the present embodiment is manufactured, in each process of rough rolling and finish rolling, it is preferable to control the temperature, the rolling reduction, and the like.

In the rough rolling process of hot rolling, it is preferable that the cumulative rolling reduction within the range from 1,050° C. to 1,150° C. is set to 50% or larger. When the cumulative rolling reduction within the range from 1,050° C. to 1,150° C. falls short of 50%, the structure becomes inhomogeneous, and there are cases where the σHV increases and hole expansibility is degraded. The cumulative rolling reduction in the present invention is the percentage of the cumulative rolling reduction amount (difference between the inlet sheet thickness before the first pass in rolling and an outlet sheet thickness after the last pass in rolling) with respect to a reference, while the reference is an inlet sheet thickness before a first pass. In addition, the cumulative rolling reduction is calculated in each of rough rolling and finish rolling. That is, the cumulative rolling reduction in rough rolling is the percentage of the difference between the inlet sheet thickness before the first pass in rough rolling and the outlet sheet thickness after the last pass in rough rolling. The cumulative rolling reduction in finish rolling is the percentage of the difference between the inlet sheet thickness before the first pass in finish rolling and the outlet sheet thickness after the last pass in finish rolling.

<Finish Rolling Process>

In the finish rolling process of hot rolling, it is preferable that the cumulative rolling reduction at 1,050° C. or lower ranges from 20% to 80%. if the cumulative rolling reduction at 1,050° C. or lower exceeds 80%, the anisotropy of the finally obtained structure of the hot rolled steel sheet is revealed. In this case, there are cases where the σHV increases and hole expansibility is degraded. The reason is presumed to be the hardness difference which is incited by deviation of the crystal orientation of ferrite grains. Meanwhile, if the cumulative rolling reduction at 1,050° C. or lower falls short of 20%, the austenite grain size is coarsened and accumulation of distortion in austenite becomes insufficient. Accordingly, ferritic transformation after finish rolling is suppressed, and the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite deviate from the desired range, and the possibility of deterioration of hole expansibility increases.

[Rolling Reduction of Last Pass: 15% to 35%]

It is preferable that the rolling reduction of the last pass is from 15% to 35%. If the rolling reduction of the last pass exceeds 35%, the anisotropy of the structure is revealed. As a result, there are cases where the σHv increases and hole expansibility is degraded. Therefore, the rolling reduction of the last pass is set to 35% or smaller and is more preferably set to 25% or smaller. Meanwhile, if the rolling reduction of the last pass falls short of 15%, accumulation of distortion in austenite becomes insufficient. Accordingly, ferritic transformation after finish rolling is suppressed, and the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite deviate from the desired range, and the possibility of deterioration of hole expansibility increases.

[Finishing Temperature: 930° C. or Higher]

It is preferable that the finishing temperature (temperature of the steel sheet after the last pass of finish rolling) is set to 930° C. or higher. If the finishing temperature falls short of 930° C., the anisotropy of the structure is likely to be revealed in the finally obtained hot rolled steel sheet. As a result, the σHv increases, and the possibility of deterioration of hole expansibility increases. Meanwhile, in accordance with an increase of the finishing temperature, the austenite grain size is coarsened and accumulation of distortion in austenite becomes insufficient. Accordingly, ferritic transformation after finish rolling is suppressed, and the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite grow, so that the possibility of deterioration of hole expansibility increases. Therefore, it is preferable that the upper limit of the finishing temperature is set to approximately 1,000° C.

<Cooling Process>

After the finish rolling, the hot rolled steel sheet is subjected to cooling.

Within the temperature range from the finish rolling last pass temperature to 720° C., i), a change in density of the Ti-containing carbide in a grain of polygonal ferrite due to the growing (coarsening) of Ti-containing carbide precipitated in ferrite, and ii) a change in aspect ratio of long side/short side of the Ti-containing carbide present in a grain of polygonal ferrite increase. Therefore, in order to obtain the desired performance, it is effective that the average cooling rate within the temperature range from the finish rolling last pass temperature to 720° C. is set to 30° C./s.

Furthermore, after the cooling, within the temperature range from 830° C. to 720° C., cooling of the hot rolled steel sheet at a low average cooling rate for a desired time which is determined in accordance with the Mn content is effective in promoting ferritic transformation and precipitation of the Ti-containing carbide, and having the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite within the desired range.

Thereafter, cooling is further performed, and then the hot rolled steel sheet is coiled. In this case, if the cooling rate is slower than 30° C./s or the coiling temperature exceeds 650° C., Ti-containing carbide in the hot rolled steel sheet is excessively coarsened during the cooling or after the coiling, and there are cases where it becomes difficult to ensure the desired strength. Meanwhile, in a case where the coiling temperature is set to less than 450° C., accuracy of controlling the coiling temperature is degraded, and it is not preferable. Therefore, in order to be effective, the coiling temperature is set to range from 450° C. to 650° C., and cooling is performed until the temperature reaches the coiling temperature at a predetermined average cooling rate or faster.

That is, in the cooling process after finish rolling, with respect to the hot rolled steel sheet after finish rolling, it is preferable that i) as primary cooling, cooling is performed under the condition that the average cooling rate within the temperature range from a finish rolling last pass temperature to MT (720° C.≤MT≤830° C.) becomes 30° C./s or faster. Thereafter, ii) as secondary cooling, cooling is performed for t seconds which is regulated by the following Expression 1 under the cooling condition that the average cooling rate within the temperature range from MT to Tx (720° C.≤Tx<MT) becomes 10° C./s or slower. Subsequently, iii) as third cooling, cooling is performed under the cooling condition that the average cooling rate becomes 30° C./s or faster within the temperature range from the secondary cooling end temperature (Tx) to CT (450° C.≤CT≤650° C.). Then, coiling is performed within the temperature range from the 450° C. to 650° C.

(t(sec)=5·(Mn)²)  Expression 1

Here, (Mn) is the Mn content by unit mass %.

In a case where the hot rolled steel sheet according to the present embodiment is manufactured, as necessary, the following processes may be further provided.

<Plating Process>

After the coiling process, a hot-dip galvanizing process for hot-dip galvanizing a hot rolled steel sheet may be provided. It is possible to form a coating layer on a surface of the steel sheet and to improve corrosion resistance of the steel sheet by performing hot-dip galvanizing. In addition, after hot-dip galvanizing, a galvannealed layer may be formed on a surface of the steel sheet by performing alloying. In addition, in this case, in order to suppress degradation of strength of the steel sheet, the maximum heating temperature during annealing before hot-dip galvanizing dipping is preferably set to 800° C. or lower. Other hot-dip galvanizing conditions may comply with routine procedures.

<Other Processes>

In the hot rolled steel sheet according to the present embodiment, in accordance with the routine procedure, after the hot rolling process, pickling may be performed. In addition, before pickling or after pickling, skin pass rolling may be performed for flatness correction or promotion of scale peeling. The elongation rate in a case of performing skin pass rolling is not particularly regulated. However, it is preferable to set to range from 0.1% to less than 3.0%.

EXAMPLES

Hereinafter, Examples of the present invention will be described.

Example 1

Pieces of steel respectively having the chemical compositions indicated in Table 1 were each formed into ingot at a laboratory and were cast into slabs. Then, the slabs were subjected to heating, hot rolling, cooling, and coiling in the pattern as shown in FIG. 1. In this case, the conditions in each process were as indicated in Table 2. In Table 2, SRT, R1, R2, R3, FT, MT, t, and CT indicate the following, respectively.

SRT: slab heating temperature

R1: cumulative rolling reduction within range from 1,050° C. to 1,150° C.

R2: cumulative rolling reduction at 1,050° C. or lower

R3: rolling reduction at last finish pass

FT: finish rolling temperature

MT: primary cooling stop temperature

t: secondary cooling time

CT: coiling temperature

Hot rolled steel sheets obtained as described above were subjected to pickling. In regard to the condition indicated as plating in the spaces for treatment in Table 3, after hot-dip galvanizing was performed, JIS No. 5 tensile test pieces were respectively collected from the hot rolled steel sheets in a direction perpendicular to the rolling direction. A tensile test was performed using these test pieces, and the yield strength (YS), the tensile strength (TS), the yield ratio (YR), and the total elongation (EL) were measured.

In addition, a hole expanding test was performed based on “JFS T 1001 the hole expanding test method” of the Japan Iron and Steel Federation Standard, and the hole expanding rate (λ) was measured.

In addition, samples each including a cross section of the hot rolled steel sheet in the rolling direction were collected. A surface corresponding to the cross section of each sample in the rolling direction was etched using a nital solution. Thereafter, a photograph of the structure obtained in the visual field of 300 μm×300 μm at ¼ position of the depth in the sheet thickness was captured using an optical microscope or an electronic scanning microscope, and the structure was identified. From the photograph of the obtained structure, the area ratio of each structure was calculated through a point counting method. Polygonal ferrite, bainite, and bainitic ferrite were determined based on the intergranular shape and the presence or absence of the internal structure. The structure etched in black was discriminated from cementite and pearlite. In addition, by means of a Le Pera-etched sample, an image analysis was performed with respect to the photograph of the structure obtained using the optical microscope, and the total area ratio of residual austenite and martensite was thereby calculated.

In addition, a pellicle sample was collected from each of the hot rolled steel sheets. Then, carbide containing at least one of Ti, V, and Nb precipitated in a grain of ferrite was observed using the transmission electron microscope (magnification: 200,000-fold), and the number density and the percentage of the precipitated element having the aspect ratio of 3 or less were obtained.

In addition, the standard deviation of micro-hardness of the steel from which 80 area % or more polygonal ferrite could be obtained was measured through the method described above. FIGS. 3A and 3B respectively show the measurement results of micro-hardness of the sample number 14 and the sample number 15, as examples.

Tables 3 and 4 show the obtained results. In Tables 3 and 4, Vα, VPθ, VMA, B, BF, and σHV indicate the following, respectively. The blank spaces for the structures denote that no observation was performed.

Vα: area ratio of ferrite

VPθ: total area ratio of pearlite and cementite

VMA: total area ratio of martensite and austenite

B, BF: bainite and bainitic ferrite

σHV: standard deviation of micro-hardness of ferrite

In the sample numbers 1 to 3, 5, 6, 11, 17 to 19, 22, and 25 to 34, since all the chemical compositions and the structures were within the range regulated by the present invention, desired mechanical characteristics were obtained. Meanwhile, in the sample numbers 4, 10, 12 to 16, 20 to 21, 24, and 36, the σHV exceeded the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained. In the sample numbers 7, 8, 18, and 36, the area ratio of polygonal ferrite fell short of the lower limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained. In the sample number 9, the total area ratio of martensite and austenite overtook the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained. In the sample numbers 36 and 38, the total area ratio of pearlite and cementite overtook the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained.

In addition, in the sample numbers 7, 8, 12, 23, 24, 35, and 38, the number density of carbide was low. In addition, in the sample numbers 7, 8, 12, 23, 24, and 36, the percentage of the Ti-containing carbide having the aspect ratio of 3 or less increased, so that desired mechanical characteristics could not be obtained.

In the sample number 37, toughness was low and breaking occurred at the time of test piece processing. Accordingly, no test could be performed.

TABLE 1
Kind	Chemical composition (mass %) (remainder of Fe and impurities)
of steel	C	Si	Mn	P	S	Al	Ti	N	Cu	Cr	Ni
A	0.036	0.10	0.51	0.012	0.0020	0.049	0.120	0.0036
B	0.057	0.05	0.50	0.010	0.0020	0.045	0.200	0.0035
C	0.057	0.05	1.14	0.010	0.0020	0.045	0.200	0.0035
D	0.055	0.60	0.76	0.010	0.0020	0.048	0.200	0.0033
E	0.050	1.15	1.28	0.009	0.0018	0.050	0.180	0.0030
F	0.079	0.05	0.51	0.009	0.0015	0.050	0.190	0.0033
G	0.094	0.05	0.52	0.011	0.0022	0.044	0.190	0.0029
H	0.057	0.06	1.75	0.010	0.0019	0.052	0.200	0.0030
I	0.060	0.05	0.50	0.010	0.0020	0.050	0.200	0.0028
J	0.056	0.06	0.51	0.010	0.0020	0.050	0.200	0.0028	0.210
K	0.051	0.04	0.50	0.010	0.0020	0.050	0.190	0.0028		0.220
L	0.055	0.05	0.50	0.010	0.0020	0.050	0.180	0.0028			0.200
M	0.054	0.05	0.54	0.010	0.0020	0.050	0.190	0.0028
N	0.055	0.04	0.51	0.010	0.0020	0.050	0.200	0.0028
O	0.055	0.05	0.49	0.010	0.0020	0.050	0.190	0.0028
P	0.053	0.06	0.51	0.010	0.0020	0.050	0.190	0.0028
Q	0.053	0.06	0.51	0.010	0.0020	0.050	0.180	0.0028
R	0.055	0.03	1.21	0.001	0.0025	0.120	0.170	0.0025
S	0.080	0.10	0.90	0.010	0.0020	0.050	0.040	0.0028
T	0.002	0.05	0.52	0.001	0.0020	0.040	0.150	0.0028
U	0.356	0.06	0.60	0.010	0.0019	0.050	0.170	0.0030
V	0.061	0.08	0.92	0.010	0.0020	0.038	0.420	0.0028
	Kind	Chemical composition (mass %) (remainder of Fe and impurities)
	of steel	Mo	V	Nb	B	Ca	Mg	Bi	Remarks
	A								Example of
									invention
	B								Example of
									invention
	C								Example of
									invention
	D								Example of
									invention
	E								Example of
									invention
	F		0.130						Example of
									invention
	G		0.250						Example of
									invention
	H								Comparative
									Example
	I			0.040					Example of
									invention
	J								Example of
									invention
	K								Example of
									invention
	L								Example of
									invention
	M	0.150							Example of
									invention
	N				0.0007				Example of
									invention
	O					0.0022			Example of
									invention
	P						0.0021		Example of
									invention
	Q							0.0018	Example of
									invention
	R								Example of
									invention
	S								Comparative
									Example
	T								Comparative
									Example
	U				0.0003				Comparative
									Example
	V								Comparative
									Example

TABLE 2
						Primary			Secondary	Third
						average			average	average
	SRT	R1	R2	R3	FT	cooling rate	MT	t	cooling rate	cooling rate	CT
Condition	(° C.)	(%)	(%)	(%)	(° C.)	(° C./s)	(° C.)	(sec)	(° C./s)	(° C./s)	(° C.)
a	1,200	82	64	20	950	40	780	10	5	20	570
b	1,200	82	64	20	950	40	730	10	5	30	570
c	1,200	82	64	20	950	50	620	10	5	30	550
d	1,200	82	64	20	950	50	780	1	5	30	570
e	1,200	82	64	20	950	40	730	10	15	20	260
f	1,200	93	12	12	950	40	730	10	5	40	590
g	1,200	82	64	20	950	60	770	10	8	20	430
h	1,200	82	64	20	950	50	680	1	8	30	570
i	1,200	82	64	20	950	30	820	1	8	30	700
j	1,200	37	64	20	950	40	770	10	8	30	570
k	1,200	82	90	20	950	40	780	10	10	30	570
l	1,200	82	64	42	950	35	770	10	10	30	570
m	1,200	82	64	20	880	50	780	10	10	30	570

TABLE 3
	Kind		Mechanical characteristics	Steel structure
Sample	of		TS	YS	YR	EL	λ	TS · λ	Vα	VPθ
No.	steel	Condition	(MPa)	(MPa)	(%)	(%)	(%)	(MPa · %)	(%)	(%)
1	A	a	630	535	84.9	24.2	124	78120	98	1
2	B	a	746	599	80.3	19.6	102	76241.2	98
3	B	b	814	670	82.3	18.2	89	72446	96
4	B	c	801	654	81.6	18.2	60	48060	75
5	C	a	748	600	80.2	20.0	100	74800	98
6	C	b	891	765	85.9	19.6	73	65043	96
7	C	c	935	857	91.7	17.6	52	48620	69
8	C	d	890	815	91.6	18.0	87	48060	60
9	C	e	765	569	74.4	19.0	60	46053	87
10	C	f	877	774	88.3	17.9	57	49989	82
11	D	a	791	650	82.2	19.0	101	79891	99
12	D	i	712	603	84.7	18.0	66	46992	96
13	D	j	802	703	87.7	17.2	61	48922	95
14	D	k	821	673	82.0	18.9	52	42692	97
15	D	l	803	666	82.9	18.6	59	47377	98
16	D	m	780	642	82.3	17.9	52	40560	91
17	E	a	842	698	82.9	18.9	93	78306	98
18	F	a	773	671	86.8	18.6	116	89668	99
19	G	a	863	696	80.6	18.2	94	81122	99
		Steel structure
						Proportion
					Number	of Ti-
					density	containing
					of	carbide
					carbide	having
					(×107	aspect
	Sample	VMA			pieces/	ratio of 3
	No.	(%)	Remainder	σHV	mm2)	or less (%)	Treatment	Remarks
	1		B, BF	15	11.0	87		Example of
								invention
	2		B, BF	17	11.3	89	Plating	Example of
								invention
	3		B, BF	24	12.3	90		Example of
								invention
	4		B, BF	34	3.2	47		Comparative
								Example
	5		B, BF	23	9.2	78		Example of
								invention
	6		B, BF	27	10.2	89		Example of
								invention
	7		B, BF	—	2.7	29		Comparative
								Example
	8		B, BF	—	3.9	33		Comparative
								Example
	9	10	B, BF	34	5.6	78		Comparative
								Example
	10		B, BF	34	7.9	82		Comparative
								Example
	11		B, BF	18	8.9	89		Example of
								invention
	12		B, BF	34	4.3	35		Comparative
								Example
	13		B, BF	32	10.2	85		Comparative
								Example
	14		B, BF	33	10.5	89		Comparative
								Example
	15		B, BF	32	9.5	82		Comparative
								Example
	16		B, BF	31	9.3	76		Comparative
								Example
	17		B, BF	20	9.2	85		Example of
								invention
	18		B, BF	18	9.0	90	Plating	Example of
								invention
	19		B, BF	20	12.1	95		Example of
								invention

TABLE 4
	Kind		Mechanical characteristics	Steel structure
Sample	of		TS	YS	YR	EL	λ	TS · λ	Vα	VPθ
No.	steel	Condition	(MPa)	(MPa)	(%)	(%)	(%)	(MPa · %)	(%)	(%)
20	G	c	939	784	83.5	18.0	50	46950	65
21	G	f	920	745	81.0	18.4	54	49680	78
22	G	g	875	739	84.5	18.1	99	86625	98
23	H	a	910	835	91.8	17.6	47	42770	30
24	H	h	935	871	93.2	17.9	43	40205	42
25	I	a	790	654	82.8	19.1	96	75840	99
26	J	a	784	639	81.5	19.7	98	76832	99
27	K	a	798	656	82.2	20.0	91	72618	98
28	L	a	763	626	82.0	20.1	104	79352	98
29	M	a	777	643	82.8	19.5	92	71484	99
30	N	a	780	665	85.3	18.4	89	69420	98
31	O	a	742	601	81.0	19.7	118	87556	99
32	P	a	741	599	80.8	19.8	116	85956	99
33	Q	a	743	597	80.3	19.4	117	86931	98
34	R	a	762	618	81.1	19.7	98	74676	99
35	T	a	482	367	76.1	31.0	83	40006	100
36	U	a	859	758	88.2	15.0	18	15462	47	9
37	V	a	Breaking occurred at time of test piece processing
38	S	a	593	449	75.7	24.7	71	42103	84	8
		Steel structure
						Proportion
					Number	of Ti-
					density	containing
					of	carbide
					carbide	having
					(×107	aspect
	Sample	VMA			pieces/	ratio of 3
	No.	(%)	Remainder	σHV	mm2)	or less (%)	Treatment	Remarks
	20		B, BF	34	4.2	23		Comparative
								Example
	21		B, BF	35	10.2	78		Comparative
								Example
	22		B, BF	19	10.5	89		Example of
								invention
	23		B, BF	—	5.2	53		Comparative
								Example
	24		B, BF	35	4.2	38		Comparative
								Example
	25		B, BF	17	11.9	90		Example of
								invention
	26		B, BF	18	10.3	91	Plating	Example of
								invention
	27		B, BF	21	8.7	87		Example of
								invention
	28		B, BF	19	9.2	86		Example of
								invention
	29		B, BF	21	12.1	85		Example of
								invention
	30		B, BF	23	10.3	83		Example of
								invention
	31		B, BF	17	11.5	92		Example of
								invention
	32		B, BF	17	9.5	90		Example of
								invention
	33		B, BF	17	10.0	87		Example of
								invention
	34		B, BF	16	11.0	88		Example of
								invention
	35		B, BF	2	0.01	100	Plating	Comparative
								Example
	36		B, BF	35	6.3	51		Comparative
								Example
	37	Breaking occurred at time of test piece processing	Comparative
								Example
	38		B, BF	15	3.8	83		Comparative
								Example

Example 2

Next, among the pieces of steel having the chemical composition indicated in Table 1, five kinds of steel (A to C, G, and H) were subjected to hot rolling and cooling shown in FIG. 1. Thereafter, descaling was performed. Then, without performing cold rolling, heat treatment simulating the galvannealing line having the pattern shown in FIG. 2 was performed using a continuous heat treatment simulator. In this case, the conditions in each process were as indicated in Table 5. In Table 5, RA, LTH, DIP, and GA indicate the following, respectively.

RA: maximum heating temperature

LTH: low-temperature retention temperature

DIP: Zn bath temperature

GA: galvannealing temperature

From the hot rolled steel sheets obtained as described above, JIS No. 5 tensile test pieces were respectively collected in a direction perpendicular to the rolling direction. A tensile test was performed using these test pieces, and the yield strength (YS), the tensile strength (TS), the yield ratio (YR), and the total elongation (EL) were measured. In addition, a hole expanding test was performed based on “JFS T 1001 the hole expanding test method” of the Japan Iron and Steel Federation Standard, and the hole expanding rate (2) was measured.

In addition, samples each including a cross section of the steel sheet in the rolling direction was collected, and the area ratio of each structure was calculated through the same method as that in Example 1.

In addition, a pellicle sample was collected from each of the hot rolled steel sheets. Then, carbide containing at least one of Ti, V, and Nb precipitated in a grain of ferrite was observed using the transmission electron microscope (magnification: 200,000-fold), and the number density and the percentage of the precipitated element having the aspect ratio of 3 or less were obtained. The standard deviation of micro-hardness of the steel from which 80 area % or more polygonal ferrite could be obtained was measured through the method described above.

Table 6 shows the obtained results. In the sample numbers 39 to 42, and 44 to 47, since all the chemical compositions and the structures were within the range regulated by the present invention, desired mechanical characteristics were obtained. Meanwhile, in the sample number 43, the σHV exceeded the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained. In the sample number 48, the area ratio of polygonal ferrite fell short of the lower limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained.

TABLE 5
	Hot rolling conditions
							Primary		Secondary	Third
							average		average	average		Galvannealing conditions
	SRT	R1	R2	R3	FT	MT	cooling rate	t	cooling rate	cooling rate	CT	RA	LTH	DIP	GA
Condition	(° C.)	(%)	(%)	(%)	(° C.)	(° C.)	(° C./s)	(sec)	(° C./s)	(° C./s)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)
a’	1200	82	64	20	950	780	40	10	5	20	570	740	490	460	Absent
b’	1200	82	64	20	950	780	40	10	5	30	570	740	490	460	530
c’	1200	82	64	20	950	620	50	10	5	30	550	740	490	460	530

TABLE 6
				Steel
	Kind		Mechanical characteristics	structure
Sample	of		TS	YS	YR	EL	λ	TS · λ	Vα
No.	steel	Condition	(MPa)	(MPa)	(%)	(%)	(%)	(MPa · %)	(%)
39	A	a’	615	541	88.0	24.1	124	76260	98
40	A	b’	617	539	87.4	24.2	130	80210	98
41	B	a’	738	620	84.0	19.3	114	84132	98
42	B	b’	733	623	85.0	19.6	110	80630	98
43	B	c’	796	663	83.3	17.8	58	46168	95
44	C	a’	740	639	86.4	20.1	113	83620	98
45	C	b’	741	642	86.6	20.2	111	82251	99
46	G	a’	850	725	85.3	18.3	99	84150	99
47	G	b’	844	729	86.4	18.2	100	84400	99
48	H	b’	899	833	92.7	18.4	50	44950	27
		Steel structure
							Proportion
						Number	of Ti-
						density	containing
						of	carbide
						carbide	having
						(×107	aspect
	Sample	VPβ	VMA			pieces/	ratio of 3
	No.	(%)	(%)	Remainder	σHV	mm2)	or less (%)	Remarks
	39			B, BF	14	10.2	89	Example of
								invention
	40			B, BF	15	11	90	Example of
								invention
	41			B, BF	15	9.8	88	Example of
								invention
	42			B, BF	16	11.2	92	Example of
								invention
	43			B, BF	34	4.3	46	Comparative
								Example
	44			B, BF	22	8.9	88	Example of
								invention
	45			B, BF	23	9.2	86	Example of
								invention
	46			B, BF	20	9.2	93	Example of
								invention
	47			B, BF	22	8.9	89	Example of
								invention
	48		4	B, BF	—	3.2	48	Comparative
								Example

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to inexpensively manufacture a high strength hot rolled steel sheet having a high yield ratio and excellent hole expansibility. In addition, the steel sheet according to the present invention also has excellent hole expansibility even during stretch flanges forming frequently employed for automobile components, particularly chassis components and the like. Therefore, the steel sheet industrially contributes to reducing weight and ensuring collision safety of vehicle bodies particularly in automobile fields.

Claims

1-4. (canceled)

5. A high strength hot rolled steel sheet comprising, as a chemical composition, by mass %,

C: 0.010% to 0.200%,

Si: 0.001% to 2.50%,

Mn: 0.001% to 1.50%,

P: 0.050% or less,

S: 0.010% or less,

N: 0.0070% or less,

Al: 0.001% to 0.50%,

Ti: 0.050% to 0.30%,

V: 0% to 0.50%,

Nb: 0% to 0.090%,

Cr: 0% to 0.50%,

Ni: 0% to 0.50%,

Cu: 0% to 0.50%,

Mo: 0% to 0.50%,

B: 0% to 0.0050%,

Ca: 0% to 0.01%,

Mg: 0% to 0.01%,

Bi: 0% to 0.01%, and

a remainder of Fe and impurities,

wherein a structure includes, by area ratio, 80% or more of a polygonal ferrite, a total of 5% or less of a martensite and an austenite, and a total of 5% or less of a pearlite and a cementite, and the remainder is at least one selected from a bainitic ferrite and a bainite,

wherein when a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller,

wherein a grain of the polygonal ferrite contains 5×107 pieces/mm2 or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, an aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3, and

wherein a tensile strength is 540 MPa or higher.

6. The high strength hot rolled steel sheet according to claim 5,

wherein the chemical composition includes, by mass %, at least one selected from the group consisting of V: 0.010% to 0.50%, Nb: 0.001% to 0.090%, Cr: 0.001% to 0.50%, Ni: 0.001% to 0.50%, Cu: 0.001% to 0.50%, Mo: 0.001% to 0.50%, and B: 0.0001% to 0.0050%.

7. The high strength hot rolled steel sheet according to claim 5,

wherein the chemical composition includes, by mass %, at least one selected from the group consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.

8. The high strength hot rolled steel sheet according to claim 6,

wherein the chemical composition includes, by mass %, at least one selected from the group consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.

9. The high strength hot rolled steel sheet according to claim 5, further comprising:

a hot-dip galvanized layer on a surface.

10. The high strength hot rolled steel sheet according to claim 6, further comprising:

a hot-dip galvanized layer on a surface.

11. The high strength hot rolled steel sheet according to claim 7, further comprising:

a hot-dip galvanized layer on a surface.

12. The high strength hot rolled steel sheet according to claim 8, further comprising:

a hot-dip galvanized layer on a surface.