ULTRA-HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT YIELD RATIO AND WORKABILITY

Abstract

An ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Si, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 1 area % or more of a region in which martensite constitutes 90 area % or more, residual austentite constitutes 0.5 area % or more, and the local Mn concentration is at least 1.2 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more.

Description

TECHNICAL FIELD

The present invention relates to an ultra-high-strength steel sheet excellent in yield ratio and workability. The steel sheet type of the ultra-high-strength steel sheet in accordance with the present invention shall be considered to include not only cold-rolled steel sheets, but also various plated steel sheets such as hot-dip galvanized steel sheets and hot-dip galvanized and alloyed steel sheets.

BACKGROUND ART

For the purpose of improvement of fuel consumption by weight reduction of vehicle bodies, steel sheets used for skeleton components of automobiles have recently been required to be increased in strength, and in order to ensure collision safety, a high yield ratio is also required. On the other hand, in order to form parts with complicated shapes, excellent workability is also required.

It has therefore been eagerly desired to provide an ultra-high-strength steel sheet increased in elongation (EL) while having a high yield ratio. More specifically, a steel sheet having a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and an elongation of 10% or more has been required.

In addition, although steel sheets for automobiles are subjected to welding during assembly of vehicle bodies or during mounting of parts, weldability heavily depends on compositions of the steel sheets. In particular, when C and Mn are added in large amounts, it is known that the weldability is degraded. It has therefore been required for the steel sheets for automobiles to fulfill the above-mentioned mechanical properties, while having a composition satisfying 0.35 mass % or less of C and 1.5 mass % or less of Mn.

Conventionally herein, in order to increase the elongation of the high-strength steel sheet, mainly the following two means have been used.

(1) The amount of residual austenite is increased to utilize a TRIP action thereof.

(2) The amount of soft ferrite (including bainitic ferrite) is increased.

However, in order to allow a large amount of austenite to remain, the means of the above (1) requires the increase of the added amount of C or Mn, resulting in a failure to satisfy C≤0.35 mass % and Mn≤1.5 mass %. There has been therefore a problem that sufficient weldability cannot be ensured.

On the other hand, in order to ensure the elongation, the means of the above (2) requires a predetermined amount of a soft phase, resulting in a failure to satisfy a yield ratio of 0.75 or more. There has been therefore a problem that sufficient collision safety cannot be ensured.

For example, Patent Literature 1 proposes a steel sheet that is increased in resistance to hydrogen embrittlement and is also excellent in resistance to delayed fracture at a punching hole processing part, in an ultra-high-strength region having a tensile strength of 1180 MPa or more, by allowing a large amount of austenite to remain by increasing the Mn content in the steel sheet.

However, with respect to the above-mentioned steel sheet, the Mn content in the steel sheet is more than 1.5 mass % for all the invention steels as shown in the examples thereof, and there has been room for improvement in terms of the weldability.

In addition, Patent Literature 2 proposes a steel sheet that can realize a tensile strength of 1470 MPa or more and an elongation of 10% or more, in a composition satisfying 0.35 mass % or less of C and 1.5 mass % or less of Mn, by increasing the fraction of a soft ferrite phase.

However, the above-mentioned steel sheet cannot realize a yield ratio of 0.75 or more as shown in the examples thereof, and there is a problem that sufficient collision safety cannot be ensured.

CITATION LIST

Patent Literatures

Patent Literature 1: JP-A-2008-81788

Patent Literature 2: JP-A-2010-90432

SUMMARY OF INVENTION

Technical Problems

Therefore, an object of the present invention is to provide an ultra-high-strength steel sheet excellent in yield ratio and workability, which can satisfy a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more and an elongation of 10% or more.

Solution to Problems

In a first invention of the present invention which is an ultra-high-strength steel sheet excellent in yield ratio and workability, the ultra-high-strength steel sheet has a composition comprising, by mass %,

C: 0.15% to 0.35%,

Si: 0.5% to 3.0%

Mn: 0.5% to 1.5%,

Al: 0.001% to 0.10% and

the balance being iron and inevitable impurities,

wherein each of P, S and N of the inevitable impurities is limited to

P: 0.1% or less

S: 0.01% or less and

N: 0.01% or less,

the ultra-high-strength steel sheet has a structure comprising, by area ratio based on a whole structure,

martensite: 90% or more and

residual austenite: 0.5% or more,

the ultra-high-strength steel sheet has 1% or more by area ratio of a region where a local Mn concentration is at least 1.2 times a Mn content in a whole steel sheet, and

the ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more and an elongation of 10% or more.

In a second invention of the present invention which is the ultra-high-strength steel sheet excellent in yield ratio and workability according to the first invention, the composition further comprises, by mass %, one or two or more of

Cu: 0.05% to 1.0%,

Ni: 0.05% to 1.0% and

B: 0.0002% to 0.0050%.

In a third invention of the present invention which is the ultra-high-strength steel sheet excellent in yield ratio and workability according to the first or second invention, the composition further comprises, by mass %, one or two or more of

Mo: 0.01% to 1.0%,

Cr: 0.01% to 1.0%,

Nb: 0.01% to 0.3%,

Ti: 0.01% to 0.3% and

V: 0.01% to 0.3%.

In a fourth invention of the present invention which is the ultra-high-strength steel sheet excellent in yield ratio and workability according to any one of the first to third inventions, the composition further comprises, by mass %, one or two of

Ca: 0.0005% to 0.01% and

Mg: 0.0005% to 0.01%.

Advantageous Effects of Invention

In accordance with the present invention, martensite is used as a main structure of steel, and Mn is concentrated in residual austenite, without increasing the average concentration of C and Mn in the whole steel sheet, whereby it has become possible to provide an ultra-high-strength steel sheet that has a high strength and a high yield ratio and is excellent in workability, while ensuring weldability.

DESCRIPTION OF EMBODIMENTS

The present invention will be explained below in greater detail.

First, a structure characterizing an ultra-high-strength steel sheet excellent in yield ratio and workability in accordance with the present invention (hereinafter also referred to as “the steel sheet in the present invention”) will be explained.

[Structure of the Steel Sheet in the Present Invention]

As described above, in the steel sheet in the present invention, martensite is used as a matrix, and moreover residual austenite in which Mn is concentrated is contained in a predetermined amount (hereinafter, austenite is sometimes represented by γ).

<Martensite: 90% or More>

In order to realize the steel sheet having a tensile strength of 1470 MPa or more and achieve a high yield ratio of 0.75 or more, martensite is required to be, by area ratio, 90% or more, preferably 92% or more, and more preferably 94% or more. In the present description, martensite is used to mean including both fresh martensite not subjected to tempering and tempered martensite subjected to tempering.

Since all except for residual austenite may be martensite, the upper limit of the martensite area ratio is 99.5%, and it is preferably 99% or less, in consideration of the lower limit (0.5%) of residual austenite.

<Residual Austenite: 0.5% or More>

In order to use its TRIP action to thereby improve the elongation, the residual austenite is required to be, by area ratio, 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more.

Since all except for martensite may be residual austenite, the upper limit of the residual austenite area ratio is 10%, and it is preferably 5% or less, more preferably 3% or less, and particularly preferably 2% or less, in consideration of the lower limit (90%) of martensite.

As described above, although the steel sheet in the present invention may be composed of only two phases of martensite and residual austenite (the total area ratio of the two phases is 100%), it is possible to inevitably generate other phases (such as ferrite, bainite and pearlite). The presence of such other phases is allowed as long as the total area ratio thereof is 9.5% or less. The total area ratio of the other phases is preferably 7.5% or less, and more preferably 5.5% or less.

<Region Where the Local Mn Concentration is at Least 1.2 Times the Mn Content in the Whole Steel Sheet: 1% or More by Area Ratio>

Residual austenite is allowed to remain even in a high strain region by concentrating Mn in residual austenite to increase stability of the residual austenite, thereby further improving the elongation to ensure an elongation of 10% or more. On the other hand, from the viewpoint of ensuring weldability, the average Mn concentration in the steel sheet is required to fulfill 1.5 mass % or less. In the steel sheet in the present invention, therefore, a Mn-concentrated region is formed. That is, residual austenite formed in the Mn-concentrated region is stabilized while keeping low the Mn concentration in the matrix. This results in that a part of a region where the local Mn concentration is at least 1.2 times the Mn content in the whole steel sheet is present as residual austenite to contribute to further improvement of the elongation.

Then, the composition constituting the steel sheet in the present invention will be explained. All the units of chemical components are hereinafter by mass %.

[Composition of Steel Sheet in the Present Invention]

C: 0.15% to 0.35%

C is an important element having a large influence on the strength of the steel sheet. In order to ensure the strength of the steel sheet, C is contained in an amount of 0.15% or more, preferably 0.16% or more and more preferably 0.17% or more. However, when C is excessively contained, the weldability is degraded. Therefore, C is contained in an amount of 0.35% or less, preferably 0.3% or less, and more preferably 0.25% or less.

Si: 0.5% to 3.0%

Si is a useful element for suppressing the formation of carbides and promoting the formation of the residual austenite. In order to effectively exhibit such an action, Si is contained in an amount of 0.5% or more, and is preferably 0.8% or more, and is more preferably 1.1% or more. However, when Si is excessively contained, the weldability is remarkably degraded. Therefore, Si is contained in an amount of 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less.

Mn: 0.5% to 1.5%

Mn is a useful element contributing to an increase in the strength of the steel sheet as a solid solution hardening element. It has also an effect of suppressing ferrite transformation during cooling by increasing hardenability during quenching. In addition, since it has also an effect of stabilizing austenite, residual austenite having high stability can be formed. In order to effectively exhibit such actions, Mn is contained in an amount of 0.5% or more, preferably 0.7% or more, and more preferably 0.9% or more. However, the Mn amount is preferably lower from the standpoint of ensuring the weldability, and Mn is contained in an amount of 1.5% or less, preferably 1.3% or less, and more preferably 1.15% or less.

Al: 0.001% to 0.10%

Al is a useful element added as a deoxidizing agent, and in order to obtain such an action, it is contained in an amount of 0.001% or more, preferably 0.01% or more, and more preferably 0.03% or more. However, when Al is excessively contained, cleanliness of the steel is degraded. Therefore, Al is contained in an amount of 0.10% or less, preferably 0.08% or less, and more preferably 0.06% or less.

The steel sheet in the present invention contains the above-mentioned elements as essential elements, the balance being iron and inevitable impurities (such as P, S, N and O). Of the inevitable impurities, P, S and N can be contained up to respective allowable ranges as described below.

P: 0.1% or less

P is inevitably present as an impurity element, and contributes to an increase in the strength by solid solution hardening. However, the segregation thereof to prior austenite grain boundary embrittles the grain boundary, thereby degrading workability. Therefore, the P amount is limited to 0.1% or less, preferably 0.05% or less, and more preferably 0.03% or less.

S: 0.01% or less

S is also inevitably present as an impurity element, and forms MnS inclusions, which may be starting points of cracks during deformation, thereby decreasing the workability. Therefore, the S amount is limited to 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less.

N: 0.01% or less

N is also inevitably present as an impurity element, and decreases the workability of the steel sheet by strain aging. Therefore, the N amount is limited to 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less.

In addition to these, the following allowable components may be contained within the ranges not impairing the actions of the present invention.

One or two or more of

Cu: 0.05% to 1.0%,

Ni: 0.05% to 1.0% and

B: 0.0002% to 0.0050%

These elements are useful elements having an effect of increasing hardenability during quenching and suppressing transformation from austenite. In order to obtain such an action, the respective elements are preferably contained in an amount equal to or more than the above-mentioned lower limits, respectively. The above-mentioned elements may be contained either alone or as a combination of two or more thereof. However, even when these elements are excessively contained, the effect becomes saturated, resulting in an economic waste. Therefore, the respective elements are contained in an amount equal to or less than the above-mentioned upper limits, respectively.

One or two or more of

Mo: 0.01% to 1.0%,

Cr: 0.01% to 1.0%,

Nb: 0.01% to 0.3%,

Ti: 0.01% to 0.3% and

V: 0.01% to 0.3%

These elements are useful for improving the strength without degrading the workability. In order to obtain such an action, the respective elements are preferably contained in an amount equal to or more than the above-mentioned lower limits, respectively. The above-mentioned elements may be contained either alone or as a combination of two or more thereof. However, when these elements are excessively contained, coarse carbides are formed to degrade the workability. Therefore, the respective elements are contained in an amount equal to or less than the above-mentioned upper limits, respectively.

One or two of

Ca: 0.0005% to 0.01% and

Mg: 0.0005% to 0.01%

These elements are useful for improving the workability by decreasing starting points of fracture by refining inclusions. In order to obtain such an action, the elements are each preferably contained in an amount of 0.0005% or more. The above-mentioned elements may be contained either alone or as a combination of two of them. However, when excessively contained, the inclusions are coarsened on the contrary to degrade the workability. Therefore, the elements are each contained in an amount of 0.01% or less.

Then, preferred production conditions for obtaining the above-mentioned steel sheet in the present invention will be explained below.

[Preferred Production Method of Steel Sheet in the Present Invention]

First, the steel having the above-mentioned composition is melted, and a slab (steel material) is obtained by ingot making or continuous casting. Thereafter, hot rolling is performed under conditions of a soaking temperature of 1200° C. or lower (more preferably 1150° C. or lower) and a finishing temperature of 900° C. or lower (more preferably 880° C. or lower), followed by cooling from the finishing temperature to the Ac1 point or lower, thereby forming a bainite or pearlite single-phase structure or a two-phase structure as containing ferrite.

After the above-mentioned hot rolling, annealing treatment is performed under conditions of holding at 680° C. to the Ac1 point (more preferably 690° C. to [Ac1-10° C.]) for 0.8 hours or longer (more preferably 1 hour or longer). By this annealing treatment, carbides are spheroidized and coarsened, and Mn is concentrated in the carbides to at least 1.2 times the amount of Mn added to the steel sheet. This annealing treatment may be performed by holding as such in the above-mentioned temperature region after cooling to the Ac1 point or lower, may be performed by gradual cooling in this temperature region, or may be performed after once cooled to lower than 680° C. after the hot rolling.

The Ac1 point can be determined from chemical components of the steel sheet using the following formula (1) described in Leslie, “The Physical Metallurgy of Steels”, translated by Shigeyasu Kouda, Maruzen, 1985, p. 273.

Ac1 (° C.)=723−10.7×Mn−16.9×Ni+29.1×Si+16.9×Cr  (1)

Here, each element symbol in the above-mentioned formula represents the content (mass %) of each element.

After the above-mentioned annealed sheet is cold rolled, the cold-rolled sheet is subjected to heat treatment (y-transformation heat treatment) under conditions of holding it at an austenite single-phase region temperature (the Ac3 point or higher) for 52 s or longer, thereby austenitizing the carbides. Since Mn has been concentrated in the carbides by the annealing treatment in the prior stage, austenite having a high Mn concentration is formed. By rapid cooling from the austenite single-phase region temperature to room temperature at a cooling rate of 100° C./s or more, residual austenite where Mn has been concentrated to at least 1.2 times the amount of Mn added to the steel sheet can be formed in martensite that is the matrix.

The Ac3 point can be determined from chemical components of the steel sheet using the following formula (2) described in Leslie, “The Physical Metallurgy of Steels”, translated by Shigeyasu Kouda, Maruzen, 1985, p. 273.

Ac3 (° C.)=910−203×√C−30×Mn+44.7×Si+700×P+400×Al−15.2×Ni−11×Cr−20×Cu+400×Ti+31.5×Mo+104×V  (2)

Here, each element symbol in the above-mentioned formula represents the content (mass %) of each element.

Then, tempered martensite is formed by tempering the above-mentioned heat-treated sheet under conditions of holding it at 150 to 300° C. for 30 to 1200 s, and strength-elongation balance can be improved to obtain the steel sheet in the present invention (the ultra-high-strength steel sheet excellent in the yield ratio and workability).

The present invention will be explained below in greater detail with reference to Examples, but it goes without saying that the present invention is not limited to the Examples described below and can be implemented with appropriate modifications without departing from the spirit described above and later, and all such modification are included in the technical scope of the present invention.

EXAMPLES

[Test Method]

Steels having respective compositions of A to K shown in Table 1 described below were melted, and ingots having a thickness of 120 mm were prepared. Using these ingots, hot rolling was performed to a thickness of 2.8 mm, and thereafter, annealing was performed under the annealing conditions shown in Table 2 described below. After the annealed sheets were pickled, they were cold rolled to a thickness of 1.0 mm to obtain cold-rolled sheets. Then, the cold-rolled sheets were subjected to y-transformation heat treatment and tempering under the respective conditions shown in Table 2 described below.

TABLE 1
		Transformation
Steel	Chemical composition* (mass %)	temperature (° C.)
type	C	Si	Mn	Al	P	S	N	Others	Ac1	Ac3
A	0.20	1.78	0.99	0.045	0.015	0.0015	0.0041	B: 0.002, Ti: 0.015	743	898
B	0.20	1.84	1.28	0.041	0.011	0.0015	0.0042	—	740	887
C	0.19	1.75	1.35	0.045	0.012	0.0012	0.0037	Ca: 0.004, Mg: 0.005	739	886
D	0.25	1.20	1.08	0.046	0.008	0.0016	0.0041	Ti: 0.05	742	854
E	0.10	1.45	1.02	0.045	0.011	0.0017	0.0038	—	743	906
F	0.22	1.44	0.49	0.045	0.009	0.0011	0.0035	—	748	889
G	0.21	1.53	0.95	0.046	0.013	0.0008	0.0041	Cr: 0.50	752	900
H	0.22	1.64	1.25	0.045	0.010	0.0016	0.0042	Cu: 0.10	740	874
I	0.21	1.46	1.11	0.045	0.009	0.0012	0.0041	Ni: 0.10	740	871
J	0.22	1.39	1.06	0.045	0.016	0.0008	0.0037	Nb: 0.05	742	874
K	0.20	1.52	1.08	0.043	0.011	0.0011	0.0041	Mo: 0.10	742	920
L	0.19	1.44	1.03	0.045	0.009	0.0012	0.0037	V: 0.05	743	884
(Underlined: outside the range of the present invention, *: balance: iron and inevitable impurities, —: not added)

TABLE 2
Production	Steel	Annealing after hot rolling	γ-transformation heat treatment	Tempering
No.	type	Temperature (° C.)	Time (h)	Temperature (° C.)	Time (s)	Cooling rate (° C./s)	Temperature (° C.)	Time (s)
1	A	500	1	930	90	>150	200	360
2	A	700	0.5	930	90	>150	200	360
3	A	700	1	930	90	>150	200	360
4	A	700	1	850	90	>150	200	360
5	A	800	1	930	90	>150	200	360
6	B	500	1	930	90	>150	200	360
7	B	700	0.5	930	90	>150	200	360
8	B	700	1	930	90	>150	200	360
9	B	700	1	850	90	>150	200	360
10	B	800	1	930	90	>150	200	360
11	C	700	1	930	90	>150	200	360
12	D	700	1	930	90	>150	200	360
13	E	700	1	930	90	>150	200	360
14	F	700	1	930	90	>150	200	360
15	G	700	1	930	90	>150	200	360
16	H	700	1	930	90	>150	200	360
17	I	700	1	930	90	>150	200	360
18	J	700	1	930	90	>150	200	360
19	K	700	1	930	90	>150	200	360
20	L	700	1	930	90	>150	200	360
(Underlined: outside the range of the present invention, Hatched: outside the recommended conditions of the present invention)

[Measurement Methods]

Using each steel sheet obtained, the area ratio of martensite and residual austenite and the local Mn concentration were measured. In order to evaluate mechanical properties of the steel sheet, the yield strength (YS), the tensile strength (TS) and the elongation (EL) were also measured. These measurement methods are shown below.

(Area Ratio of Martensite)

The area ratio of martensite was measured as follows. Each steel sheet was mirror polished, and a surface thereof was corroded with a 3% Nital liquid to expose a metal structure. Thereafter, using an SEM (scanning electron microscope), a structure of a portion of ¼ the sheet thickness was observed under a magnification of 2000 for 5 fields of view of an approximately 40 μm×30 μm region, and a region looking grey was defined as martensite. The area ratios determined for the respective fields of view were arithmetically averaged as the area ratio of martensite.

(Area Ratio of Residual Austenite)

The area ratio of residual austenite was determined by grinding and polishing each steel sheet to ¼ the sheet thickness in a sheet thickness direction and measuring X-ray diffraction intensity.

(Local Mn Concentration)

The local Mn concentration was determined by quantitatively analyzing 3 fields of view of an approximately 20 μm×20 mm region using a field emission electron probe microanalyzer (FE-EPMA), dividing a measurement region to small regions of 1 μm×1 mm in each field of view, and averaging the Mn concentrations in the respective small regions. The ratio of small regions where the average Mn concentration is at least 1.2 times the Mn content in the steel sheet was defined as the area ratio of the Mn-concentrated region in each field of view, and calculated. Evaluation was performed by arithmetically averaging the area ratios of the Mn-concentrated regions in the 3 fields of view.

(Yield Strength, Tensile Strength and Elongation)

Using each steel sheet to be evaluated, a No. 5 testpiece described in JIS Z 2201 was prepared while taking a major axis to a direction perpendicular to a rolling direction, and measurement was performed in accordance with JIS Z 2241 to determine the yield strength (YS), tensile strength (TS) and elongation (EL), and then, yield ratio (YR) was determined from YS/TS.

[Measurement Results]

The measurement results are shown in Table 3 described below. In these examples, the sheet having a tensile strength (TS) of 1470 MPa or more, a yield ratio (YR) of 0.75 or more and an elongation (EL) of 10% or more was represented by “A” and evaluated as passed, and determined as an ultra-high-strength steel sheet that excelled in the yield ratio and the workability. On the other hand, the sheet having a tensile strength (TS) of less than 1470 MPa, a yield ratio (YR) of less than 0.75 or an elongation (EL) of less than 10% was represented by “B” and determined as failed.

TABLE 3
			Area ratio in structure (%)	Mechanical properties
Steel	Steel	Production			Mn-concentrated	YS	TS	YR	EL
No.	type	No.	Martensite	Residual γ	region	(MPa)	(MPa)	(−)	(%)	Evaluation
1	A	1	95	1.1	0.0	1176	1512	0.78	8.2	B
2	A	2	95	1.1	0.6	1174	1513	0.78	9.2	B
3	A	3	95	0.7	1.3	1177	1506	0.78	10.5	A
4	A	4	84	1.0	1.5	845	1355	0.62	11.4	B
5	A	5	94	0.9	0.0	1168	1510	0.77	7.9	B
6	B	6	98	1.3	0.0	1194	1535	0.78	7.8	B
7	B	7	97	1.1	0.7	1187	1533	0.77	8.8	B
8	B	8	98	0.8	1.4	1224	1545	0.79	10.1	A
9	B	9	85	1.1	1.7	897	1398	0.64	11.2	B
10	B	10	96	1.0	0.0	1187	1520	0.78	7.5	B
11	C	11	99	0.8	1.5	1235	1556	0.79	10.2	A
12	D	12	99	0.6	1.1	1156	1487	0.78	10.0	A
13	E	13	31	0.0	0.2	412	845	0.49	19.1	B
14	F	14	78	0.0	1.8	752	1233	0.61	10.6	B
15	G	15	98	0.7	1.0	1203	1534	0.78	10.5	A
16	H	16	99	0.5	1.1	1208	1522	0.79	10.3	A
17	I	17	98	0.9	1.3	1194	1535	0.78	10.2	A
18	J	18	99	1.0	1.0	1223	1555	0.79	10.1	A
19	K	19	98	1.1	1.4	1234	1565	0.79	10.1	A
20	L	20	99	1.0	1.2	1242	1574	0.79	10.3	A
(Underlined: outside the range of the present invention, Hatched: outside the recommended conditions of the present invention)

As shown in Table 3, all the invention steels (steel Nos. 3, 8, 11, 12 and 15 to 20) fulfilling the requirements of the present invention (the above-mentioned component requirements and the above-mentioned structure requirements) satisfy a tensile strength TS of 1470 MPa or more, a yield ratio YR of 0.75 or more and an elongation EL of 10% or more, and the ultra-high-strength steel sheets excellent in the yield ratio and the workability have been obtained.

By contrast, the comparative steels (steel Nos. 1, 2, 4 to 7, 9, 10, 13 and 14) not satisfying at least one of the requirements of the present invention (the above-mentioned component requirements and the above-mentioned structure requirements) are degraded in at least any one property of the tensile strength TS, the yield ratio YR and the elongation EL.

For example, in steel Nos. 1 and 6, the annealing temperature after hot rolling is too low and is outside the recommended range as shown in production Nos. 1 and 6 of Table 2, respectively. Thus, Mn is not sufficiently concentrated in residual austenite to degrade the elongation EL, as shown in Table 3.

On the other hand, in steel Nos. 5 and 10, the annealing temperature after hot rolling is too high and is outside the recommended range as shown in production Nos. 5 and 10 of Table 2, respectively. Thus, Mn is homogenized by diffusion, and Mn is not sufficiently concentrated in residual austenite to degrade the elongation EL, as shown in Table 3.

Further, in steel Nos. 2 and 7, the annealing holding time after hot rolling is too short and is outside the recommended range as shown in production Nos. 2 and 7 of Table 2, respectively. Thus, Mn is not sufficiently concentrated in residual austenite to degrade the elongation EL, as shown in Table 3.

In addition, in steel Nos. 4 and 9, the y-transformation heat treatment temperature is too low and is outside the recommended range as shown in production Nos. 4 and 9 of Table 2, respectively. Thus, austenitization is not sufficiently achieved, and martensite is insufficient, resulting in poor tensile strength TS and yield ratio YR, as shown in Table 3.

Furthermore, in steel No. 13, the C content is too low as shown in steel type E of Table 1. Thus, both martensite and residual austenite are insufficient, and Mn is not sufficiently concentrated in residual austenite, resulting in poor tensile strength TS and yield ratio YR, as shown in Table 3.

Further, in steel No. 14, the Mn content is too low as shown in steel type F of Table 1. Thus, both martensite and residual austenite are insufficient, resulting in poor tensile strength TS and yield ratio YR, as shown in Table 3.

As described above, it has been confirmed that the ultra-high-strength steel sheets excellent in the yield ratio and the workability are obtained by satisfying the requirements of the present invention.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2015-026736 filed on Feb. 13, 2015, the entire subject matter of which is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The ultra-high-strength steel sheet of the present invention is excellent in yield ratio and workability, and is useful for vehicle bodies as cold-rolled steel sheets and various plated steel sheets.

Claims

1. An ultra-high-strength steel sheet excellent in yield ratio and workability, having a composition comprising, by mass %,

C: 0.15% to 0.35%,

Si: 0.5% to 3.0%

Mn: 0.5% to 1.5%,

Al: 0.001% to 0.10% and

the balance being iron and inevitable impurities,

wherein each of P, S and N of the inevitable impurities is limited to

P: 0.1% or less

S: 0.01% or less and

N: 0.01% or less,

the ultra-high-strength steel sheet having a structure comprising, by area ratio based on a whole structure,

martensite: 90% or more and

residual austenite: 0.5% or more,

the ultra-high-strength steel sheet having 1% or more by area ratio of a region where a local Mn concentration is at least 1.2 times a Mn content in a whole steel sheet, and

the ultra-high-strength steel sheet having a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more and an elongation of 10% or more.

2. The ultra-high-strength steel sheet excellent in yield ratio and workability according to claim 1, wherein the composition further comprises, by mass %, at least one of the following (a) to (c):

(a) one or two or more of Cu: 0.05% to 1.0%, Ni: 0.05% to 1.0% and B: 0.0002% to 0.0050%, (b) one or two or more of Mo: 0.01% to 1.0%, Cr: 0.01% to 1.0%, Nb: 0.01% to 0.3%, Ti: 0.01% to 0.3% and V: 0.01% to 0.3%, and

(c) one or two of Ca: 0.0005% to 0.01% and Mg: 0.0005% to 0.01%.