JP7215647B1 - High-strength steel plate and its manufacturing method - Google Patents

Abstract

An object of the present invention is to provide a high-strength steel sheet having a TS of 1320 MPa or more, a YR of 85% or more, and an excellent appropriate clearance range against delayed fracture, and a method for producing the same. A high-strength steel sheet that contains specific components, has a specific structure, and has a structure that satisfies the following formulas (1) and (2). KAM(S)/KAM(C) < 1.00 (1) Here, KAM(S) is the KAM (Kernel Average Misorientation) value of the surface layer of the steel sheet, and KAM(C) is the core of the steel sheet. KAM values are shown. Hv(Q)−Hv(S)≧8 (2) Here, Hv(Q) indicates the hardness of the ¼ part of the plate thickness, and Hv(S) indicates the hardness of the surface layer of the steel plate.

Description

TECHNICAL FIELD The present invention relates to a high-strength steel sheet having excellent tensile strength and delayed fracture resistance, and a method for producing the same. The high-strength steel sheet of the present invention can be suitably used as structural members such as automobile parts.

With the aim of reducing _CO2 emissions by reducing the weight of vehicles and improving collision resistance by reducing the weight of automobile bodies, the strength of steel sheets for automobiles is being increased, and new regulations are being introduced one after another. . Therefore, for the purpose of increasing the strength of the vehicle body, the application of high-strength steel sheets with a tensile strength (TS) of 1320 MPa class or higher is increasing in the main structural parts forming automobiles.

High-strength steel sheets used in automobiles are required to have an excellent yield ratio (YR=yield strength YS/tensile strength TS) from the viewpoint of the performance of parts. For example, frame parts such as bumpers of automobiles are required to have excellent impact absorption at the time of collision, so it is preferable to use a steel plate having excellent YR, which correlates with impact absorption.

In addition, there are many end faces formed by shearing in frame parts of automobiles. The morphology of the sheared edge depends on the shear clearance. The morphology of the sheared edge affects the delayed fracture resistance. Here, delayed fracture means that when a molded part is placed in an environment where hydrogen penetrates, hydrogen penetrates into the steel sheet that makes up the part, reducing the interatomic bonding force and causing local deformation. This is a phenomenon in which microcracks are generated as a result, and breakage occurs as the microcracks propagate. High-strength steel sheets used in automobiles are required to have a wide appropriate clearance range against delayed fracture.

In response to these demands, for example, Patent Document 1 provides a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent bending workability, and a method for producing the same. However, the technique described in Patent Literature 1 does not consider YR and the proper clearance range for delayed fracture. In addition, none of the steel sheets described in Patent Document 1 achieves YR≧85%.

For example, Patent Document 2 provides a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent delayed fracture resistance of sheared edges, and a method for producing the same. However, the technique described in Patent Literature 2 does not consider the appropriate clearance range for delayed fracture.

For example, Patent Document 3 provides a high-strength steel sheet having a tensile strength of 1100 MPa or more and excellent YR, surface properties and weldability, and a method for producing the same. However, the technique described in Patent Literature 3 does not consider the appropriate clearance range for delayed fracture.

Japanese Patent No. 6354909 Japanese Patent No. 6112261 Japanese Patent No. 6525114

The present invention was developed in view of such circumstances, and an object of the present invention is to provide a high-strength steel sheet having a TS of 1320 MPa or more, a YR of 85% or more, and an excellent appropriate clearance range for delayed fracture, and a method for producing the same. .

In order to achieve the above-described problems, the present inventors have made intensive studies and found the following.
(1) A TS of 1320 MPa or more can be realized by making the tempered martensite content 85% or more.
(2) Retained austenite is less than 5%, KAM(S)/KAM(C) is less than 1.00, and Hv(Q)-Hv(S) is 8 or more, so that 85% or more YR can be realized.
(3) By setting KAM(S)/KAM(C) to less than 1.00 and Hv(Q)-Hv(S) to be 8 or more, an excellent appropriate clearance range against delayed fracture can be realized.

The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows.
[1] % by mass,
C: 0.15% or more and 0.45% or less,
Si: 0.10% or more and 2.00% or less,
Mn: 0.5% or more and 3.5% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 1.000% or less,
N: 0.0100% or less,
H: 0.0020% or less,
a component composition with the balance being Fe and unavoidable impurities;
Tempered martensite has an area fraction of 85% or more,
Retained austenite is less than 5% by volume,
The total area fraction of ferrite and bainitic ferrite is 10% or less,
A high-strength steel sheet having a structure that satisfies the following formulas (1) and (2).
KAM(S)/KAM(C) < 1.00 (1)
Here, KAM(S) is the KAM (Kernel Average Misorientation) value of the surface layer of the steel sheet, and KAM(C) is the KAM value of the center of the steel sheet.
Hv(Q) - Hv(S) ≥ 8 (2)
Here, Hv(Q) indicates the hardness of the 1/4 portion of the plate thickness, and Hv(S) indicates the hardness of the surface layer of the steel plate.
[2] Further, as a component composition, in mass%,
Ti: 0.100% or less,
B: 0.0100% or less,
Nb: 0.100% or less,
Cu: 1.00% or less,
Cr: 1.00% or less,
V: 0.100% or less,
Mo: 0.500% or less,
Ni: 0.50% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
As: 0.100% or less,
Ta: 0.100% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less,
Zn: 0.020% or less,
Co: 0.020% or less,
Zr: 0.020% or less,
REM: The high-strength steel sheet according to [1], containing one or more elements selected from 0.0200% or less.
[3] The high-strength steel sheet according to [1] or [2], which has a plating layer on the surface of the steel sheet.
[4] A method for producing a high-strength steel sheet according to [1] or [2] above,
Cold-rolled steel sheets produced by subjecting steel slabs to hot rolling, pickling and cold rolling,
The temperature T1 is 850° C. or higher and 1000° C. or lower,
After annealing under the condition that the holding time t1 at T1 is 10 seconds or more and 1000 seconds or less,
Cool to 100° C. or less,
Starting processing within an elapsed time t2 of 1000 seconds or less after reaching 100° C.,
The processing has a starting temperature T2 of 80° C. or less,
After processing under the condition that the equivalent plastic strain is 0.10% or more and 5.00% or less,
The temperature T3 is 100° C. or higher and 400° C. or lower,
Tempering under the condition that the holding time t3 at T3 is 1.0 seconds or more and 1000.0 seconds or less,
A method for producing a high-strength steel sheet, wherein the cooling rate θ1 from T3 to 80°C is 100°C/sec or less.
[5] In the processing step before tempering, strain is applied by processing in two or more steps, and processing is performed under the condition that the total equivalent plastic strain of each processing is 0.10% or more. [4] A method for producing a high-strength steel sheet.
[6] The method for producing a high-strength steel sheet according to [4] or [5], wherein plating is performed during or after annealing.

According to the present invention, it is possible to obtain a high-strength steel sheet having a TS of 1320 MPa or more, a YR of 85% or more, and an excellent appropriate clearance range for delayed fracture. Further, by applying the high-strength steel sheet of the present invention to automobile structural members, for example, it is possible to improve fuel consumption by reducing the weight of the vehicle body. Therefore, the industrial utility value is extremely large.

Embodiments of the present invention will be described below.

First, the appropriate range of the chemical composition of the high-strength steel sheet and the reason for its limitation will be described. In the following description, "%" representing the content of constituent elements of steel means "% by mass" unless otherwise specified.

C: 0.15% or more and 0.45% or less C is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects TS. If the C content is less than 0.15%, it becomes difficult to achieve a TS of 1320 MPa or more. Therefore, the C content should be 0.15% or more. The C content is preferably 0.16% or more. The C content is more preferably 0.17% or more. The C content is more preferably 0.18% or more. The C content is most preferably 0.19% or more. On the other hand, if the C content exceeds 0.45%, the ultimate deformability of the steel is lowered, and the appropriate clearance range for delayed fracture is lowered. Therefore, the C content should be 0.45% or less. The C content is preferably 0.40% or less. The C content is more preferably 0.35% or less. The C content is more preferably 0.30% or less. The C content is most preferably 0.26% or less.

Si: 0.10% or more and 2.00% or less Si is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects TS and retained austenite. If the Si content is less than 0.10%, it becomes difficult to achieve a TS of 1320 MPa or more. Therefore, the Si content should be 0.10% or more. The Si content is preferably 0.15% or more. The Si content is more preferably 0.20% or more. The Si content is more preferably 0.30% or more. The Si content is most preferably 0.40% or more. On the other hand, if the Si content exceeds 2.00%, retained austenite increases excessively, making it difficult to achieve a YR of 85% or more. Therefore, the Si content should be 2.00% or less. The Si content is preferably 1.80% or less. The Si content is more preferably 1.60% or less. The Si content is more preferably 1.50% or less. The Si content is most preferably 1.20% or less.

Mn: 0.5% or more and 3.5% or less Mn is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects the ferrite fraction and the bainite fraction. If the Mn content is less than 0.5%, the ferrite fraction and bainite fraction increase, making it difficult to achieve a TS of 1320 MPa or more and a YR of 85% or more. Become. Therefore, the Mn content should be 0.5% or more. The Mn content is preferably 0.7% or more. The Mn content is more preferably 1.0% or more. The Mn content is more preferably 1.1% or more. The Mn content is most preferably 1.5% or more. On the other hand, when the Mn content exceeds 3.5%, macro segregation of Mn occurs and the ultimate deformability of the steel is lowered, so that the appropriate clearance range for delayed fracture is lowered. Therefore, the Mn content should be 3.5% or less. The Mn content is preferably 3.3% or less. The Mn content is more preferably 3.1% or less. The Mn content is more preferably 3.0% or less. The Mn content is most preferably 2.8% or less.

P: 0.100% or less When the content of P exceeds 0.100%, P segregates at grain boundaries and embrittles the steel sheet, so that the proper clearance range for delayed fracture decreases. Therefore, the P content should be 0.100% or less. The P content is preferably 0.080% or less. The P content is more preferably 0.060% or less. Although the lower limit of the P content is not particularly limited, it is preferably 0.001% or more due to production technology restrictions.

S: 0.0200% or less When the S content exceeds 0.0200%, it exists as a sulfide and lowers the ultimate deformability of the steel, thereby lowering the appropriate clearance range for delayed fracture. Therefore, the S content should be 0.0200% or less. The S content is preferably 0.0100% or less. The S content is more preferably 0.0050% or less. Although the lower limit of the S content is not particularly limited, it is preferably 0.0001% or more due to production technology restrictions.

Al: 0.010% or more and 1.000% or less By containing Al, the strength of the steel sheet increases, making it easier to achieve a TS of 1320 MPa or more. In order to obtain this effect, the Al content must be 0.010% or more. Therefore, the Al content should be 0.010% or more. The Al content is preferably 0.012% or more. Al content is more preferably 0.015% or more. The Al content is more preferably 0.020% or more. On the other hand, when the Al content exceeds 1.000%, the ferrite fraction and bainite fraction increase, making it difficult to achieve a TS of 1320 MPa or more and a YR of 85% or more. become difficult. Therefore, the Al content should be 1.000% or less. The Al content is preferably 0.500% or less. Al content is more preferably 0.100% or less.

N: 0.0100% or less When the N content exceeds 0.0100%, the cast slab becomes embrittled and easily cracked, resulting in a significant drop in productivity. Therefore, the N content should be 0.0100% or less. The N content is preferably 0.0080% or less. The N content is more preferably 0.0070% or less. The N content is more preferably 0.0060% or less. The N content is most preferably 0.0050% or less. Although the lower limit of the N content is not particularly limited, it is preferably 0.0010% or more due to production technology restrictions.

H: 0.0020% or less When the H content exceeds 0.0020% or less, the ultimate deformability of the steel is lowered, and the appropriate clearance range for delayed fracture is lowered. Therefore, the H content should be 0.0020% or less. The H content is preferably 0.0015% or less. The H content is more preferably 0.0010% or less. Although the lower limit of the H content is not particularly limited, it may be 0% because the smaller the H content, the better the appropriate clearance range against delayed fracture.

In addition to the above chemical composition, the high-strength steel sheet of the present invention further has, in mass%,
Ti: 0.100% or less, B: 0.0100% or less, Nb: 0.100% or less, Cu: 1.00% or less, Cr: 1.00% or less, V: 0.100% or less, Mo: 0.500% or less Ni: 0.50% or less Sb: 0.200% or less Sn: 0.200% or less As: 0.100% or less Ta: 0.100% or less Ca: 0.100% or less 0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less,
REM: It is preferable to contain one or more elements selected from 0.0200% or less.

Ti: 0.100% or less When the Ti content exceeds 0.100%, the cast slab becomes embrittled and easily cracked, resulting in a significant drop in productivity. Therefore, when Ti is added, its content shall be 0.100% or less. The Ti content is preferably 0.075% or less. The Ti content is more preferably 0.050% or less. The Ti content is more preferably less than 0.050%. On the other hand, the inclusion of Ti increases the strength of the steel sheet, making it easier to achieve a TS of 1320 MPa or more. In order to obtain this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. The Ti content is more preferably 0.010% or more.

B: 0.0100% or less When the content of B exceeds 0.0100%, the cast slab becomes brittle and easily cracked, resulting in a significant drop in productivity. Therefore, when B is added, its content should be 0.0100% or less. The B content is preferably 0.0080% or less. The B content is more preferably 0.0050% or less. On the other hand, the inclusion of B increases the strength of the steel sheet, making it easier to achieve a TS of 1320 MPa or more. In order to obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more.

Nb: 0.100% or less When the Nb content exceeds 0.100%, the crude cast slab becomes embrittled and easily cracked, resulting in a marked decrease in productivity. Therefore, when Nb is added, its content should be 0.100% or less. The Nb content is preferably 0.090% or less. The Nb content is more preferably 0.050% or less. The Nb content is more preferably 0.030% or less. On the other hand, the inclusion of Nb increases the strength of the steel sheet, making it easier to achieve a TS of 1320 MPa or more. In order to obtain this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.002% or more.

Cu: 1.00% or less When the Cu content exceeds 1.00%, the cast slab becomes embrittled and easily cracked, resulting in a significant decrease in productivity. Therefore, when Cu is added, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less. On the other hand, containing Cu suppresses penetration of hydrogen into the steel sheet and improves the appropriate clearance range for delayed fracture. In order to obtain this effect, the Cu content is preferably 0.01% or more. The Cu content is preferably 0.03% or more. Cu content is more preferably 0.10% or more.

Cr: 1.00% or less When the Cr content exceeds 1.00%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Lower range. Therefore, when Cr is added, its content should be 1.00% or less. The Cr content is preferably 0.70% or less. The Cr content is more preferably 0.50% or less. On the other hand, Cr not only plays a role as a solid-solution strengthening element, but also stabilizes austenite and suppresses formation of ferrite in the cooling process during continuous annealing, thereby increasing the strength of the steel sheet. In order to obtain such effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more.

V: 0.100% or less When the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions are formed, and the ultimate deformability of the steel is reduced. Clearance range is reduced. Therefore, when V is added, its content should be 0.100% or less. Preferably, it is 0.060% or less. On the other hand, V increases the strength of the steel sheet. In order to obtain such effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is more preferably 0.010% or more.

Mo: 0.500% or less When the Mo content exceeds 0.500%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Lower range. Therefore, when adding Mo, the content shall be 0.500% or less. The Mo content is preferably 0.450% or less. Mo content is more preferably 0.400% or less. On the other hand, Mo not only plays a role as a solid-solution strengthening element, but also stabilizes austenite and suppresses the formation of ferrite in the cooling process during continuous annealing, thereby increasing the strength of the steel sheet. In order to obtain such effects, the Mo content is preferably 0.010% or more. Mo content is more preferably 0.020% or more.

Ni: 0.50% or less When the Ni content exceeds 0.50%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Lower range. Therefore, when Ni is added, its content is made 0.50% or less. The Ni content is preferably 0.45% or less. The Ni content is more preferably 0.30% or less. On the other hand, Ni stabilizes austenite and suppresses the formation of ferrite in the cooling process during continuous annealing, thereby increasing the strength of the steel sheet. In order to obtain such effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.02% or more.

Sb: 0.200% or less When the Sb content exceeds 0.200%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Lower range. Therefore, when Sb is added, its content should be 0.200% or less. The Sb content is preferably 0.100% or less. The Sb content is more preferably 0.050% or less. On the other hand, Sb suppresses the formation of surface layer softening and increases the strength of the steel sheet. In order to obtain such effects, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more.

Sn: 0.200% or less When the Sn content exceeds 0.200%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Lower range. Therefore, when Sn is added, its content should be 0.200% or less. The Sn content is preferably 0.100% or less. The Sn content is more preferably 0.050% or less. On the other hand, Sn suppresses the formation of surface layer softening and increases the strength of the steel sheet. In order to obtain such effects, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more.

As: 0.100% or less When the content of As exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of steel. Clearance range is reduced. Therefore, when As is added, its content should be 0.100% or less. As content is preferably 0.060% or less. The As content is more preferably 0.010% or less. As increases the strength of the steel sheet. In order to obtain such effects, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more.

Ta: 0.100% or less When the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are formed, and the ultimate deformability of the steel is reduced. Lower range. Therefore, when Ta is added, its content should be 0.100% or less. The Ta content is preferably 0.050% or less. The Ta content is more preferably 0.010% or less. On the other hand, Ta increases the strength of the steel sheet. In order to obtain such effects, the Ta content is preferably 0.001% or more. Ta content is more preferably 0.005% or more.

Ca: 0.0200% or less When the Ca content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are formed, and the ultimate deformability of the steel is reduced. Lower range. Therefore, when Ca is added, its content should be 0.0200% or less. The Ca content is preferably 0.0100% or less. On the other hand, Ca is an element used for deoxidation, and is an element effective in making sulfides spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, the content of Ca is preferably 0.0001% or more.

Mg: 0.0200% or less When the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Lower range. Therefore, when Mg is added, its content should be 0.0200% or less. On the other hand, Mg is an element used for deoxidation, and is an element effective in making the shape of sulfides spherical, improving the ultimate deformability of the steel sheet, and improving the proper clearance range for delayed fracture. In order to obtain such effects, the content of Mg is preferably 0.0001% or more.

Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less When the contents of Zn, Co and Zr each exceed 0.020%, large amounts of coarse precipitates and inclusions are produced. Since it forms in the steel and lowers the ultimate deformability of the steel, the appropriate clearance range for hole expansion deformation decreases. Therefore, when Zn, Co and Zr are added, their contents should be 0.020% or less. On the other hand, Zn, Co and Zr are all effective elements for making inclusions spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, the contents of Zn, Co and Zr are each preferably 0.0001% or more.

REM: 0.0200% or less When the REM content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Lower range. Therefore, when REM is added, its content should be 0.0200% or less. On the other hand, REM is an element effective in making the shape of inclusions spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, the content of REM is preferably 0.0001% or more.

The balance other than the above components is Fe and unavoidable impurities. If the content of the above optional components is less than the lower limit, the effect of the present invention is not impaired. Therefore, if the content of these optional elements is less than the lower limit, these optional elements are included as unavoidable impurities.

Next, the steel structure of the high-strength steel sheet of the present invention will be explained.

Tempered martensite: 85% or more in terms of area fraction In the present invention, this is an extremely important invention constituent feature. By using martensite as the main phase, it is possible to achieve a TS of 1320 MPa or more. In order to obtain such effects, the area fraction of tempered martensite must be 85% or more. Therefore, the area fraction of tempered martensite is set to 85% or more. The area fraction of tempered martensite is preferably 90% or more. The area fraction of tempered martensite is more preferably 92% or more. More preferably, it is 95% or more. On the other hand, the upper limit is not particularly limited, but the area fraction of tempered martensite may be 100%.

Here, the method for measuring tempered martensite is as follows. After polishing the L cross section of the steel plate, 3 vol. % nital, and 1/4 part of the plate thickness (position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields of view at a magnification of 2000 using SEM. In the above structure image, the tempered martensite is a structure having fine unevenness inside the structure and having carbide inside. The tempered martensite can be determined from the average of these values.

Retained austenite: less than 5% in volume fraction In the present invention, this is a very important invention constituent feature. When the volume fraction of retained austenite is 5% or more, it becomes difficult to achieve a YR of 85% or more. The cause of the decrease in YR is that an increase in retained austenite causes a decrease in YS due to deformation-induced transformation of retained austenite. Therefore, retained austenite should be less than 5%. It is preferably 4% or less. Although the lower limit of the retained austenite is not particularly limited, the retained austenite is preferably as low as possible, and may be 0%.

Here, the method for measuring retained austenite is as follows. Retained austenite is obtained by polishing a steel plate from 1/4 part of the plate thickness to a surface of 0.1 mm, and then chemically polishing the surface to 0.1 mm. }, {220}, {311} planes and {200}, {211}, {220} planes of bcc iron. Measure the integrated intensity ratio of each diffraction peak, and average the nine integrated intensity ratios obtained. I asked for it.

Total of ferrite and bainitic ferrite: 10% or less in terms of area fraction This is an extremely important inventive constituent element in the present invention. If the total content of ferrite and bainitic ferrite exceeds 10%, it becomes difficult to achieve a TS of 1320 MPa or more and a YR of 85% or more. The cause of the decrease in YR is that ferrite and bainitic ferrite are soft structures and yield early. Therefore, the total content of ferrite and bainitic ferrite should be 10% or less. It is preferably 8% or less. More preferably, it is 5% or less. Although the lower limit of the total content of ferrite and bainitic ferrite is not particularly limited, the lower the content, the better, and the lower limit of the total content of ferrite and bainitic ferrite may be 0%.

Here, the method for measuring the sum of ferrite and bainitic ferrite is as follows. After polishing the L cross section of the steel plate, 3 vol. % nital, and 1/4 part of the plate thickness (position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields of view at a magnification of 2000 using SEM. In the above tissue images, ferrite and bainitic ferrite are recessed structures with a flat interior. The sum of ferrite and bainitic ferrite can be obtained from the average of those values.

Pearlite, fresh martensite, acicular ferrite, and the like are conceivable as structures other than the whole structure described above. These structures do not affect the characteristics as long as they do not exceed 5%, so they may be included.
KAM(S)/KAM(C)<1.00
KAM(S) is the KAM (Kernel Average Misorientation) value of the surface layer of the steel sheet, and KAM(C) is the KAM value of the center of the steel sheet. The steel plate surface layer portion is a position moved 100 μm from the steel plate surface toward the plate thickness central portion side. The central portion of the steel plate is the position of 1/2 of the plate thickness. As a result of the inventor's research, it is effective to change the dislocation distribution state from the surface layer to the inside to make KAM(S)/KAM(C) less than 1.00 in order to improve the appropriate clearance range for YR and delayed fracture. was confirmed. Therefore, KAM(S)/KAM(C) should be less than 1.00. Although the lower limit of KAM(S)/KAM(C) is not particularly limited, it is preferably 0.80 or more due to production technology restrictions.

Here, the method for measuring the KAM value is as follows. First, a test piece for structure observation was taken from the cold-rolled steel sheet. Next, the sampled test piece was polished by colloidal silica vibration polishing so that the cross section in the rolling direction (L cross section) was the observation surface. The observation surface was a mirror surface. Electron backscatter diffraction (EBSD) measurements were then performed to obtain local crystallographic orientation data. At this time, the SEM magnification was 3000 times, the step size was 0.05 μm, the measurement area was 20 μm square, and the WD was 15 mm. Analysis software: OIM Analysis 7 was used to analyze the obtained local orientation data. The analysis was performed for each of 10 fields of view for the target plate thickness, and the average value was used.

Prior to data analysis, the analysis software Grain Dilation function (Grain Tolerance Angle: 5, Minimum Grain Size: 2, Single Iteration: ON) and Grain CI Standardization function (Grain Tolerance Angle: 5, Minimum Grain Size: 5) clean The up treatment was performed once in order. Afterwards, only measurement points with a CI value >0.1 were used for the analysis. A chart of KAM values was displayed, and the average KAM value of the bcc phase was obtained. The analysis at that time was performed under the following conditions.
Nearest neighbor: 1st
Maximum misorientation: 5
Perimeter only
Check Set 0-point kernels to maximum misorientation Hv(Q)-Hv(S)≧8
Hv(Q) is the hardness of 1/4 part of the plate thickness, and Hv(S) is the hardness of the surface layer of the steel plate. The steel plate surface layer portion is a position moved 100 μm from the steel plate surface toward the plate thickness central portion side. As a result of the inventor's investigation, it was confirmed that it is effective to change the hardness from the surface layer to the inside and set Hv (Q) - Hv (S) to 8 or more in order to improve the appropriate clearance range for YR and delayed fracture. . Therefore, Hv(Q)-Hv(S) should be 8 or more. Although the upper limit of Hv(Q)-Hv(S) is not particularly limited, it is preferably 30 or less due to production technology restrictions. The preferred ranges of Hv(Q) and Hv(S) are 400-600 and 400-600, respectively.
Here, the method for measuring hardness is as follows. First, a test piece for structure observation was taken from the cold-rolled steel sheet. Next, the sampled test piece was polished so that the cross section in the rolling direction (L cross section) was the observation surface. The observation surface was a mirror surface. Then, the hardness was determined using a Vickers tester with a load of 1 kg. The hardness was measured at 10 points at intervals of 20 μm with respect to the target plate thickness, and the average value of the 8 points was used, excluding the maximum hardness and the minimum hardness.

Next, the manufacturing method of the present invention will be described.

In the present invention, the method of melting the steel material (steel slab) is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. Steel slabs (slabs) are preferably produced by continuous casting to prevent macro-segregation.

In the present invention, the slab heating temperature, slab soaking holding time and coiling temperature in hot rolling are not particularly limited. Methods for hot rolling steel slabs include a method of rolling after heating the slab, a method of directly rolling the slab after continuous casting without heating, and a method of subjecting the slab after continuous casting to heat treatment for a short period of time before rolling. etc. The slab heating temperature, slab soaking holding time, finish rolling temperature and coiling temperature in hot rolling are not particularly limited, but the slab heating temperature is preferably 1100° C. or higher. The slab heating temperature is preferably 1300° C. or less. The slab soaking holding time is preferably 30 minutes or longer. The slab soaking holding time is preferably 250 minutes or less. The finish rolling temperature is preferably the Ar ₃ transformation point or higher. Moreover, the winding temperature is preferably 350° C. or higher. The winding temperature is preferably 650°C or lower.

The hot-rolled steel sheet thus produced is pickled. Since pickling can remove oxides from the surface of the steel sheet, it is important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet. Also, the pickling may be performed once, or may be divided into a plurality of times. Further, the hot-rolled pickling-treated sheet may be cold-rolled, or the cold-rolled sheet may be heat-treated and then cold-rolled.

The rolling reduction in cold rolling and the sheet thickness after rolling are not particularly limited, but the rolling reduction in cold rolling is preferably 30% or more. The rolling reduction in cold rolling is preferably 80% or less. The number of rolling passes and the rolling reduction of each pass are not particularly limited, and the effects of the present invention can be obtained.

The cold-rolled steel sheet obtained as described above is annealed. Annealing conditions are as follows.

Annealing temperature T1: 850° C. or higher and 1000° C. or lower In the present invention, this is an extremely important invention constituent feature. When the annealing temperature T1 is less than 850°C, the total area fraction of ferrite and bainitic ferrite exceeds 10%, making it difficult to achieve a TS of 1320 MPa or more and a YR of 85% or more. become difficult to do. Therefore, the annealing temperature T1 is set to 850° C. or higher. T1 is preferably 860° C. or higher. T1 is more preferably 870° C. or higher. On the other hand, if the annealing temperature T1 exceeds 1000° C., the prior austenite grain size increases excessively, and the appropriate clearance range for delayed fracture decreases. Therefore, the annealing temperature T1 is set to 1000° C. or lower. Annealing temperature T1 is preferably 970° C. or lower. T1 is more preferably 950° C. or less.

Holding time t1 at annealing temperature T1: 10 seconds or more and 1000 seconds or less This is an extremely important invention constituent feature in the present invention. When the holding time t1 at the annealing temperature T1 is less than 10 seconds, the austenitization is insufficient, the total area fraction of ferrite and bainitic ferrite exceeds 10%, and it is difficult to achieve a TS of 1320 MPa or more. and it becomes difficult to realize YR of 85% or more. Therefore, the holding time t1 at the annealing temperature T1 is set to 10 seconds or longer. The holding time t1 at the annealing temperature T1 is preferably 30 seconds or longer. t1 is more preferably 45 seconds or longer. t1 is more preferably 60 seconds or more. t1 is most preferably 100 seconds or more. On the other hand, if the holding time at the annealing temperature T1 exceeds 1000 seconds, the grain size of prior austenite excessively increases, and the proper clearance range for delayed fracture decreases. Therefore, the holding time t1 at the annealing temperature T1 is set to 1000 seconds or less. The holding time t1 at the annealing temperature T1 is preferably 800 seconds or less. t1 is more preferably 500 seconds or less.

Cooling down to 100°C or lower after annealing In the cooling process down to 100°C or lower, austenite is transformed into martensite. In order to obtain 85% or more martensite, it is necessary to cool to 100°C or less after annealing. Therefore, it cools to 100 degrees C or less after annealing. Although the lower limit of the cooling completion temperature is not particularly limited, it is preferably 0° C. or higher due to production technology restrictions.

Elapsed time t2 from the time the temperature reaches 100° C. to the start of processing: 1000 seconds or less This is a very important invention constituent feature in the present invention. When the elapsed time t2 from the time when the temperature reaches 100° C. to the start of working exceeds 1000 seconds, aging of the martensite structure progresses, and the amount of strain introduced into the steel sheet surface layer and the steel sheet center due to working changes. KAM(S)/KAM(C) becomes 1.00 or more, and the proper clearance range for YR and delayed fracture decreases. Therefore, the elapsed time t2 from the time when the temperature reaches 100° C. to the start of processing is set to 1000 seconds or less. The elapsed time t2 from the time when the temperature reaches 100° C. to the start of processing is preferably 900 seconds or less. t2 is more preferably 800 seconds or less. Although the lower limit of the elapsed time t2 from the time when the temperature reaches 100° C. to the start of processing is not particularly limited, it is preferably 5 seconds or more due to production technology restrictions. As a result of investigation by the inventor, it was found that the elapsed time from the time when the temperature reaches 100° C. to the end of working does not affect the amount of strain introduced into the steel sheet surface layer portion and the steel plate center portion due to working.

Processing start temperature T2 is 80° C. or less This is an extremely important invention constituent feature in the present invention. When the working start temperature T2 exceeds 80 ° C., the steel plate is soft, so the amount of strain introduced into the steel plate surface layer and the steel plate center due to working changes, and KAM (S) / KAM (C) is 1.00 or more. As a result, the appropriate clearance range for YR and delayed fracture is lowered. Therefore, the processing start temperature T2 is set to 80° C. or lower. The processing start temperature T2 is preferably 60° C. or less. T2 is more preferably 50° C. or less. Although the lower limit of the processing start temperature T2 is not particularly limited, it is preferably 0° C. or higher due to restrictions on production technology.

Equivalent plastic strain: 0.10% or more and 5.00% or less In the present invention, this is an extremely important invention constituent feature. If the equivalent plastic strain is less than 0.10%, the working amount is insufficient, KAM(S)/KAM(C) is 1.00 or more, and the appropriate clearance range for YR and delayed fracture is lowered. Therefore, the equivalent plastic strain should be 0.10% or more. The plastic equivalent strain is preferably 0.15% or more. The plastic equivalent strain is more preferably 0.20% or more. When the equivalent plastic strain exceeds 5.00%, the effect of processing becomes equal at the steel plate surface and the steel plate center, and KAM (S) / KAM (C) is 1.00 or more, and the appropriate clearance range for YR and delayed fracture decreases. Note that the upper limit of the equivalent plastic strain is set to 5.00% or less due to production technology restrictions. Therefore, the equivalent plastic strain should be 5.00% or less. The equivalent plastic strain is preferably 4.00% or less. The equivalent plastic strain is more preferably 2.00% or less. The equivalent plastic strain is more preferably 1.00% or less.

It is preferable that strain is imparted by working twice or more in the working step before tempering, and the total equivalent plastic strain of each working is 0.10% or more.

Even if the equivalent plastic strain in the first processing is less than 0.10%, if the total equivalent plastic strain becomes 0.10% or more in the second and subsequent processing, KAM (S) / KAM (C) It becomes less than 1.00, and the appropriate clearance range for YR and delayed fracture is improved. Therefore, in the working process before tempering, the strain may be imparted by working twice or more, and the total equivalent plastic strain of each working should be 0.10% or more. In addition, the time from the time when the temperature reaches 100° C. to the time when the second and subsequent processing is started is not particularly limited. This is because the mobility of dislocations in martensite decreases due to the first working.

Here, the representative processing methods for the above processing include temper rolling and a tension leveler. The equivalent plastic strain in temper rolling is the elongation rate of the steel sheet, and can be obtained from the change in length of the steel sheet before and after working. The method of calculating the equivalent plastic strain of the steel sheet during leveling was calculated by the method of Reference 1 below. The following data inputs were used in the calculations, and the work hardening behavior of the material was assumed to be linear hardening elastoplastic, ignoring Bausinger hardening and tension reduction due to bend loss. Misaka's formula was used as the processing curvature formula.
・The number of plate thickness divisions: 31
・Young's modulus: 21000 kgf/mm ²
・Poisson's ratio: 0.3
・Yield stress: 111 kgf/mm ²
・ Plastic coefficient: 1757 kgf / mm ²
[Reference 1] Keisuke Misaka, Takeshi Masui: Plasticity and Processing, 17 (1976), 988.
In addition, the above processing may be performed by a general method of imparting strain other than the above, and for example, a continuous stretcher leveler or a roller leveler can also be used.

Tempering temperature T3: 100° C. or higher and 400° C. or lower In the present invention, this is an extremely important invention constituent feature. When the tempering temperature T3 is less than 100 ° C., the carbon diffusion distance is short, so the hardness of the steel plate surface and the steel plate interior is small, and Hv (Q) - Hv (S) is less than 8, which is suitable for YR and delayed fracture. Clearance range is reduced. Therefore, the tempering temperature T3 is set to 100° C. or higher. Tempering temperature T3 is preferably 150° C. or higher. T3 is more preferably 170° C. or higher. T3 is more preferably 200° C. or higher. On the other hand, if the tempering temperature T3 exceeds 400° C., the tempering of martensite progresses, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the tempering temperature T3 is set to 400° C. or lower. Tempering temperature T3 is preferably 350° C. or lower. T3 is more preferably 300° C. or less. T3 is more preferably 280° C. or less.

Holding time t3 at tempering temperature T3: 1.0 seconds or more and 1000.0 seconds or less This is an extremely important invention constituent feature in the present invention. When the holding time t3 at the tempering temperature T3 is less than 1.0 seconds, the diffusion distance of carbon is short, so the hardness of the steel sheet surface and the steel sheet interior becomes small, and Hv(Q)-Hv(S) becomes less than 8. , YR and delayed fracture are reduced. Therefore, the holding time t3 at the tempering temperature T3 is set to 1.0 seconds or longer. The holding time t3 at the tempering temperature T3 is preferably 5.0 seconds or longer. t3 is more preferably 50.0 seconds or more. t3 is more preferably 100.0 seconds or more. On the other hand, if the holding time t3 at the tempering temperature T3 exceeds 1000.0 seconds, the tempering of martensite progresses, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the holding time t3 at the tempering temperature T3 is set to 1000.0 seconds or less. The holding time t3 at the tempering temperature T3 is preferably 800.0 seconds or less. t3 is more preferably 600.0 seconds or less. t3 is more preferably 500.0 seconds or less.

Cooling rate θ1 from tempering temperature T3 to 80° C.: 100° C./sec or less This is an extremely important constituent feature of the present invention. When the cooling rate θ1 from the tempering temperature T3 to 80° C. exceeds 100° C./sec, the diffusion distance of carbon is short, so the hardness of the steel plate surface and the steel plate interior becomes small, and Hv(Q)−Hv(S) becomes It becomes less than 8, and the proper clearance range for YR and delayed fracture is lowered. Therefore, the cooling rate θ1 from the tempering temperature T3 to 80°C is set to 100°C/sec or less. The cooling rate θ1 from the tempering temperature T3 to 80°C is preferably 50°C/sec or less. Although the lower limit of the cooling rate θ1 from the tempering temperature T3 to 80° C. is not particularly limited, it is preferably set to 10° C./second or more due to production technology restrictions.

Cooling below 80° C. does not have to be specified, and may be cooled to a desired temperature by any method. It should be noted that the desired temperature is desirably about room temperature.

Further, the above high-strength steel sheet may be worked again under the condition that the equivalent plastic strain amount is 0.10% or more and 5.00% or less. Further, the processing to achieve the target equivalent plastic strain amount may be performed at once, or may be performed in several steps.

When high-strength steel sheets are traded, they are usually traded after being cooled to room temperature.

The high-strength steel sheet may be plated during or after annealing. The term "during annealing" means from the end of holding t1 at the annealing temperature T1 to the completion of cooling to room temperature after the end of holding t3 at the tempering temperature T3. After annealing means after cooling to room temperature is completed.
Examples of the plating treatment during annealing include hot-dip galvanizing treatment during cooling to 100° C. or less after holding at the annealing temperature T1, and treatment in which alloying is performed after hot-dip galvanizing. In addition, as the plating treatment after annealing, for example, Zn—Ni electro-alloy plating treatment after cooling to room temperature after finishing t3 holding at tempering temperature T3, or pure Zn electroplating treatment after cooling to room temperature is exemplified. can. A plating layer may be formed by electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. In addition, in the above-described plating treatment, the case of zinc plating was mainly described, but the type of plating metal such as Zn plating and Al plating is not particularly limited. Other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, a series of treatments such as the above-mentioned annealing, hot-dip galvanizing, galvanizing treatment, etc. Line). After hot-dip galvanization, wiping is possible in order to adjust the basis weight of the plating. In addition, the conditions of plating etc. other than the above-mentioned conditions can be based on the usual method of hot-dip galvanization.

After the plating treatment during or after annealing, processing may be performed again under the condition that the equivalent plastic strain amount is 0.10% or more and 5.00 or less. Further, the processing to achieve the target equivalent plastic strain amount may be performed at once, or may be performed in several steps.

Steel having the chemical compositions shown in Tables 1-1 and 1-2, with the balance being Fe and unavoidable impurities, was melted in a converter and made into slabs by continuous casting. Next, the obtained slab was heated, subjected to pickling treatment after hot rolling, cold rolling, and annealing treatment, working and processing shown in Tables 2-1, 2-2 and 2-3. A tempering treatment was applied to obtain a high-strength cold-rolled steel sheet having a thickness of 0.6 to 2.2 mm. In addition, some steel sheets are manufactured by applying a plating treatment after annealing.

Example no. Tests 77, 82, 85, 88, and 91 were discontinued due to slab fracture during the casting process.

Using the high-strength cold-rolled steel sheets obtained as described above as test steels, tensile properties and delayed fracture resistance properties were evaluated according to the following test methods.

(Organization observation)
The sum of the tempered martensite area fraction, the retained austenite volume fraction, the ferrite area fraction and the bainitic ferrite area fraction was obtained according to the method described above.

(KAM value)
The KAM value of the surface layer of the steel sheet and the KAM value of the center of the steel sheet were obtained according to the method described above.

(Hardness test)
According to the method described above, the hardness of the 1/4 part of the plate thickness and the hardness of the surface layer of the steel plate were obtained.

(Tensile test)
For the tensile test, a JIS No. 5 test piece (gauge length: 50 mm, width of parallel part: 25 mm) was sampled so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and the test was performed according to JIS Z 2241. A tensile test was performed at a crosshead speed of 1.67×10 ⁻¹ mm/sec to measure YS and TS. In the present invention, a TS of 1320 MPa or more was judged to be acceptable. A yield ratio (YR) of 85% or more was judged to be acceptable. Note that YR is obtained by the following equation (3).
YR=100×YS/TS (3)

(Appropriate clearance range for delayed fracture)
The appropriate clearance range for delayed fracture was obtained by the following method. A test piece of 16 mm×75 mm was prepared by shearing with the longitudinal direction perpendicular to the rolling direction. The rake angle during shearing was uniform at 0°, and the shearing clearance was changed to 5, 10, 15, 20, 25, 30, and 35%. Four-point bending was performed according to ASTM (G39-99), and a stress of 1000 MPa was applied to the bending vertex. The stress-applied test piece was immersed in hydrochloric acid of pH 3 at 25° C. for 100 hours. A shear clearance range of less than 10% where cracking does not occur is evaluated as “×”, a range of 10% or more and less than 15% is evaluated as “○”, and a shear clearance range where cracking does not occur is 15% or more is evaluated as “◎”. A specimen having a shear clearance range of 10% or more in which cracking does not occur was judged to have an excellent appropriate clearance range for delayed fracture.

As shown in Tables 3-1, 3-2 and 3-3, the examples of the present invention have a TS of 1320 MPa or more, a YR of 85% or more, and an appropriate clearance range for delayed fracture. On the other hand, the comparative examples are inferior in one or more of TS, YR, and appropriate clearance range for delayed fracture.

Claims (7)

in % by mass,
C: 0.15% or more and 0.45% or less,
Si: 0.10% or more and 2.00% or less,
Mn: 0.5% or more and 3.5% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 1.000% or less,
N: 0.0100% or less,
H: 0.0020% or less,
a component composition with the balance being Fe and unavoidable impurities;
Tempered martensite has an area fraction of 85% or more,
Retained austenite is less than 5% by volume,
The total area fraction of ferrite and bainitic ferrite is 10% or less,
A high-strength steel sheet having a structure that satisfies the following formulas (1) and (2).
KAM(S)/KAM(C) < 1.00 (1)
Here, KAM(S) is the KAM (Kernel Average Misorientation) value of the steel plate surface layer located 100 μm from the steel plate surface toward the center of the thickness , and KAM(C) is the KAM value of the center of the steel plate.
Hv(Q) - Hv(S) ≥ 8 (2)
Here, Hv(Q) indicates the hardness of the 1/4 part of the plate thickness, and Hv(S) indicates the hardness of the steel plate surface portion located 100 μm from the steel plate surface toward the plate thickness central portion . As a component composition, further, in mass%,
Ti: 0.100% or less,
B: 0.0100% or less,
Nb: 0.100% or less,
Cu: 1.00% or less,
Cr: 1.00% or less,
V: 0.100% or less,
Mo: 0.500% or less,
Ni: 0.50% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
As: 0.100% or less,
Ta: 0.100% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less,
Zn: 0.020% or less,
Co: 0.020% or less,
Zr: 0.020% or less,
REM: The high-strength steel sheet according to claim 1, containing one or more elements selected from 0.0200% or less. The high-strength steel sheet according to claim 1 or 2, which has a plating layer on the surface of the steel sheet. A method for manufacturing a high-strength steel sheet according to claim 1 or 2,
Cold-rolled steel sheets produced by subjecting steel slabs to hot rolling, pickling and cold rolling,
The temperature T1 is 850° C. or higher and 1000° C. or lower,
After annealing under the condition that the holding time t1 at T1 is 10 seconds or more and 1000 seconds or less,
Cool to 100° C. or less,
Starting processing within an elapsed time t2 of 1000 seconds or less after reaching 100° C.,
The processing has a starting temperature T2 of 80° C. or less,
After processing under the condition that the equivalent plastic strain is 0.10% or more and 5.00% or less,
The temperature T3 is 100° C. or higher and 400° C. or lower,
Tempering under the condition that the holding time t3 at T3 is 1.0 seconds or more and 1000.0 seconds or less,
A method for producing a high-strength steel sheet, wherein the cooling rate θ1 from T3 to 80°C is 100°C/sec or less. 4. In the working process before tempering, strain is imparted by working in two or more times, and working is performed under the condition that the total of the equivalent plastic strain of each working is 0.10% or more. A method for producing a high-strength steel sheet according to 1. The method for producing a high-strength steel sheet according to claim 4, wherein plating is applied during or after annealing. The method for producing a high-strength steel sheet according to claim 5, wherein plating is applied during or after annealing.