KR20240005883A - High-strength steel plate and manufacturing method thereof - Google Patents

Abstract

The purpose is to provide a high-strength steel sheet with 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 manufacturing the same. A high-strength steel sheet that contains specific components, has a specific structure, and has a structure that satisfies the formulas specified in (1) and (2) below.
KAM(S)/KAM(C)＜1.00 ·····(1)
Here, KAM(S) represents the KAM (Kernel Average Misorientation) value of the surface layer of the steel plate, and KAM(C) represents the KAM value of the center of the steel plate.
Hv(Q)－Hv(S)≥8·····(2)
Here, Hv(Q) represents the hardness of 1/4 of the plate thickness, and Hv(S) represents the hardness of the surface layer of the steel plate.

Description

High-strength steel plate and manufacturing method thereof

The present invention relates to a high-strength steel plate having excellent tensile strength and delayed fracture resistance and a method of manufacturing the same. The high-strength steel sheet of the present invention can be suitably used as a structural member for automobile parts and the like.

For the purpose of both reducing _CO2 emissions by reducing the weight of vehicles and improving crash resistance by reducing the weight of the car body, the strengthening of automotive steel sheets is progressing, and new laws and regulations are being introduced one after another. Therefore, for the purpose of increasing car body strength, the number of applications of high-strength steel sheets with a tensile strength (TS) of 1320 MPa or higher is increasing in major structural parts forming automobiles.

High-strength steel sheets used in automobiles are required to have excellent yield ratios (YR = yield strength YS/tensile strength TS) from the viewpoint of component performance. For example, in skeletal parts such as automobile bumpers, excellent shock absorption in the event of a collision is required, so it is appropriate to use a steel plate with excellent YR, which is related to shock absorption.

In addition, there are many end faces formed by shear processing in the skeletal parts of automobiles. The shape of the sheared end face depends on the shear clearance. The shape of the shear cross section affects the delayed fracture resistance. Here, delayed fracture means that when a molded part is placed in a hydrogen intrusion environment, hydrogen penetrates into the steel sheet constituting the part, lowering the bonding force between atoms and causing local deformation, resulting in microcracks. This is a phenomenon in which microcracks advance and lead to destruction. High-strength steel sheets used in automobiles are required to have a wide appropriate clearance range for delayed fracture.

In response to these requirements, for example, Patent Document 1 provides a high-strength steel plate with a tensile strength of 980 MPa or more and excellent bending workability, and a method for manufacturing the same. However, the technology described in Patent Document 1 does not consider the appropriate clearance range for YR and delayed fracture. In addition, none of the steel plates described in Patent Document 1 achieve YR≥85% or more.

For example, Patent Document 2 provides a high-strength steel plate with a tensile strength of 1320 MPa or more and excellent delayed fracture resistance in the shear cross section, and a method for manufacturing the same. However, the technology described in Patent Document 2 does not consider the appropriate clearance range for delayed destruction.

For example, Patent Document 3 provides a high-strength steel plate that has a tensile strength of 1,100 MPa or more and is excellent in YR, surface properties, and weldability, and a method for manufacturing the same. However, the technology described in Patent Document 3 does not consider the appropriate clearance range for delayed destruction.

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

The present invention was developed in consideration of these circumstances, and its purpose is to provide a high-strength steel sheet with 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 manufacturing the same.

In order to achieve the above-mentioned problem, the present inventors conducted intensive studies and discovered the following.

(1) By setting the tempered martensite to 85% or more, a TS of 1320 MPa or more can be achieved.

(2) By setting the retained austenite to less than 5%, KAM(S)/KAM(C) to less than 1.00, and Hv(Q)-Hv(S) to 8 or more, a YR of 85% or more is obtained. It can be realized.

(3) By setting KAM(S)/KAM(C) to less than 1.00 and Hv(Q)-Hv(S) to 8 or more, an appropriate clearance range for excellent delayed destruction can be realized.

The present invention has been made based on the above recognition. That is, the main structure of the present invention is as follows.

[1] In mass%,

C: 0.15% or more, 0.45% or less,

Si: 0.10% or more, 2.00% or less,

Mn: 0.5% or more, 3.5% or less,

P: 0.100% or less,

S: 0.0200% or less,

Al: 0.010% or more, 1.000% or less,

N: 0.0100% or less,

H: Contains 0.0020% or less,

A component composition with the balance consisting of Fe and inevitable impurities,

Tempered martensite is more than 85% by area fraction,

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 plate with a structure that satisfies the formulas specified in (1) and (2) below.

KAM(S)/KAM(C)＜1.00 ·····(1)

Here, KAM(S) represents the KAM (Kernel Average Misorientation) value of the surface layer of the steel plate, and KAM(C) represents the KAM value of the center of the steel plate.

Hv(Q)－Hv(S)≥8·····(2)

Here, Hv(Q) represents the hardness of 1/4 of the plate thickness, and Hv(S) represents the hardness of the surface layer of the steel plate.

[2] As the component composition, additionally 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 two 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 manufacturing a high-strength steel plate according to [1] or [2] above,

Cold rolled steel sheet produced by hot rolling, pickling, and cold rolling on a steel slab,

The temperature T1 is 850℃ or more and 1000℃ or less,

After annealing under the condition that the preservation time t1 at T1 is 10 seconds or more and 1000 seconds or less,

Cool to below 100℃,

Processing is started when the elapsed time t2 is 1000 seconds or less from the time the temperature reaches 100°C,

In the above processing, the starting temperature T2 is 80°C or lower,

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℃ or more and 400℃ or less,

Tempering under the condition that the preservation time t3 at T3 is 1.0 seconds or more and 1000.0 seconds or less,

A method of manufacturing a high-strength steel sheet in which cooling is performed under the condition that the cooling rate θ1 from T3 to 80°C is 100°C/sec or less.

[5] The high-strength steel sheet described in [4], in which strain is applied by processing in two or more parts in the processing process before tempering, and processing is performed under the condition that the sum of the equivalent plastic strain for each processing is 0.10% or more. Manufacturing method.

[6] The method for producing a high-strength steel sheet according to [4] or [5], in which plating is performed during or after annealing.

According to the present invention, it is possible to obtain a high-strength steel sheet with a TS of 1320 MPa or more, a YR of 85% or more, and an excellent appropriate clearance range for delayed fracture. In addition, by applying the high-strength steel sheet of the present invention to, for example, automobile structural members, fuel efficiency can be improved by reducing the weight of the automobile body. Therefore, the industrial use value is very large.

(Form for carrying out the invention)

Hereinafter, embodiments of the present invention will be described.

First, the appropriate range of the component composition of a high-strength steel sheet and the reason for its limitation will be explained. In addition, in the following description, “%” indicating the content of the component elements of steel means “mass%” unless otherwise specified.

C: 0.15% or more, 0.45% or less

C is one of the important basic components of steel, and especially 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 realize a TS of 1320 MPa or more. Therefore, the C content is set to 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, when the C content exceeds 0.45%, the ultimate deformation capacity of the steel decreases, and the appropriate clearance range for delayed fracture decreases. Therefore, the C content is set to 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, 2.00% or less

Si is one of the important basic components of steel, and especially 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 realize a TS of 1320 MPa or more. Therefore, the Si content is set to 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 realize YR of 85% or more. Therefore, the Si content is set to 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, 3.5% or less

Mn is one of the important basic components of steel, and is an important element that affects the ferrite fraction and bainite fraction, especially in the present invention. If the Mn content is less than 0.5%, the ferrite fraction and bainite fraction increase, making it difficult to realize TS of 1320 MPa or more, and also difficult to realize YR of 85% or more. Therefore, the Mn content is set to 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, if the Mn content exceeds 3.5%, macro-segregation of Mn occurs, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for delayed fracture decreases. Therefore, the Mn content is set to 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

If the P content exceeds 0.100%, P segregates at grain boundaries and brittle the steel sheet, thereby reducing the appropriate clearance range for delayed fracture. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less. The P content is more preferably 0.060% or less. The lower limit of the P content is not particularly limited, but is preferably 0.001% or more due to constraints in production technology.

S: 0.0200% or less

If the S content exceeds 0.0200%, it exists as sulfide and reduces the ultimate deformation capacity of the steel, so the appropriate clearance range for delayed fracture decreases. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less. The S content is more preferably 0.0050% or less. The lower limit of the S content is not particularly limited, but is preferably 0.0001% or more due to constraints in production technology.

Al: 0.010% or more, 1.000% or less

By containing Al, the strength of the steel sheet increases, making it easier to realize a TS of 1320 MPa or more. In order to obtain this effect, the Al content needs to be 0.010% or more. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.012% or more. The 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 realize TS of 1320 MPa or more, and furthermore, it becomes difficult to realize YR of 85% or more. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.500% or less. The Al content is more preferably 0.100% or less.

N: 0.0100% or less

If the N content exceeds 0.0100%, the cast slab becomes embrittled and prone to cracking, which significantly reduces productivity. Therefore, the N content is set to 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. Additionally, the lower limit of the N content is not particularly limited, but is preferably 0.0010% or more due to constraints in production technology.

H: 0.0020% or less

If the H content exceeds 0.0020% or less, the ultimate deformation capacity of the steel decreases, and the appropriate clearance range for delayed fracture decreases. Therefore, the H content is set to 0.0020% or less. The H content is preferably 0.0015% or less. The H content is more preferably 0.0010% or less. Additionally, the lower limit of the H content is not particularly limited, but as the H content decreases, the appropriate clearance range for delayed fracture also improves, so it may be 0%.

In addition to the above component 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: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less Hereinafter, it is preferable that one or two or more types of elements selected from REM: 0.0200% or less are contained.

Ti: 0.100% or less

If the Ti content exceeds 0.100%, the cast slab becomes embrittled and prone to cracking, which significantly reduces productivity. Therefore, when adding Ti, its content is set to 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, by containing Ti, the strength of the steel sheet increases, making it easier to realize 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

If the B content exceeds 0.0100%, the cast slab becomes embrittled and prone to cracking, which significantly reduces productivity. Therefore, when adding B, its content is set to 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, by containing B, the strength of the steel sheet increases, making it easier to realize 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

If the Nb content exceeds 0.100%, the rough cast slab becomes embrittled and prone to cracking, which significantly reduces productivity. Therefore, when adding Nb, its content is set to 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, by containing Nb, the strength of the steel sheet increases, making it easier to realize a TS of 1320 MPa or more. In order to obtain this effect, it is preferable that the Nb content is 0.001% or more. The Nb content is more preferably 0.002% or more.

Cu: 1.00% or less

If the Cu content exceeds 1.00%, the cast slab becomes embrittled and prone to cracking, which significantly reduces productivity. Therefore, when adding Cu, the Cu content is set to 1.00% or less. Cu content is preferably 0.50% or less. On the other hand, by containing Cu, hydrogen penetration into the steel sheet is suppressed and the appropriate clearance range for delayed fracture is improved. In order to obtain this effect, the Cu content is preferably 0.01% or more. Cu content is preferably 0.03% or more. The Cu content is more preferably 0.10% or more.

Cr: 1.00% or less

If the Cr content exceeds 1.00%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Cr, its content is set to 1.00% or less. 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 serves as a solid solution strengthening element, but also stabilizes austenite during the cooling process during continuous annealing, and can suppress the formation of ferrite, thereby increasing the strength of the steel sheet. In order to obtain this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more.

V: 0.100% or less

If the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expanding deformation decreases. Therefore, when V is added, its content is set to 0.100% or less. Preferably it is 0.060% or less. On the other hand, V increases the strength of the steel plate. In order to obtain this effect, 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

If the Mo content exceeds 0.500%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when Mo is added, its content is set to 0.500% or less. Mo content is preferably 0.450% or less. Mo content is more preferably 0.400% or less. On the other hand, Mo not only serves as a solid solution strengthening element, but also increases the strength of the steel sheet by stabilizing austenite and suppressing the formation of ferrite during the cooling process during continuous annealing. In order to obtain this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.020% or more.

Ni: 0.50% or less

If the Ni content exceeds 0.50%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Ni, its content is set to 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 during the cooling process during continuous annealing, thereby increasing the strength of the steel sheet. In order to obtain this effect, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.02% or more.

Sb: 0.200% or less

If the Sb content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Sb, its content is set to 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 this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more.

Sn: 0.200% or less

If the Sn content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Sn, its content is set to 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 this effect, 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 As content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding As, its content is set to 0.100% or less. The As content is preferably 0.060% or less. The As content is more preferably 0.010% or less. As increases the strength of steel sheets. In order to obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more.

Ta: 0.100% or less

If the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Ta, its content is set to 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 this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more.

Ca: 0.0200% or less

If the Ca content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Ca, its content is set to 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 improving the ultimate deformation capacity of a steel sheet by making the shape of sulfide spherical and improving the appropriate clearance range for delayed fracture. In order to obtain this effect, the Ca content is preferably 0.0001% or more.

Mg: 0.0200% or less

If the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, thereby reducing the appropriate clearance range for hole expansion deformation. Therefore, when adding Mg, its content is set to 0.0200% or less. On the other hand, Mg is an element used for deoxidation and is an element effective in improving the ultimate deformation capacity of a steel sheet by making the shape of sulfide spherical and improving the appropriate clearance range for delayed fracture. In order to obtain this effect, the Mg content is preferably 0.0001% or more.

Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less

If the contents of Zn, Co, and Zr each exceed 0.020%, large amounts of coarse precipitates and inclusions are generated, which lowers the ultimate deformation capacity of the steel, and thus the appropriate clearance range for hole expansion deformation decreases. Therefore, when adding Zn, Co, and Zr, their contents are each set to 0.020% or less. On the other hand, Zn, Co, and Zr are all elements that are effective in sphericalizing the shape of inclusions, improving the ultimate deformation capacity of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain this effect, it is preferable that the contents of Zn, Co, and Zr are each 0.0001% or more.

REM: 0.0200% or less

If the REM content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformation capacity of the steel, thereby reducing the appropriate clearance range for hole expansion deformation. Therefore, when REM is added, its content is set to 0.0200% or less. On the other hand, REM is an element effective in improving the ultimate deformation capacity of a steel sheet by sphericalizing the shape of inclusions and improving the appropriate clearance range for delayed fracture. In order to obtain this effect, the REM content is preferably 0.0001% or more.

The remainder other than the above components is Fe and inevitable impurities. In addition, since the effect of the present invention is not impaired when the content of the above-mentioned optional elements is less than the lower limit, when these optional elements are contained less than the lower limit, these optional elements are included as inevitable impurities.

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

Tempered martensite: 85% or more by area fraction

In the present invention, this is a very important invention configuration requirement. By using martensite as the main phase, it becomes possible to realize a TS of 1320 MPa or more. In order to obtain this effect, it is necessary to set the tempered martensite to 85% or more in terms of area fraction. Therefore, the tempered martensite is set to be 85% or more in terms of area fraction. 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. Meanwhile, the upper limit is not particularly limited, but tempered martensite may be 100% in terms of area fraction.

Here, the measuring method of tempered martensite is as follows. After polishing the L cross section of the steel plate, etching it with 3 vol.% nital, 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) was scanned using SEM. Observe at 10 o'clock with a magnification of 2000x. In addition, in the above structure image, tempered martensite is a structure with fine irregularities on the inside of the structure and also has carbide inside. From the average of those values, tempered martensite can be obtained.

Retained austenite: less than 5% by volume

In the present invention, this is a very important invention configuration requirement. When the retained austenite is 5% or more by volume, it becomes difficult to realize 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 processing-induced transformation of the retained austenite. Therefore, retained austenite is set to less than 5%. Preferably it is 4% or less. The lower limit of retained austenite is not particularly limited, but the lower the retained austenite, the more preferable it is, and may be 0%.

Here, the method for measuring retained austenite is as follows. Retained austenite was determined by polishing the steel sheet from 1/4 of the sheet thickness to a 0.1 mm surface, then chemically polishing the surface to an additional 0.1 mm, using CoKα rays with an The integrated intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes and the {200}, {211}, and {220} planes of bcc iron were measured, and the obtained nine integrated intensity ratios were averaged. So I saved it.

Total of ferrite and bainitic ferrite: 10% or less as an area fraction

In the present invention, this is a very important invention configuration requirement. If the total of ferrite and bainitic ferrite exceeds 10%, it becomes difficult to realize TS of 1320 MPa or more, and it also becomes difficult to realize YR of 85% or more. The reason for the decrease in YR is that ferrite and bainitic ferrite have a soft structure, so early yielding occurs. Therefore, the total of ferrite and bainitic ferrite is 10% or less. Preferably it is 8% or less. More preferably, it is 5% or less. Additionally, the lower limit of the total of ferrite and bainitic ferrite is not particularly limited, but it is preferable that these are less, and the lower limit of the total of ferrite and bainitic ferrite may be 0%.

Here, the method of measuring the sum of ferrite and bainitic ferrite is as follows. After polishing the L cross section of the steel plate, etching it with 3 vol.% Nital, 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) was examined at 2000 times magnification using an SEM. Observe at 10 o'clock with magnification. In addition, in the above structure image, ferrite and bainitic ferrite have a flat structure inside the concave portion. From the average of those values, the sum of ferrite and bainitic ferrite can be obtained.

As structures other than the above overall structure, pearlite, fresh martensite, needle-shaped ferrite, etc. can be considered. 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 plate, and KAM(C) is the KAM value of the center of the steel plate.

In the present invention, this is a very important invention configuration requirement. The steel sheet surface layer is a position moved 100 μm from the surface of the steel sheet toward the center of the sheet thickness. The center of the steel plate is the location of 1/2 the plate thickness. As a result of the inventor's investigation, it was confirmed that it is effective to improve the appropriate clearance range for YR and delayed fracture by changing the dislocation distribution state from the surface layer to the interior and making KAM(S)/KAM(C) less than 1.00. Therefore, KAM(S)/KAM(C) is set to less than 1.00. Additionally, the lower limit of KAM(S)/KAM(C) is not particularly limited, but is preferably set to 0.80 or more due to constraints in production technology.

Here, the method of measuring the KAM value is as follows. First, a test piece for structure observation was collected from a cold rolled steel sheet. Next, the collected test piece was polished by colloidal silica vibration polishing so that the cross section in the rolling direction (L cross section) became the observation surface. The observation surface was a mirror surface. Next, electron backscattering diffraction (EBSD) measurements were performed to obtain local crystal 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 Analysis7 was used to analyze the obtained local orientation data. The analysis was performed for each of 10 views for the target plate thickness, and the average value was used.

Prior to data analysis, the grain dilation function (Grain Tolerance Angle: 5, Minimum Grain Size: 2, Single Iteration: ON) and the Grain CI Standarization function (Grain Tolerance Angle: 5, Minimum Grain Size: 5) of the analysis software are used. Clean-up processing was performed one time in order. After that, only measurement points with a CI value >0.1 were used for 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 conducted 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 of the plate thickness, and Hv(S) is the hardness of the surface layer of the steel plate.

In the present invention, this is a very important invention configuration requirement. The steel sheet surface layer is a position moved 100 μm from the surface of the steel sheet toward the center of the sheet thickness. As a result of the inventor's investigation, it was confirmed that changing the hardness from the surface layer to the inside and setting Hv(Q)-Hv(S) to 8 or more is effective in improving the appropriate clearance range for YR and delayed fracture. Therefore, Hv(Q)-Hv(S) is set to 8 or more. The upper limit of Hv(Q)-Hv(S) is not particularly limited, but is preferably set to 30 or less due to constraints in production technology. Additionally, the preferable ranges of Hv(Q) and Hv(S) are 400 to 600 and 400 to 600, respectively.

Here, the method for measuring hardness is as follows. First, a test piece for structure observation was collected from a cold rolled steel sheet. Next, the collected test piece was polished so that the cross section in the rolling direction (L cross section) became the observation surface. The observation surface was a mirror surface. Next, 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 for the target plate thickness, and the average value of 8 points excluding the maximum and minimum hardness was used.

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

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

In the present invention, the slab heating temperature, slab crack retention time, and coiling temperature in hot rolling are not particularly limited. Methods for hot rolling a steel slab include a method of heating the slab and then rolling it, a method of directly rolling the slab after continuous casting without heating it, and a method of subjecting the slab after continuous casting to a short-time heat treatment and rolling it. . The slab heating temperature, slab crack retention 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 lower. The slab crack preservation time is preferably 30 minutes or more. The slab crack preservation time is preferably 250 minutes or less. The finish rolling temperature is preferably equal to or higher than the Ar ₃ transformation point. Additionally, the coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower.

The hot rolled steel sheet manufactured in this way is subjected to acid cleaning. Since acid cleaning can remove oxides from the surface of a steel sheet, it is important for ensuring good chemical conversion processability and plating quality in the high-strength steel sheet of the final product. In addition, acid washing may be performed once or may be divided into multiple times. In addition, after hot rolling, cold rolling may be performed as is on the pickled plate, or cold rolling may be performed after heat treatment.

The reduction ratio in cold rolling and the sheet thickness after rolling are not particularly limited, but the reduction ratio in cold rolling is preferably 30% or more. It is preferable that the reduction ratio in cold rolling is 80% or less. Additionally, the effect of the present invention can be obtained without any particular limitation on the number of rolling passes and the reduction ratio of each pass.

Annealing is performed on the cold rolled steel sheet obtained as described above. Annealing conditions are as follows.

Annealing temperature T1: 850℃ or higher and 1000℃ or lower

In the present invention, this is a very important invention configuration requirement. When the annealing temperature T1 is less than 850°C, the total of ferrite and bainitic ferrite exceeds 10% in terms of area fraction, making it difficult to realize TS of 1320 MPa or more, and also difficult to realize YR of 85% or more. 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, when 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 lower.

Preservation holding time t1 at annealing temperature T1: 10 seconds or more and 1000 seconds or less

In the present invention, this is a very important invention configuration requirement. When the storage time t1 at the annealing temperature T1 is less than 10 seconds, austenitization becomes insufficient, the total of ferrite and bainitic ferrite exceeds 10% in area fraction, and it becomes difficult to realize a TS of 1320 MPa or more, and , it becomes difficult to realize YR of 85% or more. Therefore, the storage time t1 at the annealing temperature T1 is set to 10 seconds or more. The preservation time t1 at the annealing temperature T1 is preferably 30 seconds or more. t1 is more preferably 45 seconds or more. t1 is more preferably 60 seconds or more. t1 is most preferably 100 seconds or more. On the other hand, when the storage time at the annealing temperature T1 is more than 1000 seconds, the prior austenite grain size increases excessively, and the appropriate clearance range for delayed fracture decreases. Therefore, the storage time t1 at the annealing temperature T1 is set to 1000 seconds or less. The preservation time t1 at the annealing temperature T1 is preferably 800 seconds or less. t1 is more preferably 500 seconds or less.

After annealing, cool to below 100℃

In the cooling process to 100°C or lower, austenite is transformed into martensite. In order to obtain martensite of 85% or more, it is necessary to cool to 100°C or lower after annealing. Therefore, after annealing, it is cooled to 100°C or lower. The lower limit of the cooling completion temperature is not particularly limited, but is preferably 0°C or higher due to constraints in production technology.

Elapsed time t2 from the point at which the temperature reached 100℃ to the start of processing: 1000 seconds or less

In the present invention, this is a very important invention configuration requirement. If the elapsed time t2 from the point at which the temperature reaches 100°C to the start of processing exceeds 1000 seconds, the aging of the martensite structure progresses and the amount of strain introduced into the surface layer of the steel sheet and the center of the steel sheet due to processing changes, so KAM(S) /KAM(C) becomes 1.00 or more, and the appropriate clearance range for YR and delayed destruction decreases. Therefore, the elapsed time t2 from the point at which the temperature reaches 100° C. to the start of processing is set to 1000 seconds or less. The elapsed time t2 from the point at which the temperature reaches 100°C to the start of processing is preferably 900 seconds or less. t2 is more preferably 800 seconds or less. In addition, the lower limit of the elapsed time t2 from the point at which the temperature reaches 100°C to the start of processing is not particularly limited, but is preferably 5 seconds or more due to constraints in production technology. Additionally, as a result of the inventor's investigation, it became clear that the elapsed time from the point at which the temperature reached 100°C to the end of processing did not affect the amount of strain introduced into the surface layer of the steel sheet and the center of the steel sheet due to processing.

Processing start temperature T2 is 80℃ or lower

In the present invention, this is a very important invention configuration requirement. When the processing start temperature T2 exceeds 80°C, since the steel sheet is soft, the amount of strain introduced into the surface layer of the steel sheet and the center of the steel sheet due to processing changes, and KAM(S)/KAM(C) becomes 1.00 or more, and YR and The appropriate clearance range for delayed destruction decreases. Therefore, the processing start temperature T2 is set to 80°C or lower. The processing start temperature T2 is preferably 60°C or lower. T2 is more preferably 50°C or lower. Additionally, the lower limit of the processing start temperature T2 is not particularly limited, but is preferably 0°C or higher due to constraints in production technology.

Equivalent plastic strain: 0.10% or more and 5.00% or less

In the present invention, this is a very important invention configuration requirement. When the equivalent plastic strain is less than 0.10%, the processing amount is insufficient, KAM(S)/KAM(C) becomes 1.00 or more, and the appropriate clearance range for YR and delayed fracture decreases. Therefore, the equivalent plastic strain is set to 0.10% or more. The plastic equivalent strain is preferably 0.15% or more. The equivalent plastic strain is more preferably 0.20% or more. When the equivalent plastic strain exceeds 5.00%, the influence of processing becomes the same on the surface layer of the steel sheet and the center of the steel sheet, KAM(S)/KAM(C) becomes 1.00 or more, and the appropriate clearance range for YR and delayed fracture decreases. Additionally, the upper limit of equivalent plastic strain is set to 5.00% or less due to constraints in production technology. Therefore, the equivalent plastic strain is set to 5.00% or less. Equivalent plastic strain is preferably 4.00% or less. Equivalent plastic strain is more preferably 2.00% or less. Equivalent plastic strain is more preferably 1.00% or less.

In the processing process before the tempering, it is preferable that the strain is applied by processing in two or more processes, and that the total equivalent plastic strain of each processing 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) becomes less than 1.00, and YR and the appropriate clearance range for delayed destruction is improved. Therefore, the strain may be applied by machining in two or more stages in the machining process before tempering, as long as the total equivalent plastic strain of each machining is 0.10% or more. Additionally, the time from the moment the temperature reaches 100°C to the start of processing after the second time is not particularly limited. This is because the ease of movement of dislocations in martensite decreases due to the first processing.

Here, representative processing methods for the above processing include temper rolling and tension leveling. The equivalent plastic strain in temper rolling is the elongation of the steel sheet, and can be obtained from the change in length of the steel sheet before and after processing. The method for calculating the equivalent plastic strain of the steel plate during leveler processing was calculated using the method in Reference 1 below. In the calculation, the following data input values were used, the work hardening behavior of the material was assumed to be a linear hardening elastoplastic, Bausinger hardening was ignored, and tension reduction due to bend loss was ignored. Additionally, Misaka's formula was used as the processing curvature equation.

· Number of plate thickness divisions: 31

·Young’s modulus: 21000kgf/㎟

·Poisson’s ratio: 0.3

·Yield stress: 111kgf/㎟

·Plastic coefficient: 1757kgf/㎟

[Reference 1] Yoshisuke Misaka, Takeshi Masui: Plasticity and Processing, 17 (1976), 988.

In addition, the above processing may be performed by any general method of imparting deformation other than the above, and may also be performed using, for example, a continuous stretcher leveler or roller leveler.

Tempering temperature T3: 100℃ or higher and 400℃ or lower

In the present invention, this is a very important invention configuration requirement. When the tempering temperature T3 is less than 100℃, the diffusion distance of carbon is short, so the hardness on the surface and inside the steel sheet decreases, and Hv(Q)-Hv(S) becomes less than 8, which is optimal for YR and delayed fracture. The clearance range decreases. Therefore, the tempering temperature T3 is set to 100°C or higher. The 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, when the tempering temperature T3 exceeds 400°C, the tempering of martensite progresses, making it difficult to realize a TS of 1320 MPa or more. Therefore, the tempering temperature T3 is set to 400°C or lower. The tempering temperature T3 is preferably 350°C or lower. T3 is more preferably 300°C or lower. T3 is more preferably 280°C or lower.

Preservation holding time t3 at tempering temperature T3: 1.0 seconds or more and 1000.0 seconds or less

In the present invention, this is a very important invention configuration requirement. When the storage time t3 at the tempering temperature T3 is less than 1.0 seconds, the diffusion distance of carbon is short, so the hardness on the surface of the steel sheet and inside the steel sheet decreases, and Hv(Q)-Hv(S) becomes less than 8, YR and the appropriate clearance range for delayed destruction decreases. Therefore, the storage time t3 at the tempering temperature T3 is set to 1.0 seconds or more. The storage time t3 at the tempering temperature T3 is preferably 5.0 seconds or more. t3 is more preferably 50.0 seconds or more. t3 is more preferably 100.0 seconds or more. On the other hand, when the storage time t3 at the tempering temperature T3 is more than 1000.0 seconds, the tempering of martensite progresses, making it difficult to realize a TS of 1320 MPa or more. Therefore, the storage time t3 at the tempering temperature T3 is set to 1000.0 seconds or less. The storage 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℃: 100℃/sec or less

In the present invention, this is a very important invention configuration requirement. 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 on the surface and inside the steel plate decreases, and Hv(Q)-Hv(S) is 8. becomes less than that, the appropriate clearance range for YR and delayed destruction decreases. 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. Additionally, the lower limit of the cooling rate θ1 from the tempering temperature T3 to 80°C is not particularly limited, but is preferably set to 10°C/sec or more from constraints in production technology.

Cooling below 80°C does not need to be specifically defined, and cooling to a desired temperature may be performed by any method. Additionally, the desired temperature is preferably around room temperature.

Additionally, the above high-strength steel sheet may be processed again under conditions where the equivalent plastic deformation amount is 0.10% or more and 5.00% or less. In addition, the processing to obtain the desired equivalent plastic deformation amount may be performed at once, or may be performed several times.

Additionally, when a high-strength steel plate becomes an object of trade, it usually becomes an object of trade after it is cooled to room temperature.

Plating may be performed on the high-strength steel sheet during or after annealing. During annealing, it means from the end of storage t1 at the annealing temperature T1 to the time when cooling to room temperature is completed after the end of storage t3 at the tempering temperature T3. After annealing means after cooling to room temperature is completed.

Examples of the plating treatment during annealing include, for example, hot-dip galvanizing treatment while cooling to 100°C or lower after storage at an annealing temperature T1, and alloying treatment after hot-dip galvanizing. In addition, as a plating treatment after annealing, for example, Zn-Ni electroplating treatment or pure Zn electroplating treatment after cooling to room temperature by cooling to room temperature after completion of storage at tempering temperature T3. It can be exemplified. The plating layer may be formed by electroplating, or molten zinc-aluminum-magnesium alloy plating may be performed. In addition, although the above plating treatment was mainly explained in the case of zinc plating, the type of plating metal such as Zn plating and Al plating is not particularly limited. The conditions of other manufacturing methods are not particularly limited, but from the viewpoint of productivity, the series of processes such as annealing, hot-dip galvanizing, and zinc plating alloying treatment are performed in CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. It is advisable to do so. After hot dip galvanizing, wiping is possible to adjust the weight per unit area of the plating. In addition, conditions such as plating other than the above-mentioned conditions may depend on the commercial method of hot-dip galvanizing.

After plating treatment during or after annealing, processing may be performed again under the condition that the equivalent plastic deformation amount is 0.10% or more and 5.00 or less. In addition, the processing to obtain the desired equivalent plastic deformation amount may be performed at once, or may be performed several times.

Example

Steel with the component composition shown in Table 1-1 and Table 1-2, the balance being Fe and inevitable impurities, was melted in a converter and made into a slab by continuous casting. Next, the obtained slab is heated, and after hot rolling, acid cleaning treatment is performed, cold rolling is performed, and annealing treatment, processing and tempering treatment shown in Table 2-1, Table 2-2 and Table 2-3 are performed. , high-strength cold-rolled steel sheets with sheet thicknesses of 0.6 to 2.2 mm were obtained. In addition, some steel sheets are manufactured by performing plating treatment after annealing.

Example No. Tests 77, 82, 85, 88, and 91 were discontinued because the slabs fractured during the casting process.

The high-strength cold-rolled steel sheet obtained as described above was used as a test steel, and the tensile properties and delayed fracture resistance were evaluated according to the test method below.

(tissue observation)

According to the method described above, the sum of the tempered martensite area fraction, retained austenite volume fraction, ferrite area fraction, and bainitic ferrite area fraction was determined.

(KAM value)

According to the method described above, the KAM value of the surface layer of the steel sheet and the KAM value of the center of the steel sheet were determined.

(hardness test)

According to the above-described method, the hardness of 1/4 of the plate thickness and the hardness of the surface layer portion of the steel plate were determined.

(tensile test)

For the tensile test, a JIS No. 5 test piece (gauge length 50 mm, parallel portion width 25 mm) was taken so that the length of the test piece was in the direction perpendicular to the rolling direction, and tested according to JIS Z 2241. A tensile test was performed under the condition that the crosshead speed was 1.67 x 10 ^-1 mm/sec, and YS and TS were measured. In addition, in the present invention, 1320 MPa or more was judged as passing by TS. A yield ratio (YR) of 85% or higher was judged to be acceptable. Additionally, YR is obtained from the following equation (3).

YR=100×YS/TS····(3)

(Adequate clearance range for delayed destruction)

The appropriate clearance range for delayed destruction was obtained by the following method. A test piece was created by shearing it to 16 mm x 75 mm with the direction perpendicular to the rolling direction as the length. The rake angle during shearing was unified at 0°, and the shear 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 peak. The test piece under stress was immersed in hydrochloric acid at 25°C and pH 3 for 100 hours. A shear clearance range without cracks of less than 10% was evaluated as “×”, a range of 10% to less than 15% was evaluated as “○”, a shear clearance range without cracks of more than 15% was evaluated as “◎”, and those with no cracks were evaluated as “◎”. A shear clearance range of 10% or more was judged to have an excellent appropriate clearance range for delayed fracture.

As shown in Table 3-1, Table 3-2, and Table 3-3, in the present invention example, TS is 1320 MPa or more, YR is 85% or more, and the appropriate clearance range for delayed fracture is excellent. On the other hand, in the comparative example, TS, YR, or at least one of the appropriate clearance ranges for delayed destruction are inferior.

(Table 1-1)

(Table 1-2)

(Table 2-1)

(Table 2-2)

(Table 2-3)

(Table 3-1)

(Table 3-2)

(Table 3-3)

Claims (6)

In mass%,
C: 0.15% or more, 0.45% or less,
Si: 0.10% or more, 2.00% or less,
Mn: 0.5% or more, 3.5% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more, 1.000% or less,
N: 0.0100% or less,
H: Contains 0.0020% or less,
A component composition with the balance consisting of Fe and inevitable impurities,
Tempered martensite is more than 85% by area fraction,
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 plate with a structure that satisfies the formulas specified in (1) and (2) below.
KAM(S)/KAM(C)＜1.00 ·····(1)
Here, KAM(S) represents the KAM (Kernel Average Misorientation) value of the surface layer of the steel plate, and KAM(C) represents the KAM value of the center of the steel plate.
Hv(Q)－Hv(S)≥8·····(2)
Here, Hv(Q) represents the hardness of 1/4 of the plate thickness, and Hv(S) represents the hardness of the surface layer of the steel plate. According to paragraph 1,
As the 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: High-strength steel sheet containing one or two or more elements selected from 0.0200% or less. According to claim 1 or 2,
A high-strength steel plate with a plating layer on the steel plate surface. A method for manufacturing a high-strength steel sheet according to claim 1 or 2,
Cold-rolled steel sheets produced by hot rolling, pickling, and cold rolling on steel slabs,
The temperature T1 is 850℃ or more and 1000℃ or less,
After annealing under the condition that the preservation time t1 at T1 is 10 seconds or more and 1000 seconds or less,
Cool to below 100℃,
Processing is started when the elapsed time t2 is 1000 seconds or less from the time the temperature reaches 100°C,
In the above processing, the starting temperature T2 is 80°C or lower,
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℃ or more and 400℃ or less,
Tempering under the condition that the preservation time t3 at T3 is 1.0 seconds or more and 1000.0 seconds or less,
A method of manufacturing a high-strength steel sheet in which cooling is performed under the condition that the cooling rate θ1 from T3 to 80°C is 100°C/sec or less. According to paragraph 4,
A method for producing a high-strength steel sheet in which the processing step before tempering is divided into two or more processes to impart strain, and the processing is performed under the condition that the sum of the equivalent plastic strain for each processing is 0.10% or more. According to clause 4 or 5,
A method of manufacturing a high-strength steel sheet in which plating is performed during or after annealing.