WO2023181640A1 - High strength steel sheet and manufacturing method therefor - Google Patents

Abstract

The purpose of the present invention is to provide a high strength steel sheet and a method for manufacturing the same, the high strength steel sheet having the TS of more than or equal to 1180 MPa and the YR of more than or equal to 85%, and having excellent width-direction flatness and anti-working embrittlement property.　This high strength steel sheet comprises a predetermined component composition, wherein, at a 1/4 plate-thickness position, the amount of tempered martensite is more than or equal to 90% in area fraction, the amount of retained austenite is less than 3% in volume fraction, the total of the amount of ferrite and the amount of bainitic ferrite is less than 10% in area fraction, the average crystal grain size of prior austenite is less than or equal to 20 μm, and the average value of the occupancies of packets having a maximum occupancy in the prior austenite grains is less than or equal to 70% in area fraction.

Description

High strength steel plate and its manufacturing method

The present invention relates to a high-strength steel plate that is excellent in tensile strength, flatness in the width direction, and resistance to work embrittlement, and a method for 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.

With the aim of both reducing _CO2 emissions by reducing the weight of vehicles and improving collision resistance by reducing the weight of the car body, the strength of thin steel sheets for automobiles is progressing, and new laws and regulations are being introduced one after another. . Therefore, in order to increase the strength of the car body, high-strength steel plates with a tensile strength (TS) of 1180 MPa or higher are increasingly being used in the main structural parts forming automobiles.

High-strength steel sheets used in automobiles are required to have excellent work embrittlement resistance and yield ratio from the viewpoint of component performance. For example, for frame parts such as automobile bumpers, high-strength steel plates are used, which have excellent resistance to mechanical embrittlement that does not cause embrittlement due to press forming, and have excellent impact absorption properties in the event of a collision, which is correlated with YR. It is preferable to do so.

Furthermore, high-strength steel sheets used in automobiles are required to have excellent flatness. Patent Document 1 describes that warpage of a steel plate adversely affects operational troubles in a forming line and dimensional accuracy of products. As a result of extensive studies, the present inventors found that the dimensional accuracy of a product is affected not only by the warpage of the steel plate but also by the flatness in the width direction of the plate, which is evaluated using steepness. For example, in order to achieve excellent dimensional accuracy, the steepness in the width direction is preferably 0.02 or less.

In response to these demands, for example, Patent Document 2 provides a high-strength steel plate having a tensile strength of 1100 MPa or more and excellent YR, surface texture, and weldability, and a method for manufacturing the same. However, the technique described in Patent Document 2 does not take into account flatness in the width direction of the plate and resistance to work embrittlement.

Patent Document 3 provides a hot-dip galvanized steel sheet with excellent press formability and low-temperature toughness and a tensile strength of 980 MPa or more, and a method for manufacturing the same. However, although the technique described in Patent Document 3 can improve the embrittlement of the steel plate due to a temperature drop, it does not take into account the embrittlement of the steel plate due to processing. Flatness in the width direction of the plate is also not considered.

Patent No. 4947176 Patent No. 6525114 Patent No. 6777272

Smart Process Society Journal 2013, Volume 2, Issue 3, p. 110-118

The present invention was developed in view of the above circumstances, and provides a high-strength steel plate having a TS of 1180 MPa or more, a YR of 85% or more, and excellent flatness in the width direction and resistance to work embrittlement, and a method for manufacturing the same. The purpose is to

In order to achieve the above-mentioned problems, the present inventors have made the following findings as a result of extensive studies.
(1) By setting the amount of tempered martensite to 90% or more, a TS of 1180 MPa or more can be achieved.
(2) By setting the amount of retained austenite to less than 3% and the total amount of ferrite and bainitic ferrite to less than 10%, YR of 85% or more can be achieved.
(3) By setting the occupancy of the largest packet of tempered martensite in prior austenite grains to 70% or less, excellent flatness in the sheet width direction can be achieved.
(4) Excellent resistance to work embrittlement by setting the occupancy rate of the largest packet of tempered martensite in prior austenite grains to 70% or less and the average prior austenite grain size of tempered martensite to 20 μm or less characteristics can be realized.

The present invention has been made based on the above findings. That is, the gist of the present invention is as follows.
[1] In mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0. Contains 100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities. Regarding the component composition, at the 1/4 position of the plate thickness, the amount of tempered martensite is 90% or more in area fraction, the amount of retained austenite is less than 3% in volume fraction, and the total amount of ferrite and bainitic ferrite. is less than 10% in terms of area fraction, the average crystal grain size of prior austenite is 20 μm or less, and the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is 70% or less in area fraction. , high strength steel plate.
[2] The component composition further includes, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0. 10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Cu: 1.00% Below, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, The high-strength steel plate according to [1], containing at least one element selected from Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less.
[3] The high-strength steel plate according to [1] or [2], which has a plating layer on the surface of the steel plate.
[4] The method for producing a high-strength steel plate according to [1] or [2], wherein a cold-rolled plate produced by subjecting steel having the above-mentioned composition to hot rolling, pickling, and cold rolling, Heating is performed under the conditions that the annealing temperature T1 is 750°C or more and 950°C or less, the holding time t1 at the annealing temperature T1 is 10 seconds or more and 1000 seconds or less, and the average cooling rate from 750°C to 600°C is 20°C/s or more, (Ms +50℃) to average cooling rate of quenching start temperature T2 of 5℃/s to 30℃/s, quenching start temperature T2 of The specified martensitic transformation start temperature (°C), the average cooling rate from the quenching start temperature T2 to 80°C is 300°C/s or more, the tempering temperature T3 is 100°C or more and 400°C or less, and the tempering temperature is A method for manufacturing a high-strength steel plate, which is heated for a holding time t3 at T3 of 10 seconds or more and 10,000 seconds or less.
Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]... (1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.
[5] The method for producing a high-strength steel plate according to [4], which comprises performing a plating treatment.

According to the present invention, it is possible to obtain a high-strength steel plate having a TS of 1180 MPa or more, a YR of 85% or more, and excellent flatness in the width direction and resistance to work embrittlement. Furthermore, by applying the high-strength steel plate of the present invention to, for example, automobile structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Therefore, the industrial value is extremely large.

FIG. 1 is a diagram showing the structure of a packet having the maximum occupancy in prior austenite grains and a method for calculating the same according to the present invention. FIG. 2 is a diagram showing the concept of steepness θ of a steel plate according to the present invention and its calculation method.

Hereinafter, embodiments of the present invention will be described.

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

[C: 0.030% or more and 0.500% or less]
C is one of the important basic components of steel, and in particular, in the present invention, it is an important element that affects the fraction of tempered martensite and the resistance to work embrittlement. If the C content is less than 0.030%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, when the C content exceeds 0.500%, the tempered martensite becomes brittle and the work embrittlement resistance deteriorates. Therefore, the content of C is 0.030% or more and 0.500% or less. Preferably it is 0.050% or more. Preferably it is 0.400% or less. More preferably, it is 0.100% or more. More preferably, it is 0.350% or less.

[Si: 0.01% or more and 2.50% or less]
Si is one of the important basic components of steel, and in particular in the present invention, it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite, so it is an important element that affects TS and the amount of retained austenite. It is an element. If the Si content is less than 0.01%, it becomes difficult to achieve a TS of 1180 MPa or more. On the other hand, when the Si content exceeds 2.50%, retained austenite increases excessively, making it difficult to achieve YR of 85% or more. Therefore, the Si content is set to 0.01% or more and 2.50% or less. Preferably it is 0.05% or more. Preferably it is 2.00% or less. More preferably, it is 0.10% or more. More preferably, it is 1.20% or less.

[Mn: 0.10% or more and 5.00% or less]
Mn is one of the important basic components of steel, and in particular, in the present invention, it is an important element that affects the fraction of tempered martensite and the resistance to work embrittlement. If the Mn content is less than 0.10%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, when the Mn content exceeds 5.00%, the tempered martensite becomes brittle and the work embrittlement resistance deteriorates. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. Preferably it is 0.50% or more. Preferably it is 4.50% or less. More preferably, it is 0.80% or more. More preferably, it is 4.00% or less.

[P: 0.100% or less]
Since P segregates at prior austenite grain boundaries and embrittles the grain boundaries, it lowers the ultimate deformability of the steel sheet, resulting in a decrease in work embrittlement resistance. Therefore, the content of P needs to be 0.100% or less. Although the lower limit of the P content is not particularly defined, it is preferably 0.001% or more since P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the content of P is 0.100% or less. Preferably it is 0.001% or more. Preferably it is 0.070% or less.

[S: 0.0200% or less]
S exists as a sulfide and reduces the ultimate deformability of the steel sheet, thereby reducing the work embrittlement resistance. Therefore, the S content needs to be 0.0200% or less. Although the lower limit of the S content is not particularly specified, it is preferably 0.0001% or more due to production technology constraints. Therefore, the S content is set to 0.0200% or less. Preferably it is 0.0001% or more. Preferably it is 0.0050% or less.

[Al: 1.000% or less]
Since Al raises the _A3 transformation point and contains a large amount of ferrite in the microstructure, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the Al content needs to be 1.000% or less. Although the lower limit of the Al content is not particularly defined, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is set to 1.000% or less. Preferably it is 0.001% or more. Preferably it is 0.500% or less.

[N: 0.0100% or less]
N exists as a nitride and reduces the ultimate deformability of the steel sheet, thereby reducing the work embrittlement resistance. Therefore, the N content needs to be 0.0100% or less. Although the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0001% or more due to constraints on production technology. Therefore, the N content is set to 0.0100% or less. Preferably it is 0.0001% or more. Preferably it is 0.0050% or less.

[O: 0.0100% or less]
O exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing the work embrittlement resistance. Therefore, the content of O needs to be 0.0100% or less. Although the lower limit of the O content is not particularly defined, it is preferable that the O content is 0.0001% or more due to production technology constraints. Therefore, the O content is set to 0.0100% or less. Preferably it is 0.0001% or more. Preferably it is 0.0050% or less.

A high-strength steel plate according to an embodiment of the present invention has a composition containing the above-mentioned components, with the remainder containing Fe and inevitable impurities. Here, unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. A total content of these impurities of 0.100% or less is allowed.

In addition to the above-mentioned composition, the high-strength steel plate of the present invention further includes, in mass%,
Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0. 200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% At least one element selected from the following and Bi: 0.200% or less may be contained alone or in combination.

If each of Ti, Nb and V is 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated and the ultimate deformability of the steel plate will not be reduced, so the work embrittlement resistance will not deteriorate. Therefore, it is preferable that the contents of Ti, Nb, and V are each 0.200% or less. Although the lower limits of the contents of Ti, Nb, and V are not particularly defined, the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, it is more preferable that the contents of Ti, Nb, and V are each 0.001% or more. Therefore, when Ti, Nb, and V are contained, their contents are each 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If Ta and W are each 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated and the ultimate deformability of the steel sheet will not be reduced, so the work embrittlement resistance will not deteriorate. Therefore, it is preferable that the contents of Ta and W are each 0.10% or less. Note that there is no particular lower limit to the content of Ta and W, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. It is more preferable that the contents of Ta and W are each 0.01% or more. Therefore, when Ta and W are contained, their contents are each 0.10% or less. More preferably, it is 0.01% or more. More preferably, it is 0.08% or less.

If B is 0.0100% or less, it will not generate cracks inside the steel sheet during casting or hot rolling, and will not reduce the ultimate deformability of the steel sheet, so the work embrittlement resistance will not deteriorate. Therefore, the content of B is preferably 0.0100% or less. Note that the lower limit of the B content is not particularly specified, but since it is an element that segregates at austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content is 0.0003% or more. preferable. Therefore, when B is contained, its content should be 0.0100% or less. More preferably, it is 0.0003% or more. More preferably, it is 0.0080% or less.

If each of Cr, Mo, and Ni is 1.00% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, it is preferable that the contents of Cr, Mo, and Ni are each 1.00% or less. Note that the lower limit of the content of Cr, Mo, and Ni is not particularly specified, but since they are elements that improve hardenability, it is more preferable that the content of Cr, Mo, and Ni is each 0.01% or more. . Therefore, when Cr, Mo, and Ni are contained, their contents are each 1.00% or less. More preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

If Co is 0.010% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not decrease. Therefore, the Co content is preferably 0.010% or less. Although the lower limit of the Co content is not particularly specified, since it is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content should be 0.010% or less. More preferably, it is 0.001% or more. More preferably, it is 0.008% or less.

If Cu is 1.00% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, the Cu content is preferably 1.00% or less. Although the lower limit of the Cu content is not particularly specified, since it is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, if Cu is contained, the content should be 1.00% or less. More preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

If Sn is 0.200% or less, it will not generate cracks inside the steel sheet during casting or hot rolling, and will not reduce the ultimate deformability of the steel sheet, so the work embrittlement resistance will not deteriorate. Therefore, the content of Sn is preferably 0.200% or less. Note that the lower limit of the Sn content is not particularly specified, but since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content should be 0.001% or more. It is more preferable. Therefore, if Sn is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If Sb is 0.200% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not decrease. Therefore, the content of Sb is preferably 0.200% or less. Although the lower limit of the Sb content is not particularly defined, it is more preferable that the Sb content is 0.001% or more since it is an element that controls the softening thickness of the surface layer and enables strength adjustment. Therefore, if Sb is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If each of Ca, Mg and REM is 0.0100% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, the content of Ca, Mg and REM is preferably 0.0100% or less. Note that the lower limits of the contents of Ca, Mg, and REM are not particularly stipulated, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of steel sheets, the contents of Ca, Mg, and REM are More preferably, each amount is 0.0005% or more. Therefore, when Ca, Mg and REM are contained, their contents are each 0.0100% or less. More preferably, it is 0.0005% or more. More preferably, it is 0.0050% or less.

If each of Zr and Te is 0.100% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, the contents of Zr and Te are preferably 0.100% or less. Note that the lower limits of the contents of Zr and Te are not particularly specified, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the contents of Zr and Te are respectively 0. More preferably, the content is .001% or more. Therefore, when Zr and Te are contained, their contents are each 0.100% or less. More preferably, it is 0.001% or more. More preferably, it is 0.080% or less.

If Hf is 0.10% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, the Hf content is preferably 0.10% or less. Note that there is no particular lower limit to the Hf content, but since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of steel sheets, the Hf content should be 0.01% or more. It is more preferable to do so. Therefore, if Hf is contained, the content should be 0.10% or less. More preferably, it is 0.01% or more. More preferably, it is 0.08% or less.

If Bi is 0.200% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, the Bi content is preferably 0.200% or less. Although the lower limit of the Bi content is not particularly defined, since it is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

In addition, each content of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi is preferable. If it is less than the lower limit, the effect of the present invention will not be impaired, and therefore it is included as an unavoidable impurity.

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

[Tempered martensite: 90% or more in area fraction]
This is an extremely important feature of the invention. By using tempered martensite as the main phase, it is possible to achieve a TS of 1180 MPa or more. In order to obtain such an effect, it is necessary that the area fraction of tempered martensite be 90% or more. Therefore, the area fraction of tempered martensite is 90% or more. Preferably it is 94% or more. More preferably, it is 96% or more.

Here, the method for measuring tempered martensite is as follows. After polishing the L cross section of the steel plate, 3vol. % nital, and 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields at a magnification of 2000 times using an SEM. Note that in the above structure image, the tempered martensite is a structure that has fine irregularities inside and has carbides inside. Tempered martensite can be determined from the average value of those values.

[Retained austenite amount is less than 3%]
This is an extremely important feature of the invention. When the volume fraction of retained austenite is 3% or more, it becomes difficult to achieve YR of 85% or more. The reason for the decrease in YR is that an increase in retained austenite causes a decrease in YS due to deformation-induced transformation of the retained austenite. Therefore, the retained austenite content should be less than 3%. Preferably it is 1% or less. Note that the lower limit of retained austenite is not particularly limited. It may be 0%.

Here, the method for measuring retained austenite is as follows. Retained austenite was determined by polishing the steel plate from 1/4 part of the plate thickness to a surface of 0.1 mm, and then chemically polishing the surface to a further 0.1 mm using an X-ray diffractometer using CoKα rays. }, {220}, {311} planes and the diffraction peaks of {200}, {211}, {220} planes of BCC iron were measured, and the nine integrated intensity ratios obtained were averaged. Convert and seek.

[Total of ferrite and bainitic ferrite: less than 10% in area fraction]
This is an extremely important feature of the invention. If the total content of ferrite and bainitic ferrite is 10% or more, it becomes difficult to achieve a TS of 1180 MPa or more, and it becomes difficult to achieve a YR of 85% or more. The reason for the decrease in YR is that ferrite and bainitic ferrite have soft structures, so that yielding occurs early. Therefore, the total amount of ferrite and bainitic ferrite is less than 10%. Preferably it is 8% or less. More preferably, it is 5% or less. Note that the lower limit of the total of ferrite and bainitic ferrite is not particularly limited. It may be 0%.

Here, the method for measuring the total amount of ferrite and bainitic ferrite is as follows. After polishing the L cross section of the steel plate, 3vol. % nital, and 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields at a magnification of 2000 times using an SEM. In the above structure image, ferrite and bainitic ferrite have concave portions and a flat structure inside, and have no carbide inside. The sum of ferrite and bainitic ferrite can be determined from the average value of those values.

Possible structures other than all the above structures include pearlite, fresh martensite, and acicular ferrite. These structures may be included because they do not affect the properties as long as they are in a range of 5% or less.

[Prior austenite average grain size is 20 μm or less]
This is an extremely important feature of the invention. By reducing the average grain size of prior austenite, crack propagation can be suppressed, thereby improving the work embrittlement resistance of the steel sheet. In order to obtain these effects, it is necessary to make the average crystal grain size of prior austenite 20 μm or less. The lower limit of the average prior austenite grain size is not particularly defined, but if the prior austenite average grain size is less than 2 μm, residual austenite may increase, so it is preferably 2 μm or more. Therefore, the average crystal grain size of prior austenite is set to 20 μm or less. Preferably it is 2 μm or more. Preferably it is 15 μm or less. More preferably, the thickness is 3 μm or more. More preferably, the thickness is 10 μm or less.

Here, the method for measuring the average grain size of prior austenite is as follows. After polishing the L cross section of the steel plate, etching it with a mixed solution of picric acid and ferric chloride to expose the prior austenite grain boundaries, 3 to 10 fields of view are photographed using an optical microscope at a magnification of 400x. A total of 20 straight lines (10 vertically x 10 horizontally) are drawn at equal intervals on the obtained image data and determined by a cutting method.

[The average value of the occupancy of the packets with the maximum occupancy in the prior austenite grains is 70% or less in terms of area fraction]
This is an extremely important feature of the invention. The occupancy of the packets having the maximum occupancy within the prior austenite grains influences the flatness in the width direction and the resistance to work embrittlement. The packet with the maximum occupancy rate in the prior austenite grains means that, as shown in Figure 1, there are up to four regions in the prior austenite grains, called packets, that have the same crystal habit plane during transformation. indicates the packet with the largest occupancy rate. The occupancy of one packet within a prior austenite grain is determined by dividing the area of a specified packet by the total area within the prior austenite grain. As a result of extensive studies, the present inventors found that by reducing the occupancy of the packets with the maximum occupancy within the prior austenite grains, the strain between the packets was alleviated and the flatness in the sheet width direction was improved. I discovered that. They also found that by reducing the occupancy of packets, which have the highest occupancy within prior austenite grains, the structure becomes finer and crack propagation can be suppressed, thereby improving the work embrittlement resistance of the steel sheet. . Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is set to be 70% or less. Preferably it is 60% or less. Note that the lower limit of the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is not particularly limited. There are a maximum of four types of packets, and when the four packets are evenly distributed, the occupancy rate of the packet having the maximum occupancy rate in the prior austenite grains is 25%. Therefore, the lower limit of the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is set to 25% or more, but there is no need to limit it to this.

Here, the method for measuring the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is as follows. First, a test piece for microstructural observation is taken from a cold-rolled steel sheet. Next, the sampled test piece is polished by colloidal silica vibration polishing so that the cross section in the rolling direction (L cross section) becomes the observation surface. The observation surface is a mirror surface. Next, electron beam backscatter diffraction (EBSD) measurement is performed on a 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) to obtain local crystal orientation data. At this time, the SEM magnification is 1000 times, the step size is 0.2 μm, the measurement area is 80 μm square, and the WD is 15 mm. The obtained local orientation data is analyzed using OIM Analysis 7 (OIM), and a color-coded diagram (CP map) for each Close-packed Plane group (CP group) is created using the method described in Non-Patent Document 1. . In the present invention, packets are defined as areas to which the same CP group belongs. The area of the packet with the largest occupancy is determined from the obtained CP map and divided by the total area within the prior austenite grains, thereby obtaining the occupancy of the packet with the maximum occupancy within the prior austenite grains. This analysis is performed on ten or more adjacent prior austenite grains, and the average value is taken as the average value of the occupancy of the packets having the maximum occupancy within the prior austenite grain.

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

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. The steel slab (slab) is preferably manufactured by a continuous casting method in order to prevent macro segregation.

In the present invention, the slab heating temperature, slab soaking time and coiling temperature in hot rolling are not particularly limited. Methods for hot rolling steel slabs include rolling the slab after heating, directly rolling the slab after continuous casting without heating it, and rolling after subjecting the slab after continuous casting to a short heat treatment. Examples include. Although the slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, the lower limit of the slab heating temperature is preferably 1100° C. or higher. The upper limit of the slab heating temperature is preferably 1300°C or less. The lower limit of the slab soaking time is preferably 30 min or more. The upper limit of the slab soaking time is preferably 250 min or less. The lower limit of the finish rolling temperature is preferably equal to or higher than the Ar ₃ transformation point. Further, the lower limit of the winding temperature is preferably 350°C or higher. Moreover, the upper limit of the winding temperature is preferably 650° C. or less.

The hot-rolled steel sheet produced in this way is pickled. Since pickling can remove oxides on 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. Further, the pickling may be carried out once or may be carried out in multiple steps. Moreover, cold rolling may be performed on the pickled plate after hot rolling, or cold rolling may be performed after heat treatment.

The rolling reduction in cold rolling and the plate thickness after rolling are not particularly limited, but the lower limit of the rolling reduction is preferably 30% or more. Further, the upper limit of the rolling reduction ratio is preferably 80% or less. Note that the effects of the present invention can be obtained without any particular limitations on the number of rolling passes and the rolling reduction rate of each pass.

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

[Annealing temperature T1: 750°C or higher and 950°C or lower]
When the annealing temperature T1 is less than 750°C, the total area fraction of ferrite and bainitic ferrite becomes 10% or more, making it difficult to achieve a TS of 1180 MPa or more and achieving a YR of 85% or more. becomes difficult to do. On the other hand, when the annealing temperature T1 exceeds 950° C., the prior austenite grain size increases excessively, exceeding 20 μm, and the work embrittlement resistance deteriorates. Therefore, the annealing temperature T1 is set to 750°C or more and 950°C or less. Preferably it is 800°C or higher. Preferably it is 900°C or less.

[Holding time t1 at annealing temperature T1: 10 seconds or more and 1000 seconds or less]
If the holding time t1 at the annealing temperature T1 is less than 10 seconds, austenitization will be insufficient, and the total area fraction of ferrite and bainitic ferrite will be 10% or more, making it difficult to achieve a TS of 1180 MPa or more. , and it becomes difficult to achieve a YR of 85% or more. On the other hand, when the holding time at the annealing temperature T1 exceeds 1000 seconds, the prior austenite grain size increases excessively, and the resistance to work embrittlement deteriorates. Therefore, the holding time t1 at the annealing temperature T1 is 10 seconds or more and 1000 seconds or less. Preferably it is 50 seconds or more. Preferably it is 500 seconds or less.

[Average cooling rate from 750°C to 600°C is 20°C/s or more]
When the average cooling rate from 750°C to 600°C is less than 20°C/s, the total area fraction of ferrite and bainitic ferrite becomes 10% or more, making it difficult to achieve a TS of 1180 MPa or more, and , it becomes difficult to achieve a YR of 85% or more. Therefore, the average cooling rate from 750°C to 600°C is set to 20°C/s or more. Preferably it is 30°C/s or more. The upper limit does not need to be particularly limited, but is preferably 2000° C./s or less.

[(Ms+50°C) to average cooling rate of quenching start temperature T2 of 5°C/s or more and 30°C/s or less]
This is an extremely important feature of the invention. The average cooling rate from (Ms + 50° C.) to the quenching start temperature T2 affects the average value of the total area fraction of ferrite and bainitic ferrite and the occupancy of the packet with the maximum occupancy in the prior austenite grains. (Ms + 50°C) - If the average cooling rate of the rapid cooling start temperature T2 is less than 5°C/s, the total area fraction of ferrite and bainitic ferrite becomes 10% or more, making it difficult to achieve a TS of 1180 MPa or more. , and it becomes difficult to achieve a YR of 85% or more. On the other hand, when the average cooling rate from (Ms+50℃) to quenching start temperature T2 exceeds 30℃/s, the average value of the occupancy of the packets with the maximum occupancy in the prior austenite grains exceeds 70%, and The flatness of the plate deteriorates, and the resistance to work embrittlement decreases. Therefore, the average cooling rate from (Ms+50°C) to the rapid cooling start temperature T2 is set to be 5°C/s or more and 30°C/s or less. Preferably it is 10°C/s or more. Preferably it is 20°C/s or less.

[Rapid cooling start temperature T2 is (Ms-80°C) or more and less than Ms]
This is an extremely important feature of the invention. The quenching start temperature T2 is set to (Ms-80°C) or more and less than Ms, and the quenching is performed in a state where the martensitic transformation rate before the start of quenching is 1% or more and 80% or less. As a result, it is possible to obtain a structure in which the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is 70% or less, and the volume fraction of retained austenite is less than 3%. If the quenching start temperature T2 is less than (Ms-80℃), the martensite transformation rate before the start of quenching exceeds 80%, so the retained austenite becomes 3% or more in volume fraction, achieving YR of 85% or more. becomes difficult to do. On the other hand, when the quenching start temperature T2 exceeds Ms, the martensite transformation rate before the start of quenching is less than 1%, the average value of the occupancy of the packets with the maximum occupancy in the prior austenite grains exceeds 70%, and the plate The flatness in the width direction deteriorates, and the resistance to work embrittlement decreases. Therefore, the quenching start temperature T2 is set to be at least (Ms-80°C) and less than Ms. Preferably it is (Ms-50°C) or higher. Preferably it is (Ms-5°C) or less. Note that the martensitic transformation start temperature Ms (° C.) is defined by the following equation (1).
Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]... (1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.

[Average cooling rate from quenching start temperature T2 to 80°C is 300°C/s or more]
If the average cooling rate from the quenching start temperature T2 to 80°C is less than 300°C/s, the volume fraction of retained austenite will be 3% or more, making it difficult to achieve YR of 85% or more. Therefore, the average cooling rate from the rapid cooling start temperature T2 to 80°C is set to 300°C/s or more. Preferably it is 800°C/s or more. The upper limit does not need to be particularly limited, but is preferably 2000° C./s or less.

[Tempering temperature T3 is 100°C or more and 400°C or less]
In the present invention, tempered martensite refers to a structure in which martensite at 80° C. or lower is heat-treated at a tempering temperature of 100° C. or higher and for a holding time of 10 seconds or longer. Therefore, when the tempering temperature T3 is less than 100° C., martensite is not sufficiently tempered, resulting in a structure consisting mainly of as-quenched martensite, and the as-quenched martensite deteriorates the work embrittlement resistance. On the other hand, when the tempering temperature T3 exceeds 400°C, the tempered martensite decomposes and becomes ferrite, making the area fraction of the tempered martensite less than 90%, making it difficult to achieve a TS of 1180 MPa or more. Become. Therefore, the tempering temperature T3 is set at 100°C or more and 400°C or less. Preferably the temperature is 150°C or higher. Preferably the temperature is 350°C or less.

[Holding time t3 at tempering temperature T3 is 10 seconds or more and 10,000 seconds or less]
In the present invention, tempered martensite refers to a structure in which martensite at 80° C. or lower is heat-treated at a tempering temperature of 100° C. or higher and for a holding time of 10 seconds or longer. Therefore, when the holding time t3 at the tempering temperature T3 is less than 10 seconds, martensite is not sufficiently tempered, resulting in a structure consisting mainly of as-quenched martensite, and the as-quenched martensite deteriorates the work embrittlement resistance. On the other hand, when the tempering temperature T3 exceeds 10,000 seconds, the tempered martensite decomposes and becomes ferrite, making the area fraction of the tempered martensite less than 90%, making it difficult to achieve a TS of 1180 MPa or more. Become. Therefore, the holding time t3 at the tempering temperature T3 is set to 10 seconds or more and 10,000 seconds or less. Preferably it is 50 seconds or more. Preferably it is 5000 seconds or less.

Cooling after tempering does not need to be particularly specified, and may be cooled to a desired temperature by any method. Note that the desired temperature is preferably about room temperature.

Furthermore, the above-mentioned high-strength steel plate may be processed under conditions that result in an equivalent plastic strain amount of 0.10% or more and 5.00% or less. Further, after processing, reheating may be performed again under the conditions of 100° C. or more and 400° C. or less.

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

The high-strength steel plate may be subjected to plating treatment during or after annealing.

Examples of the plating treatment during annealing include hot-dip galvanizing during or after cooling at 750° C. to 600° C. at an average cooling rate of 20° C./s or more, and alloying after hot-dip galvanizing. Further, as the plating treatment after annealing, for example, Zn-Ni electroplating treatment or pure Zn electroplating treatment can be exemplified after tempering. The plating layer may be formed by electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. In addition, although the above-mentioned 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. Conditions for other manufacturing methods are not particularly limited, but from the viewpoint of productivity, a series of treatments such as annealing, hot-dip galvanizing, and galvanizing alloying are performed on a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. It is preferable to carry out the process using Line). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Note that conditions for plating and the like other than the conditions described above can be based on a conventional method for hot-dip galvanizing.

After the plating treatment after annealing, processing may be performed again under conditions that result in an equivalent plastic strain amount of 0.10% or more and 5.00 or less. Further, after processing, reheating may be performed again under the conditions of 100° C. or more and 400° C. or less.

A steel having the composition shown in Tables 1 and 2, with the remainder consisting of Fe and unavoidable impurities, was melted in a converter and made into a slab by continuous casting. Next, the obtained slab was heated, hot-rolled, pickled, cold-rolled, and annealed and tempered as shown in Tables 3 to 6 to give a plate thickness of 0.6. A high-strength cold-rolled steel plate with a thickness of ~2.2 mm was obtained. Note that some steel plates are manufactured by performing plating treatment during or after annealing.

Using the high-strength cold-rolled steel sheet obtained as described above as a test steel, tensile properties, flatness in the sheet width direction, and work embrittlement resistance were evaluated according to the following test methods.

(organizational observation)
According to the method described above, the sum of the amount of tempered martensite, the amount of retained austenite, the amount of ferrite, the amount of bainitic ferrite, and the average grain size of prior austenite were determined.

(Occupancy of packets with maximum occupancy in prior austenite grains)
According to the method described above, the average value of the occupancy of the packets having the maximum occupancy within the prior austenite grains was determined.

(Tensile test)
For the tensile test, a JIS No. 5 test piece (gauge length 50 mm, parallel part width 25 mm) was taken so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and tested in accordance with JIS Z 2241. A tensile test was conducted at a crosshead speed of 1.67×10 ⁻¹ mm/sec, and YS and TS were measured. In addition, in the present invention, a TS of 1180 MPa or more was determined to be acceptable. A yield ratio (YR) of 85% or more was judged to be acceptable. Note that YR is determined by the following equation (2).
YR=100×YS/TS...(2)
(Flatness in the board width direction)
The flatness of the various cold-rolled steel sheets obtained as described above in the sheet width direction was measured by the method shown in FIG. 2. Specifically, a plate with a length of 500 mm in the rolling direction (coil width x 500 mm L x plate thickness) is cut out from the coil, placed on a surface plate so that the warp of the end face faces upward, and the stylus moves over the object to be measured. The height of the steel plate was continuously measured over the entire width direction using a moving contact displacement meter. Based on the results, the steepness θ, which is an index indicating the flatness of the steel plate shape, was measured according to the method shown in FIG. If the steepness exceeds 0.02, it is evaluated as "×", if the steepness exceeds 0.01 and is less than 0.02, it is evaluated as "○", and if the steepness is less than 0.01, it is evaluated as "◎". Those with a flatness of 0.02 or less were judged to have "excellent flatness in the board width direction."

(Processing brittleness resistance)
The resistance to work brittleness was evaluated by Charpy test. For the Charpy test piece, a plurality of steel plates were stacked together and fastened together with bolts, and after confirming that there were no gaps between the steel plates, a test piece with a V-notch having a depth of 2 mm was prepared. The number of steel plates to be stacked was set so that the thickness of the test piece after stacking was closest to 10 mm. For example, if the plate thickness is 1.2 mm, 8 plates are laminated, resulting in a test piece thickness of 9.6 mm. The laminated Charpy test piece was taken with the width direction of the plate as the longitudinal direction. As an index showing the resistance to work embrittlement, the ratio vE _0% /vE _10% of the impact absorption energy at room temperature between the as-manufactured (unprocessed) steel plate and the 10% rolled steel plate was measured. "x" indicates that vE _0% /vE _10% is less than 0.6; "○" indicates that vE _0% /vE _{10 %} is _0.6 or more and less than 0.7; Those with vE _0% /vE _10% of 0.6 or more were evaluated as "excellent in resistance to work brittleness". Note that conditions other than the above were in accordance with JIS Z 2242:2018.

The results are shown in Tables 7 to 10. As shown in these tables, the examples of the present invention have a TS of 1180 MPa or more, a YR of 85% or more, and are excellent in flatness in the sheet width direction and resistance to work embrittlement. On the other hand, in the comparative example, any one or more of TS, YR, flatness in the plate width direction, or work embrittlement resistance is inferior.

Claims (5)

In mass%,
C: 0.030% or more and 0.500% or less,
Si: 0.01% or more and 2.50% or less,
Mn: 0.10% or more and 5.00% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0100% or less, and
A component composition containing O: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities;
At the plate thickness 1/4 position,
The amount of tempered martensite is 90% or more in terms of area fraction,
The amount of retained austenite is less than 3% in volume fraction,
The total amount of ferrite and bainitic ferrite is less than 10% in area fraction,
The average crystal grain size of prior austenite is 20 μm or less,
A high-strength steel sheet in which the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is 70% or less in terms of area fraction. The component composition further includes, in mass%,
Ti: 0.200% or less, Nb: 0.200% or less,
V: 0.200% or less, Ta: 0.10% or less,
W: 0.10% or less, B: 0.0100% or less,
Cr: 1.00% or less, Mo: 1.00% or less,
Co: 0.010% or less, Ni: 1.00% or less,
Cu: 1.00% or less, Sn: 0.200% or less,
Sb: 0.200% or less, Ca: 0.0100% or less,
Mg: 0.0100% or less, REM: 0.0100% or less,
Zr: 0.100% or less, Te: 0.100% or less,
Hf: 0.10% or less, Bi: 0.200% or less,
The high-strength steel plate according to claim 1, containing at least one element selected from the following. The high-strength steel plate according to claim 1 or 2, which has a plating layer on the surface of the steel plate. A method for manufacturing a high-strength steel plate according to claim 1 or 2, comprising:
A cold-rolled plate produced by subjecting steel having the above-mentioned composition to hot rolling, pickling and cold rolling,
annealing temperature T1 is 750°C or more and 950°C or less,
Heating under conditions where the holding time t1 at the annealing temperature T1 is 10 seconds or more and 1000 seconds or less,
Average cooling rate from 750°C to 600°C is 20°C/s or more,
(Ms+50°C) to average cooling rate of quenching start temperature T2 of 5°C/s or more and 30°C/s or less,
The quenching start temperature T2 is (Ms-80°C) or more and less than Ms, where Ms is the martensitic transformation start temperature (°C) defined by formula (1),
Cooling at an average cooling rate of 300°C/s or more from the rapid cooling start temperature T2 to 80°C,
Tempering temperature T3 is 100°C or more and 400°C or less,
A method for producing a high-strength steel plate, which is heated at a tempering temperature T3 for a holding time t3 of 10 seconds or more and 10,000 seconds or less.
Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]... (1)
Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0. The method for manufacturing a high-strength steel plate according to claim 4, which comprises performing a plating treatment.

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