JP7597273B1 - Steel plates and members, and their manufacturing methods - Google Patents

Abstract

The present invention provides a steel sheet having a TS of 1,180 MPa or more and excellent dimensional accuracy and shear angle range. The steel sheet has a predetermined chemical composition and a structure having an area fraction of tempered martensite of 83% or more, an area fraction of retained austenite of less than 3%, a combined area fraction of ferrite and bainitic ferrite of 5% or more and less than 15%, and a prior γ grain boundary occupancy rate of 20% or more.

Description

The present invention relates to a steel plate, a component made from the steel plate, and a method for manufacturing the same.

In order to achieve both reduction in _CO2 emissions by reducing the weight of automobile bodies and improvement in crashworthiness, efforts are being made to increase the strength of steel sheets, which are the raw material for automobile parts. In addition, new legal regulations are being introduced one after another. As a result, the number of cases in which steel sheets with a tensile strength (TS) of 1180 MPa or more are used in the main structural parts of automobiles (hereinafter also referred to as automobile frame parts) is increasing.

Steel sheets, which are used as raw materials for automotive parts, are also required to have excellent dimensional accuracy (hereinafter simply referred to as dimensional accuracy) when formed into parts. For example, in automotive frame parts such as bumpers, it is possible to suppress springback and improve dimensional accuracy by controlling the yield ratio (hereinafter also referred to as YR) of the steel sheet within a certain range.

As an example of a steel sheet used as a material for automobile parts, Patent Document 1 describes:
"In mass percent,
C: 0.09% or more and 0.37% or less,
Si: more than 0.70% and less than 2.00%,
Mn: 2.60% or more and 3.60% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
A composition comprising Al: 0.010% or more and 1.000% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities;
The area ratio of martensite having a carbon concentration greater than 0.7×[%C] and smaller than 1.5×[%C] is 55% or more,
The area ratio of tempered martensite having a carbon concentration of 0.7×[% C] or less is 5% or more and 40% or less,
a ratio of a carbon concentration in the retained austenite to a volume fraction of the retained austenite is 0.05 or more and 0.40 or less;
The martensite and the tempered martensite have an average grain size of 5.3 μm or less,
The steel structure further has a surface softened thickness of 10 μm or more and 100 μm or less,
A high-strength steel plate having a tensile strength of 1180 MPa or more.
In addition, [%C] indicates the content (mass%) of the component element C in the steel.
has been disclosed.

In Patent Document 2,
"The composition contains, by mass%, C: 0.15% or more and 0.40% or less, Si: 1.5% or less, Mn: 0.9 to 1.7%, P: 0.03% or less, S: less than 0.0020%, sol.Al: 0.2% or less, N: less than 0.0055%, and O: 0.0025% or less, and satisfies the relationship of the following formula (1), with the balance being Fe and unavoidable impurities;
The area ratio of tempered martensite and bainite to the entire structure is 95% or more and 100% or less in total,
The inclusion particles are composed of one or more inclusion particles having a major axis of 0.3 μm or more, which are extended in the rolling direction and/or distributed in a dotted row pattern, and when the inclusion particles are composed of two or more particles, the distance between the inclusion particles is 30 μm or less, and the number of inclusion groups having a total length of more than 120 μm in the rolling direction is 0.8 particles/mm2 or less,
The aspect ratio is 2.5 or less, the major axis is 0.20 μm or more and 2 μm or less, and the number of carbides mainly composed of Fe is 3500 pieces/mm2 ^or less.
The number of carbides having a diameter of 10 to 50 nm distributed inside the tempered martensite and/or the bainite is 0.7 x 107 pieces/ ^mm2 or more,
The average grain size of prior γ grains is 18 μm or less.
A cold-rolled steel sheet with a thickness of 0.5 to 2.6 mm, a tensile strength of 1320 MPa or more, and excellent resistance to delayed fracture.
5[%S]＋[%N]＜0.0115・・・(1)
Here, [%S] and [%N] are the S and N contents (mass%) in the steel, respectively.
Here, excellent resistance to delayed fracture means that the delayed fracture time when cold press forming involving shearing and punching is:
Tensile strength: 1320MPa or more but less than 1530MPa: over 200hrs
Tensile strength: 24 hours or more for 1530 MPa or more but less than 1550 MPa,
Tensile strength: 12hr or more for 1550MPa or more but less than 1570MPa,
Tensile strength: 9hr or more for 1570MPa or more but less than 1610MPa,
Tensile strength: 1.0 hr or more for 1610 MPa or more but less than 1960 MPa,
Tensile strength: 1960 MPa or more means 0.2 hours or more. The delayed fracture time is the time from the start of immersion to the beginning of the appearance of microcracks when immersed in hydrochloric acid (hydrogen chloride aqueous solution) of pH 1 at an aqueous solution temperature of 20°C.
has been disclosed.

In Patent Document 3,
"In mass percent,
C: 0.12% or more and 0.40% or less,
Si: 0.01% or more and 1.5% or less,
Mn: more than 1.7% and not more than 3.5%;
P: 0.05% or less,
S: 0.010% or less,
sol. Al: 1.00% or less,
N: 0.010% or less,
B: 0.0002% or more and 0.0050% or less, and one or two of Nb and Ti in total of 0.010% or more and 0.080% or less, with the balance being Fe and unavoidable impurities;
and a steel structure having an area ratio of martensite of 70% or more, an area ratio of bainite of 30% or less, and a total area ratio of ferrite and retained austenite of 10% or less,
The number density of carbides having a major axis of 0.5 μm or more at a 1/4 position of the sheet thickness of the steel sheet is 60,000 pieces/mm2 ^or less,
the number density of inclusion particles having a circle equivalent diameter of 4.0 μm or more in a range of 1/4 to 3/4 of the sheet thickness of the steel sheet is 10 particles/mm2 or ^more and 30 particles/mm2 ^or less,
the number density of inclusion particles having a circular equivalent diameter of 4.0 μm or more in the range from the surface of the steel plate to 1/4 of the plate thickness is 27 particles/ ^mm2 or less,
A steel plate with a tensile strength of 1,310 MPa or more.
has been disclosed.

Patent No. 6747612 Patent No. 6112261 Patent No. 7001197

Meanwhile, automobile parts, particularly automobile frame parts, have many end faces (hereinafter also referred to as shear end faces) formed by shear processing. For this reason, the steel sheets that are used as the raw materials for automobile parts are also required to have excellent delayed fracture resistance properties after shear processing. Delayed fracture is a phenomenon that leads to destruction in the following manner. That is, when a part is placed in a hydrogen intrusion environment while a high stress is applied to the part by forming processing or the like, hydrogen penetrates into the part. The hydrogen that penetrates into the part causes a decrease in interatomic bonding strength and local deformation. This causes microcracks to form in the part, and the microcracks grow, leading to destruction.

Delayed fracture resistance is affected by the shape of the sheared end surface. The shape of the sheared end surface is also affected by the shear angle used during shearing (hereinafter simply referred to as the shear angle). In other words, delayed fracture resistance is affected by the shear angle. For example, even for parts made from the same steel plate, delayed fracture resistance decreases if the shear angle is outside the appropriate range. The shear angle is the angle (opening angle) between the upper and lower blades used in shearing.

Steel sheets used as raw materials for automotive parts are sheared at various shear angles depending on the required dimensional accuracy, productivity, equipment constraints, etc. Therefore, there is a demand for a wide range of appropriate shear angles for delayed fracture (i.e., the range of shear angles at which excellent delayed fracture resistance can be obtained in the steel sheet after shear processing, hereinafter also referred to as shear angle range), i.e., excellent shear angle range.

However, none of the steel sheets disclosed in Patent Documents 1 to 3 take into consideration the wide range of shear angles. Therefore, there is currently a demand for the development of a steel sheet with a TS of 1,180 MPa or more, which also has excellent dimensional accuracy and wide range of shear angles.

The present invention has been developed in view of the above-mentioned current situation, and aims to provide a steel plate having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range, together with an advantageous manufacturing method thereof.
Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022.
Excellent dimensional accuracy means that YR is 65% or more and 85% or less, where YR is calculated by the following formula:
YR=100×YS/TS
In the formula, YS is the yield stress, which, like TS, is measured by a tensile test in accordance with JIS Z 2241:2022.
The term "excellent shear angle range" means that the appropriate range of the shear angle at which delayed fracture does not occur when the load stress is 1000 MPa is 0 to 0.5° or more.
Details of the measurement methods are as described in the Examples below.

In order to achieve the above object, the inventors have conducted extensive research and have obtained the following findings.
(A) In order to obtain TS: 1180 MPa or more, it is important that the area fraction of tempered martensite is 83% or more and the total area fraction of ferrite and bainitic ferrite is less than 15%. This makes it possible to obtain TS: 1180 MPa or more while ensuring the specified required properties.
(B) In order to obtain excellent dimensional accuracy, it is important that the total area fraction of ferrite and bainitic ferrite is 5% or more. This makes it possible to obtain excellent dimensional accuracy while ensuring the required characteristics.
(C) In order to obtain an excellent shear angle range, it is important that the area fraction of retained austenite is less than 3% and the occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite is 20% or more. This makes it possible to obtain an excellent shear angle range while ensuring the specified required properties.
The present invention was completed based on the above findings and through further investigation.

That is, the gist and configuration of the present invention are as follows.
1. In mass percent,
C: 0.030% or more and 0.500% or less,
Si: 0.010% or more and 2.500% or less,
Mn: 0.10% or more and 5.00% or less,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.0100% or less,
A composition comprising O: 0.0100% or less and Al: 1.000% or less, with the balance being Fe and unavoidable impurities;
Area fraction of tempered martensite: 83% or more,
Area fraction of retained austenite: less than 3%;
Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%;
and
and a structure in which an occupancy rate of prior austenite grain boundaries by the ferrite and the bainitic ferrite is 20% or more.

2. The composition further comprises, 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,
2. The steel plate according to 1 above, containing at least one selected from Hf: 0.10% or less and Bi: 0.200% or less.

3. The steel plate described in 1 or 2 above, having a plating layer on the surface.

4. A component made using a steel plate described in any one of 1 to 3 above.

5. A method for producing the steel plate according to any one of 1 to 3 above,
A preparation step of preparing a base steel sheet having the component composition according to 1 or 2;
Next, the base steel sheet is
Average heating rate in the temperature range of 700°C to 750°C: 5.0°C/s or less;
A heating step in which the material is heated under the condition of a maximum temperature T1 of 800° C. or higher and 900° C. or lower;
Next, the base steel sheet is
A first cooling step in which the average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2 is 0.10° C./s or more and 5.00° C./s or less;
Next, the base steel sheet is
Intermediate holding temperature T2: 600°C or more and 750°C or less,
Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less,
An intermediate holding step of holding the steel sheet at a tension of 5 MPa or more;
Next, the base steel sheet is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
A second cooling step of cooling the mixture to a second cooling end temperature under the conditions of
Next, the base steel sheet is
Tempering temperature T3: 100°C or more and 400°C or less,
A tempering process in which tempering is performed under the condition of a tempering time t3 of 10 seconds or more and 10,000 seconds or less;
A manufacturing method for steel plates that provides the following:

6. The method for manufacturing steel plate described in 5, further comprising a plating process for applying a plating treatment to the base steel plate between the intermediate holding process and the second cooling process, or after the tempering process.

7. A method for manufacturing a component, comprising the step of subjecting a steel plate described in any one of 1 to 3 above to at least one of forming and joining processes to form a component.

According to the present invention, a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range can be obtained. Furthermore, the steel sheet of the present invention can be applied to a wider range of materials for automobile parts, so that the weight of the automobile body can be reduced, thereby further improving fuel efficiency and greatly contributing to reducing _CO2 emissions. Therefore, the industrial utility value is extremely high.

FIG. 2 is a schematic diagram for explaining the definition of a prior γ grain boundary occupancy rate.

The present invention will be described based on the following embodiments.
[1] Steel Plate First, the composition of the steel plate according to one embodiment of the present invention will be described. Note that the unit of the composition is "mass%", but hereinafter, unless otherwise specified, it will be simply shown as "%".

[C: 0.030% or more and 0.500% or less]
C is one of the important basic components of steel. In particular, in the steel plate according to one embodiment of the present invention, C is an important element that affects the area fraction of tempered martensite. If the C content is less than 0.030%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. The tempered martensite becomes embrittled, making it difficult to realize an excellent shear angle range. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably The C content is preferably 0.050% or more, and more preferably 0.100% or more. The C content is preferably 0.400% or less, and more preferably 0.350% or less.

[Si: 0.010% or more and 2.500% or less]
Si is one of the important basic components of steel. In particular, in a steel sheet according to an embodiment of the present invention, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si Si is an important element that affects the area fraction of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a TS of 1180 MPa or more. If the Si content exceeds 500%, the amount of retained austenite increases excessively, making it difficult to realize an excellent shear angle range. Therefore, the Si content is set to 0.010% or more and 2.500% or less. The Si content is preferably 0.050% or more, more preferably 0.100% or more. The Si content is preferably 2.000% or less, more preferably 1.200% or less. .

[Mn: 0.10% or more and 5.00% or less]
Mn is one of the important basic components of steel. In particular, in a steel sheet according to an embodiment of the present invention, Mn is an important element that affects the area fraction of tempered martensite and the shear angle range. If the Mn content is less than 0.10%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. If the content of Mn exceeds 0.10%, the tempered martensite becomes embrittled, and it becomes difficult to realize an excellent shear angle range. Therefore, the content of Mn is set to 0.10% or more and 5.00% or less. The Mn content is preferably 0.50% or more, and more preferably 0.80% or more. The Mn content is preferably 4.50% or less, and more preferably 4.00% or less.

[P: 0.100% or less]
P segregates at prior austenite grain boundaries, embrittling the grain boundaries and becoming the starting point of delayed fracture. Therefore, if the P content is excessive, it becomes difficult to achieve an excellent shear angle range. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.070% or less. There is no particular lower limit for the P content. However, P is P is a solid solution strengthening element that can increase the strength of the steel sheet, so the P content is preferably 0.001% or more.

[S: 0.0200% or less]
S exists as sulfide and becomes the starting point of delayed fracture. Therefore, if the S content is excessive, it becomes difficult to realize an excellent shear angle range. Therefore, the S content is set to 0. The S content is preferably 0.0050% or less. There is no particular lower limit for the S content. However, due to production technology constraints, the S content is It is preferable that the content is 0.0001% or more.

[N: 0.0100% or less]
N exists as a nitride and becomes the starting point of delayed fracture. Therefore, if the N content is excessive, it becomes difficult to realize an excellent shear angle range. Therefore, the N content is set to 0. The N content is preferably 0.0050% or less. There is no particular lower limit for the N content. However, due to production technology constraints, the N content is It is preferable that the content is 0.0001% or more.

[O: 0.0100% or less]
O exists as an oxide and becomes the starting point of delayed fracture. Therefore, if the O content is excessive, it becomes difficult to realize an excellent shear angle range. Therefore, the O content is set to 0. The O content is preferably 0.0050% or less. There is no particular lower limit for the O content. However, due to production technology constraints, the O content is It is preferable that the content is 0.0001% or more.

[Al: 1.000% or less]
Al exists as an oxide and becomes the starting point of delayed fracture. Therefore, if the content of Al is excessive, it becomes difficult to realize an excellent shear angle range. Therefore, the content of Al is set to 1. The Al content is preferably 0.500% or less. There is no particular lower limit for the Al content. However, due to restrictions in production technology, the Al content is It is preferable that the content is 0.001% or more.

The basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities. In addition to the above basic components, the steel sheet according to one embodiment of the present invention may contain at least one element selected from the following, either alone or in combination, as an optional added element.
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% or less and Bi: 0.200% or less. The above-mentioned optional elements are not particularly limited in terms of lower limit because the effects of the present invention can be obtained as long as they are contained in amounts not exceeding the upper limits. In addition, when the above-mentioned optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.

[Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less]
If the Ti, Nb and V contents are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting points of delayed fracture. Therefore, the shear angle range is not reduced. When Ti, Nb and V are contained, the content of each is preferably 0.200% or less. The content of each of Ti, Nb and V is more preferably 0.100% or less. There is no particular lower limit for the content of Nb and V. However, Ti, Nb and V form fine carbides, nitrides or carbonitrides during hot rolling or annealing, thereby improving the strength of the steel sheet. Therefore, the contents of Ti, Nb and V are each preferably 0.001% or more.

[Ta: 0.10% or less, W: 0.10% or less]
If the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not generated, and they do not become the starting points of delayed fracture. Therefore, the shear angle range is not reduced. When Ta and W are contained, their contents are preferably 0.10% or less. The contents of Ta and W are more preferably 0.08% or less. The lower limits of the contents of Ta and W are particularly Not specified. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the contents of Ta and W are Each of them is preferably at least 0.01%.

[B: 0.0100% or less]
If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and will not become the starting point of delayed fracture. Therefore, there will be no decrease in the shear angle range. When B is contained, its content is preferably 0.0100% or less. The content of B is more preferably 0.0080% or less. There is no particular lower limit for the content of B. B is an element that segregates at austenite grain boundaries during annealing to improve hardenability, and therefore the B content is preferably 0.0003% or more.

[Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less]
If the content of Cr, Mo and Ni is 1.00% or less, a large amount of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not decreased. When Cr, Mo and Ni are contained, the content of each is preferably 1.00% or less. The content of Cr, Mo and Ni is more preferably 0.80% or less. The lower limits of the Mo and Ni contents are not particularly specified. However, since Cr, Mo and Ni are elements that improve hardenability, the Cr, Mo and Ni contents are each set to 0.01% or more. It is preferable to do so.

[Co: 0.010% or less]
If the Co content is 0.010% or less, large precipitates and inclusions are not generated in large quantities, and they do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, the Co content is In this case, the Co content is preferably 0.010% or less. The Co content is more preferably 0.008% or less. There is no particular lower limit for the Co content. However, Co has an effect on hardenability. Therefore, the Co content is preferably 0.001% or more.

[Cu: 1.00% or less]
If the Cu content is 1.00% or less, large precipitates and inclusions are not generated in large quantities, and they do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, the inclusion of Cu is In this case, the Cu content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less. There is no particular lower limit for the Cu content. However, Cu has the effect of improving hardenability. Therefore, the Cu content is preferably 0.01% or more.

[Sn: 0.200% or less]
If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and will not become the starting point of delayed fracture. Therefore, the shear angle range will not be reduced. When Sn is contained, the content is preferably 0.200% or less. The Sn content is more preferably 0.100% or less. There is no particular lower limit for the Sn content. However, Sn is an element that improves hardenability and generally improves corrosion resistance, so the Sn content is preferably 0.001% or more.

[Sb: 0.200% or less]
If the Sb content is 0.200% or less, large precipitates and inclusions are not generated in large quantities, and they do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, the inclusion of Sb is In this case, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.100% or less. There is no particular lower limit for the Sb content. However, Sb is effective in preventing surface softening. Sb is an element that controls the thickness and enables strength adjustment. Therefore, the Sb content is preferably 0.001% or more.

[Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less]
If the Ca, Mg and REM contents are each 0.0100% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting points of delayed fracture. Therefore, the shear angle range is not reduced. When Ca, Mg and REM are contained, the content of each is preferably 0.0100% or less. The content of each of Ca, Mg and REM is more preferably 0.0050% or less. The lower limits of the contents of Mg and REM are not particularly specified. However, Ca, Mg and REM are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. The content of each of S and REM is preferably 0.0005% or more.

[Zr: 0.100% or less, Te: 0.100% or less]
If the Zr and Te contents are each 0.100% or less, large amounts of coarse precipitates and inclusions are not generated, and they do not become the starting points of delayed fracture. Therefore, the shear angle range is not reduced. When Zr and Te are contained, their contents are preferably 0.100% or less. The contents of Zr and Te are more preferably 0.080% or less. The lower limits of the contents of Zr and Te are However, Zr and Te are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. Therefore, the content of Zr and Te is set to 0.001% or more. It is preferable to set the above.

[Hf: 0.10% or less]
If the Hf content is 0.10% or less, large precipitates and inclusions are not generated in large quantities, and they do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. In some cases, the Hf content is set to 0.10% or less. The Hf content is more preferably set to 0.08% or less. There is no particular lower limit for the Hf content. However, Hf is an element that makes the shape of nitrides and sulfides spheroidal and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more.

[Bi: 0.200% or less]
If the Bi content is 0.200% or less, large precipitates and inclusions are not generated in large quantities, and they do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. In some cases, the Bi content is set to 0.200% or less. The Bi content is more preferably set to 0.100% or less. There is no particular lower limit for the Bi content. However, Bi is Bi is an element that reduces segregation. Therefore, the Bi content is preferably 0.001% or more.

The elements other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr and Cs. These unavoidable impurities are permitted to be contained if their total amount is 0.100% or less.

Next, the structure of a steel sheet according to one embodiment of the present invention will be described.
The structure of the steel plate according to one embodiment of the present invention is
Area fraction of tempered martensite: 83% or more,
Area fraction of retained austenite: less than 3%;
Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%;
and
This is a structure in which the occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite is 20% or more.
The reasons for each of the limitations will be explained below. The area fraction of each phase is the area ratio that each phase occupies with respect to the entire structure.

[Area fraction of tempered martensite: 83% or more]
In the steel sheet according to one embodiment of the present invention, it is extremely important that the area fraction of tempered martensite is 83% or more. That is, by making tempered martensite the main phase, and in particular by making its area fraction 83% or more, it is possible to realize TS: 1180 MPa or more. Therefore, the area fraction of tempered martensite is 83% or more. The area fraction of tempered martensite is preferably 85% or more, more preferably 87% or more. There is no particular upper limit for the area fraction of tempered martensite. The area fraction of tempered martensite is, for example, preferably less than 95%, more preferably 94% or less, and even more preferably 93% or less.

[Area fraction of retained austenite: less than 3%]
In the steel sheet according to one embodiment of the present invention, it is extremely important that the area fraction of the retained austenite is less than 3%. That is, if the area fraction of the retained austenite is 3% or more, it becomes difficult to realize an excellent shear angle wide range. One of the causes of the deterioration of the shear angle wide range is that the retained austenite is transformed into high-hardness martensite during shear processing due to processing-induced martensite transformation, which becomes the starting point of fracture. Therefore, the area fraction of the retained austenite is less than 3%. The area fraction of the retained austenite is preferably 1% or less. The lower limit of the area fraction of the retained austenite is not particularly specified. The area fraction of the retained austenite may be 0%.

[Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%]
In the steel sheet according to one embodiment of the present invention, it is extremely important that the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. That is, when the total area fraction of ferrite and bainitic ferrite is 15% or more, it is difficult to realize a TS of 1180 MPa or more. On the other hand, when the total area fraction of ferrite and bainitic ferrite is less than 5%, it is difficult to realize excellent dimensional accuracy. Therefore, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. The total area fraction of ferrite and bainitic ferrite is preferably 6% or more, more preferably 7% or more. In addition, the total area fraction of ferrite and bainitic ferrite is preferably 14% or less, more preferably 13% or less. In addition, ferrite and bainitic ferrite may be contained alone, or both of them may be contained.

It is preferable that the area fraction of the remaining structure other than the above is 5% or less. Examples of the remaining structure include pearlite, fresh martensite, and acicular ferrite. These remaining structures may be included as long as they are 5% or less and do not affect the properties. The area fraction of the remaining structure may be 0%.

Here, the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite are measured, for example, as follows.
That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished. Next, the observation surface of the sample is corroded with 1 vol. % nital to reveal the structure. Next, the 1/4 plate thickness position of the steel plate (the position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed by SEM at a magnification of 2000 times in 10 fields of view. In addition, in the observation image, the tempered martensite has fine irregularities inside the structure and has carbides inside the structure. In addition, the ferrite and bainitic ferrite are structures in which the inside of the structure is flat at the recesses and does not have carbides inside. Then, the areas of the regions occupied by the tempered martensite, ferrite, and bainitic ferrite are obtained for each field of view. Next, the areas of the regions occupied by tempered martensite, and ferrite and bainitic ferrite are each divided by the area of the entire observation field, and multiplied by 100. Then, the average values are determined as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, respectively.
In addition, the structure of a steel sheet is generally symmetrical in the thickness direction, so any one of the surfaces (front surface and back surface) of the steel sheet may be set as the starting point of the thickness position (0 thickness position), such as a 1/4 thickness position or a position at a depth of 100 μm from the steel sheet surface.

The area fraction of retained austenite is measured, for example, as follows.
That is, the steel plate is mechanically ground to a depth of 1/4-0.1 mm of the plate thickness so that the observation position is the 1/4 position of the plate thickness of the steel plate, and then further polished by 0.1 mm by chemical polishing. The polished surface is used as the observation surface, and the ratio of the integrated intensity of the diffraction peaks of the {200}, {211}, and {220} planes of fcc iron (austenite) to the integrated intensity of the diffraction peaks of the {200}, {220}, and {311} planes of bcc iron is obtained using CoKα radiation by an X-ray diffractometer. Next, the volume fraction of the retained austenite is calculated from the ratio of the integrated intensities of the respective planes. Then, the retained austenite is considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

Furthermore, the area fraction of the remaining structure is determined by subtracting the area fraction of tempered martensite, the total area fraction of ferrite and bainitic ferrite, and the area fraction of retained austenite determined as described above from 100%.
[Area fraction of remaining structure (%)] = 100 - [Area fraction of tempered martensite (%)] - [Total area fraction of ferrite and bainitic ferrite (%)] - [Area fraction of retained austenite (%)]

[Occupation rate of prior austenite grain boundaries by ferrite and bainitic ferrite (hereinafter also referred to as prior γ grain boundary occupation rate): 20% or more]
In the steel sheet according to one embodiment of the present invention, it is extremely important to set the prior γ grain boundary occupancy rate to 20% or more in order to realize an excellent shear angle wide range. Prior austenite grain (hereinafter also referred to as prior γ grain) boundaries are the starting points of delayed fracture. Here, it is important that the prior γ grain boundaries are occupied by soft ferrite and bainitic ferrite, and in particular, that the prior γ grain boundary occupancy rate is set to 20% or more. This makes it possible to minimize the influence of the shear angle during shear processing and suppress the occurrence of delayed fracture, thereby realizing an excellent shear angle wide range. Therefore, the prior γ grain boundary occupancy rate is set to 20% or more. The prior γ grain boundary occupancy rate is preferably 22% or more, more preferably 24% or more. The upper limit of the prior γ grain boundary occupancy rate is not particularly specified. The prior γ grain boundary occupancy rate may be 100%.

Here, the prior γ grain boundary occupancy rate is determined, for example, as follows (see FIG. 1).
For one prior γ grain (hereinafter also referred to as the prior γ grain) identified in the observation image in measuring the above-mentioned area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, the total length of the prior γ grain (the perimeter of the prior γ grain, which is the sum of the solid line (prior austenite grain boundary not occupied by ferrite and bainitic ferrite) and the dotted line in Fig. 1, hereinafter also referred to as L _T ) and the length of the interface between the prior γ grain and the ferrite and bainitic ferrite in contact with the prior γ grain (the total prior γ grain boundary length of the dotted lines in Fig. 1, hereinafter also referred to as L _F ) are measured. Then, the prior γ grain boundary occupancy rate of the prior γ grain is calculated using the following formula.
Prior γ grain boundary occupancy rate (%) of the prior γ grain = (L _F /L _T ) x 100
This measurement is performed on 30 prior γ grains in order from the closest to the prior γ grain, and the average of the prior γ grain boundary occupancy ratios measured for each prior γ grain is regarded as the prior γ grain boundary occupancy ratio of the steel sheet to be measured.

L _T and L _F are measured, for example, as follows.
A sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Then, the observation surface of the sample is polished. Then, the observation surface of the sample is corroded with 1 vol. % nital to reveal the structure. Then, the structure is observed at a 1/4 position of the plate thickness of the steel plate (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) with a SEM at a magnification of 2000 times. From the obtained structure image, L _T and L _F are measured using the object function of Adobe Illustrator.

The mechanical properties of the steel plate according to one embodiment of the present invention are as described above.

The steel sheet according to one embodiment of the present invention may have a plating layer on the surface. The plating layer may be provided on only one surface of the steel sheet, or on both surfaces. The plating layer is not particularly limited. An example of the plating layer is a zinc plating layer mainly composed of Zn (Zn content is 50.0 mass% or more). Examples of the zinc plating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer. The steel sheet having a zinc plating layer can also be called a zinc-plated steel sheet. The above-mentioned steel sheets having the hot-dip galvanized layer, alloyed hot-dip galvanized layer, and electrogalvanized layer can also be called hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), and electrogalvanized steel sheet (EG), respectively.

Examples of plating layers other than zinc plating layers include aluminum plating layers and alloy plating layers. Examples of alloy plating layers include hot-dip zinc-aluminum-magnesium alloy plating layers and Zn-Ni electric alloy plating layers.

In addition, the plating weight per side of the plating layer is not particularly limited, but is preferably 20 g/m ² or more and 80 g/m ² or less.

The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.50 mm or more and 2.50 mm or less.

[2] Members Next, members according to one embodiment of the present invention will be described.
A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member.
Here, the above steel sheet has a TS of 1180 MPa or more, and is also excellent in dimensional accuracy and shear angle range. Therefore, the member according to one embodiment of the present invention is particularly suitable for use as a material for automobile parts, for example. This allows for improved fuel efficiency by reducing the weight of the automobile body, which contributes greatly to reducing _CO2 emissions.

[3] Manufacturing Method of Steel Sheet Next, a manufacturing method of a steel sheet according to one embodiment of the present invention will be described.

A method for producing a steel sheet according to one embodiment of the present invention includes the steps of:
A preparation step of preparing a base steel sheet having the above-mentioned composition;
Next, the base steel sheet is
Average heating rate in the temperature range of 700°C to 750°C: 5.0°C/s or less;
A heating step in which the material is heated under the condition of a maximum temperature T1 of 800° C. or higher and 900° C. or lower;
Next, the base steel sheet is
A first cooling step in which the average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2 is 0.10° C./s or more and 5.00° C./s or less;
Next, the base steel sheet is
Intermediate holding temperature T2: 600°C or more and 750°C or less,
Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less,
An intermediate holding step of holding the steel sheet at a tension of 5 MPa or more;
Next, the base steel sheet is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
A second cooling step of cooling the mixture to a second cooling end temperature under the conditions of
Next, the base steel sheet is
Tempering temperature T3: 100°C or more and 400°C or less,
A tempering process in which tempering is performed under the condition of a tempering time t3 of 10 seconds or more and 10,000 seconds or less;
It has the following characteristics.
Unless otherwise specified, the above temperatures are the surface temperatures of the steel sheet. Furthermore, unless otherwise specified, the average heating rate and the average cooling rate are also based on the surface temperatures of the steel sheet.

Preparation step: First, a base steel sheet having the above-mentioned composition is prepared. For example, the base steel sheet can be prepared by hot rolling a steel slab to obtain a hot-rolled steel sheet, optionally pickling and heat treating the hot-rolled steel sheet, and then cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet. The conditions for these steps are not particularly limited, and may be in accordance with conventional methods.

For example, any known melting method such as converter or electric furnace is suitable for melting steel slabs (steel material). In order to prevent macrosegregation, it is preferable to melt steel slabs using a continuous casting method.

Examples of hot rolling include a method of rolling a steel slab after heating, a method of directly rolling a steel slab after continuous casting without heating, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment. In addition, the slab heating temperature, slab soaking holding time, and coiling temperature in hot rolling are not particularly limited. The slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab soaking holding time is preferably 30 min or more. The slab soaking holding time is preferably 250 min or less. The finish rolling temperature is preferably the _Ar3 transformation point or higher. The coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower. The _Ar3 transformation point is calculated by the following formula.
Ar ₃ transformation point (°C) = 868-396×[%C]+24.6×[%Si]-68.1×[%Mn]-36.1×[%Ni]-20.7×[%Cu]-24.8×[%Cr]
In the above formula, the symbol [% element symbol] represents the content (mass %) of the corresponding element in the above composition.

Pickling can remove oxides from the surface of the hot-rolled steel sheet, and is preferably performed to ensure good chemical conversion treatability and plating quality in the final steel sheet product. Pickling may be performed once or multiple times. The hot-rolled steel sheet after pickling may be subjected to heat treatment.

The total reduction ratio in cold rolling is preferably 30% or more. The total reduction ratio in cold rolling is preferably 80% or less. The number of rolling passes and the reduction ratio of each pass are not particularly limited, and the desired effect can be obtained.

Heating Step Next, the steel plate prepared in the preparation step is heated to a maximum temperature T1 at an average heating rate of 5.0°C/s or less in a temperature range of 700°C to 750°C.

[Average heating rate in the temperature range of 700°C to 750°C: 5.0°C/s or less]
After extensive research, the inventors found that the average heating rate in the temperature range of 700°C to 750°C (hereinafter also simply referred to as the average heating rate) affects the prior γ grain boundary occupancy rate. That is, by setting the average heating rate to 5.0°C/s or less, the dissolution of carbides is promoted. This refines the prior γ grains, contributing to an increase in the prior γ grain boundary occupancy rate. As a result, the shear angle range is improved. Therefore, the average heating rate is set to 5.0°C/s or less. The average heating rate is preferably 3.0°C/s or less. There is no particular upper limit to the average heating rate. For example, the average heating rate is preferably 0.1°C/s or more.

[Maximum temperature T1: 800°C or more and 900°C or less]
When the maximum reaching temperature T1 is less than 800° C., the total area fraction of ferrite and bainitic ferrite becomes 15% or more, and it becomes difficult to realize a TS of 1180 MPa or more. If the maximum temperature T1 exceeds 800° C., the total area fraction of ferrite and bainitic ferrite will be less than 5%, making it difficult to achieve excellent dimensional accuracy of the parts. The maximum attainable temperature T1 is preferably 810° C. or higher. The maximum attainable temperature T1 is preferably 890° C. or lower.

After the maximum temperature T1 is reached, the process may immediately proceed to the cooling process described below, or the maximum temperature T1 may be held for a certain period of time, for example, 1.0 to 5.0 seconds, before proceeding to the cooling process described below.

First Cooling Step Next, the base steel sheet is cooled under the condition of an average cooling rate of 0.10° C./s or more and 5.00° C./s or less in the temperature range from the maximum attainable temperature T1 to the intermediate holding temperature T2.

[Average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2 (hereinafter also referred to as the first average cooling rate): 0.10° C./s or more and 5.00° C./s or less]
As a result of intensive research by the inventors, it was found that the first average cooling rate affects the prior γ grain boundary occupancy rate. That is, by setting the first average cooling rate to 5.00°C/s or less, the nucleation of ferrite from the prior γ grain boundary is promoted, which contributes to an increase in the prior γ grain boundary occupancy rate. As a result, the shear angle wideness is improved. On the other hand, when the first average cooling rate is less than 0.10°C/s, the total area fraction of ferrite and bainitic ferrite becomes 15% or more, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the first average cooling rate is set to 0.10°C/s or more and 5.00°C/s or less. The first average cooling rate is preferably 0.20°C/s or more. The first average cooling rate is preferably 3.00°C/s or less.

The first cooling end temperature may be 600° C. or higher and 750° C. or lower. For example, the first cooling end temperature may be the intermediate holding temperature T2.

- Intermediate holding process Next, the base steel plate is
Intermediate holding temperature T2: 600°C or more and 750°C or less,
Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less,
Tension applied to material steel plate: maintained at 5 MPa or more.

[Intermediate holding temperature T2: 600°C or higher and 750°C or lower]
If the intermediate holding temperature T2 is less than 600°C, the transformation of ferrite and bainitic ferrite from locations other than the prior γ grain boundaries may be promoted. Therefore, it becomes difficult to achieve a prior γ grain boundary occupancy rate of 20% or more. On the other hand, when the intermediate holding temperature T2 exceeds 750° C., the total area fraction of ferrite and bainitic ferrite is less than 5%, and it is difficult to obtain an excellent part. It becomes difficult to achieve dimensional accuracy. Therefore, the intermediate holding temperature T2 is set to 600° C. or higher and 750° C. or lower. The intermediate holding temperature T2 is preferably 610° C. or higher. The intermediate holding temperature T2 is preferably 740° C. The intermediate holding temperature is a holding temperature in the intermediate holding step. The intermediate holding temperature may be constant during holding. The intermediate holding temperature is 600° C. or higher. As long as the temperature is within a range of 750° C. or less and the temperature fluctuation is within ±10° C. of the set temperature, the temperature does not have to be constant during the holding period.

[Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less]
When the intermediate holding time t2 is less than 1.0 second (including the case where no intermediate holding is performed), the prior γ grain boundary occupancy rate becomes less than 20%, and an excellent shear angle wide range cannot be realized. If the intermediate holding time t2 exceeds 2000.0 seconds, the total area fraction of ferrite and bainitic ferrite becomes 15% or more, making it difficult to achieve a TS of 1180 MPa or more. The intermediate holding time t2 is set to 1.0 seconds or more and 2000.0 seconds or less. The intermediate holding time t2 is preferably 10.0 seconds or more. The intermediate holding time t2 is preferably 1500.0 seconds or less. The holding time t2 is the holding time at the intermediate holding temperature T2.

[Tension applied to material steel plate: 5 MPa or more]
As a result of intensive research, the inventors have found that applying tension to the material steel sheet during intermediate holding affects the prior γ grain boundary occupancy rate. In this case, by applying a tension to the material steel sheet (hereinafter also simply referred to as the applied tension) of 5 MPa or more, the nucleation of ferrite from the prior γ grain boundaries is promoted, which contributes to an increase in the prior γ grain boundary occupancy rate. This makes it possible to make the prior γ grain boundary occupancy rate 20% or more, and to realize an excellent shear angle range. Therefore, the applied tension is set to 5 MPa or more. The applied tension is preferably 10 MPa or more. The upper limit of the applied tension is not particularly specified. The applied tension is preferably, for example, 100 MPa or less.

In addition, between the intermediate holding step and the second cooling step described below, the base steel sheet may be subjected to a plating treatment. The plating treatment is described below.

Second cooling process: Next, the base steel plate is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
The mixture is cooled to the second cooling end temperature under the above condition.

[Average cooling rate in the temperature range of 300° C. to 100° C. (hereinafter also referred to as second average cooling rate): 300° C./s or more]
If the second average cooling rate is less than 300° C./s, the area fraction of the retained austenite becomes 3% or more, and it becomes difficult to realize an excellent shear angle wide range. Therefore, the second average cooling rate is set to 300° C./s or more. The second average cooling rate is preferably 800° C./s or more. The upper limit of the second average cooling rate is not particularly specified. For example, the second average cooling rate is preferably 2000° C./s or less.

The second cooling end temperature may be, for example, less than 100°C. The second cooling end temperature may be, for example, about room temperature.

Tempering Step Next, the base steel sheet is tempered under the conditions of a tempering temperature T3: 100° C. or more and 400° C. or less, and a tempering time t3: 10 seconds or more and 10,000 seconds or less.

[Tempering temperature T3: 100°C or more and 400°C or less]
Tempered martensite is produced by tempering martensite through a tempering process. If the tempering temperature T3 is less than 100° C., the martensite is not sufficiently tempered, and the steel remains mainly composed of as-quenched martensite. In such a structure mainly composed of martensite as quenched, an excellent shear angle range cannot be obtained. On the other hand, when the tempering temperature T3 exceeds 400°C, the tempering of martensite proceeds excessively, It becomes difficult to realize a TS of 1180 MPa or more. Therefore, the tempering temperature T3 is set to 100° C. or more and 400° C. or less. The tempering temperature T3 is preferably set to 150° C. or more. The tempering temperature T3 is preferably set to The tempering temperature is 350° C. or less. The tempering temperature here is the holding temperature in the tempering process. The tempering temperature may be constant during holding. The tempering temperature is 100° C. or more. As long as the temperature is within a range of 400° C. or less and the temperature fluctuation is within ±10° C. of the set temperature, the temperature does not have to be constant during the holding period.

[Tempering time t3: 10 seconds or more and 10,000 seconds or less]
As described above, tempered martensite is produced by tempering martensite through a tempering process. If the tempering time t3 is less than 10 seconds, the martensite is not sufficiently tempered, and the quenching In such a structure mainly composed of martensite as quenched, excellent shear angle range cannot be obtained. On the other hand, when the tempering time t3 exceeds 10,000 seconds, the tempering of martensite is If the tempering process progresses excessively, it becomes difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering time t3 is set to 10 seconds or more and 10,000 seconds or less. The tempering time t3 is preferably set to 50 seconds or more. The time t3 is preferably 5000 seconds or less. Note that the tempering time t3 here refers to the holding time at the tempering temperature T3.

There are no particular regulations regarding the cooling after tempering. For example, cooling may be performed by any method according to conventional methods. The end temperature of cooling after tempering may be, for example, about room temperature.

After the tempering process, the steel sheet may be processed under conditions that result in an equivalent plastic strain of 0.10% to 5.00%. After the processing, the steel sheet may be reheated at a temperature of 100°C to 400°C.

Furthermore, after the tempering process, the base steel sheet may be subjected to a plating process, as described below.

- Plating process The steel sheet may be optionally plated. The plating process is not particularly limited. Examples of the plating process include galvanizing processes such as hot-dip galvanizing, alloyed hot-dip galvanizing, and electric galvanizing. Examples of plating processes other than galvanizing include aluminum plating and alloy plating. Examples of alloy plating processes include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electric alloy plating. All of the treatment conditions may be in accordance with conventional methods. As described above, the plating process is preferably performed between the intermediate holding step and the second cooling step, or after the tempering step. For example, the hot-dip galvanizing process and the alloyed hot-dip galvanizing process are preferably performed between the intermediate holding step and the second cooling step. Moreover, the electric galvanizing process and the Zn-Ni electric alloy plating process are preferably performed after the tempering step.

In the case of hot-dip galvanizing and alloyed hot-dip galvanizing, from the viewpoint of productivity, it is preferable to carry out a series of processes such as the heating process and the plating process in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping is possible to adjust the coating weight.

In addition, after the plating process, the base steel sheet may be processed under conditions that result in an equivalent plastic strain of 0.10% to 5.00%. Furthermore, after the processing, the base steel sheet (plated steel sheet) may be reheated under conditions of 100°C to 400°C.

Conditions other than those mentioned above are not particularly limited and may be made in accordance with conventional methods. According to the method for manufacturing steel sheet according to one embodiment of the present invention described above, a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range can be obtained. The obtained steel sheet can be suitably used, for example, as a material for automobile parts. When the steel sheet is to be traded, it is usually cooled to room temperature before being traded.

[4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described.
A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component.
Here, the molding method is not particularly limited, and for example, a general processing method such as press molding can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.

A steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The steel slab was then heated. The steel slab was then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then pickled. The hot-rolled steel sheet was then cold-rolled to obtain a cold-rolled steel sheet. In this manner, a base steel sheet was prepared. The base steel sheet thus prepared was then subjected to a heating process, a first cooling process, an intermediate holding process, a second cooling process, and a tempering process under the conditions shown in Table 2, to obtain a steel sheet (thickness: 0.6 to 2.2 mm) that was to become the final product. In addition, some of the steel sheets (those in the type column in Table 2, which are GI, GA, and EG) were subjected to a plating process. Of these, those in the type column in Table 2, which are GI and GA, were subjected to a plating process between the intermediate holding process and the second cooling process. In addition, for those in the type column of Table 2 that are marked as EG, plating was performed after the tempering process. Conditions not specified were those according to conventional methods.

Using the steel sheets thus obtained, the area fraction of tempered martensite, the area fraction of retained austenite, the total area fraction of ferrite and bainitic ferrite, and the prior γ grain boundary occupancy rate were determined in the manner described above. The results are shown in Table 3.

In addition, each evaluation was performed according to the following procedures. The evaluation results are shown in Table 3.

(TS Evaluation)
From the obtained steel plate, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction of the test piece. Next, a tensile test according to JIS Z 2241:2022 was performed using the taken test piece, and TS and YS were measured. The crosshead speed was set to 1.67×10 ⁻¹ mm/sec. Then, TS was evaluated according to the following criteria.
Good (passed, excellent): TS is 1180 MPa or more. Poor (failed): TS is less than 1180 MPa.

(Evaluation of dimensional accuracy)
From the TS and YS measured in the above evaluation of TS, YR was calculated according to the following formula.
YR=100×YS/TS
The dimensional accuracy was evaluated according to the following criteria.
Good (pass, excellent): YR is 65% or more and 85% or less. Poor (fail): YR is less than 65% or YR is more than 85%.

(Evaluation of shear angle range)
The obtained steel plate was sheared to a size of 16 mm x 75 mm with the direction perpendicular to the rolling direction as the longitudinal direction to prepare a test piece. The clearance during shearing was 15% in all cases. The shear angle during shearing was changed in a range of 0° to 2.0° at 0.25° intervals. Next, a four-point bending test was performed according to ASTM (G39-99), and a stress of 1000 MPa was applied to the bending apex of the test piece. Next, while the stress was applied, the test piece was immersed in hydrochloric acid at 25°C and pH: 3 for 100 hours. After immersion, the presence or absence of cracks was visually confirmed for each test piece. Then, the shear angle range was evaluated according to the following criteria.
Excellent (passed, particularly excellent): the appropriate range of shear angles to prevent delayed fracture is 0° to 1.0° or more Good (passed, excellent): the appropriate range of shear angles to prevent delayed fracture is 0° to 0.5° or more and less than 1.0° Poor (failed): the appropriate range of shear angles to prevent delayed fracture is the range of shear angles in which no cracks were observed in the test specimens in the above test. For example, if no cracks were observed in any of the test specimens prepared with a shear angle of 0° to 0.75° during shearing, but cracks were observed in the test specimens prepared with a shear angle of 1.00° or more during shearing, the appropriate range of shear angles to prevent delayed fracture would be "0° to 0.75°", and the test specimens would be evaluated as "good (passed, excellent)". Furthermore, when no cracks were observed in any of the test pieces prepared with a shear angle of 0° to 0.25° during shearing, but cracks were observed in the test pieces prepared with a shear angle of 0.50° or more during shearing, the appropriate range of shear angles for delayed fracture becomes "0° to 0.25°", and the test piece is evaluated as "poor (failed)".

As shown in Table 3, all of the inventive examples passed the test for TS, dimensional accuracy, and shear angle range. In addition, all of the members obtained by forming or joining using the steel plates of the inventive examples had the desired shape without any cracks. Furthermore, in these members, no delayed fracture occurred even when the shear angle of the shear process was changed. The dimensional accuracy was also good.
On the other hand, in the comparative example, at least one of TS, dimensional accuracy, and shear angle range was unacceptable.

Claims (8)

In mass percent,
C: 0.030% or more and 0.500% or less,
Si: 0.010% or more and 2.500% or less,
Mn: 0.10% or more and 5.00% or less,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.0100% or less,
A composition comprising O: 0.0100% or less and Al: 1.000% or less, with the balance being Fe and unavoidable impurities;
Area fraction of tempered martensite: 83% or more,
Area fraction of retained austenite: less than 3%;
Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%;
and
and a structure in which an occupancy rate of prior austenite grain boundaries by the ferrite and the bainitic ferrite is 20% or more.
Here, the occupancy rate of the prior austenite grain boundary by the ferrite and the bainitic ferrite is an average value of the occupancy rates of the prior austenite grain boundary by ferrite and bainitic ferrite in prior austenite grains calculated by the following formula.
Occupancy rate (%) of prior austenite grain boundaries by ferrite and bainitic ferrite in prior austenite grains = (L _F /L _T ) x 100
In the formula, L _T is the total length of the prior austenite grain, and L _F is the length of the interface between the prior austenite grain and the ferrite and bainitic ferrite adjacent to the prior austenite grain. The composition further comprises, 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,
The steel sheet according to claim 1, containing at least one selected from Hf: 0.10% or less and Bi: 0.200% or less. The steel sheet according to claim 1, having a plating layer on its surface. The steel sheet according to claim 2, having a plating layer on its surface. A member made using the steel plate according to any one of claims 1 to 4. A method for producing a steel sheet according to any one of claims 1 to 4, comprising the steps of:
A preparation step of preparing a base steel sheet having the component composition according to claim 1 or 2;
Next, the base steel sheet is
Average heating rate in the temperature range of 700°C to 750°C: 5.0°C/s or less;
A heating step in which the material is heated under the condition of a maximum temperature T1 of 800° C. or higher and 900° C. or lower;
Next, the base steel sheet is
A first cooling step in which the average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2 is 0.10° C./s or more and 5.00° C./s or less;
Next, the base steel sheet is
Intermediate holding temperature T2: 600°C or more and 750°C or less,
Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less,
An intermediate holding step of holding the steel sheet at a tension of 5 MPa or more;
Next, the base steel sheet is
Average cooling rate in the temperature range of 300°C to 100°C: 300°C/s or more;
A second cooling step of cooling the mixture to a second cooling end temperature under the conditions of
Next, the base steel sheet is
Tempering temperature T3: 100°C or more and 400°C or less,
A tempering process in which tempering is performed under the condition of a tempering time t3 of 10 seconds or more and 10,000 seconds or less;
A manufacturing method for steel plates that provides the following: The method for manufacturing steel sheet according to claim 6, further comprising a plating process for plating the base steel sheet between the intermediate holding process and the second cooling process or after the tempering process. A method for manufacturing a component, comprising the steps of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining processes to produce a component.

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