JP7759017B2 - Steel plates and parts - Google Patents

Description

The present disclosure relates to a steel plate and a part, and more particularly to a steel plate having high strength, excellent ductility and hole expandability, and suppressing the occurrence of unevenness in bending, and a part manufactured using the same.
This application claims priority based on Japanese Patent Application No. 2023-171497, filed on October 2, 2023, the contents of which are incorporated herein by reference.

In recent years, efforts have been made to reduce the weight of automobile and machine parts. Designing parts with optimal shapes to ensure rigidity makes it possible to reduce the weight of automobile and machine parts. Furthermore, for blank-formed parts such as press-formed parts, weight can be reduced by reducing the thickness of the part material.

However, if one attempts to maintain the strength properties of parts, such as static fracture strength and yield strength, while reducing plate thickness, it becomes necessary to use high-strength materials. In particular, the use of higher-strength steel plates is beginning to be considered for automotive suspension parts such as lower arms, trail links, and knuckles. These automotive suspension parts are manufactured by subjecting steel plates to burring, stretch flange forming, bending, and other processes. Therefore, steel plates used for these automotive suspension parts are required to have excellent formability, particularly ductility and hole expandability.

In addition, when manufacturing the above-mentioned automotive suspension parts, if high-strength steel plates are used, fine cracks (irregularities) are more likely to occur within the bend during forming. If fine cracks occur within the bend, the crash resistance characteristics of the part will deteriorate.

For example, Patent Document 1 discloses a high-strength steel sheet that contains a martensite phase at an area ratio of 95% or more, has a structure in which the prior austenite grains have an average aspect ratio of 3.0 or more, has a 5-minute relaxation stress value of 20 MPa or less when subjected to 400 MPa in a stress relaxation test, and has a tensile strength of 1180 MPa or more. Patent Document 1 also discloses that the above configuration enables the production of a high-strength steel sheet with excellent delayed fracture resistance that has a high strength (TS: 1180 MPa or more) while also significantly improving delayed fracture resistance, making it suitable as a material for automotive parts.

Patent Document 2 discloses a method for manufacturing a high-strength steel sheet characterized by an average aspect ratio of prior austenite grains of 1.3 to 5.0 and an area ratio of bainite phase of 80% or more. Patent Document 2 also discloses that the above configuration results in a high-strength steel sheet with excellent hole expandability.

Patent document 3 discloses a steel plate with excellent fatigue crack growth resistance, which contains 1 to 60% acicular ferrite by area fraction and has a number ratio of 80% or more of acicular ferrite particles with a major axis in the range of 5 to 100 μm.

International Publication No. 2021/193310 Japanese Patent Application Publication No. 2021-116476 Japanese Patent Application Publication No. 2007-321220

However, Patent Documents 1 to 3 do not take into consideration the occurrence of unevenness within the bend.

This disclosure has been made in consideration of the above-mentioned problems, and aims to provide a steel sheet and a part made therefrom that have high strength, excellent ductility and hole expandability, and that suppress the occurrence of unevenness within the bend during forming.

The inventors have discovered that by controlling the aspect ratio distribution of prior austenite grains, it is possible to suppress the occurrence of irregularities within the bent section during forming, while ensuring high strength, as well as excellent ductility and hole expandability. The inventors have also discovered that suppressing the occurrence of irregularities within the bent section during forming can improve the crash resistance properties of the steel sheet.

The gist of the present disclosure, which was made based on the above findings, is as follows.
(1) Chemical composition, in mass%,
C: 0.045-0.120%,
Si: 0-3.00%,
Mn: 1.20-3.00%,
sol. Al: 0.001 to 0.500%,
P: 0 to 0.0800%,
S: 0 to 0.0100%,
N: 0 to 0.0050%,
O: 0 to 0.0100%,
Ti: 0 to 0.180%,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Cu: 0 to 1.000%,
Cr: 0-2.000%,
Mo: 0-3.000%,
Ni: 0 to 0.500%,
B: 0 to 0.0100%,
Ca: 0-0.0500%,
Mg: 0 to 0.050%,
REM: 0-0.100%,
Bi: 0-0.100%,
Ta: 0-0.100%,
Zr: 0 to 0.500%,
Co: 0-3.000%,
Zn: 0-0.200%,
W: 0-0.200%,
Sb: 0 to 0.500%,
As: 0 to 0.050% and Sn: 0 to 0.050%;
the balance being Fe and impurities;
In the metal structure at 1/4 of the plate thickness from the surface, in area%,
Prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 are less than 20%;
Prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40% or more,
Prior austenite grains having an aspect ratio of 6.0 or more are less than 30%;
Bainite is 70 to 95%;
A steel plate characterized in that martensite is 5 to 30%.
(2) The chemical composition is, in mass%,
Ti: 0.001 to 0.180%,
Nb: 0.001 to 0.100%,
V: 0.001 to 1.000%,
Cu: 0.001 to 1.000%,
Cr: 0.001-2.000%,
Mo: 0.001-3.000%,
Ni: 0.001 to 0.500%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0500%,
Mg: 0.001-0.050%,
REM: 0.001-0.100%,
Bi: 0.001-0.100%,
Ta: 0.001 to 0.100%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 3.000%,
Zn: 0.001-0.200%,
W: 0.001-0.200%,
Sb: 0.001 to 0.500%,
The steel sheet according to (1), characterized in that it contains one or more selected from the group consisting of As: 0.001 to 0.050% and Sn: 0.001 to 0.050%.
(3) In the metal structure at a 1/4 position of the plate thickness from the surface, in area%,
Prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more,
The steel sheet according to (1) or (2), characterized in that prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10% or more.
(4) In the metal structure at a position of 1/4 of the plate thickness from the surface,
The steel sheet according to any one of (1) to (3), characterized in that the maximum value of the length of the major axis of the prior austenite grains is 200 μm or less.
(5) A part comprising the steel sheet according to any one of (1) to (4).

The above aspects of the present disclosure make it possible to provide steel sheets and parts using the same that have high strength, excellent ductility and hole expandability, and that suppress the occurrence of unevenness within the bend during forming.

The following describes a steel plate and a part according to one embodiment of the present disclosure (hereinafter, sometimes referred to as the steel plate and part according to this embodiment). However, this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the spirit of this disclosure.

The individual constituent elements of the present disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to the present embodiment will be described.
Below, numerical ranges defined by "to" include the lower and upper limits. Numerical values indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

The chemical composition of the steel sheet according to this embodiment is, in mass %, C: 0.045-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, sol. Al: 0.001-0.500%, P: 0-0.0800%, S: 0-0.0100%, N: 0-0.0050%, O: 0-0.0100%, and the balance: Fe and impurities. Each element is described in detail below.

C: 0.045-0.120%
C is an important element for improving the strength of the steel sheet. To obtain the desired strength, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, 0.55% or more, or 0.60% or more.
On the other hand, if the C content exceeds 0.120%, the hole expandability of the steel sheet deteriorates. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.110% or less, more preferably 0.100% or less.

Si: 0-3.00%
Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the steel sheet. Since Si may not be contained, the Si content may be 0%. In order to reliably obtain the above effect, the Si content is preferably 0.10% or more. The Si content is more preferably 0.20% or more, or 0.50% or more.
On the other hand, if the Si content exceeds 3.00%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, 2.00% or less, or 1.50% or less.

Mn: 1.20-3.00%
Mn is an element effective in improving the hardenability and strength of the steel sheet by solid solution strengthening. To obtain the desired strength, the Mn content is set to 1.20% or more. The Mn content is preferably 1.30% or more, 1.50% or more, or 1.70% or more.
On the other hand, if the Mn content exceeds 3.00%, the bainite transformation is delayed, making it difficult to obtain the desired amount of bainite. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or 2.30% or less.

Sol. Al: 0.001 to 0.500%
Al has the effect of improving the quality of steel by deoxidation and also has the effect of controlling ferrite transformation. To obtain these effects, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more, more preferably 0.020% or more.
On the other hand, if the Al content exceeds 0.500%, ferrite tends to be excessively formed, resulting in a decrease in the strength of the hot-rolled steel sheet. Therefore, the Al content is set to 0.500% or less. The Al content is preferably 0.400% or less, 0.300% or less, or 0.200% or less.

P: 0-0.0800%
P is an element that affects the weldability of steel sheets. In particular, if the P content exceeds 0.0800%, the weldability of the steel sheets deteriorates significantly. Furthermore, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the P content is set to 0.0800% or less. The P content is preferably 0.0600% or less, or 0.0400% or less.
The P content may be 0%. From the viewpoint of refining costs, the P content may be 0.0010% or more.

S: 0~0.0100%
S is an element that affects the ductility and hole expandability of steel sheets. In particular, if the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are harmful to the ductility and hole expandability of steel sheets, are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less.
The S content may be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more.

N: 0 to 0.0050%
N is an element that combines with Ti to form Ti nitrides. In particular, if the N content exceeds 0.0050%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, or 0.0030% or less.
The N content may be 0%. From the viewpoint of refining costs, the N content may be 0.0001% or more.

O: 0 to 0.0100%
O is an element that, when contained in large amounts in steel, forms coarse oxides that become fracture initiation sites, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
Since O may not be contained, the O content may be 0%. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.

The steel sheet according to this embodiment may contain the above chemical components, with the balance being Fe and impurities. In this embodiment, impurities refer to substances mixed in from raw materials such as ore, scrap, or the manufacturing environment, and/or substances that are acceptable to the extent that they do not adversely affect the properties of the steel sheet according to this embodiment, for example, properties related to the problem that this disclosure is intended to solve.

Although not essential for achieving the desired properties, the following optional elements may be added to reduce manufacturing variations and further improve the strength of the steel sheet. However, since the addition of these elements is not essential, the lower limit of the content of these elements is 0%.

Ti: 0.001-0.180%
Ti precipitates in steel as carbides or nitrides, and has the effect of refining the metal structure through a pinning effect and increasing the strength and yield ratio of the steel sheet through precipitation strengthening. To reliably obtain these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.180%, excessive precipitation of TiC deteriorates the ductility and hole expandability of the steel sheet. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, 0.150% or less, or 0.130% or less.

Nb: 0.001-0.100%
Nb has the effect of increasing the strength of the steel sheet by refining the crystal grain size of the steel sheet and by precipitation strengthening of NbC. To reliably obtain this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Nb content exceeds 0.100%, the above effect saturates. Therefore, even when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, more preferably 0.060% or less.

V:0.001~1.000%
V has the effect of increasing the strength of the steel sheet by strengthening through precipitation, strengthening through grain refinement by inhibiting the growth of ferrite crystal grains, and strengthening through dislocations by inhibiting recrystallization. To reliably obtain these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the V content is excessive, a large amount of carbonitrides precipitates, deteriorating the ductility and hole expandability of the steel sheet. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.800% or less, or 0.600% or less.

Cu: 0.001-1.000%
Cu exists in the form of fine particles in steel and has the effect of increasing the strength of the steel sheet. To reliably obtain this effect, the Cu content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Cu content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Cu content is set to 1.000% or less, and preferably 0.800% or less, and 0.600% or less.

Cr:0.001~2.000%
Cr is an element effective in improving the strength of steel sheets. To reliably obtain this effect, the Cr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Cr content is excessive, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.500% or less, 1.200% or less, or 1.000% or less.

Mo: 0.001~3.000%
Mo is an element effective in strengthening ferrite precipitation. To reliably obtain this effect, the Mo content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Mo content is excessive, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Mo content is set to 3.000% or less. The Mo content is preferably 2.500% or less, 2.000% or less, or 1.500% or less.

Ni: 0.001-0.500%
Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, the Ni content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Ni content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Ni content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.150% or less.

B: 0.0001-0.0100%
B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.
On the other hand, if the B content is excessive, B precipitates are formed, reducing the strength of the steel sheet. Therefore, the B content is set to 0.0100% or less, and preferably 0.0080% or less, and 0.0050% or less.

Ca: 0.0001-0.0500%
Ca has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the steel sheet. Ca also fixes S in the steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the hole expandability of the steel sheet. To reliably obtain these effects, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, or 0.0010% or more.
On the other hand, even if the Ca content exceeds 0.0500%, the above effect saturates. Therefore, the Ca content is set to 0.0500% or less, and preferably 0.0300% or less, and 0.0200% or less.

Mg: 0.001-0.050%
Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the steel sheet. To reliably obtain this effect, the Mg content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the steel sheet. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less, more preferably 0.030% or less.

REM: 0.001~0.100%
REM has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in steel to a preferred shape. To reliably obtain this effect, the REM content is preferably 0.001% or more. The REM content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the REM content exceeds 0.100%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the steel sheet. Therefore, the REM content is set to 0.100% or less. The REM content is preferably 0.080% or less, and more preferably 0.060% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.

Bi:0.001~0.100%
Bi has the effect of increasing the yield ratio of the steel sheet by refining the solidification structure. To reliably obtain this effect, the Bi content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Bi content exceeds 0.100%, the effect of the above action saturates, which is economically undesirable. Therefore, the Bi content is set to 0.100% or less. The Bi content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

Ta: 0.001~0.100%
Ta, like V, has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. To reliably obtain this effect, the Ta content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Ta content exceeds 0.100%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less, more preferably 0.050% or less.

Zr: 0.001-0.500%
Zr has the effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the Zr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Zr content exceeds 0.500%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less, more preferably 0.100% or less.

Co:0.001~3.000%
Co has the effect of increasing the strength of the steel sheet through solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Co content exceeds 3.000%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 1.000% or less, and more preferably 0.500% or less.

Zn: 0.001-0.200%
Zn has the effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the Zn content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Zn content exceeds 0.200%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0.150% or less, more preferably 0.100% or less.

W: 0.001-0.200%
W has the effect of increasing the strength of the steel sheet by solid solution strengthening. To reliably obtain this effect, the W content is preferably 0.001% or more, and more preferably 0.005% or 0.010% or more.
On the other hand, if the W content exceeds 0.200%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less, more preferably 0.100% or less.

Sb: 0.001-0.500%
Sb has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of the steel sheet. To reliably obtain this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, even if Sb is contained in a large amount, the above effect saturates, so the Sb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.100% or less.

As: 0.001-0.050%
As has the effect of reducing the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of the steel sheet. To reliably obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more, more preferably 0.010% or more.
On the other hand, since the above effects are saturated even when As is contained in a large amount, the As content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

Sn: 0.001-0.050%
Sn has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of the steel sheet. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, even if Sn is contained in a large amount, the above effect saturates, so the Sn content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

The chemical composition of the steel sheet described above may be analyzed using a spark discharge optical emission spectrometer or the like. Values of C and S are identified by burning the steel sheet in an oxygen stream using a gas component analyzer or the like and measuring by an infrared absorption method. Values of O and N are identified by melting a test piece taken from the steel sheet in a helium stream and measuring by a thermal conductivity method.
When the steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

Next, the metal structure of the steel sheet according to this embodiment will be described.
In the steel plate according to this embodiment, in the metal structure at a position from the surface to 1/4 of the plate thickness, prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for less than 20%, prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40%, prior austenite grains having an aspect ratio of 6.0 or more account for less than 30%, bainite accounts for 70 to 95%, and martensite accounts for 5 to 30%.

In this embodiment, the "position 1/4 of the plate thickness from the surface" refers to a region from 1/8 of the plate thickness depth from the surface to 3/8 of the plate thickness depth from the surface. In other words, it refers to a region starting from 1/8 of the plate thickness depth from the surface and ending at 3/8 of the plate thickness depth from the surface. The reason for specifying the metallographic structure at this position is that the metallographic structure at this position represents a typical metallographic structure of a steel plate.
When the steel sheet has a plating layer, a coating film, or the like on its surface, the surface here refers to the interface between the steel sheet and the plating layer, the coating film, or the like.

Prior austenite grains with an aspect ratio of 1.0 or more and less than 2.0: less than 20 area% Prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0: 40 area% or more Prior austenite grains with an aspect ratio of 6.0 or more: less than 30 area% If the area ratio of prior austenite grains having a specified aspect ratio is outside the above range, it is not possible to suppress unevenness that occurs within the bend during forming. Therefore, the prior austenite grains with an aspect ratio of 1.0 or more and less than 2.0 are set to less than 20 area%, the prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0 are set to 40 area% or more, and the prior austenite grains with an aspect ratio of 6.0 or more are set to less than 30 area%.

The more prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0, the better, so they may be 100% by area. Prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0 may be 95% by area or less, or 90% by area or less.

Prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 preferably account for 15% by area or less, and more preferably 10% by area or less. The fewer prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, the better, and so they may account for 0% by area.
Furthermore, the amount of prior austenite grains having an aspect ratio of 6.0 or more is preferably 25 area % or less, more preferably 20 area % or less, and even more preferably 15 area % or less, more preferably 10 area % or less. Since the number of prior austenite grains having an aspect ratio of 6.0 or more is preferably as small as possible, it may be 0 area %.

Prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0: 30 area % or more Prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0: 10 area % or more It is preferable that the prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40 area % or more, the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30 area % or more, and the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10 area % or more. By accounting for 30 area % or more of the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 and 10 area % or more of the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, the occurrence of irregularities in the bent portion can be further suppressed.

The aspect ratio of a prior austenite grain is the value obtained by dividing the major axis of the prior austenite grain by the minor axis, and is a value of 1.0 or more.

The aspect ratio of the prior austenite grains is measured by the following method.
A sample is taken at a quarter position from the end face in the width direction of the steel plate so that the metal structure of the cross section (thickness direction x rolling direction cross section) normal to the width direction can be observed. The size of the sample depends on the measuring device, but it may be, for example, a rectangular parallelepiped measuring the entire thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface is mirror-polished and then corroded using a saturated aqueous solution of picric acid using the Bechet-Beaujard method in accordance with JIS G 0551:2020. Grains that turn black due to corrosion are identified as prior austenite grains. The observation surface revealing the prior austenite grains is observed using an optical microscope, and eight fields of view, 200 μm in the thickness direction x 600 μm in the rolling direction, are photographed at a magnification of 500x from the surface to a quarter position in the thickness direction (the region from 1/8 of the thickness depth from the surface to 3/8 of the thickness depth from the surface). The aspect ratio of each prior austenite grain is obtained by calculating the value of the major axis/minor axis for each prior austenite grain from the photographed structure. The area ratios of prior austenite grains having each aspect ratio are obtained by calculating the area ratios of prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0, prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, and prior austenite grains having an aspect ratio of 6.0 or more.

When the prior austenite grains cannot be sufficiently revealed by the above-mentioned method, the prior austenite grains are identified by the reconstruction method described in "Study on High-Precision Reconstruction Method of Austenite Structure of Steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the aspect ratio of the prior austenite grains is determined.
The EBSD measurement data used in the reconstruction method is obtained by the following method.
After colloidal polishing or electrolytic polishing is performed on the above-mentioned observation field (field of view of 200 μm in the sheet thickness direction×600 μm in the rolling direction), crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.1 μm.
For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector) is used. At this time, the degree of vacuum inside the device is set to 9.6 × 10 ^-5 Pa or less, the acceleration voltage is 15 kV, and the probe current is 36 nA. Note that after determining the acceleration voltage, the probe current may be adjusted to obtain a clearer pattern.
To display the EBSD map, version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX/TSL Solution is used for the obtained crystal orientation information.

In addition, when prior austenite grains having an equivalent circle diameter of less than 2 μm are contained, these are excluded from the above measurement because prior austenite grains having an equivalent circle diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment.
Furthermore, when only a portion of the prior austenite grains is included in the edge of the structural photograph, the prior austenite grains are excluded from the measurement.
These exclusion conditions also apply to the measurement of the major axis length of prior austenite grains, which will be described later.
The rolling direction of the steel sheet is determined by the following method.
Test specimens are taken so that the thickness cross section of the steel plate can be observed. The direction perpendicular to the plate surface is the Z direction, and a total of 12 test specimens are taken by rotating the plate every 30° around this Z direction. The thickness cross sections of the taken test specimens are polished, and the prior austenite grain boundaries are revealed using the above-mentioned etching solution. The average aspect ratio of the prior austenite grains is calculated using the intercept method. The test specimen with the largest average aspect ratio of the prior austenite grains is identified, and the direction from which the test specimen was taken is determined to be the rolling direction of the steel plate. In other words, the direction parallel to the thickness cross section of the test specimen and perpendicular to the thickness direction is determined to be the rolling direction of the steel plate.

Maximum long axis length of prior austenite grains: 200 μm or less In the metal structure at a position 1/4 of the plate thickness from the surface, the maximum long axis length of prior austenite grains is preferably 200 μm or less. By setting the maximum long axis length of prior austenite grains to 200 μm or less, it is possible to further suppress the occurrence of irregularities within the bent portion during forming.

The maximum value of the long axis length of the prior austenite grains is obtained by measuring the long axis length of each prior austenite grain in the structural photograph taken using the method described above and calculating the maximum value.

Area ratio of bainite: 70 to 95%
Bainite is a structure consisting of fine crystal grains and carbides. If the area fraction of bainite is less than 70%, the desired ductility cannot be obtained in the steel plate. Therefore, the area fraction of bainite is set to 70% or more. The area fraction of bainite is preferably 75% or more, or 80% or more.
On the other hand, if the area fraction of bainite exceeds 95%, the steel plate cannot obtain the desired strength. Therefore, the area fraction of bainite is set to 95% or less. The area fraction of bainite is preferably 93% or less, 90% or less, or 87% or less.

Area ratio of martensite: 5 to 30%
Martensite is a structure that increases the strength of a steel sheet. If the area fraction of martensite is less than 5%, the desired strength cannot be obtained. Therefore, the area fraction of martensite is set to 5% or more. The area fraction of martensite is preferably more than 5%, 6% or more, 7% or more, 10% or more, or 13% or more.
On the other hand, if the area fraction of martensite exceeds 30%, the desired ductility cannot be obtained. Therefore, the area fraction of martensite is set to 30% or less. The area fraction of martensite is preferably 25% or less, more preferably 20% or less.

Residual tissue: 0-5%
The metal structure of the inner region of the steel plate according to this embodiment may contain ferrite and pearlite as a remaining structure in addition to bainite and martensite. The area ratio of the remaining structure may be 0 to 5%.

The area ratios of bainite, martensite and the remaining structure are measured by the following method.
A sample is taken from a quarter-way point from the end face in the width direction of the steel plate so that the metal structure of the cross section (thickness direction × rolling direction cross section) normal to the width direction can be observed. The size of the sample depends on the measuring device, but may be, for example, a rectangular parallelepiped measuring the entire thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface is mirror-polished and then etched with nital. After etching, eight visual fields of 200 μm in the thickness direction × 600 μm in the rolling direction are photographed at a magnification of 1000 times or more using an optical microscope and a scanning electron microscope (SEM) from the surface to a quarter-way point in the thickness direction (the region from 1/8 of the thickness depth from the surface to 3/8 of the thickness depth from the surface). Image analysis of the obtained microstructure photographs is performed to obtain the area fractions of ferrite, pearlite, and bainite.
Then, at the same observation position, only the corroded layer is removed by polishing, followed by a mirror finish, and after REPELLER corrosion, the structure is observed using an optical microscope and a scanning electron microscope, and the area ratio of martensite is obtained by performing image analysis on the obtained structure photograph.

In the above-mentioned structure observation, each structure is identified by the following method.
Martensite has a high dislocation density and a substructure such as blocks and packets within the grains, so it can be distinguished from other metal structures using electron channeling contrast images taken with a scanning electron microscope.

Bainite is defined as a structure that is an aggregate of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more within the structure, and is not martensite, or a structure that contains Fe-based carbides with a major axis of 20 nm or more within the structure, and the Fe-based carbides have a single variant, i.e., a structure that contains Fe-based carbides elongated in the same direction.
Here, the Fe-based carbides elongated in the same direction refer to Fe-based carbides whose elongation directions differ by no more than 5°.

A structure consisting of massive crystal grains that does not contain substructures such as laths inside the structure is considered to be ferrite.
A structure in which plate-like ferrite and Fe-based carbides are layered is considered to be pearlite.

Tensile strength (TS): 980 MPa or more The steel sheet according to this embodiment may have a tensile strength of 980 MPa or more. The tensile strength is more preferably 1000 MPa or more. By making the tensile strength 980 MPa or more, the applicable parts are not limited, and the contribution to vehicle body weight reduction can be increased.
There is no particular need to set an upper limit to the tensile strength, but from the viewpoint of suppressing die wear, it may be set to 1500 MPa or less or 1300 MPa or less.

Total elongation (El): 10.0% or more Hole expansion ratio (λ): 50% or more The steel sheet according to this embodiment may have a total elongation (total elongation at maximum test force) of 10.0% or more and a hole expansion ratio of 50% or more. The total elongation is preferably 12.0% or more, or 14.0% or more. The hole expansion ratio is preferably 55.0% or more, or 60.0% or more.

The tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241: 2022. The test specimen is a No. 5 test specimen of JIS Z 2241: 2022. The tensile test specimen is taken from a quarter portion from the end in the plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction.
When a No. 5 test piece cannot be obtained because the steel plate or part is small or has a complex shape, a small rectangular piece having a parallel portion of any width may be obtained and the tensile strength and total elongation may be determined by performing a tensile test using the small rectangular piece. The direction perpendicular to the rolling direction is the longitudinal direction of the small rectangular piece.
The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

Evaluation of unevenness within bend In the steel plate according to this embodiment, when a bending test described later is performed and the surface roughness within the bend is measured, the maximum valley depth (Rv) of the roughness curve may be 20 μm or less. If the maximum valley depth of the roughness curve within the bend after the bending test described later is 20 μm or less, it can be determined that crack generation within the bend is suppressed and the impact resistance characteristics are excellent.

The bending test is carried out by the following method.
A 100 mm × 30 mm rectangular test piece is cut from the steel plate at half the width direction. The test is performed in accordance with the V-block method of JIS Z 2248:2022 (bending angle θ is 90°) for bending (L-axis bending) in which the bending ridgeline is parallel to the rolling direction (L direction).

Next, a section is cut parallel to the bend ridgeline so that the apex of the bend can be observed, and the roughness is measured using a contact roughness meter. Within the bend at the apex of the bend, the surface roughness is measured at two points 2.0 mm apart, perpendicular to the ridgeline, using a contact roughness meter. A roughness curve is obtained by applying a profile curve filter with a cutoff value λc to the measured cross-sectional curve obtained. Specifically, components with a wavelength λc of 0.8 mm or less are removed from the measurement results to obtain a roughness curve. Based on the obtained roughness curve, the maximum valley depth Rv of the roughness curve is calculated in accordance with JIS B 0601:2013.

The bending test is performed with an R/t ratio of 1.3, since the effect is considered equivalent if the value obtained by dividing the bending radius R by the plate thickness t is the same. As an example, if the plate thickness t is 3.0 mm, R is 3.9 mm.

The thickness of the steel plate according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the steel plate is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to this embodiment may be 1.2 mm or more. Preferably, it is 1.4 mm or more.
On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the plate thickness may be 8.0 mm or less, and is preferably 6.0 mm or less.

The steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a plating layer on the surface to provide a surface-treated steel sheet for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized plating and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized plating, alloyed hot-dip galvanized plating, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights.
Furthermore, it is also possible to further enhance corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, by applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

The steel sheet according to this embodiment has high strength, excellent ductility, and hole expandability. It is also thought to suppress the occurrence of irregularities within the bend during forming and have excellent crashworthiness, making it suitable for use in parts, particularly automobile parts. Among automobile parts, it is particularly suitable for use in automobile suspension parts such as lower arms, trail links, and knuckles. These automobile parts may consist solely of the steel sheet according to this embodiment, or may be formed by joining the steel sheet according to this embodiment with other steel sheets.

Parts manufactured using the steel plate according to this embodiment have the same chemical composition as the steel plate described above. Furthermore, parts may contain both processed and unprocessed portions. The unprocessed portions have the same metallurgical structure as the steel plate described above. Processed portions generally have the same metallurgical structure as the steel plate described above, but heavily processed parts or parts at the edges of the part may not have the metallurgical structure described above. Therefore, when measuring the metallurgical structure of a part, the edges are avoided and measurements are taken of unprocessed parts. If there are no unprocessed parts, measurements are taken of parts that have not been heavily processed. Unprocessed or heavily processed parts refer to, for example, flat parts of the part and parts that have not been subjected to punching, hole expansion, bending, etc. As an example, in the case of the above-mentioned part, a test piece is taken from the flat, largest-area part near the center of gravity and examined. However, for the surface roughness on the inner periphery of the bent part, which is thought to have a significant impact on crashworthiness characteristics, the roughness is measured at the part processed with the smallest bending radius within the part.

Next, a preferred manufacturing method for the steel plate according to this embodiment will be described. According to the manufacturing method described below, the steel plate according to this embodiment can be manufactured stably. Note that the temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.

A preferred method for manufacturing the steel sheet according to this embodiment is as follows.
(1) Rough rolling is performed with a total reduction of 50% or more and a rough rolling completion temperature of 1100° C. or more. However, these rough rolling conditions are more preferable.
(2) Finish rolling is carried out under the following conditions:
The finish rolling start temperature is set to 1020 to 1100°C.
The reduction rate in the first stage is set to 40% or more.
The total reduction in the final two stages is set to less than 40%.
The finish rolling completion temperature is set to 900°C or higher and lower than 980°C.
In the finish rolling, it is more preferable to perform rolling at a temperature range of 980°C or higher and lower than 1020°C at a reduction ratio of 30% or higher two or more times.
(3) After the finish rolling is completed, cooling is started within 1 second and is continued at an average cooling rate of 30°C/s or more until the temperature reaches 200°C.
Each step will be described below.

(1) Rough Rolling In rough rolling, it is more preferable to set the total reduction rate to 50% or more and the rough rolling completion temperature to 1100° C. or more. By performing rough rolling under these conditions, the maximum value of the major axis length of the prior austenite grains can be preferably controlled.

The slab is not particularly limited except for having the above-mentioned chemical composition. For example, a slab can be produced by melting molten steel of the above-mentioned chemical composition using a converter or electric furnace, and then using a continuous casting method. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used. When heating the slab before rough rolling, the heating temperature should be maintained at a temperature range of 1100°C or higher for 3000 seconds or more.

(2) Finish Rolling In finish rolling, it is preferable that the finish rolling start temperature (first stage entry temperature) is 1020 to 1100° C., the first stage reduction is 40% or more, the total reduction of the final two stages is less than 40%, and the finish rolling completion temperature (final stage delivery temperature) is 900° C. or more and less than 980° C. By performing finish rolling under these conditions, the distribution of the aspect ratio of the prior austenite grains can be favorably controlled.
Furthermore, in the finish rolling, it is more preferable to perform rolling at a reduction ratio of 30% or more twice or more in a temperature range of 980° C. or more and less than 1020° C. By also satisfying this condition, it is possible to preferably control the area ratio of prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 and prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0.

The reduction ratio referred to here can be expressed as (1- _t1 / _t0 ) x 100(%), where _t0 is the plate thickness before rolling and t1 is the plate thickness after rolling. The total reduction ratio of the final two stages can be expressed as (1- _t3 / _t2 ) x 100(%), where _t2 is the entry thickness _one stage before the final stage and _t3 is the delivery thickness of the final stage.

(3) Cooling After the finish rolling is completed, it is preferable to start cooling within 1 second and cool the steel sheet at an average cooling rate of 30° C./s or more until the temperature reaches 200° C. By cooling under these conditions, it is possible to obtain the desired amount of bainite.
After cooling to 200°C, the wire may be wound into a coil.

In this embodiment, the average cooling rate is the temperature difference between the start and end points of the set range divided by the elapsed time from the start point to the end point.

Next, the effects of one aspect of the present disclosure will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and achieve the objectives of the present disclosure.

Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. The resulting slabs were used to obtain steel plates shown in Tables 4A and 4B under the production conditions shown in Tables 3A and 3B. The thicknesses of the resulting steel plates were 1.2 to 8.0 mm.
After the finish rolling was completed, cooling was started within 1 second.

The area ratios of the prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, the area ratios of the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0, the area ratios of the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, the area ratios of the prior austenite grains having an aspect ratio of 6.0 or more, the maximum length of the major axis of the prior austenite grains, and the area ratios of bainite and martensite were measured for the obtained steel sheets at a position 1/4 of the sheet thickness from the surface by the above-mentioned methods. In addition, the tensile strength, total elongation, and hole expansion ratio were measured, and unevenness in the bent portion was evaluated by the above-mentioned methods.
In some cases, the metal structure at the quarter position from the surface of the steel plate contained ferrite and pearlite as the remaining structure in addition to bainite and martensite.
The measurement results are shown in Table 5.

Method for Evaluating Steel Sheet Properties When the tensile strength was 980 MPa or more, the steel sheet was judged to be high strength and passed, whereas when the tensile strength was less than 980 MPa, the steel sheet was judged not to be high strength and passed.

If the total elongation was 10.0% or more, the steel sheet was deemed to have excellent ductility and was judged to have passed. On the other hand, if the total elongation was less than 10.0%, the steel sheet was deemed to have poor ductility and was judged to have failed.

If the hole expansion ratio was 50% or more, the steel sheet was deemed to have excellent hole expandability and was judged to have passed. On the other hand, if the hole expansion ratio was less than 50%, the steel sheet was deemed to have poor hole expandability and was judged to have failed.

After the bending test, the maximum valley depth of the roughness curve within the bend was evaluated according to the following criteria. When the evaluation was A or higher, the steel sheet was judged to be pass, as it was deemed that the occurrence of irregularities within the bend during forming was suppressed. On the other hand, when the evaluation was B, the steel sheet was judged to be fail, as it was deemed that the occurrence of irregularities within the bend during forming was not suppressed.
Furthermore, when the evaluation was AA, it was determined that the steel sheet was one in which the occurrence of unevenness inside the bend during forming was further suppressed.
AA: The maximum valley depth of the roughness curve is 15 μm or less. A: The maximum valley depth of the roughness curve is more than 15 μm and 20 μm or less. B: The maximum valley depth of the roughness curve is more than 20 μm.

Looking at Tables 1 to 5, it can be seen that the steel sheets according to the examples of the present invention have high strength, excellent ductility and hole expandability, and also suppress the occurrence of unevenness inside the bend.
On the other hand, it is clear that the steel sheets according to the comparative examples are inferior in one or more of the above properties.

In addition, for all examples, lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated using the same method as described above. The measurement and evaluation results were the same as those shown in Tables 4A, 4B, and 5.

The above aspects of the present disclosure make it possible to provide a steel plate and parts manufactured using the same that have high strength, excellent ductility and hole expandability, and suppress the occurrence of unevenness within the bend.

Claims (9)

The chemical composition, in mass%, is
C: 0.045-0.120%,
Si: 0-3.00%,
Mn: 1.20-3.00%,
sol. Al: 0.001 to 0.500%,
P: 0 to 0.0800%,
S: 0 to 0.0100%,
N: 0 to 0.0050%,
O: 0 to 0.0100%,
Ti: 0 to 0.180%,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Cu: 0 to 1.000%,
Cr: 0-2.000%,
Mo: 0-3.000%,
Ni: 0 to 0.500%,
B: 0 to 0.0100%,
Ca: 0-0.0500%,
Mg: 0 to 0.050%,
REM: 0-0.100%,
Bi: 0-0.100%,
Ta: 0-0.100%,
Zr: 0 to 0.500%,
Co: 0-3.000%,
Zn: 0-0.200%,
W: 0-0.200%,
Sb: 0 to 0.500%,
As: 0 to 0.050% and Sn: 0 to 0.050%;
the balance being Fe and impurities;
In the metal structure at 1/4 of the plate thickness from the surface, in area%,
Prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 are less than 20%;
Prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40% or more,
Prior austenite grains having an aspect ratio of 6.0 or more are less than 30%;
Bainite is 70 to 95%;
A steel plate characterized in that martensite is 5 to 30%. The chemical composition is, in mass %,
Ti: 0.001 to 0.180%,
Nb: 0.001 to 0.100%,
V: 0.001 to 1.000%,
Cu: 0.001 to 1.000%,
Cr: 0.001-2.000%,
Mo: 0.001-3.000%,
Ni: 0.001 to 0.500%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0500%,
Mg: 0.001-0.050%,
REM: 0.001-0.100%,
Bi: 0.001-0.100%,
Ta: 0.001 to 0.100%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 3.000%,
Zn: 0.001-0.200%,
W: 0.001-0.200%,
Sb: 0.001 to 0.500%,
2. The steel sheet according to claim 1, further comprising at least one selected from the group consisting of As: 0.001 to 0.050% and Sn: 0.001 to 0.050%. In the metal structure at a position of 1/4 of the plate thickness from the surface, in terms of area%,
Prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more,
3. The steel sheet according to claim 1, wherein the content of prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 is 10% or more. In the metal structure at a position of 1/4 of the plate thickness from the surface,
3. The steel sheet according to claim 1, wherein the maximum value of the length of the major axis of the prior austenite grains is 200 μm or less. In the metal structure at a position of 1/4 of the plate thickness from the surface,
4. The steel plate according to claim 3, wherein the maximum value of the length of the major axis of the prior austenite grains is 200 μm or less. A part characterized by including the steel plate according to claim 1 or 2. A part characterized by including the steel plate described in claim 3. A part characterized by including the steel plate described in claim 4. A part characterized by including the steel plate described in claim 5.