JP2002060889A - High tensile steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide, without raising cost, a high tensile steel plate in which anisotropy in a plane orthogonal to Z direction is decreased to increase safety required of structural materials and sufficient resistance to crack propagation from all directions is provided. SOLUTION: The high tensile steel plate 1 is composed of a martensitic single-phase structure or a mixed structure of martensite and bainite. Moreover, the shape aspect ratio of an old austenitic grain boundary 3 existing in a Z cross section 2 orthogonal to the Z direction is <=7.0, and the area occupancy of A inclusions in a cross section 4 orthogonal to the cross section 2 and parallel to an L direction is <=0.05%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a high-tensile steel sheet. More specifically, the present invention relates to, for example, 780 N / m
a high-strength steel plate having a tensile strength of at least m ² or
The present invention relates to a high-strength steel plate such as a high-strength steel plate having a tensile strength of 950 N / mm ² or more.

[0002]

2. Description of the Related Art As pen stocks and pressure vessels become larger, steel materials are required to have higher strength in order to save resources and reduce transportation and construction costs. For example, 7
In some cases, not only high-strength steel plates having a tensile strength of 80 N / mm ² or more but also high-strength steel plates having a tensile strength of 950 N / mm ² or more are actually applied to penstocks. This type of high-strength steel sheet is generally subjected to quenching / tempering after re-heating after hot rolling, or after controlled rolling as disclosed in, for example, JP-A-9-263828. It has been manufactured by direct quenching.

However, according to these conventional methods, in particular, a method of directly quenching after controlled rolling, the austenitic grains of the resulting high-tensile steel plate usually have austenite grains in a rolling direction (hereinafter, referred to as a rolling direction).
Both extend in two directions, the "L direction") and a direction orthogonal to the L direction (hereinafter, referred to as "C direction"), and are extremely flat in the thickness direction (hereinafter, referred to as "Z direction"). Because of the form, the mechanical properties of the high-tensile steel plate, which is the final product, such as anisotropy such as strength and toughness, are significantly increased. Therefore, for example, Japanese Patent Application Laid-Open No. 5-222454 discloses an invention in which the anisotropy in the Z direction is improved by means such as heating temperature control, thereby reducing particularly the anisotropy in toughness.

[0004]

As a result of detailed investigation and examination of the mechanical properties of this high-tensile steel plate, the present inventors found that the mechanical properties represented by the tensile properties and the Charpy impact test properties were in the Z direction. The effect of anisotropy on the cross section orthogonal to
It turned out to be noticeable.

Conventionally, most of experimental examples focusing on anisotropy in a plane orthogonal to the Z direction are almost 600 N / mm ^2.
Grade ferrite-pearlite steel is the target, and hardly used for high-strength steel sheets of 800 N / mm ² or higher having martensite as a main structure. Japanese Patent Application Laid-Open No. 10-265893 discloses a few examples.
An invention is disclosed in which a temper type is used to achieve equiaxed graining and suppress anisotropy of toughness in a plane orthogonal to the Z direction. However, in the present invention, although it is possible to reduce the anisotropy of toughness by employing the direct quenching-reheating quenching step, the processing pitch of the heat treatment is significantly reduced, so that economic efficiency is significantly impaired. ,
Actual implementation is difficult in terms of cost.

The present invention relates to a high-strength steel plate having a tensile strength of, for example, 780 N / mm ² or more, or 950 N / mm ²
High-strength steel plate, which is preferably used as a structural strength member, such as a high-tensile steel plate having a tensile strength of at least / mm ² , reduces the anisotropy of mechanical properties at low cost without increasing costs. The purpose is to provide.

[0007] From another viewpoint, the present invention reduces anisotropy in a plane perpendicular to the Z-direction and enhances crack propagation from all directions in order to enhance the safety as a structural material. An object of the present invention is to provide a high-tensile steel plate having resistance at a low price without a rise in cost.

[0008]

Means for Solving the Problems In order to maintain economical efficiency, the present inventors assume that heat treatment after direct quenching is performed only by tempering, and that the anisotropy of mechanical properties is reduced. Means were considered. Specifically, first, anisotropy in a plane orthogonal to the Z direction was investigated for a large number of direct-quenched high-tensile steel sheets. The present inventors have obtained a new finding as described below by comparing and investigating those having a large anisotropy and those having a small anisotropy.

That is, the anisotropy shows a remarkable difference particularly in toughness. Regarding the anisotropy in each direction, the anisotropy in the L direction is the best, and the anisotropy in the C direction or 45 ° direction is the lowest. Such a difference in anisotropy in each direction is remarkable. Even if the characteristics are good in the L direction, the resistance to crack propagation in the C direction and the 45 degree direction is significantly impaired. When a crack propagation test was performed on the assumption of a stress load in the L direction, it was found that the crack was bent and propagated in the C direction and the 45 ° direction, and the resistance remained low. To increase the reliability of structural safety, it is, of course, necessary to obtain problem-free toughness in all directions.

Therefore, the present inventors have further studied and found that the shape aspect ratio of the former austenite grain boundary existing in the cross section orthogonal to the Z direction is perpendicular to this cross section and parallel to the L direction. The inventors have found that the above problem can be solved by limiting both the occupied area ratio of the A-based inclusions in the cross section to a specific range, and completed the present invention.

The present invention comprises a martensite single phase structure or a mixed structure of martensite and bainite,
The occupied area ratio of the A-based inclusions in the cross section orthogonal to the cross section and parallel to the L direction in which the shape aspect ratio of the prior austenite grain boundary existing in the cross section orthogonal to the Z direction is 7.0 or less. Is not more than 0.05%. The high-tensile steel sheet according to the present invention has a small anisotropy of toughness including the L direction and the C direction in a plane orthogonal to the Z direction.

The “shape aspect ratio” of the high-strength steel sheet according to the present invention is defined as the L-direction grain size of the prior austenite grain boundary existing in the cross section orthogonal to the Z direction.
Means the ratio to the particle size in the C direction perpendicular to the direction.
The “A-based inclusion” in the high-strength steel sheet according to the present invention means an inclusion that has been viscously deformed by working, and specifically, is an inclusion defined by JIS G 0555.

The high-strength steel sheet according to the present invention has a mass%
, C: 0.06-0.19%, Si: 0.15-0.
60%, Mn: 0.60 to 1.80%, Cr: 0.05
１．２1.20%, Mo: 0.05-1.00% and B:
It is exemplified to contain 0.0005 to 0.0025%.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a high-tensile steel sheet according to the present invention will be described below in detail with reference to the accompanying drawings.

First, the composition of the high-strength steel sheet in the present embodiment will be described. [C: 0.06 to 0.19%] C is the most effective and inexpensive element for improving strength. C content is 0.06
If it is less than%, it is necessary to increase the content of other elements contributing to the strength to maintain the required strength, and as a result, economic efficiency is significantly impaired. On the other hand, C content is 0.19%
If it exceeds, the weldability is significantly impaired, and the toughness is remarkably deteriorated, so that practical problems increase. Therefore, in the present embodiment, the C content is limited to 0.06% to 0.19%.

[Si: 0.15 to 0.60%] Si is
It is an element that contributes to strength improvement, but the Si content is 0.1
If it is less than 5%, such an effect is not recognized, while, on the other hand, 0.
If it exceeds 60%, the base material toughness will be significantly impaired.
Therefore, in the present embodiment, the Si content is limited to 0.15% or more and 0.60% or less.

[Mn: 0.60 to 1.80%] Mn is:
Although 0.60% or more is added to secure the strength, if the Mn content exceeds 1.80%, the toughness is remarkably deteriorated.
Therefore, in the present embodiment, the Mn content is limited to 0.60% to 1.80%.

[Cr: 0.05-1.20%] Cr is
By containing 0.05% or more, it exerts a function of enhancing hardenability and is effective for improving strength and toughness. However, when the Cr content exceeds 1.20%, toughness is significantly impaired. Therefore, in the present embodiment, the Cr content is limited to 0.05% or more and 1.20% or less.

[Mo: 0.05-1.00%] Mo is
Addition of 0.05% or more is effective for improving the strength, but addition of more than 1.00% impairs toughness. Therefore, in the present embodiment, the Mo content is limited to 0.05% or more and 1.00% or less.

[B: 0.0005 to 0.0025%] B
Is an indispensable element in practical high-strength steels, since hardenability is significantly improved by the addition of a small amount of 0.0005% or more. However, if the B content exceeds 0.0025%, the toughness is significantly impaired. Therefore, in the present embodiment,
The B content is limited to 0.0005% or more and 0.0025% or less.

The high-strength steel sheet of the present embodiment
u, Ni, V, Nb, sol. Al, Ti, Zr, Ca
, Etc., may be contained in an appropriate amount as an optional additive element as needed. Hereinafter, these optional elements will be sequentially described.

[Cu: 1.0% or less] The Cu content is 1.
If it exceeds 0%, scale is generated and the surface properties of the steel material are remarkably deteriorated, and further, the toughness is deteriorated. So, C
When adding u, its content is desirably limited to 1.0% or less.

[Ni: 5.0% or less] Ni is an effective element for improving both strength and toughness. However, if the content exceeds 5.0%, the cost is increased, and furthermore, Deteriorates toughness. Therefore, when adding Ni, it is desirable to limit the content to 5.0% or less.

[V: 0.10% or less] V precipitates as VC during tempering and has a function of improving strength.
Even if added in excess of 0.10%, the effect is saturated and the toughness is significantly deteriorated. Therefore, when V is added, its content is preferably limited to 0.10% or less.

[Nb: 0.10% or less] Nb not only suppresses crystal grain coarsening during slab heating, but also exerts the same effect during quenching, and is effective in the production of fine steel materials in units of fracture surface. . However, if added in excess of 0.10%, the toughness is significantly impaired. Therefore, when Nb is added, its content is desirably limited to 0.10% or less.

[Sol. Al: 0.1% or less] sol.
Al is effective as an element contributing to refinement of austenite grains, but if added in excess of 0.1%, toughness is deteriorated. Then, sol. It is desirable that the content of Al be limited to 0.1% or less.

[Ti: 0.1% or less] Ti plays a role in fixing N and enhancing the effect of improving hardenability by B.
Excessive addition exceeding 0.1% causes toughness degradation. Therefore, when Ti is added, the content is 0.1%.
It is desirable to limit to the following.

[Zr: 0.15% or less] Zr contributes to the improvement of the strength of steel by precipitation, but when added in a large amount, it causes an increase in cost and a deterioration in toughness. Therefore, when Zr is added, its content is desirably limited to 0.15% or less.

[Ca: 0.008% or less] By controlling the form of sulfide-based nonmetallic inclusions, Ca can increase crack propagation resistance, and as a result contributes to improvement in toughness. However, if the Ca content exceeds 0.008%, the amount of nonmetallic inclusions increases, adversely affecting toughness. Therefore, when Ca is added, its content is 0.00
It is desirable to limit it to 8% or less.

Other than the above, Fe and impurities. The composition of the high-strength steel sheet according to the present embodiment is an example of the composition of the high-strength steel sheet according to the present invention. That is, the composition of the high-strength steel sheet according to the present embodiment reliably obtains a martensite single-phase structure or a mixed structure of martensite and bainite, which is a premise of the high-strength steel sheet of the present embodiment as described later. It is possible,
This shows one composition that can obtain desired strength, toughness and weldability on an industrial scale. Therefore, the composition of the high-tensile steel sheet according to the present invention is not limited to the above-described composition of the high-tensile steel sheet according to the present embodiment.

The high-strength steel sheet of the present embodiment having such a steel composition has the following structural features (i) to (iii). Hereinafter, these features will be described. FIG.
FIG. 1 is an explanatory diagram illustrating a configuration example of a high-tensile steel sheet 1 according to the present embodiment. FIG. 2 is an explanatory diagram showing an example of the metal structure of the Z section 2 in FIG. FIG. 3 is an explanatory view conceptually showing the direction of anisotropy in the Z section 2 which is improved and improved by the high-tensile steel sheet 1 of the present embodiment.

(I) It is composed of a martensite single phase structure or a mixed structure of martensite and bainite. This is to ensure that the high-strength steel sheet of the present embodiment has desired strength and toughness.

Incidentally, martensite and bainite are
In any case, the old austenite grain boundary 3 is left to form a lath-like transformed structure, and the following method is exemplified as a method for determining these. That is, thin film transmission observation or extraction replica observation is performed by an electron microscope, and the precipitation form of cementite in the structure is observed. In the case of as-quenched martensite, carbon remains in a supersaturated solid solution state, and therefore, no precipitation of cementite is observed inside the lath-like transformation structure. In the cooling process, a temper-equivalent effect called auto-tempering may be exhibited. In this case, precipitation is observed in the form of being arranged in four variants inside a lath-like transformed structure, like tempered martensite.

On the other hand, as for the bainite structure, cementite precipitation is observed inside bainitic ferrite like tempered martensite, but upper bainite or lath transformation structure in which cementite precipitation is observed between lath transformation structures. The bainite ferrite end face (the face of the tip of the lath bainitic ferrite which grows at a constant angle in austenite) forms an angle of 60 ° with lower bainite where cementite precipitates.

In some cases, a small amount of retained austenite remains between the laths of the upper bainite, but it is regarded as a part of the upper bainite, and the case where the retained austenite remains between the laths is also included in the martensite-bainite mixed structure. Shall be.

Therefore, the high-strength steel sheet according to the present embodiment is limited to a single-phase structure of martensite or a mixed structure of martensite and bainite. (ii) The shape aspect ratio (D _ZL / D _ZC ) of the prior austenite grain boundary 3 existing in the Z section 2 orthogonal to the Z direction is 7.
Here, the “shape aspect ratio” is
Z section 2 orthogonal to the Z direction (the thickness direction in FIG. 1)
L of old austenite grain boundary 3 existing in (see FIG. 2)
Of the particle diameter D _{ZL in} the direction parallel to the direction
The ratio to the particle size D _{ZC in the} direction parallel to the direction (D _ZL /
D _ZC ). That is, for example, in a mixed structure of martensite and bainite, by performing etching using picric acid or the like, the former austenite grain boundary 3
The “aspect ratio” means that the grain size D _{ZL of} the old austenite grain boundary 3 developed by etching in the direction parallel to the L direction and the particle size D _ZL parallel to the C direction. It means the ratio (D _ZL / D _ZC ) to the particle size D _{ZC in the} direction.

As described above, the shape of the austenite grains particularly when controlled rolling is performed in the late recrystallization region becomes a so-called "pancake" shape which is extremely thin in the Z direction. When the structure observation of the Z section 2 is performed using an optical microscope after the austenite grain boundaries 3 are revealed by picric acid and a surfactant or the like, a large ellipse that has developed much in the direction parallel to the L direction due to the restriction of the rolling mill. Austenite grain boundaries 3 having a shape of shape are often observed. Of course, such an elliptical shape is not recognized in the case where quenching is performed directly from the recrystallization region or the case where the final structure is formed by reheating and quenching.

Low-grade steels up to 600 N / mm ² grade are also disclosed, for example, in “Iron and Steel No. 61,7, pp. 991-1011“ Strength toughness and texture of non-heat treated high-strength steels ””. However, the texture is considered to be the decisive cause of the increase in anisotropy.However, the results of the investigation of the texture in a sample obtained by subjecting a steel mainly composed of martensite to heavy working in the non-recrystallized region are considered. However, it was found that the transformed texture did not develop unexpectedly, and even if the texture was suppressed, the increase in the anisotropy of the mechanical properties in the Z section 2 intersecting the Z direction could not be reduced. The present inventors further studied and found that one of the causes of the increase in anisotropy is due to the shape of the austenite grains.

In other words, since the martensite structure and the bainite structure allow the austenite grain boundaries, which are large-angle grain boundaries, to clearly remain in the final structure, the supersaturated carbon in the martensite is transformed into the austenite grain boundaries after transformation. Cementite F remarkably precipitated during cooling and tempering processes
e ₃ C is observed. Austenite grain boundaries are linear (3
Cementite Fe ₃
C is also distributed linearly.

Here, considering the crack propagation resistance form,
It is considered that the interface between the cementite and the matrix is likely to cause a pre-crack, and the shape of the austenite grains has a great influence on the mechanical properties. When the austenite grains observed in the Z section 2 have a shape elongated extremely in the L direction, and when a Charpy test piece is collected in the C direction, the crack propagates along the austenite grain boundary because the distance propagates. Crack propagation resistance is significantly impaired.

Here, by limiting the aspect ratio of the austenite grain size in the Z section to 7.0 or less, the anisotropy of the mechanical properties can be reduced. Therefore, the high-tensile steel sheet according to the present embodiment has a Z section 2 perpendicular to the Z direction.
It is limited that the shape aspect ratio (D _ZL / D _ZC ) of the prior austenite grain boundary 3 existing in the inside is 7.0 or less.

In the present embodiment, there is a prior art which mentions the “aspect ratio” of the prior austenite grain size, which is one of the most important management parameters. That is, JP-A-10-298707 and JP-A-11-172365 disclose that the deformation zone in the austenite grains is secured by securing the aspect ratio of the prior austenite grain size to a specified amount or more, and the lower bainite after transformation is secured. / Focus on controlling the packet size of martensite to be small. However,
The minor axis when calculating the "aspect ratio" in these prior arts is clearly the thickness of the prior austenite grain size in the thickness direction, and it is specified that this aspect ratio is flatness. That is, the “aspect ratio” in these prior arts is different from the “aspect ratio” in the present embodiment.

(Iii) The occupied area ratio of the A-based inclusions in the L-direction section 4 which is orthogonal to the Z direction and which is orthogonal to the Z direction and which is parallel to the L direction is 0.05% or less. “A-based inclusion” means an inclusion that has been viscously deformed by processing, and is specifically JIS G 055
5 are inclusions defined by

That is, by limiting the shape aspect ratio (D _ZL / D _ZC ) of the prior austenite grain boundary 3 to 7.0 or less and controlling the form of the inclusion to a specific form as described above, High strength steel sheet 1 according to the embodiment
Can be reduced to a desired degree.

Conventionally, even when the austenite grains are a direct quenched material or a reheat quenched material from a recrystallized region which is almost spherical, the cause of the anisotropy of the mechanical properties in the Z section 2 is particularly a cause. It has been attributed to elongated inclusions.

In other words, the inclusions which are softer than the matrix at the time of rolling, such as MnS in steel, are rolled out by rolling and remain in the final structure in a shape as expanded by rolling. In particular, in the actual production process, it is inevitable that the inclusions extend in the L direction much, which causes a decrease in toughness particularly in the test piece in the C direction along the progress of the crack.

This expanded inclusion is compliant with JIS G05
55, it is inevitable that the A-based inclusions cause anisotropy of the mechanical properties in the steel material subjected to the rolling process. The only way to effectively reduce the amount is to reduce the amount of the A-based inclusion itself. Anisotropy can be effectively reduced by suppressing the A-based inclusions to an occupied area ratio of 0.05% or less.

Therefore, in the high-tensile steel sheet according to the present embodiment, the occupied area ratio of the A-based inclusion in the L-direction section 4 which is perpendicular to the Z section 2 which is perpendicular to the Z direction and parallel to the L direction is 0%. 0.05% or less.

The high-strength steel sheet 1 of the present embodiment has the steel composition and structure described above. Next, an example of a method for manufacturing the high-strength steel sheet 1 of the present embodiment will be described. First,
The rolling material having the steel composition described above is charged into a heating furnace,
For example, it is heated to a heating temperature of about 1150 ° C. or less. Next, for example, the thickness is reduced to a desired thickness by performing hot rolling at a temperature of 900 ° C. or less at a draft of 50% or more,
Thereafter, accelerated cooling (for example, water cooling) is performed using a suitable accelerated cooling device.

This controlled rolling method is an important process for improving the toughness of the manufactured high-tensile steel sheet.
In order to improve the toughness of the high-strength steel sheet to a desired degree, it is effective to make the effective crystal grain size of the steel material fine and make the fracture surface unit fine. Therefore, in the present embodiment, the non-recrystallized region is rolled before online quenching, and a deformed zone is introduced so that the microstructure after quenching is remarkably refined. Specifically, the rolling reduction at 900 ° C. or less is specified as 50% or more, and water cooling is performed after hot rolling in order to transform into a structure mainly composed of martensite.

If necessary, a tempering treatment is performed to give a desired toughness. Thus, the high-tensile steel sheet 1 according to the present embodiment is manufactured. The high-strength steel sheet 1 according to the present embodiment has small anisotropy of toughness in the Z section 2 including the L direction and the C direction. As described above, in order to increase the guarantee for the safety of the structure, the anisotropy of toughness including the L direction and the C direction in the Z section 2 is reduced, and sufficient crack propagation from all directions is achieved. It is important to be able to provide a steel sheet having resistance without special high-cost processing. The high-strength steel sheet 1 according to the present embodiment can sufficiently meet such demands.

It should be noted that the crack propagation should be finally evaluated by a large-scale fracture toughness test such as an ESSO test or a double tensile test, which is suitable for the substance.
However, since a good correlation with a large-sized test is generally shown by a test such as WES3003, the evaluation may be made by the characteristics of the Charpy impact test.

As described above, according to the present embodiment, the anisotropy of the toughness in the Z section 2 including the L direction and the C direction is reliably reduced to such a degree that there is no practical problem, and the high tensile strength excellent in the low temperature toughness is obtained. The steel plate could be provided without fail.

Further, the present invention will be described in detail with reference to examples, but these are exemplifications of the present invention, and the present invention is not limited thereto.

[0055]

EXAMPLE A slab having the steel composition shown in Table 1 was heated to the slab heating temperature shown in Table 2 and then reduced to the unrecrystallized region reduction rate shown in Table 2 and shown in Table 2. By performing hot rolling at the hot rolling end temperature, the thickness is reduced to the product sheet thickness shown in Table 2, and then the accelerated cooling start temperature shown in Table 2,
Water cooling was performed at an accelerated cooling end temperature and a 1/2 t part cooling rate, and further tempering was performed at a tempering temperature shown in Table 2.

From the obtained samples, a notch Charpy impact test piece (2 mm V notch test piece taken from 1/4 t (JIS Z22024)) and a JIS No. 4 test piece (tensile test piece taken from 1/4 t (JIS Z220
No. 14)). Using these test pieces, a Charpy impact test (JIS Z2242) and a tensile test (JIS Z2241) were performed at a pitch of 15 ° from 0 ° (L direction) to 180 °, and the resulting structure and FIG. Z section 2 orthogonal to the Z direction at
The shape aspect ratio of the prior austenite grain boundary 3 existing in the inside and the occupied area ratio of the A-based inclusion in the section 4 which is orthogonal to the Z section 2 and parallel to the L direction were measured.

The results are summarized in Table 1. Sample No. in Table 1 1-4, 7-8, 11-12, 15-16,
19 to 20, 23 to 24, and 27 to 28 are all examples of the present invention satisfying the scope of the present invention. Each of these samples has low anisotropy in mechanical properties.

On the other hand, the sample No. 5, 9, 13, 1
7, 21, 25, and 29 are all comparative examples in which the shape aspect ratio of the prior austenite grain size on the Z section 2 is out of the range of the present invention. Each of these samples,
The anisotropy of the mechanical properties has increased and the uniformity has been impaired.

Further, the sample No. 6, 10, 14, 1
8, 22, 26, and 30 are comparative examples in which the occupied area ratio of the A-based inclusion in the cross section 4 in the L direction is out of the range of the present invention. In all of these samples, the anisotropy of the mechanical properties is large, and the uniformity is impaired.

[0060]

[Table 1]

[0061]

[Table 2]

[0062]

As described [Effect Invention above in detail, the present invention, for example, high-tensile steel plates having a 780N / mm ² or more tensile strength or high tension with 950 N / mm ² tensile strength of not less than A high-tensile steel sheet suitably used as a structural strength member, such as a thick steel plate, could be provided at low cost without an increase in cost and with reduced anisotropy in mechanical properties.

Further, according to the present invention, in order to enhance the safety as a structural material, anisotropy in a plane perpendicular to the Z direction is reduced, and sufficient resistance to crack propagation from all directions is obtained. It was possible to provide a high-tensile steel plate at a low cost without a rise in cost.

The significance of the present invention having such effects is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a configuration example of a high-tensile steel sheet according to an embodiment.

FIG. 2 is an explanatory diagram showing an example of a metal structure of a Z section in FIG.

FIG. 3 is an explanatory diagram conceptually showing an anisotropic direction in a Z section, which is improved / improved by the high-tensile steel sheet of the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 High-strength steel plate 2 Z section 3 Former austenite grain boundary 4 Section

Claims (2)

[Claims] 1. A martensite single-phase structure or a mixed structure of martensite and bainite, wherein a shape aspect ratio of a prior austenite grain boundary existing in a cross section orthogonal to a sheet thickness direction is 7.0 or less, and A high-strength steel sheet, wherein an occupied area ratio of the A-based inclusion in a cross section orthogonal to the cross section and parallel to a rolling direction is 0.05% or less. 2. C: 0.06 to 0.19% by mass%,
Si: 0.15 to 0.60%, Mn: 0.60 to 1.8
0%, Cr: 0.05 to 1.20%, Mo: 0.05 to
The high-strength steel sheet according to claim 1, containing 1.00% and B: 0.0005 to 0.0025%.