WO2024053729A1 - Steel plate - Google Patents

Abstract

Provided is a steel plate characterized by having a predetermined chemical composition and having a metal structure including, in terms of area percentage, 80-95% of ferrite, 5-20% of martensite, and a total of 0-10% of at least one of bainite, pearlite, and retained austenite, and having the average grain spacing of martensite of 2.5 μm or less, and the standard deviation of 1.5% or less in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction.

Description

steel plate

The present invention relates to a steel plate, and more particularly, to a steel plate with an excellent appearance and a tensile strength of 400 MPa or more, which is mainly used as an outer panel member of an automobile, for example.

In order to reduce carbon dioxide emissions from automobiles, attempts are being made to use high-strength steel plates to reduce the weight of automobile bodies while ensuring safety. The strength of automotive steel sheets has been significantly improved in automobile frame parts, but steel sheets with a tensile strength of 300 MPa or less are mainly used for exterior panel parts such as doors and hoods. No progress has been made in increasing strength. Such outer panel members are required to have high moldability and appearance. Generally, when the strength of a steel plate is increased, its formability and appearance after forming are reduced. Therefore, in high-strength steel sheets, it is difficult to achieve both strength, formability, and appearance, especially appearance after forming. Conventionally, several means have been proposed to solve these problems.

For example, in Patent Document 1, in mass %, C: 0.02 to 0.3%, Si: 0.1 to 2.0%, Mn: less than 1.0%, Cr: more than 1.0 to 3.0%. 0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, N: 0.001 to 0.008%, and 2.5≦1.5Mn %+Cr%, 4.1-2.3Mn%-1.2Cr%≦Si%, and the steel sheet for hot-dip galvanizing is characterized in that the balance is Fe and unavoidable impurities. Furthermore, Patent Document 1 teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both workability and post-processing appearance of a hot-dip galvanized steel sheet having a tensile strength of 390 MPa or more. Furthermore, in Patent Document 1, by setting the area ratio of ferrite, which is the main phase, to 70% or more and setting the area ratio of the hard second phase containing martensite to 30% or less, strength, yield strength, yield ratio, - It is taught that it is possible to achieve a good range of all ductility balances.

In Patent Document 2, in mass%, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0 .005 to 0.03%, Ti: 0.02 to 0.1%, Al: 0.01 to 0.05%, N: 0.005% or less, Sb: 0.03% or less, Cu: 0. Ti* that is more than 0.005% and 0.03% or less and is represented by Ti* = (Ti%) - 3.4 x (N%) - 1.5 x (S%) - 4 x (C%) in a range that satisfies 0<Ti*<0.02, and further in a range that satisfies (Sb%)≧(Cu%)/5, with the remainder consisting of Fe and unavoidable impurities; On both sides, the content (mass%) of Ti element contained in precipitates with a size of less than 20 nm in the surface layer part of the plate thickness up to 10 μm from each surface is 9% or less of the total Ti content (mass%) in the steel plate. A cold-rolled steel sheet is described that is characterized by: In addition, in Patent Document 2, the content (mass%) of Ti element contained in precipitates with a size of less than 20 nm in the surface layer part of the plate thickness up to 10 μm from each surface of both sides of the steel plate is calculated as the total Ti content (mass%) in the steel plate. %) to 9% or less, it is possible to avoid appearance unevenness caused by such fine Ti-based precipitates and obtain a cold-rolled steel sheet with excellent surface properties. It has been taught that this method can be suitably used for parts that require excellent post-molding surface quality, mainly automobile outer panels.

Japanese Patent Application Publication No. 2009-249737 International Publication No. 2011/142473

For example, in the case of composite structure steel having a metal structure containing soft ferrite and hard martensite as described in Patent Document 1, the soft ferrite and its surroundings are preferentially deformed during processing such as press forming. Non-uniform deformation is likely to occur. Therefore, when using a steel with a composite structure consisting of soft and hard structures, minute irregularities may occur on the surface of the steel sheet after forming, resulting in appearance defects called ghost lines. be. In this regard, for example, Patent Document 1 discusses improving formability and appearance after forming mainly from the viewpoint of chemical composition, but from the viewpoint of making the metal structure appropriate, Sufficient consideration has not necessarily been given. Therefore, with the steel sheets of the prior art, there is still room for improvement in terms of formability and improved appearance after forming.

Therefore, an object of the present invention is to provide a steel plate that can achieve both strength, formability, and appearance after forming using a novel configuration.

In order to achieve the above object, the present inventors conducted studies focusing on the distribution state of martensite in addition to optimizing the proportion of martensite, which is a hard structure, in the metal structure. As a result, the present inventors were able to uniformly disperse martensite, which is included in a predetermined proportion in the metal structure, in both the micro and macro areas of the metal structure, thereby improving the structure based on such a hard structure. In addition to achieving the desired high strength and formability, the present inventors have discovered that the formation of minute irregularities on the steel plate surface is significantly suppressed even when strain is applied by press forming, etc., and the present invention has been completed. .

The gist of the invention is as follows.
(1) In mass%,
C: 0.03-0.08%,
Si: 0.01-1.00%,
Mn: 0.50-3.00%,
P: 0.1000% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0200% or less,
O: 0 to 0.020%,
Cr: 0-2.000%,
Mo: 0-1.000%,
Ti: 0 to 0.500%,
Nb: 0 to 0.500%,
B: 0 to 0.0100%,
Cu: 0 to 1.000%,
Ni: 0 to 1.00%,
W: 0-0.100%,
V: 0-1.000%,
Ta: 0-0.100%,
Co: 0-3.000%,
Sn: 0-1.000%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0-0.0500%,
Y: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%,
Has a chemical composition consisting of Bi: 0 to 0.0500%, and the balance: Fe and impurities,
In area ratio,
Ferrite: 80-95%,
Martensite: 5 to 20%, and at least one of bainite, pearlite, and retained austenite: 0 to 10% in total,
The average particle spacing of martensite is 2.5 μm or less,
A steel sheet having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
(2) the chemical composition is in mass%;
Cr: 0.001-2.000%,
Mo: 0.001 to 1.000%,
Ti: 0.001 to 0.500%,
Nb: 0.001-0.500%,
B: 0.0001 to 0.0100%,
Cu: 0.001 to 1.000%,
Ni: 0.001 to 1.00%,
W: 0.001-0.100%,
V: 0.001-1.000%,
Ta: 0.001 to 0.100%,
Co: 0.001 to 3.000%,
Sn: 0.001 to 1.000%,
Sb: 0.001 to 0.500%,
As: 0.001 to 0.050%,
Mg: 0.0001-0.050%,
Zr: 0.0001 to 0.050%,
Ca: 0.0001-0.0500%,
Y: 0.0001-0.0500%,
La: 0.0001 to 0.0500%,
Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%
The steel plate described in (1) above, characterized by containing at least one of the following.

According to the present invention, it is possible to provide a steel plate that can achieve both strength, formability, and appearance after forming.

<Steel plate>
The steel plate according to the embodiment of the present invention has, in mass%,
C: 0.03-0.08%,
Si: 0.01-1.00%,
Mn: 0.50-3.00%,
P: 0.1000% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0200% or less,
O: 0 to 0.020%,
Cr: 0-2.000%,
Mo: 0-1.000%,
Ti: 0-0.500%,
Nb: 0 to 0.500%,
B: 0 to 0.0100%,
Cu: 0-1.000%,
Ni: 0 to 1.00%,
W: 0-0.100%,
V: 0-1.000%,
Ta: 0-0.100%,
Co: 0-3.000%,
Sn: 0-1.000%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0-0.0500%,
Y: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%,
Has a chemical composition consisting of Bi: 0 to 0.0500%, and the balance: Fe and impurities,
In area ratio,
Ferrite: 80-95%,
Martensite: 5 to 20%, and at least one of bainite, pearlite, and retained austenite: 0 to 10% in total,
The average particle spacing of martensite is 2.5 μm or less,
It is characterized by having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.

For exterior panel members such as doors and hoods, composite structure steel (DP steel), which has a relatively low yield strength, is often used in order to avoid surface defects called surface strain that occur during press forming. However, in the case of DP steel, in which a soft structure consisting of ferrite and a hard structure consisting of martensite coexist, non-uniform deformation occurs where the soft structure and its surroundings are preferentially deformed during processing such as press forming, and the steel plate after forming The appearance of minute irregularities on the surface may cause appearance defects called ghost lines. To explain in more detail, during processing such as press molding, the soft tissue made of ferrite is greatly deformed and dented, while the hard tissue made of martensite is deformed small. Therefore, compared to soft tissues, hard tissues do not become depressed, but swell up in a convex manner. As a result, variations in the amount of deformation occur, particularly in the width direction of the steel plate, resulting in band-like (striped) ghost lines. On the other hand, as the strength of steel sheets increases, a relatively large amount of elements such as Mn may be added in order to improve the hardenability of the steel sheets. Mn is an element that tends to segregate in the form of streaks in steel sheets. More specifically, during casting, Mn-enriched regions such as center segregation and micro-segregation are formed, and during hot rolling and cold rolling, Mn enriched regions are formed in the rolling direction. By being stretched, Mn segregates into streaks. Therefore, due to such segregation of Mn, there are regions with high hardenability and regions with low hardenability in the steel sheet. As a result, a relatively large number of striped hard structures are generated in the metal structure of the steel sheet after quenching. In this case, the occurrence of ghost lines becomes particularly noticeable. On the other hand, if Mn segregation in the steel sheet could be sufficiently suppressed, it would be possible to reduce the formation of such striped hard structures and to disperse the hard structures more uniformly in the metal structure. . In this case, even when strain is applied by press forming or the like, it is possible to sufficiently reduce the generation of minute irregularities on the surface of the steel sheet, and it is considered possible to suppress the generation of ghost lines. However, with the demand for higher strength, it is actually very difficult to reliably and sufficiently suppress Mn segregation, especially when the amount of Mn added in the steel sheet increases. In addition, as the strength increases, the moldability itself also decreases, so it is generally very difficult to achieve both strength, moldability, and appearance after molding.

Therefore, the present inventors first achieved the desired high strength by optimizing the chemical composition of the steel sheet and optimizing the ratio of ferrite, which is a soft structure, and martensite, which is a hard structure, in the metal structure. We investigated ways to improve the appearance after molding while achieving moldability. Specifically, the present inventors focused on the distribution state of martensite, which is a hard structure in the metal structure, and more specifically, controlled the distribution of martensite from a different perspective than reducing Mn segregation. We examined the following. As a result, as will be explained in detail later regarding the method of manufacturing the steel sheet, the present inventors configure the metal structure in the steel sheet before final annealing with bainite and/or martensite, and then add such metal structure to the steel sheet. By final annealing a steel plate having a It has been found that uniform dispersion can be achieved. More specifically, the present inventors finally annealed a steel plate having a metal structure consisting of bainite and/or martensite under predetermined conditions, thereby reducing the average particle spacing of martensite to 2.0 mm in the microscopic region. It has been found that the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to be 1.5% or less in the macro region. By controlling the average particle spacing of martensite to 2.5 μm or less, the hard structure can be densely and uniformly dispersed in the micro region. By controlling the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction to 1.5% or less, variations in the hard structure in the macro region can be significantly reduced. By satisfying both of these requirements, it is possible to form a metal structure in which martensite, which is a hard structure, is finely and uniformly dispersed throughout the steel sheet. As a result, according to the steel plate according to the embodiment of the present invention, the amount of deformation of the steel plate can be made more uniform especially in the width direction even during forming such as press forming, and appearance defects such as ghost lines are significantly reduced. It becomes possible to achieve a suppressed and excellent appearance after molding. For example, even if the uniformity of martensite is ensured in the microscopic area, if the uniformity of martensite is not ensured in the macroscopic area, martensite will be dispersed finely and uniformly throughout the steel sheet. cannot be formed. Similarly, even if the uniformity of martensite is ensured in the macroscopic area, if the uniformity of martensite is not ensured in the microscopic area, martensite may exist locally unevenly. Therefore, it is impossible to form a metal structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Therefore, in order to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed in the steel sheet according to the embodiment of the present invention, the average particle spacing of martensite should be controlled to 2.5 μm or less. It is necessary to satisfy both the requirements of controlling the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction to 1.5% or less.

Although we do not intend to be bound by any particular theory, in order to finely and uniformly disperse martensite throughout the steel sheet in the metallographic structure of the final steel sheet, it is necessary to It is considered extremely important to form austenite nucleation sites in a highly dispersed manner. In this regard, the martensitic structure further has substructures such as packets, blocks, and laths within the prior austenite grains, and therefore has many different internal interfaces compared to structures such as ferrite. It is an organization that has Like martensite, bainite is also a structure that has many various interfaces inside. Therefore, by configuring the metal structure of the steel sheet before final annealing with bainite and/or martensite, when such metal structure is heated during final annealing, the interface between these can become a nucleation site for austenite. It becomes possible to disperse and generate a very large amount of carbide. Therefore, it is considered possible to generate austenite finely and uniformly over the entire steel plate by generating a large amount of carbide on the interface and then heating the temperature to a two-phase region of ferrite and austenite. Finally, by rapidly cooling a steel sheet with such a metal structure, martensite is generated from these austenites, so the average particle spacing of martensite is controlled to 2.5 μm or less in the final metal structure. At the same time, the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to be 1.5% or less. In other words, it is considered that a metal structure in which martensite is uniformly dispersed in both micro and macro regions can be obtained. It is thought that by performing such heat treatment, it becomes possible to disperse martensite finely and uniformly throughout the steel sheet to the extent that the influence of Mn segregation is negated. Conventionally, it has been common to consider the distribution control of hard structures from the perspective of reducing Mn segregation itself, so it is not necessary to depend on the presence or degree of Mn segregation. The fact that martensite can be uniformly dispersed in both the micro and macro regions is quite surprising and surprising.

According to the steel sheet according to the embodiment of the present invention, in addition to the above findings, good formability is ensured by controlling the area ratio of ferrite, which is a soft structure, to 80 to 95%, and at the same time, good formability is ensured, and the area ratio of ferrite, which is a soft structure, is controlled to 80 to 95%. By controlling the area ratio of martensite to 5 to 20% and further controlling the chemical composition of the steel plate within a predetermined range, a high tensile strength of 400 MPa or more can be ensured. As a result, it becomes possible to achieve a high level of both strength, moldability, and appearance after molding. Hereinafter, each component of the steel plate according to the embodiment of the present invention will be explained in more detail.

First, the metal structure of the steel plate according to the embodiment of the present invention will be explained. Hereinafter, the tissue fraction will be expressed as an area ratio, so the unit of tissue fraction "%" means area %. Furthermore, as will be described later, the metallographic structure is controlled in the 1/4th part of the thickness of the steel plate. The 1/4th part of the thickness of the steel plate means the area between the plane at a depth of 1/8 of the thickness of the steel plate and the plane at a depth of 3/8 of the thickness from the rolling surface of the steel plate. Hereinafter, unless otherwise specified, all tissue fractions mean values at 1/4 part of the plate thickness.

[Ferrite: 80-95%]
Since ferrite has a soft structure, it is easily deformed and contributes to improving elongation. When the area ratio of ferrite is 80% or more, sufficient formability can be obtained. From the viewpoint of improving formability, the higher the area ratio of ferrite is, the more preferable it is, and may be, for example, 82% or more, 85% or more, 87% or more, or 90% or more. On the other hand, if too much ferrite is contained, the steel sheet may not be able to achieve the desired strength. Therefore, the area ratio of ferrite is set to 95% or less. The area ratio of ferrite may be 94% or less or 92% or less.

[Martensite: 5-20%]
Martensite has a high dislocation density and is a hard structure, so it is a structure that contributes to improving tensile strength. By setting the area ratio of martensite to 5% or more, a tensile strength of 400 MPa or more can be ensured. From the viewpoint of improving strength, the higher the martensite area ratio is, the more preferable it is, and may be, for example, 7% or more, 10% or more, or 13% or more. On the other hand, when the area ratio of martensite is 20% or less, moldability and appearance can be ensured. The area ratio of martensite may be 17% or less or 15% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[At least one of bainite, pearlite, and retained austenite: 0 to 10% in total]
The residual structure other than ferrite and martensite may have an area ratio of 0%, but when the residual structure exists, the residual structure is at least one of bainite, pearlite, and retained austenite. From the viewpoint of ensuring the above-mentioned effects based on ferrite and martensite, the area ratio of at least one of residual structures, ie, bainite, pearlite, and retained austenite, is set to be 10% or less in total, for example, 8% or less, 6% or less, 4% or less. % or less, 3% or less, or 2% or less. In particular, the area percentage of retained austenite may be between 0 and 3%. For example, the area percentage of retained austenite may be 2% or less, 1% or less, 0.5% or less, 0.3% or less, or 0.1% or less. On the other hand, setting the area ratio of the remaining structure to 0% requires sophisticated control in the manufacturing process of the steel plate, which may lead to a decrease in yield. Therefore, the area ratio of the remaining tissue may be 0.5% or more or 1% or more.

[Identification of metallographic structure and calculation of area ratio]
Identification of metal structure and calculation of area ratio are performed using FE-SEM (measured using a field emission scanning electron microscope, e.g., JEOL JSM-7200F, accelerating voltage 15 kV) after corrosion using nital reagent or Repeller liquid. This is carried out using an optical microscope and X-ray diffraction method. Structure observation using FE-SEM and an optical microscope is performed at a magnification of 1,000 to 50,000 times on a 100 μm×100 μm area in a steel plate cross section parallel to the rolling direction and perpendicular to the plate surface. For each metal structure, measurement points are set at three locations, and the area ratio is determined by calculating the average value of the measured values. For example, if it is not possible to secure a measurement area of 100 μm in the thickness direction because the steel plate to be measured is thin, a measurement area of 10,000 μm ² will be secured while reducing the length in the thickness direction. . For example, a measurement area of 20 μm in the plate thickness direction and 500 μm in the rolling direction may be observed. However, if the number of crystal grains included in the plate thickness direction is too small, measurement accuracy may decrease, so the measurement length in the plate thickness direction is set to 10 μm or more, preferably 50 μm or more. The same applies to the "100 μm x 100 μm area" in the following description.

The area ratio of ferrite is within the range of 1/8th position to 3/8th position of the plate thickness, centered at the 1/4th position of the plate thickness, in an electron channeling contrast image by FE-SEM (field emission scanning electron microscope). It is determined by observing a 100 μm x 100 μm area. More specifically, it can be calculated by image analysis using image analysis software Image J.

The area ratio of martensite is determined by the following procedure. First, the observation surface of the sample is etched with repeller liquid, and then an area of 100 μm x 100 μm is observed using FE-SEM within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 plate thickness position. do. In repeller corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded region corresponds to the total area ratio of martensite and retained austenite. Specifically, the image analysis software Image J was used to binarize the metal structure based on differences in brightness.The black part of the image data is ferrite, and the white part that has not been corroded by the repeller is martensite and retained austenite. Total organization. Therefore, the area ratio of martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method, which will be described later, from the area ratio of this uncorroded region. The area ratio of martensite determined by this method also includes the area ratio of tempered martensite.

The area ratio of retained austenite is calculated by X-ray diffraction method. First, the sample is removed by mechanical polishing and chemical polishing from the surface of the sample to a depth of 1/4 in the thickness direction. Next, the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220) and (311) of the fcc phase obtained using MoKα rays at a position of 1/4 of the plate thickness. From this, the tissue fraction of retained austenite is calculated. A general 5-peak method is used as this calculation method. The calculated microstructure fraction of retained austenite is determined as the area fraction of retained austenite.

Identification of bainite and calculation of area ratio are performed in the following steps. First, the observation surface of the sample is corroded with a nital reagent, and then an area of 100 μm x 100 μm is observed using FE-SEM within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 plate thickness position. do. Bainite is identified in the following manner from the position and arrangement of cementite contained within the tissue in this observation region. Bainite is classified into upper bainite and lower bainite, and in upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite. In lower bainite, cementite exists inside lath-like bainitic ferrite, and the crystal orientation relationship between bainitic ferrite and cementite is one type, and cementite has the same variant. Based on these characteristic points, upper bainite and lower bainite can be respectively identified. In the present invention, these are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis. Note that cementite is observed as a region with high brightness on the SEM image. By analyzing the chemical composition of cementite using energy dispersive X-ray spectroscopy (EDS), it can be confirmed that cementite is a carbonitride mainly composed of iron.

Identification of pearlite and calculation of area ratio are performed in the following steps. First, the observation surface of the sample is corroded with a nital reagent, and then a range from 1/8 to 3/8 of the plate thickness, centered at the 1/4 plate thickness position, is observed using an optical microscope. Images observed with an optical microscope are binarized based on differences in brightness, and areas where black and white areas are dispersed in a lamellar manner are identified as pearlite, and the area ratio of this area is calculated based on image analysis. More specifically, the image analysis software Image J was used to binarize the difference in brightness, and an image captured at a measurement magnification of 500 times including an imaging range of 100 μm x 100 μm was used to calculate the area of pearlite using a point counting method. Find the rate. In the above imaging range, draw 8 lines parallel to the rolling direction at equal intervals and 8 lines perpendicular to the rolling direction at equal intervals, and calculate the proportion of pearlite among the 64 intersections of these lines. It can be calculated as an area fraction.

[Average particle spacing of martensite: 2.5 μm or less]
In an embodiment of the present invention, the average particle spacing of martensite, which is a hard structure, is controlled to be 2.5 μm or less. The average particle spacing of martensite is an index representing the uniformity of hard structure distribution in the micro region. The smaller the average particle spacing of martensite, the more densely and uniformly the hard structure is dispersed, and therefore, it can be said that the uniformity is higher. The appearance of the steel plate after press forming becomes better as the amount of deformation of the steel plate during press forming is more uniform, especially in the width direction of the steel plate. The amount of deformation of a steel plate is strongly influenced by the distribution of the hard structure, so in order to make the amount of deformation of the steel plate uniform in the width direction of the steel plate, it is necessary to make the distribution of the hard structure in the metal structure uniform. . In addition to controlling the standard deviation in the area ratio of martensite, which will be explained later, by controlling the average particle spacing of martensite to 2.5 μm or less, the amount of deformation of the steel plate in the width direction can be reduced even during forming such as press forming. As a result, a good appearance after molding can be achieved. The average particle spacing of martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, most preferably 2.0 μm or less or 1.8 μm or less. Although the lower limit is not particularly limited, for example, the average particle spacing of martensite may be 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.

[Measurement of average particle spacing of martensite]
The average particle spacing of martensite is determined as follows. First, a sample having a steel plate cross section in a direction parallel to the rolling direction and perpendicular to the plate surface is taken, and this cross section is used as an observation surface. On this observation surface, an area of 100 μm x 100 μm within the range of 1/8 to 3/8 of the plate thickness centered at the 1/4 position of the plate thickness was set as the observation area, and martensite was identified using FE-SEM. do. Specifically, using image analysis software Image J, the metal structure is binarized based on the difference in brightness, and martensite is identified. When using repeller liquid, the black part of the image data is ferrite, and the white part not corroded by repeller is the total structure of martensite and retained austenite. However, in the steel sheet according to the embodiment of the present invention, the area ratio of retained austenite is sufficiently low compared to the area ratio of martensite, so the white structure can be regarded as martensite. Next, among the identified martensite grains, the distance between the centers (centers of gravity) of all adjacent martensite grains is calculated based on image analysis as the grain spacing, and the average value of the calculated grain spacing is Strictly speaking, it is determined as the average particle spacing of particles containing martensite and/or retained austenite.

[The standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less]
In the embodiment of the present invention, the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to be 1.5% or less. The standard deviation is an index representing the uniformity of the hard tissue in the macro region. Appearance, which is an issue during press forming, depends on minute irregularities on the surface of the steel sheet due to differences in the amount of deformation in the width direction of the steel sheet. Therefore, if there is a large variation in the area ratio of hard structures included in the sheet thickness in the direction perpendicular to the rolling direction and the sheet thickness direction, a difference will occur in the amount of deformation in the width direction of the steel sheet, resulting in minute This results in unevenness. Therefore, it is effective to reduce the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction, that is, the width direction of the steel sheet. More specifically, in addition to controlling the average particle spacing of martensite as described above, by controlling the standard deviation to 1.5% or less, the width of the steel plate can be improved even during forming such as press forming. Variations in the amount of deformation can be further reduced, and as a result, a good appearance after molding can be achieved. The standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less. Although the lower limit is not particularly limited, for example, the standard deviation may be 0.1% or more, 0.3% or more, or 0.5% or more.

[Measurement of standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction]
The standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is determined as follows. First, a metallographic image in a cross section of a steel plate in a 50 mm area in a direction perpendicular to the rolling direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be acquired and stitched together to form a 50 mm image. Next, the acquired image is divided into every 100 μm (0.1 mm) in the direction perpendicular to the rolling direction, and the area ratio of martensite in the entire plate thickness is calculated for each divided range. The standard deviation of the martensite area ratio is calculated based on the martensite area ratio calculated from each of the 500 divided images in total. This operation is performed for three regions having different positions in the rolling direction, and the average value of the standard deviations determined for each region is determined as the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction.

When the rolling direction of the steel plate is not clear, the following method, for example, is adopted as a method for specifying the rolling direction of the steel plate. After finishing the thickness section of the steel plate by mirror polishing, the S concentration is measured using an electron probe micro analyzer (EPMA). The measurement conditions are an accelerating voltage of 15 kV, a measurement pitch of 1 μm, and a distribution image in a 500 μm square range at the center of the plate thickness. At this time, the stretched region with a high S concentration is determined to be an inclusion such as MnS. When observing, you may observe from multiple fields of view. Next, using the plate thickness cross section first observed using the above method as a reference, rotate the plate thickness direction in 5° increments in the range of 0° to 180° as an axis, and then take a cross section using the above method. Observe. The average value of the lengths of the long axes of the plurality of inclusions in each of the obtained cross sections is calculated for each cross section, and the cross section in which the average value of the lengths of the long axes of the inclusions is maximum is identified. A direction parallel to the longitudinal direction of the inclusion in the cross section is determined as the rolling direction. For example, if a measurement area of 500 μm square cannot be secured because the steel plate to be measured is thin, a measurement area of 250,000 μm ² is secured while reducing the length in the thickness direction.

Next, the reason for limiting the chemical composition of the steel plate according to the embodiment of the present invention will be explained. Hereinafter, % in chemical composition means mass %.

[C:0.03-0.08%]
C is an element that secures a predetermined amount of martensite and improves the strength of the steel plate. In order to sufficiently obtain such effects, the C content is set to 0.03% or more. The C content may be 0.04% or more or 0.05% or more. On the other hand, if too much C is contained, the strength may become too high and the elongation may decrease. Therefore, the C content is set to 0.08% or less. The C content may be 0.07% or less or 0.06% or less.

[Si: 0.01 to 1.00%]
Si is an element that improves the strength of steel sheets through solid solution strengthening. In order to sufficiently obtain such effects, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if Si is contained excessively, it becomes difficult to remove scale generated during hot rolling, which may lead to deterioration in appearance. Therefore, the Si content is set to 1.00% or less. The Si content may be 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.

[Mn: 0.50-3.00%]
Mn is an element that improves hardenability and contributes to improving steel sheet strength. In order to sufficiently obtain such effects, the Mn content is set to 0.50% or more. The Mn content may be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred manufacturing method for steel sheets, which will be explained later, the metal structure in the steel sheet before final annealing is changed in order to uniformly disperse martensite in both the micro and macro regions in the final metal structure. It must be composed of bainite and/or martensite. Therefore, improving hardenability by adding Mn is also important in improving the appearance after molding. On the other hand, when Mn is contained excessively, ferrite transformation is excessively suppressed, a desired amount of ferrite cannot be secured, and elongation properties may decrease. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[P: 0.1000% or less]
P is an impurity element, and is an element that causes embrittlement of the welded portion and deteriorates the plating properties. Therefore, the P content is set to 0.1000% or less. The P content may be 0.0600% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less. The lower the P content, the better, and the lower limit is not particularly limited and may be 0%. On the other hand, if the P content is reduced to less than 0.0001% in a practical steel plate, the manufacturing cost will significantly increase, resulting in an economic disadvantage. Therefore, the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[S: 0.0200% or less]
S is an impurity element that inhibits weldability and also inhibits manufacturability during casting and hot rolling. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0120% or less, 0.0100% or less, or 0.0080% or less. The lower the S content, the better, and the lower limit is not particularly limited and may be 0%. On the other hand, if the S content is reduced to less than 0.0001% in a practical steel plate, the manufacturing cost will significantly increase, resulting in an economic disadvantage. Therefore, the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[Al: 1.000% or less]
Al is an element that functions as a deoxidizing agent and is an effective element for increasing the strength of steel. Although the Al content may be 0%, in order to fully obtain these effects, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. On the other hand, when Al is contained excessively, coarse oxides are formed, which may reduce toughness. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.

[N: 0.0200% or less]
N is an element that causes blowholes to occur during welding. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The lower the N content, the better, and the lower limit is not particularly limited and may be 0%. On the other hand, if N is reduced to less than 0.0001% in a practical steel plate, the manufacturing cost will significantly increase, resulting in an economic disadvantage. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[O: 0-0.020%]
O is an element that causes blowholes to occur during welding. Therefore, the O content is set to 0.020% or less. The O content may be 0.018% or less, 0.015% or less, 0.010% or less, or 0.008% or less. The lower the O content, the better, and the lower limit is not particularly limited and may be 0%. On the other hand, if O is reduced to less than 0.0001% in a practical steel plate, the manufacturing cost will significantly increase, resulting in an economic disadvantage. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

The basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe for the purpose of improving properties, if necessary. For example, the steel plate has Cr: 0 to 2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0-1.000%, Ni: 0-1.00%, W: 0-0.100%, V: 0-1.000%, Ta: 0-0.100%, Co: 0-3 .000%, Sn: 0-1.000%, Sb: 0-0.500%, As: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca Contains at least one of: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, and Bi: 0 to 0.0500%. But that's fine. These optional elements will be explained in detail below.

[Cr: 0-2.000%]
Cr, like Mn, is an element that improves hardenability and contributes to improving the strength of the steel sheet. The Cr content may be 0%, but in order to obtain the above effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, even if Cr is contained excessively, the effect may be saturated, leading to an increase in manufacturing costs. Therefore, the Cr content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, or 0.500% or less.

[Mo: 0-1.000%]
Mo, like Cr, is an element that contributes to increasing the strength of the steel sheet. This effect can be obtained even with a small amount. Although the Mo content may be 0%, in order to obtain the above effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, if Mo is contained excessively, hot workability may be reduced and productivity may be reduced. Therefore, the Mo content is preferably 1.000% or less. The Mo content may be 0.800% or less, 0.400% or less, or 0.200% or less.

[Ti: 0 to 0.500%]
Ti is an element effective in controlling the morphology of carbides. Ti can help increase the strength of ferrite. Although the Ti content may be 0%, in order to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, even if Ti is contained excessively, the effect may be saturated and the manufacturing cost may increase. Therefore, the Ti content is preferably 0.500% or less, and may be 0.400% or less, 0.200% or less, or 0.100% or less.

[Nb: 0 to 0.500%]
Like Ti, Nb is an element effective in controlling the morphology of carbides, and is also effective in improving the toughness of steel sheets by refining the structure. These effects can be obtained even in minute amounts. Although the Nb content may be 0%, in order to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more or 0.010% or more. On the other hand, when Nb is contained excessively, coarse carbides and the like are generated in the steel, which may reduce the toughness of the steel sheet. For this reason, the Nb content is preferably 0.500% or less. The Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less.

[B: 0 to 0.0100%]
B is an element that suppresses the formation of ferrite and pearlite and promotes the formation of martensite in the cooling process from austenite. Further, B is an element useful for increasing the strength of steel. These effects can be obtained even in minute amounts. The B content may be 0%, but in order to obtain the above effects, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more or 0.0010% or more. On the other hand, if B is contained excessively, toughness and/or weldability may deteriorate. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Cu: 0-1.000%]
Cu is an element that contributes to improving the strength of steel sheets. This effect can be obtained even with a small amount. The Cu content may be 0%, but in order to obtain the above effects, the Cu content is preferably 0.001% or more. The Cu content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if Cu is contained excessively, red-hot brittleness may occur and productivity in hot rolling may be reduced. Therefore, the Cu content is preferably 1.000% or less. The Cu content may be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[Ni: 0-1.00%]
Ni is an element effective in improving the strength of steel sheets. Although the Ni content may be 0%, in order to obtain the above effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.005% or more or 0.010% or more. On the other hand, if Ni is contained excessively, the weldability of the steel plate may deteriorate. For this reason, the Ni content is preferably 1.00% or less. The Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.

[W: 0-0.100%]
W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. Although the W content may be 0%, in order to obtain these effects, the W content is preferably 0.001% or more. The W content may be 0.005% or more or 0.010% or more. On the other hand, if W is contained excessively, weldability may deteriorate. Therefore, the W content is preferably 0.100% or less. The W content may be 0.080% or less, 0.040% or less, or 0.020% or less.

[V: 0-1.000%]
Like Ti and Nb, V is an element effective in controlling the morphology of carbides, and is also effective in improving the toughness of steel sheets by refining the structure. Although the V content may be 0%, in order to obtain the above effects, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if V is contained excessively, a large amount of precipitates may be generated, which may reduce toughness. Therefore, the V content is preferably 1.000% or less. The V content may be 0.400% or less, 0.200% or less, or 0.100% or less.

[Ta: 0-0.100%]
Ta, like W, is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.005% or more or 0.010% or more. On the other hand, even if Ta is contained excessively, the effect will be saturated, and if Ta is contained in the steel sheet more than necessary, the manufacturing cost will increase. Therefore, the Ta content is preferably 0.100% or less. The Ta content may be 0.080% or less, 0.040% or less, or 0.020% or less.

[Co: 0-3.000%]
Co, like Ni, is an element effective in improving the strength of steel sheets. Although the Co content may be 0%, in order to obtain the above effects, the Co content is preferably 0.001% or more. The Co content may be 0.005% or more, 0.010% or more, or 0.100% or more. On the other hand, if Co is contained excessively, hot workability may decrease, leading to an increase in raw material cost. For this reason, the Co content is preferably 3.000% or less. The Co content may be 2.000% or less, 1.000% or less, 0.500% or less, or 0.200% or less.

[Sn: 0 to 1.000%]
Sn is an element that can be contained in a steel plate when scrap is used as a raw material for the steel plate. Furthermore, Sn may cause embrittlement of ferrite. Therefore, the Sn content is preferably as low as possible, and is preferably 1.000% or less. The Sn content may be 0.100% or less, 0.040% or less, or 0.020% or less. Although the Sn content may be 0%, reducing the Sn content to less than 0.001% causes an excessive increase in refining cost. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[Sb: 0 to 0.500%]
Sb, like Sn, is an element that can be contained in a steel plate when scrap is used as a raw material for the steel plate. Furthermore, Sb strongly segregates at grain boundaries and may cause embrittlement of the grain boundaries. Therefore, the Sb content is preferably as low as possible, and is preferably 0.500% or less. The Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less. Although the Sb content may be 0%, reducing the Sb content to less than 0.001% causes an excessive increase in refining cost. Therefore, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[As: 0 to 0.050%]
As, like Sn and Sb, is an element that can be contained in a steel plate when scrap is used as a raw material for the steel plate. Furthermore, As is an element that strongly segregates at grain boundaries, and the smaller the As content, the more preferable it is. The As content is preferably 0.050% or less, and may be 0.040% or less or 0.020% or less. Although the As content may be 0%, reducing the As content to less than 0.001% causes an excessive increase in refining cost. Therefore, the As content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[Mg: 0 to 0.050%]
Mg controls the morphology of sulfides and oxides and contributes to improving the bending formability of the steel sheet. This effect can be obtained even with a small amount. The Mg content may be 0%, but in order to obtain the above effects, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, 0.001% or more, or 0.005%. On the other hand, even if Mg is contained excessively, the effect will be saturated, and if Mg is contained in the steel sheet more than necessary, the manufacturing cost will increase. Therefore, the Mg content is preferably 0.050% or less. The Mg content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[Zr: 0 to 0.050%]
Zr is an element that can control the form of sulfide in trace amounts. The Zr content may be 0%, but in order to obtain the above effects, the Zr content is preferably 0.0001% or more. The Zr content may be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, even if Zr is contained excessively, the effect will be saturated, and containing Zr in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, the Zr content is preferably 0.050% or less. The Zr content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[Ca: 0-0.0500%]
[Y: 0 to 0.0500%]
[La: 0 to 0.0500%]
[Ce: 0 to 0.0500%]
Ca, Y, La, and Ce are elements that can control the form of sulfide in trace amounts. The Ca, Y, La, and Ce contents may be 0%, but in order to obtain the above effects, it is preferable that the Ca, Y, La, and Ce contents are each 0.0001% or more, and 0.0005%. The content may be 0.0010% or more, 0.0020% or more, or 0.0030% or more. On the other hand, even if these elements are contained in excess, the effect will be saturated, and if they are contained in the steel sheet in excess of necessary, manufacturing costs will increase. Therefore, the Ca, Y, La, and Ce contents are each preferably 0.0500% or less, and may be 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[Bi:0 to 0.0500%]
Bi is an element that has the effect of improving formability by making the solidified structure finer. The Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0050%. % or more. On the other hand, even if Bi is contained excessively, the effect will be saturated, and if Bi is contained in the steel sheet more than necessary, the manufacturing cost will increase. Therefore, the Bi content is preferably 0.0500% or less, and may be 0.0400% or less, 0.0200% or less, or 0.0100% or less.

In the steel sheet according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are elements that are mixed in from steel raw materials and/or during the steel manufacturing process and are allowed to exist within a range that does not impede the properties of the steel sheet according to the embodiment of the present invention.

The chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the steel plate may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N using an inert gas melting-thermal conductivity method, and O using an inert gas melting-non-dispersive infrared absorption method.

[Plate thickness]
The steel plate according to the embodiment of the present invention has a thickness of, for example, 0.2 to 2.0 mm, although it is not particularly limited. A steel plate having such a thickness is suitable for use as a material for lid members such as automobile doors and hoods. The plate thickness may be 0.3 mm or more or 0.4 mm or more. Similarly, the plate thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. The thickness of the steel plate is measured with a micrometer.

[Plating]
The steel plate according to the embodiment of the present invention may further have a plating layer on the surface for the purpose of improving corrosion resistance. The plating layer may be any suitable plating layer, such as a hot-dip plating layer or an electroplating layer. The hot-dip plating layer may be, for example, a hot-dip galvanized layer, a hot-dip zinc alloy plating layer (a hot-dip plating layer composed of an alloy of zinc and additional elements such as Si and Al), or an alloy formed by alloying these platings. It may be a hot dip galvanized layer (alloyed plating layer). The hot-dip galvanized layer and the hot-dip zinc alloy plated layer are preferably plating layers containing less than 7% by mass of Fe, and the alloyed plating layer is a plating layer containing 7% by mass or more and 15% by mass or less of Fe. It is preferable that In the hot-dip galvanized layer, the hot-dip zinc alloy plated layer, and the alloyed plating layer, components other than zinc and Fe are not particularly limited, and various configurations can be adopted within a normal range. Furthermore, the plating layer may be, for example, an aluminum plating layer. Further, the amount of the plating layer to be deposited is not particularly limited and may be a general amount to be deposited.

[Mechanical properties]
According to the steel plate according to the embodiment of the present invention, high tensile strength, specifically, a tensile strength of 400 MPa or more can be achieved. The tensile strength is preferably 440 MPa or more or 480 MPa or more, more preferably 540 MPa or more or 600 MPa or more. Although the upper limit is not particularly limited, for example, the tensile strength may be 980 MPa or less or 900 MPa or less. Similarly, according to the steel sheet according to the embodiment of the present invention, excellent formability can be achieved, and more specifically, a total elongation of 20% or more can be achieved. The total elongation is preferably 22% or more, more preferably 25% or more or 30% or more. Although the upper limit is not particularly limited, for example, the total elongation may be 50% or less or 45% or less. Tensile strength and total elongation are measured by conducting a tensile test in accordance with JIS Z 2241:2011 based on a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate. .

Although the steel plate according to the embodiment of the present invention has high strength, specifically, a tensile strength of 400 MPa or more, it can maintain excellent formability and appearance even after forming such as press forming. Therefore, the steel sheet according to the embodiment of the present invention is very useful for use as outer panel members of automobiles, such as roofs, hoods, fenders, and doors, which require a high level of design, for example.

<Manufacturing method of steel plate>
Next, a preferred method for manufacturing a steel plate according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a steel plate according to an embodiment of the present invention, and is limited to those manufactured by the manufacturing method described below. It is not intended to.

The method for manufacturing a steel plate according to an embodiment of the present invention includes:
A hot rolling process comprising heating a slab having the chemical composition described above in connection with a steel plate to a temperature of 1100 to 1400°C for finish rolling and then winding at a temperature of 500 to 700°C, a hot rolling step in which the finishing temperature of the finish rolling is 800 to 1350°C;
A pickling process of pickling the obtained hot rolled steel sheet,
A cold rolling process in which pickled hot rolled steel sheets are cold rolled at a reduction rate of 20 to 90%;
The step of primary annealing the obtained cold-rolled steel sheet, the primary annealing is to heat the cold-rolled steel sheet and hold it at a maximum heating temperature of 3 to 950°C for 10 to 500 seconds, and then to 500 to 950°C. A primary annealing process that includes cooling to a cooling stop temperature of 350°C or less by controlling the average cooling rate in a temperature range of 700°C to 50°C/sec or more, and a secondary annealing of the cold rolled steel plate after the primary annealing. The secondary annealing is a step of heating the cold-rolled steel sheet and holding it at a maximum heating temperature of (Ac1+20) to 820°C for 10 to 500 seconds, followed by average cooling in a temperature range of 500 to 700°C. It is characterized by including a secondary annealing step in which the cooling rate is controlled at 30° C./second or higher, and the average cooling rate in the temperature range of 200 to 500° C. is controlled to 40° C./second or higher. Each step will be explained in detail below.

[Hot rolling process]
[Slab heating]
First, a slab having the chemical composition described above in connection with steel plate is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method. The slabs used contain relatively high amounts of alloying elements in order to obtain high strength steel sheets. For this reason, it is necessary to heat the slab to dissolve the alloying elements in the slab before hot rolling. If the heating temperature is less than 1100° C., the alloying elements will not be fully dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1400° C. or lower from the viewpoint of the capacity of the heating equipment and productivity.

[Rough rolling]
In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness or the like. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be ensured.

[Final rolling/winding]
The heated slab, or the optionally rough rolled slab, is then subjected to finish rolling. Since the slab used as described above contains a relatively large amount of alloying elements, it is necessary to increase the rolling load during hot rolling. For this reason, hot rolling is preferably performed at a high temperature. In particular, the finishing temperature of finish rolling is important in terms of controlling the metallographic structure of the steel sheet. If the finishing temperature of finish rolling is low, the metal structure may become non-uniform and formability may deteriorate. For this reason, the finishing temperature of finish rolling is set to 800°C or higher. On the other hand, in order to suppress coarsening of austenite, the finishing temperature of finish rolling is set to 1350° C. or lower. Next, the finish-rolled hot rolled steel sheet is wound up at a winding temperature of 500 to 700°C. The growth of oxide scale can be suppressed by setting the winding temperature to 500 to 700°C.

[Acid washing process]
Next, the obtained hot-rolled steel sheet is pickled to remove oxidized scale formed on the surface of the hot-rolled steel sheet. Pickling may be carried out under conditions suitable for removing oxide scale, and may be carried out once or in multiple steps to ensure removal of oxide scale.

[Cold rolling process]
The pickled hot rolled steel sheet is cold rolled at a rolling reduction of 20 to 90% in a cold rolling process. By setting the rolling reduction ratio in cold rolling to 20% or more, the shape of the cold rolled steel sheet can be kept flat and a decrease in ductility in the final product can be suppressed. On the other hand, by setting the rolling reduction ratio in cold rolling to 90% or less, it is possible to prevent the rolling load from becoming excessive and making rolling difficult. The number of rolling passes and the rolling reduction rate for each pass are not particularly limited, and may be appropriately set so that the rolling reduction rate of the entire cold rolling falls within the above range.

[Primary annealing process]
The obtained cold-rolled steel sheet is heated in the next primary annealing step, held at a maximum heating temperature of 3 to 950°C for 10 to 500 seconds, and then reduced to an average cooling rate of 50°C in the temperature range of 500 to 700°C. The cooling temperature is controlled to be at least 350° C./sec to a cooling stop temperature of 350° C. or less. Here, the Ac3 point (°C) is determined by cutting out a small piece from a cold-rolled steel plate and from the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/sec. By holding the temperature at Ac3 point or higher for a sufficient period of time to promote austenitization, and then rapidly cooling it to a temperature of 350°C or lower, the metal structure in the steel sheet after cooling is reliably changed to bainite and/or It is possible to construct a structure mainly composed of martensite, for example, full bainite or full martensite. Here, a structure mainly composed of bainite and/or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more, and full bainite is a structure containing at least one of bainite and martensite in an area ratio of 90% or more. It refers to a structure consisting of 100% bainite, and full martensite refers to a structure consisting of 100% martensite in terms of area ratio. A bainite and/or martensitic structure has many different interfaces inside it, compared to a structure such as ferrite. Therefore, by making the metal structure of the steel sheet before the secondary annealing process, that is, the final annealing process, consist of bainite and/or martensite, the metal structure is heated at the stage of secondary annealing. It becomes possible to disperse and generate a large number of carbides that can serve as nucleation sites for austenite. As a result, austenite is generated finely and uniformly throughout the steel sheet from these widely dispersed nucleation sites, and then martensite is generated from these austenites, so that in the metal structure obtained after secondary annealing, The average particle spacing of martensite is controlled to be 2.5 μm or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to be 1.5% or less. That is, it becomes possible to achieve a metal structure in which martensite is uniformly dispersed in both micro and macro regions.

If the primary annealing process is not performed, the metal structure in the steel sheet before the final annealing (secondary annealing) process cannot be composed of a structure mainly composed of bainite and/or martensite. In addition, even if the primary annealing process is performed, if the maximum heating temperature in the primary annealing process is less than 3 points Ac or the holding time is less than 10 seconds, the austenitization will be insufficient, and even if the steel sheet is cooled even after cooling. The metal structure inside cannot be composed of a structure mainly composed of bainite and/or martensite. That is, the total area ratio of bainite and martensite cannot be made 90% or more. On the other hand, since heating and holding at a higher temperature and for a longer time reduces productivity, the maximum heating temperature in the primary annealing step is set to 950° C. or lower, and the holding time is set to 500 seconds or lower.

In addition, if the average cooling rate in the temperature range of 500 to 700°C in the primary annealing step is less than 50°C/second or the cooling stop temperature is over 350°C, ferrite will be generated during cooling and the The metal structure cannot have a total area ratio of bainite and martensite of 90% or more. Therefore, the average cooling rate needs to be 50°C/second or more, and the upper limit is preferably 300°C/second. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, and may be, for example, room temperature (25°C), and preferably 200°C.

[Secondary annealing process (final annealing process)]
The cold-rolled steel sheet after the primary annealing is heated again in the next secondary annealing step, held at the maximum heating temperature of (Ac1+20) ~ 820°C for 10 ~ 500 seconds, and then heated at the average temperature in the temperature range of 500 ~ 700°C. Cooling is achieved by controlling the cooling rate to 30°C/second or more, and further controlling the average cooling rate in the temperature range of 200 to 500°C to 40°C/second or more. Here, the Ac1 point (°C) is determined by cutting out a small piece from a cold-rolled steel sheet and calculating the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/sec, as in the case of the Ac3 point. First, in the step of heating the steel plate after primary cooling to the maximum heating temperature of (Ac1+20) to 820°C, carbides are dispersed on many interfaces contained inside bainite and/or martensite in the metal structure. It can be generated by Next, by holding the maximum heating temperature corresponding to the two-phase region of ferrite and austenite for 10 to 500 seconds, austenite is transferred from the carbides to the entire steel sheet while maintaining the state in which the carbides are dispersed on the interface. It can be produced finely and uniformly. Finally, by controlling the average cooling rate in the temperature range of 500 to 700°C to 30°C/second or more, and further controlling the average cooling rate in the temperature range of 200 to 500°C to 40°C/second or more, fine dispersion is achieved. As a result, the average particle spacing of martensite can be controlled to 2.5 μm or less, and the area of martensite in the direction perpendicular to the rolling direction and the plate thickness direction can be appropriately generated. The standard deviation in the ratio is controlled to 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both the micro and macro regions.

If the maximum heating temperature in the secondary annealing step is less than Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure as described above cannot be obtained, and in particular, martensite cannot be properly generated. I can't. On the other hand, when the maximum heating temperature exceeds 820° C., the area ratio of austenite becomes too high and the area ratio of ferrite cannot be increased to 80% or more. Furthermore, due to the high temperature, it is no longer possible to maintain the dispersed state of carbides on the interface, and the uniform dispersion of martensite in both the micro and macro regions is prevented in the final metal structure. be unable to achieve it. Moreover, if the holding time exceeds 500 seconds, the austenite grains will become coarse, and the martensite grains obtained by subsequent cooling will also become relatively coarse. In such a case, it is not possible to obtain a fine martensite structure in which the average particle spacing of martensite is controlled to be 2.5 μm or less.

Furthermore, if the average cooling rate in the temperature range of 500 to 700°C in the secondary annealing step is less than 30°C/second, the transformation from austenite to bainite etc. will be accelerated, and even if the subsequent cooling is performed appropriately, However, the desired amount of martensite may not be obtained. In this case, the desired strength cannot be achieved and/or a homogeneous distribution of martensite, especially in the microscopic region, cannot be achieved. Therefore, the average cooling rate in the temperature range of 500 to 700°C needs to be 30°C/second or more, and the upper limit is, for example, 200°C/second or less, preferably 60°C/second or less. On the other hand, if the average cooling rate in the temperature range of 200 to 500°C is less than 40°C/sec, the transformation from austenite to martensite cannot be promoted, and the formation of other structures such as bainite is similarly inhibited. It ends up being too many. Therefore, the average cooling rate in the temperature range of 200 to 500°C needs to be 40°C/second or more, and the upper limit is, for example, 200°C/second or less, preferably 80°C/second or less.

In the above method, the steel plate according to the embodiment of the present invention is manufactured by two annealing treatments including primary annealing and secondary annealing. It is not necessarily limited to what is manufactured by, for example, it is also possible to manufacture by one annealing treatment. More specifically, by configuring the metal structure of the steel sheet after the hot rolling process to be full bainite or full martensite, it is possible to omit the primary annealing described above. However, in this case, it is necessary to appropriately control the cooling conditions and coiling temperature after hot rolling, and it is also important to control the rolling reduction in the subsequent cold rolling. This is because if the rolling reduction in cold rolling becomes high, recrystallization occurs during heating in the subsequent annealing process, making it impossible to maintain the metal structure formed in the hot rolling process.

[Plating process]
For the purpose of improving corrosion resistance, etc., the surface of the obtained cold rolled steel sheet may be subjected to plating treatment. The plating process may be hot-dip plating, alloyed hot-dip plating, electroplating, or the like. For example, the steel plate may be subjected to hot-dip galvanizing treatment, or alloying treatment may be performed after hot-dip galvanizing treatment. Specific conditions for the plating treatment and alloying treatment are not particularly limited, and may be any suitable conditions known to those skilled in the art. For example, in hot-dip galvanizing, the temperature of the plate immersed in the galvanizing bath (temperature of the steel plate when immersed in the hot-dip galvanizing bath) ranges from 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C). A temperature range of up to 50° C. higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature + 50° C.) is preferred. When alloying the hot-dip galvanized layer, it is preferable to heat the steel plate on which the hot-dip galvanized layer is formed to a temperature in the range of 400 to 600°C.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way.

In the following examples, steel plates according to embodiments of the present invention were manufactured under various conditions, and the tensile strength, formability, and appearance properties of the obtained steel plates were investigated.

First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to a predetermined temperature of 1100 to 1400°C and hot rolled. Hot rolling was carried out by performing rough rolling and finish rolling, and the finish rolling temperature and coiling temperature were as shown in Table 2. Next, the obtained hot rolled steel sheets were pickled and then cold rolled at the rolling reduction ratio shown in Table 2 to obtain cold rolled steel sheets having a thickness of 0.4 mm. Next, the obtained cold rolled steel sheets were subjected to primary annealing and secondary annealing under the conditions shown in Table 2. Finally, hot-dip galvanizing was appropriately performed as a plating treatment, and some of them were further alloyed at the alloying temperatures shown in Table 2.

The properties of the obtained steel plate were measured and evaluated by the following methods.

[Tensile strength (TS) and total elongation (El)]
Tensile strength (TS) and total elongation (El) were determined by a tensile test in accordance with JIS Z 2241:2011 based on a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel plate. It was measured by doing.

[Appearance after molding]
The appearance after molding was evaluated based on the degree of ghost lines generated on the surface of the door outer member to which approximately 5% strain was applied by press molding. The surface after press molding was ground with a grindstone, and the linear striped pattern that appeared on the surface and extending substantially parallel to the rolling direction was determined to be a ghost line and evaluated. An arbitrary area of 100 mm x 100 mm was visually confirmed, and when no streak pattern was observed, it was determined to be passed (〇), and when a streak pattern was observed, it was determined to be failed (×). In this test, we evaluated the appearance after forming by actually press forming the door outer member, but the evaluation target may also be a formed member that can be estimated to have been subjected to approximately 5% strain due to press forming. Similarly, a test piece taken from a specimen with a prestrain of 5% may be evaluated, and similar results can be obtained by such a test method. In addition, in the case of a test piece taken from a steel plate, it is possible to evaluate a JIS No. 5 test piece whose longitudinal direction is perpendicular to the rolling direction and the plate thickness direction, with a pre-strain of 5%.

A steel plate with a tensile strength (TS) of 400 MPa or more, a total elongation (El) of 20% or more, and a passing evaluation of appearance after forming was evaluated as a steel plate that can achieve both strength, formability, and appearance after forming. The results are shown in Table 3.

Referring to Tables 1 to 3, in Comparative Example 16, the TS was too high due to the high C content, and the El was decreased. In Comparative Example 17, ferrite transformation was suppressed due to the high Mn content, and El was similarly reduced. In Comparative Example 18, the maximum heating temperature in the primary annealing step was low, resulting in insufficient austenitization, and even after subsequent cooling, the metal structure in the steel sheet was composed of a structure mainly composed of bainite and/or martensite. It is thought that it was not possible. As a result, the average particle spacing of martensite in the metal structure obtained after secondary annealing is more than 2.5 μm, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5 μm. %, and the appearance after molding deteriorated. It is considered that in Comparative Example 19, the cooling stop temperature of the primary annealing step was high, and therefore the transformation from austenite to bainite and/or martensite could not proceed sufficiently. As a result, the average particle spacing of martensite in the metal structure obtained after secondary annealing is more than 2.5 μm, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1. The content exceeded .5%, and the appearance after molding deteriorated. In Comparative Example 20, the average cooling rate in the temperature range of 500 to 700 ° C. in the primary annealing step was slow, so it is thought that the transformation from austenite to bainite and/or martensite could not proceed sufficiently. It will be done. As a result, the average particle spacing of martensite in the metal structure obtained after secondary annealing is more than 2.5 μm, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5 μm. %, and the appearance after molding deteriorated. In Comparative Example 21, since the maximum heating temperature in the secondary annealing step was high, austenitization progressed too much, and the desired amount of ferrite could not be obtained in the metal structure after cooling. Furthermore, a large amount of residual structure was generated, making it impossible to achieve uniform dispersion of martensite in both the micro and macro regions. As a result, both moldability and post-molding appearance deteriorated. It is thought that in Comparative Example 22, the holding time in the secondary annealing step was long, so that the austenite grains became coarse. As a result, the average particle spacing of martensite in the metal structure obtained after secondary annealing was more than 2.5 μm, and the appearance after molding was deteriorated. In Comparative Example 23, the average cooling rate in the temperature range of 500 to 700°C in the secondary annealing step was low, so the transformation from austenite to bainite etc. was promoted, and the desired amount of martensite could not be obtained. As a result, the average particle spacing of martensite in the metal structure obtained after secondary annealing was more than 2.5 μm, and the appearance after molding was deteriorated. In Comparative Example 24, the average cooling rate in the temperature range of 200 to 500°C in the secondary annealing step was low, so the transformation from austenite to martensite could not be promoted, and similarly, a large amount of bainite was generated. Ta. As a result, similarly, in the metal structure obtained after secondary annealing, the average particle spacing of martensite exceeded 2.5 μm, and the appearance after molding deteriorated. In Comparative Examples 22 to 24, although the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction was 1.5% or less, the average particle spacing of martensite was more than 2.5 μm. That is, although the uniformity of martensite was ensured in the macro area, the uniformity of martensite was not ensured in the micro area. Therefore, in these comparative examples, it is considered that the appearance after molding deteriorated due to the presence of locally non-uniform martensite. In Comparative Example 25, since the first annealing step was not performed, the metal structure in the steel sheet before the second annealing step could not be composed of a structure mainly composed of bainite and/or martensite. As a result, the average particle spacing of martensite in the metal structure obtained after secondary annealing is more than 2.5 μm, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5 μm. %, and the appearance after molding deteriorated. In Comparative Example 26, since the maximum heating temperature in the secondary annealing step was low, martensite could not be appropriately generated in the metal structure after cooling, and the desired TS could not be obtained.

In contrast, the steel sheets according to all the invention examples have a predetermined chemical composition and furthermore, by appropriately controlling the proportions of ferrite and martensite in the metal structure, the steel sheets have a TS of 400 MPa or more and a TS of 20% or more. In addition to achieving an El of By controlling the deviation to 1.5% or less, we were able to suppress the formation of minute irregularities on the steel plate surface and significantly suppress the generation of ghost lines even when strain was applied by press forming. . When cross-sectional observation of the metallographic structure of the cold-rolled steel sheets before secondary annealing according to all the invention examples, all of them were composed of martensite with an area ratio of 90% or more.

Claims (2)

1. In mass%,
   C: 0.03-0.08%,
   Si: 0.01-1.00%,
   Mn: 0.50-3.00%,
   P: 0.1000% or less,
   S: 0.0200% or less,
   Al: 1.000% or less,
   N: 0.0200% or less,
   O: 0 to 0.020%,
   Cr: 0-2.000%,
   Mo: 0-1.000%,
   Ti: 0 to 0.500%,
   Nb: 0 to 0.500%,
   B: 0 to 0.0100%,
   Cu: 0 to 1.000%,
   Ni: 0 to 1.00%,
   W: 0-0.100%,
   V: 0-1.000%,
   Ta: 0-0.100%,
   Co: 0-3.000%,
   Sn: 0-1.000%,
   Sb: 0 to 0.500%,
   As: 0 to 0.050%,
   Mg: 0 to 0.050%,
   Zr: 0 to 0.050%,
   Ca: 0-0.0500%,
   Y: 0 to 0.0500%,
   La: 0 to 0.0500%,
   Ce: 0 to 0.0500%,
   Has a chemical composition consisting of Bi: 0 to 0.0500%, and the balance: Fe and impurities,
   In area ratio,
   Ferrite: 80-95%,
   Martensite: 5 to 20%, and at least one of bainite, pearlite, and retained austenite: 0 to 10% in total,
   The average particle spacing of martensite is 2.5 μm or less,
   A steel sheet having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
2. The chemical composition is in mass%,
   Cr: 0.001-2.000%,
   Mo: 0.001 to 1.000%,
   Ti: 0.001 to 0.500%,
   Nb: 0.001-0.500%,
   B: 0.0001 to 0.0100%,
   Cu: 0.001 to 1.000%,
   Ni: 0.001 to 1.00%,
   W: 0.001-0.100%,
   V: 0.001-1.000%,
   Ta: 0.001 to 0.100%,
   Co: 0.001 to 3.000%,
   Sn: 0.001 to 1.000%,
   Sb: 0.001 to 0.500%,
   As: 0.001 to 0.050%,
   Mg: 0.0001-0.050%,
   Zr: 0.0001 to 0.050%,
   Ca: 0.0001-0.0500%,
   Y: 0.0001-0.0500%,
   La: 0.0001 to 0.0500%,
   Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%
   The steel plate according to claim 1, characterized in that it contains at least one of the following.