JP2017214624A - Steel plate with good formability, corrosion resistance and ridging property, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Cr-added steel plate which has high corrosion resistance, formability and ridging properties, and an efficient manufacturing method for the steel plate.SOLUTION: After a steel material containing less than 3-13 mass% of Cr is strained with a shot blast etc., after cold rolling and then a Cr film is formed, a heat treatment of an α-γ transformation point to 1,300°C or less is carried out. Denoting a mean Cr concentration of the whole steel plate as X mass%, a region having a Cr concentration equal to or larger than X is represented as an A layer and a region having a Cr concentration less than X is represented as a B layer, A layers and a B layer forming a stack constitution A-B-A. The steel plate is characterized in that: a mean composition of the A layers containing 10.5 mass% or more of Cr; a mean composition of the B layer contains less than 3-13 mass% of Cr and is an α-γ transformation component-based composition which is an α phase at room temperature; a {222} plane integration degree of an αFe phase of the A layers is less than 60%, and a {222} plane integration degree of an αFe phase of the B layer is less 60-99%; and further (mean crystal particle size of B layer)/(mean crystal particle size of A layer) is 1.5 or larger.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel sheet containing Cr that is excellent in workability and corrosion resistance, such as deep drawing and press molding, and also excellent in ridging resistance, and a method for producing the same.

Conventionally, ferritic stainless steel contains 10.5% or more of Cr in a steel plate in order to impart corrosion resistance. In order to further improve the corrosion resistance, it is necessary to increase the Cr content.
However, as the Cr content increases, there is a problem that the workability deteriorates due to a decrease in the accumulation of {111} oriented crystal grains in the αFe layer necessary for ensuring the workability of the steel sheet. In addition, as the price of rare metals has risen, there has been a problem of increased alloy costs.

Conventionally, as a technique for ensuring corrosion resistance while suppressing alloy costs, a technique for increasing the Cr concentration only in the surface layer portion of a steel sheet is known from Patent Documents 1 and 2 and the like.
With these technologies, the corrosion resistance of the steel sheet can be secured to some extent with a small amount of Cr, but if the Cr content of the steel material in the center layer is reduced to increase the accumulation of {111} orientation grains, the corrosion resistance of the steel sheet as a whole decreases. End up.

  On the other hand, the inventors of the present invention disclosed in Patent Document 3 that a specific heat treatment was performed on a steel sheet composed of an α-γ transformation component system containing 3 mass% or more and less than 13 mass% of Cr. A Cr-enriched portion having a Cr concentration of 10.5% by mass or more is formed over a depth range of 0.1 to 50 μm, and the {222} plane integration degree of αFe phase is 60% over almost the entire plate thickness. A Cr-added highly corrosion-resistant steel sheet is proposed in which the corrosion resistance and workability are both balanced at a high level by 99% or less.

  On the other hand, ferritic stainless steel has a big problem of securing ridging resistance. Ridging is a surface unevenness that occurs after molding such as pressing. When ridging occurs, not only the beauty is impaired, but also the problem of increasing the load of the surface polishing process after processing occurs, which becomes a big problem when making a product. In Patent Document 3, it is possible to improve the workability such as deep drawing by improving the {222} plane integration degree of the entire steel sheet, but since ridging resistance is not taken into consideration at all, texture control is performed. In the process, a group of colonies with similar crystal orientations (colonies) are formed continuously in the rolling direction, and the colonies are deformed like coarse crystal grains during processing, so ridging occurs depending on the type of processing. That was a problem.

  Patent Document 4 discloses a technique for increasing the {200} plane integration degree of the entire steel sheet by performing blast processing on the surface layer as texture control in Si-containing steel. This is to increase the magnetic flux density. In this case as well, the {100} oriented grains on the surface grow in a columnar shape in the center of the steel sheet while maintaining the grain size projected on the surface. A group (colony) is continuously present in the rolling direction with a relatively large particle size. Since the technology of Patent Document 4 is not related to workability and ridging resistance at all, these properties are not considered at all, but even if these properties are considered, Improvements in processability and ridging resistance when processed are not expected.

Japanese Patent Publication No. 6-27318 JP-A-5-70926 JP 2014-088661 A International Publication No. 2011/052654

  It is an object of the present invention to provide a steel sheet that improves both workability and corrosion resistance in a steel sheet containing Cr, such as a ferritic stainless steel sheet, and further suppresses generation of ridging.

  The present inventors previously suppressed Cr content by concentrating Cr on the surface layer portion of a steel sheet having a relatively low Cr content of less than 13% as disclosed in Patent Document 3. The means to increase the degree of {111} crystal orientation to improve the workability at the same time as securing the corrosion resistance is studied, and (i) if the rolling reduction ratio of the cold rolling is optimized in the steel sheet manufacturing process, at least the steel sheet {222} texture can be formed in the surface layer part, and (ii) the {222} texture in the region is preserved by increasing the Cr concentration in the surface part to form an α single phase structure, and the temperature exceeds the A3 transformation point. It has been found that by performing a heat treatment for heating and cooling, the aggregate structure can be grown on the entire steel sheet, and a steel sheet structure excellent in workability can be obtained.

In the present invention, further, attention is paid to the fact that ridging is suppressed by the anisotropy of crystal grains of the steel sheet surface layer, and (iii) {111} orientation including {100} orientation grains in the {222} texture of the surface layer. Grains other than grains, that is, orientations used when calculating the {222} plane integration degree, {110}, {200}, {211}, {310}, {321}, {411}, {420}, {332} , {521}, {442} (hereinafter referred to as grains of orientation group other than {222}), the orientation group other than {222} on the surface layer does not grow greatly even by the heat treatment. The steel plate center layer has a structure including large crystal grains grown from the {111} orientation grains of the surface layer toward the inside of the steel plate, and the size of the crystal grains of the steel plate surface layer and the steel plate center layer can be made different. Be able to solve gender issues Heading was.
The gist of the present invention as a result of such examination is as follows.

(1) In a steel sheet having a plurality of regions having different compositions in the thickness direction,
When the average Cr concentration of the whole steel sheet is X mass%,
The region where the Cr concentration is X or more is the A layer, the region less than X is the B layer, and the A layer and the B layer have a laminated structure of A-B-A in the plate thickness direction.
The average composition of layer A contains Cr: 10.5% by mass or more, or Cr: 10.5% by mass or more, and at least Al, Ga, Mo, Nb, Si, Sn, Ti, V, W, Zn A composition containing one or more ferrite forming elements,
The average composition of B layer contains Cr: 3 mass% or more and less than 13 mass%, or Cr: 3 mass% or more and less than 13 mass%, Ni: 0.1 mass% or more and less than 1 mass%, and Mn: 0.00%. It is a composition of an α-γ transformation component system containing either one or both of 6% by mass or more and less than 1% by mass and being an α phase at room temperature,
The {222} plane integration degree of the αFe phase of the A layer is less than 60%,
{222} plane integration degree of the αFe phase of the B layer is 60% or more and 99% or less,
Furthermore, a steel sheet excellent in workability, corrosion resistance, and ridging resistance, wherein (average grain size of layer B) / (average grain size of layer A) is 1.5 or more.
Here, the {222} plane integration degree is 11 planes {110}, {200}, {211}, {310}, {222}, {321} with respect to the plane orientation of the αFe layer parallel to the steel plate surface. , {411}, {420}, {332}, {521}, {442} integrated intensity is measured, and each measured value is divided by the theoretical integrated intensity of the sample having a random orientation, The ratio of {222} strength to the sum total of 11 surfaces is obtained as a percentage.

(2) In manufacturing a steel sheet having a plurality of regions having different compositions in the thickness direction,
Cr: 3 mass% or more and less than 13 mass%, or Cr: 3 mass% or more and less than 13 mass%, Ni: 0.1 mass% or more and less than 1 mass%, Mn: 0.6 mass% or more and 1 mass% B1 is a steel sheet containing either or both of less than% and having a strain introduced into the surface layer on one or both surfaces of a steel sheet composed of an α-γ transformation component system that is α phase at room temperature,
A steel sheet in which a film made of Cr or a film made of ferrite and at least one element of Cr, Al, Ga, Mo, Nb, Si, Sn, Ti, V, W, and Zn is formed on the surface of the steel sheet B1. Is B2,
The steel plate B2 is subjected to a heat treatment for heating and cooling to a temperature of A3 point or higher and 1300 ° C. or lower of the composition of the steel plate B1, and at least Cr is diffused from the coating into the steel plate B1 in the heat treatment process. The Cr concentration in the surface layer region is increased, and at the time of cooling, the grain growth accompanying transformation is caused from the steel sheet surface layer side toward the steel plate inside, and excellent in workability, corrosion resistance, and ridging resistance as described in (1) above A method of manufacturing a steel sheet.

  In the present invention, {111} orientation grains having excellent workability in the steel sheet inner layer after having a structure in which the Cr concentration is different in the plate thickness direction of the steel plate with the layer having a relatively high Cr concentration as the surface layer and the low layer as the inner layer. In addition to making the steel sheet with excellent workability and corrosion resistance, and by making the grain size of the steel sheet surface layer finer than that of the inner layer of the steel sheet, it has excellent workability, corrosion resistance and ridging resistance A steel plate can be provided at low cost.

It is a figure for demonstrating the process for obtaining the steel plate excellent in workability, corrosion resistance, and ridging resistance.

  In the following description,% of element content means mass%. Further, the crystal orientation in the steel sheet and the measured plane integration degree are described by the crystal plane orientation parallel to the steel sheet surface. The degree of surface integration is expressed by applying the extinction rule in the X-ray measurement on the crystal plane caused by the body-centered cubic crystal structure which is the α phase of Fe. That is, for example, {100} and {111} are used for the crystal orientation, and {200} and {222} are used for the texture and the degree of surface integration determined by measurement. It represents information about crystal grains.

In the present invention, the surface layer has a high Cr concentration and a low {222} plane integration degree, the steel sheet center layer has a high {222} plane integration degree, and the ratio of the grain size between the steel sheet center and the steel sheet surface is 1.5 or more. By doing so, it is possible to obtain a steel sheet that is excellent in corrosion resistance and workability and also excellent in ridging resistance.
It should be noted that the Cr concentration, the surface integration degree, and the particle size may be changed independently in the thickness direction. That is, the change behaviors of the above characteristic values in the plate thickness direction do not necessarily coincide with each other. As will be described later, the A layer and the B layer are distinguished by the Cr concentration. At this boundary, the {222} plane integration degree is It is not necessary to change from less than 60% to 60% or more, or to change the particle size clearly. Thus, in the present invention, the fact that the change in concentration, the change in texture, and the change in crystal grain size do not occur abruptly at the same boundary is also a cause of improving ridging resistance more than before. it is conceivable that.
Of course, depending on the boundary clearly distinguished in the plate thickness direction, the Cr concentration, the degree of surface integration, and the particle size change greatly at the same time, and even within the region between the boundaries, even if these characteristics change with the same behavior, The effect is not lost.

  Further, the present invention has been made by finding that, in the crystal orientation distribution in the thickness direction of the steel sheet, ridging resistance can be improved by keeping the orientation group other than the {111} orientation in the surface layer region and not distributing it in the central region of the steel sheet. It is a thing. As a means for realizing such a crystal orientation distribution, a method in which {111} orientation grains formed in the surface layer region are selectively grown in the central layer region using the change in transformation behavior due to the component is industrially efficient. It was made by finding that it is good.

Below, about the structure of the steel plate of this invention, and the manufacturing method of the steel plate, the reason for limitation of each condition and preferable conditions are demonstrated.
This steel plate is composed of an A layer having a relatively high Cr concentration and a B layer having a relatively low Cr concentration in the Cr concentration distribution in the plate thickness direction. First, the definition of this boundary will be described.

A layer prescribed | regulated by this invention is a area | region where Cr density | concentration is more than the steel plate average in Cr density | concentration distribution of a plate | board thickness direction.
Further, in the Cr concentration distribution in the plate thickness direction, a region where the Cr concentration is less than the average of the steel plates is defined as the B layer. That is, the region that is not the A layer is the B layer, and the steel plate of the present invention is a steel plate in which the A layer and the B layer are formed in layers.
The Cr concentration distribution and the steel plate average of the Cr concentration can be determined by performing a line analysis on the cross section in the plate thickness direction of the steel plate using EPMA.

Next, the laminated structure of the steel plate of this invention is demonstrated.
The steel sheet of the present invention has a laminated structure in which the A layer is present on the steel sheet surface side of the B layer. As the simplest laminated structure, an A-B-A laminated structure is given. By adopting such a configuration, it becomes possible to achieve both workability, corrosion resistance, and ridging resistance.
Below, the conditions that should be satisfied in the present invention will be described for the A layer and the B layer. However, when a plurality of A layers or B layers are present in the steel sheet, all of the layers must satisfy the respective conditions. is there.

(Composition of layer A)
The A layer of the steel sheet of the present invention is a region where the Cr concentration is singly or the concentration of ferrite forming elements together with Cr is relatively high in the steel plate. The ferrite forming element is at least one element selected from Al, Ga, Mo, Nb, Si, Sn, Ti, V, W, and Zn.

In the present invention, the Cr concentration of the A layer is defined by the average concentration of the A layer. This is because the present invention allows the Cr concentration fluctuation in the A layer. In the present invention, this concentration is 10.5% or more. Needless to say, the Cr concentration at an arbitrary position in the thickness direction in the A layer is higher than the average Cr concentration of the entire steel sheet, and more than the Cr concentration at an arbitrary position in the thickness direction in the B layer. high. Therefore, the average Cr concentration of the A layer defined here is higher than the average Cr concentration of the entire steel plate and higher than the average concentration of the B layer. The average Cr concentration of the A layer is preferably 13% or more from the viewpoint of corrosion resistance, and is preferably 18% or more, and more preferably more than 20%, in order to obtain higher corrosion resistance.
In addition, if the average Cr concentration of the A layer is 10.5% or more in relation to a production method for controlling the texture by the transformation described later, the {222} plane integration degree of the B layer is finally improved and good Easy processability.

  In the present invention, the concentration distribution of the contained elements in the thickness direction of the steel sheet is not particularly limited. However, as will be described later, Cr diffusion from the steel sheet surface layer toward the steel sheet interior (and from the steel sheet interior toward the steel sheet surface). In a production method using (Fe diffusion), it is conceivable that the Cr concentration increases from the steel sheet center toward the steel sheet surface. The corrosion resistance is particularly affected by the Cr concentration on the outermost surface of the steel sheet. Therefore, the Cr concentration on the outermost surface alone, not the entire Cr-concentrated portion, is controlled to be 13% or more, further 18% or more, and more than 20%. It is an effective means from the viewpoint of achieving both alloy cost and corrosion resistance.

In addition, when the layer A contains Cr and the above ferrite forming element, the content of the ferrite forming element is preferably in the following range.
Al: 0.6-8%, Mo: 0.5-2.5%, Ga: 0.9-3.5%, Nb: 0.4-1%, Si: 0.9-4%, Sn : 0.1-1.8%, Ti: 0.7-2%, V: 0.6-2%, W: 1.2-6%, Zn: 0.8-4%.
The lower limit of the addition amount of each element is to maintain the α single-phase structure of the A layer during the heat treatment when the Cr concentration of the A layer is 10.5% in connection with a manufacturing method for controlling the texture by transformation described later. This is an effective amount. The upper limit is an amount that does not deteriorate the workability. When these ferrite forming elements are contained in the A layer, ridging resistance can be improved even with a small amount of Cr, and Mo, Sn, and Nb are also effective in improving corrosion resistance.

(B layer composition)
In the present invention, the Cr concentration of the B layer is defined by the average concentration of the B layer. This is because the present invention allows the Cr concentration fluctuation in the B layer. In the present invention, this concentration is 3% or more and less than 13%. Needless to say, the Cr concentration at an arbitrary position in the thickness direction in the B layer is lower than the average Cr concentration of the entire steel sheet, and more than the Cr concentration at an arbitrary position in the thickness direction in the A layer. Low. Therefore, the average Cr concentration of the B layer defined here is lower than the average Cr concentration of the entire steel sheet and lower than the average concentration of the A layer. The average Cr concentration of the B layer is 3% or more from the viewpoint of corrosion resistance.

  In relation to the manufacturing method of controlling the texture by the transformation described later, the average composition of the B layer should be 3% or more and less than 13% of Cr, and the steel should have an α-γ transformation component system composition that is an α phase at room temperature. preferable. Alternatively, it is preferable that either one or both of Ni and Mn is contained in a range of Ni: 0.1% by mass or more and less than 1% by mass and Mn: 0.6% by mass or more and less than 1% by mass.

If Cr is less than 3%, it is difficult to increase the {222} plane integration degree of the B layer. Further, if it is 13% or more, it becomes an α single-phase component, so that texture formation due to the progress of transformation does not occur in the heat treatment, and it is difficult to ensure a high degree of {222} plane integration.
Ni and Mn are elements that are particularly preferable for making the effects of the present invention remarkable in terms of crystal orientation selectivity and grain growth behavior in a production method in which a texture is controlled by the transformation described later. As the composition of the central layer, when Cr is contained in a range of 3% or more and less than 13%, and further containing Ni or Mn, if Ni: 0.1% or more and Mn: 0.6% or more, workability In addition, the corrosion resistance is remarkably improved. Since the workability deteriorates when the content of Ni and Mn is 1% or more, it is preferably less than 1%.

  As for the composition of the A layer and the B layer, in order to obtain impurity elements inevitably mixed from raw materials or in the refining process and known predetermined characteristics as long as the content of Cr, which is an essential element, is in the above range A known Cr steel or stainless steel composition containing various elements other than Cr can be applied.

(Thickness of layer A)
The thickness of the layer is 0.05 μm or more. Moreover, it is preferable that an upper limit shall be 1000 micrometers. If the thickness of the A layer is less than 0.05 μm, the inner layer is sufficiently inside as shown in FIG. 1c, starting from the {111} -oriented grains of the surface layer, in connection with the production method of controlling the texture by transformation described later. It cannot be developed, and it is difficult to make the {222} plane integration degree inside the steel plate 60% or more. Moreover, 0.05 micrometer or more is required also from the point of ensuring corrosion resistance.
The upper limit of the thickness of the Cr enriched portion is preferably 1000 μm. If it exceeds 1000 μm, the effect of improving the corrosion resistance and workability (texture controllability) will be saturated, although the Cr content of the whole steel sheet increases and the alloy cost increases.

(B layer thickness)
The thickness of the B layer is preferably 10 μm or more and 3 mm or less. When the thickness is less than 10 μm, it becomes very difficult to preferentially grow the {111} -oriented grains in the surface layer inside the steel sheet in connection with a manufacturing method in which the texture is controlled by transformation described later. On the other hand, if the thickness exceeds 3 mm, {111} oriented grains cannot be sufficiently grown to the inside of the steel sheet, making it difficult to obtain a steel sheet with good workability.

(Gathering organization)
In the present invention, for the A layer and the B layer, the {222} plane integration degree of the αFe phase with respect to the plate surface is defined. The degree of {222} plane integration is such that, at an arbitrary position in the plate thickness direction of the steel plate, the Fe α-crystal 11 plane {110}, {200}, {211}, {310}, {310}, { 222}, {321}, {411}, {420}, {332}, {521}, {442} integrated intensities were measured, and each of the measured values was divided by the theoretical integrated intensity of the sample in a random orientation. Later, it can be obtained as a percentage of the {222} strength with respect to the sum of the 11 surfaces of the divided value. Note that the integrated intensity of a sample having a random orientation may be obtained by preparing a sample and actually measuring it.

That is, the {222} plane integration degree is expressed by the following formula (1).
{222} surface integration = [{i (222) / I (222)} / {Σi (hkl) / I (hkl)}] × 100 (1)
However, the symbols are as follows.
i (hkl): Measured integrated strength of {hkl} plane in the measured sample I (hkl): Theoretical integrated strength of {hkl} plane in the sample with random orientation Σ: Sum of αFe crystal 11 plane where The integrated intensity of each crystal plane at an arbitrary position in the plate thickness direction is obtained by applying a general EBSD method to a plate surface whose plate thickness cross section has been polished. The plane integration degree of the crystal orientation of the A layer and the B layer was calculated at the center of each layer, that is, at the 1/2 thickness position of each layer.

(A surface integration degree of A layer)
In general, in order to increase the {222} plane integration degree in order to improve workability, it is necessary to grow {111} oriented grains coarsely. Further, when the {222} plane integration degree is increased, colonies of {111} orientation grains are also formed.
In the present invention, the surface integration degree of the A layer is defined by the {222} surface integration degree measured at the center position in the thickness direction of the A layer, and this is less than 60%. Depending on the manufacturing method, the texture may vary greatly depending on the position in the thickness direction within the A layer. In the present invention, however, the degree of surface integration measured by the texture at the central position in the thickness direction of the A layer is It becomes an index of the magnitude of the invention effect.

By keeping the {222} plane integration degree low in this way, the A layer has a relatively large number of grains in the orientation group other than {222}, the average crystal grain size becomes fine, and the colony of {111} orientation grains Can be decomposed to prevent ridging. If the {222} plane integration degree of the A layer is 60% or more, it becomes difficult to set the ratio of the average crystal grain size at the steel sheet center to the steel sheet surface to 1.5 or more, and the effect of improving ridging resistance is small. Become.
The lower limit is not particularly limited. In the manufacturing method in which the texture is controlled by the transformation described later, the {111} -oriented grains in the A layer remain as a starting point for the {111} -oriented grains that grow toward the B layer, but they exhibited their functions. It may be lost later. Even if {111} orientation grains disappear, grains of orientation groups other than {222} are formed so as to be relatively fine and have various orientations, so that ridging resistance is not deteriorated.

(B layer integration degree)
The {222} plane integration degree of the B layer is 60% or more and 99% or less. This surface integration degree is measured at the center position in the thickness direction of the B layer, similarly to the A layer.
When the {222} plane integration degree is less than 60%, the workability is not sufficient. For example, as shown in the examples described later, the ear height after cylindrical deep drawing with a drawing ratio of 2 is 1 A moldability of less than 5 mm cannot be obtained.
Further, if the degree of integration exceeds 99%, it is difficult to manufacture and improvement in workability cannot be expected.

(Ratio of particle size of A layer and B layer)
In the steel of the present invention, (average crystal grain size of layer B) / (average crystal grain size of layer A) is 1.5 or more. If the above particle size ratio is increased to 3 to 5, ridging resistance can be further improved.
If the particle size ratio is less than 1.5, ridging resistance cannot be improved.
The average crystal grain size of each layer is determined by observing the structure of each of the A layer and the B layer determined by EPMA line analysis in the cross section in the plate thickness direction of the steel sheet. The crystal grain size was measured based on JIS G 0552.

(Other)
If necessary, the surface of the steel sheet of the present invention may be subjected to a known surface treatment such as plating for a known purpose. As a result, the effects of the present invention are not lost.

Then, the manufacturing method of this invention is demonstrated with reference to drawings. The control of the Cr concentration distribution and crystal orientation described below utilizes the same phenomenon as the technique disclosed by the present inventors in Patent Documents 3 and 4. That is, the basic principle is crystal growth along the direction of element concentration by utilizing diffusion and transformation in heat treatment.
Below, the example of the formation method of the material from which a component, crystal structure, and a texture like this invention differ in a plate | board thickness direction is shown. In particular, as a manufacturing method for controlling the texture distribution in the plate thickness direction by transformation, a manufacturing method using a steel plate having a film formed thereon will be described.

(Creation of steel plates B1 and B2)
Cr is 3% by mass or more and less than 13% by mass, and further, if necessary, one or both of Ni and Mn, Ni: 0.1% by mass or more and less than 1% by mass, Mn: 0.6% by mass or more and 1% by mass A slab such as a continuous cast slab or ingot made of Cr-containing steel of α-γ transformation component system that is α phase at room temperature is prepared, and the slab is hot-rolled in the γ region and then cooled. In the step of obtaining a steel sheet by sequentially reducing the thickness by hot rolling, a steel sheet in which {111} orientation grains are formed at least in the surface layer portion by cold rolling in a range where the rolling reduction is 50% or more and 95% or less. obtain.

Next, the surface layer on one or both sides of the steel plate is distorted by shot blasting to obtain a steel plate B1 (see the state in FIG. 1a). This strain must be “characteristic” in the sense that it differs from the strain imparted by cold rolling under general lubrication. In addition to shot blasting, it is also effective to impart a strong shear strain to the surface layer by, for example, cold rolling without lubrication or different peripheral speed rolling.
Since the state of deformation stress becomes complicated, the dislocation density is increased, and the invention effect can be remarkably obtained when the dislocation density introduced into the steel sheet surface layer is 1 × 10 ¹⁵ / m ² or more. .

The conditions for shot blasting and the rolling conditions such as lubrication and different peripheral speeds may be appropriately determined for generally known factors. The greater the deviation from the strain imparted by cold rolling under general lubrication, generally the plane strain with a small shear component in the plane of the plate thickness direction and rolling direction, The effect of the invention becomes remarkable.
By giving such strain, the {111} orientation grains are preferentially grown from the surface layer toward the steel sheet center layer while retaining the grains of the orientation group other than {222} in the surface layer portion of the steel sheet in the heat treatment described later. It becomes possible. If this characteristic strain is not applied, {111} oriented grains preferentially grow in the surface layer in the heat treatment described later, and the entire surface is covered with coarse {111} oriented grains. Improvement cannot be expected.

On the surface of the steel plate B1 with a characteristic strain on the surface layer, Cr alone or Cr and at least one ferrite forming element of Al, Ga, Mo, Nb, Si, Sn, Ti, V, W, Zn are contained. By making it adhere, steel plate B2 is obtained (refer the state of Drawing 1b). The ferrite forming element may be attached as an alloy with Cr, or each element may be attached alone. An essential element for obtaining the effect of the present invention is Cr, and in the present invention, a metal layer including the ferrite forming element that adheres to the surface of the steel plate B1 simultaneously with Cr is described as a “Cr layer”.
The elements in the Cr layer have an α single phase composition in at least a part of the region diffused and alloyed in the steel plate B1 in the subsequent heat treatment. Thereby, the {111} -oriented crystal structure formed in the surface layer of the steel plate B1 grows toward the inside of the steel plate, and the {222} plane integration degree of the steel plate central region can be remarkably increased.

  As a method for forming the Cr layer on the surface of the steel plate B1, various methods such as a plating method such as hot dipping and electrolytic plating, a dry process such as PVD and CVD, and powder coating can be employed. A plating method is suitable as a method for efficiently attaching a diffusing element for industrial implementation.

  The thickness of the Cr layer is desirably 0.05 μm or more and 1000 μm or less. If the thickness is less than 0.05 μm, a sufficient {222} plane integration degree cannot be obtained in the steel sheet center layer after the heat treatment. Moreover, when it exceeds 1000 μm, it takes time to completely alloy with the steel plate B1, and when the Cr layer remains, the thickness becomes larger than necessary.

  In order to set the {222} plane integration degree to 60% or more, it is not always necessary to alloy all of the Cr film, and high corrosion resistance can be imparted. Thus, when a part of the film remains on the surface, the remaining Cr film is also included in the A layer, but this is only equivalent to a layer like a general metal plating, The effects of the present invention such as processability and ridging resistance are not lost.

(Formation of A layer and B layer)
The steel plate B2 on which the Cr layer is formed is heated to A3 point of the steel plate B1, and the elements in the Cr layer including Cr are diffused into the steel plate B1 to form a Cr concentrated portion in the surface layer portion of the steel plate B1. In this temperature rising process, the region of the steel plate B1 to which the characteristic strain of the surface layer is imparted is recrystallized. At this time, grains of orientation groups other than {111} are included in the surface layer structure mainly having the {111} orientation. Is formed.
In a region where Cr from the Cr layer is alloyed and the Cr concentration exceeds 13%, it becomes an α single phase component and does not undergo γ transformation, and the {111} -oriented grains preferentially grow as the temperature of the steel plate increases. However, at this time, the grains in the orientation group other than {111} on the surface layer remain until they disappear, the {222} plane integration degree in the surface region does not increase significantly, and the crystal grain size is relatively small. (See the state of FIG. 1c). Finally, a layer corresponding to the A layer of the present invention is formed in this surface layer region.

When the steel plate is further heated and held at a temperature of not less than A3 and not more than 1300 ° C. of the steel plate B1, the central layer region that is not the α single phase component is transformed from the α phase to the γ phase.
When the holding time is lengthened, with the diffusion of Cr, the region having the α single phase composition spreads toward the center of the steel sheet, and the region that has been the γ phase transforms again into the α phase. When transforming from the γ phase to the α phase, the transformation is performed by preferentially taking over the {111} orientation among the crystal orientations of the adjacent α grains. As a result, the holding time becomes longer and the {222} plane integration degree of the inner region greatly increases. (See state in Fig. 1d)

  Thereafter, when the steel sheet is cooled, the γ phase in the inner region is transformed into the α phase. Also at this time, the transformation is performed in such a way that the {111} orientation is preferentially taken over among the crystal orientations of the adjacent α grains. For this reason, the {222} plane integration degree increases even in the steel plate internal region where the Cr concentration is not so high. Since the cooling is performed from the steel sheet surface and a temperature gradient is generated in the thickness direction, the transformation occurs from the steel sheet surface side toward the central layer. For this reason, the {111} oriented grains have a columnar coarse structure toward the steel sheet central layer. Develop as. As a result, a high {222} plane integration degree is obtained in the steel sheet center layer (see the state of FIG. 1e), and a layer corresponding to the B layer of the present invention is formed in this surface layer region.

In the above heat treatment, the rate of temperature rise to the A3 point is preferably 0.1 ° C./sec or more and 500 ° C./sec or less. At the temperature rising rate within this range, {111} oriented grains for causing the above action are efficiently formed.
The holding temperature after the temperature rise is set to A3 point or higher and 1300 ° C or lower. If it is not more than the A3 point, as described above, the effect of further increasing the {222} plane integration degree using the transformation from the γ phase to the α phase during cooling cannot be used. Even if heating at a temperature exceeding 1300 ° C., the effect is not only saturated, but also the shape of the product steel plate after cooling is not preferable.
As for the holding time, cooling may be started immediately after reaching the holding temperature (substantially holding for 0.01 second or more). There is no particular upper limit on the holding time, but if it exceeds 600 seconds, not only the heat treatment cost increases, but also the influence on the characteristics is saturated.
The cooling rate is preferably 0.1 ° C./sec or more and 500 ° C./sec or less. When cooled in this temperature range, the preferential growth of {111} oriented grains in the transformation from the γ phase to the α phase during the cooling of the central layer occurs effectively, and the orientation in the {222} plane orientation further proceeds.

As a result, a region having a relatively high Cr concentration and a low {222} plane integration degree and a relatively fine crystal structure is formed on the steel sheet surface layer side, and at the same time, the Cr concentration is relatively on the steel sheet center side. And a region having a relatively coarse crystal structure with a low {222} plane integration degree.
The above behavior is manifested by the method and principle described here, and with regard to Cr concentration, crystal structure and crystal orientation, those skilled in the art usually have knowledge of diffusion, recrystallization, and grain growth, and with general steel materials. It is easy to adjust as well.
And about the composition including Cr density | concentration, crystal orientation, and crystal structure, the steel plate excellent in workability, corrosion resistance, and ridging resistance can be obtained by satisfy | filling said conditions.

  In the above description, the Cr layer has been described as one layer on one surface. However, the concentration distribution, crystal structure, and crystal orientation of the A layer finally formed on the steel sheet surface layer can be freely set by using multiple layers having different components. It is possible to control. Even in such a case, the effect of the present invention can be obtained as long as the A layer does not depart from the definition of the present invention.

Hereinafter, the present invention will be described in more detail by way of examples.
A 230 mm thick ingot containing the remaining Fe and inevitable impurities having the composition shown in Tables 1 to 3 is melted by vacuum melting, heated to 1000 ° C. and hot-rolled, and variously processed from this hot-rolled sheet by machining. After cutting out a plate material having a thickness, cold rolling was performed at various cold rolling rates to obtain cold rolled plates of 0.03 mm to 2.5 mm. In the obtained cold-rolled sheet, it was confirmed that the main phase at normal temperature was an α-Fe phase in any of the steel sheets. The A3 point of each steel is shown in Tables 1-3. Ingredients E, F, K, and P have no A3 point and are in the α single phase in the temperature range from room temperature to 1300 ° C.

A surface strain was applied to these cold-rolled sheets by a shot blast method to obtain a steel sheet B1.
In shot blasting, an iron ball having a diameter of 100 to 500 μm was projected at a speed of 40 to 60 m / sec for 3 minutes. The projected surface was observed with an electron microscope, and the amount of strain was quantified as a dislocation density. The dislocation density was calculated by determining the number of dislocations by the line segment method.
In the examples, those having a dislocation density of 1 × 10 ¹⁵ / m ² or more were considered to have shot blasting (having “characteristic distortion” required by this method).

Next, Cr coatings having different thicknesses were formed on both surfaces of each steel plate B1 by sputtering to obtain a steel plate B2. Also, when a film of another element was formed on the Cr film, the sputtering method was used. The thickness of each coating was calculated from the change in weight before and after sputtering. A combination of the Cr film and the other element film corresponds to the Cr layer of the present invention. The Cr layer has the same thickness on both surfaces, and the numerical values in the table are the total thickness of both surfaces. In other words, the thickness on one side is half the value in the table.
Further, the composition, crystal orientation and crystal grain size were also measured for each of the A layers on both surface sides, and the average values were evaluated.
Then, the heat processing which heat-cools with the temperature increase rate of Table 4-1, 2 and a holding time, and the cooling rate was performed.

The steel plate after the heat treatment was confirmed to be α single phase under all conditions by XRD measurement.
The plate thickness sections of these steel plates were subjected to line analysis in the plate thickness direction by EPMA, and the region corresponding to the A layer and the region corresponding to the B layer were determined. The average Cr concentration of the steel sheet, the A layer which is a region where the Cr concentration is higher than this Cr concentration, and the B layer which is a region where the Cr concentration is lower than this Cr concentration, the thickness average composition, {222} plane strength, each layer Tables 4-1 and 2 show the ratio of the average crystal grain size. Needless to say, each numerical value is a numerical value measured according to the provisions of the present invention described in the above-mentioned [Description of Embodiments].

  The moldability was evaluated based on the height of the ear after cylindrical deep drawing with a drawing ratio of 2. When a cylindrical drawing is performed from a disk having a diameter D to an inner diameter bunch of a molded product having a diameter d, D / d is referred to as a drawing ratio. When the ear height is small, good in-plane anisotropy at the time of molding, rough skin resistance, and ridging resistance can be obtained. If the ear height is more than 1.5 mm, any of the above characteristics is inferior. The conditions of cylindrical deep drawing were as follows. That is, punch diameter: φ50 mm, punch shoulder R: 5 mm, blank diameter φ100 mm, wrinkle holding force: 1 ton, friction coefficient: 0.11 to 0.13.

The corrosion resistance was evaluated by a salt dry combined cycle corrosion test CCT (Cyclic Corrosion Test). In the test, salt spray (5% NaCl aqueous solution spray state, temperature 35 ° C., 30 minutes) → drying (60 ° C., humidity 30%, 60 minutes) → wet (40 ° C., humidity 95%, 1 hour) was performed 100 cycles. This is the condition. Evaluation was made by observing the surface of the steel plate after 100 cycles, determining the area ratio of rusting, and determining the following criteria.
No rusting, that is, when the film remaining rate is 100% ◎ When the rusting rate is less than 5% (95% or more, less than 100% film remaining rate) ○: 5% or more and less than 30% The rusting rate (70% or more, film remaining rate of less than 95%) was Δ, and the rusting rate of 30% or more (film remaining rate of less than 70%) was ×. Here, when the film remaining rate is 100%, ◎ is accepted when the rusting rate is less than 5% (the film remaining rate is 95% or more and less than 100%).

  For ridging resistance, a JIS No. 5 tensile test piece was taken in parallel with the rolling direction, and then a tensile strain of 15% was given by a tensile tester. The unevenness height of the plate surface at the center of the parallel part of the test piece was measured by scanning with a contact roughness meter in the direction perpendicular to the rolling direction to evaluate ridging resistance. The scanning conditions were a scanning length of 10 mm, a scanning speed of 0.2 mm / second, and a cutoff of 0.8 mm. When the uneven height was less than 6 μm, the ridging resistance was defined as pass (◯), and 6 μm or more was defined as reject (×).

The results are shown in Tables 4-1.
In Comparative Examples 1 to 5, the result of CCT was acceptable, but the α {222} plane integration degree of the B layer was less than 60%, and the ear height of the moldability index was higher than 1.5 mm. Processability was not obtained.
In Comparative Examples 6 to 8, the CCT result was unsuccessful, the α {222} plane integration degree of the B layer was less than 60%, and the ear height of the moldability index was higher than 1.5 mm. No workability was obtained.
In Comparative Examples 9 to 11, the CCT was acceptable, the α {222} plane integration degree of the B layer was 60% or more, and the ear height of the moldability index was higher than 1.5 mm. 222} The surface integration degree was 60% or more and the particle size ratio between the B layer and the A layer was less than 1.5.

  In Examples 1 to 44, the result of CCT is acceptable, the α {222} plane integration degree of the B layer is 60% or more, the {222} plane integration degree of the A layer is less than 60%, and the particle size When the ratio was 1.5 or more, all the ridging unevenness heights were less than 6 μm, which was acceptable. Moreover, since the ear height of the moldability index was 1.5 mm or less, sufficient workability was obtained. In Examples 34 to 36, the thickness of the A layer and the Cr concentration were higher than those in Examples 31 to 33, but the characteristics were saturated in the ear height and the corrosion resistance as indicators of formability.

  The present invention is industrially effective because it can provide a steel sheet that is excellent in corrosion resistance and has excellent workability and ridging resistance by using less Cr.

Claims (2)

In a steel sheet having a plurality of regions having different compositions in the plate thickness direction,
When the average Cr concentration of the whole steel sheet is X mass%,
The region where the Cr concentration is X or more is the A layer, the region less than X is the B layer, and the A layer and the B layer have a laminated structure of A-B-A in the plate thickness direction.
The average composition of layer A contains Cr: 10.5% by mass or more, or Cr: 10.5% by mass or more, and at least Al, Ga, Mo, Nb, Si, Sn, Ti, V, W, Zn A composition containing one or more ferrite forming elements,
The average composition of B layer contains Cr: 3 mass% or more and less than 13 mass%, or Cr: 3 mass% or more and less than 13 mass%, Ni: 0.1 mass% or more and less than 1 mass%, and Mn: 0.00%. It is a composition of an α-γ transformation component system containing either one or both of 6% by mass or more and less than 1% by mass and being an α phase at room temperature,
The {222} plane integration degree of the αFe phase of the A layer is less than 60%,
{222} plane integration degree of the αFe phase of the B layer is 60% or more and 99% or less,
Furthermore, a steel sheet excellent in workability, corrosion resistance, and ridging resistance, wherein (average grain size of layer B) / (average grain size of layer A) is 1.5 or more. In the production of a steel sheet having a plurality of regions having different compositions in the thickness direction,
Cr: 3 mass% or more and less than 13 mass%, or Cr: 3 mass% or more and less than 13 mass%, Ni: 0.1 mass% or more and less than 1 mass%, Mn: 0.6 mass% or more and 1 mass% B1 is a steel sheet containing either or both of less than% and having a strain introduced into the surface layer on one or both surfaces of a steel sheet composed of an α-γ transformation component system that is α phase at room temperature,
A steel sheet in which a film made of Cr or a film made of ferrite and at least one element of Cr, Al, Ga, Mo, Nb, Si, Sn, Ti, V, W, and Zn is formed on the surface of the steel sheet B1. Is B2,
The steel plate B2 is subjected to a heat treatment for heating and cooling to a temperature of A3 point or higher and 1300 ° C. or lower of the composition of the steel plate B1, and at least Cr is diffused from the coating into the steel plate B1 in the heat treatment process. The steel sheet excellent in workability, corrosion resistance, and ridging resistance according to claim 1, wherein Cr concentration in the surface layer region is increased and grain growth accompanying transformation is caused from the steel sheet surface layer side toward the inside of the steel sheet during cooling. Manufacturing method.

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