The present invention relates to a steel sheet and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2019-000671 filed in Japan on January 07, 2019, and the contents thereof are incorporated herein by reference.
Background technology
[0002]
In recent years, in order to protect the global environment, it has been required to improve the fuel efficiency of automobiles. With regard to improving the fuel efficiency of automobiles, steel sheets for automobiles are required to have higher strength in order to reduce the weight of the vehicle body while ensuring safety. The demand for higher strength is increasing not only for members and pillars, which are structural members, but also for outer panel parts (roofs, hoods, fenders, doors, etc.) of automobiles. To meet such demands, materials have been developed for the purpose of achieving both strength and elongation (formability).
[0003]
On the other hand, the modeling of automobile outer panel parts tends to become more and more complicated. If the steel sheet is made stronger and thinner in order to reduce the weight, unevenness is likely to occur on the surface of the steel sheet when it is formed into a complicated shape. If the surface becomes uneven, the appearance after molding is deteriorated. Since not only properties such as strength but also designability and surface quality are important for the outer panel parts, it is required to have an excellent appearance after molding. The unevenness generated after molding described here is unevenness generated on the surface of molded parts by molding even if there is no unevenness on the surface of the steel sheet after manufacturing, and even if the formability of the steel sheet is improved, the occurrence is not necessarily suppressed. This was not a major issue in applying high-strength steel sheets to the outer panels of automobiles.
[0004]
Regarding the relationship between the appearance after molding and the material properties of the steel sheet applied to the outer panel, for example, in Patent Document 1, in order to improve the surface texture after the overhanging process, from the {001} surface parallel to the surface of the steel sheet. A ferrite-based thin steel sheet in which the area fraction of a crystal having a crystal orientation within ± 15 ° is 0.25 or less and the average particle size of the crystal is 25 μm or less is disclosed.
However, Patent Document 1 relates to a ferritic thin steel sheet having a C content of 0.0060% or less. In order to increase the strength of the steel sheet, it is effective to increase the C content and make the structure a double-phase structure consisting of ferrite and a hard phase. When the C content is increased in order to obtain the crystal, it is not possible to reduce the area fraction of the crystal having a crystal orientation within ± 15 ° from the {001} plane parallel to the steel plate surface as described in Patent Document 1. It turned out. That is, the method of Patent Document 1 cannot simultaneously achieve high strength and improvement of surface texture after processing (suppression of occurrence of unevenness).
[0005]
For example, Patent Document 2 discloses a double-phase structure steel having a ferrite and a second phase, and describes that it is effective to lower the yield point as a countermeasure against surface strain during molding. However, Patent Document 2 does not disclose the relationship between the appearance after molding and the structure from the viewpoint of measures against surface roughness and patterns.
[0006]
That is, conventionally, a high-strength double-phase structure steel having improved surface roughness and pattern defects after molding has not been proposed.
Prior art literature
Patent documents
[0007]
Patent Document 1: Japanese Patent Application Laid-Open No. 2016-156079
Patent Document 2: International Publication No. 2013/0464676
Outline of the invention
Problems to be solved by the invention
[0008]
The present invention has been made in view of the above problems. An object of the present invention is to provide a high-strength steel plate in which the occurrence of surface irregularities during molding is suppressed and a method for producing the same.
Means to solve problems
[0009]
The present inventors examined a method for solving the above problems. In particular, we focused on the relationship between the surface unevenness of the manufactured steel sheet and the microstructure and aggregate structure of the steel sheet on the surface unevenness after molding. Ii) Surface unevenness after molding occurs due to non-uniform deformation in the range from the surface of the steel sheet to the position of 20 μm in the plate thickness direction, iii) The cause of non-uniform deformation is the non-uniformity of the hard structure. We found that it was caused by dispersal and the development of specific aggregates.
Further, as a result of further studies by the present inventors, it is desirable to use DP steel composed of ferrite and the second phase in order to achieve both strength and formability, and the range is 0 to 20 μm in the plate thickness direction from the surface. In the metal structure of the surface layer region (range from the surface to the position of 20 μm in the plate thickness direction from the surface), the fraction of the second phase, the average crystal grain size of the second phase, and the aggregate structure of ferrite are different from those inside the steel plate. The present inventors have found that a steel plate having an excellent appearance (surface quality) after molding can be obtained by using a metal structure while ensuring strength and suppressing the occurrence of surface irregularities during molding.
[0010]
Further, as a result of studies by the present inventors, in order to control the metallographic structure of the surface layer region, strain is applied not after cold rolling but after hot rolling, and the subsequent cold rolling ratio is applied according to the amount of processing. And it was found that it is effective to set the heat treatment conditions.
[0011]
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] The steel plate according to one aspect of the present invention has a chemical composition of C: 0.050% or more, 0.145% or less, Mn: 0.70% or more, 2.25% or less, Al in mass%. Total of Si: 0.60% or less, P: 0.030% or less, S: 0.020% or less, N: 0.0050% or less, Mo: 0% or more, 0.80% or less, B: 0 % Or more, 0.0050% or less, Ti: 0% or more, 0.200% or less, Nb: 0% or more, 0.100% or less, Cr: 0% or more, 0.700% or less and Ni: 0% or more. , 0.200% or less, the balance is composed of iron and impurities, and the metal structure of the surface layer region ranging from the surface to the position of 20 μm in the plate thickness direction from the surface is ferrite and 1 in volume fraction. The interior is composed of a second phase of 0.0 to 15.0%, and ranges from a position of more than 20 μm in the plate thickness direction from the surface to a position of 1/4 of the plate thickness in the plate thickness direction from the surface. The metallographic structure of the region is composed of ferrite and a second phase having a volume fraction of 5.0 to 25.0%, and the volume fraction of the second phase of the surface layer region is the volume fraction of the internal region. It is smaller than the volume fraction of the two phases, the average crystal grain size of the second phase is 0.5 to 4.0 μm in the surface layer region, and the {001} orientation and the {111} orientation of the ferrite. A texture with an intensity ratio of X ODF {001} / {111} of 0.70 to 2.50 is included.
[2] In the steel sheet according to the above [1], the average crystal grain size of the second phase in the internal region is 1.0 to 5.0 μm, and the second phase in the surface layer region is said to have an average crystal grain size of 1.0 to 5.0 μm. It may be larger than the average crystal grain size.
[3] The steel plate according to the above [1] or [2] has a chemical composition of Mo: 0.001% or more, 0.80% or less, B: 0.0001% or more, 0. 0050% or less, Ti: 0.001% or more, 0.200% or less, Nb: 0.001% or more, 0.100% or less, Cr: 0.001% or more, 0.700% or less and Ni: 0. Any one or more of 001% or more and 0.200% or less may be contained.
[4] The steel sheet according to any one of the above [1] to [3] has a chemical composition satisfying the following formula (1), a tensile strength of 550 MPa or more, and a plate thickness of 0.10 mm or more. , 0.55 mm or less, and the plate width may be 1300 mm or more and 2000 mm or less.
7.00 ≧ [C] × 20 + [Si] × 3.0 + [Mn] × 2.0 + [Al] × 2.0 + [Cr] + [Ti] × 20 + [Nb] × 40 + [Mo] × 2. 0 + [Ni] x 2.0 + [B] x 200 ... (1)
However, the element symbol in the above formula (1) is the content of each element in mass%, and if it is not contained, 0 is substituted.
[5] In the steel sheet according to any one of the above [1] to [4], the second phase in the surface layer region comprises one or more of martensite, bainite, and tempered martensite. May be good.
[6] The steel sheet according to any one of the above [1] to [5] may have a plating layer on the surface thereof.
[7] The method for producing a steel plate according to another aspect of the present invention includes a heating step of heating a steel piece having the chemical composition according to the above [1] to 1000 ° C. or higher, and a rolling end temperature of the steel piece. The absolute value of σ s, which is the residual stress on the surface of the hot-rolled steel plate after the hot-rolling step of hot-rolling to 950 ° C. or lower to obtain a hot-rolled steel plate and the hot-rolled steel plate, is 165. The hot-rolled steel plate after the stress-applying step and the stress-applying step is cold-rolled so that the cumulative reduction rate is 70 to 90%, so that the cumulative reduction rate is about 400 MPa. In the cold rolling process for obtaining a steel plate and for the cold-rolled steel plate, the average heating rate from 300 ° C. to a soaking temperature T1 ° C. satisfying the following equation (2) is 1.5 to 10.0 ° C./sec. After heating to the temperature of T1 ° C. for 30 to 150 seconds, and the cold-rolled steel plate after the baking step, the average cooling rate of the cold-rolled steel plate to the soaking temperature of T1 ° C. to 650 ° C. After cooling to a temperature range of 550 to 650 ° C so that the temperature is 1.0 to 10.0 ° C / sec, the average cooling rate is 200 to 490 ° C so as to be 5.0 to 500.0 ° C / sec. It is provided with a cooling process for cooling to a temperature range.
1275-25 x ln (σ s) -4.5 x R CR ≤ T1 ≤ 1275-25 x ln (σ s) -4 x R CR ... (2)
[8] In the method for manufacturing a steel sheet according to the above [7], the stress applying step may be performed at 40 to 500 ° C.
[9] In the method for manufacturing a steel sheet according to the above [7] or [8], the finish rolling start temperature may be 850 ° C. or lower in the hot rolling step.
[10] The method for manufacturing a steel sheet according to any one of [7] to [9] above holds the cold-rolled steel sheet after the cooling step in a temperature range of 200 to 490 ° C. for 30 to 600 seconds. A holding step may be further provided.
The invention's effect
[0012]
Compared with the conventional material, the steel sheet of the above aspect of the present invention suppresses the occurrence of surface irregularities even after various deformations caused by press deformation. Therefore, the steel sheet of the above aspect of the present invention is excellent in the beauty of the surface after molding, and can contribute to the improvement of the sharpness and design of the coating. Further, the steel sheet of the above aspect of the present invention has high strength and can contribute to further weight reduction of the automobile. In the present invention, high strength means having a tensile strength of 550 MPa or more.
Further, according to the method for manufacturing a steel sheet according to the above aspect of the present invention, it is possible to manufacture a high-strength steel sheet in which the occurrence of surface irregularities is suppressed even after various deformations caused by press deformation.
A brief description of the drawing
[0013]
[Fig. 1] Fig. 1 is a diagram showing the relationship between surface texture after molding and texture parameters.
Embodiment for carrying out the invention
[0014]
The steel plate according to the embodiment of the present invention (the steel plate according to the present embodiment) has a chemical composition of 0.05% or more, C: 0.050% or more, 0.145% or less, Mn: 0.70% or more, and 2 in mass%. .25% or less, total of Al and Si: 0.60% or less, P: 0.030% or less, S: 0.020% or less, N: 0.0050% or less, Mo: 0% or more, 0.80 % Or less, B: 0% or more, 0.0050% or less, Ti: 0% or more, 0.200% or less, Nb: 0% or more, 0.100% or less, Cr: 0% or more, 0.700% or less And Ni: Contains 0% or more and 0.200% or less, and the balance is composed of iron and impurities.
Further, in the steel plate according to the present embodiment, the metal structure of the surface layer region in the range from the surface to the position of 20 μm in the plate thickness direction from the surface is ferrite and the volume fraction is 1.0 to 15.0%. The metal structure of the internal region, which is composed of the second phase and ranges from the position of more than 20 μm in the plate thickness direction from the surface to the position of 1/4 of the plate thickness in the plate thickness direction from the surface, is ferrite. , The volume fraction is 5.0 to 25.0%, and the volume fraction of the second phase in the surface layer region is higher than the volume fraction of the second phase in the internal region. Is also small.
Further, the steel sheet according to the present embodiment has an average crystal grain size of the second phase of 0.5 to 4.0 μm in the surface layer region, and has the {001} orientation and the {111} orientation of the ferrite. Includes aggregates with an intensity ratio of X ODF {001} / {111} of 0.70 to 2.50.
In the steel sheet according to the present embodiment, the average crystal grain size of the second phase in the internal region is 1.0 to 5.0 μm, and the average crystal grain size is 1.0 to 5.0 μm.It is preferably larger than the average crystal grain size of the second phase in the surface layer region.
[0015]
Hereinafter, the steel sheet according to this embodiment will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention. The numerical limitation range described below includes the lower limit value and the upper limit value. Numerical values ​​indicated as "super" and "less than" do not include the value in the numerical range. All% of the chemical composition indicate mass%. First, the reason for limiting the chemical composition of the steel sheet according to the present embodiment will be described.
[0016]

[C: 0.050% or more, 0.145% or less]
C (carbon) is an element that enhances the strength of the steel sheet and is an essential element for ensuring the volume fraction of the second phase. In order to secure the desired volume fraction of the second phase, the C content is set to 0.050% or more. It is preferably 0.070% or more.
On the other hand, when the C content exceeds 0.145%, the number of particles in the hard phase (second phase) increases, and the hard phase becomes easy to connect. Since the portions other than the connected hard phase are deformed at the time of molding, if the particles of the hard phase are unevenly dispersed, pattern defects on the surface after molding are likely to occur. Further, when the C content exceeds 0.145%, the cold rolling load when cold rolling is performed at a high pressure lowering rate increases, the productivity decreases, and the formability and weldability of the steel sheet deteriorate. Therefore, the C content is set to 0.145% or less. Preferably, the C content is 0.130% or less, more preferably 0.110% or less.
[0017]
[Mn: 0.70% or more and 2.25% or less]
Mn (manganese) is an element effective in increasing the mechanical strength of steel sheets. Mn is also an element that prevents cracking during hot rolling by fixing S (sulfur) in steel as MnS or the like. In order to obtain these effects, the Mn content is set to 0.70% or more. It is preferably 0.90% or more.
On the other hand, when the Mn content exceeds 2.25%, the cold rolling load when cold rolling is performed at a high pressure reduction rate increases, and the productivity decreases. In addition, since Mn segregation is likely to occur, the hard phase is likely to aggregate after annealing and pattern defects on the surface after molding are likely to occur. Therefore, the Mn content is set to 2.25% or less. It is preferably 2.00% or less, more preferably 1.75% or less.
[0018]
[Total of Al and Si: 0.60% or less]
Al (aluminum) is a deoxidizing element for steel and is an effective element for increasing the mechanical strength of steel sheets. Further, Si (silicon) is a deoxidizing element of steel and is an effective element for increasing the mechanical strength of a steel sheet. However, when the total content of Al and Si exceeds 0.60%, the scale peelability at the time of production is lowered, and surface defects are likely to occur in the product. In addition, the cold rolling load when cold rolling is performed at a high pressure reduction rate increases, and productivity decreases. Further, the weldability and deformability of the steel sheet are reduced. Therefore, the total content of Al and Si is set to 0.60% or less. It is preferably 0.50% or less.
Further, by setting the Si content to 0.10% or less, the scale peelability at the time of production can be improved and the occurrence of surface defects in the product can be suppressed. Therefore, the Si content is preferably 0.10% or less, and more preferably 0.05% or less.
[0019]
[P: 0.030% or less]
P (phosphorus) is an impurity. If P is excessively contained in the steel, cracking during hot rolling or cold rolling is promoted, and the ductility and weldability of the steel sheet are deteriorated. Therefore, the P content is limited to 0.030% or less. Preferably, the P content is limited to 0.020% or less. Since it is preferable that the P content is low, it may be 0%, but considering the current general refining (including secondary refining), the P content may be 0.0005% or more.
[0020]
[S: 0.020% or less]
S (sulfur) is an impurity. If S is excessively contained in the steel, MnS stretched by hot rolling is generated, and the deformability of the steel sheet is lowered. Therefore, the S content is limited to 0.020% or less. Since it is preferable that the S content is low, it may be 0%, but considering the current general refining (including secondary refining), the S content may be 0.0005% or more.
[0021]
[N: 0.0050% or less]
N (nitrogen) is an impurity and is an element that reduces the deformability of the steel sheet. Therefore, the N content is limited to 0.0050% or less. Since it is preferable that the N content is low, it may be 0%. However, considering the current general refining (including secondary refining), the N content may be 0.0005% or more.
[0022]
The steel sheet according to the present embodiment may contain the above elements and the balance may be Fe and impurities. However, in order to improve various properties, the following elements (arbitrary elements) may be contained instead of a part of Fe. Since it is not necessary to intentionally add these optional elements to the steel in order to reduce the alloy cost, the lower limit of the content of these arbitrary elements is 0%. Impurities refer to components that are unintentionally contained from raw materials or other manufacturing processes in the manufacturing process of steel sheets.
[0023]
[Mo: 0% or more, 0.80% or less]
Mo (molybdenum) is an element that contributes to improving the mechanical strength of steel sheets. Further, when the Mo content is smaller than the Mn content, it is an element that is less likely to segregate than Mn and contributes to uniform dispersion of the hard phase. Therefore, Mo may be contained as needed. When the above effect is obtained, the Mo content is preferably 0.001% or more.
On the other hand, if the Mo content is excessive, the deformability of the steel sheet may decrease. Therefore, even when it is contained, the Mo content is set to 0.80% or less. In addition, Mo is an expensive element, and an increase in Mo content leads to an increase in alloy cost. From this viewpoint, the Mo content is preferably 0.15% or less.
[0024]
[B: 0% or more, 0.0050% or less]
B (boron) is an element that immobilizes carbon and nitrogen in steel to form fine carbonitrides. Fine carbonitrides contribute to steel precipitation strengthening, microstructure control, fine grain strengthening, and the like. Therefore, B may be contained as needed. When the above effect is obtained, the B content is preferably 0.0001% or more.
On the other hand, if the B content exceeds 0.0050%, not only the above effect is saturated, but also the workability (deformability) of the steel sheet may decrease. Further, since the strength of the steel sheet to be subjected to cold rolling is increased by containing B, the cold rolling load when cold rolling is performed at a high pressure reduction rate is increased. Therefore, when B is contained, the B content is set to 0.0050% or less.
Further, by setting the Al content to 0.10% or more and the B content to 0.0010% or more and 0.0030% or less, the strength of the steel sheet is made more efficient while reducing the cold spreading load. Can be improved. Therefore, it is preferable that the Al content is 0.10% or more and the B content is 0.0010% or more and 0.0030% or less. In this case, the upper limit of the Al content may be 0.50% in relation to the total amount with Si described above.
[0025]
[Ti: 0% or more, 0.200% or less]
Ti (titanium) is an element that immobilizes carbon and nitrogen in steel to form fine carbonitrides. Fine carbonitrides contribute to steel precipitation strengthening, microstructure control, fine grain strengthening, and the like. Therefore, Ti may be contained as needed. When the above effect is obtained, the Ti content is preferably 0.001% or more.
On the other hand, when the Ti content exceeds 0.200%, not only the above effect is saturated, but also the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load when cold rolling is performed at a high pressure lowering rate increases. Increase. Therefore, even when Ti is contained, the Ti content is set to 0.200% or less.
[0026]
[Nb: 0% or more, 0.100% or less]
Nb (niobium) is an element that immobilizes carbon and nitrogen in steel to form fine carbonitrides. The fine Nb carbonitride contributes to steel precipitation strengthening, microstructure control, fine grain strengthening, and the like. Therefore, Nb may be contained as needed. When the above effect is obtained, the Nb content is preferably 0.001% or more.
On the other hand, when the Nb content exceeds 0.100%, not only the above effect is saturated, but also the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load when cold rolling is performed at a high pressure lowering rate increases. Increase. Therefore, even when Nb is contained, the Nb content is set to 0.100% or less.
[0027]
[Cr: 0% or more, 0.700% or less]
Cr (chromium) is an element that contributes to improving the mechanical strength of steel sheets. Therefore, Cr may be contained as needed. When the above effect is obtained, the Cr content is preferably 0.001% or more.
On the other hand, when the Cr content becomes excessive, the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load when cold rolling is performed at a high pressure reduction rate increases. Further, if Cr is excessively contained, the alloy cost increases. Therefore, even when Cr is contained, the Cr content is set to 0.700% or less.
[0028]
[Ni: 0% or more, 0.200% or less]
Ni (nickel) is an element that contributes to improving the mechanical strength of steel sheets. Therefore, Ni may be contained as needed. When the above effect is obtained, the Ni content is preferably 0.001% or more.
On the other hand, when the Ni content becomes excessive, the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load when cold rolling is performed at a high pressure reduction rate increases. Further, if Ni is excessively contained, the alloy cost increases. Therefore, even when Ni is contained, the Ni content is set to 0.200% or less.
[0029]
The chemical composition of the above-mentioned steel sheet may be measured by a general analysis method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrum) may be used for measurement. C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method. When the steel sheet has a plating layer on the surface, the plating layer on the surface may be removed by mechanical grinding, and then the chemical composition may be analyzed.
[0030]

In the steel sheet according to the present embodiment, when the plate thickness is t, the depth range from the surface to t / 4 in the plate thickness direction is divided into two regions, and the depth position is 20 μm in the depth direction starting from the surface. The depth range with the end point is the surface layer region, and the range on the center side of the steel sheet with respect to the surface layer region is the internal region.
As a result of the study by the present inventors, it was found that the occurrence of surface unevenness during molding is caused by non-uniform deformation during molding due to non-uniform strength in the micro region. In particular, regarding the occurrence of surface irregularities, the influence of the metal structure in the surface layer region, which is in the range of 0 to 20 μm in the plate thickness direction from the surface (range from the surface to the position of 20 μm in the plate thickness direction from the surface), is large. Do you get it. Therefore, in the steel sheet according to the present embodiment, the metal structure of the surface layer region is controlled as follows.
[0031]
[Composed of ferrite and the second phase with a volume fraction of 1.0 to 15.0%, and the volume fraction of the second phase is smaller than the volume fraction of the second phase in the internal region]
If the volume fraction of the second phase in the surface layer region is less than 1.0%, the strength of the steel sheet will not be sufficiently improved. Therefore, the volume fraction of the second phase is set to 1.0% or more. On the other hand, when the volume fraction of the second phase exceeds 15.0%, the hard phase tends to be unevenly dispersed, so that surface irregularities during molding occur and the appearance after molding deteriorates.
Further, the volume fraction of the second phase in the metal structure of the surface layer region is smaller than the volume fraction of the second phase in the internal region. Only by making the volume fraction of the second phase in the surface layer smaller than the volume fraction of the second phase in the internal region and increasing the volume fraction in the internal region as described later, the generation of surface irregularities and the material can be suppressed. It is possible to achieve both strength and strength.
[0032]
In the steel sheet according to the present embodiment, the second phase in the surface layer region has a hard structure other than ferrite, for example, pearlite and martensa. One or more of it, retained austenite, bainite, and tempered martensite. From the viewpoint of improving the strength, the second phase in the surface layer region is preferably composed of one or more of martensite, bainite, and tempered martensite, and more preferably composed of martensite.
[0033]
The volume fraction of the second phase in the surface layer region can be obtained by the following method.
For observing the metal structure (microstructure) from the W / 4 position or 3 W / 4 position of the plate width W of the obtained steel sheet (that is, the position of W / 4 in the width direction from any of the widthwise ends of the steel sheet). A sample (generally the size is 20 mm in the rolling direction x 20 mm in the width direction x thickness of the steel sheet) is collected, and the metal structure (microstructure) at a thickness of 1/4 from the surface is observed from the surface using an optical microscope. , Calculate the area fraction of the second phase from the surface of the steel sheet (the surface excluding the plating layer if plating is present) to 20 μm. As a sample preparation, the plate thickness cross section in the direction perpendicular to rolling is polished as an observation surface and etched with a repeller reagent.
Classify "microstructure" from optical micrographs with a magnification of 500 times. When observing with an optical microscope after corrosion of the repeller, for example, bainite is black, martensite (including tempered martensite) is white, and ferrite is gray. Can be easily discriminated from. In the optical micrograph, the region other than gray showing ferrite is the second phase.
From the surface of the steel plate etched with the repeller reagent to the region from the surface to the position of 1/4 of the plate thickness in the plate thickness direction, 10 fields of observation were observed at a magnification of 500 times, and from the surface of the steel plate in the obtained optical microscope photograph. A region portion of 20 μm is designated, and image analysis is performed using image analysis software of “Photoshop CS5” manufactured by Adobe to obtain the area fraction of the second phase. As an image analysis method, for example, the maximum lightness value L max and the minimum lightness value L min of an image are acquired from an image, and pixels having a lightness of L max-0.3 × (L max-L min) to L max are obtained. The portion having is defined as a white region, the portion having pixels of L min to L min + 0.3 × (L max-L min) is defined as a black region, and the other portion is defined as a gray region, which is a region other than the gray region. Calculate the area fraction of the two phases. Image analysis is performed on a total of 10 observation fields in the same manner as described above to measure the area fraction of the second phase, and the average value is calculated by averaging these area fractions. Assuming that the area fraction is equal to the volume fraction, this average value is taken as the volume fraction of the second phase in the surface layer region.
[0034]
[The average crystal grain size of the second phase is 0.5 to 4.0 μm]
If the average crystal grain size of the second phase exceeds 4.0 μm, the appearance after molding deteriorates. Therefore, the average crystal grain size of the second phase in the surface layer region is set to 4.0 μm or less.
On the other hand, if the average crystal grain size of the second phase is less than 0.5 μm, the particles of the second phase are likely to aggregate and be generated. Even if the individual particles of the second phase are small, if they are aggregated and formed, the appearance after molding is deteriorated. Therefore, the average crystal grain size of the second phase in the surface layer region is set to 0.5 μm or more. It is preferably 1.0 μm or more.
[0035]
The average crystal grain size of the second phase in the surface layer region can be obtained by the following method.
In the same manner as above, 10 fields of observation were observed at a magnification of 500 times in the region from the surface of the steel plate etched with the reperer reagent to the position of 1/4 of the plate thickness in the plate thickness direction, and the steel plate in the optical microscope photograph was observed. A region of (20 μm from the surface) × 200 μm was selected, and image analysis was performed in the same manner as above using the image analysis software of “Photoshop CS5” manufactured by Adobe, and the area fraction occupied by the second phase and the particles of the second phase. Calculate the number and each. By adding them together and dividing the area fraction occupied by the second phase by the number of particles in the second phase, the average area fraction per particle in the second phase is calculated. The circle-equivalent diameter is calculated from the average area fraction and the number of particles, and the obtained circle-equivalent diameter is used as the average crystal grain size of the second phase.
[0036]
[In the surface layer region, the texture of ferrite having X ODF {001} / {111}, which is the intensity ratio between the {001} orientation and the {111} orientation, is 0.70 to 2.50 is included].
In the surface layer region, X ODF {001} / {111}, which is the intensity ratio (ratio of the maximum value of the X-ray random intensity ratio) between the {001} orientation and the {111} orientation of the ferrite, is 0.70 to 2. By including the texture of 50, the appearance after molding is improved. The reason for this is not clear, but it is considered that the non-uniform deformation on the surface is suppressed by the interaction between the existence form of the second phase and the crystal orientation distribution of ferrite.
If X ODF {001} / {111} is less than 0.70, non-uniform deformation due to the orientation distribution and strength difference of each crystal of the material (steel plate) is likely to occur, and the orientation is closer to {001} of ferrite. Deformation concentration is remarkable. On the other hand, even if X ODF {001} / {111} exceeds 2.50, non-uniform deformation due to the orientation distribution and intensity difference of each crystal of the material is likely to occur, and the boundary between ferrite and the second phase, And, it is considered that non-uniform deformation to the boundary between the crystal grains in the direction near {111} of ferrite and the crystal grains in other directions is likely to occur, and the unevenness of the surface is likely to develop.
Further, when the difference between the ferrite X ODF {001} / {111} in the surface layer region and the ferrite X ODF {001} / {111} in the internal region is −0.30 to 0.40, the plate thickness direction It is more preferable because it is considered that the deformation non-uniformity in the ferrite of the above is suppressed and it contributes to the improvement of the work hardening characteristics of the material.
[0037]
Whether or not the ferrite in the surface layer region contains an texture having an intensity ratio X ODF {001} / {111} of 0.70 to 2.50 is determined by using the EBSD (Electron Backscattering Diffraction) method as follows. You can ask for it in the same way.
For the sample to be used for the EBSD method, the steel plate is polished by mechanical polishing, and then the strain is removed by chemical polishing or electrolytic polishing, and at the same time, the range from the surface to the surface to the position of 1/4 of the plate thickness in the plate thickness direction is set. The sample is adjusted so that the included plate thickness direction cross section is the measurement surface, and the texture is measured. Regarding the sampling position in the plate width direction, a sample is sampled near the plate width position of W / 4 or 3 W / 4 (a position separated from the end face of the steel plate by a distance of 1/4 of the plate width of the steel plate).
The crystal orientation distribution of the sample is measured from the surface of the steel sheet to the area from the surface to 20 μm in the plate thickness direction at a pitch of 0.5 μm or less by the EBSD method. Ferrites are extracted using an IQ (Image Quality) value map that can be analyzed by EBSP-OIM (registered trademark, Electron Backscatter Diffraction Pattern-Orientation Image Microscope). Since ferrite has a characteristic of having a large IQ value, it can be easily separated from other metal structures by this method. The threshold value of the IQ value is set so that the area fraction of ferrite calculated by observing the microstructure due to the above-mentioned leveler corrosion and the area fraction of ferrite calculated based on the IQ value match.
The maximum value of the X-ray random intensity ratio of the {001} orientation group and the {111} orientation in the φ2 = 45 ° cross section of the three-dimensional texture (ODF) display calculated using the crystal orientation of the extracted ferrite. Ratio with the maximum value of the X-ray random intensity ratio of the group (γ-fiber) ({001} Maximum value of the X-ray random intensity ratio of the orientation group / {111} X-ray random intensity ratio of the orientation group (γ-fiber) X ODF {001} / {111}, which is the maximum value of). The X-ray random intensity ratio is obtained by measuring the diffraction intensity of a standard sample having no accumulation in a specific orientation and the diffraction intensity of the test material by an X-ray diffraction method or the like under the same conditions. It is a numerical value obtained by dividing the diffraction intensity by the diffraction intensity of the standard sample. For example, when a steel sheet is rolled and annealed at a high pressure reduction rate of 70% or more, an texture develops and the X-ray random intensity of the {111} orientation group (γ-fiber) increases.
Here, {hkl} indicates that the normal direction of the plate surface is parallel to when the sample is collected by the above method. As the crystal orientation, the orientation perpendicular to the plate surface is usually indicated by (hkl) or {hkl}. {Hkl} is a general term for equivalent planes, and (hkl) refers to individual crystal planes. That is, since the body-centered cubic structure (bcc structure) is targeted in this embodiment, for example, (111), (-111), (1-11), (11-1), (-1-11). ), (-11-1), (1-1-1), and (1-1-1) are equivalent and indistinguishable. In such a case, these directions are collectively referred to as a {111} direction group. Since the ODF display is also used for the direction display of other crystal structures having low symmetry, it is common to display the individual directions in (hkl) [uvw] in the ODF display, but in the present embodiment, the individual directions are generally displayed. We focused on the normal direction direction {hkl}, which was found to have a great influence on the development of unevenness after molding. {Hkl} and (hkl) are synonymous.
If the product is a steel sheet with a plating layer, the surface of the steel sheet excluding the plating layer is defined as the starting point of the surface layer region.
[0038]

In the steel plate according to the present embodiment, after controlling the metal structure of the surface layer region as described above, a position of more than 20 μm in the plate thickness direction from the surface to a position of 1/4 of the plate thickness in the plate thickness direction from the surface (plate thickness). When t: t / 4), it is necessary to control the metal structure of the internal region.
[0039]
[Consists of ferrite and the second phase with a volume fraction of 5.0 to 25.0%]
If the volume fraction of the second phase in the internal region is less than 5.0%, the strength of the steel sheet cannot be sufficiently improved. Therefore, the volume fraction of the second phase is set to 5.0% or more.
On the other hand, if the volume fraction of the second phase exceeds 25.0%, the volume fraction of ferrite becomes too small, and the workability such as elongation and hole expansion of the steel sheet deteriorates. Therefore, the volume fraction of the second phase is set to 25.0% or less.
[0040]
[The average crystal grain size of the second phase is 1.0 to 5.0 μm and is larger than the average crystal grain size of the second phase in the surface structure]
When the average grain size in the inner region is 1.0 to 5.0 μm and is larger than the average crystal grain size of the second phase in the surface layer structure, the average crystal grain size of the second phase in the surface layer structure is in the inner region. On the other hand, the smaller size suppresses non-uniform deformation in the surface layer region, which is preferable.
Therefore, the average particle size in the internal region may be controlled within the above range.
[0041]
For the volume fraction and average crystal grain size of the second phase in the internal region, a steel plate etched with a reperer reagent was used, and the plate thickness was increased from a position exceeding 20 μm in the plate thickness direction from the surface of the sample to the plate thickness direction from the surface. It can be obtained by selecting a range up to the 1/4 position and analyzing by the same method as the surface layer region.
Also, regarding the texture of ferrite in the internal region, using the above-mentioned EBSD method, the range from the position of more than 20 μm in the plate thickness direction from the surface of the sample to the position of 1/4 of the plate thickness in the plate thickness direction from the surface. Can be obtained by selecting and analyzing by the same method as the surface area.
When the plate thickness of the product exceeds 0.40 mm, the internal region is preferably in the range from the position of more than 20 μm in the plate thickness direction from the surface to the position of 100 μm in the plate thickness direction from the surface.
[0042]

The steel sheet according to the present embodiment has a chemical composition satisfying the following formula (1), a tensile strength of 550 MPa or more, a plate thickness of 0.10 mm or more and 0.55 mm or less, and a plate width of 1300 mm or more and 2000 mm or less. Is preferable. By satisfying all of these conditions, a steel sheet having excellent surface quality can be obtained over the entire width in the plate width direction.
[0043]
7.00 ≧ [C] × 20 + [Si] × 3.0 + [Mn] × 2.0 + [Al] × 2.0 + [Cr] + [Ti] × 20 + [Nb] × 40 + [Mo] × 2. 0 + [Ni] x 2.0 + [B] x 200 ... (1)
However, the above formula (1) The element symbol inside is the content of each element in mass%, and if it is not contained, 0 is substituted.
[0044]
Tensile strength is obtained by the method described in "JIS Z 2241: 2011 Metallic Material Tensile Test Method". As the test piece, a JIS No. 5 test piece cut out in the direction perpendicular to the rolling direction was used.
For the plate thickness of the steel plate, the plate is sampled from the end in the longitudinal direction of the steel plate coil, and a sample for plate thickness measurement is obtained from a position 300 mm in the plate width direction from the end and measured with a micrometer. Obtained by. The plate width is calculated from the positional relationship between both ends of the steel sheet by detecting the positions of both ends in the width direction of the steel sheet by image analysis using two cameras on the exit side of the cold-rolled line. It is obtained by measuring at any time on the line and averaging the plate width data for one coil. If this method is difficult, the plate may be sampled from the longitudinal end of the steel plate coil and the plate width may be measured with a caliper.
[0045]

The steel sheet according to this embodiment may have a plating layer on the surface. Having a plating layer on the surface is preferable because the corrosion resistance is improved.
The plating to be applied is not particularly limited, but is limited to hot-dip zinc plating, alloyed hot-dip zinc plating, electric zinc plating, Zn-Ni plating (electrical alloy zinc plating), Sn plating, Al-Si plating, alloyed electric zinc plating, etc. Examples thereof include hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zinc-aluminum-magnesium alloy-Si plated steel plate, and zinc-deposited Al plating.
[0046]

Next, a preferable manufacturing method of the steel sheet according to the present embodiment will be described. The steel sheet according to the present embodiment can obtain the effect as long as it has the above-mentioned characteristics regardless of the manufacturing method. However, the following method is preferable because it can be stably produced.
[0047]
Specifically, the steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps (i) to (vi).
(I) A heating step of heating a steel piece having the above chemical composition to 1000 ° C. or higher.
(Ii) A hot rolling step of hot rolling a steel piece so that the rolling end temperature is 950 ° C. or lower to obtain a hot-rolled steel sheet.
(Iii) A stress applying step of applying stress to a hot-rolled steel sheet after a hot rolling step so that σ s, which is a residual stress on the surface, has an absolute value of 165 to 400 MPa.
(Iv) A cold rolling step of cold-rolling a hot-rolled steel sheet after a stress-applying step to obtain a cold-rolled steel sheet having an RCR of 70 to 90%, which is a cumulative reduction rate.
(V) The cold-rolled steel sheet is heated so that the average heating rate from 300 ° C. to the soaking temperature T1 ° C. satisfying the following equation (2) is 1.5 to 10.0 ° C./sec, and then the soaking heat is obtained. Annealing step, which performs annealing at a temperature of T1 ° C. for 30 to 150 seconds.
1275-27 x ln (σ s) -4.5 x R CR ≤ T1 ≤ 1275-25 x ln (σ s) -4 x R CR ... (2)
(Vi) The cold-rolled steel sheet after the annealing step was cooled to a temperature range of 550 to 650 ° C. so that the average cooling rate from the soaking temperature T1 ° C. to 650 ° C. was 1.0 to 10.0 ° C./sec. After that, a cooling step of cooling to a temperature range of 200 to 490 ° C. so that the average cooling rate is 5.0 to 500.0 ° C./sec.
Further, in the case of producing a cold-rolled steel sheet or a plated steel sheet having more excellent formability by improving the ductility by tempering martensite, a manufacturing method including the following steps may be used.
(Vii) A holding step of holding the cold-rolled steel sheet after the cooling step in a temperature range of 200 to 490 ° C. for 30 to 600 seconds.
Hereinafter, each process will be described.
[0048]
[Heating process]
In the heating step, steel pieces having a predetermined chemical composition are heated to 1000 ° C. or higher prior to rolling. If the heating temperature is less than 1000 ° C., the rolling reaction force increases in the subsequent hot rolling, sufficient hot rolling cannot be performed, and the desired product thickness may not be obtained. Alternatively, it may not be possible to wind up due to deterioration of the plate shape.
It is not necessary to limit the upper limit of the heating temperature, but it is economically unfavorable to make the heating temperature excessively high. For this reason, the heating temperature of the steel piece is preferably less than 1300 ° C. Further, the steel pieces used in the heating step are not limited. For example, a molten steel having the above chemical composition can be melted using a converter, an electric furnace, or the like, and a steel piece produced by a continuous casting method can be used. Instead of the continuous casting method, an ingot forming method, a thin slab casting method, or the like may be adopted.
[0049]
[Hot rolling process]
In the hot rolling process, steel pieces heated to 1000 ° C. or higher by the heating process are hot rolled and wound to obtain a hot-rolled steel sheet.
If the rolling end temperature exceeds 950 ° C, the average crystal grain size of the hot-rolled steel sheet becomes too large. In this case, the average crystal grain size of the final product plate also becomes large, which causes a decrease in yield strength and deterioration of the surface quality after molding, which is not preferable. Therefore, the rolling end temperature is set to 950 ° C. or lower.
Further, it is preferable that the finish rolling start temperature is 850 ° C. or lower.
[0050]
When the temperature change in the hot rolling process (finish rolling end temperature-finish rolling start temperature) is + 5 ° C or higher, recrystallization is promoted by the processing heat generation in the hot rolling process, and the crystal grains are refined, which is preferable.
Further, the winding temperature in the winding step is preferably 750 ° C. or lower, more preferably 650 ° C. or lower, in order to make the crystal grains finer. Further, the winding temperature is preferably 450 ° C. or higher, more preferably 500 ° C. or higher, in terms of reducing the strength of the steel sheet to be cold-rolled.
[0051]
[Stress application process]
In the stress applying step, stress is applied to the hot-rolled steel sheet after hot rolling so that the residual stress σ s on the surface becomes 165 to 400 MPa in absolute value. For example, stress can be applied by hot rolling or pickling and then grinding a hot-rolled steel sheet with a surface grinding brush. At that time, the contact pressure of the grinding brush to the surface of the steel sheet may be changed, and the surface residual stress may be measured online using a portable X-ray residual stress measuring device and controlled so as to be within the above range. By cold rolling, annealing, and cooling while residual stress is applied to the surface so as to be within the above range, a steel sheet having a ferrite having a desired texture and a desired hard phase distribution can be obtained. ..
If the residual stress σ s is less than 165 MPa or more than 400 MPa, the desired texture cannot be obtained after subsequent cold rolling, annealing and cooling. Further, when the residual stress is applied not after hot rolling but after cold rolling, the residual stress is widely distributed in the plate thickness direction, so that a desired hard phase distribution and texture can be obtained only on the surface layer of the material. Can not.
The method of applying residual stress to the surface of the hot-rolled steel sheet is not limited to the above-mentioned grinding brush, and there is also a method of performing shot blasting, for example. However, in the case of shot blasting, there is a risk that fine irregularities may occur on the surface due to the collision of the projecting material, or that defects may occur due to the subsequent cold rolling or the like due to the biting of the projecting material. Therefore, it is preferable to apply stress by grinding with a brush.
Further, under pressure by a roll such as a skin pass, stress is applied to the entire thickness direction of the steel sheet, and a desired hard phase distribution and texture cannot be obtained only on the surface layer of the material.
[0052]
The stress applying step is preferably performed at a steel sheet temperature of 40 to 500 ° C. By performing this in this temperature range, residual stress can be efficiently applied to the range of the surface layer region, and cracking due to the residual stress of the hot-rolled steel sheet can be suppressed.
[0053]
[Cold rolling process]
In the cold rolling process, cold rolling is performed in which the cumulative reduction ratio RCR is 70 to 90% to obtain a cold-rolled steel sheet. By cold rolling a hot-rolled steel sheet to which a predetermined residual stress is applied at the above cumulative reduction rate, ferrite having a desired texture can be obtained after annealing and cooling.
If the cumulative reduction rate RCR is less than 70%, the texture of the cold-rolled steel sheet does not develop sufficiently, so that the desired texture cannot be obtained after annealing. Further, when the cumulative reduction rate RCR is more than 90%, the texture of the cold-rolled steel sheet is excessively developed, and the desired texture cannot be obtained after annealing. In addition, the rolling load increases and the uniformity of the material in the plate width direction decreases. In addition, production stability is reduced. Therefore, the cumulative rolling reduction R CR in cold rolling is set to 70 to 90%.
[0054]
[Annealing process]
In the annealing process, the cold-rolled steel sheet is heated to a soaking temperature at an average heating rate according to the residual stress applied in the stress application process and the cumulative reduction rate RCR in the cold rolling process, and then applied in the stress application process. It is held at a soaking temperature according to the residual stress and the cumulative reduction rate RCR in the cold rolling process.
Specifically, in the annealing step, the average heating rate of the cold-rolled steel sheet from 300 ° C. to the soaking temperature T1 ° C. satisfying the following equation (2) is 1.5 to 10.0 ° C./sec. After heating, annealing is performed at a soaking temperature of T1 ° C. for 30 to 150 seconds.
[0055]
1275-25 x ln (σ s) -4.5 x R CR ≤ T1 ≤ 1275-25 x ln (σ s) -4 x R CR ... (2)
[0056]
If the average heating rate is less than 1.5 ° C / sec, it takes time to heat and productivity decreases, which is not preferable. Further, if the average heating rate exceeds 10.0 ° C./sec, the temperature uniformity in the plate width direction is lowered, which is not preferable.
Further, when the soaking temperature T1 is lower than 1275-25 × ln (σ s) −4.5 × R CR, recrystallization of ferrite and reverse transformation from ferrite to austenite do not proceed sufficiently, and a desired set is obtained. I can't get the tissue. Further, it is not preferable because the difference in strength between the unrecrystallized grains and the recrystallized grains promotes non-uniform deformation during molding. On the other hand, when the soaking temperature T1 is higher than 1275-25 × ln (σ s) -4 × R CR, recrystallization of ferrite and reverse transformation from ferrite to austenite proceed sufficiently, but the crystal grains become coarse. This is not preferable because the desired texture cannot be obtained.
The average heating rate is calculated by (heating end temperature-heating start temperature) / (heating time).
[0057]
[Cooling process]
In the cooling process, the cold-rolled steel sheet after soaking in the annealing process is cooled. In cooling, after cooling to a temperature range of 550 to 650 ° C so that the average cooling rate from the soaking temperature T1 ° C to 650 ° C is 1.0 to 10.0 ° C / sec, the average cooling rate is further increased. Cool to a temperature range of 200 to 490 ° C. to 5.0 to 500.0 ° C./sec.
If the average cooling rate from T1 ° C to 650 ° C is less than 1.0 ° C / sec, the ferrite transformation is excessively promoted, and the desired volume fraction of the second phase cannot be obtained. On the other hand, if the temperature exceeds 10.0 ° C., the ferrite transformation does not proceed sufficiently and the carbon concentration to austenite does not proceed sufficiently, so that the desired volume fraction of the second phase cannot be obtained.
Further, when the average cooling rate from the temperature range to the temperature range of 200 to 490 ° C. after cooling to the temperature range of 550 to 650 ° C. is less than 5.0 ° C./sec, the ferrite transformation is excessively promoted. Therefore, the desired volume fraction of the second phase cannot be obtained. On the other hand, it is difficult to set the temperature above 500.0 ° C due to equipment restrictions, so the upper limit is set to 500.0 ° C / sec.
The average cooling rate is calculated by (cooling start temperature-cooling end temperature) / (cooling time).
[0058]
[Holding process]
The cold-rolled steel sheet after cooling to 200 to 490 ° C. may be held in the temperature range of 200 to 490 ° C. for 30 to 600 seconds.
It is preferable to keep the martensite in the temperature range for a predetermined time because the effect of improving the ductility by tempering the martensite can be obtained.
The cold-rolled steel sheet after cooling to 200 to 490 ° C. or the cold-rolled steel sheet after the holding step may be cooled to room temperature at 10 ° C./sec or higher.
[0059]
The cold-rolled steel sheet obtained by the above method may be further subjected to a plating step of forming a plating layer on the surface. Examples of the plating step include the following steps.
[0060]
[Electroplating process]
[Alloying process]
For the cold-rolled steel sheet after the cooling step or the holding step, electroplating may be performed to form an electroplating layer on the surface. The electroplating method is particularly limited. No. The conditions may be determined according to the required characteristics (corrosion resistance, adhesion, etc.).
Alternatively, the cold-rolled steel sheet after electroplating may be heated to alloy the plated metal.
[0061]
[Hot-dip galvanizing process]
[Alloying process]
The hot-dip galvanized steel sheet after the cooling step or the holding step may be hot-dip galvanized to form a hot-dip galvanized layer on the surface. The hot-dip galvanizing method is not particularly limited. The conditions may be determined according to the required characteristics (corrosion resistance, adhesion, etc.).
Alternatively, the plated layer may be alloyed by heat-treating the cold-rolled steel sheet after hot-dip galvanizing. When alloying, it is preferable to heat-treat the cold-rolled steel sheet in a temperature range of 400 to 600 ° C. for 3 to 60 seconds.
[0062]
According to the above manufacturing method, the steel sheet according to the present embodiment can be obtained.
Example
[0063]
Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one condition example. The present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0064]
Steel piece No. in Table 1 Steels having the chemical compositions shown in A to AB were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. The resulting slab was heated to the temperatures shown in Tables 2A and 2B. The heated slab was hot-rolled under the conditions shown in Tables 2A and 2B and wound up.
After that, the coil was rewound to apply stress to the hot-rolled steel sheet. At that time, using the processing temperature (steel plate temperature) portable X-ray residual stress measuring device shown in Tables 2A and 2B, while measuring the surface residual stress online, the residual stress σ s shown in Tables 2A and 2B is obtained. In addition, the contact pressure of the grinding brush on the steel plate surface was changed. Then, cold rolling was performed at the cumulative rolling reduction rate RCR shown in Tables 2A and 2B to obtain steel sheets A1 to AB1.
Note that the "temperature change in the hot rolling process" in Tables 2A and 2B indicates the temperature change in the hot rolling process (finish rolling end temperature-finish rolling start temperature).
[0065]
After that, annealing and cooling were performed under the conditions shown in Tables 3A to 3C, and some steel sheets were further held at 200 to 490 ° C. for 30 to 600 seconds. After cooling or holding, the mixture was allowed to cool to room temperature. After that, various types of plating were performed on some of the steel sheets to form a plating layer on the surface. In Tables 3A to 3C, CR is no plating, GI is hot-dip zinc plating, GA is alloyed hot-dip zinc plating, EG is electric plating, EGA is alloyed electric zinc plating, Zn-Al-Mg, Al-Si, etc. , Indicates that plating containing these elements was performed. Further, the phosphate-treated EG in Tables 3A to 3C indicates that the phosphate-treated electrogalvanizing was performed, and the lubrication-treated GA indicates that the lubrication-treated alloyed hot-dip galvanizing was performed.
[0066]
The obtained product board No. For A1a to AB1a, the metallographic structure observation of the surface layer region and the internal region, X ODF {001} / {111}, plate thickness, plate width and tensile strength were measured by the above-mentioned methods. The results are shown in Tables 4A to 4C.
“ΔX ODF {001} / {111} surface layer-inside” in Tables 4A to 4C refers to the ferrite X ODF {001} / {111} in the surface layer region and the ferrite X ODF {001} / {111 in the internal region. } Shows the difference.
[0067]
[Evaluation of surface properties of steel sheet]
In addition, the surface properties of the steel sheet were evaluated for the manufactured product board.
Specifically, the surface of the manufactured steel sheet was visually observed and the surface texture was evaluated. The evaluation criteria for the surface texture of the steel sheet are as follows. When the surface property evaluation of the steel sheet was C or D, it was judged to be unacceptable because it could not be used as an exterior material or a part.
A: No pattern generation (more desirable, it can be used as an exterior material)
B: Allowable minute pattern generation (can be used as an exterior material)
C: Unacceptable pattern generation (can be used as a part, but not as an exterior material)
D: Significant pattern defects (cannot be used as parts)
The results are shown in Tables 4A to 4C.
[0068]
[Sheet steel forming test]
Materials with a steel sheet surface quality evaluation of C or D (product plates No. S2a, No. X1a to No. Z1a) were not subjected to a forming test, and were formed only for materials with a steel sheet surface quality evaluation of A or B. A test was conducted.
Regarding molding, a deep drawing tester, a φ50 mm cylindrical punch, and a φ54 mm cylindrical die were used for the steel sheet whose surface texture was measured above, and a 10% in the rolling width direction was obtained in a cylindrical draw forming test by the Marcinianc method. A plastic strain was applied.
A test piece of 100 mm in the rolling width direction × 50 mm in the rolling direction is prepared from the portion deformed by molding, and the arithmetic average height Pa of the cross-sectional curve specified in JIS B0601: 2001 is defined as the rolling direction in accordance with the JIS B0633: 2001 standard. Measured in the perpendicular direction. The evaluation was performed on the portion deformed by molding, and the evaluation length was set to 30 mm.
Further, in the flat portion of the molded product, a test piece having a rolling width direction of 100 mm and a rolling direction of 50 mm is prepared, and the arithmetic average of the cross-sectional curve specified in JIS B0601 (2001) according to the JIS B0633 (2001) standard. The height Pa was measured in the direction perpendicular to the rolling direction. The evaluation length was 30 mm.
The roughness increase amount ΔPa (ΔPa = Pa of the molded product-Pa of the steel plate) was calculated using the Pa of the molded product and the Pa of the steel plate obtained in the above measurement test.
[0069]
The surface texture of the steel sheet after molding was evaluated based on ΔPa. The evaluation criteria are as follows. When the surface evaluation of the steel sheet after molding was C or D, it was judged to be unacceptable because it could not be used as an exterior material or a part.
A: ΔPa ≦ 0.25 μm (more desirable, it can be used as an exterior material)
B: 0.25 μm <ΔPa ≦ 0.35 μm (can be used as an exterior material)
C: 0.35 μm <ΔPa ≦ 0.55 μm (Can be used as a part, but cannot be used as an exterior material.)
D: 0.55 μm <ΔPa (cannot be used as a component)
[0070]
As shown in Tables 1 to 4C, in the example (Example) in which the chemical composition, the metal structure of the surface layer region, the metal structure of the internal region, and X ODF {001} / {111} are in the preferable range, the surface texture evaluation and the surface texture evaluation and The surface property evaluation after molding was A or B, and the stage of the steel sheet and the formation of surface irregularities after processing were suppressed. On the other hand, for an example (comparative example) in which any one or more of the chemical composition, the metal structure of the surface layer region, the metal structure of the internal region, and X ODF {001} / {111} is out of the scope of the present invention, the stage of the steel sheet. Or, after molding, a pattern was generated or unevenness was generated, and it was in a state where it could not be used as an exterior material or a part.
FIG. 1 is a diagram showing the relationship between the surface texture after molding obtained in this example and the texture parameters. Looking at FIG. 1, the texture parameter is within the range of the present invention (intensity ratio X ODF {001} / {111} of ferrite between {001} orientation and {111} orientation is 0.70 to 2.50). In this example, it can be seen that the surface texture after molding is excellent. In FIG. 1, even if X ODF {001} / {111} is 0.70 to 2.50, there is a point where ΔPa exceeds 0.35 μm, but this is because the second phase fraction of the surface layer is outside the scope of the present invention. This is a comparative example.
[0071]
[table 1]

[0072]
[Table 2A]

[0073]
[Table 2B]

[0074]
[Table 3A]

[0075]
[Table 3B]

[0076]
[Table 3C]

[0077]
[Table 4A]

[0078]
[Table 4B]

[0079]
[Table 4C]

Industrial applicability
[0080] [0080]
With the steel sheet according to the above aspect of the present invention, it is possible to manufacture a high-strength steel sheet in which the occurrence of surface irregularities is suppressed even after various deformations caused by press deformation. Therefore, it has high industrial applicability.
The scope of the claims
[Claim 1]
The chemical composition is mass%,
C: 0.050% or more, 0.145% or less,
Mn: 0.70% or more, 2.25% or less,
Total of Al and Si: 0.60% or less,
P: 0.030% or less,
S: 0.020% or less,
N: 0.0050% or less,
Mo: 0% or more, 0.80% or less,
B: 0% or more, 0.0050% or less,
Ti: 0% or more, 0.200% or less,
Nb: 0% or more, 0.100% or less,
Cr: 0% or more, 0.700% or less, and
Ni: 0% or more, 0.200% or less
Containing, the balance consists of iron and impurities,
The metal structure of the surface layer region, which ranges from the surface to the position of 20 μm in the plate thickness direction from the surface, is composed of ferrite and the second phase having a volume fraction of 1.0 to 15.0%.
5. The metal structure of the internal region, which ranges from the position of more than 20 μm in the plate thickness direction from the surface to the position of 1/4 of the plate thickness in the plate thickness direction from the surface, is ferrite and the volume fraction. Consists of 0-25.0% Phase 2
The volume fraction of the second phase in the surface layer region is smaller than the volume fraction of the second phase in the internal region.
In the surface area
The average crystal grain size of the second phase is 0.5 to 4.0 μm.
Includes an aggregate structure in which the intensity ratio X ODF {001} / {111} of the ferrite between the {001} orientation and the {111} orientation is 0.70 to 2.50.
A steel plate characterized by that.
[Claim 2]
The claim is characterized in that the average crystal grain size of the second phase in the internal region is 1.0 to 5.0 μm and is larger than the average crystal grain size of the second phase in the surface layer region. Item 1. The steel plate according to Item 1.
[Claim 3]
The chemical composition is by mass%
Mo: 0.001% or more, 0.80% or less,
B: 0.0001% or more, 0.0050% or less,
Ti: 0.001% or more, 0.200% or less,
Nb: 0.001% or more, 0.100% or less,
Cr: 0.001% or more, 0.700% or less and
Ni: 0.001% or more, 0.200% or less
The steel sheet according to claim 1 or 2, wherein the steel sheet contains any one or more of the above.
[Claim 4]
The chemical composition satisfies the following formula (1),
The tensile strength is 550 MPa or more,
The plate thickness is 0.10 mm or more and 0.55 mm or less, and the plate width is 1300 mm or more and 2000 mm or less.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet is characterized by the above.
7.00 ≧ [C] × 20 + [Si] × 3.0 + [Mn] × 2.0 + [Al] × 2.0 + [Cr] + [Ti] × 20 + [Nb] × 40 + [Mo] × 2. 0 + [Ni] x 2.0 + [B] x 200 ... (1)
However, the element symbol in the above formula (1) is the content of each element in mass%, and if it is not contained, 0 is substituted.
[Claim 5]
The steel sheet according to any one of claims 1 to 4, wherein the second phase in the surface layer region is composed of any one or more of martensite, bainite, and tempered martensite.
[Claim 6]
The steel sheet according to any one of claims 1 to 5, wherein the surface has a plating layer.
[Claim 7]
A heating step of heating a steel piece having the chemical composition according to claim 1 to 1000 ° C. or higher,
A hot rolling process in which the steel pieces are hot-rolled so that the rolling end temperature is 950 ° C. or lower to obtain a hot-rolled steel sheet.
A stress applying step of applying stress to the hot-rolled steel sheet after the hot rolling step so that the residual stress σ s on the surface becomes 165 to 400 MPa in absolute value.
A cold rolling step of cold-rolling the hot-rolled steel sheet after the stress-applying step to obtain a cold-rolled steel sheet having an RCR of 70 to 90%, which is a cumulative reduction rate.
The cold-rolled steel sheet is heated so that the average heating rate from 300 ° C. to the soaking temperature T1 ° C. satisfying the following equation (2) is 1.5 to 10.0 ° C./sec, and then the soaking temperature. The annealing step of performing annealing at T1 ° C. for 30 to 150 seconds,
The cold-rolled steel sheet after the annealing step was cooled to a temperature range of 550 to 650 ° C. so that the average cooling rate from the soaking temperature T1 ° C. to 650 ° C. was 1.0 to 10.0 ° C./sec. After that, it is provided with a cooling step of cooling to a temperature range of 200 to 490 ° C. so that the average cooling rate is 5.0 to 500.0 ° C./sec.
A method for manufacturing a steel sheet, which is characterized by the fact that.
1275-25 x ln (σ s) -4.5 x R CR ≤ T1 ≤ 1275- 25 x ln (σ s) -4 x R CR ... (2)
[Claim 8]
Perform the stress applying step at 40 to 500 ° C.
The method for manufacturing a steel sheet according to claim 7, wherein the steel sheet is manufactured.
[Claim 9]
In the hot rolling process
The method for manufacturing a steel sheet according to claim 7 or 8, wherein the finish rolling start temperature is 850 ° C. or lower.
[Claim 10]
The steel sheet according to any one of claims 7 to 9, further comprising a holding step of holding the cold-rolled steel sheet after the cooling step in a temperature range of 200 to 490 ° C. for 30 to 600 seconds. Production method