This announcement relates to high-strength steel sheets.
Background technology
[0002]
In recent years, from the viewpoint of improving the fuel cost of automobiles, which leads to environmental conservation, steel sheets for automobiles have become stronger and thinner, and the car body has become lighter. It deteriorates from one to another. However, in recent years, high-strength steel plates used for the bone structure of automobiles, etc., have improved workability such as hole widening and ductility, and good flexibility is also important. NS.
[0003]
Therefore, in the bending process of the steel sheet, the condition of the outer peripheral surface layer of the bending is affected by the direction in which the tensile force is applied in the circumferential direction. .. Therefore, by installing a soft layer on the surface layer of high-strength steel sheet, the tension applied to the surface layer during bending is applied.
[0004]
Regarding high-strength steel sheets that have a soft layer on the surface layer, the following patent documents 1 to 3 refer to the following steel sheets and theirs. How to show
[0005]
First, in Patent Document 1, from the interface between the steel plate and the plating layer, in order from the interface to the steel plate side, a vinyl containing an oxide of Si and / or Mn is used. It has a hard layer composed of the main structure, and the average depth T of the above-mentioned soft layer is 20 μm or more, and it is a high-strength steel sheet with the above-mentioned features. It also describes how to make it.
[0006]
Next, in Patent Document 2, the bicker hardness at a position 100 μm from the surface of the steel sheet and the bicker hardness at a depth of 20 μm from the surface of the steel sheet are subleaded. The manufacturing method of lead is described.
[0007]
Next, in Patent Document 3, the bicker hardness at the position 5 μm from the surface layer in the plate thickness direction is 80% or less of the hardness at the position 1/2 in the plate thickness direction, and the surface layer or the plate 15 is High-strength molten-lead steel sheet and its manufacturing method, which is characterized by the fact that the position is ½ of the thickness direction and the vehicle hardness is 90% or more of the hardness. Is described
Prior art literature
Patent literature
[0008]
Patent Document 1: Japanese Patent Application Laid-Open No. 2015-343334
Patent Document 2: Japanese Patent Application Laid-Open No. 2015-117403
Patent Document 3: International Publication No. 2016/013145
Summary of Ming dynasty
Issues to be solved by the clarification
[0009]
However, the high strength required for the bone structure of automatic vehicles is not only to improve the flexibility and workability, but also to the bending load.
[0010]
Therefore, this development aims to provide high-strength steel sheets with high workability, bending load, and bending, based on the current state of conventional technology. Eyes Eyes High-strength eyes
Means to solve the problem
[0011]
The authors manufactured a steel sheet with a soft part on the surface layer, which was previously known, and investigated its flexibility. In addition, the authors manufactured a steel sheet with a surface soft part in which the structure of the song was distributed, and investigated the flexibility. As a result, the surface soft part was used. By utilizing the parry as a distributed rigid structure and controlling the average spacing between the parlite and the parry, the bending load of the steel sheet is used. We have found that the flexibility can be increased at the same time. Furthermore, we have found that a high-strength steel plate that is superior to the plate thickness plate thickness and plate thickness can be obtained in addition to the above-mentioned metal structure control of the surface layer soft part. The summary of this publication obtained in this way is as follows.
[0012]
(1) Plate thickness A high-strength steel plate with a central part and a surface soft part formed on one side or both sides of the above-mentioned plate thickness center part. Steel plate The structure of the steel plate is tempered in terms of area ratio. 1 type or 2 types or more: Total 15% unsatisfied, and not hardened Martinite: 5% unsatisfied, the above-mentioned surface layer soft metal structure , Area ratio, Ferrite: 65% or more, Parlite: 5% or more 20% Unsatisfied, Tempered Martinite, Bainite, Residual case One or more tenites: 10% unsatisfied in total, and hardened Martensite: 5% unsatisfied, one or both sides The thickness of each of the above-mentioned surface layer soft parts formed in is more than 10 .mu.m, and the thickness is 15% or less of the plate thickness. The average distance between the light and the metal is 3 μm or more, and the kicker hardness (Hc) at the center of the plate thickness mentioned above and the bead of the surface layer soft part described above. However, the composition of the components in the center of the plate thickness is C: 0.10% or more, 0.30% or less, Si: 0.01% or more, 2.5% or less, Al: 0% or more. , 2.50% or less, Mn: 0.1% or more, 10.0% or less, P: 0.10% or less, S: 0.050% or less, N: 0.0100% or less, O: 0.0060 % Or less, Cr: 0% or more, 5.0% or less, Mo: 0% or more, 1.00% or less, B: 0% or more, 0.0100% or less, Nb: 0% or more , 0.30% or less, Ti: 0% or more, 0.30% or less, V: 0% or more, 0.50% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1 .00% or less, Ca: 0% or more, 0.040% or less, Mg: 0% or more, 0.040% or less, and REM: 0% or more, 0.040% or less, and the balance is Fe. This is an extra-high-strength steel plate that is an impure material.
[0013]
(2) The composition of the components in the center of the plate thickness described above is, by mass%, Cr: 0.1% or more, 5.0% or less,
Mo: 0.01% or more, 1.00% or less, B: 0.0001% or more, 0.0100% or less, Nb: 0.001% or more, 0.30% or less, Ti: 0.001% or more, 0.30% or less, V: 0.001% or more, 0.50% or less, Ni: 0.0001% or more, 1.00% or less, Cu: 0.001% or more, 1.00% or less, Ca: 0.001% or more, 0.040% or less, Mg: 0.001% or more, 0.040% or less, and REM: 0.001% or more, 0.040% or less. The high-strength steel plate according to claim 1, which is characterized by containing the above.
[0014]
(3) The C content of the soft part of the surface layer mentioned above is 0.9 times or less of the C content of the central part of the plate thickness mentioned above. .. .. Of steel plate.
[0015]
(4) A high-strength steel plate with any of the above (1) to (3), which is characterized by having a molten lead layer on the surface.
[0016]
(5) A high-strength steel sheet of any of the above (1) to (3), which is characterized by having an alloyed molten lead metal layer on the surface.
The invention's effect
[0017]
According to the present invention, it is possible to provide high-strength steel sheets with high workability, bending load, and bending. This is the high level of the Ming dynasty
Form for the Ming dynasty
[0018]
The following is an explanation of the actual form of this announcement. This announcement is not limited to the following implementation forms.
[0019]
The high-strength steel plate of this embodiment has a plate thickness center and a surface soft part formed on one side or both sides of the plate thickness center. Plate thickness Plate thickness Plate thickness Plate thickness Explains. In addition, the fraction of the metallographic structure shown below indicates the proportion of each structure in the cross section of the high-strength steel sheet as the area ratio. It means "explanatory area%".
[0020]
[Center of plate thickness]
First, the metallographic structure at the center of the plate thickness is 5% unsatisfied with more than 85% tempered martensite and one or more of ferrite, bainite, pearlite and retained austenite with a total of less than 15%. It is made up of martensite.
[0021]
"Tempering martensite: 85% or more"
Tempering martensite has a high dislocation density and is hard, and when the area ratio of tempered martensite, which is a structure that contributes to the improvement of the tensile strength of the steel sheet, is less than 85%, the steel sheet satisfies the tensile strength of 1180 MPa or more. Sufficient perforation may not be obtained. This is due to the deterioration of tissue uniformity. Therefore, the area ratio of tempered martensite is It can be 85% or more, or 100%.
[0022]
"One or more types of ferrite, bainite, pearlite, retained austenite: less than 15% in total"
Since ferrite has a soft structure, it is easily deformed and contributes to improving the ductility of the steel sheet. However, the interface between the hard structure and ferrite can be the starting point of fracture. If the ferrite content is 15% or more, it can be the starting point of fracture. Since the number of interfaces increases, the hole malleability of the steel sheet may deteriorate. Therefore, the ferrite content is less than 15% and may be zero.
[0023]
The vanite is the interface between the carbonized vanity and the vanity carbon with a high transfer density, or the inside. It contains carbonized material and is hard. Therefore, the veneer contributes to the improvement of the tensile strength of the steel sheet. If the veneer is 15% or more, the steel sheet is Although the tensile strength of 1180 MPa or more can be satisfied, the uniformity of the tissue may be deteriorated, which may lead to the deterioration of the hole widening property. That
[0024]
Residual Australian is an organization that contributes to the improvement of ductility of steel sheet by the effect of Transformation Induced Plasticity (TRIP). In some cases, the hole widening property of the steel sheet may be deteriorated because it is hardened due to the processing induction and is changed to the maltense. Therefore, the residual austenite is 15% unsatisfied, and it is okay to use zero.
[0025]
The party is a hard structure in which a soft interface and a hard interface are arranged in layers, and contributes to the improvement of the tensile strength of the steel sheet. However, the interface between the soft ferry and the hard cement can be the starting point of the collapse. If the value is 15% or more, there are many interfaces that can be the starting point of such fractures, which may lead to deterioration of the hole widening property of the steel sheet. Therefore, the party is 15% unsatisfied, and it may be a zero.
[0026]
If two or more types of ferite, vanite, partner, and residual austenite are contained, the total of these tissues is a steel plate if the total is 15% or more. Since the hole widening may deteriorate, the total of these tissues should be 15% unsatisfied.
[0027]
"Quenched martensite: less than 5%"
As-quenched martensite is a very hard structure with a high dislocation density and contributes to the improvement of the tensile strength of the steel sheet. However, as-quenched martensite is very hard. Therefore, as-quenched martensite and other structures. Therefore, the interface between martensite and other structures can be the starting point of fracture as it is hardened. If the number of such interfaces increases, the hole expandability of the steel sheet may deteriorate. Therefore, the quenching is 5% unsatisfied, and it is okay to use zero.
[0028]
[Surface soft part]
The metallographic structure of the surface layer soft part is a mixed structure consisting of at least a ferite and a partite. It is composed of 5% or more and less than 20% of pearlite, one or more of tempered martensite, bainite and retained austenite, and less than 5% of hardened martensite in total. ..
[0029]
"Ferrite: 65% or more"
Since ferrite has a soft structure, it is easily deformed, and if ferrite, which contributes to improving the bendability of the steel sheet, is less than 65%, the surface soft part cannot be sufficiently deformed when the steel sheet is bent, and the steel sheet cannot be deformed sufficiently. Flexibility may be low. Therefore, 65% or more of ferrite in the soft surface layer is required. However, as will be described later, 5% or more of pearlite is also required in the soft surface layer. The upper limit of the light is 95%.
[0030]
"Pearlite: 5% or more and less than 20%"
Pearlite is a hard structure in which soft ferrite and hard cementite are arranged in layers, which are dispersed in ferrite. Pearlite can increase the bending load of a steel sheet. It is a structure. Pearlite on the surface soft part If it is less than 5%, the bending load of the steel sheet may not be sufficiently increased. On the other hand, if the pearlite on the surface soft part is 20% or more, the interface between pearlite and soft ferrite increases. Since this interface can be the starting point of cracking, if there are many of these interfaces, the flexibility of the steel sheet may deteriorate.
[0031]
"Tempering martensite: less than 10%"
Tempering martensite is a structure that has a high dislocation density and is hard and can increase the bending load of the steel sheet. However, if the tempered martensite is 10% or more, the interface between the hard tempered martensite and the soft ferrite increases. Since the interface can be the starting point of cracking, if the number of interfaces increases, the bendability of the steel sheet may deteriorate. Therefore, the tempered martensite is less than 10%, and gero is acceptable.
[0032]
"Bainite: less than 10%"
Bainite contains carbides at the interface or inside of lath-shaped bainite ferrite with high dislocation density and is hard, so bainite can increase the bending load of the steel sheet. However, bainite. If the value is 10% or more, the interface between bainite and soft ferrite increases. Since the interface can be the starting point of cracking, if the interface increases, the bendability of the steel sheet may deteriorate. Therefore, the vanite is 10% unsatisfied, and it may be a zero.
[0033]
"Residual austenite: less than 10%"
Residual austenite is a structure that contributes to the improvement of ductility of steel sheet by the work-induced transformation effect, while retained austenite is transformed into martensite as it is hardened by work-induced transformation, so that the flexibility of the steel sheet is deteriorated. There are cases. Therefore, the residual austenite is 10% unsatisfied, and it may be zero.
[0034]
If two or more types of tempered maltensite, bainite, and residual austenite are contained, the total of these tissues will be 10% or more. , The flexibility of the steel sheet may deteriorate. Therefore, it is preferable that the total of these tissues is less than 10%.
[0035]
"Quenched martensite: less than 5%"
As-quenched martensite is a very hard structure with a high dislocation density and can increase the bending load of the steel sheet. However, as-quenched martensite is very hard. There is a large difference in strength from the structure. Therefore, the interface between martensite and other structures as hardened can be the starting point of fracture. If there are many such interfaces, the bendability of the steel sheet may be significantly deteriorated. NS. Therefore, the quenching is 5% unsatisfied, and it is okay to use zero.
[0036]
In this publication, the same and area ratio of each metal structure in the center of the plate thickness is calculated as follows.
[0037]
"Ferite"
First, collect a sample having a plate thickness cross section parallel to the rolling direction of the steel plate, and use this cross section as an observation surface. The area of ​​100 .mu.m x 100 .mu.m is the observation area. This observation area should be observed with a scanning electron microscope at a magnification of 1000 to 50000. The electron channeling contrast image seen in is an image that displays the crystal orientation difference of the crystal grains as the difference in contrast. In this electron channeling contrast image, the uniform contrast portion is ferrite. And thus The area ratio of the identified ferrite is calculated by the point counting method (according to ASTM E562).
[0038]
"Parite"
First, eat the above-mentioned inspection surface with a natural drug. From the surface of the rotted observation surface, the observation area from the surface of the steel plate or the heart to the heart is observed with an optical microscope at a magnification of 1000 to 50000, and the observation image has a dark contrast. Let the region be pearlite. Then, the area ratio of pearlite identified in this way is calculated by the point counting method.
[0039]
"Bainite and tempered martensite"
Observe the observation area corroded by the Nital reagent as described above by 1000 to 50000 times with a field emission scanning electron microscope (FE-SEM). From the position of the sensor contained inside and the arrangement of the sentence, the following can be seen as follows.
[0040]
As for the existence state of the vanite, there is a cement or a residual crystal at the interface of the lattice-shaped vanity. In some cases, or if there is a cement inside the laser-shaped interface. Interface of vanity if there is a cement or residual crystal at the interface of the market. Therefore, it is possible to specify the vanity. In addition, a sensor is installed inside the laser-shaped vector interface. If it exists, there is only one type of crystal orientation relationship between the vinyl interface and the cement, and the interface is the same. The interface is the same as the interface.The area ratio of the vanite, which is the same as this, is calculated by the point counting method.
[0041]
In the tempered matrix, the crystal exists inside the matrix, but the crystal and the cement are present. Since there are two or more crystal orientations of the site and the cement has multiple variants, the tempered multi-unit is specified. The area ratio of the tempered and tempered crystal, which is the same as this, is calculated by the point counting method. do.
[0042]
"Quenched martensite"
First, the observation surface similar to the observation surface used for the identification of the above ferrite is etched with the repera solution, and the area similar to the identification of the above ferrite is set as the observation area. Therefore, the observation area corroded by the repera solution is observed with FE-SEM, and the uncorroded area is defined as martensite and retained austenite. And the sum of martensite and retained austenite thus identified. The area ratio is calculated by the point counting method. Next, the volume ratio of retained austenite calculated as follows is regarded as the area ratio of retained austenite, and the area ratio is subtracted from the above total area ratio. Therefore, it is possible to calculate the area ratio of martensite before it is hardened.
[0043]
"Residual Austin"
The volume fraction of the residual Australian can be obtained by the X-ray folding method. First, it is removed by polishing as described above, and the surface at the position of 1/4 of the plate thickness is exposed from the surface of the steel plate. Then, the MoK line is applied to the exposed surface, and the (200) surface, (220) surface, and (311) surface of the fcc phase are folded. The integral strength ratio of the key is obtained. As a method for calculating this rounded peak, a general 5-peak method can be used. Residual Austin obtained in this way
[0044]
In addition, in this publication, the same and area ratio of each metal structure in the surface layer soft part is calculated as follows.
[0045]
First, collect the sample in the same way as the same method for determining the metal structure in the core of the above plate. Select multiple observation areas for the land, as described later, for the observation of the trial. The total area of ​​these observation areas is 2.0 x 10-9 m 2 or more, except that the above-mentioned method is the same as the method for defining the metal structure in the center of the plate thickness. be.
[0046]
The volume fraction of the residual Australian in the surface layer soft part is obtained by the crystal orientation information of the observation region region by the electron backscatter diffraction method (EBSD). NS
[0047]
Specifically, first, a sample having a plate thickness section parallel to the rolling direction of the steel sheet is sampled. Wet polishing with emery paper using the cross section as the observation surface, and diamond polishing with an average particle size of 1 .mu.m. Polishing with grains and chemical polishing are sequentially applied to the observation surface. Then, within the range determined to be the surface soft part by the method described later among the observation surfaces polished in this way, there is no bias in the plate thickness direction. Multiple observation regions are randomly selected, and the crystal orientations of regions with a total of 2.0 × 10-9 m 2 or more are obtained at 0.05 μm intervals.
[0048]
At this time, the software "IOM Data Collection TM (ver) manufactured by TSL Solutions Co., Ltd. is used as the software for acquiring data on the crystal orientation. .7) ”, etc. were used. The obtained crystal orientation information was obtained from the software“ IOM Analysis TM (ver.7) ”manufactured by TSL Solutions Co., Ltd. in the bcc phase. This fcc phase is the residual crystal. The volume ratio of the residual crystal obtained by this is retained. Areas that are considered to be the area ratio of the Australian and are judged to be the Martinite or the residual Austin, as described above. By subtracting from the area ratio of the product, it is possible to obtain the area ratio of the molten metal.
[0049]
In addition, in the high-strength steel plate of this Ming dynasty, the thickness of the surface layer soft part on one side is 10 μm or more, and 15% or less of the plate thickness. Well. In addition, in the surface soft part, the bicker hardness Hc in the center of the plate thickness and the bicker hardness Hs in the surface soft part / / / /
[0050]
"Thickness of surface soft part: more than 10 μm, cut, 15% or less of plate thickness"
The soft part of the surface layer has the effect of improving the flexibility. Surface layer Soft part The effect of the surface layer and below is hardly obtained. On the other hand, if the thickness of the surface soft part exceeds 15% of the plate thickness, the bending load and tensile strength will be large, large, large, large, and the thickness of the soft part will be 10 μm or more. It should be 15% or less of the plate thickness. If there are surface soft parts on both sides of the steel sheet, the percentage shall be 15% or less of each.
[0051]
"The average distance between pearlite on the surface soft part is 3 microns or more"
Since the interface between hard pearlite and soft ferrite has a large difference in hardness, the interface may become the starting point of fracture and cause deterioration of the bendability of the steel sheet. However, the average of pearlite and pearlite on the surface soft part. By setting the spacing to 3 microns or more, the connection between the voids generated at the interface can be suppressed and the deterioration of the bendability of the steel sheet can be suppressed. The upper limit of the above-mentioned average spacing is not particularly limited. From the viewpoint of suppressing the decrease in hardness Hs and bending load of the surface layer soft part, the above-mentioned average interval is 50 or less.
[0052]
It is difficult to directly obtain the average spacing D of pearlite. Therefore, the average spacing D of pearlite can be obtained as follows. That is, the average spacing D of pearlite is the area ratio of pearlite Ap (0 ≤ 0 ≤). It is defined by the following equation (1) using A p ≤ 1) and the average length d of the pearlite. Here, in the surface soft part defined as a pearlite, multiple observation areas are randomly selected so that there is no bias in the plate thickness direction. The observation area is 2.0 × in total. 10 -9 m 2 or more. Next, the value obtained by dividing the total area of ​​pearlite contained in the observation area by the total number of pearlite contained in the observation area is taken as the average area of ​​pearlite. And the average of this pearlite. Let the square root of the area be the average length d of pearlite.
[0053]
[Number 1]

[0054]
"0.50 ≤ Hs / Hc ≤ 0.75"
The Bicker hardness Hs of the surface soft part is an important factor for improving the bending property and securing the bending load. If the ratio Hs / Hc to the vehicle hardness Hc at the center of the surface layer is less than 0.50, the flexibility is improved, but the bending load is greatly reduced. Rua invites you. Ah Hs Ah Ah The load is higher, but it is difficult to obtain a large improvement in flexibility.
[0055]
In this expression, the bicker hardness Hc in the center of the plate thickness and the bicker hardness Hs in the surface soft part are determined by the following methods. It is determined by measuring with a cache hardness tester.
[0056]
First, measure at 5 points from the position of 1/2 of the plate thickness to the surface at an interval of 2% of the plate thickness, perpendicular to the plate thickness direction and parallel to the compression direction. Find the average value of the bicker hardness of the five points measured in each of the plate thickness directions, and use the average value of each position. The bicker hardness of the plate thickness is defined as the bicker hardness Hc at the center of the plate thickness.
[0057]
Next, the surface is more than the position in the plate thickness direction where the bicker hardness is 0.9 times or less with respect to the bicker hardness at the position of 1/2 of the plate thickness. Definition "" In the surface soft part defined in ", 10 points of bicker hardness were measured in random, and 10 points of bicker hardness were measured.
[0058]
The composition of the components of the steel sheet of this invention is not particularly limited as long as it is within the range where the above-mentioned structure can be obtained. The following means a suitable example, example, example, example, example, and “mass%” for the steel sheet of this Ming dynasty.
[0059]
First, I will explain the composition of the ingredients in the center of the plate thickness.
[0060]
"C: 0.10% or more, 0.30% or less"
C is an elemental element that secures a fixed amount of tempered heat and improves the strength of the steel sheet. C is contained in order to improve the temperature to 1180 MPa or more. It is preferable that the amount is 0.10% or more. On the other hand, the C content may be excessive and the hole widening property of the steel sheet may be reduced. Therefore, it is preferable that the C content is 0.30% or less.
[0061]
"Si: 0.1% or more, 2.5% or less"
Si is an element that acts as a deoxidizing agent. In addition, it is preferable that Si is 3% or more of carbonized material and residual oil after heat treatment. In addition, the Si content is reduced to 0.1%, which means that the current refining process leads to embrittlement of the steel sheet and reduces workability. Therefore, it is preferable that the Si content is 2.5% or less.
[0062]
"Al: 0% or more, 2.500% or less"
Al is an element that acts as a deoxidizing agent for steel and stabilizes the ferrite, and if necessary, the content of Al is 0.001% or more. I like this. On the other hand, if the Al content exceeds 2,500%, coarse Al oxides are produced, which may result in steel sheets, steel sheets, and processable steel sheets. Therefore, it is preferable that the Al content is 2.50% or less.
[0063]
"Mn: 0.1% or more, 10.0% or less"
Mn is an element that acts as a deoxidizing agent. In addition, it is preferable that Mn has a Mn content of 0.1% or more as an element that improves hardenability. On the other hand, if the Mn content exceeds 10.0%, the crude Mn oxide will be released from the steel, and the workability of the steel sheet will deteriorate as it becomes the starting point of fracture during formation during formation. There are cases where it is squeezed. Therefore, it is preferable that the Mn content is 10.0% or less.
[0064]
"P: 0.100% or less"
P is an impure element and may segregate in the central part of the sheet thickness of the steel sheet to reduce the toughness. In addition, if the content of P exceeds 0.100%, the strength of the welded part and the hole widening property may be significantly reduced. , The content of P is 0.10% or less. P is better than 0.010% or less. The P content is low, preferably 0%. However, at 0.0001, the manufacturing cost will rise significantly, which will be economically disadvantageous. Therefore, the qualitative lower limit of the P content is 0.0001%.
[0065]
"S: 0.050% or less"
S is an impure element and may reduce the solubility. In addition, S may reduce the hole widening property of the steel sheet by forming a coarse MnS and the manufacturability during casting and hot spreading. If the S content exceeds 0.050%, the adhesion will be reduced and the manufacturing and manufacturing will be significantly reduced. Therefore, the S content is 0.050% or less. I like Rukoto. The S content is preferably 0.010% or less. S. S. S. S 0% is fine. However, in order to reduce the S content of the actual steel sheet to 0.0001%, the manufacturing cost has increased. It is disadvantageous. The practical lower limit of the trowel is 0.0001%.
[0066]
"N: 0.01000% or less"
N may form a coarse chonized material and reduce the bendability and hole widening property of the steel sheet. Also, N is a contact element. If the content of N exceeds 0.01000%, the hole widening of the steel sheet will be reduced and the occurrence of blowholes will be noticeable. The content of N is low, preferably 0%. However, the content of N in the actual steel sheet is reduced by 0.00050%, to the width, to the width, to the width, to the width, to the width, and to the width. Width Rumeny Rumeny Rumeny Width Rumeny Width Rumeny Width Therefore, the qualitative lower limit of the N content is 0.00050%.
[0067]
"O: 0.0060% or less"
O may form a coarse oxide and reduce the bendability and hole widening property of the steel sheet. Also, O is an element of time and time. If the content of O exceeds 0.0060%, the hole widening of the steel sheet will be reduced and the occurrence of blowholes will be noticeable. The content of O is low, preferably 0%. However, the content of O in the actual steel sheet is reduced by 0.0005% to the extent that it is wide, and the width is wide. Width Rumeny Rumeny Rumeny Width Rumeny Width Rumeny Width Therefore, the qualitative lower limit of the O content is 0.0005%.
[0068]
"Cr: 0% or more, 5.00% or less"
Cr is an element that is effective in increasing the strength of steel sheets by increasing the quenchability in the same way as Mn, and is required to increase the strength of steel sheets by quenching as necessary. It is preferable that the Cr content is 0.10% or more. On the other hand, when the Cr content is 5.00%, the Cr core may be segregated to form coarse Cr carbonized material, which may reduce the cold formability. Therefore, the Cr content should be 5.00% or less. ..
[0069]
"Mo: 0% or more, 1,000% or less"
Mo is an element effective for strengthening steel sheets like Mn and Cr, and is added as needed. It is preferable that the Mo content is 0.010% or more, depending on the Mo content. On the other hand, if the Mo content exceeds 1,000%, coarse Mo carbonized matter may be formed, resulting in a steel plate, a steel plate, or a steel plate. Therefore, it is preferable that the Mo content is 1,000% or less.
[0070]
"B: 0% or more, 0.0100% or less"
B is an element that suppresses the formation of ferrite and pearlite in the cooling process from austenite and promotes the formation of low-temperature transformation structures such as bainite or martensite. It is added as needed. In order to obtain the above-mentioned effect by B, the content of B is 0.0001 and 0.0001% is not satisfied. In addition to the need for analysis, depending on the analyzer, the lower limit of detection and the lower limit may lead to the formation of coarse B oxides in the steel. When the workability of the steel sheet deteriorates due to the origin of the void during resin molding BB. I like this one, which is B.
[0071]
"Nb: 0% or more, 0.300% or less"
Nb is an element that is effective in controlling the morphology of carbonized materials, and its addition makes the structure finer, so that the effect of Nb can be obtained by improving the toughness element of the steel sheet. Therefore, it is preferable that the Nb content is 0.001% or more. On the other hand, the Nb content is 0.300%, and the ductility is markedly deteriorated as the strength of the steel sheet increases, and the workability may decrease. Therefore, the content of Nb is 0.300%.
[0072]
"Ti: 0% or more, 0.300% or less"
Like Nb, Ti is an element that is important for controlling the morphology of carbonized substances. It's not a little, but it's okay to have 0%. However, the practical lower limit of the Ti content of 0.001% and the unrefined amount is 0.001%. On the other hand, if the Ti content exceeds 0.300%, coarse Ti oxides or TiN may be present in the steel for processing. Therefore, it is preferable that the Ti content is 0.300% or less.
[0073]
"V: 0% or more, 0.500% or less"
V, like Ti and Nb, is an element that is effective in controlling the morphology of carbonized substances, and is an effective element that makes the structure finer by its addition. In order to obtain the effect of V, it is preferable that the content of V is 0.001% or more. One, The amount of V 0.500 is large, and a large amount of V carbonized material is deposited, and the strength of the steel sheet is increased, but the ductility is markedly deteriorated, and the workability may be deteriorated. For this reason, V Kohaha
[0074]
"Ni: 0% or more, 1.00% or less"
Ni is an element that is effective in improving the strength of steel sheets, and is added as needed. It is preferable that the content of the steel sheet, which makes the steel sheet stronger and stronger than Ni, is 0.01% or more. On the other hand, if the Ni content exceeds 1.00%, the ductility of the steel sheet will decrease and the workability will decrease. In this case, the content should be 1.00% or less. Shii.
[0075]
"Cu: 0% or more, 1,000% or less"
Cu is an element that is effective in improving the strength of steel sheets, and is added as needed. It is preferable that the content of the steel sheet that is made stronger by using Cu is 0.001% or more. On the other hand, if the Cu content exceeds 1,000%, it causes reddish brittleness and the production decreases due to thermal spread. It is preferable that the Cu content is 1,000% or less.
[0076]
"Ca: 0% or more, 0.040% or less"
Ca is an element that can control the morphology of sulfide by adding a small amount, and is added as needed. Form obtained by Ca For morphology, it is preferable that the content of Ca is 0.001% or more. On the other hand, if the Ca content is excessive, the crude Ca oxide may be produced, and the processability may be deteriorated as the starting point. Therefore, it is preferable that the Ca content is 0.040% or less.
[0077]
"Mg: 0% or more, 0.040% or less"
Mg is an element that can control the morphology of sulfide by adding a small amount like Ca, and is added as needed. In order to obtain the effect of Mg, it is preferable that the content of Mg is 0.001% or more. On the other hand, if the Mg content is excessive, it may be mediated. Therefore, it is preferable that the Mg content is 0.040% or less.
[0078]
"REM: 0% or more, 0.040% or less"
REM (Rare-Earth Metal) is an element that can control the morphology of sulfide by adding a small amount like Ca and Mg, and is added. Examples of the REM contained in the high-strength steel plate of the present invention are W, Ta, Sn, Sb, As, Zr, Y, La, and Ce. In order to obtain the effect of controlling the morphology of REM, it is preferable that the content of REM is 0.001% or more. On the other hand, the REM content may be excessively coarse or coarse, or the workability of coarse or coarse coarse may be reduced. Therefore, it is preferable that the REM content is 0.040% or less. REM may be added in combination with a multi-series element of the simulator.
[0079]
The rest of the chemical composition at the center of the plate thickness is Fe and impure. As for impure substances, high-strength steel sheets that cannot be made from steel raw materials or scraps are allowed to contain elements within the range in which the above-mentioned effects can be obtained. An example can be shown.
[0080] [0080]
Next, the composition of the components of the soft part of the surface layer will be explained. The component range of the surface soft part is the same as that of the central part of the plate thickness, but the C of the surface soft part is C of C. I like the fact that it is 9 times or less. Since the C content of the surface layer soft part is 0.9 times or less of the C content of the central part of the plate thickness, the surface layer part and the surface layer center soft part can be easily improved. , It is easy to improve the flexibility of the steel sheet.
[0081]
In addition, the component range other than C in the surface layer soft part is the same as the component range in the central part of the plate thickness mentioned above for the content range and the reason. The content of this strength is almost the same as that of the central part of the plate thickness.
[0082]
Next, we will explain an example of the manufacturing method of the high-strength steel sheet of this announcement. The high-strength steel sheets of the present invention can be manufactured as follows. ..
[0083]
First, a cast slab having the above-mentioned composition of the high-strength steel sheet of the present invention is prepared. Then, the slab is directly or once cooled and then heated to 1100 ° C. or higher and subjected to hot rolling. Ar 3 transformation point Complete the hot rolling in the above temperature range, and wind the hot-rolled steel sheet after the completion of the hot rolling in the temperature range of 700 ° C or lower. In addition, the hot-rolled steel sheet is pickled at a temperature of 70 ° C or higher and 100 ° C or lower for 50 to 50 and 50 to 50.
[0084]
The hot-rolled steel sheet after pickling is used as a cold-rolled steel sheet with a reduction rate of 30% or more and 80% or less. Following this "in the case of cold-rolled steel sheet", the log P O2 (atm) log: -26 or more, -22 or less in the atmosphere, "Ac3-30". ℃ ”or higher and 950 ℃ or lower. Ac3 is the temperature at which the reverse transformation of the Australian is completed, and the small pieces cut out from the hot-rolled steel sheet are heated to 1100 1100 at 1 ° C / sec.
[0085]
After annealing as described above, perform the cooling process described in 1) or 2) below.
1) At an average cooling rate of 20 ° C / sec or higher, the steel plate is stopped at a temperature range of 25 ° C or higher and 600 ° C or lower for 1000 seconds or lower at a temperature range of 400 ° C or higher.
2) At an average cooling rate of 0.5 ° C / sec or higher and 20 ° C / sec or lower, the region of 600 ° C or higher and 750 ° C or lower is cooled (first stage cooling), and then 20 ° C / sec. At the above average cooling rate, cool the steel plate in the temperature range of 25 ° C or higher and 600 ° C or lower (second stage, next)) and keep the steel plate in the temperature range of ° C or higher and 400 ° C or lower for 1000 seconds or less.
[0086]
Below, each process condition described above will be explained in detail.
[0087]
"Casting slab"
The cast slab used for hot rolling may be a cast slab and is not limited to a specific cast slab. For example, a continuous cast slab or a slab manufactured by a thin slab caster may be used.
[0088]
"Casting slave heating temperature: 1100 ° C or higher"
When the cast slab that has been cooled once is heated and then subjected to hot compression, the cast slab shall have a tensile strength of 1180 MPa or more for the high-strength steel plate of the present invention. Therefore, it contains a relatively large amount of alloying elements. Therefore, it is necessary to dissolve the cast element in the cast slave. If the heating temperature of the casting slab is less than 1100 ° C, the alloying elements will be added to the casting slab, coarsely, coarsely, coarsely, coarsely, coarsely, and coarsely. Embrittlement cracks may occur in the coarse. For this reason, it is preferable that the heating temperature of the cast slave is 1100 ° C or higher. From the viewpoint of castability, it is preferable that the temperature is 1250 ° C or lower.
[0089]
"Hot pressure expansion completion temperature range: Ar 3 transformation point or higher"
As mentioned above, the cast slave used for the production of high-strength steel sheets of the present invention needs to be loaded with alloying elements. For this reason, it is preferable that the hot pressure is applied at a high temperature. In the hot compression completion temperature range, if the metal metal is in the two-phase temperature range of (austenite + ferrite), the non-uniformity of the metallographic structure may increase and the formability after heat treatment may decrease. Therefore, the hot rolling completion temperature range is defined as the temperature range above the Ar 3 transition point. In addition, during hot compression, coarsely compressed steel plates may be joined to continuously perform hot compression.
[0090]
"Heat-rolled steel sheet winding temperature range: 700 ° C or less"
When the winding temperature of the hot-rolled steel sheet exceeds 700 ° C, the non-uniformity of the metallographic structure becomes large, and the post-heat treatment post-heat treatment post-heat treatment post-heat treatment post-winding temperature range is 700 ° C or less. I like this. The lower limit of the take-up temperature range is not particularly limited, but the take-up temperature is below room temperature, which is the actual lower limit of the temperature.
[0091]
[Pickling: 50 seconds or more and 300 seconds or less at a temperature of 70 ° C or higher]
In the pickling and pickling process of the hot-rolled steel sheet wound as described above for 50 seconds or more and 300 seconds or less at a temperature of 70 ° C or higher, the oxides on the surface of the hot-rolled steel sheet are removed. And cold rolling
We will improve the chemical conversion processability and plating properties of steel sheets. In addition, it is possible to control the pickling conditions and to efficiently introduce shearing to the surface layer by cold compression in the subsequent process. NS.
[0092]
According to the above-mentioned pickling conditions, the number of peak points (PPc) on the surface of the steel sheet before cold rolling by pickling becomes 60 (/ mm) or more, and cold rolling (PPc). Is measured using a stylus type surface roughness measuring machine according to JIS B 0601 (2013). Under normal pickling conditions, the average pickling method, but according to the pickling method described above, the surface soft part of the surface layer and the color of the surface soft part may be subjected to annealing in the subsequent process. It is possible to control the average interval between the acid and the acid.
[0093]
The solution used for pickling should be the solution used for normal pickling, for example, 5 vol. % Or more salt acid and sulfuric acid are listed. Or you can go. The above pickling time means the time of the pickling if one pickling is performed, and the total of these times of pickling if multiple picklings are performed. By setting the total washing temperature to 70 ° C or higher, the oxides on the surface layer can be sufficiently removed. The upper limit of the pickling temperature is not particularly limited, but in practice, the surface roughness should be increased to the surface layer by cold compression. The introduction of shear is easy. If the lower limit is more than 300 seconds, the surface roughness will be excessively rough and the surface cleanliness will be deteriorated, and the unevenness left after cooling will not be noticed. The upper limit of the raw pickling time is preferably 200 seconds.
[0094]
"Cold compression reduction rate: 30% or more, 80% or less"
For hot-rolled steel sheets that have been pickled as described above, the reduction rate should be 30% or more, and the reduction rate should be 30% or more for cold rolling and 80% or less. Therefore, the shape of the cold-rolled steel sheet can be kept flat and the decrease in final ductility can be suppressed. .. On the other hand, the reduction rate of cold compression is 80% or less, which makes it difficult to spread because the compression load is excessive. The number of compression pressures and the reduction rate for each pass are not particularly limited, and the reduction rate for cold compression is not limited to the above range.
[0095]
"Atmospheric acid elemental pressure P O2 log P O2: -26 or more, -22 or less"
"Annealing temperature range:" Ac3-30 ° C "or higher, 950 ° C or lower"
Continuous firing slow furnace heating furnace heating furnace heating furnace heating furnace The logPO2 of the acid component pressure PO2 in the atmosphere inside the furnace is maintained at -26 or more and -22 or less, "Ac3-30. The steel plate is heated in a temperature range of ℃ or more and 950 ℃ or less, and then the furnace is heated.
[0096]
In the annealing by the heating furnace and the soaking furnace, decarburization of the surface of the steel sheet progresses and the amount of carbon in the surface layer decreases in the temperature range of "Ac3-30 ° C" or higher. Surface layer input Surface layer amount Amount A proper amount of ferrite and pearlite can be obtained in the surface layer by reducing the amount. To promote such decarburization, the oxygen partial pressure in the furnace atmosphere. Limited to the range.
[0097]
If the log P O2 of the oxygen partial pressure of the atmosphere is -26 or more, the acid potential is sufficiently high and the above-mentioned decarburization can be ensured. NS . logP O2 is preferably -25 or higher. On the other hand, when logP O2 is -22 or less, the acid potential is high, and the steel sheet itself is also oxidized, which suppresses the desired surface condition. Is easy to obtain.
[0098]
By setting the annealing temperature range to "Ac3-30 ° C" or higher, austenite is generated during annealing, and a fixed amount of tempering is performed as the final structure. It is easy to obtain the temperature. Therefore, it is easy for the steel sheet to satisfy the desired tensile strength. On the other hand, if the annealing temperature range exceeds 950 ° C, there is no problem with the characteristics of the steel sheet, but the productivity decreases. For this reason, it is preferable to have an annealing temperature range of ℃ ℃.
[0099]
The cooling process of 1) above
"Average cooling rate: 20 ° C / sec or more"
"Cooling stop temperature: 25 ° C or higher, 450 ° C or lower"
Cool the annealed steel sheet to a temperature range of 450 ° C or less with an average cooling of 25 at 20 ° C / sec or more. This cooling is an important part of the quenching that is the source of the fixed amount of tempering and quenching.
[0100]
In the quenched temperature range above 25 ° C and below 450 ° C, the trace amount of translocation present in the austenite grains before the transformation is observed. It is generated as a nucleus. The average cooling rate until it reaches the temperature range of 25 ° C or higher and 450 ° C or lower from the annealing temperature is 20 ° C / sec or higher. Therefore, the above-mentioned translocation contained in the austenite grains before the transformation disappears and the temperature is suppressed. The average cooling rate in this cooling process. Is preferably above 35 ° C / sec.

The scope of the claims
[Claim 1]
A high-strength steel plate having a plate thickness center portion and a surface soft portion formed on one side or both sides of the plate thickness center portion.
In the cross section of the high-strength steel sheet,
The metal structure at the center of the plate thickness is tempered martensite: 85% or more, one or more of ferrite, bainite, pearlite, retained austenite: less than 15% in total, and as-quenched martensite: Consists of less than 5%,
The metallographic structure of the surface soft part is ferrite: 65% or more, pearlite: 5% or more and less than 20%, tempered martensite, bainite, one or more types of retained austenite: less than 10% in total, And as-quenched martensite: consisting of less than 5%,
The thickness of each of the surface soft parts formed on one side or both sides is more than 10.mu.m and 15% or less of the plate thickness.
The average distance between pearlite and pearlite on the surface soft part is 3 micron or more.
The Vickers hardness (Hc) at the center of the plate thickness and the Vickers hardness (Hs) at the surface soft part satisfy 0.50 ≤ Hs / Hc ≤ 0.75.
The composition of the ingredients in the center of the plate thickness is, by mass%,
C: 0.10% or more, 0.30% or less,
Si: 0.01% or more, 2.5% or less,
Al: 0% or more, 2.50% or less,
Mn: 0.1% or more, 10.0% or less,
P: 0.10% or less,
S: 0.050% or less,
N: 0.0100% or less,
O: 0.0060% or less,
Cr: 0% or more, 5.0% or less,
Mo: 0% or more, 1.00% or less,
B: 0% or more, 0.0100% or less,
Nb: 0% or more, 0.30% or less,
Ti: 0% or more, 0.30% or less,
V: 0% or more, 0.50% or less,
Ni: 0% or more, 1.00% or less,
Cu: 0% or more, 1.00% or less,
Ca: 0% or more, 0.040% or less,
Mg: 0% or more, 0.040% or less, and
REM: 0% or more, 0.040% or less
High-strength steel sheet containing Fe and the rest of which is impure.
[Claim 2]
The composition of the ingredients in the center of the plate thickness is, by mass%,
Cr: 0.1% or more, 5.0% or less,
Mo: 0.01% or more, 1.00% or less,
B: 0.0001% or more, 0.0100% or less,
Nb: 0.001% or more, 0.30% or less,
Ti: 0.001% or more, 0.30% or less,
V: 0.001% or more, 0.50% or less,
Ni: 0.0001% or more, 1.00% or less,
Cu: 0.001% or more, 1.00% or less,
Ca: 0.001% or more, 0.040% or less,
Mg: 0.001% or more, 0. 040% or less, and
REM: 0.001% or more, 0.040% or less
The high-strength steel plate according to claim 1, which is characterized by containing one or more of the selected types.
[Claim 3]
The high strength described in claim 1 or 2 is characterized by the fact that the C content of the soft part of the surface layer is 0.9 times or less of the C content of the central part of the plate thickness. Strength
[Claim 4]
The high-strength steel plate described in any one of claims 1 to 3, which is characterized by having a melted lead metal layer on the surface.
[Claim 5]
The high-strength steel plate described in any one of claims 1 to 3, which is characterized by having an alloyed molten lead metal layer on the surface.