JP5124865B2 - High tensile cold-rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile cold-rolled steel sheet and a method for producing the same. In particular, the present invention relates to a high-tensile cold-rolled steel sheet excellent in formability and chemical conversion treatment and a method for producing the same.

  In recent years, there has been a strong demand for weight reduction of the vehicle body in order to improve the fuel consumption of automobiles and reduce exhaust gas, and the application of high-tensile steel plates to vehicle body structural members has been promoted as one effective means. Since the weight reduction effect is enhanced as the strength of the steel sheet used is increased, a high-tensile steel sheet having a tensile strength of 590 MPa or more has been applied to vehicle body structural member applications such as members and frames. It was. However, the strength and workability of a steel sheet are generally in a contradictory relationship, and there is a problem that the workability decreases as the strength of the steel sheet increases, making it difficult to apply a high-strength steel sheet. On the other hand, steel sheets have been proposed in which the elongation is improved by utilizing the processing-induced transformation of retained austenite and the workability is improved.

  For example, in Patent Document 1 below, a steel sheet containing 0.4 to 1.8% Si and 0.2 to 2.5% Mn is heated to the [ferrite + austenite] two-phase region, and then in the middle of cooling. A method of realizing a mixed structure containing the retained austenite by holding in a temperature range of 500 to 350 ° C. for 30 seconds to 30 minutes to form a high-tensile steel sheet exhibiting high ductility is disclosed.

  In Patent Document 2 below, as a method for producing a high-strength steel sheet exhibiting high ductility, a steel sheet containing 0.7 to 2.0% Si and 0.5 to 2.0% Mn is annealed [ferrite + Austenite] After heating to the two-phase region, hold constant temperature for 10 to 50 seconds between 650 and 450 ° C. in the cooling process, and at least 10% ferrite and residual austenite by volume in martensite or bainite. A method of making a mixed structure steel sheet is disclosed.

  However, since the steel sheets described in Patent Documents 1 and 2 both contain a large amount of Si and have sufficient retained austenite, they exhibit good ductility, but the chemical conversion processability is not always good, and the coating of automobile manufacturers Numerous problems have occurred at the stage.

Regarding chemical conversion property, Patent Document 3 below shows the relationship between Si and Al in the steel composition. On the other hand, Patent Document 4 below discloses a Si reduction method for retained austenitic steel.
JP-A 61-157625 Japanese Patent Laid-Open No. 60-43430 JP 2003-193192 A JP 2000-345288 A

  An object of the present invention is to provide a high-tensile steel plate excellent in formability and chemical conversion treatment and a method for producing the same. More specifically, high tensile cold rolling having a tensile strength of 540 MPa class to 1180 MPa class having improved ductility while having excellent ductility equivalent to that of conventional retained austenitic steel having a high Si content. It is an object to provide a steel plate and a method for manufacturing the steel plate.

  The invention described in Patent Document 3 relates to a so-called DP steel, and the level of Si content in the steel is low, so that the chemical conversion processability is not a big problem from the beginning. Therefore, it does not suggest the problem of the present invention and the means for solving the problem of suppressing deterioration of chemical conversion properties when the Si content is high.

  The invention described in Patent Document 4 relates to a retained austenitic steel as in the present invention, but by adding Al, the Si content is minimized to 0.01 to 0.1 necessary for austenite residue. %. Therefore, although improvement of chemical conversion property can be expected, improvement of the original ductility of retained austenitic steel cannot be expected, and it cannot be a problem solving means of achieving both formability and chemical conversion property targeted by the present invention.

  As a result of intensive studies to achieve the above object, the present inventor can suppress the concentration of Si on the steel sheet surface by increasing the Al content in the steel, thereby increasing the Si content. The new knowledge that favorable chemical conversion property was realizable also in the steel plate which it has was acquired.

The present invention based on the above-mentioned new knowledge is, in mass%, C: 0.08 to 0.30%, Si: 0.30 to 1.0%, Mn: 1.0 to 2.8%, P: 0.0. Containing 05% or less, S: 0.01% or less, Al: 0.20 to 1.5%, and N: 0.01% or less, the balance being Fe and impurities, and the total content of Si and Al comprising a quantity chemical composition 1.2-1.8%, and the residual austenite containing 5% or more by area, provided with a Ru steel structure name from the remainder ferrite and bainite, the mass of Si and Al in addition the surface of the steel sheet A high-tensile cold-rolled steel sheet having a concentration ratio Si / Al of 0.5 or less.

  Here, the mass concentration ratio Si / Al of Si and Al on the steel sheet surface is the ratio of the Si mass concentration and the Al mass concentration on the steel sheet surface determined by ESCA. Moreover, the area ratio of a retained austenite can be calculated | required from the intensity ratio of the austenite phase peak by the ESCA measurement in the steel plate cross section in plate | board thickness 1/4 position, as shown in an Example.

The chemical composition may further contain an element selected from the group consisting of one or more of the following (1) to (3) in mass%, instead of a part of Fe:
(1) One or more selected from the group consisting of V: 0.1% or less, Ti: 0.1% or less, and Nb: 0.1% or less,
(2) One or more selected from the group consisting of Mo: 0.5% or less, Cr: 0.5% or less, and B: 0.005% or less,
(3) One or more selected from the group consisting of Ca: 0.004% or less, Zr: 0.05% or less, and rare earth elements: 0.05% or less.

The present invention also relates to a method for producing a high-tensile cold-rolled steel sheet comprising the following steps (A) to (G):
(A) A rough hot rolling step in which a rough bar is obtained by subjecting a steel ingot or steel slab having the above chemical composition to rough hot rolling;
(B) a first descaling step of performing a descaling process after holding the coarse bar in a temperature range of 1050 ° C. or more for 1 second or more;
(C) Finishing hot rolling step of subjecting the rough bar subjected to the descaling treatment to finish hot rolling to obtain a hot-rolled steel sheet;
(D) a winding step of winding the hot-rolled steel sheet in a temperature range of 500 to 680 ° C;
(E) a second descaling process in which the wound hot-rolled steel sheet is discharged and subjected to a descaling process;
(F) a cold rolling step in which cold rolling is performed on the hot-rolled steel sheet subjected to the descaling treatment; and (G) the cold-rolled steel sheet is Ac ₁ point or more and Ac ₃ points or less. A continuous annealing step in which the substrate is cooled to a temperature range of 350 to 450 ° C. at an average cooling rate of 5 ° C./second or more and held for 30 seconds or more in the temperature range.

  In the step (A), it is preferable to carry out rough hot rolling after the steel ingot or steel slab is set to 1050 ° C. or higher.

  According to the present invention, in a steel sheet having a high Si content of 0.30 to 1.0% and containing 5% by area or more of retained austenite and exhibiting excellent ductility inherent to retained austenitic steel, By suppressing the concentration, the chemical conversion processability, which is a characteristic of the steel sheet surface, can be remarkably improved. This makes it possible to achieve both formability and chemical conversion treatment for a high-tensile cold-rolled steel sheet having a tensile strength of 540 MPa to 1180 MPa.

  The high-strength cold-rolled steel sheet according to the present invention and the high-strength cold-rolled steel sheet produced by the method of the present invention can be subjected to high-level press forming and can be coated without any trouble after chemical conversion treatment. It is suitable for application to structural members, thereby contributing to improved fuel efficiency and reduced exhaust gas by reducing the weight of the vehicle body. However, this cold-rolled steel sheet can of course be used for other applications including home appliances and building materials.

The present invention is described in detail below. First, the reasons for limiting the chemical composition and steel structure of the high-strength steel sheet of the present invention will be described. In the following description,% related to the steel composition is mass%.
C: 0.08 to 0.3%
C is a component required to act together with Si and Al to stabilize austenite to room temperature. In addition, it is an essential component as a basic element from the viewpoint of securing strength. If the C content is less than 0.08%, it is difficult to ensure the intended strength, and it is difficult to ensure the retained austenite in an amount of 5 area% or more, which is an amount capable of obtaining sufficient ductility. It becomes. On the other hand, if the C content exceeds 0.3%, the hardness of the molten metal portion is remarkably increased when welded, and the weldability is remarkably deteriorated, so that the range of application as an industrial material is remarkably limited. The C content is preferably 0.09 to 0.20%.

Mn: 1.0-2.8%
Mn is a solid solution strengthening element and is contained for securing the intended strength. If the Mn content is less than 1.0%, it may be difficult to ensure the intended strength. On the other hand, if the Mn content exceeds 2.8%, segregation of Mn at the center of the steel sheet becomes significant, and ductility and bendability deteriorate. Further, since Mn also has a quenching action, if its content exceeds 2.8%, martensite is generated, causing an unnecessary increase in strength and deteriorating ductility.

Si: 0.30 to 1.0%, Al: 0.20 to 1.5%, Si + Al: 1.2 to 1.8%, Si / Al mass concentration ratio on steel plate surface: 0.5 or less Si and Al is the most important element in the chemical composition of steel in the present invention.

  Si is a ferrite stabilizing element and is an essential element for stabilizing austenite to room temperature. By increasing the volume of ferrite during the two-phase region annealing, the C concentration of the austenite phase is increased and the austenite phase is stabilized to room temperature. Al is an element having the same action as Si. In order to produce retained austenite, it is necessary to contain Si in an amount of 0.30% or more and Al in an amount of 0.20% or more. According to our experience, the total content of Si and Al is 1.2%. If it is more than the above, 5% by area or more of retained austenite can be secured, and good ductility can be obtained. On the other hand, if the total content of Si and Al exceeds 1.8%, soft ferrite tends to be formed in the heat-affected zone during welding (called HAZ), the HAZ zone softens, and the origin of weld cracking It becomes easy to become. Therefore, the total content of Si and Al is set to 1.2% or more and 1.8% or less.

  Considering the influence on chemical conversion processability, Si is an element that deteriorates chemical conversion processability. In general, Si concentrates on the surface of the steel sheet and easily combines with oxygen on the surface of the steel sheet to form a thin Si oxide layer. This thin Si oxide layer hardly dissolves in the acid (mainly hydrofluoric acid) in the chemical conversion treatment solution, and remarkably hinders the growth of chemical conversion crystals. Therefore, chemical conversion processability can be improved by reducing the Si content in the steel. However, as described above, Si is an element indispensable for improving ductility, and the reduction of Si in steel immediately leads to deterioration of ductility and workability.

Therefore, the inventors conducted intensive investigations on the effect of Al, which has the same action as Si, and obtained the following new knowledge.
Al is concentrated in the surface layer of the steel sheet together with Si. When Al is hardly contained in the steel components and the Si content is high (Si: 1.3%, Al: 0.03%), Si is strongly concentrated on the surface layer (FIG. 2). When a large amount of Al is contained (Si: 0.7%, Al: 0.8%), the concentration of the surface layer of Al suppresses the concentration of Si, so that the surface layer Si mass concentration level becomes low (FIG. 1). ). Furthermore, the content of the surface layer of Al is not much different from the Al content in the central portion of the plate thickness, and the change in mass concentration in the cross-sectional direction of the thickness of Al, and therefore the degree of concentration of the surface layer is small compared to Si (Fig. 1). In addition, even if the Al concentrated on the surface layer becomes an Al oxide layer, it is more soluble than the Si oxide in the acid (mainly hydrofluoric acid) in the chemical conversion treatment solution, so that the growth of chemical crystals is remarkably increased. There is no hindrance.

  As described above, the effect of improving the chemical conversion processability by adding Al is such that when the Si content exceeds 1.0%, the action of preventing chemical conversion crystal growth by Si is overcome. It becomes difficult to secure. On the other hand, when the Al content is less than 0.20%, the surface layer concentration of Al is insufficient, and the above effects cannot be obtained sufficiently. On the other hand, if the Al content exceeds 1.5%, an oxide is formed at the weld interface during butt welding, and it becomes difficult to ensure sufficient welding strength.

From the above, the Si content is 0.30% or more and 1.0% or less, and the Al content is 0.20% or more and 1.5% or less.
The size of the conversion crystal is about several μm. Therefore, in the element concentration distribution in the depth direction in the vicinity of the surface of the plate thickness, it is considered that deterioration of chemical conversion treatment due to Si can be prevented if Si concentration on the surface of the steel plate is suppressed. As a result of further investigation from this viewpoint, it was found that the Si / Al mass concentration ratio on the steel sheet surface is important. That is, when the mass concentration ratio of Si / Al is 0.5 or less, good chemical conversion property can be obtained. Here, the mass concentration ratio Si / Al of Si and Al on the steel sheet surface is the ratio of the Si mass concentration and the Al mass concentration on the steel sheet surface determined by ESCA.

P: 0.05% or less, S: 0.01% or less, N: 0.01% or less All of these elements are impurities.
Since P segregates at the grain boundaries and deteriorates ductility, it is preferable that the P content is low. P also degrades weldability. Therefore, the P content is set to 0.05% or less.

  S not only degrades ductility and weldability by generating MnS, but also consumes Mn, which is an austenite stabilizing element, so a lower S content is preferred. Therefore, the S content is set to 0.01% or less.

  N is also an impurity and its content is preferably low. In particular, when the N content exceeds 0.01%, the amount of Al consumed as AlN is large, and the effect of adding Al becomes small. Therefore, the N content is 0.01% or less.

One or more selected from the group consisting of V: 0.1% or less, Ti: 0.1% or less, and Nb: 0.1% or less,
V, Ti and Nb are all elements having a precipitation strengthening action, and bring about an effect of increasing the strength of the steel sheet. Therefore, 1 type (s) or 2 or more types can be contained as needed. However, excessive content causes deterioration of ductility and is economically disadvantageous, so the upper limit of the content of each element in the case of containing these is set to 0.1%. In order to surely obtain the above effect, it is preferable to set the lower limit of each element to 0.001%.

One or more selected from the group consisting of Mo: 0.5% or less, Cr: 0.5% or less, and B: 0.005% or less,
Mo, Cr and B are all elements having a transformation strengthening effect, and bring about an effect of increasing the strength of the steel sheet. Therefore, 1 type (s) or 2 or more types can be contained as needed. However, excessive content causes deterioration of ductility and is also economically disadvantageous. Therefore, the upper limit of the content of each element when containing these is 0.5% for Mo and Cr, and about B Set to 0.005%. In order to surely obtain the above effect, it is preferable that the lower limit of the content is 0.01% for Mo and Cr and 0.0003% for B.

One or more selected from the group consisting of Ca: 0.004% or less, Zr: 0.05% or less, and rare earth elements (REM): 0.05% or less.
Ca, Zr, and REM are all elements that have the effect of controlling the shape of inclusions and improving corrosion resistance, and have the effect of increasing ductility and improving end face corrosion resistance. Therefore, 1 type (s) or 2 or more types can be contained as needed. However, since the effect is saturated and economically disadvantageous even if contained in excess, the upper limit of the content of each element in the case of containing these is 0.004% for Ca and 0.05 for Zr. % And REM are set to 0.005%. In order to obtain the above-mentioned effect reliably, the lower limit of the content is preferably 0.0002% for Ca, 0.005% for Zr, and 0.002% for REM.

Next, suitable production conditions for the high-tensile cold-rolled steel sheet according to the present invention will be described.
(Rough hot rolling process, first descaling process, finish hot rolling process, winding process)
The steel ingot or steel slab having the above chemical composition is subjected to rough hot rolling to form a rough bar. After holding the rough bar in a temperature range of 1050 ° C. or higher for 1 second, a descaling treatment is performed, and then the finish heat Hot-rolled steel sheet is formed by hot rolling in a temperature range of 500 to 680 ° C.

  By holding for 1 second or more in a temperature range of 1050 ° C. or higher before descaling the coarse bar, generation of an iron scale containing Si oxide on the steel surface is promoted, and at the same time as the scale thickness increases, The amount of Si increases. Al also forms an oxide similar to Si, but scale generation is not promoted as much as Si. Then, the production scale containing a large amount of Si is effectively obtained by the first process of descaling (typically descaling process by high-pressure water spray), which is the next process, and the subsequent machining such as hot rolling. Removed. As a result, the Si mass concentration in the surface layer portion of the steel sheet obtained by hot rolling decreases. The coarse bar descaling process (first descaling process) may be a mechanical descaling process such as grinding.

  If the holding temperature of the coarse bar before descaling is less than 1050 ° C. and / or the holding time is less than 1 second, the generation of iron scale containing Si oxide is insufficient, and the scale thickness and hardness are reduced. In some cases, the scale removal by machining depending on the thickness is not effectively performed, and the mass concentration ratio Si / Al of Si and Al on the surface of the steel sheet exceeds 0.5. The upper limit of the holding temperature is practically 1400 ° C., and the upper limit of the holding time is practically 1 minute.

  In the present invention, as a method of holding the rough bar in the temperature range of 1050 ° C. for 1 second or longer, a rough bar heating apparatus using induction heating, current heating, furnace heating or the like between the rough hot rolling mill and the finishing hot rolling mill And heating the coarse bar is exemplified. From the viewpoint of temperature controllability and operability, coarse bar heating by induction heating is preferred. In addition, since it is sufficient to perform descaling after holding the rough bar in the temperature range of 1050 ° C. for 1 second or longer, in addition to heating the rough bar by the above method, a steel ingot or steel slab used for rough hot rolling The present invention also includes the case where the temperature of is set to a high temperature so that the rough bar after the rough hot rolling is at a temperature of 1050 ° C. or more for 1 second or more and no special heating is performed.

  Also, the mass concentration ratio Si / Al on the surface of the coarse bar is reduced by the same mechanism as described above by subjecting the steel ingot or steel slab to be subjected to the coarse hot rolling process to 1050 ° C. or higher and then subjecting it to hot hot rolling. This is preferable because the mass concentration ratio Si / Al on the steel sheet surface of the final product can be further reduced. The steel ingot or steel slab may be subjected to rough hot rolling after being heated to 1050 ° C. or higher in a heating furnace after being lowered in temperature to less than 1050 ° C., or after ingot or partial rolling after continuous casting The steel slab may be subjected to rough hot rolling without lowering the temperature to less than 1050 ° C.

  The hot rolled steel sheet obtained by hot finish rolling is wound up in a temperature range of 500 to 680 ° C. In the winding stage, a Si-enriched scale is generated by the same mechanism as described above. This scale removal is performed in the next pickling process (second descaling process). When the winding temperature is as low as less than 500 ° C., Si concentration on the scale is not noticeable, and the effect of reducing the surface layer Si concentration cannot be sufficiently obtained. Moreover, baking starts and intensity | strength rises and the cold rolling property in the next cold rolling process is inhibited. On the other hand, when the coiling temperature is higher than 680 ° C., excessive scale is generated in the coiling process, which causes surface defects and is a quality problem. Therefore, the winding temperature is set in the range of 500 to 680 ° C.

(Second descaling process)
By applying a descaling process to the hot-rolled steel sheet obtained by the above process, the Si-concentrated scale formed during winding is removed, and the mass concentration ratio of Si and Al in the surface layer portion of the steel sheet is Si / Reduce Al. As the descaling process after the hot rolling (second descaling process), pickling (spraying or dipping) is usually used, but machining such as grinding may be used. Skin pass rolling may be performed before pickling.

(Cold rolling process)
The hot-rolled steel sheet obtained by the above process is cold-rolled to obtain a cold-rolled steel sheet. If the rolling reduction of cold rolling is less than 30%, recrystallization cannot be performed completely in the subsequent annealing process, and mechanical properties and low temperature toughness may deteriorate. On the other hand, when the rolling reduction exceeds 80%, the rolling load increases and the rolling mill may be overloaded. For this reason, it is preferable that the rolling reduction in cold rolling shall be 30 to 80%. The rolling reduction is more preferably 40 to 70%.

(Continuous annealing process)
The cold-rolled steel sheet obtained by the above process is held in a temperature range of Ac ₁ point or more and Ac ₃ point or less for 30 seconds or more and then cooled to a temperature range of 350 to 450 ° C. at an average cooling rate of 5 ° C./second or more. Then, a continuous annealing process is performed in the temperature range for 30 seconds or more.

In the continuous annealing process, in order to form a two-phase structure of [ferrite + austenite] once, it is held for 30 seconds or more in a temperature range of Ac ₁ point or more and Ac ₃ point or less. If the holding temperature is too low, the re-solution of the carbide produced by hot rolling is delayed, and if it is too high, the volume fraction of austenite becomes too large, and the C concentration in the austenite decreases, and the room temperature of the austenite in the final product. Stability at is reduced. The steel of the present invention having the above chemical composition is preferably annealed at 760 to 850 ° C.

  Subsequently, it cools to the temperature range of 350-450 degreeC with an average cooling rate of 5 degree-C / sec or more, and hold | maintains for 30 seconds or more in the said temperature range. When the average cooling rate is less than 5 ° C./sec, the pearlite transformation of austenite may progress, and it may be difficult to secure the desired retained austenite. In order to grow ferrite and increase the C concentration in austenite, it is desirable to set the average cooling rate up to 700 ° C. at which the ferrite formation rate is maximized to 10 ° C./second or less. On the other hand, the cooling from 700 ° C. to 350 to 450 ° C. is desirably performed at an average cooling rate of 50 ° C./second or more in order to suppress the pearlite transformation of austenite.

  By holding for 30 seconds or more in a temperature range of 350 to 450 ° C., the concentration of C to the remaining austenite is promoted while part of the austenite is transformed to bainite. When the holding time is less than 30 seconds, the progress of bainite transformation becomes insufficient, and it may be difficult to concentrate C to austenite. The upper limit of the holding time does not need to be specified in particular. However, if the holding time is excessively long, the length of the equipment is increased and the productivity is deteriorated. Therefore, the holding time is preferably 10 minutes or less. When the holding temperature is higher than 450 ° C., the progress of the bainite transformation becomes insufficient, and it may be difficult to concentrate C to austenite. When the holding temperature is lower than 350 ° C., the bainite to be formed becomes lower bainite, which may make it difficult to concentrate C into austenite. The cooling rate to room temperature after holding is not particularly limited.

  By the above method, it is possible to produce a high-tensile cold-rolled steel sheet having 5% by area or more of retained austenite and having a Si / Al mass concentration ratio on the steel sheet surface of 0.5 or less. As described above, this high-tensile cold-rolled steel sheet has high ductility and excellent formability. Furthermore, since chemical conversion property is also favorable, the coating film which was excellent in adhesiveness can be formed by coating after performing chemical conversion treatment before or after press molding.

  The chemical conversion treatment is generally performed by a well-known phosphate treatment such as zinc phosphate and manganese phosphate. The coating method is not particularly limited, and an appropriate method may be selected according to the situation. For example, electrodeposition coating, spray coating, powder coating, roll coating, etc. can be employed. The high-tensile cold-rolled steel sheet of the present invention can also be used as a base material for various plated steel sheets including zinc-based plating and aluminum-based plating.

  Test slabs having the chemical compositions shown in Table 1 were produced in a vacuum melting furnace. The test slab was heated to a temperature of 1050 ° C. or higher and subjected to rolling imitating rough hot rolling with a test hot rolling facility to obtain a rough bar. The coarse bar is heated to the coarse bar heating temperature shown in the same table and held for 2 seconds, subjected to the first descaling treatment by water spray, and then subjected to rolling imitating finish hot rolling to obtain a hot rolled steel sheet. It was.

  The hot-rolled steel sheet was cooled to 600 ° C. and held at this temperature for 1 hour to reproduce the winding heat treatment of the hot-rolled steel sheet. The hot-rolled steel sheet that had been subjected to the winding heat treatment was removed from the scale by pickling (second descaling process) and cold-rolled at a reduction rate of 70%. Then, using a continuous annealing simulator, annealing was performed at 800 ° C. for 60 seconds, and after cooling to 400 ° C. at an average cooling rate of 60 ° C./second, the temperature was 10 seconds for steel type R and 100 seconds for other steel types. After holding, it was further cooled to room temperature to obtain a cold-rolled steel sheet.

The tensile properties of each cold-rolled steel sheet were evaluated by L-direction tension of a JIS No. 5 tensile test piece, and the case where the product of TS (MPa) and EL (%) was 18000 MPa% or more was considered good.
The steel structure was determined by observing with a light microscope after corroding the cross-sectional specimen in the plate thickness direction. Nittal corrosion solution was used for the observation of ferrite, and repeller corrosion solution was used for the observation of austenite.

  The element mass concentration of the surface layer was measured by ESCA (ESCA3200 manufactured by Shimadzu Corporation: tube condition: 8 kV, 30 mA). The ratio of the measured values of Si mass concentration and Al mass concentration on the steel sheet surface was determined as the Si / Al mass concentration ratio.

The amount of residual γ (residual austenite area ratio) was calculated from the strength ratio of ESCA in the cross section of the steel sheet at the 1/4 position of the thickness.
The chemical conversion treatment is performed with standard specifications using a zinc phosphate-based chemical conversion treatment liquid (Bt3080: manufactured by Nihon Parker Rising Co., Ltd.), which is a normal automotive chemical, and then the properties of the chemical conversion film are visually and scanned. Observed with a scanning electron microscope, “○” indicates that the steel sheet substrate is densely coated, and “×” indicates that the chemical conversion film has a partial defect.

The above results are also shown in Table 1.
As can be seen from the results in Table 1, the steel sheet according to the present invention has excellent chemical conversion treatment properties, and both have excellent strength / ductility balance.

The figure which shows the change of the depth direction of Fe, Si, and Al density | concentration of the surface vicinity measured by ESCA of the cold rolled steel plate of high Si and high Al. The figure which shows the change of the depth direction of Fe and Si density | concentration of the surface vicinity measured by ESCA of the cold rolled steel plate which hardly contains Al at high Si.

Claims (6)

In mass%, C: 0.08 to 0.30%, Si: 0.30 to 1.0%, Mn: 1.0 to 2.8%, P: 0.05% or less, S: 0.01 %, Al: 0.20 to 1.5%, and N: 0.01% or less, the balance being Fe and impurities, and the total content of Si and Al is 1.2 to 1.8. % and is provided with a chemical composition, and the residual austenite containing 5% or more by area, provided with a Ru steel structure name from the remainder ferrite and bainite, the mass concentration ratio Si / Al of Si and Al in addition the surface of the steel sheet 0.5 A high-tensile cold-rolled steel sheet characterized by:   The chemical composition may be one selected from the group consisting of V: 0.1% or less, Ti: 0.1% or less, and Nb: 0.1% or less in mass%, instead of part of Fe. The high-tensile cold-rolled steel sheet according to claim 1, containing two or more kinds.   The chemical composition may be one selected from the group consisting of Mo: 0.5% or less, Cr: 0.5% or less, and B: 0.005% or less in mass% instead of a part of Fe. The high-tensile cold-rolled steel sheet according to claim 1 or 2, containing two or more kinds.   The chemical composition is one selected from the group consisting of Ca: 0.004% or less, Zr: 0.05% or less, and rare earth elements: 0.05% or less in mass%, instead of a part of Fe. Or the high-tensile cold-rolled steel sheet according to any one of claims 1 to 3, comprising two or more kinds. A method for producing a high-tensile cold-rolled steel sheet comprising the following steps (A) to (G):
(A) A rough hot rolling step in which the steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is subjected to rough hot rolling to obtain a rough bar;
(B) a first descaling step of performing a descaling process after holding the coarse bar in a temperature range of 1050 ° C. or more for 1 second or more;
(C) Finishing hot rolling step of subjecting the rough bar subjected to the descaling treatment to finish hot rolling to obtain a hot-rolled steel sheet;
(D) a winding step of winding the hot-rolled steel sheet in a temperature range of 500 to 680 ° C;
(E) a second descaling process in which the wound hot-rolled steel sheet is discharged and subjected to a descaling process;
(F) a cold rolling step in which cold rolling is performed on the hot-rolled steel sheet subjected to the descaling treatment; and (G) the cold-rolled steel sheet is Ac ₁ point or more and Ac ₃ points or less. A continuous annealing step in which the substrate is cooled to a temperature range of 350 to 450 ° C. at an average cooling rate of 5 ° C./second or more and held for 30 seconds or more in the temperature range.   The method for producing a high-tensile cold-rolled steel sheet according to claim 5, wherein in the step (A), the steel ingot or the steel slab is heated to 1050 ° C or higher and then subjected to rough hot rolling.

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