JP5637530B2 - Ultra-high strength cold-rolled steel sheet with a tensile strength of 780 MPa or more that has high ductility and excellent chemical conversion properties - Google Patents

Description

  The present invention relates to an ultra-high-strength cold-rolled steel sheet having a tensile strength TS of 780 to 1180 MPa class, which is suitable for automobile parts that are press-formed into a strict shape and has high ductility and excellent chemical conversion properties.

  Super high strength cold-rolled steel sheets with a TS of 780 to 1180 MPa used for automobile parts and the like need to have high ductility such as stretch flangeability and excellent chemical conversion properties in addition to high strength due to their application characteristics. It is.

  For high-strength, high-strength cold-rolled steel sheets, for example, C: 0.05 to 0.35%, Si: 0.5 to 3.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% Less than N, 0.0020 to 0.0100%, Ti: 0.001 to 0.20%, Ca and / or REM: 0.0010 to 0.010%, ferrite phase of 30% or more by volume ratio, residual austenite phase of 2% or more by volume ratio In addition, the secondary adhesion of the coating film has a composite structure consisting of a low-temperature transformation phase, a ferrite grain size of 15 μm or less, and a strength ratio of Si and Fe of 6.0 or less by GDS analysis from the steel sheet surface layer to a depth of 10 nm. An excellent high strength and high ductility cold rolled steel sheet is disclosed (Patent Document 1). Also, C: 0.06-0.25%, Si: 2.5% or less, Mn: 0.5-3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1-2.5%, Ti: 0.003-0.08%, N: Surface properties containing 0.01% or less and satisfying (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0.01 and having a retained austenite phase of 5% or more by volume and A high ductility type high-tensile cold-rolled steel sheet excellent in shock absorption is disclosed (Patent Document 2). Furthermore, it contains C: 0.04-0.14%, Si: 0.2-2.0%, Mn: 0.5-2.0%, P: 0.008% or less, B: 0.0003-0.0050%, Al: 0.010-2.00%, and in area ratio A high-strength cold-rolled steel sheet excellent in workability and weldability composed of a composite structure of 4 to 15% or more of retained austenite phase, ferrite phase, and bainite phase is disclosed (Patent Document 3).

On the other hand, regarding the chemical conversion treatment, high-strength cold-rolled steel sheets are not necessarily targeted, but surface-treated steel sheets with excellent chemical conversion treatment with Ni oxide or Ni hydroxide attached to the surface are disclosed (patents) Reference 4). Also, C: 0.001-0.25%, Si: 0.1-1.5%, Mn: 0.2-3.0%, and Cu: 0.5-1.5%, Ni: 0.5-1.5%, Mo: 0.2-1.0%, Cr: 0.1-2.5 %, V: 0.01% to 0.1%, B: 0.0005% to 0.0030%, Ti: 0.01% to 0.20%, Nb: 0.01% to 0.20%, and an iron coating layer is formed on the surface. A high-strength cold-rolled steel sheet having excellent post-coating corrosion resistance has been disclosed (Patent Document 5). Furthermore, C: 0.001-0.25%, Si: 0.1-1.5%, Mn: 0.2-3.0%, and Cu: 0.5-1.5%, Ni: 0.5-1.5%, Mo: 0.2-1.0%, Cr: 0.1-2.5 %, V: 0.01 to 0.1%, B: 0.0005 to 0.0030%, Ti: 0.01 to 0.20%, Nb: nickel containing nickel metal particles on the surface, containing at least one element selected from 0.01 to 0.20% A high-strength cold-rolled steel sheet having a metal layer and excellent phosphatability is disclosed (Patent Document 6). Further, regarding the Si distribution, the surface coverage of the Si-based oxide is disclosed (Patent Document 7). Furthermore, regarding the TRIP steel, the average value of the surface Si concentration, and the area ratio of the portion where the concentration ratio to the Si concentration in the steel in the surface Si concentration distribution is 10 or more are disclosed (Patent Document 8).
JP2003-201538 Japanese Patent Laid-Open No. 10-130776 JP 2001-64748 A JP 59-159987 A JP-A-5-320952 JP-A-6-93472 Japanese Patent Laid-Open No. 2004-323969 JP 2004-204350 A

  However, the high ductility, high-strength cold-rolled steel sheets described in Patent Documents 1 to 3 have insufficient chemical conversion properties. In particular, in the high-strength cold-rolled steel sheet described in Patent Document 1, the strength ratio of Si and Fe is reduced to 6.0 or less by pickling or brushing to improve the secondary adhesion of the coating film. An excellent chemical conversion treatment property is not obtained, and a non-uniform chemical conversion treatment film is formed on the steel sheet surface. Further, with the high-strength cold-rolled steel sheet described in Patent Document 3, a TS of 780 MPa or more cannot be obtained.

  The surface-treated steel sheets and high-strength cold-rolled steel sheets described in Patent Documents 4 to 6 require special plating treatments such as Ni oxides, Ni hydroxides, iron coating layers, nickel metal layers, and are expensive. Productivity is poor. Further, only the components of the steel sheet are disclosed, and depending on the production conditions, an ultra-high strength cold-rolled steel sheet having a high ductility and a TS of 780 MPa or more cannot be obtained.

  In the steel sheet described in Patent Document 7, zinc phosphate crystals are generated without gaps, but since the Si concentration is not controlled, a sufficiently excellent corrosion resistance after coating cannot be obtained. Since the steel sheet described in Patent Publication 8 has TRIP characteristics, it is excellent only in ductility and inferior in stretch flangeability. In addition, the surface Si average concentration level is extremely high at 11 to 42, and there are sites where the concentration ratio to the Si concentration in the steel is 10 or more, which is more expensive than usual, such as twice pickling, grinding, descaling, etc. In spite of going through various processes, Si concentration has not been sufficiently reduced. Even if the steel sheet surface Si is reduced before annealing, it is a big problem that Si oxides are generated on the steel sheet surface during annealing.

  It is an object of the present invention to provide an ultra-high strength cold-rolled steel sheet having a TS of 780 to 1180 MPa class that can be manufactured at low cost and has high ductility and excellent chemical conversion properties.

  When the inventors proceeded to study a super-high strength cold-rolled steel sheet having a TS of 780 to 1180 MPa, which can be manufactured at a low cost and has high ductility and excellent chemical conversion property, the following knowledge was obtained.

  i) In order to achieve high ductility and high strength of TS of 780 MPa or more, the composition of the components should be optimized, the microstructure should be a composite structure composed of a ferrite phase and a tempered martensite phase, and the volume of the tempered martensite phase. The rate should be 20-70%.

  ii) In order to achieve excellent chemical conversion, the ratio of Si concentration Si (0) on the steel sheet surface to Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface, that is, [Si (0) / Si ( 0.1)] should be 1.0 to 1.6, and the standard deviation should be 0.50 or less.

  iii) The microstructure and surface Si concentration as described above can be controlled for the first time by, for example, performing annealing by continuous annealing to optimize the annealing conditions and performing pickling treatment and alkali treatment after annealing. .

  The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.0%, Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 ~ 0.005%, Al: 0.005 ~ 0.08%, N: 0.001 ~ 0.01%, the balance consists of Fe and inevitable impurities, has a composite structure consisting of ferrite phase and tempered martensite phase, tempered martensite phase The ratio of the Si concentration Si (0) on the steel sheet surface to the Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface [Si (0) / Si (0.1 )] Is 1.0 to 1.6, and the standard deviation of this ratio is 0.50 or less, and it provides an ultra-high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, which is excellent in chemical conversion processability.

  The ultra-high-strength cold-rolled steel sheet of the present invention further contains at least one element selected from Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, and V: 0.001 to 0.2% by mass%. Can be contained.

  Further, the ultra-high-strength cold-rolled steel sheet of the present invention further includes, in mass%, Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: It can also contain at least one element selected from 0.0001 to 0.005% and Ca: 0.0001 to 0.005%.

  According to the present invention, an ultra-high strength cold-rolled steel sheet having a TS of 780 to 1180 MPa class, which has high ductility and excellent chemical conversion property, can be produced at low cost. The ultra-high-strength cold-rolled steel sheet of the present invention is suitable for automobile parts that are press-formed into a strict shape.

  Below, the ultra high strength cold-rolled steel sheet of the present invention will be described in detail.

1) ingredients
C: C is indispensable for strengthening steel using low-temperature transformation phase such as martensite phase. In general, the strength of the low temperature transformation phase tends to be proportional to the C content. If the amount is less than 0.05%, the microstructure does not become a composite structure composed of a ferrite phase and a martensite phase, a TS of 780 MPa or more cannot be obtained, and the TS-El balance is low. Here, El represents the elongation obtained by the tensile test. Further, in order to obtain TS of 780 MPa or more, it is preferable that the amount is large. However, if it exceeds 0.2%, the spot weldability is remarkably deteriorated, or the martensite phase is excessively hardened and the ductility tends to be lowered. Therefore, the C content is 0.05 to 0.2%, preferably 0.06 to 0.16%.

  Si: Si is an element that contributes to strengthening by solid solution strengthening. Also, by strengthening the ferrite phase, the hardness difference from the martensite phase is reduced, and at the time of stretch flange molding, cracking and crack propagation at the boundary between the ferrite phase and the martensite phase are suppressed to promote uniform deformation and elongation. There is also an effect of improving flangeability. Therefore, the amount needs to be 0.5% or more. On the other hand, when the amount exceeds 2.0%, not only is this effect saturated, but the following problems occur. That is, it is strengthened excessively and the ductility of the ferrite phase is greatly reduced. Spot weldability decreases. A hard-to-peel scale is generated during hot rolling, causing surface defects. If the Si oxide is unevenly distributed on the surface of the steel sheet, it will inhibit the adsorption of Ti colloids and charged phosphate particles during surface preparation, which is the pre-coating treatment stage, and will deteriorate the chemical conversion treatment performance, thereby improving the post-coating corrosion resistance. Reduce. Therefore, the Si content is 0.5 to 2.0%, preferably 0.8 to 1.6%.

  Mn: Mn is an element necessary for the formation of a composite structure. It enhances the hardenability of steel and forms a hard martensite phase, improving the TS-El balance. It also affects the final tempered martensite fraction by controlling the volume fraction of the ferrite and austenite phases at the start of heating and cooling during annealing. Therefore, in order to obtain desired material characteristics and structure, the amount needs to be 1.5% or more. On the other hand, if the amount exceeds 3.5%, spot weldability is reduced, or a layered structure of ferrite phase and martensite phase is formed due to segregation of Mn, and ductility is reduced. Therefore, the Mn content is 1.5 to 3.5%, preferably 1.7 to 2.4%.

  P: P is a solid solution strengthening element. However, segregation to the grain boundary lowers the bond strength of the grain boundary and lowers formability and spot weldability. Therefore, the amount needs to be 0.05% or less. On the other hand, reducing the amount to less than 0.001% significantly increases the manufacturing cost. Therefore, the P content is 0.001 to 0.05%, preferably 0.001 to 0.02%.

  S: S exists in the steel sheet as a plate-like inclusion MnS, which reduces the ultimate deformability of the steel sheet and reduces stretch flangeability. Therefore, the amount needs to be 0.005% or less. On the other hand, reducing the amount to less than 0.0001% is accompanied by a decrease in productivity and an increase in cost. Therefore, the S content is 0.0001 to 0.005%, preferably 0.0001 to 0.002%.

  Al: Al is effective as a deoxidizer for steel, and also has the effect of removing non-metallic inclusions that reduce local ductility into the slag. In addition, it is easier to oxidize than Si, which adversely affects chemical conversion properties, and has the effect of suppressing Si from being localized or excessively concentrated on the steel sheet surface. Furthermore, Al partially dissolves in the ferrite phase, strengthens the ferrite phase, reduces the hardness difference between the ferrite phase and the hard martensite phase, and contributes to the improvement of stretch flangeability. Therefore, the amount needs to be 0.005% or more. On the other hand, if the amount exceeds 0.08%, the steel plate surface is excessively covered with an Al passivated film, the chemical conversion property is lowered, and the corrosion resistance is also deteriorated. Also, the weldability is reduced. Therefore, the Al content is 0.005 to 0.08%, preferably 0.01 to 0.05%.

  N: N is an impurity element that causes strain aging, but in a composite steel sheet, the effect is not so great that strain aging does not become a problem. N forms nitrides and has the effect of suppressing surface cracking of the slab. Therefore, the amount needs to be 0.001% or more. On the other hand, when the amount exceeds 0.01%, the effect of suppressing slab surface cracking tends to be saturated. Therefore, the N content is 0.001 to 0.01%, preferably 0.001 to 0.0050%.

  The balance is Fe and inevitable impurities.

  The above components are sufficient to achieve the object of the present invention, but at least selected from Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, and V: 0.001 to 0.2% for the following reasons. It is preferable that one element is contained. That is, when Ti is contained in excess of 0.04%, Ti carbonitride precipitates in the ferrite phase, reduces the ductility of the ferrite phase, or reduces the amount of C to be concentrated to the austenite phase during annealing, Reduce the volume fraction of the martensite phase. On the other hand, when Ti is contained in an amount of 0.001% or more, the structure becomes uniform due to the refinement of crystal grains and the ductility is improved. In particular, grain growth is suppressed during slab heating prior to hot rolling, which is effective for subsequent refinement of the structure. Ti also precipitates as carbonitrides and sulfides at high temperatures during slab cooling, preventing the precipitation of AlN and the precipitation of Nb and V carbides at relatively low temperatures to prevent surface cracks in the slab. It is also an effective element for doing so. Therefore, the Ti content is 0.001 to 0.04%, more preferably 0.001 to 0.02%. Nb and V, like Ti, precipitate as carbides such as NbC and VC when contained over 0.001%, suppress the growth of the ferrite phase during annealing, and make the structure finer and homogenously improve stretch flangeability. Is an effective element. On the other hand, when Nb exceeds 0.04%, and when V exceeds 0.2%, YS increases due to precipitation strengthening, workability decreases, and the martensite phase decreases. Reduce ductility. Therefore, the Nb amount is 0.001 to 0.04%, more preferably 0.001 to 0.02%, and the V amount is 0.001 to 0.2%, more preferably 0.001 to 0.01%.

  Furthermore, at least one selected from Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001 to 0.005% for the following reasons These elements are preferably contained. That is, when containing 0.01% or more of Cu, Ni, Mo, Cr, the formation of the martensite phase is promoted, the martensite phase itself is strengthened, and when B is contained by 0.0001% or more, the hardenability is increased. It becomes higher and the formation of martensite phase is promoted, which contributes to higher strength. On the other hand, if Cu, Ni, Mo, Cr is contained in excess of 1%, and if B is contained in excess of 0.005%, the hardenability becomes too high and the formation of the ferrite phase is suppressed. , The martensite phase is excessively cured and the workability is lowered. It is also disadvantageous in terms of cost. Therefore, the Cu amount is 0.01 to 1%, more preferably 0.01 to 0.5%, the Ni amount is 0.01 to 1%, more preferably 0.01 to 0.5%, and the Mo amount is 0.01 to 1%, more preferably 0.01 to 0.5%. %, The Cr content is 0.01 to 1%, more preferably 0.01 to 0.5%, and the B content is 0.0001 to 0.005%, more preferably 0.0001 to 0.002%.

  Furthermore, when Ca is contained in an amount of 0.0001 to 0.005%, more preferably 0.0001 to 0.002%, the shape of sulfide such as MnS is controlled to improve ductility such as stretch flangeability.

  In addition, REM having the same effect as Ca, Sb having an effect of regulating the crystal of the steel sheet surface layer, and the like can be contained in a range of 0.0001 to 0.1%. Thereby, chemical conversion property is not impaired significantly.

2) Microstructure As described above, in order to achieve high ductility and high strength of TS of 780 MPa or more, the microstructure is a composite structure of ferrite phase and tempered martensite phase, and the volume of tempered martensite phase. The rate needs to be controlled. The tempered martensite phase is a structure obtained by reheating the martensite phase, which is a low temperature transformation phase obtained by quenching the austenite phase to the martensite transformation temperature Ms point or less by water quenching, etc., and is harder than the ferrite phase Therefore, it contributes to strength. In order to obtain TS of 780 MPa or more, the volume ratio of the tempered martensite phase needs to be 20% or more. On the other hand, when the volume fraction of the tempered martensite phase exceeds 70%, the volume fraction of the soft ferrite phase is lowered and excessively strengthened, and the TS-El balance is lowered. Therefore, the volume ratio of the tempered martensite phase is 20 to 70%, preferably 30 to 65%.

  Here, the volume ratio of the tempered martensite phase was measured at a magnification of 1000 with an optical microscope or a scanning electron microscope at the position of the thickness 1/4 of the thickness cross section parallel to the rolling direction of the steel plate. The area ratio occupied by the tempered martensite phase present in the film was obtained by image processing and used as the volume ratio.

3) [Si (0) / Si (0.1)]
As described above, in order to achieve excellent chemical conversion, the ratio of Si concentration Si (0) on the steel sheet surface to Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface [Si (0) / Si (0.1)] and its standard deviation need to be controlled. Si is an easily oxidizable element. If SiO ₂ is excessively present on the surface of the steel sheet, insulators will be scattered on the surface of the steel sheet. Inhibits adsorption. Further, the Si oxide on the surface of the steel sheet inhibits the etching property of the steel sheet at the initial stage of formation of the chemical conversion film with zinc phosphate or the like, which is performed as a base treatment for electrodeposition coating. Further, when the Si oxide is locally present at a high concentration, unevenness occurs in etching, and formation of a chemical conversion crystal is hindered at a location with a high Si concentration. That is, the chemical conversion treatment film is not sufficiently finely and densely formed on the surface of the steel sheet, and when exposed to a poor environment such as salt warm water after electrodeposition coating, the secondary adhesion of the coating film is remarkably deteriorated. Therefore, in order to optimize the Si concentration on the steel sheet surface, the relationship between the above ratio [Si (0) / Si (0.1)] and chemical conversion treatment was examined. [Si (0) / Si (0.1)] It was found that excellent chemical conversion treatment properties can be obtained by setting the ratio to 1.0 to 1.6, preferably 1.0 to 1.4. When this ratio exceeds 1.6, the chemical conversion treatment performance is remarkably lowered. When this ratio is 1.0, that is, the Si concentration at the surface is the same as the Si concentration at a depth of 0.1 mm, and excellent chemical conversion property is obtained.

  In addition to the Si concentration on the steel sheet surface, the distribution of Si is extremely important. This is because even if the Si concentration is the same, there may be many regions having a high Si concentration, or the Si concentration may be uniform. Therefore, when the relationship between the standard deviation of [Si (0) / Si (0.1)] and the corrosion resistance after painting was examined, if this standard deviation exceeded 0.50, there was a local region with high Si concentration, and the corrosion resistance after painting. Was found to tend to deteriorate rapidly. Therefore, this standard deviation is 0.50 or less, preferably 0.30 or less.

  If Si is present on average, the etching property is poor, but the reaction proceeds uniformly and more uniform fine and dense chemical crystals are formed. Therefore, it is preferable that the standard deviation is small, that is, the Si concentration is uniform on the steel plate surface.

Here, Si (0) and Si (0.1) were measured as follows. That is, using EPMA JXA8600MX manufactured by JEOL Ltd., analysis area: 200 μm × 200 μm, electron beam acceleration energy: 15 kV, current: 1.2 × 10 ⁻⁷ A, analysis time: 50 msec / point, Si The element distribution was investigated, and the Si distribution was measured in the range of Si count 0 (MIN) to 500 (MAX) according to the strength level. The average Si count on the steel sheet surface was Si (0), and the average Si count on the surface ground to a depth of 0.1 mm from the steel sheet surface was Si (0.1). The standard deviation of the ratio between Si (0) and Si (0.1) was obtained from the distribution of Si counts relative to Si (0).

4) Manufacturing method The ultra-high-strength cold-rolled steel sheet of the present invention is, for example, hot-rolled a steel slab having the above-described component composition, and then cold-rolled into a steel sheet. After heating to a temperature of 900 ° C, quench from a temperature of 550-800 ° C at a cooling rate of 500 ° C / second or more, reheat at a temperature of 150-500 ° C for 100-1400 seconds, and then at a pH of 0-4 After pickling treatment at a temperature of 10 to 100 ° C. for 5 to 150 seconds, an alkali treatment is carried out at a pH of 10 to 14 at a temperature of 10 to 100 ° C. for 2 to 50 seconds, and tempered at an elongation rate of 0.2 to 1.5%. It can be manufactured by rolling.

  The steel slab is manufactured by continuous casting or ingot-making and split rolling, and the manufactured slab is cooled and reheated or hot-rolled as it is to form a hot-rolled sheet. The hot-rolled sheet is cooled and wound, pickled, and cold-rolled to obtain a cold-rolled sheet having a desired thickness. At this time, the conditions from hot rolling to cold rolling are not particularly limited, but the finishing temperature in hot rolling is 850 ° C. or higher in order to make the structure of the hot rolled sheet uniform and improve workability such as bendability. The coiling temperature after hot rolling is preferably 450 ° C to 650 ° C in order to reduce cold deformation resistance and improve cold rolling properties, and the cold rolling rate promotes recrystallization of the ferrite phase. In order to improve the ductility, 30% or more is desirable. Next, the cold-rolled sheet after cold rolling is annealed in a continuous annealing furnace, but the continuous annealing conditions shown below are extremely important for achieving the microstructure of the steel sheet and the Si concentration of the steel sheet surface necessary for the present invention. is important.

  4-1) Annealing temperature: When the annealing temperature is lower than 700 ° C., a band-like non-uniform structure is formed from the structure expanded by cold rolling, and stretch flangeability and bendability deteriorate. Further, since there is almost no austenite phase at the time of heating, a martensite phase is not formed, and high strength and high ductility cannot be achieved. On the other hand, when the annealing temperature is higher than 900 ° C., the crystal grains are excessively coarsened, the amount of ferrite phase formed is also reduced, and ductility such as stretch flangeability is lowered. Accordingly, the annealing temperature is 700 to 900 ° C.

  4-2) Rapid cooling start temperature: In order to increase the ductility and strength of the steel sheet after heating at the above annealing temperature, after cooling, the steel sheet is rapidly cooled from the desired rapid cooling start temperature, and the ferrite phase and martensite A phase complex is formed. When the quenching start temperature is less than 550 ° C., the volume ratio of the martensite phase is small, and a TS of 780 MPa or more cannot be obtained. On the other hand, when the quenching start temperature exceeds 800 ° C., the volume ratio of the martensite phase increases, leading to a decrease in ductility. Therefore, the rapid cooling start temperature is 550 to 800 ° C.

  4-3) Cooling rate during quenching: In the present invention, since the ferrite stabilizing element Si is added, if the cooling rate is lower than 500 ° C., the ferrite phase is excessively formed, and the volume ratio of the martensite phase is reduced. Decreases and TS above 780MPa cannot be obtained. Therefore, the cooling rate at the time of quenching is 500 ° C./second or more, preferably 1000 ° C./second or more. This cooling rate is an average cooling rate from the cooling start temperature to 100 ° C. The cooling is preferably water cooling, but it is also possible to perform cooling by combining using natural cooling, gas cooling, mist cooling, roll cooling, or the like.

  4-4) Reheating conditions: When the reheating temperature is lower than 150 ° C, the martensite phase is not tempered sufficiently and the TS-El balance is lowered. In addition, since the hard phase increases, the hardness difference from the ferrite phase also increases, and stretch flangeability and bendability deteriorate. On the other hand, when the reheating temperature exceeds 500 ° C., tempering proceeds excessively rapidly, the martensite phase decomposes into a ferrite phase and carbide, and softens, so a TS of 780 MPa or more cannot be obtained. Therefore, the reheating temperature is 150 to 500 ° C, preferably 200 to 400 ° C. If the reheating time is less than 100 seconds, the tempering of the martensite phase becomes insufficient, the strength becomes excessively high, and the TS-El balance and stretch flangeability deteriorate. On the other hand, when the reheating time exceeds 1400 seconds, not only does the effect tend to saturate, but tempering proceeds excessively, and a TS of 780 MPa or more cannot be obtained. Therefore, the reheating time is 100 to 1400 seconds, preferably 300 to 1000 seconds. The cooling to room temperature after reheating can be performed by air cooling, furnace cooling, gas cooling, mist cooling, water cooling, or the like.

  4-5) Conditions for pickling treatment: The purpose of pickling treatment is to remove the Fe-based oxide so-called scale formed on the surface layer of the steel plate to clean the steel plate surface, and to colloid Ti during surface conditioning at the pre-painting treatment stage. It is to remove the complex oxide of Si and Mn that inhibits adsorption of charged phosphate fine particles or inhibits nucleation of chemical crystals when immersed in chemical solution for chemical conversion treatment. For this purpose, it is necessary to carry out pickling treatment at a pH of 4 or less and a temperature of 10 ° C. or more for 5 seconds or more. On the other hand, when the treatment temperature exceeds 100 ° C or the treatment time exceeds 150 seconds, the effect is not only saturated, but also the perforation and the surface of the steel sheet become uneven, resulting in reduced corrosion resistance after chemical electrodeposition coating. . The closer the pH is to 0, the faster the reaction takes place. Accordingly, the pickling treatment is performed at a pH of 0 to 4 and at a temperature of 10 to 100 ° C. for 5 to 150 seconds. The pickling is preferably performed with hydrochloric acid, sulfuric acid, nitric hydrochloric acid or the like.

4-6) Alkali treatment conditions: The alkali treatment is carried out to remove the hardly acid-soluble SiO ₂ remaining after pickling. According to the pH-potential diagram, SiO ₂ is inactive in the pH range of 0-4 and is not removed by pickling. Therefore, in the conventional technique, Ni plating or the like is applied on SiO ₂ to ensure the corrosion resistance after chemical conversion coating. The present inventors have found that it is possible to completely remove the hardly acid-soluble SiO ₂ produced during hot rolling or annealing by performing an alkali treatment after pickling and remaining after pickling. For this purpose, it is necessary to perform an alkali treatment at a pH of 10 or higher and a temperature of 10 ° C. or higher for 2 seconds or longer. On the other hand, when the processing temperature exceeds 100 ° C. or the processing time exceeds 50 seconds, the effect is saturated. The closer the pH is to 14, the faster the reaction takes place. Accordingly, the alkali treatment is performed at a pH of 10 to 14 and at a temperature of 10 to 100 ° C. for 2 to 50 seconds. The alkaline chemical liquid type is not limited, but caustic soda, condensed phosphate, and the like are preferable.

  4-7) Elongation ratio of temper rolling: In general, temper rolling is performed for the purpose of removing yield elongation, but it also affects the chemical conversion processability. If the elongation rate of temper rolling is less than 0.2%, the number of lattice defects that become one of the sites of adsorption of Ti colloid and charged phosphate particles during surface preparation, which is the pretreatment stage, is small, and the subsequent chemical conversion treatment process In this case, it becomes difficult to produce fine and dense chemical crystals. Moreover, unevenness | corrugation remains on the steel plate surface, and it becomes difficult to produce a chemical conversion crystal uniformly. On the other hand, when the elongation rate is 1.5% or more, such effects tend to saturate and a decrease in El is observed. Therefore, the elongation rate of temper rolling is set to 0.2 to 1.5%.

  Slabs of steels A to K having the components shown in Table 1 were heated to 1250 ° C, hot-rolled at a finishing temperature of 880 ° C, wound at a winding temperature of 620 ° C, and with a cold rolling reduction ratio of 50%. Cold rolling was performed to obtain a steel plate having a thickness of 1.8 mm. Subsequently, continuous annealing, pickling treatment, alkali treatment, and temper rolling were performed on the cold-rolled steel sheet under the conditions shown in Table 2 to produce cold-rolled steel sheets 1-18. In addition to investigating the microstructure and [Si (0) / Si (0.1)] by the above methods, the following methods are also used to examine tensile properties, stretch flangeability, chemical conversion properties, and corrosion resistance after chemical electrodeposition coating. Went. Further, the ferrite average crystal grain size was measured according to JIS Z 0522.

  Tensile properties: Using a JIS Z 2201 No. 5 test piece with the rolling direction of the steel sheet and the 90 ° direction as the longitudinal direction, a tensile test based on JIS Z 2241 was performed to measure TS and El. Here, when TS × El ≧ 17500 MPa ·%, it was determined that the TS-El balance was excellent.

  Stretch flangeability: Based on the Japan Iron and Steel Federation standard JFS T1001, punch a 10mm diameter hole into a steel plate, expand this hole using a 60 ° conical punch, and determine the hole diameter d (mm) when the crack penetrates the plate thickness. The hole expansion ratio λ = [(d-10) / 10] × 100 was determined. A similar test was performed three times, and the average value was evaluated. Here, when TS × λ ≧ 70000 MPa ·%, it was determined that the TS-λ balance was excellent.

Chemical conversion treatment and corrosion resistance after chemical conversion electrodeposition: Using a surface conditioning chemical (5N-10) and chemical conversion solution (SD2800) manufactured by Nippon Paint Co., Ltd., test specimens of 75 mm x 150 mm with zinc phosphate After chemical conversion treatment, apply electrodeposition coating (paint: V-50 black) with a thickness of 25μm, make two cuts with a length of 100mm with a cutter knife, immerse in 50% 5% NaCl solution for 240 hours, The adhesive tape was cut and pasted on the cut, and the peel width of the coating film was measured. When the chemical conversion grain size was 2 to 10 μm, the coating weight was 1.8 to 2.6 g / m ² , and the maximum peel width was 2.5 mm or less, it was determined that the chemical conversion treatment property and the corrosion resistance after electrodeposition coating were good. Here, the film weight was obtained by dissolving the chemical film after the chemical conversion treatment and measuring the weight before and after dissolution. The chemical crystal grain size was measured by a cutting method after observing the structure at 1000 times with SEM.

  The results are shown in Table 3. Steel plates 1, 2, 10 to 17 as examples of the present invention are TS × El ≧ 17500 MPa ·%, TS × λ ≧ 70000 MPa ·%, maximum peel width is 2.5 mm or less, high ductility, chemical conversion treatment property It can be seen that this is a super-high-strength cold-rolled steel sheet having an excellent TS of 780 to 1180 MPa. On the other hand, the steel plates 3, 4, 5, 6, 7, 8, 9 and 18 as comparative examples have [Si (0) / Si (0.1)] and their standard deviations outside the scope of the present invention, and chemical conversion. Inferior in processability and corrosion resistance after painting.

  The ultra-high-strength cold-rolled steel sheet of the present invention is suitable not only for automobile parts but also for members that require severe bending and chemical conversion properties in the fields of building materials and home appliances.

Claims (4)

  In mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.0%, Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 to 0.005%, Al: 0.005 to 0.08%, N: 0.001 to 0.01%, and the balance consists of Fe and inevitable impurities, has a composite structure consisting of a ferrite phase and a tempered martensite phase, the volume ratio of the tempered martensite phase is 20 to 70%, and the steel sheet surface The ratio [Si (0) / Si (0.1)] of the Si concentration Si (0) and the Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface is 1.0 to 1.6, and the standard deviation of the ratio An ultra-high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, which has a high ductility and excellent chemical conversion properties, characterized in that is a 0.50 or less.   Furthermore, it contains at least one element selected from Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, and V: 0.001 to 0.2% by mass%. An ultra-high strength cold-rolled steel sheet with a tensile strength of 780 MPa or more that has excellent ductility and excellent chemical conversion.   Further, at least one kind selected from Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001 to 0.005% by mass% 3. The ultra-high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, which has high ductility and excellent chemical conversion properties, according to claim 1 or 2, characterized by containing an element.   Further, by mass%, Ca: 0.0001 to 0.005% is contained, The high ductility according to any one of claims 1 to 3, and having a tensile strength of 780 MPa or more that is excellent in chemical conversion treatment property High strength cold rolled steel sheet.