JP4725376B2 - High-strength cold-rolled steel sheet excellent in formability, chemical conversion treatment and post-coating corrosion resistance, and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in formability and excellent in post-coating corrosion resistance evaluated by chemical conversion treatment, salt warm water immersion test and combined cycle corrosion test, and a method for producing the same.

  In recent years, improvement in fuel efficiency of automobiles has been demanded from the viewpoint of conservation of the global environment. There is also a demand for improving the safety of automobiles from the viewpoint of occupant protection in the event of a collision. For this reason, it is necessary to reduce the weight and the strength of the automobile body, and recently, the thickness and the strength of the automobile parts have been actively promoted.

  On the other hand, since many automobile parts are manufactured by press-forming steel sheets, the steel sheets are strongly required to have high press formability, particularly high strength and high ductility, that is, excellent strength-ductility balance. A high strength cold-rolled steel sheet having high ductility often contains a large amount of Si as a strengthening element, and an oxide of Si is formed on the steel sheet surface during annealing. Therefore, these high-strength cold-rolled steel sheets with a high Si content are inferior in chemical conversion treatment, and are exposed to harsh environments such as a salt-water immersion test and a combined cycle corrosion test that repeats wet-drying after electrodeposition coating. And compared with a normal steel plate, a coating film peels and the corrosion resistance after a coating tends to fall.

Therefore, in Patent Document 1, for example, the slab is heated at a temperature of 1200 ° C. or higher during hot rolling, descaled at a high pressure, and the surface of the hot rolled steel sheet is ground with a nylon brush containing abrasive grains before pickling. A high-strength cold-rolled steel sheet is proposed in which the Si concentration on the steel sheet surface is lowered by dipping twice in a hydrochloric acid bath and pickling. Patent Document 2 proposes a high-strength cold-rolled steel sheet in which the line width of a linear oxide containing Si observed at 1 to 10 μm from the steel sheet surface is 300 nm or less to improve corrosion resistance.
JP 2004-204350 A JP 2004-244698 A

  However, in the high-strength cold-rolled steel sheet described in Patent Document 1, even if Si oxide on the steel sheet surface is reduced before cold rolling, Si oxide is formed on the steel sheet surface by subsequent annealing, and chemical conversion treatment and Corrosion resistance after painting cannot be improved sufficiently. In the high-strength cold-rolled steel sheet described in Patent Document 2, corrosion resistance does not become a problem in environments such as chemical conversion treatment and the salt spray test of JIS Z 2371, but the salt warm water immersion test and the combined cycle corrosion test In such a severe environment, the corrosion resistance after painting is not sufficient. Thus, reducing the amount of Si on the steel sheet surface does not ensure sufficient chemical conversion properties and post-coating corrosion resistance, resulting in a high-strength cold-rolled steel plate with excellent formability, chemical conversion properties, and post-coating corrosion resistance. I can't.

  The present invention has a high tensile strength TS of 590 MPa or higher, TS × El (El: Elongation) of 23000 MPa ·% or more, excellent moldability, chemical conversion treatment, salt warm water immersion test, and combined cycle corrosion test. An object of the present invention is to provide a high-strength cold-rolled steel sheet excellent in corrosion resistance after coating in a severe environment and a method for producing the same.

The purpose is mass%, C: 0.05-0.25%, Si: 0.8-3.0%, Mn: 0.5-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.06% or less, balance Fe and It consists of inevitable impurities, and has a structure containing, by volume, 30% or more of ferrite, 2% or more of retained austenite, and 3 to 50% in total of bainite and / or martensite, and the following formula (1) moldability, chemical conversion treatability and corrosion resistance after coating tHAT defined by the surface of the steel sheet Si content Cs (Si) is 2.5% or less, wherein the Zn of 10-100 mg / m ² is present on the surface of the steel sheet Can be achieved by a high-strength cold-rolled steel sheet with excellent resistance.
Cs (Si) = Cb (Si) × [Rs (Si / Fe) / Rb (Si / Fe)] (1)
Where Cb (Si) is the amount of Si in the steel, Rs (Si / Fe) is the GDS count integrated value ratio of Si and Fe from the steel sheet surface to a depth of 50 nm, and Rb (Si / Fe) is in the steel. Represents the GDS count ratio of Si and Fe.

  Further, the high-strength cold-rolled steel sheet of the present invention contains at least one element selected from Ti: 0.005-0.3%, Nb: 0.005-0.3%, and V: 0.005-0.3% by mass%. Can be made.

  Furthermore, the high-strength cold-rolled steel sheet of the present invention can contain Mo: 0.005 to 0.3% by mass.

  Furthermore, the high-strength cold-rolled steel sheet of the present invention can contain at least one element selected from Ca: 0.001 to 0.1% and REM: 0.001 to 0.1% by mass.

The high-strength cold-rolled steel sheet of the present invention is, for example, a steel slab having the above composition, heated to a temperature of 1170 ° C. or less, and then hot-rolled into a hot-rolled steel sheet, and then the hot-rolled steel sheet is 30 to 60% After cold rolling at a reduction rate of 700 ° C, heat to 700 ° C or higher and hold for 30s or more, then cool to 300 to 480 ° C with an average cooling rate of 10 ° C / s or more, and at that temperature 60 to 600s After holding, cool, and after pickling so that Cs (Si) of the above formula (1) is 2.5% or less, further plating with Zn so that 10 to 100 mg / m ² of Zn exists on the steel plate surface Can be produced by a method for producing a high-strength cold-rolled steel sheet excellent in formability, chemical conversion property and corrosion resistance after coating.

  By this invention, TS × El is 23000MPa ・% or more, excellent moldability, and high strength cooling excellent in chemical conversion treatment and post-coating corrosion resistance in harsh environments such as salt warm water immersion test and combined cycle corrosion test. It became possible to manufacture rolled steel sheets.

  Details of the present invention will be described below. In the present invention, “%” in the component composition and concentration means “% by mass” unless otherwise specified.

1) ingredients
C: C is an essential element in high strength steel, further TRIP (transformation induced plasticity: Tr ansformation I nduced P lasticity) residual austenite has an effect, bainite, is an essential element for the generation of martensite. However, if the C content is less than 0.05%, the desired high strength cannot be obtained, and if it exceeds 0.25%, weldability is deteriorated. For this reason, the amount of C is limited to 0.05 to 0.25%, preferably 0.10 to 0.20%.

  Si: Si has the effect of strengthening steel by solid solution strengthening, stabilizing austenite, and promoting the formation of residual austenite phase. Such an effect is recognized when the Si content is 0.8% or more, but when it exceeds 3.0%, the ductility is lowered. For this reason, Si amount is limited to 0.8 to 3.0%, preferably 1.0 to 2.5%.

  Mn: Mn strengthens the steel by solid solution strengthening, improves the hardenability of the steel, and promotes the formation of retained austenite, bainite, and martensite. Such an effect is recognized when the Mn content is 0.5% or more, but when it exceeds 3.0%, it is saturated and the cost is increased. For this reason, the amount of Mn is limited to 0.5 to 3.0%, preferably 1.0 to 2.0%.

  P: P is a solid solution strengthening element and is usually an element effective for obtaining a high-strength steel sheet. Therefore, P is preferably contained in an amount of 0.005% or more, but if it exceeds 0.05%, spot weldability is lowered. For this reason, the amount of P is limited to 0.05% or less, preferably 0.02% or less.

  S: S precipitates as MnS in the steel and lowers the stretch flangeability of the steel sheet. For this reason, the amount of S is limited to 0.01% or less, preferably 0.005% or less, more preferably 0.003% or less.

  Al: Al is an element added as a deoxidizer in the steelmaking stage, and is an effective element for separating nonmetallic inclusions that reduce stretch flangeability as slag. However, if it exceeds 0.06%, cost increases. For this reason, the Al content is limited to 0.06% or less, but is preferably 0.02 to 0.06%.

  The balance is Fe and inevitable impurities, but for the following reasons, at least one element selected from Ti: 0.005-0.3%, Nb: 0.005-0.3%, V: 0.005-0.3%, Ca: At least one selected from 0.001 to 0.1% and REM: 0.001 to 0.1%, Mo: 0.005 to 0.3% can be appropriately combined or individually contained.

  Ti, Nb, V: Ti, Nb, V forms carbides and nitrides, suppresses the growth of ferrite in the heating stage during annealing, refines the structure, and remarkably improves formability, especially stretch flangeability Let Therefore, it is effective to contain at least one of these elements. At this time, each element needs to be contained by 0.005% or more. However, if it exceeds 0.3%, the yield strength YS increases due to precipitation strengthening, the formability decreases, and the retained austenite for expressing the TRIP effect decreases. Therefore, the amount of these elements is limited to 0.005 to 0.3%, preferably 0.01 to 0.2%, respectively.

  Mo: Mo is an element that has the effect of improving the hardenability of steel and promoting the formation of bainite and martensite. Such an action is recognized when the Mo content is 0.005% or more, but when it exceeds 0.3%, the effect is saturated and the cost is increased. For this reason, the amount of Mo is limited to 0.005 to 0.3%, preferably 0.01 to 0.2%.

  Ca, REM: Ca and REM have the effect of controlling the form of sulfide inclusions and improving the stretch flangeability of the steel sheet. Such an effect can be obtained by including at least one of these elements. At this time, each element needs to be contained by 0.001% or more. However, the effect is saturated when it exceeds 0.1%. Therefore, the amount of these elements is limited to 0.001 to 0.1%, preferably 0.001 to 0.05%, respectively.

2) Microstructure In order to obtain excellent formability, specifically, TS x El of 23000 MPa ·% or more, in addition to the above components, the volume ratio of ferrite is 30% or more and residual austenite is added for the following reasons. It is necessary to make it a structure containing 2% or more and 3 to 50% in total of bainite and / or martensite.

  Ferrite: Ferrite is a soft phase containing no iron carbide, has high deformability, and improves the ductility of the steel sheet. However, if the ferrite volume fraction is less than 30%, a significant ductility improvement effect cannot be expected. Therefore, the ferrite volume fraction is limited to 30% or more, preferably 50% or more.

  Residual austenite: Residual austenite has a so-called TRIP effect that causes strain-induced transformation to martensite during forming, disperses locally applied strain widely, and improves the ductility of the steel sheet. However, if the volume fraction of retained austenite is less than 2%, a significant improvement in ductility cannot be expected. Therefore, the volume fraction of retained austenite is limited to 2% or more, preferably 5% or more.

  Bainite and / or martensite: Both bainite and martensite are hard and have the effect of increasing the steel sheet strength by strengthening the structure. Moreover, since it involves the generation of movable dislocations during transformation, it also has the effect of reducing the yield ratio of the steel sheet. Such an effect is recognized by setting the volume fraction of bainite and / or martensite to 3% or more in total, but if it exceeds 50%, the steel sheet strength becomes too high and the ductility is lowered. The upper limit. For this reason, the volume fraction of bainite and / or martensite is limited to 3 to 50% in total, preferably 10 to 30%.

3) Si content on steel sheet surface Cs (Si)
After electrodeposition coating, scratches that reach the base steel plate with a cutter and soak in salt warm water (5% saline, 60 ° C) for 240 hours, or a combined cycle corrosion test that repeats dry-wetting When Cs (Si) defined by the above formula (1) exceeds 2.5% when exposed to an unfavorable environment, film peeling occurs greatly from the cut part, and the corrosion resistance after coating is significantly reduced. The inventors have found. This decrease in post-coating corrosion resistance is thought to be due to the fact that Si on the surface of the steel sheet inhibits the etching of the steel sheet and inhibits the formation of a healthy chemical conversion film in the zinc phosphate treatment that is performed as a base treatment for electrodeposition coating. It is done. Therefore, in order to improve the corrosion resistance after painting, Cs (Si) needs to be 2.5% or less, preferably 2.2% or less.

  The reason why Rs (Si / Fe) in the above formula (1) was obtained from the GDS (Glow Discharge Spectroscopy) count integrated value of Si and Fe from the steel sheet surface to a depth of 50 nm is that the steel sheet by etching during chemical conversion treatment was used. This is because the dissolution is about 50 nm deep, and the amount of Si existing from the steel sheet surface to a depth of 50 nm greatly affects the corrosion resistance after coating.

  Here, in order to obtain the GDS count integrated value of Si and Fe from the steel sheet surface to a depth of 50 nm, separately obtain the relationship between the sputtering depth by GDS and the sputtering time, and the GDS count integrated value up to the sputtering time corresponding to 50 nm. You can ask for. Rb (Si / Fe) is the GDS count ratio of Si and Fe in the steel. The GDS count of Si and Fe is almost constant with respect to the sputtering time, and the effect of surface enrichment is no longer observed. The GDS count values of Si and Fe at Cb (Si) is the Si content (% by mass) in the steel.

4) Amount of Zn on the surface of the steel sheet A phosphate film is formed on the surface of the steel sheet by chemical conversion treatment. At this time, the zinc phosphate crystal nuclei are chemically treated to form zinc phosphate crystals densely. It is important to produce a large number in the initial stage. The formation of zinc phosphate crystal nuclei is thought to have started from cathode sites such as cementite and sulfide existing on the steel sheet surface. On the other hand, the Si oxide formed on the surface of the steel sheet during annealing hinders the etching of the steel sheet and degrades the chemical conversion processability. Therefore, it is necessary to remove it by pickling in advance. The cathode sites such as cementite and sulfide are dissolved and removed at the same time, and the number of zinc phosphate crystal nuclei is reduced. For this reason, the film crystal becomes coarse, the scale is generated, and a good chemical conversion treatment property cannot be obtained only by the pickling treatment, and the corrosion resistance after coating is lowered.

The present inventors increase the number of zinc phosphate crystal nuclei by making Zn in the surface of the steel sheet 10 to 100 mg / m ² , thereby achieving refinement and densification of zinc phosphate crystals, and phosphates. It was found that the corrosion resistance after coating can be improved by improving the processability. At this time, when the amount of Zn present on the steel sheet surface is less than 10 mg / m ² , there is almost no effect of improving the phosphate treatment property, and when it exceeds 100 mg / m ² , the X-ray diffraction intensity H of the hopite (020) plane is The P ratio [= P / (H + P)] of the zinc phosphate coating, which is an index of chemical conversion treatment calculated from the X-ray diffraction intensity P of the phosphophyllite (100) surface, decreases, and the corrosion resistance after coating is reduced. descend.

Although the details of the mechanism by which Zn is present on the steel sheet surface improves chemical conversion properties are unclear, it is considered as follows. In the zinc phosphate coating, when the steel plate surface is dissolved in the acidic chemical conversion treatment solution, the pH of the steel plate surface rises, and Zn ions and phosphate ions in the treatment solution react and precipitate. For this reason, it is considered that the greater the amount of Zn ions present on the surface of the steel sheet, the more advantageous is the precipitation of the zinc phosphate film.However, increasing the amount of Zn ions causes problems such as increased sludge generation. There is a limit to increasing the amount of Zn ions in the treatment liquid from the current amount. However, if Zn is present on the steel sheet surface, Zn is a base metal electrochemically than Fe, so Zn is preferentially dissolved, resulting in an increase in the amount of Zn ions on the steel sheet surface, Precipitation of zinc phosphate crystal nuclei easily occurs, the number of zinc phosphate crystal nuclei increases, and chemical conversion treatment performance improves. On the other hand, when the amount of Zn on the steel sheet surface exceeds 100 mg / m ² , only dissolution of Zn occurs in the chemical conversion solution, and Fe does not dissolve, so that only hopite is formed and phosphophyllite is not formed. . As a result, the P ratio decreases and the corrosion resistance after coating decreases.

5) Manufacturing method An example of the manufacturing method of the high-strength cold-rolled steel sheet according to the present invention has been described above. The reasons for limiting each condition in the above manufacturing method example will be described below. In addition, the manufacturing method of this invention steel plate is not limited to the said example.

  Slab heating temperature: When the slab heating temperature exceeds 1170 ° C, Si concentrates on the surface, forming a scale that is difficult to remove by descaling during hot rolling, pickling after hot rolling, etc., which is cold rolling -Remains after annealing and chemical conversion processability deteriorates. For this reason, the heating temperature of a slab shall be 1170 degrees C or less.

  Hot rolling conditions other than the slab heating temperature are not particularly limited, but are preferably the following conditions.

Finishing temperature: When the finishing temperature of hot rolling is less than the Ar ₃ transformation point, rolling is performed in a two-phase region of austenite + ferrite, and formability tends to deteriorate. On the other hand, if the finishing temperature exceeds (Ar ₃ transformation point +100) ° C., the structure of the steel becomes coarse and the formability and surface properties are likely to deteriorate. For this reason, the finishing temperature is preferably Ar ₃ transformation point to (Ar ₃ transformation point + 100) ° C.

  Cooling rate after hot rolling: The hot-rolled steel sheet is cooled and austenite is transformed into ferrite. At this time, if the cooling rate is slow, the ferrite produced by the transformation becomes coarse and may adversely affect the formability. Therefore, the average cooling rate is preferably 20 ° C./s or more. Moreover, it is preferable to cool to 400-650 degreeC and to wind up a steel plate after that. At this time, if the temperature is lower than 400 ° C, the strength of the hot-rolled steel sheet becomes too high, the rolling load in the subsequent cold rolling is remarkably increased, and problems such as difficulty in cold rolling are likely to occur. The lower limit of the temperature is 400 ° C, and if it exceeds 650 ° C, grain boundary oxidation in the hot-rolled steel sheet becomes significant, and problems such as deterioration of surface properties and deterioration of fatigue characteristics are likely to occur. The upper limit of the temperature is preferably 650 ° C.

  Cold rolling reduction: The hot-rolled steel sheet is cold-rolled to the desired thickness, but if the rolling reduction is less than 30%, the introduced strain is insufficient and the post-annealing properties are inferior. If it exceeds, the properties will not be affected, but rather the rolling load of the cold rolling mill will increase. For this reason, the rolling reduction of cold rolling is 30 to 60%. In addition, in order to remove the scale generated on the surface of the hot-rolled steel sheet, it is preferable that the hot-rolled steel sheet be pickled according to a conventional method before cold rolling.

Annealing temperature and holding time: It is necessary to heat the steel sheet after cold rolling to the austenite + ferrite two-phase region by continuous annealing, that is, to a temperature of 700 ° C or higher, and then to obtain residual austenite by cooling. is there. In addition, when it heats exceeding 850 degreeC, since a ferrite particle size becomes coarse and a moldability deteriorates, it is preferable that an annealing temperature shall be 850 degrees C or less.
In addition, when the two-phase region is cooled immediately after heating, retained austenite cannot be obtained. Therefore, it is necessary to maintain the annealing temperature for 30 seconds or more. However, if the holding time is long, the ferrite grain size becomes coarse and the moldability may be deteriorated. Therefore, the holding time is preferably within 300 s.

  Cooling rate after annealing: The steel plate after annealing is cooled at an average cooling rate of 10 ° C / s or higher, preferably 20 ° C / s or higher, from the annealing temperature to the quenching stop temperature described below in order to generate retained austenite. There is a need.

  Quenching stop temperature and holding time: When the quenching stop temperature is less than 300 ° C, all austenite transforms into martensite, and when it exceeds 480 ° C, most of the austenite transforms into pearlite or bainite, resulting in no residual austenite and a TRIP effect. You can't expect. Therefore, the rapid cooling stop temperature is 300 to 480 ° C, preferably 350 to 450 ° C. The retention time is less than 60 s, and when the next cooling starts, most of the retained austenite transforms to martensite, and when it exceeds 600 s, the bainite transformation is generated and the retained austenite is reduced, so the TRIP effect can be expected. Disappear. Therefore, the holding time at the rapid cooling stop temperature is 60 to 600 s, preferably 60 to 300 s.

  Pickling: The steel sheet cooled in this way is pickled with hydrochloric acid, sulfuric acid, nitric acid + hydrochloric acid, etc. in order to remove the Si oxide formed on the steel sheet surface during annealing and make the above Cs (Si) 2.5% or less. There is a need. The type of acid, pickling temperature, pickling time and the like are not particularly limited. For example, it is preferable to use 10% hydrochloric acid or 1% hydrochloric acid + 25% nitric acid and immerse at 30 to 70 ° C. for 5 to 20 seconds.

Zn plating: In order to make 10-100 mg / m < ² > Zn exist on the steel plate surface after pickling, it is necessary to carry out Zn plating. The method of Zn plating is not particularly limited. For example, the steel plate after pickling is used as a cathode in a plating bath at a temperature of 60 ° C. in which a ZnSO ₄ .7H ₂ O aqueous solution having a concentration of 200 g / 1 is adjusted to pH 2.0 with sulfuric acid. Thus, by electroplating for ^{2 s} at a current density of 5 A / dm ² , about 30 mg / m ² of Zn can be present on the steel sheet surface.

Steels a to q having the compositions shown in Table 1 were melted to form slabs. These slabs were hot-rolled under the hot rolling conditions shown in Table 2 to obtain hot rolled plates having a thickness of 3 to 4 mm. These hot-rolled sheets were cold-rolled under the cold rolling conditions shown in Table 2 to obtain cold-rolled sheets having a thickness of 1.8 mm. These cold-rolled sheets are annealed under the annealing conditions shown in Table 2, then pickled under the pickling conditions shown in Table 2, and Zn-plated in a plating bath of ZnSO4 · 7H ₂ O at 200g / liter, washed and dried Thereafter, temper rolling of 0.7% was performed. Here, the Zn plating was performed by changing the plating conditions as shown in Table 2 to adjust the Zn content on the steel sheet surface. Then, the Zn amount, Cs (Si), structure, mechanical properties, chemical conversion property, and post-coating corrosion resistance of the obtained steel plate surface were investigated by the following methods.

(1) Zn content on steel plate surface A Zn count of fluorescent X-rays and a calibration curve of Zn content were prepared in advance and calculated from the Zn count value of fluorescent X-rays on the steel plate.

(2) Cs (Si)
As described above, Rs (Si / Fe) and Rb (Si / Fe) were measured by GDS analysis and calculated using the above formula (1).

(3) Microstructure The cross section in the rolling direction of the steel sheet was examined by observing it with an optical microscope or a scanning electron microscope (SEM). Using a cross-sectional structure photograph at a magnification of 1000 times, the occupied area ratios of ferrite and bainite / martensite existing in a 100 mm square area on a photograph arbitrarily set by image analysis were determined and used as the respective volume ratios. The amount of retained austenite was determined by measuring the X-ray diffraction intensity after polishing to 1/4 of the thickness direction of the steel sheet. MoKα rays were used as incident X-rays, and the X-ray diffraction intensity ratio of each surface of retained austenite {111}, {200}, {220}, {311} was determined, and the volume fraction of retained austenite was determined from these. .

(4) Mechanical properties YS, TS, and El were measured in accordance with the method specified in JIS Z 2241 using No. 5 test piece specified in JIS Z 2201 collected in a direction perpendicular to the rolling direction. Further, the yield ratio YR (= YS / TS) and the strength-ductility balance TS × El were calculated.

(5) Chemical conversion treatment, post-coating corrosion resistance Chemical conversion treatment uses a degreasing agent manufactured by Nippon Paint Co., Ltd .; Surf Cleaner EC90, surface conditioner; Surffine 5N-10, chemical conversion treatment agent; Surfdyne SD2800. Concentration conditions were performed under conditions worse than the standard conditions. As an example of standard conditions, the degreasing process has a concentration of 16 g / l, a processing temperature of 42 to 44 ° C., a processing time of 120 s, a spray degreasing, and a surface adjustment process of 1.5 to 2.5 points of total alkalinity, and a temperature of 20 to 25 ° C. The treatment time was 30 s, the immersion, and the chemical conversion treatment step were performed with a total acidity of 21 to 24 points, a free acidity of 0.7 to 0.9 points, an accelerator concentration of 2.8 to 3.5 points, a treatment temperature of 44 ° C., and a treatment time of 120 s. As an inferior condition, the treatment temperature in the chemical conversion treatment step was lowered to 38 ° C. Then, electrodeposition coating was performed using an electrodeposition paint; V-50 manufactured by Nippon Paint. The amount of chemical conversion coating was ^{2 to} 2.5 g / m ² , and electrodeposition coating was aimed at a film thickness of 25 μm.

  Evaluation of chemical conversion property was performed by calculating | requiring P ratio of the steel plate which performed chemical conversion treatment on the said standard conditions. The P ratio was calculated from the P ratio = P / (H + P) by obtaining the X-ray diffraction intensity H of the hoplite (020) plane and the X-ray diffraction intensity P of the phosphophyllite (100) plane. The P ratio was 0.85 or higher, and chemical conversion treatment was good.

The corrosion resistance after coating was evaluated in three ways: a salt warm water immersion test, a salt spray test (SST), and a combined cycle corrosion test (CCT). Each condition is shown below.
Salt warm water immersion test: Samples that have been subjected to chemical conversion treatment and electrodeposition coating were provided with cut shears (length: 45 mm) with a cutter, immersed in a 5% NaCl solution at 60 ° C for 240 hours, washed with water, dried, and the cut collar After sticking the adhesive tape on, the tape was peeled off, and the maximum peeled width on the left and right sides of the cut collar was measured. If the maximum peel width is 5.0 mm or less, it can be said that the salt warm water adhesion is good.
Salt spray test (SST): Applying cross-cut wrinkles with a sample cutter with chemical conversion treatment and electrodeposition coating, using a 5% NaCl solution, spraying salt water for 1000h according to JIS Z 2371, The maximum width of the entire peel when the left and right cross cuts were taken when the cut collar was peeled was measured. SST is good when the maximum peel width is 4.0 mm or less.
Combined cycle corrosion test (CCT): Samples that have been subjected to chemical conversion treatment and electrodeposition coating are subjected to crosscut wrinkles with a cutter and sprayed with salt water (5% NaCl: 35 ° C to 98% RH) for 2 hours → dried (60 ° C- 30% RH) 2h → Wet (50 ° C-95% RH) 2h as one cycle After 90 cycles of repeated tests, rinsed with water, dried, and then peeled off the tape from the cut collar. It was measured. If the maximum peel width is 6.0 mm or less, it can be said that the combined cycle corrosion resistance is good.

  The results are shown in Table 3. In all of the invention examples satisfying the requirements of the present invention, TS has a high strength of 590 MPa or more, and TS × El has a very good strength-ductility balance of 23000 MPa ·% or more, and the P ratio is 0.90 or more. Excellent chemical conversion treatment properties are exhibited, and the maximum total peel width is small in any of the salt warm water immersion test, salt spray test, and combined cycle corrosion test, and extremely excellent corrosion resistance after coating.

Claims (5)

In mass%, C: 0.05-0.25%, Si: 0.8-3.0%, Mn: 0.5-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.06% or less, remaining Fe and inevitable impurities And having a structure containing, in volume ratio, 30% or more of ferrite, 2% or more of retained austenite, and 3 to 50% in total of bainite and / or martensite, and is defined by the following formula (1) Excellent in formability, chemical conversion treatment and post-coating corrosion resistance, characterized in that the Si content Cs (Si) on the steel sheet surface is 2.5% or less and 10-100 mg / m ² of Zn exists on the steel sheet surface. Strength cold-rolled steel sheet;
Cs (Si) = Cb (Si) × [Rs (Si / Fe) / Rb (Si / Fe)] (1)
Where Cb (Si) is the amount of Si in the steel, Rs (Si / Fe) is the GDS count integrated value ratio of Si and Fe from the steel sheet surface to a depth of 50 nm, and Rb (Si / Fe) is in the steel. Represents the GDS count ratio of Si and Fe.   In addition to the above composition, it further contains at least one element selected from Ti: 0.005-0.3%, Nb: 0.005-0.3%, and V: 0.005-0.3% by mass%. Item 5. A high-strength cold-rolled steel sheet excellent in formability, chemical conversion property and post-coating corrosion resistance.   In addition to the above composition, the composition further comprises Mo: 0.005 to 0.3% by mass%, and the high-strength cooling excellent in formability, chemical conversion treatment property and post-coating corrosion resistance according to claim 1 or 2. Rolled steel sheet.   The composition according to any one of claims 1 to 3, further comprising at least one element selected from Ca: 0.001 to 0.1% and REM: 0.001 to 0.1% by mass% in addition to the above composition. A high-strength cold-rolled steel sheet excellent in formability, chemical conversion property and post-coating corrosion resistance according to any one of the items. The steel slab having the composition according to any one of claims 1 to 4 is heated to a temperature of 1170 ° C or lower, and then hot-rolled to form a hot-rolled steel sheet, and then the hot-rolled steel sheet is 30 to 60 After cold rolling at a reduction rate of%, heated to a temperature of 700 ° C. or higher and held for 30 s or more, then cooled to a temperature of 300 to 480 ° C. at an average cooling rate of 10 ° C./s or more, and at that temperature 60 to 60 After holding for 600 s, it is cooled, pickled so that Cs (Si) of the above formula (1) is 2.5% or less, and further plated with Zn so that 10 to 100 mg / m ² of Zn exists on the steel plate surface. A method for producing a high-strength cold-rolled steel sheet excellent in formability, chemical conversion property and post-coating corrosion resistance.