JP4807088B2 - Cold-rolled steel sheet excellent in deep drawability and post-coating corrosion resistance and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet having excellent deep drawability and excellent post-coating corrosion resistance evaluated by a salt spray test and a method for producing the same.

  Conventionally, cold rolled steel sheets having a tensile strength of less than 350 MPa and high deep drawability have been used as exterior panels for automobiles. In order for a steel sheet to exhibit excellent deep drawability, it is required that it has a high r-value (Rankford value) of 1.2 or more as mechanical characteristics. As a cold-rolled steel sheet having such a high r value, generally, an ultra-low carbon steel containing a carbonitride-forming element is hot-rolled to produce carbonitride at the hot-rolling stage, so that the solid solution in the steel is dissolved. A cold-rolled steel sheet manufactured by performing cold rolling-recrystallization annealing with C and N reduced, so-called IF (Interstitial Free) steel is known. As a measure for improving the deep drawability in this IF steel, various methods have been proposed so far, for example, Patent Document 1 discloses cold rolling by precipitating precipitates coarsely in the hot rolling stage. A method of manufacturing a cold-rolled steel sheet is disclosed in which the ferrite grain growth is promoted in the subsequent annealing stage to improve the deep drawability.

  However, the ultra-low carbon IF steel as disclosed in Patent Document 1 is phosphoric acid treatability (chemical conversion treatability) as compared with the conventionally used low carbon steel sheet with a C content of 0.02 to 0.08%. It is known that when it is exposed to a severe environment such as a salt spray test after electrodeposition coating, excellent corrosion resistance after coating cannot be obtained. This is thought to be because cementite does not precipitate because the amount of C is small, and the cathode portion that is the site of formation of phosphate conversion crystal nuclei decreases.

  On the other hand, in recent years, there has been a rapid increase in the tendency to use high-strength steel sheets having a tensile strength of 350 to 600 MPa for the purpose of reducing the weight of automobile bodies and improving safety. For this reason, research on IF steel with high strength and a high r value equal to or higher than that of conventional cold-rolled steel sheets is being actively pursued. Such high-strength IF steels with excellent deep drawability contain solid solution strengthening elements such as Si, Mn, and P. Among them, Si increases strength while suppressing deterioration of workability of the steel sheet. Because it can be raised, it is often used. However, when a large amount of Si is added, an oxide of Si is easily formed on the steel sheet surface during annealing. For this reason, steel sheets with a high Si content have a problem that the corrosion resistance after coating tends to be lower than that of ordinary steel sheets when exposed to harsh environments such as a salt spray test after electrodeposition coating. .

  To date, technologies have been proposed to improve chemical conversion properties and post-coating corrosion resistance by reducing the amount of Si on the steel sheet surface for steel sheets containing more C than those contained in low-carbon steel sheets and steel sheets containing high concentrations of Si. ing. For example, in Patent Document 2, the slab is heated to 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, 9% A high-strength cold-rolled steel sheet that has been dipped twice in a hydrochloric acid bath and pickled to reduce the Si concentration on the steel sheet surface has been proposed. In Patent Document 3, the line width of a linear oxide containing Si observed in a region 1 to 10 μm deep from the steel sheet surface is set to 300 nm or less, and by reducing the Si amount on the steel sheet surface, JIS Z 2371 A high-strength cold-rolled steel sheet with improved corrosion resistance by a salt spray test has been proposed.

In addition, a technique has been proposed in which an S compound is attached to the surface of a steel plate to improve chemical conversion property and post-coating corrosion resistance. For example, Patent Document 4 proposes a method for producing an ultra-low carbon cold-rolled steel sheet having excellent phosphate treatment properties by attaching an S compound to the steel sheet surface before continuous annealing. In Patent Document 5, the surface contaminants that inhibit the formation of phosphate films such as graphite and Si are electrolytically removed, and the S compound is adsorbed on the surface of the steel sheet. Cold-rolled steel sheets and methods for producing the same have been proposed.
Japanese Patent No. 3428318 JP 2004-204350 A JP 2004-244698 A Japanese Patent Publication No. 3-68951 Japanese Patent Publication No. 61-4199

  However, in the high-strength cold-rolled steel sheet described in Patent Document 2, 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 the corrosion resistance after coating is reduced. Cannot improve. If the technique of the high-strength cold-rolled steel sheet described in Patent Document 3 is applied to IF steel with a small amount of C in order to improve deep drawability, sufficient post-coating corrosion resistance cannot be obtained. In the method for producing an ultra-low carbon cold-rolled steel sheet described in Patent Document 4 and the cold-rolled steel sheet described in Patent Document 5, a cold-rolled steel sheet having an r value of 1.2 or more and excellent in deep drawability is not necessarily obtained.

  An object of the present invention is to provide a cold-rolled steel sheet having an r value of 1.2 or more, excellent deep drawability, and excellent corrosion resistance after coating even in a severe environment such as a salt spray test, and a method for producing the same.

The above purpose is by mass%, C: 0.008% or less, Mn: 0.05 to 1.0%, Si: 0.2% or more and less than 0.8%, S: 0.005% or less, N: 0.01% or less, Al: 0.01 to 0.1% Further, it contains at least one element selected from Ti: 0.010 to 0.1%, Nb: 0.015 to 0.15%, the balance is made of Fe and unavoidable impurities, and the contents of Ti and Nb are as described above. The amount of Si Cs (Si) on the steel plate surface defined by the following formula (2) satisfying the formula (1) is 1.6% or less, and 0.1 to 100 mg / m ² of S compound in terms of S is present on the steel plate surface. It can be achieved by a cold-rolled steel sheet excellent in deep drawability and post-coating corrosion resistance characterized by being present.
Cs (Si) = Cb (Si) × [Rs (Si / Fe) / Rb (Si / Fe)] (2)
Here, Cb (Si) is the amount of Si in the steel, Rs (Si / Fe) is the GDS (Glow Discharge Spectroscopy) count integrated value ratio of Si and Fe from the steel sheet surface to a depth of 50 nm, and Rb (Si / Fe Fe) represents the GDS count ratio of Si and Fe in the steel. The reason why Rs (Si / Fe) was obtained from the GDS count integrated value of Si and Fe from the surface of the steel sheet to a depth of 50 nm is because the dissolution of the steel sheet by etching during the chemical conversion treatment is about 50 nm deep. This is because the amount of Si existing from the steel sheet surface to a depth of 50 nm greatly affects the corrosion resistance after coating.

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

Cold-rolled steel sheet of the present invention, for example, a hot-rolled steel sheet having the above composition is cold rolled at a reduction rate of 50% to 95%, after recrystallization annealing at annealing temperatures of 750 to 950 ° C., and pickling, Cold rolling excellent in deep drawability and post-coating corrosion resistance, characterized in that an aqueous solution containing an S compound is brought into contact with the steel sheet surface and 0.1 to 100 mg / m ² of S compound is present on the steel sheet surface in terms of S amount It can manufacture with the manufacturing method of a steel plate.

  According to the present invention, a cold-rolled steel sheet having an r value of 1.2 or more and excellent deep drawability and excellent corrosion resistance after coating can be produced even in a severe environment such as a salt spray test.

  Details of the present invention will be described below.

1) Component (“%” below represents “% by mass”)
C: 0.008% or less
The smaller the amount of C, the better the deep drawability. If it is 0.008% or less, an r value of 1.2 or more can be obtained. Therefore, the C content is 0.008% or less, preferably 0.005% or less.

Mn: 0.05-1.0%
Mn is an effective element for preventing red heat embrittlement due to S during hot rolling. In order to obtain this effect, the Mn content needs to be 0.05% or more. However, if the Mn content exceeds 1.0%, MnS precipitates during continuous casting, causing hot brittleness and causing slab cracking. Therefore, the Mn content is 0.05 to 1.0%, preferably 0.2 to 0.7%.

N: 0.01% or less
The smaller the amount of N, the better the deep drawability, and if it is 0.01% or less, an r value of 1.2 or more can be obtained. Therefore, the C content is 0.01% or less, preferably 0.005% or less.

S: 0.005% or less
The smaller the amount of S, the better the deep drawability. If it is 0.005% or less, an r value of 1.2 or more can be obtained. Therefore, the S content is 0.005% or less, preferably 0.003% or less.

Al: 0.01-0.1%
Al is added as a steel deoxidizer, and is an effective element for improving the cleanliness of steel. Further, since nitride is formed to reduce the solid solution N, it is an element effective for improving deep drawability and aging. In order to obtain such an effect, the Al content needs to be 0.01% or more. However, if it exceeds 0.1%, the effect is saturated and the cost is increased. Therefore, the Al content is 0.01 to 0.1%, preferably 0.02 to 0.07%.

At least one element selected from Ti: 0.010 to 0.1%, Nb: 0.015 to 0.15%
Ti and Nb have the effect of preferentially forming {111} -oriented grains that are advantageous for deep drawability by precipitating solute C or N in steel as carbonitrides. If the Ti content is less than 0.010% or the Nb content is less than 0.015%, the effect is poor.If the Ti content exceeds 0.1% or the Nb content exceeds 0.15%, solid solution Ti or Nb increases, and processing Worsen sex. Therefore, the Ti amount is 0.010 to 0.1%, preferably 0.05 to 0.08%, and the Nb amount is 0.015 to 0.15%, preferably 0.045 to 0.12%.

  In order to reliably obtain an r value of 1.2 or more, it is necessary to satisfy the above formula (1) while controlling the Ti and Nb amounts within the above ranges. In other words, if the sum of Ti * and Nb, which is effective for precipitating solid solution C, is not stoichiometrically greater than the amount of C, solid solution C will exist during recrystallization and deep drawability It is not possible to preferentially form {111} -oriented crystal grains advantageous to the above, and an r value of 1.2 or more cannot be obtained.

In the present invention, the following reasons, the Si Ru is contained less than 0.8% to 0.2%. Further, at least one element selected from Ca: 0.001 to 0.1% and REM: 0.001 to 0.1% may be contained.

Si: 0.2% or more and less than 0.8% In order to increase the tensile strength of the cold-rolled steel sheet of the present invention to 350 MPa or more, it is effective to contain Si content by 0.2% or more. However, if the Si content is 0.8% or more, , Surface properties and workability deteriorate. Therefore, in order to increase the strength, the Si content is preferably 0.2% or more and less than 0.8%, and more preferably 0.2% or more and less than 0.6%. When Si is contained in an amount of 0.2% or more and less than 0.8%, Cs (Si) defined by the above formula (2) is 1.6 to ensure excellent post-coating corrosion resistance even in a severe environment such as a salt spray test. % Or less, preferably 1.2%. When Cs (Si) exceeds 1.6%, in the zinc phosphate treatment that is performed as a base treatment for electrodeposition coating, Si on the surface of the steel sheet inhibits the etching of the steel sheet and prevents the formation of a healthy chemical conversion coating film. It is considered that the corrosion resistance after painting is not obtained.

  The reason why Rs (Si / Fe) in the above formula (2) was obtained from the GDS (Glow Discharge Spectroscopy) count integrated value of Si and Fe from the surface of the steel sheet to a depth of 50 nm is the reason for the steel sheet by etching during chemical conversion treatment. 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.

Ca: 0.001 to 0.1%, REM: at least one selected from 0.001 to 0.1%
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 is preferably contained in an amount of 0.001% or more. However, the effect is saturated when it exceeds 0.1%. Therefore, the amount of these elements is 0.001 to 0.1%, preferably 0.001 to 0.05%, respectively.

  The balance is Fe and inevitable impurities. In addition, if it is a range which does not affect the effect of this invention, it is satisfactory even if it contains a component other than the above.

2) Amount of S compound on steel plate surface A phosphate film is formed on the steel plate surface by chemical conversion treatment. At this time, in order to form zinc phosphate crystals densely, the zinc phosphate crystal nuclei are formed by chemical conversion. It is important to produce a large number in the initial stage of processing. 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. However, in the ultra-low carbon steel plate such as the steel of the present invention, the amount of C is small compared to the conventionally used low carbon steel plate of 0.02 to 0.08%, and cementite is not generated. There are few cathode parts which become a production site. In addition, in steel sheets to which Si is added, the Si oxide formed on the surface of the steel sheet during annealing inhibits the etching of the steel sheet and deteriorates the chemical conversion treatment property. By the pickling, the cathode part such as cementite and sulfide existing on the surface of the steel sheet is also dissolved and removed. Therefore, in any case, coarsening of the film crystal, generation of scale, etc. occur, and good chemical conversion property cannot be obtained by pickling or electrolytic removal, resulting in poor corrosion resistance after coating.

The inventors have made the zinc phosphate crystal finer and more dense by increasing the number of zinc phosphate crystal nuclei by allowing 0.1 to 100 mg / m ² of S compound in terms of S on the steel sheet surface. The present inventors have found that the corrosion resistance after coating can be improved by improving the phosphatability. At this time, when the amount of S compound present on the steel sheet surface is less than 0.1 mg / m ^{2 in} terms of S, there is almost no phosphate treatment improvement effect, and when it exceeds 100 mg / m ² , the improvement effect is saturated, rather the appearance Make it worse. The amount of S compound present on the steel sheet surface is preferably 0.1 to 50 mg / m ² in terms of S. Examples of the S compound to be present on the steel sheet surface include FeS and MnS.

4) Manufacturing method As described above, the cold-rolled steel sheet of the present invention is, for example, cold-rolled hot-rolled steel sheet having the above composition at a reduction rate of 50 to 95% and re-applied at an annealing temperature of 750 to 950 ° C. after recrystallization annealing and pickling can be produced by a method in which the presence of S compounds of 0.1-100 mg / m ² in the amount of S in terms touched let in the surface of the steel sheet in an aqueous solution to the steel sheet surface containing S compound. The reasons for limiting each manufacturing condition will be described below.

Cold rolling reduction: 50-95%
In order to obtain a high r value of 1.2 or more, it is necessary to perform cold rolling at a reduction rate of at least 50%. The r value improves as the rolling reduction increases, but deteriorates when the rolling reduction exceeds 95%. Therefore, the rolling reduction of cold rolling is 50 to 95%.

Annealing temperature: 750-950 ° C
The cold-rolled sheet after cold rolling is recrystallized and annealed to develop a {111} recrystallized texture and obtain a high r value. At this time, if the annealing temperature is less than 750 ° C., the recrystallization is not completed and the r value is low. On the other hand, if it exceeds 950 ° C., the austenite single phase region is annealed. The tissue is randomized and only low r values are obtained. Therefore, the annealing temperature is 750 to 950 ° C.

S compound treatment: The steel sheet after annealing is immersed in an aqueous solution of thiourea, thioglycolic acid, dimethyl sulfide, etc. in order to make 0.1 to 100 mg / m ² of S compound in S conversion on the steel sheet surface, spray, roll coater It is necessary to wash with water after making contact.

Note that, before shrink blunt after S compound treatment, for reliably below 1.6% of Cs to remove Si oxide formed on the surface of the steel sheet (Si) during annealing, hydrochloric, sulfuric, acid such as nitric acid + hydrochloric acid Need to wash. At this time, the type of acid, pickling temperature, pickling time and the like are not particularly limited, but for example, 10% hydrochloric acid or 1% hydrochloric acid + 25% nitric acid is used, and it is preferable to immerse at 30-70 ° C. for 5-20 s. .

  Manufacturing conditions other than those described above are not particularly limited, but are preferably the following conditions.

Hot rolling: A slab whose components are adjusted in the steelmaking process and manufactured by continuous casting is hot-rolled directly or after reheating to obtain a hot-rolled steel sheet. At this time, finishing temperature of hot rolling, the Ar less than ₃ transformation point becomes a mixed structure of austenite and ferrite, and is easy to degrade the workability, it is preferable that the Ar ₃ transformation point or more. In addition, when the finishing temperature exceeds (Ar ₃ transformation point +100) ° C., the steel structure is coarsened and the workability and surface properties are liable to deteriorate, so it is preferable to set it to (Ar ₃ transformation point +100) ° C. or less. . The steel sheet after hot rolling is cooled, and austenite is ferrite-transformed and wound. At this time, if the cooling rate is slow, the ferrite produced by the transformation becomes coarse and may adversely affect workability. Therefore, the average cooling rate is preferably 20 ° C./s or more. The winding temperature is preferably 400 to 650 ° C. The hot-rolled steel sheet after winding is pickled according to a conventional method before cold rolling in order to remove the scale generated on the surface. The annealing is preferably performed by continuous annealing that can provide high productivity.

Table 1 shows hot-rolled steel sheets A to D manufactured with a finishing temperature of 910 ° C. that is not less than Ar ₃ transformation point (Ar ₃ transformation point +100) ° C. and a coiling temperature of 620 ° C. After cold rolling at a reduction ratio shown in 2 to obtain a cold-rolled steel sheet having a thickness of 0.7 mm, continuous annealing was performed at the annealing temperatures shown in Table 2. After that, as shown in Table 2, some steel plates were pickled in a pickling solution at 35 ° C. (1% hydrochloric acid + 25% nitric acid) for 10 s, or S was soaked in 0.5 g / l thioglycolic acid for 5 s. After compound treatment, washing and drying, temper rolling with an elongation of 0.7% was performed to prepare Sample Nos. 1 to 12. Then, the surface S amount, Cs (Si), r value, and post-coating corrosion resistance of the obtained samples were investigated by the following methods.

(1) Surface S amount A fluorescent X-ray S count and an S amount calibration curve were prepared in advance and calculated from the fluorescent X-ray S count value of the steel sheet sample.

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

(3) r value Method specified in JIS Z 2241 using No. 5 test specimen specified in JIS Z 2204 taken from the rolling direction, 45 ° to rolling direction, and 90 ° to rolling direction Measure the rolling direction, the 45 ° direction with respect to the rolling direction, and the r value in the rolling direction, that is, rL, rD, rC, and calculate the average r value = (rL + 2rD + rC) / 4 R value.

(4) Corrosion resistance after painting Chemical conversion treatment uses degreasing agent: Surf Cleaner EC90, surface conditioner: Surffine 5N-10, chemical conversion treatment agent: Surfdyne SD2800 manufactured by Nippon Paint Co., Ltd. Degreasing process: Concentration 16g / l, Treatment temperature 42-44 ° C, treatment time 120s, spray, surface adjustment process: total alkalinity 1.5-2.5 points, temperature 20-25 ° C, treatment time 30s, immersion, chemical conversion treatment process: total acidity 21-24 points, free acidity The test was carried out under the conditions of 0.7 to 0.9 point, accelerator concentration of 2.8 to 3.5 point, processing temperature of 44 ° C. and processing time of 120 s. Thereafter, electrodeposition coating was performed using Nippon Paint's electrodeposition paint: V-50. 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.

  After coating, the corrosion resistance was evaluated by a salt spray test, i.e., a sample that had been subjected to chemical conversion treatment and electrodeposition coating was subjected to a test where a crosscut wrinkle was applied with a cutter and immersed in a 5% NaCl solution at 60 ° C for 240 hours, followed by washing Then, the tape was peeled from the cut collar and the maximum width of the maximum peel 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 corrosion resistance after coating is good (OK).

  The results are shown in Table 2. Inventive Sample Nos. 1, 3, 7, and 9 satisfying the requirements of the present invention all exhibit excellent deep drawability with an r value of 1.2 or more, and extremely excellent corrosion resistance after coating.

Claims (4)

In mass%, C: 0.008% or less, Mn: 0.05 to 1.0%, Si: 0.2% or more and less than 0.8%, S: 0.005% or less, N: 0.01% or less, Al: 0.01 to 0.1%, further Ti : 0.010 to 0.1%, Nb: contains at least one element selected from 0.015 to 0.15%, the balance consists of Fe and unavoidable impurities, the content of Ti and Nb is the following formula (1) And the Si amount Cs (Si) on the steel sheet surface defined by the following formula (2) is 1.6% or less, and 0.1 to 100 mg / m ² of S compound in terms of S is present on the steel sheet surface. Cold-rolled steel sheet with excellent deep drawability and post-coating corrosion resistance characterized by:
Ti * / 48 + [Nb] / 93> [C] / 12 ・ ・ ・ (1)
Cs (Si) = Cb (Si) × [Rs (Si / Fe) / Rb (Si / Fe)] (2)
Here, Ti * = [Ti] -48 [S] / 32-48 [N] / 14, [M] is the content of element M (% by mass), and Cb (Si) is the amount of Si in the steel. Rs (Si / Fe) represents 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) represents the GDS count ratio of Si and Fe in the steel. The deep drawing according to claim 1, 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 composition. Cold-rolled steel sheet with excellent corrosion resistance and post-coating corrosion resistance. The hot-rolled steel sheet having the component composition according to claim 1 is cold-rolled at a reduction ratio of 50 to 95%, recrystallized and annealed at an annealing temperature of 750 to 950 ° C, and then pickled and contains an S compound. A method for producing a cold-rolled steel sheet excellent in deep drawability and post-coating corrosion resistance, wherein an aqueous solution is brought into contact with the steel sheet surface, and 0.1 to 100 mg / m ^{2 of} an S compound is present on the steel sheet surface in terms of S amount. The hot-rolled steel sheet having the component composition according to claim 2 is cold-rolled at a reduction rate of 50 to 95%, recrystallized and annealed at an annealing temperature of 750 to 950 ° C, and then pickled and contains an S compound. A method for producing a cold-rolled steel sheet excellent in deep drawability and post-coating corrosion resistance, wherein an aqueous solution is brought into contact with the steel sheet surface, and 0.1 to 100 mg / m ^{2 of} an S compound is present on the steel sheet surface in terms of S amount.