JP2521553B2 - Method for producing cold-rolled steel sheet for deep drawing having bake hardenability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet for deep drawing, which is provided with bake hardenability (hereinafter abbreviated as BH property).

[Conventional technology]

Cold-rolled steel sheets for deep drawing are used for outer and inner panels of automobiles. The cold-rolled steel sheet here means a cold-rolled steel sheet in a narrow sense that is not surface-treated, and a cold-rolled steel sheet that has been subjected to surface treatment such as Zn plating or alloyed Zn plating for rust prevention. The material properties required for these steel sheets are i) excellent press formability and ii) BH property. That is, i) relates to the recent improvement in integral moldability and sterling of automobile parts. That is, these steel sheets are required to have more and more severe formability recently, and cold-rolled steel sheets having excellent formability are required. On the other hand, when the moldability is improved, the material is generally softened and the dent resistance is deteriorated. The above-mentioned ii) BH property is an important property that solves such problems, and as a result, the workability during pressing is ensured, and the strength increases in the painting process after pressing, which is a problem of dent resistance. Is improved. Also, as global environmental problems have come to the surface these days, weight reduction of the vehicle body is important for improving fuel efficiency. For that purpose, a high-strength cold-rolled steel sheet for panels having excellent BH property with excellent formability is required.

Super-processable steel sheets are usually manufactured by continuous annealing based on high-purity steel (IF steel) or batch-type decarburization annealing based on IF steel or low carbon steel. is there. It is well known that continuous annealing is preferable from the viewpoint of material uniformity and economy, but in this case solid solution C that contributes to BH property by adding Ti or Nb to satisfy excellent workability, N
Is controlled to zero or an extremely low amount. Therefore, the BH property is generally lost in the super-workable steel sheet.

However, many studies have been made in the past to impart BH property to such continuously annealed IF steel. The present invention is premised on a steel sheet to which Ti and Nb are added in combination, which is considered to have a good balance between the value which is an index of deep drawability and elongation. Conventional techniques for imparting BH properties to such steel sheets include
There are basically the following three ideas.

The first is T as disclosed in JP-A-59-31827.
First, fix N with i and leave excess solid solution C so that stretcher strain does not occur and BH property is expressed.
This is a technique of adjusting and adding Nb. In connection with the above technique, Japanese Patent Laid-Open No. 60-47328 aims to improve BH property by further adding B to this to contribute to BH property of solid solution B. Secondly, Japanese Patent Laid-Open Nos. 61-26757 and 62-7822.
Both of them are characterized by increasing the BH property by changing the precipitation behavior of sulfides and carbides of Ti by extremely reducing the S content to 30 ppm or less. Thirdly, JP-A-61-2769.
No. 31 gazette discloses that the soaking temperature of continuous annealing is 85
0 ° C. to Ac ₃ and BH by high temperature to dissolve the carbides
It is characterized by imparting sex. However, all of these conventional methods have the following problems.

[Problems to be Solved by the Invention]

In the first conventional method, N is Ti and C is partially fixed with Nb. However, Ti and Nb are added only in an equivalent amount or less, and in particular, Nb is adjusted to a small amount so that an excessive amount of solid solution C contributing to BH property remains. Therefore, a large amount of solute C also exists in the hot rolled sheet, and such solute C is extremely harmful to the development of the {111} texture of the cold rolled / annealed sheet, and the value, that is, the deep drawability deteriorates. I have a problem to do. In addition, since Ti is added in an equivalent amount or less with respect to N, it is no longer possible to fix S, and it is not possible to utilize Ti sulfide, which is important for workability and BH property. In addition, adding a large amount of B to impart BH property is
It deteriorates the quality and raises the cost. In the second conventional method, when S is set to 30 ppm or less, BH property is imparted to the Ti-added ultra-low carbon steel, and this technique is extended to the case of the Ti- and Nb-added ultra-low carbon steel. There is. However, as a result of detailed study by the present inventors, in the ultra-low carbon steel to which Ti and Nb are added in combination, i) even if the S content is reduced to 30 ppm or less, the BH content does not increase, but rather decreases. Ii) It was also found that the BH property was not sufficient. Furthermore, reducing the amount of S to 30 ppm or less has a problem of causing a significant cost increase. The third conventional method is premised on that the annealing temperature is as high as 850 ° C or higher. High temperature annealing improves workability,
Although it is preferable for imparting BH, there are many problems such as plate breakage in terms of operation technology, and reduction in flatness of the steel plate, and energy costs are high.

The present invention solves the above problems and provides a technique for stably and economically producing a steel sheet having BH characteristics and having excellent workability.

[Means for solving the problem]

In the present invention, first, as a basic condition for ensuring extremely excellent workability, it is sufficient to fix a part of N and S.
Add Ti. This is because the Ti content below the N equivalent
This is different from the first conventional method in which only addition is made. This allows
N is not only stably fixed as TiN but also a considerable amount of S is fixed as TiS. Since these precipitates are formed at high temperature, they are coarse and do not hinder grain growth. Therefore, the crystal grain size of the final product is larger than when fine TiC and NbC are precipitated, and the workability is significantly improved. Further, as a result of fixing S by Ti, Mn, which has been conventionally added for the purpose of preventing hot brittleness, can also be lowered, and thereby workability can be further improved. In addition, in the ultra low carbon steel in which Ti and Nb are added in combination, differently from the Ti added ultra low carbon steel, we have obtained new knowledge that increasing the S amount rather increases the BH amount. That is, Ti added in an amount equal to or more than N can fix C as TiC when the amount of S is low.
The amount decreases. On the other hand, when the amount of S is increased, Ti equal to or more than N is first used for fixing S, so that it becomes impossible to fix C anymore. Therefore, C is fixed by Nb. Under such a composition, even if the annealing temperature is lower than 850 ° C., when Nb is equal to or less than the equivalent amount of C, the solid solution C easily remains and the BH property is imparted. In this case, Ti fixes N and S as relatively large precipitates, and also fixes C with Nb, so that the grain growth property becomes good and extremely excellent workability can be obtained. As a matter of course, it is possible to operate at a temperature of less than 850 ° C, which is accompanied by high temperature annealing. The problems of operating technology and economic problems caused by the rise of energy costs will be solved.

Further, the metallurgical principle described above can be applied not only to the production of cold-rolled steel sheet that is not subjected to surface treatment, but also to the original plate of electric Zn-plated cold-rolled steel sheet, and further to Zn plating by continuous hot-dip Zn plating equipment of in-line annealing method. It can also be applied to the production of alloyed Zn-plated cold-rolled steel sheets.

The present invention is constructed based on such an idea and a new finding, and the gist thereof is as follows.

(1) By weight, C: 0.0005 to 0.0040%, Si: 0.8% or less, M
n: 0.03-1.0%, P: 0.15% or less, S: 0.004-0.015%, Al:
0.01-0.1%, N: 0.0005-0.0060%, balance Fe and unavoidable impurities, and Ti and Nb must be added together. Ti is 0.009-0.05% and 48 / 14N <48
Nb contained within the range that satisfies Ti <48/14 ・ N + 48/32 ・ S
Is 0.009 to 0.02%, and 93/12 · (C-0.0015) ≦ Nb ≦
The steel containing in a range satisfying 93/12 · C (Ar ₃ -100)
Characterized by hot rolling at a finishing temperature of ℃ or more, winding at a temperature of 500 to 750 ℃, cold rolling at a rolling reduction of 60% or more, and continuous annealing at 700 ℃ or more and less than 850 ℃. A method for manufacturing a cold-rolled steel sheet for deep drawing having bake hardenability.

(2) By weight, C: 0.0005 to 0.0040%, Si: 0.8% or less, M
n: 0.03-1.0%, P: 0.15% or less, S: 0.004-0.015%, Al:
0.01-0.1%, N: 0.0005-0.0060%, B: 0.0002-0.0010
% And balance Fe and unavoidable impurities, and Ti and Nb
It is an essential condition that Ti is 0.009-0.05.
%, And within the range of 48/14 · N <Ti <48/14 · N + 48/32 · S, Nb is 0.009 to 0.02%, and 93/12
・ (C-0.0015) ≦ Nb ≦ 93/12 ・ Steel contained within the range that satisfies C is hot-rolled at a finishing temperature of (Ar ₃ −100) ° C. or higher and then wound at a temperature of 500 to 750 ° C. , Then the reduction rate
A method for producing a deep-drawn cold-rolled steel sheet having bake hardenability, which comprises cold rolling at 60% or more and then continuous annealing at 700 ° C or more and less than 850 ° C.

According to the present invention, a value of 1.8 or more and a BH property of 3.0 kgf / mm ² can be obtained.

The reasons for limiting numerical values will be described below to further clarify the present invention.

C is an element that contributes to the BH property, and its range is extremely important. That is, the C content must be 0.0005 to 0.0040%. It is technically extremely difficult to reduce the amount of C to less than 0.0005% and causes a significant cost increase.
If the C content exceeds 0.0040%, the Nb content for fixing the C content increases, and i) the alloy cost of Nb increases, and i
i) Since fine NbC is increased, grain growth is hindered, and the value is lowered and elongation is significantly deteriorated, which falls outside the range of super-workable steel sheets.

Si is an effective element for high strength because it suppresses deterioration of elongation and increases strength, but if it exceeds 0.8%,
The chemical conversion treatment property deteriorates, and the plating property deteriorates when a hot-dip galvanized steel sheet is manufactured. Therefore, the amount should be 0.8% or less. The lower the Si, the better the workability, so there is no lower limit.

The amount of Mn is 0.03 to 1.0%. If the amount is less than 0.03%, hot embrittlement occurs. Further, if it exceeds 1.0%, it hardens and the workability deteriorates.

The amount of P should be 0.15% or less. P is an effective element that reduces the deterioration of workability and increases strength, but if it exceeds 0.15%, it becomes too hard and causes secondary work embrittlement.

The amount of S is extremely important in the present invention.
It is 0.004 to 0.015%. When the amount of S is less than 0.004%, N
Even if is fixed, excess Ti forms Ti carbide before forming Ti sulfide. As a result, solid solution C does not remain after annealing and the BH property becomes poor. Further, since such Ti carbide is finer than Ti sulfide, the crystal grain size of the annealed plate becomes finer and the workability also deteriorates. On the other hand, S amount is 0.015
If it exceeds%, hot embrittlement occurs, and the workability after cold rolling / annealing remarkably deteriorates. The range of the present invention is 0.004 ~
At 0.015% S, excess Ti even if N is fixed
Form Ti sulfide. As a result, C is fixed to Nb or fixed in total amount. By the way, C of the present invention
In the composition range of Nb and Nb and annealing conditions, solid solution C remains before annealing, or NbC is redissolved during annealing and solid solution C exists, so that BH property is exhibited. Also relatively large
Since Ti sulfide is present, the grain growth property is good, and the material of the present invention is also excellent in workability.

The amount of Al is 0.01 to 0.1%. Al is added as a molten steel deoxidizer before adding Ti and Nb, but if it is less than 0.01% and too small, Ti and Nb are oxidized and the yields thereof are reduced. on the other hand,
If it is added in excess of 0.1%, the amount of Al ₂ O ₃ inclusions increases,
Deteriorate the material.

The N content is 0.0005 to 0.0060%. From the viewpoint of workability, the smaller the N content, the more preferable, but if it is less than 0.0005%, the steelmaking cost will remarkably increase. On the other hand, the amount of N is 0.
If it is too much, that is, too much, the amount of Ti for fixing the Ti will increase and the cost will increase, which is not preferable.

In the present invention, the combined addition of Ti and Nb is an essential condition. First, the Ti addition amount is 0.009 to 0.05% and 48/14 · N.
It must be <Ti <48/14 ・ N + 48/32 ・ S. That is, Ti is added after deoxidation of Al to fix N in steel as TiN first, but it is necessary to adjust the composition so that excess Ti always exists. This is because excess Ti forms Ti sulfides (TiS etc.) as described in the reason for limiting the amount of S, and as a result, the workability of the steel sheet is improved and BH property is imparted. Therefore, Ti must first be> 48/14 · N. On the other hand, when the Ti amount exceeds 48/14 · N + 48/32 · S, the remaining Ti fixes C even when N and S are fixed, resulting in a decrease in BH amount and workability due to a small amount of TiC. Since it deteriorates, Ti <48/14 ・ N + 48/32 ・ S must be satisfied.
Even if this condition is satisfied, it becomes difficult to form TiN or Ti sulfide if the Ti addition amount is less than 0.009%.
Therefore, it is necessary to add 0.009% or more of Ti.
On the other hand, when the amount of Ti added exceeds 0.05%, TiN and Ti sulfide are formed, but there is an excess of Ti, which fixes C in the form of TiC or Ti ₄ C ₂ S _2. Therefore, the amount of remaining solid solution C becomes extremely small, and it becomes impossible to impart the BH property effective for improving the dent resistance. Therefore, the Ti addition amount is set to 0.05% or less.

The amount of Nb added is 0.009 to 0.02% and 93/12 · (C-0.001
5) It must be ≤ Nb ≤ 93/12 · C. The deep drawability is improved by adding a small amount of Nb, but the effect is slight when the addition amount is less than 0.009%. On the other hand, when Nb exceeds 0.02%, the ductility is significantly deteriorated, the BH property is also extremely low, and the dent resistance is deteriorated. That is, in the present invention, since Ti is basically consumed by fixing a part of N and S, C is fixed by Nb. In this case, in addition to the ranges of C and Nb amounts already described, 93/12. (C-
0.0015) ≤ Nb ≤ 93/12 · C is required. N in atomic ratio
If b / C is in the range of 1 or less, Nb will be insufficient and solid solution C will remain, resulting in a high BH amount. These solute C contribute to the BH characteristic effective for dent resistance. Also, the amount of Nb is Nb <93/12
When it becomes [C-0.0015], the amount of BH increases, but the workability deteriorates due to solid solution C and fine NbC. On the other hand, the upper limit of Nb amount must be 93/12 · C. When the Nb amount becomes Nb> 93/12 · C, almost all C is fixed by Nb, and a sufficient BH amount cannot be obtained.

B is added as necessary for the purpose of preventing deterioration of secondary workability, and the addition amount is 0.0002 to 0.0010%. If the addition amount is less than 0.0002%, there is no effect in preventing deterioration of the secondary workability, while if over 0.0010% is added, the workability deteriorates and slab cracking occurs.

A slab having the above chemical composition is hot rolled. The finishing temperature for hot rolling needs to be (Ar ₃ -100) ° C or higher from the viewpoint of ensuring workability after cold rolling and annealing. Also,
The winding temperature is 500-750 ℃. The lower limit of the winding temperature is determined for the purpose of ensuring deep drawability and ductility, and the upper limit thereof is determined from the viewpoint of preventing the yield reduction due to the deterioration of the material at both ends of the coil.

Cold rolling may be performed under normal conditions, and the rolling reduction is 60% or more for the purpose of ensuring deep drawability after annealing.

The annealing temperature of continuous melting or in-line annealing continuous melting Zn plating equipment shall be 700 ℃ or more and less than 850 ℃. If the annealing temperature is less than 700 ° C, recrystallization is insufficient. Also,
The workability and BH property improve as the annealing temperature increases, but 85
If the temperature is 0 ° C or higher, the temperature is too high and the plate breaks or the flatness of the plate deteriorates.

 Next, the present invention will be described with reference to examples.

(Example 1) Steel having the chemical composition shown in Table 1 was tapped in a converter,
A slab was made with a continuous casting machine, then heated to 1070 ° C, and hot rolling was performed so that the finishing temperature was 910 ° C near the Ar ₃ transformation point and the plate thickness was 4.2 mm. The average cooling rate on the run-out table was 40 ° C / s, after which it was coiled at 700 ° C. After pickling, cold rolling was performed at 0.8 mm, followed by continuous annealing in an actual machine. Continuous annealing conditions are annealing temperature: 830 ℃,
Soaking: 1 min, cooling rate: 100 ℃ / s up to room temperature. Then, temper rolling was performed at a rolling reduction of 0.6%, and the steel was subjected to a tensile test.

The test results are shown in Table 2. Here, the tensile test is JIS5
Evaluation was performed using a No. test piece. The BH property is 2 in the rolling direction.
% Tensile prestrain was added, the load was once removed, heat treatment equivalent to paint baking was performed at 170 ° C. for 20 minutes, and then the tensile test was performed again, and the increase amount of the yield stress at this time was obtained and evaluated. The r value was obtained by a generally well known method. The secondary workability was evaluated by cooling a cup drawn at a drawing ratio of 2.5 (shear edge) to −50 ° C., crushing the cup, and observing the crack occurrence state.

Steel Nos. A-1 to E-1 in Tables 1 and 2 have P of about 0.00
8% Included intensity level is cold-rolled steel sheet 30 kgf / mm ² grade, A-2~E-2 is a high strength cold rolled steel sheet of about 0.07% added to the intensity level of 35 kgf / mm ² class P Is.

As can be seen from these tables, the steel produced according to the scope of the invention has a desired BH content of 3 kgf / mm ² or higher and 1.8
The above is obtained. That is, the steels A-1 and E-1 are superformable cold-rolled steel sheets having a strength level of 30 kgf / mm ² and BH property, and the steels A-2 and E-2 have a strength level of 35k.
It is a gf / mm ² grade high strength cold rolled steel sheet with BH properties and excellent workability. Here, the steels B-1 and B-2 have an insufficient amount of S and thus have an insufficient BH property. On the other hand, steel C-1, C-
In No. 2, since the amount of Nb added was too large, the workability, particularly ductility, was poor, and the BH property was insufficient. Further, Steel D-1 has almost no BH property because the Ti addition amount is too large. In Steel D-2, since the amounts of C and Nb do not satisfy Nb ≧ 93/12 · (C−0.0015), the workability, particularly the value, is low.

(Example) Among the test materials shown in Table 1, A-1 and B-1 were cold-rolled under the same conditions as in Example 1, and then hot-dipped Zn-plated cold-rolled steel sheets were used in an actual machine. Manufactured. The maximum temperature of annealing was 810 ° C, and the temperature of the Zn plating bath was 460 ° C.
When the alloying treatment was performed, the steel sheet was immersed in a Zn plating bath, reheated to 520 ° C., held for about 20 seconds, and then immediately cooled. BH evaluated in the same manner as in Example 1 in Table 3
The result of sex is shown. As is clear from the table, the present invention steel A-
No. 1 has a desired amount of BH regardless of the presence or absence of alloying treatment.

[Effects of the Invention] As is apparent from the above description, according to the production method of the present invention, i) a superformable cold-rolled steel sheet having a BH property added and a strength level of 30 kgf / mm ² grade, and ii) a BH property A high strength cold rolled steel sheet for deep drawing having a strength level of 35 kgf / mm ² grade can be obtained. INDUSTRIAL APPLICABILITY The present invention not only enables molding of a difficult-to-mold component for automobiles and further enables integral moldability, but also exerts many effects in improving dent resistance. Further, by using the high-strength cold-rolled steel sheet having BH property obtained by the present invention, it becomes possible to reduce the thickness of the automobile panel steel sheet and reduce the vehicle body weight. Therefore, the present invention contributes to the improvement of fuel consumption and eventually to the countermeasure against the global warming problem. Furthermore, since such a cold-rolled steel sheet can be manufactured by continuous annealing or in-line annealing type continuous hot-dip Zn plating equipment, the advantages of continuous annealing can be enjoyed in terms of material uniformity and economical efficiency.

Claims (2)

(57) [Claims] 1. By weight, C: 0.0005 to 0.0040%, Si: 0.8% or less, Mn: 0.03 to 1.0%, P: 0.15% or less, S: 0.004 to 0.015
%, Al: 0.01 to 0.1%, N: 0.0005 to 0.0060% and balance Fe
And inevitable impurities, and the essential addition of Ti and Nb must be 0.009-0.05% and 48/14
・ N <Ti <48/14 ・ N + 48/32 ・ S contained within the range that satisfies S, Nb is 0.009-0.02%, and 93/12 ・ (C-0.0015)
Steel containing within the range of ≦ Nb ≦ 93/12 · C (Ar ₃ −
After hot rolling at a finishing temperature of 100) ° C or higher, 500 to 75
A cold-rolled steel sheet for deep-drawing having bake hardenability, which is characterized in that it is wound at a temperature of 0 ° C, then cold-rolled at a rolling reduction of 60% or more, and then continuously annealed at 700 ° C or more and less than 850 ° C. Production method. 2. By weight, C: 0.0005 to 0.0040%, Si: 0.8% or less, Mn: 0.03 to 1.0%, P: 0.15% or less, S: 0.004 to 0.015
%, Al: 0.01 to 0.1%, N: 0.0005 to 0.0060%, B: 0.0002 to
0.0010% and balance Fe and inevitable impurities, and
The combined addition of Ti and Nb is an essential condition, Ti is 0.009
~ 0.05%, and 48/14 ・ N <Ti <48/14 ・ N + 48/32 ・
Steel containing N within the range satisfying S, 0.009 to 0.02%, and satisfying 93/12 · (C−0.0015) ≦ Nb ≦ 93/12 · C (Ar ₃ −100) After hot rolling at a finishing temperature of ℃ or more, winding at a temperature of 500 to 750 ℃, then cold rolling at a rolling reduction of 60% or more, then 700 ℃ or more, 850 ℃
A method for producing a cold-rolled steel sheet for deep drawing having bake hardenability, which comprises performing continuous annealing at a temperature of less than 1.