JP3288514B2 - Manufacturing method of high-strength cold-rolled steel sheet for deep drawing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention proposes a method for producing a high-strength cold-rolled steel sheet for deep drawing, which is high in strength and excellent in press formability. It proposes an excellent method. The cold-rolled steel sheet according to the present invention is appropriately subjected to surface treatment and press working, and then used, for example, for automobiles and home electric appliances, and in some cases, spot-welded,
In particular, since the required formability and strength can be simultaneously imparted to them, it is possible to achieve a reduction in the thickness of the steel sheet and, consequently, a reduction in weight, and specifically, a tensile strength of 38 kgf / mm, which is excellent in economic efficiency. There is a demand for a cold-rolled steel sheet having a Rankford value, that is, an average r value of 1.8 or more, which is ² or more.

[0002]

2. Description of the Related Art Conventionally, decarburization has been sufficiently performed at the steel making stage to obtain extremely low carbon, and solid solution C and solid solution N in steel have been reduced to Ti.
High-strength cold-rolled steel sheets with increased strength, based on steel containing C and N fixed by precipitating them as carbonitrides and containing Si, Mn and P as solid solutions, A proposal has been made.

[0003] For example, in Japanese Patent Application Laid-Open No. 63-190141, a high-tensile cold-rolled steel sheet having good formability and a method for producing the same include a cold-rolled steel sheet obtained by adding a large amount of Mn and P to the ultra-low carbon Ti-added steel. The proposal has been disclosed. In this proposal, by adding an appropriate amount of Mn and P, a small amount of solute C remains after annealing, which significantly improves the r-value. It is described that brittleness is effectively prevented, and B may be added as necessary for strengthening the grain boundary. However, when P is added without adding Si, there is a problem that spot weldability deteriorates.

[0004] In addition, Si, Mn,
And cold-rolled steel sheets to which a solid solution strengthening component of P is added in combination. However, the addition of Si, Mn, and P is merely for solid solution strengthening, and although the influence on the average r value of each component has been clarified, the addition between these solid solution components having high deep drawability has been described. At present, it is not clear yet the balance of the compound addition of the above.

[0005] In addition to these problems related to tensile properties, ultra low carbon steel containing a large amount of such a solid solution strengthening component has been obtained because austenite is in an unrecrystallized state during hot rolling. The strength of the hot-rolled steel sheet increases significantly.
Therefore, the cold rolling property is deteriorated. That is, in addition to the difficulty in cold rolling, there is a problem that cracks are likely to occur at the edge in the width direction of the steel sheet.

[0006]

The inventors have already disclosed in Japanese Patent Application No. 4-211483 (high-strength cold-rolled steel sheet for deep drawing and a method for producing the same) in accordance with the amounts of Si and P. Has been shown to prevent the formation of a quenched structure in a hot-rolled sheet, but it has not been sufficient yet, and it is necessary to further improve the cold rolling property. Therefore, the present invention adds Nb for more securely fixing solid solution C in addition to Ti for fixing solid solution C and solid solution N, and further adds B for suppressing the brittleness of secondary working. Very low carbon Ti added,
Nb, and the B complex added steel based, which a low-cost alloy additives containing Si, moreover tensile strength a target in a conventional continuous annealing 38kgf / mm ² or more, the average r value 1.8 or more deep drawability An object of the present invention is to propose a method for producing an excellent high-strength cold-rolled steel sheet, and to significantly improve the cold rolling property in the production process.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have focused on the ultra-low carbon Ti, Nb, and B-added steels and have been diligently conducting research. As a result, extremely low carbon Ti,
In the Nb and B composite-added steel, by maintaining the addition amount of Mn with respect to the addition amount of Si and P, a high average r value is obtained in combination with the more reliable fixation of solid solution C by the addition of Nb. It was newly found that it could be obtained.

The reason for this is that the addition of Mn results in the refinement of the hot-rolled base sheet, which results in the development of a (111) texture effective for improving the average r value after annealing. In general, when a solid solution strengthening component is added alone, P and Si have little effect on the average r value, but rather increase the average r value, whereas Mn greatly deteriorates the average r value. It is known. However, when a composite is added, the transformation point rises rapidly only with Si and P, and it is necessary to regulate the rolling conditions and the annealing conditions.
For this reason, it has become necessary to add a component that lowers the transformation point. In order to lower the transformation point temperature effectively at low cost, the addition of Mn is effective. However, if the added amount of Mn is too large, a low-temperature transformation phase appears to make the hot-rolled sheet high in strength and greatly deteriorates the deep drawability of the annealed material.

Therefore, the present invention provides a method for adding Mn in an amount corresponding to the addition amounts of Si and P, and after hot rolling,
By limiting the cooling rate up to the winding process, an appropriate balance of Si, Mn and P is maintained, the ferrite grain size is reduced, and a high average r value is combined with the more secure fixation of solid solution C by adding Nb. At the same time, the structure of the hot-rolled sheet is not made into a quenched structure, but is made into a ferrite structure, and the hot-rolled sheet is softened to improve the cold rolling property.

That is, the gist of the present invention is as follows: C: 0.001 wt% or more, 0.005 wt% or less, Si: 0.2 wt% or more, 1.0 wt% or less, Mn: 0.5 wt% or more, 2.0 wt% or less, Ti: 0.01 wt% or more, 0.2 wt% or less, Nb: 0.001 wt% or more, 0.02 wt% or less, B: 0.0002 wt% or more, 0.005 wt% or less, P: 0.05 wt% or more, 0.15 wt% or less, S: 0.02 wt% The following contains sol Al: 0.1 wt% or less and N: 0.005 wt% or less, and Cr: 1.0 wt% or less, Ni: 1.5 wt% or less, Mo: 0.5 wt% or less, and Cu: 1.5 wt% as required. One or more selected from the following are contained, and the content of Si, Mn and P is 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0 satisfies the relationship, the balance as material a steel slab having a composition of iron and inevitable impurities, Ar ₃ point (℃) or higher, Ar ₃
Hot rolling is completed at a rolling finishing temperature range of not more than the point (° C) + 100 ° C, and a temperature range from the temperature to at least 700 ° C is cooled at a cooling rate of 8 ° C / s or less, and the coil is formed at a temperature of 650 ° C or less. After winding, cold-rolled, then 750 ° C
A method for producing a high-strength cold-rolled steel sheet for deep drawing, characterized by performing continuous annealing at the above temperature.

[0011]

The reasons for limiting the chemical composition of steel in the present invention will be described. C: 0.001 to 0.005 wt% C is an average r when a large amount of solid solution C remains during recrystallization.
Significantly degrade the value. Also, Ti for fixing solid solution C,
Since Nb also needs to be added according to the C content, the C content is preferably as low as possible, and the allowable upper limit is 0.005 wt.
%. On the other hand, the lower limit is better as low as possible, but the lower limit is 0.001 wt% based on the current steelmaking technology.

Si: 0.2 to 1.0 wt% Si has a large solid solution strengthening ability and does not deteriorate the average r value, so it is optimal as a solid solution strengthening component and suppresses deterioration of spot weldability. It is necessary to contain at least 0.2% by weight to achieve the effect. However, the content is 1.0 wt%
If it exceeds 300, the surface treatment properties will deteriorate. Therefore, the content is set to 0.2 wt% or more and 1.0 wt% or less.

Mn: 0.5 to 2.0 wt% Mn is an important component in the present invention. Mn is Si or P
Unlike this, since it is a component that lowers the transformation point, by effectively utilizing it, the particle size of the hot-rolled sheet can be made extremely fine. This grain refinement of the hot-rolled sheet is very effective in improving the average r value because the (111) texture develops from the crystal grain boundaries. The effect is obtained when the content is 0.5 wt% or more. On the other hand, since Mn itself is a component that deteriorates the average r value, it is harmful to include it in a large amount, and if the content exceeds 2.0 wt%, a low-temperature transformation phase is likely to appear and the ferrite structure is lost. Greatly deteriorates. Therefore, its content is 0.5 wt% or more, 2.0 wt%
The following is assumed.

Further, the balance between the content of Si and the following P and the content of Mn is set to 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0. The above-mentioned effectiveness of Mn can be exhibited. This is because when the ratio of the sum of the Si and P contents to the Mn content is smaller than 0.2, the average r value deteriorates, and conversely 1.
If it exceeds 0, the transformation point temperature becomes high, and it becomes impossible to reduce the grain size of the hot-rolled sheet.

Ti: 0.01-0.2 wt% Ti is contained for fixing solid solution C, S and N as TiC, TiS and TiN. The effect is that the content is 0.01wt%
If the amount is less than 0.2 wt%, phosphide is generated, and the elongation and the average r value are deteriorated. Therefore, its content should be 0.01 wt% or more and 0.2 wt% or less.

Nb: 0.001 to 0.02 wt% Nb is an effective component for fixing solid solution C as NbC in the same manner as Ti. Although solid solution C can be fixed with Ti alone, C can be fixed more reliably by adding Nb in combination, and the average r value can be improved. The effect is 0.001 content
If the content is less than 0.02% by weight, hot rolling is performed in an austenite non-recrystallized state, which adversely affects the formability of the annealed material. Therefore, its content is made 0.001 wt% or more and 0.02 wt% or less.

B: 0.0002 to 0.005 wt% B is contained to suppress the brittleness in secondary processing. In particular, when a solid solution strengthening component is added to an ultra-low carbon steel sheet to which Ti and Nb are added, secondary work embrittlement resistance is deteriorated, so that it is essential to add B. The effect is manifested when the content is 0.0002 wt% or more. However, if the content exceeds 0.005 wt%, the recrystallization of austenite is delayed and the load at the time of hot rolling is increased. Deteriorate. Therefore, its content should be 0.0002 wt% or more and 0.005 wt% or less.

P: 0.05 to 0.15 wt% P is an important component as a solid solution strengthening component, and its solid solution strengthening ability is higher than that of Si and Mn, and is also a component for improving the average r value. In order to obtain the effect, it is necessary to contain 0.05 wt% or more. However, if it exceeds 0.15 wt%, it segregates at the grain boundary, embrittles the grain boundary, and further causes central segregation during solidification. . Therefore its content is 0.05wt%
At least 0.15 wt%.

S: not more than 0.02 wt% S does not affect the average r value, but when its content increases, sulfides such as MnS increase, and the local ductility represented by stretch flangeability decreases. Therefore, the upper limit of the content is set to 0.02 wt%.

Sol Al: 0.1 wt% or less Al is a component necessary for deoxidation.
If the content exceeds wt%, not only the deoxidizing effect is saturated, but also the number of inclusions increases, which adversely affects the moldability. Therefore, its content should be 0.1 wt% or less.

N: 0.005 wt% or less N is an impurity component inevitably mixed into steel.
By adding Ti, it is fixed as TiN to improve formability. However, when a large amount of TiN is formed, workability is deteriorated. Therefore, the upper limit of the allowable content is 0.005 wt%
And

Cr: 1.0 wt% or less, Ni: 1.5 wt% or less, M
o: 0.5 wt% or less, Cu: 1.5 wt% or less, one or more of Cr, Ni, Mo and Cu are both high without significantly affecting the cold rolling property and deep drawability in the manufacturing process. It is a component effective for strengthening, and can be selectively added alone or in combination as needed. However, excessive addition causes a cost increase, so the upper limit of the content is C
r: 1.0 wt%, Ni: 1.5 wt%, Mo: 0.5 wt% and Cu: 1.5
wt%.

Next, the manufacturing method will be described. Steelmaking and casting conditions may be performed according to a conventional method. Hot rolling finish temperature: Ar ₃ point to Ar ₃ point + 100 ° C. The hot rolling finish temperature needs to be changed according to the Ar ₃ transformation point. Below the Ar ₃ transformation point, it becomes two-phase zone rolling,
A texture that adversely affects the average r value of the annealed material develops, which is not preferable. On the other hand, when the temperature is relatively high with respect to the Ar ₃ transformation point, that is, higher than 100 ° C., the grain size of the hot-rolled sheet structure becomes coarse, and the development of a texture effective for deep drawability during annealing is insufficient. Is unsuitable for Therefore, the hot rolling finish temperature is set from the Ar ₃ point (° C.) to the Ar ₃ point (° C.) + 100 ° C.

Cooling conditions after hot rolling: The cooling rate in the temperature range from the rolling finishing temperature to at least 700 ° C. is 8 ° C./s.
The hot rolling is completed at 3 points or more of Ar and then 50 ° C /
By performing cooling at a cooling rate of about s, a fine ferrite structure favorable for deep drawability can be obtained. However, when the cooling rate is higher than 8 ° C./s, the ferrite transformation occurs in the cooling process without sufficiently releasing the strain accumulated in austenite during hot rolling. The property is deteriorated. That is, the present invention has newly found that the strain accumulated during the hot rolling can be released by gentle cooling after the hot rolling and the softening of the hot rolled sheet can be achieved, and the cooling rate after the hot rolling is set to 8 ° C. / S or less. Further, since the effect of strain release by slow cooling cannot be exerted at a temperature lower than 700 ° C., the temperature range from the rolling finishing temperature to 700 ° C. may be slow cooling, that is, 8 ° C./s or less.

Winding temperature: 650 ° C. or less It has been found that the limiting of the winding temperature is that when Ti and P are added in a complex manner as in the present invention, the average r value is greatly deteriorated when the temperature exceeds 650 ° C. It is due to. This is presumed to be due to the fact that a large amount of Ti and Fe phosphides precipitate on the hot-rolled sheet at a winding temperature exceeding 650 ° C., and details thereof are unknown, but this has an adverse effect on the average r value of the annealed material. Therefore, the winding temperature should be 650 ° C or less.

The cold rolling may be performed according to a conventional method.
The rolling reduction may be in the range of common sense. Annealing is preferably continuous annealing. In this case, the annealing temperature is set to 750 ° C. or higher so that recrystallization is completed, and the upper limit is preferably set to Ac ₁ point (° C.) + 50 ° C. The tempered rolling may be performed on the annealed steel sheet within a range of common sense.

As described above, according to the present invention, cold rolling subsequent to hot rolling is extremely facilitated, and the obtained steel sheet has excellent characteristics and is used as a high-strength cold-rolled steel sheet for deep drawing for automobiles and the like. It can be advantageously applied to home appliances and the like. Further, the cold-rolled steel sheet is also suitable as a hot-dip galvanized steel sheet or an electro-galvanized steel sheet.

[0028]

EXAMPLE A steel slab containing the chemical composition shown in Table 1 was used as a raw material, hot-rolled under various conditions, cold-rolled with a rolling reduction of 80% after pickling, and a sheet thickness of 0.8. mm steel plate. Subsequently, the steel sheet was subjected to continuous annealing at a temperature of 800 ° C. for a time of 40 seconds, and then temper rolling at a rolling reduction of 0.8%.

[0029]

[Table 1]

The tensile strength of the hot-rolled material thus obtained was examined and used as an index of cold rolling property, and the tensile strength, total elongation and average r value of the temper-rolled material were investigated. Here, the tensile test was performed using a JIS No. 5 test piece, and the average r value was 15% tensile pre-strain, and then the L direction (rolling direction) and the D direction (rolling direction relative to the rolling direction) measured by a three-point method. 45 ° direction) and C direction (90 ° to rolling direction)
The average r value = (r _L + 2r _D + r _c ) / 4. Table 2 summarizes the hot rolling conditions and the inspection results.

[0031]

[Table 2]

As is clear from Tables 1 and 2, when the compliant steel of the present invention was used, the hot-rolled sheet of the suitable example, which was hot-rolled and cooled slowly under the conditions suitable for the present invention, was the hot-rolled sheet of the comparative example. Compared with the sheet, the tensile strength is low and the cold rollability is excellent, and the temper rolled material of the applicable example of the present invention has a tensile strength of 45 kgf / mm ² or more, and a total elongation of: 35% or more, average r
The value of 1.8 or more shows extremely excellent characteristics.

[0033]

According to the present invention, a low-cost alloy-added steel in which the relationship between the contents of Si, Mn, and P is specified and contained based on the ultra-low carbon Ti, Nb, and B composite-added steel is used. By specifying the cold rolling and subsequent cooling conditions, the cold rolling property after hot rolling is improved, and an excellent high-strength cold-rolled steel sheet for deep drawing is obtained. Since it is easy to manufacture rolled steel sheets and has excellent economic efficiency, it is extremely advantageous for use in automobiles and home electric appliances.

Continuation of the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corp. Technical Research Division (56) References JP-A-6-10094 (JP, A) JP-A-6-106 57372 (JP, A) JP-A-6-25798 (JP, A) JP-A-2-173247 (JP, A) JP-A-5-345954 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46-9/48 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.001 wt% or more, 0.005 wt% or less, Si: 0.2 wt% or more, 1.0 wt% or less, Mn: 0.5 wt% or more, 2.0 wt% or less, Ti: 0.01 wt% or more, 0.2 wt% %, Nb: 0.001 wt% or more, 0.02 wt% or less, B: 0.0002 wt% or more, 0.005 wt% or less, P: 0.05 wt% or more, 0.15 wt% or less, S: 0.02 wt% or less, sol Al: 0.1 wt% or less and N: 0.005 wt% or less, the content of Si, Mn and P is defined as 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0. met, the balance as material a steel slab having a composition of iron and inevitable impurities, Ar ₃ point (℃) above, Ar ₃
Hot rolling is completed at a rolling finishing temperature range of not more than the point (° C) + 100 ° C, and a temperature range from the temperature to at least 700 ° C is cooled at a cooling rate of 8 ° C / s or less, and the coil is formed at a temperature of 650 ° C or less. After winding, cold-rolled, then 750 ° C
A method for producing a high-strength cold-rolled steel sheet for deep drawing, characterized by performing continuous annealing at the above temperature. 2. C: 0.001 wt% or more, 0.005 wt% or less, Si: 0.2 wt% or more, 1.0 wt% or less, Mn: 0.5 wt% or more, 2.0 wt% or less, Ti: 0.01 wt% or more, 0.2 wt% %, Nb: 0.001 wt% or more, 0.02 wt% or less, B: 0.0002 wt% or more, 0.005 wt% or less, P: 0.05 wt% or more, 0.15 wt% or less, S: 0.02 wt% or less, sol Al: 0.1 1% or less selected from the group consisting of Cr: 1.0 wt% or less, Ni: 1.5 wt% or less, Mo: 0.5 wt% or less, and Cu: 1.5 wt% or less. Containing more than one species,
The content of Si, Mn and P satisfies the relationship of 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0, and the balance is a steel slab composed of iron and unavoidable impurities. With Ar ₃ points (° C) or higher, Ar ₃
Hot rolling is completed in a rolling finishing temperature range of not more than the point (° C) + 100 ° C, and a temperature range from the temperature to at least 700 ° C is cooled at a cooling rate of 8 ° C / s or less, and the coil is cooled at a temperature of 650 ° C or less. And then cold rolled, then 750
A method for producing a high-strength cold-rolled steel sheet for deep drawing, characterized by performing continuous annealing at a temperature of at least ℃.