JPH0333774B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high-strength cold-rolled steel sheet for deep drawing, which has excellent press workability and has a tensile strength of 35 kg/mm ² or more, suitable for automobile panels, etc. This invention relates to cold-rolled steel sheets that are also used as base plates for surface-treated steel sheets. In recent years, with the aim of reducing the weight of automobile bodies and improving safety, there has been an increasing demand for high strength automobile panel materials, and the use of high-strength cold-rolled steel sheets with a tensile strength of 35 to 45 Kg/mm ² grade is becoming more important. is increasingly required. Recently, steel sheets for this type of use are increasingly being used as surface-treated steel sheets that have been subjected to surface treatments such as galvanizing in order to further improve corrosion resistance. By the way, it is generally known that improving the strength of steel sheets impairs press formability, but among the cold-rolled steel sheets for deep drawing that are used after press working, especially for automotive panel materials, extremely good pressability is required. Processability is required. Therefore, for this type of use, an important issue is how to combine high strength and excellent press formability. The following cold-rolled steel sheets have been known that meet the above-mentioned requirements to some extent. Firstly, so-called reflux steel, which is a low carbon aluminum killed steel with P added thereto, is known.
Although this rifosu steel has excellent deep drawability, it is not possible to obtain excellent material quality without box annealing, and sufficient material quality cannot be obtained by continuous annealing and continuous hot-dip galvanizing methods, which are highly productive. There were some drawbacks that I couldn't overcome. Second, a ferrite-martensite two-phase steel sheet is known, which is obtained by adding Mn, Cr, etc. to low carbon steel and performing a two-phase region annealing method. Although this steel plate has excellent stretch formability,
It has disadvantages such as poor deep drawability and galvanizing properties. Third, we added strong carbide-forming elements such as Ti and Nb to ultra-low carbon steel with a C content of 0.01% or less to obtain ultra-deep drawability, and solid solution reinforcement such as P and Si. Cold-rolled steel sheets that have high strength by adding elements are known, and examples of this type of steel sheet using Ti as a carbide-forming element are disclosed in Japanese Patent Publication No. 57945/1983 and Japanese Patent Application Laid-open No. 57-57- Examples using Nb are disclosed in Japanese Patent Publication No. 57-58427, Japanese Patent Application Laid-open No. 139654-1984, etc. This type of steel sheet has various advantages, such as excellent deep drawability and high strength, and can be manufactured by continuous annealing and continuous hot-dip galvanizing. However, recent automobile panel materials are required to have even better press formability, and in particular improved stretch formability. has not yet fully met the requirements. In other words, in recent years, panels have been integrated and the blank area has been reduced in order to improve efficiency and reduce material costs in the automobile manufacturing process, and this has created a demand for further improvements in the press formability of steel sheets. In particular, there is a strong demand for improvement in stretch formability, and for this purpose, improvement in elongation properties is required as a mechanical property, but the above-mentioned steels are still insufficient in terms of elongation properties. This invention was made in view of the above circumstances, and the purpose is to provide a high-strength cold-rolled steel sheet for deep drawing that has even better elongation properties than conventional cold-rolled steel sheets for deep drawing and has sufficient strength. It is something to do. In order to achieve the above-mentioned object, the present inventor has conducted various experiments and studies, and as a result, the present inventor has combinedly added P and Cu to Ti-containing aluminum killed steel, and the P, Cu
By adjusting the content within a specific range that correlates with each other, a cold-rolled steel sheet with an excellent strength-ductility balance, that is, a cold-rolled steel sheet with sufficient strength and good elongation properties, can be obtained. I discovered that
This led to this invention. Specifically, the high-strength cold-rolled steel sheets for deep drawing of the present application and the first invention contain C0.015% or less, Si0.10% or less,
Contains Mn0.20% or less, Al0.005-0.10%, and
0.03~0.150% P and 0.21% or more Cu, Cu
(%) + 13 x P (%) is in the range of 0.55 to 2.50, and further contains 0.002 to 0.10% Ti.
It is characterized by containing so that the value of Ti (%) / C (%) is 4 or more, and the balance consists of Fe and unavoidable impurities. By containing them in a specific relationship, it was possible to obtain a cold-rolled steel sheet with an excellent strength-ductility balance. Further, the cold-rolled steel sheet for deep drawing of the second invention is
In addition to the ingredients of the invention, B of 0.0050% or less,
It contains one or two types of Nb in the range of 0.002% or more and less than 3 × C (%), and by adding B and/or Nb in this way, the strength - While maintaining ductility balance,
The deep drawability expressed by the r value (Rankford value) was further improved. This invention will be explained in more detail below. First, to explain the experimental results that formed the basis of this invention, the present inventors determined that the basic composition was 0.004%.
C-0.01%Si-0.15%Mn-0.008%S-0.03%Al
Ti-containing aluminum killed steel with -0.05% Ti was added with various amounts of Cu and P, and its mechanical properties were investigated using JIS No. 5 test pieces when hot-rolled, cold-rolled and continuously annealed in a conventional manner. The thickness of the steel plate is 0.80mm. When we organized the relationship between tensile strength (TS) and total elongation (El) for these cold-rolled steel sheets with various P and Cu contents, we found that TS and
The relationship of El correlates with the amount of Cu (%) + 13 × P (%),
It has been found that by setting the value of Cu (%) + 13 x P (%) within the range of 0.55 to 2.50, a cold rolled steel sheet with an extremely excellent strength-ductility balance can be obtained. In other words, there is a general tendency for El to decrease as TS increases, but when the value of Cu (%) + 13 × P (%) is particularly within the range of 0.55 to 2.50, it is different from other cases. By comparison, it was found that El was large even with the same TS value, that is, the ductility was superior even with the same strength. As a result of further research, it was possible to obtain a high-strength cold-rolled steel sheet for deep drawing with a particularly excellent strength-ductility balance by using the steel composition as described above. Next, the reason for limiting the range of components in this invention will be explained. C: C is an element that inhibits the development of the {111} texture, which is a preferable crystal orientation for deep drawability, and must be contained at 0.015% or less in order to obtain a steel sheet with excellent deep drawability. Si: Si is a solid solution strengthening element that is effective in increasing the strength of steel sheets, but it is an element that significantly inhibits the chemical conversion treatment and galvanizing properties of steel sheets, and is used as a base plate for surface-treated steel sheets. In the cold-rolled steel sheet of the present invention, in which carbon dioxide is also used, the content must be 0.10% or less. Mn: Mn is also a solid solution strengthening element and is effective in improving the strength of steel sheets, but if it exceeds 0.20%, it will have a significant negative effect on deep drawability, so it must be kept at 0.20% or less. Al: 0.005% or more of Al is necessary for deoxidizing steel, but if it exceeds 0.10%, non-metallic inclusions will rapidly increase and the surface quality will deteriorate.
Must be within the range of ~0.10%. Ti: Ti is an element necessary to fix carbon in steel as TiC and improve deep drawability, and its weight ratio
The effect will not be exhibited unless the Ti/C ratio is 4 or more, but if it is added in excess of 0.10%, nonmetallic inclusions will rapidly increase and the surface quality will deteriorate. Also
Even if the absolute amount of Ti is less than 0.002%, the above-mentioned effect of adding Ti cannot be obtained. Therefore, it is necessary that Ti be within the range of 0.002 to 0.10% and that Ti/C be 4 or more. P, Cu: The combined addition of P and Cu is the most important point in this invention. Adding only one of these elements alone will not achieve the desired strength.
The effect of improving ductility balance cannot be obtained. If P is less than 0.03%, no effect can be obtained from the combined addition of P and Cu, and on the other hand, if P exceeds 0.150%, the spot weldability will be significantly deteriorated, so the P amount should be 0.03~
Must be within 0.150%. Also, Cu
If the amount is less than 0.21%, the effect of the combined addition of P and Cu cannot be obtained, so the amount of Cu needs to be 0.21% or more. Within this range of P amount and Cu amount, especially the value of Cu (%) + 13 × P (%) is 0.55 ~
By adjusting it within the range of 2.50, a high-tensile cold-rolled steel sheet for deep drawing with an excellent strength-ductility balance as shown in FIG. 1 can be obtained. If the value of Cu (%) + 13 × P (%) is less than 0.55,
If it exceeds 2.50, the balance between strength and ductility is poor, and especially the ductility becomes low even at the same strength. Although the mechanism by which such a combined addition effect of P and Cu is brought about is not yet clear, the combined addition of P and Cu particularly improves uniform elongation. It is thought that this has some effect of preventing localization of strain during deformation. In addition, conventional steels with only P added or steels with only Cu added were said to have problems such as brittleness and deterioration of surface properties, respectively, but in this invention, P and Cu are added in combination within the specified range as mentioned above. It has been confirmed that there are no problems with steel. By having the above-mentioned composition, it is possible to obtain a cold-rolled steel sheet for deep drawing having the desired properties with an excellent balance of strength and ductility. Also good. The reason is as follows. That is, by adding appropriate amounts of B and Nb, the deep drawability expressed by the Rankford value (r value) can be further improved while maintaining an excellent strength-ductility balance. However, B is 0.0050
%, the effect of B addition will not only be saturated, but also cause deterioration of deep drawability, so the amount of B is
The upper limit is 0.0050%. In addition, the effect is not recognized when Nb is less than 0.002%, but on the other hand, when it is more than 3×C (%)
Adding Nb not only saturates the effect of the addition, but also unnecessarily increases costs.
Must be within the range of 0.002% or more and less than 3×C (%). Note that even if either B or Nb is added alone,
Further, even if a compound is added, the effect can be obtained as long as the composition is within the above composition range. Steel having the above composition can be manufactured as a cold-rolled steel sheet by a normal process, and preferred manufacturing conditions will be described below. First, in order to reduce C in the steelmaking process, a combination of converter steelmaking and degassing treatment is desirable. Next, in the casting process for producing steel slabs, a continuous casting method is preferable from the viewpoint of cost and homogeneity. In the hot rolling process, a method of reheating the steel slab may be adopted, or direct hot rolling may be used. Alternatively, a method of directly melting a thin steel piece with a thickness of 100 mm or less and directly hot rolling it may be used.
The soaking temperature during hot rolling is preferably 1200 to 800°C, particularly preferably 1050 to 850°C. The hot rolling finishing temperature is preferably 950 to 600℃, and the coiling temperature is
750-200°C is suitable. The obtained hot rolled steel strip is cold rolled at a reduction rate of 50% or more after pickling, and annealed at a temperature higher than the recrystallization temperature.The annealing method is determined from the viewpoint of production method. Preferably, continuous annealing is applied. After annealing, it is possible to perform temper rolling at a rolling reduction of 2% or less for the purpose of shape correction, etc. The above are suitable conditions for manufacturing the cold rolled steel sheet of the present invention as a normal cold rolled steel sheet. It goes without saying that the present invention can also be applied to original sheets of various surface-treated steel sheets, such as electrogalvanized steel sheets and aluminized steel sheets. Examples of the present invention will be described below along with comparative examples. Steels having the compositions shown in sample numbers 1 to 10 in Table 1 were melted by a continuous furnace RH degassing method, and steel slabs were obtained by a continuous casting method. Heating and soaking each steel piece at 950-1050℃,
Hot rolling was carried out according to a conventional method to obtain a hot rolled steel strip having a thickness of 2.8 to 3.2 mm. After pickling the steel strip, it was cold-rolled into a cold-rolled steel strip with a thickness of 0.8 mm, and the cold-rolled steel strip was continuously annealed at a soaking temperature of 790 to 830°C.
Temper rolling was carried out at a rolling reduction of 0.3 to 0.7%. Mechanical properties of the obtained steel plate (yield strength: YS, tensile strength: TS, total elongation: El, average Rankford value:
r) is shown in Table 2.

【table】

【table】

【table】

[Table] As is clear from Table 2, the steels of sample numbers 2, 3, and 4 within the composition range of this invention all have an excellent strength-ductility balance and a high tensile strength with a high r value.
It is clear that it is a high strength steel of 35-45Kg/mm class ² . In addition, the steels of sample numbers 8, 9, and 10 are those to which B and/or Nb are added, and in these steels, further improvement in material quality, especially improvement in r value, is observed. Note that comparative steel 1 of sample number 1 has a high C content, but in this case, deep drawability (r value) is inferior. In addition, all of the steels of sample numbers 5 to 7 have addition amounts of P and Cu that are outside the range of the present invention, and those with high strength have low ductility, and those with high ductility have low strength, and the steels with high strength have low strength. The steel plate has a poor ductility balance. As is clear from the above description, the high-strength cold-rolled steel sheet for deep drawing of the present invention has an excellent balance of strength and ^ductility . It has good properties and excellent stretch formability, making it ideal for automotive panel materials, etc.

[Brief explanation of the drawing]

FIG. 1 is a correlation diagram showing the relationship between tensile strength (TS) and total elongation (El) corresponding to the value of Cu (%) + 13 × P (%).

Claims (1)

[Claims] 1 C0.015% (weight%, the same applies hereinafter) or less, Si0.10
% or less, Mn 0.20% or less, Al 0.005-0.10%, and 0.03-0.150% P and 0.21% or more
Contains Cu in a range where the value of Cu (%) + 13 × P (%) is 0.55 to 2.50, and further contains 0.002 to 0.10% Ti with a value of Ti (%) / C (%) of 4 or more. 1. A high-strength cold-rolled steel sheet for deep drawing, characterized in that the content is within a range such that the remainder is Fe and unavoidable impurities. 2 Contains 0.015% or less of C, 0.10% or less of Si, 0.20% or less of Mn, 0.005 to 0.10% of Al, and 0.03 to 0.150
% P and 0.21% or more Cu, Cu (%) + 13×
Contains within a range where the value of P (%) is 0.55 to 2.50,
Furthermore, it contains 0.002 to 0.10% Ti within a range where the Ti (%) / C (%) value is 4 or more, and
B of 0.0050% or less and 3×C (%) of 0.002% or more
A high-strength cold-rolled steel sheet for deep drawing, characterized in that it contains one or two types of Nb within a range of less than or equal to or less than 1,000 yen, and the remainder is Fe and unavoidable impurities.