JPH06330238A - Cold rolled steel sheet for working excellent in baking hardenability and weather resistance - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet having extremely good workability, baking hardenability and good weather resistance. CONSTITUTION:This steel sheet has a compsn. contg., by weight, 0.010 to 0.038% C, <=0.04% Si, 0.05 to 0.25% Mn, 0.010 to 0.120% P, 0.002 to 0.020% S, 0.05 to 0.50% Cu, 0.010 to 0.10% Al and <=0.0060% N, and the balance Fe with inevitable impurities, has a particle-regulated structure in which the average particle size 15 regulated to 10 to 45mum and is incorporated with solid solution carbon in the range of 2.5 to 8.0ppm. In this way, the cold rolled steel sheet for working excellent in baking hardenability of >=4kgf/mm<2> and weather resistance can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet having excellent bake hardenability and weather resistance. More specifically, it has a characteristic that the yield point increases and hardness increases after coating baking treatment after press working. It relates to a cold-rolled steel sheet which has been significantly improved and further has weather resistance.

[0002]

2. Description of the Related Art Conventionally, cold-rolled steel sheets used for outer panels and parts of automobiles have been required to have high deep drawability and bulgeability. Therefore, Ti-added ultra-low carbon steel, Nb-added ultra-low carbon steel, etc., in which elements that sufficiently reduce the carbon content in steel and further fix solid solution carbon are added, are used as raw materials for these parts. There are many.

On the other hand, due to the recent demand for energy saving, it has become possible to reduce the weight of a finished product by using a steel plate having a thickness thinner than that of a conventional one as a component. In this case, it becomes necessary to compensate for the strength decrease due to the decrease in plate thickness by some method. As a method therefor, there has been considered a method of increasing the yield strength of a finished product by utilizing an increase in yield point (hereinafter referred to as BH) caused by passing through a coating drying line after press molding (generally 170 to 200 ° C.). This utilizes the strain age-hardening property resulting from the interaction between C and N that form a solid solution in the steel sheet and dislocations in the steel, but in recent years, B
The H property requires a large increase of approximately 4 kgf / mm ² or more.

Steel sheets having such a BH property have been studied so far. For example, in the above-mentioned Ti-added ultra-low carbon steel, Nb-added ultra-low carbon steel, etc., JP-A-2-197549 and JP-A-63 are used. As disclosed in JP-A-247338, a steel sheet having a BH property can be manufactured by setting the amount of C, Ti or Nb in the steel to a specific range and annealing the steel under specific conditions. However, the BH steel sheet based on this ultra-low carbon steel is liable to change in the amount of solid solution C between lots due to slight differences in the carbide forming elements such as Ti and Nb or slight fluctuations in the annealing temperature, or even between individual lots. There is a problem that it is difficult to produce a stable BH amount between the two coils. Further, since the total amount of carbon is small, the amount of BH itself is not so high, and it is currently about 3 kgf / mm ² as disclosed in JP-A-2-197549.
Furthermore, these ultra-low carbon steels have a problem that the decarburization cost in steelmaking and the addition cost of Ti, Nb, etc. are high.

Further, as a method for manufacturing a steel sheet which reduces the cost in steel making and obtains good workability, Japanese Patent Laid-Open No. 2-9
As disclosed in Japanese Patent No. 3025, there is a method for producing a low carbon aluminum killed steel by continuous annealing, but when this method does not always provide a sufficient amount of BH or when the crystal grains become coarse, the surface quality deteriorates. There are cases. There are various plating methods in order to improve the weather resistance of the cold-rolled steel sheet for processing, but this causes a cost increase and is disadvantageous in that the price is high. In recent years, there has been a demand for a steel sheet which is inexpensive and has weather resistance without plating, which increases the price. Steel plates developed from such a viewpoint include P, Cu, as disclosed in, for example, Japanese Patent Application Laid-Open No. 3-2321.
There is a hot-rolled steel sheet containing a large amount of Cr, Ti, etc. and having high strength and weather resistance. This high weather resistance steel material rusts like the ordinary steel in the first period when it is exposed to the atmosphere, but with the passage of time, part of the rust gradually becomes extremely dense and adheres to the base material. The rust layer serves as a protective film against the environment and prevents corrosion from progressing, thereby maintaining weather resistance. It is well known that such rust can be caused by containing a small amount of P, Cu, Cr, etc.,
The structure of the rust layer is, for example, "Iron and Steel 55 (1969) p.
355 "and the like.

However, although the steel sheet produced by this method has high strength, it has a problem that it cannot be sufficiently formed when it is processed by pressing or the like. Further, the steel sheet manufactured by this method cannot sufficiently impart BH property. From the above circumstances, there has been a long-awaited demand for a cold-rolled steel sheet for working which is inexpensive and has a high BH content and which has weather resistance.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a cold rolled steel sheet for working which has high bake hardenability and high weather resistance.

[0008]

Means for Solving the Problems The inventors of the present invention have made various studies on the component composition, structure and solid solution C of a steel sheet, and as a result, have easily made a cold-rolled steel sheet for working excellent in bake hardenability and weather resistance And found that it can be supplied economically. In order to enable processing such as pressing, it is necessary to make the _rave value and elongation of the steel sheet sufficiently high. Furthermore, as a result of examining the amounts of various elements contained in the steel, the structure of the steel sheet, and the like in order to impart weather resistance without impairing the workability, the following has been clarified.

The gist of the present invention is that C: 0.010 to 0.038%, Si: 0.04% or less, Mn: 0.05 to 0.25%, P: 0. 010-
0.120%, S: 0.002-0.020%, Al:
0.010 to 0.10%, Cu: 0.05 to 0.5%,
N: contains 0.0060% or less, the balance contains Fe and unavoidable impurities, and has an average crystal grain size of 10 to 45 μm.
2.5 to 8.0 pp of solid solution carbon
It is a cold-rolled steel sheet for working excellent in bake hardenability and weather resistance, characterized in that it is contained within the range of m.

The present invention will be described in detail below. C
The higher the content, the lower the ductility. Therefore, the smaller the amount of C, the better the press workability. However, C <0.01
In the range of 0%, there is a problem that the cost of decarburization becomes high, and it is difficult to control the amount of cementite in the steel, and proper solute C cannot be obtained. Further, if the C content exceeds 0.038%, the ductility and deep drawability of the product deteriorate significantly. Therefore, in consideration of both workability and BH property, the C content is 0.01
It was limited to 0 to 0.038%.

Although a small amount of Si causes no problem, if the content of Si exceeds 0.04%, the workability is deteriorated. Therefore, S
The amount should be 0.040% or less. Mn is a component necessary to prevent hot brittleness, but 0.05%
If it is less than FeS, FeS is generated and its effect is not obtained. Also, 0.2
If it exceeds 5%, the deep drawability deteriorates. Therefore, the Mn content is limited to 0.05 to 0.25%.

When P coexists with Cu, it has the effect of forming stable rust and improving weather resistance. In order to exert this effect, 0.010% or more is required. However, if it is more than 0.120%, the ductility is largely reduced and the workability is deteriorated. Therefore, the amount of P should be 0.010 to 0.120.
Limited to%. If S is less than 0.002%, the amount of MnS produced is small, and the crystal grains of the hot-rolled sheet are likely to become coarse, causing rough skin. Further, if it exceeds 0.020%, it causes hot brittleness. Therefore, the S amount is limited to 0.002 to 0.020%.

Al is an element necessary for controlling the amounts of oxygen and nitrogen in steel, and at least 0.010% is necessary for precipitating N as AlN after winding the hot-rolled sheet.
However, if it exceeds 0.10%, the workability is deteriorated. Therefore, it is limited to 0.010 to 0.10%. In particular, in order to improve the workability by coarsening the size of precipitated AlN to such an extent that the cold-rolled steel sheet does not become rough, the workability is improved by 0.030.
The range of 0.080% is desirable.

Further, if a large amount of precipitated AlN deteriorates workability, it is preferable that the amount thereof be small. Therefore, the amount of N must be 0.0060% or less. Cu is an element necessary for coexisting with P to form a stable rust layer and impart weather resistance. The present inventors have found that in order to give a steel sheet sufficient weather resistance in a normal atmospheric environment, 0.05% or more is necessary. But,
If it exceeds 0.50%, the ductility decreases and the workability deteriorates, which is not preferable. Therefore, the Cu content is 0.05 to 0.5.
Limited to 0%.

The cold-rolled steel sheet for working needs to have a high r _ave value and ductility, and at the same time, its surface condition must be kept beautiful after being worked by pressing or the like. For this purpose, the present inventors have found that it is necessary to satisfy the above-described component conditions and also satisfy the following crystal grain conditions. That is, in order to have good formability, the crystal grains of the cold-rolled steel sheet must be sized, and the average grain size must be 10 μm or more. Average particle size is 10 μm
When it is less than 1, the r _ave value and the ductility are lowered. However, if the average particle diameter exceeds 45 μm, roughening of the surface after processing tends to occur. Therefore, the crystal grains of the steel sheet must have a sized structure and the average crystal grain size must be in the range of 10 to 45 μm. FIG. 2 shows the effect of the crystal grain size on the r _ave value and the surface properties when the average crystal grain size is changed by changing the annealing temperature and the annealing time for the steels having the compositions shown in Table 1. As is clear from FIG. 2, the average crystal grain size is 10 to 45.
It can be seen that good press formability and surface quality can be obtained at μm.

[0016]

[Table 1]

In the above investigation, it was found that good press moldability, bake hardenability, and surface properties can be obtained even when the composition is changed within the range of the present invention. Next, the present invention is characterized in that solid solution C is required to be used in order to impart a high BH property to the steel plate, and this must be within a specific range. Although the BH property increases as the amount of solute C in the steel sheet increases,
Material deterioration due to aging at room temperature becomes large and workability deteriorates. Within the chemical composition range of the present invention, if the amount of dissolved C is 8.0 ppm or less, deterioration of the material due to aging at room temperature does not pose a problem. However, the amount of dissolved C is 2.5 pp
If it is less than m, the BH property becomes low. Therefore, the amount of solid solution C must be in the range of 2.5 to 8.0 ppm. Fig. 1 shows the yield point after 6 months at room temperature when the solid solution C content was changed by changing the overaging time when hot-rolling and cold-rolling continuous annealing of steels having the chemical compositions shown in Table 1. It shows the effect of the amount of solid solution C on the elongation amount (%) and the BH amount. As is clear from FIG. 1, the amount of dissolved C is 2.5 to
Within the range of 8.0 ppm, aging deterioration is small and a high BH amount can be obtained.

As described above, according to the present invention, the composition of the cold rolled steel sheet,
By setting the crystal grain size and the amount of solute carbon in a specific range, it is possible to obtain excellent bake hardenability and weather resistance. Hereinafter, examples of the present invention will be shown together with comparative examples.

[0019]

EXAMPLE Steels having compositions shown in Tables 2 and 3 (continued from Table 2) were made into cold-rolled steel sheets by ordinary hot rolling, cold rolling and continuous annealing steps. Samples 1 to 14 are all within the scope of the present invention in terms of composition, average crystal grain size, and amount of solid solution C.
Any one or more of the composition, the average crystal grain size, and the amount of dissolved C is outside the scope of the present invention. The average crystal grain size was variously changed by the annealing temperature and the annealing time in the continuous annealing, and the solute C amount was variously changed by the overaging temperature and the overaging time.

The obtained cold rolled steel sheet was subjected to a tensile test using JIS No. 5 to measure its r _ave value, ductility and bake hardenability and to observe its surface properties. Furthermore, in order to evaluate the weather resistance in normal air, the steel sheet was left in a wet environment (humidity 85%, 50 ° C. × 15.5 hr) → dry (70 ° C. × 3 hr) → salt water immersion (50 ° C. × 50 ° C. × 3 hr). A corrosion acceleration test was performed in which a cycle consisting of 2 hr) → standing indoor (2 hr) → salt spray (1.5 hr) was repeated 15 times. The weather resistance was judged by observing the surface state of the steel sheet after this accelerated test. Table 4 shows the above test results.

As can be seen from Tables 2, 3 and 4, in order to obtain good workability, bake hardenability, surface properties and weather resistance, the chemical composition, average crystal grain size and solid solution carbon within the scope of the present invention. It is clear that an amount is needed. The average crystal grain size is changed by the annealing temperature and the annealing time in this example, but the composition, the heating temperature of the slab, the winding temperature after hot rolling, the cold rolling rate, the heating rate during annealing, and the cooling rate. It can be changed by The amount of solute carbon mainly changes depending on the total amount of carbon and the annealing conditions, but the annealing method is not particularly limited, and may be the continuous annealing method as in this example, or may be the box annealing method, for example. good.

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

[0025]

As described above, the cold-rolled steel sheet of the present invention in which the composition of steel, the average crystal grain size and the amount of solute carbon are within the specified ranges are remarkably excellent in bake hardenability, weather resistance and workability. Are better.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the amount of solute carbon on yield point elongation and bake hardenability after aging at room temperature.

FIG. 2 is a diagram showing an influence of an average crystal grain size on a surface texture and a r _ave value.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Teruaki Yamada 1 Fuji-machi, Hirohata-ku, Himeji-shi, Hyogo Shin Nippon Steel Co., Ltd. Hirohata Works

Claims (1)

[Claims] 1. A weight ratio of C: 0.010 to 0.038.
%, Si: 0.04% or less, Mn: 0.05 to 0.25
%, P: 0.010 to 0.120%, S: 0.002
0.020%, Al: 0.010 to 0.10%, Cu:
0.05 to 0.5%, N: 0.0060% or less, the balance contains Fe and unavoidable impurities, has a sized structure with an average crystal grain size of 10 to 45 μm, and forms a solid solution. A cold-rolled steel sheet for working, which is excellent in bake hardenability and weather resistance, characterized by containing carbon in the range of 2.5 to 8.0 ppm.