JPH08134590A - Cold rolled steel sheet for working, excellent in surface characteristic, baking hardenability, and corrosion resistance - Google Patents

Abstract

PURPOSE: To produce a cold rolled steel sheet for working, excellent in surface characteristic, baking hardenability, and corrosion resistance by specifying a composition consisting of C, Si, Mn, P, S, Al, Cu, N, Ni, and Fe and also specifying crystalline grain size and solid-solution carbon content, respectively. CONSTITUTION: This cold rolled steel sheet for working has a composition consisting of, by weight, 0.010-0.045% C, <=0.04% Si, 0.04-0.25% Mn, 0.008-0.120% P, 0.002-0.020% S, 0.010-0.10% Al, 0.05-0.6% Cu, <=0.0060% N, 0.1×Cu to 1.5% Ni, and the balance Fe with inevitable impurities and also has a graded structure of 10-45μm average crystalline grain size and contains 2.5-8.0ppm solid- solution carbon. In this steel sheet, the characteristic of raising yield point and increasing hardness after press working and baking finish treatment can be remarkably improved.

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 surface properties, bake hardenability, and corrosion resistance. More specifically, the yield point of the cold rolled steel sheet is increased after press baking after press working. It relates to a cold-rolled steel sheet for working which has a significantly improved hardening property and further has corrosion resistance.

[0002]

2. Description of the Related Art Due to the recent demand for energy saving of automobiles,
It has become possible to reduce the weight of finished products by using steel plates that are thinner than before as parts. in this case,
It is necessary to compensate for the strength decrease due to the decrease in plate thickness by some method. As a method thereof, 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 (generally 170 to 200 ° C.) after press molding. . this is,
It utilizes strain age hardening due to the interaction between C and N that form a solid solution in the steel sheet and dislocations in the steel.
The BH property requires a large increase of approximately 4 kgf / mm ² or more.

A steel sheet having such a BH property has been studied so far. For example, in the above-mentioned Ti-added ultra-low carbon steel and Nb-added ultra-low carbon steel, Japanese Unexamined Patent Publication No. Hei 2-197549 and Japanese Unexamined Patent Publication No. Sho. As disclosed in Japanese Patent Laid-Open No. 63-247338, it is possible to manufacture a steel sheet having BH property by setting the amount of C, Ti or Nb in the steel within a specific range and annealing the steel under specific conditions. However, BH based on this ultra low carbon steel
The steel sheet forms a solid solution C due to slight differences in the carbide forming elements such as Ti and Nb, or slight fluctuations in the annealing temperature.
There is a problem that the amount is easily changed and it is difficult to obtain a stable BH amount between each lot or between one coil. Further, since the total amount of carbon is small, the amount of BH itself is not so high, and it is the present condition that it is 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 of manufacturing a steel sheet which reduces the cost in steel making and obtains good workability, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent Publication No. 93025, there is a method of producing a low carbon number aluminum killed steel by continuous annealing, but when this method does not always provide a sufficient BH amount,
As the crystal grains become coarse, the surface quality may deteriorate.

Next, there are various plating methods for improving the corrosion resistance of the cold-rolled steel sheet for working, but this method has a drawback that it causes an increase in cost and the cost is high. In recent years, there has been a demand for a steel sheet that is inexpensive and has corrosion resistance without plating, which would increase the price. The steel sheet developed from this point of view contains a large amount of P, Cu, Cr, Ti, etc. to have high strength and weather resistance, as disclosed in, for example, Japanese Patent Laid-Open No. 3-2321. There are hot rolled steel sheets. This high weather resistance steel material rusts like the ordinary steel in the first period when exposed to the atmosphere, but with the passage of time, a part of the rust gradually and extremely closely 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 a rust can be formed by containing a small amount of P, Cu, Cr, etc., and the structure of the rust layer has been clarified in documents such as "Iron and Steel" 55 (1969), p355, etc. ing. However, although the steel plate manufactured by this method has high strength, when processed by pressing or the like,
There is a problem that molding cannot be performed sufficiently. Further, the steel sheet produced by this method has a problem that sufficient BH property cannot be imparted, cracking due to Cu is likely to occur during hot rolling, and it is difficult to obtain high surface quality.

[0006] From the above circumstances, B is inexpensive and expensive.
There has been a long-awaited demand for a corrosion-resistant cold-rolled steel sheet capable of obtaining an H content.

[0007]

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

[0008]

Means for Solving the Problems As a result of various studies on the composition, structure and solid solution C of the steel sheet, the present inventors have found that a cold rolled steel sheet having excellent surface properties, bake hardenability and corrosion resistance can be easily obtained. It has been found that it can be supplied economically and economically. In order to perform processing such as pressing, it is necessary to increase the ductility of the steel sheet, and as a result of examining the amounts of various elements contained in the steel in order to combine surface properties, bake hardenability and corrosion resistance, It became clear.

That is, the gist of the present invention is that C: 0.010 to 0.045% and Si: 0.0 by weight.
4% or less, Mn: 0.04 to 0.25%, P: 0.00
8 to 0.120%, S: 0.002 to 0.020%, A
1: 0.010 to 0.10%, Cu: 0.05 to 0.6
%, N: 0.0060% or less, and 0.1 ×
It contains Ni of Cu% or more and 1.5% or less, the balance is Fe and unavoidable impurities, and has an average crystal grain size of 10 to 4
It has a sized structure of 5 μm and contains 2.5 to 8% of solid solution carbon.
The cold-rolled steel sheet for working is excellent in surface properties, bake hardenability, and corrosion resistance, characterized by being contained in the range of 0 ppm.

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 0.045%, the ductility and deep drawability of the product are significantly deteriorated.
Therefore, considering both the workability and the BH property, the C content is set to 0.
It was limited to 010 to 0.045%.

Although a small amount of Si causes no problem, if the content of Si exceeds 0.04%, the workability is deteriorated. Therefore, S
The i content must be 0.04% or less. Mn is a component necessary to prevent hot brittleness, but 0.04%
If it is less than FeS, FeS is generated and the effect is not obtained. Also, 0.
If it exceeds 25%, the deep drawability deteriorates. Therefore, the Mn content is limited to 0.04 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, addition of 0.008% or more is necessary. However, if it is more than 0.120%, the ductility is largely lowered and the workability is lowered. Therefore, the amount of P is 0.008-0.
Limited to 120%. If S is less than 0.002%, MnS
Is small and the crystal grains of the hot-rolled sheet are likely to become coarse, which causes rough skin. Further, if it exceeds 0.020%, it causes hot brittleness. Therefore, the S amount is 0.002-0.0
Limited to 20%.

Al is an element necessary for controlling the amounts of oxygen and nitrogen in the steel, and N is replaced by A after winding the hot-rolled sheet.
A minimum of 0.010% is required for precipitation as 1N. However, if it exceeds 0.10%, the workability is deteriorated. Therefore, the amount of Al is limited to 0.010 to 0.10%. In particular, the range of 0.030 to 0.080% is preferable in order to coarsen the size of precipitated AlN to the extent that roughening of the cold-rolled steel sheet does not occur and improve workability.

Further, if a large amount of precipitated AlN deteriorates the workability, it is preferable that the amount thereof be small. For that purpose, the amount of N must be 0.0060% or less. Cu coexists with P to form a stable rust layer and is an element necessary for imparting corrosion resistance. The inventors of the present invention have found that 0.05% or more is necessary for the steel sheet to have sufficient corrosion resistance in a normal atmospheric environment. However, if it exceeds 0.60%, the ductility is lowered and the workability is lowered, which is not preferable. Therefore, the Cu content is 0.05 to
It was limited to 0.6%.

Ni is the surface of the steel sheet that is apt to occur when processing such as hot rolling when producing a steel sheet containing Cu,
It is an element necessary to prevent cracks at the edge, tip, and rear. It has been found by the present inventors that Ni has an effect of preventing cracking during hot rolling by containing 1/10 or more by weight of Cu content. However, if the Ni content exceeds 1.5%, the ductility decreases. Therefore, the amount of Ni is limited to the range of 0.1% Cu% or more and 1.5% or less.

The steel sheet of the present invention is characterized by having a high BH content by having a solid solution C within a specific range.
The higher the amount of solid solution C in the steel sheet, the higher the amount of BH, but the deterioration of the material due to aging at room temperature becomes large and the workability deteriorates. The chemical composition is within the range of the present invention, and the amount of dissolved C is 8.0.
When the content is less than or equal to ppm, deterioration of the material due to aging at room temperature is very small and does not pose a problem. However, the amount of dissolved C is 2.
When it is less than 5 ppm, the amount of BH 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 amount of BH when the steel having the chemical composition shown in Table 1 is hot-rolled, continuously cold-rolled and then continuously annealed, and the amount of solid solution C is changed by changing the overaging time, and after 6 months at room temperature. It shows the effect of the amount of solid solution C on the ductility of. As is clear from FIG. 1, the amount of dissolved C is 2.5 to 8.0 pp.
It can be seen that in the range of m, a high BH amount is secured and ductility deterioration due to aging is small. It was found that similar results were obtained when the composition of the steel was changed within the scope of the present invention.

[0017]

[Table 1]

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 solid solution carbon within the specific ranges, 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 A slab having a composition shown in Tables 2 and 3 (continued from Table 2) was heated to 1250 ° C., hot rolled, and then cold rolled and continuously annealed to obtain a cold rolled steel sheet. Sample 1
No. 14 to 14 are within the scope of the present invention in terms of composition, average crystal grain size, and amount of solid solution C, and Samples 15 to 28 are any one or more of composition, average crystal grain size, and amount of solid solution C of the present invention. Out of range. The solute C was variously changed depending on the overaging temperature and the overaging time in continuous annealing.

With respect to the obtained cold rolled steel sheet, the average r value, ductility and bake hardenability were measured and the surface properties were observed. Furthermore, in order to evaluate the weather resistance in normal air, the steel sheet was left in a wet environment (humidity 85%, 50%
℃ × 15.5hr) → drying (70 ° C × 3hr) → salt water immersion (50 ° C × 2hr) → left indoors (2hr) → saltwater spray (1.5hr)] repeated corrosion cycle 15 times went. The corrosion resistance was determined by observing the surface state of the steel sheet after this accelerated test.

Table 4 shows the above test results. Table 2 to 4
As can be seen from the above, good average r value, ductility, surface texture,
It is clear that the chemical composition and the amount of solute carbon within the scope of the present invention are necessary to obtain the bake hardenability and the corrosion resistance. 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 in addition to this, for example, a box annealing method. 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 has good surface properties, bake hardenability, and weather resistance. And has processability.

[Brief description of drawings]

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

Claims (1)

[Claims] 1. A weight ratio of C: 0.010 to 0.04.
5%, Si: 0.04% or less, Mn: 0.04 to 0.2
5%, P: 0.008 to 0.120%, S: 0.002
~ 0.020%, Al: 0.010 to 0.10%, C
u: 0.05 to 0.6%, N: 0.0060% or less, further 0.1% Cu% or more and 1.5% or less Ni, and the balance Fe and unavoidable impurities. , Having an average grain size of 10 to 45 μm, and containing solute carbon in the range of 2.5 to 8.0 ppm, is excellent in surface properties, bake hardenability, and corrosion resistance. Cold rolled steel sheet for processing.