JPH03226526A - Production of cold rolled steel sheet for working excellent in blankability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet for working excellent in blankability by subjecting a slab of a steel in which a composition consisting of C, Si, Mn, S, Al, B, and Fe and B concentration in the surface layer part are specified, respectively, to hot rolling, to cold rolling, and then to annealing in a specific atmosphere. CONSTITUTION:In a slab of a steel having a composition which consists of, by weight, <=0.05% C, <=2.0% Si, <=2.0% Mn, <=0.03% S, <=0.15% Al, <=0.0050% B, and the balance essentially Fe and in which, if necessary, <=0.2% P, one or more kinds among 0.001-0.1% each of Ti, Nb, Zr, V, Mo, and Ca, and one or more kinds among 0.005-10.0% each of Cr, Ni, and Cu are further added and blended, average B concentration in the surface layer part between the surface, at least of one side, and the part at a depth 1% of the slab thickness is regulated to a value >=1.5 times the B concentration in the central part of the slab thickness. This steel slab is subjected to hot rolling and cold rolling by the conventional methods. Then, the resulting cold rolled sheet is annealed in a nonoxidizing atmosphere of <=-10 deg.C dew point. By this method, the occurrence of burr can be effectively reduced without adversely affecting other properties, and the cold rolled steel sheet for working excellent in blankability can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a cold-rolled steel sheet for processing which has excellent punchability and is suitable for use in automobile exterior panels and the like.

(Prior Art) Cold-rolled steel sheets for processing used as exterior plates for automobiles and home appliances are required to have not only formability but also punchability. This is because most fence boards of this type are stamped in some way.

A particularly important property in terms of punching workability is that the generation of burrs at F4 during punching is small. This is because if the burrs are large, when pressed products are stacked,
This is because not only the surface of the steel sheet is damaged and the corrosion resistance is deteriorated by these burrs, but also the amount of iron powder during pressing increases, inducing star-like surface defects.

For example, in JP-A-1-230748, S is added to Ti-added low carbon steel by 0.050 to 0.150 ivt% (
Although it has been proposed to increase the S content by increasing the S content (hereinafter simply expressed in %), such high S steel is inferior in large-scale corrosion resistance.

In addition, in Japanese Patent Application Laid-open No. 1-255626, 400
Adding a desired strain of 2 to 6% below °C,
By performing nitriding treatment at a temperature range of 800°C,
Although attempts have been made to increase the hardness of the surface layer, this method has the disadvantage that the surface layer becomes too hard, resulting in large wear and damage to the mold during continuous pressing.

(Problems to be Solved by the Invention) The present invention advantageously solves the above problems, and improves punchability by effectively reducing the occurrence of burrs without adversely affecting other properties. The purpose of this paper is to propose an advantageous manufacturing method for excellent cold-rolled steel sheets for processing.

(Means for Solving the Problems) As a result of intensive research to solve the above problems, the inventors have found that they form a B-enriched layer on the surface layer of a steel billet before hot rolling, and It has been found that performing the subsequent annealing in a non-oxidizing atmosphere with a dew point of 10° C. or lower is extremely effective in achieving the intended purpose.

This invention is based on the above knowledge.

That is, the gist of the present invention is as follows.

(1) C: 0.05% or less, Si: 2.0% or less, Gate n: 2.0% or less, S: 0.03% or less, Al: 0.15% or less, and B: 0.0050%. A steel piece containing: After hot-rolling and then cold-rolling a steel billet with a temperature of at least 1.5 times the dew point of the atmospheric gas,
A method for producing a cold-rolled steel sheet for processing with excellent punchability, which comprises annealing in a non-oxidizing atmosphere at a temperature of 10° C. or less (first invention).

(2) In the first invention, additional P: 0.2% in the material
A method for manufacturing a cold-rolled steel sheet for processing with excellent punchability (second invention), which is obtained by adding and blending the following:

(3) In the first or second invention, the material further contains Ti: 0.001-0.1%, Nb: 0.001-0.1%, Zr: 0.001-0.1%, V: 0.001-0.1%, Mo: 0.001-0.1% and Ca: 0.
A method for producing a cold-rolled steel sheet for processing with excellent punchability, which comprises adding at least one selected from 0.001 to 0.1% (third invention).

(4) In the first, second or third invention, the material further contains Cr: 0.005-10.0%, Ni: 0.005-10.0% and Cu: 0
．． A method for producing a cold-rolled steel sheet for processing with excellent punchability, which comprises adding at least one selected from 0.005 to 10.0% (fourth invention).

The experimental results that formed the basis of this invention will be described below.

C: 0.003%, Si: 0.01%, Mn:
0.1%, P: 0.012%, S: 0.013%
, Al: 0.03%, Ti: 0.04% and B: 0.0003%, and 820. By adding , a concentrated layer of B is formed on the slab surface, and the average B concentration in the surface layer from the surface to a depth of 1% of the plate thickness: 0.0006% B concentration in the center of the plate thickness: 0.0002% The Toshida steel slab is hot rolled and then cold rolled according to the conventional method.
．． After forming a cold-rolled sheet with a thickness of 8 mm, it was continuously annealed in a mixed gas atmosphere of H2 and N2 (810°C) with various dew points, and then temper-rolled at a rolling reduction of 0.7%.

FIG. 1 shows the results of an investigation of the relationship between the burr height and the atmospheric dew point when the cold-rolled sheet thus obtained was punched into a disk having a diameter of 32 au.

As is clear from the figure, when the atmospheric dew point is below 110°C, the burr height is significantly reduced compared to the conventional case.

(Function) Next, in this invention, the reason why the component composition of the material is limited to the above range will be explained.

C: 0.05% or less The less C, the better the deep drawing strength.
It is preferable to reduce the contamination as much as possible, but 0.05%
The following are acceptable.

Si: 2.0% or less Si has the effect of strengthening the steel and is added in the required amount depending on the desired strength, but if the amount added exceeds 2.0%, it will have a negative effect on deep drawability. It needs to be 2.0% or less.

Mn: 2.0% or less Mn, like Si, has the effect of strengthening steel, and is added in the necessary amount depending on the desired strength.
If it exceeds 2.0%, it will have a negative effect on deep drawability, so it needs to be 2.0% or less.

S: 0.03% or less S is preferably reduced as much as possible from the viewpoint of deep drawability and large crack resistance, but it is permissible up to 0.03% or less.

Al: 0.15% or less Al is useful as a deoxidizing agent and an element for fixing N precipitation, but if the content exceeds 0.15%, no improvement in its effectiveness can be expected; .15% or less.

B: 0.0050% or less In this invention, B, especially the surface layer B, is the most important.
Content: O, ooso% or less, on at least one side, an average of 81 degrees in the surface layer from the surface to a depth of 1% of the plate thickness is 1.0% of the B concentration in the center of the plate thickness.
It is important to make it 5 times or more.

The reason why the overall B content is set to 0.0050% or less is because adding a large amount exceeding this value will lead to deterioration of workability, and the average B concentration in the surface layer is The reason why the B concentration is set to be 1.5 times or more than the B concentration at the center of the thickness is that if the B concentration is less than 1.5 times, no matter how much the atmospheric dew point is adjusted in the annealing process, a satisfactory reduction in swelling cannot be expected.

P: 0.2% or less P is an effective element for obtaining high strength without adversely affecting punching properties, but if it exceeds 0.2%, it not only becomes too hard but also becomes brittle. % or less.

Ti+ Nb+ Zr, V, Mo, Ca: 0.
001-0.1% Each of the above elements is a useful element that improves workability by forming carbides, nitrides, and sulfides, but if the content is less than 0.001%, the effect of addition will be reduced. On the other hand, if it exceeds 0.1%, it becomes hard.
0.001 to 0.001 when used alone or in combination
It was supposed to be added in a range of 0.1%.

Cr, Ni, Cu: 0.005-10.0
%Each of the above elements contributes to the improvement of corrosion resistance, but if the amount is less than 0%, the effect will be poor, and even if the amount is less than 10.0%, the effect will be poor.

Therefore, it was decided to add in the range of o, oos to 10.0%.

Now, as a surface concentration method for B, there is one method as follows.

(1) In the mold powder for casting, B, 0. 5.0
-30.0%, and 820% at the interface between this mold powder and the steel. A method to increase the surface layer B concentration in the slab shell by reducing it with AI in the steel.

This method was first developed by the inventors in Japanese Patent Application No. 1-286022M.
By using this method, which is disclosed in the specification, the average B concentration in the surface layer of the steel billet can be made 1.5 times or more the average B concentration in the center of the plate thickness.

C: 0.0025%, St: 0.01%, Mn
: 0.10%, P: 0.015%, S: 0.00
8%, Al: 0.020% Oyohi B: 0.00
Continuous casting was performed on molten steel containing 0.4% Fe and the remainder Fe while supplying mold powder having the composition shown in Table 1 into the mold. Table 2 shows the conditions for continuous casting. In addition, the mold powder thickness in the molten steel scene is 15.0 mm.
It became.

The B concentration distribution in the thickness direction of the thus obtained slab is shown in FIG.

As is clear from the figure, the B concentration is 511II1 under the surface layer.
Within the range of 1, it was possible to produce slabs with a concentration more than twice as high as that inside.

Regarding the composition of the mold powder, in addition to the usual mold powder composition for ultra-low carbon steel for deep drawing, B, 0. For example, T, C: 0.5 to 5.0.
%, 5ift: 20.0-40.0%, CaO:
20.0-40.0%, AhOs: 0 or 8.
0% or less, Na, 0:0 or 10.0% or less, Mg
O: Q or 6.0% or less, F: 0 or 10.0% or less, Btus? 5.0-30.0%, TiO2:
Q or a composition of 12.0% or less.

Here B20 in mold powder. The content is 5.
If it is less than 0%, the dissociation reaction of 8.03 will be insufficient and concentration of B on the slab surface will be difficult to occur;
If B20. is exceeded, B20. The content is preferably about 5.0 to 30.0%.

(2) As a method for forming a B-concentrated layer on the surface layer of the slab, in addition to controlling the composition of the mold powder, applying a boride at the slab or hot rolling stage can be used to form a B-concentrated layer on the surface layer of the object to be coated. There are ways to increase the concentration.

Hereinafter, the manufacturing process of this invention will be specifically explained.

First, steel production can be carried out according to a conventional method (although the present invention does not require any limitations on these conditions, it is desirable to use a continuous casting method in terms of cost and quality.

Regarding hot rolling, the finishing temperature is adjusted from the viewpoint of workability.
, it is preferable to set it near just above the transformation point.

Preferably, finishing is performed within the range of Ars transformation point to Ar3 transformation point +50°C. Other conditions are not particularly stipulated and may be carried out according to conventional methods. A sufficiently excellent material can be obtained at a winding temperature of about 550 to 700°C.

Cold rolling may also be carried out according to a conventional method, and the present invention does not particularly require limitations on these conditions, but a cold rolling reduction of 50% or more is desirable for sufficient recrystallization.

Next, an annealing treatment is performed, and in the present invention, it is important to make the atmosphere non-oxidizing and to keep the dew point at -10° C. or less during such annealing.

This is because unless the atmosphere is made non-oxidizing, oxides will preferentially form on the surface, so there is a problem that the punching performance cannot be improved by the surface layer B. Also, if the dew point is higher than 110 degrees Celsius, the above-mentioned This is because, as shown in FIG. 1, the burr height cannot be sufficiently reduced.

Note that it is advantageous in terms of surface roughness and shape control to subsequently perform temper rolling at a reduction rate of about 0.3 to 2.0%.

As mentioned above, by combining the surface concentration of B and low dew point annealing, a suitable hard phase is formed only on the surface layer of the product sheet, and as a result, shear deformation during punching becomes a cleaved ring, resulting in burrs. An effective reduction is achieved.

(Example) Various steel slabs having the compositions shown in Table 3 were treated according to the conditions shown in Table 4.

Table 4 also shows the results of examining the mechanical properties and burr height of the cold-rolled sheets thus obtained.

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain a cold-rolled steel sheet for processing that has excellent workability as well as punching workability.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the dew point of the annealing atmosphere and the burr height, and FIG. 2 is a graph showing the B concentration distribution in the thickness direction of the slab slab. Figure 1

Claims (1)

[Claims] 1. C: 0.05 wt% or less, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, S: 0.03 wt% or less, Al: 0.15 wt% or less, and B: A steel piece containing 0.0050wt% or less, with the remainder being substantially Fe, and on at least one side, the average B concentration in the surface layer from the surface to a depth of 1% of the sheet thickness is A steel slab with a B concentration of 1.5 times or more in the center is hot-rolled and then cold-rolled according to a conventional method, and then the dew point of the atmospheric gas is lowered to -
A method for producing a cold-rolled steel sheet for processing with excellent punchability, the method comprising annealing in a non-oxidizing atmosphere at a temperature of 10° C. or lower. 2. A method for producing a cold-rolled steel sheet for processing according to claim 1, which further comprises adding and blending P: 0.2 wt% or less into the raw material and having excellent punchability. 3. In claim 1 or 2, the material further contains Ti: 0.001 to 0.1 wt%, Nb: 0.001 to 0.1 wt%, Zr: 0.001 to 0.1 wt%, V: 0. A cold rolled material with excellent punching properties, which is obtained by adding at least one selected from 0.001 to 0.1 wt%, Mo: 0.001 to 0.1 wt%, and Ca: 0.001 to 0.1 wt%. Method of manufacturing steel plates. 4. In claim 1, 2 or 3, the material further contains Cr: 0.005-10.0 wt%, Ni: 0.005-10.0 wt% and Cu: 0.005-10.0 wt%. A method for producing a cold-rolled steel sheet for processing with excellent punching properties, which is obtained by adding at least one of the selected materials.