KR930002739B1 - Method for making aluminium-killed cold-rolled steel having a good forming property - Google Patents

Abstract

A soft cold rolled steel plate, employing as the starting material a slab which is composed of the low carbon steel added a little content of tungsten, comprises 0.02-0.1 wt.% carbon, less than 0.3 wt.% manganese, less than 0.02 wt.% sulphur, less than 0.005 wt.% nitrogen, 0.03-0.07 wt.% aluminum, 0.01-0.20 wt.% tungsten, inevitable impurities and the balance of iron. The cold rolled steel is prepared by (a) soaking the slab continuously casted by Al-killed method at 1050-1150 deg.C, (b) finish rolling the slab at the temperature of Ar3, (c) after hot rolling to wind the slab steel at 650-750 deg.C, and cold rolling, and (d) continuously tempering at 800-850 deg.C.

Description

Manufacturing method of Al-killed cold rolled steel sheet with excellent formability

The present invention relates to a method for manufacturing an aluminum (Al) -killed cold rolled steel sheet used as a soft cold rolled steel sheet for automobiles, and more particularly, to a slab in which a small amount of tungsten is added to a low carbon steel of 0.02% or more by Al-kill method. The present invention relates to a method for producing a soft cold rolled steel sheet excellent in formability by hot rolling, continuous rolling, and subsequent annealing by continuous casting (hereinafter referred to as "casting").

Conventional cold rolled steels that improve the formability by adding alloying elements related to this technology include the method of adding a small amount of tungsten to low carbon rim steel (The Journal of the Australian Institute of Metal Vol 17, 1972 No. 3) and the poles. And a method of adding carbides such as titanium (Japanese Patent Application Laid-Open No. 60-82616) or niobium (Japanese Patent Application Laid-Open No. 54-1245) to an alloy element which is easy to form nitride. The method of adding the former tansten is manufactured by slab by steelmaking ingot and adopts annealing for recrystallization annealing after cold rolling, which makes the process complicated and takes a long time to process, and the material of the product varies in the longitudinal direction. There is a big disadvantage. In addition, the latter method of adding titanium or niobium (alloy element) controls the size of precipitates by the addition of alloying elements, and also effectively removes dissolved carbon and solid solution nitrogen in the steel (carbon + nitrogen). Titanium or niobium is added so that the content ratio of c) is about 1.0 in atomic ratio. Therefore, in the case of a cold rolled steel sheet containing titanium or niobium, when the carbon content is high, the amount of alloying elements increases, which causes a cost increase. Therefore, ultra-low carbon steel is usually used. Since the level is at least 0.015 to 0.025%, a separate decarburization process is required. Therefore, there is a problem that the increase in the treatment cost of the additional decarburization process and the productivity due to the addition of this process treatment are lowered. Accordingly, an object of the present invention is to solve the above problems, by adding a small amount of tungsten to 0.02wt% or more of low carbon steel and deoxidizing it to Al after steelmaking, unlike the conventional ingot method, it is intended to regenerate slab production. In addition, it is intended to provide an Al-killed cold rolled steel sheet having excellent molding by continuous annealing after cold rolling.

In order to achieve the above object, the present inventors completed the present invention by performing low-carbon steel in which tungsten was added in small amounts by steelmaking (slab production), hot rolling after hot rolling, and continuous annealing after cold rolling.

According to the present invention, a method of manufacturing a soft cold rolled steel sheet comprising a step of hot rolling a slab made by adding a small amount of tungsten to a low carbon steel, followed by cold rolling and annealing, wherein the slab is in weight%, C: 0.02 ~ 0.1%, Mn: 0.3% or less, Al: 0.03 ~ 0.07%, P: 0.2% or less, S: 0.02% or less, N: 0.005% or less, Al: 0.03 ~ 0.07%, W: 0.01 ~ 0.20%, etc. Composed of unavoidable impurities and Fe and continuously cast by Al-kill method; The hot rolled steel sheet produced by the hot rolling is wound at 650 ~ 750 ℃; The cold rolled cold rolled material is characterized in that the continuous annealing at a temperature of 800 ~ 850 °.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The carbon content of the slab is limited to 0.02 to 0.10%, which is difficult to require additional decarburization in the steelmaking process below 0.020%, and when it exceeds 0.10%, it is difficult to effectively remove carbon in the steel by tungsten, and thus the solid carbon This is because moldability deterioration occurs, and the amount of addition of alloying elements required to completely remove such solid solution carbon becomes excessive. In tungsten-added low carbon steels, the carbides represented are (Fe, W) ₃ C, W ₂ C, and the like. In the present invention, when tungsten with a stoichiometric ratio less than that of carbon is added (Fe, W) ) ₃ C appears, but when overaged at high temperatures, some fine W ₂ C is formed.

Also, the Mn amount is restricted to less than 0.3% is due to the development of the texture of the Mn by the force acting between the carbon to suppress the formation of (Fe, W) C ₃ decreases formability in steel. In addition, the amount of P is limited to 0.02% or less because it is an element that causes spot weldability, poor paintability and grain brittleness during secondary processing and increases yield strength and tensile strength by solid solution strengthening effect. S is an element that causes coil edge cracking due to red hot brittleness during hot rolling, but when S is precipitated in the form of MnS by Mn, cracking can be prevented. Therefore, the amount of Mn added should be less than 0.020% in consideration of the remaining content combined with oxygen in the steel. Since N is an element that increases the strength of the steel sheet and decreases the elongation rate and causes aging at room temperature, it is advantageous in terms of material to precipitate as nitride in the steel.In the present invention, Al is added as a deoxidizer after steelmaking for reoxidation. Al has a strong bonding force with N and is easy to form AlN. Therefore, it is preferable to limit within the range that can be completely precipitated as nitride by Al, but may be set considering the following Al amount. If Al is 0.07% or more, there is a problem of generating surface defects of the steel sheet by aluminum oxide. If Al is less than 0.03%, it is difficult to sufficiently precipitate N in the steel.Al is preferably 0.05 to 0.07%, and N is preferably limited to 50 ppm or less. .

On the other hand, W is a binding element of C, which is precipitated as ₃ C between steelmaking and hot rolling (Fe, W) to prevent formability deterioration due to solid solution carbon and to increase elongation. The material is degraded by the formation of carbides. Therefore, the amount of W added is preferably added at 0.01 to 0.20%, and more preferably at (W at%) / (C at%) = 0.04 to 0.10 when based on W ₃ C.

On the other hand, in the present invention, the slab heating temperature is preferably set to 1050 ~ 1150 ℃, which W precipitates, such as WC, (Fe, W) ₃ C already precipitated in the steel is completely dissolved in this temperature range. Undissolved precipitates are then coarsened during subsequent hot rolling and some remaining W is partially replaced with Fe ₃ C Fe after rolling (Fe, W) to form ₃ C, thereby forming a (111) set of annealing plates. This is because the development of tissue components is facilitated and the moldability is improved. In addition, the hot rolling finish temperature is preferably set to a temperature directly above Ar ₃ in order to obtain a uniform grain recrystallization structure. In addition, the coiling temperature has a large influence on the crystal grain size and the precipitation distribution of the hot rolled sheet, the crystal grain size and precipitate distribution has a direct relationship to the formability, the main precipitate in the present invention is (Fe, W) ₃ C as the precipitate Since it is easy to coarse when winding after hot rolling, it is necessary to wind up at 600 ~ 700 ℃ to improve the processability in order to obtain an effective precipitate size. In addition, the recrystallization annealing after cold rolling, it is desirable to apply a continuous annealing capable of very fast heating rate of 5 ℃ / second after quenching and over-aging treatment, wherein the annealing temperature is 800 ~ to ensure sufficient grain growth after recrystallization complete It is preferable to set it as 850 degreeC.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

Using a vacuum-dissolved slab having the chemical composition as shown in Table 1, after maintaining the heating temperature before hot rolling at 1100 ° C. for 1 hour, the hot rolling was finished at 890 ° C. to produce a 3.2 mm hot rolled sheet and then reproduce the winding process. In order to maintain the temperature at 700 ° C. for 1 hour, the furnace was cooled. Thereafter, the scale of the steel plate surface was removed by pickling and then cold rolled to a reduction ratio of 75%. Continuous annealing conditions were maintained at 830 ° C. at a heating rate of 5 ° C./sec for 40 seconds, then slowly cooled to 5 ° C./sec up to 680 ° C., followed by quenching to 400 ° C., and 15 ° C. from 400 ° C. to 350 ° C. for overaging treatment. The furnace was cooled at a cooling rate of / min.

TABLE 1

The mechanical properties of the tungsten additive (inventive material) and the comparative material prepared in this way are shown in Table 2 below.

TABLE 2

γ value: rank ford value representing the formability index

According to the above Table 2, it can be seen that the present invention exhibits excellent results compared to the comparative material in which the yield strength and the tensile strength are not deteriorated in comparison with the comparative material, but the elongation and? Value are not added with the alloying element. In addition, it can be seen that the greater the tungsten-to-carbon ratio of the present invention with tungsten added, the lower the yield strength and the improved workability.

As described above, according to the manufacturing method of the present invention, a conventional cold rolled steel sheet manufacturing method, that is, titanium (Ti) or niobium (Nb) is added to bond with carbon or nitrogen, which is a chip-type element of steel, to form carbon in solid solution. In addition, it is possible to secure good formability even at the low and low carbon steel level where the carbon content is relatively low compared to the conventional method of removing nitrogen, and the decarburization process such as RH-OB can be omitted in the steelmaking process, thereby reducing the processing cost and processing time. There is an effect that can shorten. In addition, the present invention has the effect that can be performed after steelmaking by deoxidizing with aluminum (Al), and productivity can be improved by adopting continuous annealing.

Claims (1)

In the method for producing a soft cold rolled steel sheet using a slab containing a small amount of tungsten in low carbon steel as a starting material, in weight%, C: 0.02 to 0.1%, Mn: 0.3% or less, P: 0.02% or less, S: 0.02 % Or less, N: 0.005% or less, Al: 0.03 ~ 0.07%, W: 0.01 ~ 0.20%, other unavoidable impurities and slabs made of Fe and continuously cast by Al-kill method at 1050 ~ 1150 ℃ Al-killed cold rolling with excellent moldability, characterized in that it is treated, finished and rolled at a temperature directly above Ar _3, hot rolled at 650 to 750 ° C., followed by cold rolling, and continuous annealing at a temperature of 800 to 850 ° C. Method of manufacturing steel sheet.