US3798076A - METHOD FOR PRODUCING Al-KILLED DEEP DRAWING COLD ROLLED STEEL PLATE BY CONTINUOUS ANNEALING - Google Patents

Abstract

A method of producing an aluminum-killed cold rolled steel sheet for deep drawing by a continuous annealing, which comprises cold rolling a hot rolled steel plate containing not more than 0.015 percent of carbon, 0.05 to 0.50 percent of manganese, 0.13 to 0.33 percent of soluble aluminum with the balance being iron and unavoidable impurities.

Description

United States Patent Shirnizu et a1.

[ Mar. 19, 1974 METHOD FOR PRODUCING AL-KILLED DEEP DRAWING COLD ROLLED STEEL PLATE BY CONTINUOUS ANNEALING Inventors: Mineo Shirnizu; Hiroshi Takechi;

Kenichiro Suemune; Setsuo Nagao; Kuniteru Ohta, all of Kitakyusyu, Japan Nippon Steel Corporation, Tokyo, Japan Filed: Apr. 27, 1972 Appl. No.: 248,242

Assignee:

Foreign Application Priority Data Apr. 27, 1971 Japan 46-027179 U.S. C1. 148/12 Int. Cl C21d 9/48 Field of Search 75/124; 148/12 [56] References Cited UNITED STATES PATENTS 3,248,270 4/1966 Laidman et a1 148/12 3,522,110 7/1970 Shimizu et a1. 148/12 3,615,925 10/1971 Garber et a] 148/12 3,666,569 5/1972 Hultgren 148/12 3,668,016 6/1972 Shimizu et a1. 148/12 Primary Examiner-W. W. Stallard Attorney, Agent, or FirmWenderoth, Lind & Ponack [5 7 ABSTRACT A method of producing an aluminum-killed cold rolled steel sheet for deep drawing by a continuous annealing, which comprises cold rolling a hot rolled steel plate containing not more than 0.015 percent of carbon, 0.05 to 0.50 percent of manganese, 0.13 to 0.33 percent of soluble aluminum with the balance being iron and unavoidable impurities.

2 Claims, 1 Drawing Figure Box Annealing (Temp, 700C, Soaking: 4 hours) Continuous Annealing (Tempi 850C,

Soaking: 5 minutes) F value value PATENTEDHAR i 9 m4 3,798,076

Box Annealing (Temp. 700C, Soaking: 4 hours) Continuous Annealing (Temp. 850C,

Soaking: 5 minutes) METHOD FOR PRODUCING AL-KILLED DEEP DRAWING COLD ROLLED STEEL PLATE BY CONTINUOUS ANNEALIING BACKGROUND OF THE INVENTION The present invention relates to a method for producing cold rolled steel plate or sheet having excellent deep-drawability.

Cold rolled steel sheets have been widely used in various applications such as in the automobile industries, and for electric applications, office utensils, etc.

Among these applications, aluminum-killed cold rolled steel sheets are commonly used in cases where deep-drawability is required.

The aluminum-killed cold rolled steel sheets have been conventionally produced without exception by recrystallization annealing with gradual heating of cold rolled steel sheet in a coil form in a box annealing furnace. There has never been developed a continuous annealing process which is a short-time annealing process with an accelerated heating and cooling rate because such process will sacrify desirous properties of aluminum-killed steel sheets.

On the other hand, the box annealing method has such defects that production per unit time is small and the production cost is increased because of the very low heating and cooling rate of the coil.

SUMMARY OF THE INVENTION In the course of extensive experiments in efforts for solving difficulties in the production of aluminumkilled cold rolled steel sheets having excellent deep-drawability by continuous annealing, the present inventors have discovered that when a cold rolled steel sheet having a higher soluble aluminum content than that of the conventionally produced aluminum-killed steel sheets (0.03 0.06 percent of sol.Al), namely when the sheet contains a soluble aluminum content between 0.13 percent and 0.33 percent, is continuously annealed with a heat cycle above the recrystallization temperature, an average plastic strain ratio (T value) similar to or more than that of the convention aluminum-killed steel sheet is obtained. The present invention is based on the above discovery.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be described in detail with reference to the attached drawing, which shows a graph indicating 'r' values obtained when aluminum-killed steel sheets having different sol.Al contents are continuously annealed and box annealed.

DETAILED DESCRIPTION OF THE INVENTION Hot rolled steel sheets used as the starting materials in the present invention may be produced by ordinary production processes, including ingot-making and breaking down from molten steel prepared in electric furnaces, convertors, etc. or from molten steel further treated by vacuum degassing; slab-making by continuous casting, hot rolling and then acid pickling. Among hot rolled steel plates produced as above, it is found that the steel composition which show excellent deep-drawability by continuous annealing must be different from the composition of the conventional aluminum-killed hot rolled steel plates. Thus the steel composition is: less than 0.015 percent of carbon, 0.05

0.50 percent of manganese, 0.13 0.33 percent of soluble aluminum with the balance being iron and unavoidable impurities.

The range of the above composition is determined from the production conditions of the hot rolled steel sheet and the deep-drawability after the continuous annealing.

More concrete explanations on the above limitations of each element of the steel composition will be made hereinunder.

Carbon is effective and necessary for giving appropriate strength to the cold rolled steel, but too high a content of carbon lowers deep-drawability due to dissolution of over-saturated carbon into the matrix during the continuous annealing where accelerated heating and cooling is done, and forms lump cementite due to the coagulation of precipitates taking place during the cooling step after the hot rolling in the production of hot rolled steel plates. This cementite is finely devided by the cold rolling and locally dispersed, which causes various defects such as lowering of deep-drawability. Therefore, for improvement of deep-drawability it is desired that the carbon content be as low as possible, and the upper limit of the carbon content range is set at 0.015 percent with consideration to other properties and production cost. For lowering the carbon content, decarburization annealing of hot rolled steel sheets as well as decarburization of molten steel may be conducted.

Manganese is effective to prevent hot embrittlement due to sulphur when it is contained in an amount more than 0.05 percent. However, too high a manganese content increases strength and lowers deep-drawability. Therefore, the upper limit of manganese is set at 0.5 percent. The range of soluble aluminum of 0.13 to 0.33 percent has been determined from the results of experiments on the present invention. This range has been selected so as to assure an value similar to or more than that obtained by box annealing conventional aluminum-killed cold rolled steel sheets, and particularly the range of 0.18 to 0.30 percent is desired. When an aluminum-killed cold rolled steel sheet containing such a soluble aluminum content is continuously annealed, deep-drawability is improved. This improvement may be attributed to the assumption that the acceralated heating and cooling rate as effected in continuous annealing is favourable for AlN present in the steel to form-a coagulated structure favourable to deep-drawing during the restoration and recrystallization period. Soluble aluminum contents excessively out of this range are considered to cause problems unfavourable to the coagulated structure or the Al precipitates, thus lowering deep-drawability. It is desirable that unavoidable impurities such as P and S be maintained as low as possible due to their adverse effect of lowering deep-drawability.

According to the present invention, a hot rolled steel sheet of the above composition is cold rolled at a reduction rate of more than 30 percent and subjected to recrystallization annealing in a continuous annealing furnace.

In this case, the heat cycle, namely the heating and cooling rate, and the soaking time in the annealing treatment and so on are not specifically limited.

The method of the present invention has a remarkable advantage in that an aluminum-killed cold rolled steel sheet having better deep-drawability than that of the conventional aluminum-killed cold steel sheet produced by box annealing can be produced in a large amount at low production cost.

This is shown in the single FIGURE which shows the effect of annealing method on theTvalues. The test was duplicated. The dotted lines show F values obtained when the steels were subjected to continuous annealing at 850C for 5 minutes, while the solid lines show ?values obtained when the steels were box-annealed at 700C for 4 hours. As is clear from the FlGURE, the effect of the sol. Al content on the F value is not remarkable when the steel is box-annealed. Thus the F value becomes high around 0.15 percent sol. Al content but lowers when so]. A1 content is more than about 0.15 percent. On the other hand, when the steel is subjected to continuous annealing, the F value is remarkably improved as the sol. Al content is more than the 0.1 1 percent, and around 0.13 percent sol. A1 content the F value exceeds that of the box-annealed steel, and it shows a peek around 0.22 percent sol. A1 content.

An example of the present invention will be set forth below. 1

Table 1 shows chemical compositions of hot rolled steel sheets produced from vacuum degassed molten steel prepared in a convertor by ingot-making, breaking-down and hot rolling. The hot rolled steel plates were cold rolled at a reduction rate of 70 percent to oband as compared with M steel and N steel which were produced by box annealing.

Table 2 Mechanical Properties of Steel Sheets As Annealed Yield Tensile Rupture T value Stress Strength Elongation (Kg/m (Kg/m A 21.2 31.5 45.2 1.65 Inventive B 19.6 30.1 46.0 1.79 Steel C 19.3 30.3 47.2 1.91 Sheets 1) 20.1 30.) 46.1 1.141 F. 21.2 31.1 44.8 1.611

F 20.11 30.6 46.2 1.41 G 21.6 31.8 45.6 1.46 H 24.8 33.1 43.2 1.29 Comparison .1 27.2 34.2 46.2 1.28 Steel K 286 34.8 43.8 1.12 Sheets Box Annealing (Temperature: 700C Soaking: 5 hours) Table 1 (weight Grades C Si Mn P sol. Al N A 0.005 0.01 0.09 0.010 0.008 0.150 0.0042 lnventive B 0.009 0.31 0.012 0.012 0.186 0.0036 Steel C 0.006 0.01 0.16 0.011 0.011 0.243 0.0052 Sheets D 0.34 0.017 0.014 0.310 0.0061 E 0.012 0.02 0.31 0.013 0.011 0.192 0.0055 F 0.005 0.01 0.21 0.016 0.017 0.072 0.0048 G 0.009 0.18 0.015 0.014 0.106 0.0062 H 0.007 0.02 0.61 0.012 0.241 0.0051 Compari- 1 0.023 0.01 0.31 0.017 0.016 0.210 0.0056 son J 0.040 0.34 0.014 0.011 0.216 0.0048 Steel K 0.058 0.29 0.012 0.014 0.056 0.0055 Sheets L 0.026 0.35 0.014 0.013 tr 0.024

(Rimmed Steel) M 0.049 0.31 0.016 0.016 0.063 0.0041 N 0.033 0.33 0.017 0.015 tr 0.0021

(Rimmed Steel) What is claimed is: l. A method of producing an aluminum-killed cold rolled steel sheet for deep drawing, said method comprising: cold rolling a hot rolled steel plate containing not more than 0.015 percent of carbon, 0.05 to 0.50 percent of manganese, 0.13 to 0.33 percent of soluble aluminum, with the balance being iron and unavoidable impurities, and continuously annealing the thus cold rolled steel sheet.

2. A method according to claim 1, wherein said hot rolled steel plate contains 0.18 to 0.30 percent of soluble aluminum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 5 Patent No. 3,798,076 7 Dated March 19, 1974 Inventor(s) Mineo Shimizu, Hiroshi Takechi, Kenichiro Suemune,

Setsuo Nagao, and Kuniteru Ohta. It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the single Figure of the drawings, change the "sol Al(%)" legends from ".01", ".02" and ".03" to .l .2 and .3 respectively.

Signed and sealed this 30th day of July 197 SEAL Attest:

MCCOY M. GIBSON, JR. Attesting Officer C. MARSHALL DANN Commissionerof Patents FdRM PC4050 'USCOEAM-DC 60376-P69 LS. GOVERNMENT PRINTING OFFICE IQ, -3-33

Claims (1)

1. 2. A method according to claim 1, wherein said hot rolled steel plate contains 0.18 to 0.30 percent of soluble aluminum.

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