US3865645A

US3865645A - Cold-rolled steel sheet for press-forming

Info

Publication number: US3865645A
Application number: US319017A
Authority: US
Inventors: Nobuyuki Takahashi; Kenitiro Suemune; Shuji Nagata; Tatumi Tomozoe
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1971-12-27
Filing date: 1972-12-27
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11
Also published as: JPS5322052B2; BR7209158D0; AU470367B2; ZA729117B; AU5056272A; JPS4870611A

Abstract

A cold-rolled steel sheet having an excellent press-formability, consisting of less than 0.08 wt. percent carbon, 0.05 to 0.4 wt. percent manganese, 0.015 to 0.10 wt. percent silicon, 0.01 to 0.10 wt. percent acid-soluble aluminum, 0.003 to 0.015 wt. percent nitrogen, the balance being iron and unavoidable impurities, said cold-rolled steel sheet being manufactured through a process step of a continuous annealing with or without an overaging treatment.

Description

United States Patent [191 Takahashi et a1.

[ COLD-ROLLE'D'STEEL SHEET FOR PRESS-FORMING [75] Inventors: Nobuyuki Takahashi; Kenitiro Suemune;,Shuji Nagata; Tatumi Tomozoe, all of Kitakyushu, Japan [73] Assigne'e: Nippon Steel-Corp., Tokyo, Japan [22] Filed: Dec. 27, 1972 [2]] Appl. No.: 319,017

30 Foreign Applicationllriority Data Dec. 27, 1971 Japan 4610537 [52] U.S. CI 148/142, 75/124, 148/2, 148/12.3, 148/36 [5 1] Int. Cl C22c 41/02, C2ld 9/48 [58] Field of Search 75/123 R, 123 B; 148/36, 148/2, 12.3, 134, 143, 142

'[ 56] References Cited UNITED STATES PATENTS 2,109,271 2/1938 Krause 75/123 2,125,128 7/1928 Robinson 148/142 [4 1 Feb. 11,1975

2,319,655 5/1943 Saylov 75/123 2,986,403 5/1961 Leslie 148/12 3,117,897 l/l964 Williams 148/142 3,239,389 3/1966 Yoshida... 75/123 3,323,953 6/1967 Lesney... 148/39 3,357,822 12/1967 Miyoshi 148/142 OTHER PUBLICATIONS Making Shaping and Treating of Steel, by US 0 Steel, 8th Edition, published 1964, pages, 552-554.

Primary ExaminerC. Lovell Attorney, Agent, or FirmWenderoth, Lind & Ponack [57] ABSTRACT 5 Claims, 1 Drawing Figure COLD-ROLLED STEEL SHEET FOR PRESS-FORMING The present invention relates to a cold-rolled steel sheet for press-forming, manufactured by a continuous annealing.

In general, cold-rolled steel sheets are used as such parts of automobiles as cold-formed by a drawing working, for instance, rear fendemfront fender and door inner, for which an excellent drawing workability is required.

As for cold-rolled steel sheets having such an excel lent drawing workability there are used at present an aluminum-killed low carbon steel containing 0.03 to 0.07 wt. percent carbon and 0.02 to 0.08 wt. percent acid-soluble aluminum and further a cold-rolled titani um-added extremely low carbon steel sheet, in which the drawing workability is not influenced by a rate of heating of an annealing and is superior to the aluminum-killed low carbon steel sheet, due to the fixation of carbon and nitrogen by titanium. However, this aluminum-killed low carbon steel sheet is almost annealed in the state of a tight coil or of an open coil by using a box annealing furnace. But, such a box annealing has following disadvantages: a long time is required for annealing, the productivity is low and a uniform heating of the whole coil or of the interior of the furnace is not obtainable for a constructional reason, whereby a uniform material quality over the whole length of coil can not be achieved. Particularly, in the case of an annealing in the state of a tight coil the contact between the surface of a steel sheet and an atmosphere gas within the furnace is bad and moveover there occur easily various kinds of hindrances in regard to the surface properties due to lack of the uniformity. Further, a titanium-added extremely low carbon steel sheet has a disadvantage that its production cost becomes higher than that of an aluminum-killed low carbon steel, because the former contains about 0.1 wt. percent titanium.

These problems caused in subjecting an aluminumkilled cold-rolled steel sheet to a box annealing are solved by a continuous annealing to be conducted in a continuous annealing furnace. In general, a continuous annealing has following advantages: the productivity is high for reason of construction; a uniformity of material qualities through one coil and that between coils are obtained; and a uniform contact between the surface of a steel sheet and atmosphere gas within the furnace is secured. However, on the other hand, the continuous annealing can not be freed from disadvantages: that is, on account of a rapid heating and a short soaking time crystal grains are refined in general, and on account of a high rate of cooling subsequent to the annealing carbon contained in the steel can not sufficiently precipitate as carbide so that the continuously annealed steel is high in yield stress and tensile strength, but low in breaking elongation and n-value and great in aging property as compared with a boxannealed steel material. Therefore, the former is inferior to the latter in the press-formability. Therefore, the continuous annealing treatment is usually not applied to the production of a cold-rolled steel sheet for pressforming, but iiiahiyfor' the production of steel sheet to be used as material for producing tin plate.

Reasons, why a cold-rolled steel sheet subjected to a continuous annealing treatment has an unfavorable press-formability, are found in the following situations: on account of rapid heating and short annealing time crystal grains are difficult to grow, but are easy to re fine; on account of a quenching after the annealing, the amount of solid solution of carbon is large, so that there is caused a hardening aging, which makes the steel material hard, and finally on account of rapid heating, the development of a recrystal collective texture, which is essentially desirable for the drawability, is checked. However, the greatest problem, with which the production of a cold-rolled steel sheet for drawing of such qualities as of the SPCD class prescribed in the Japanese Industrial Standard (MS) is confronted, is hardening aging due to quenching.

As a method for preventing a steel material from becoming hard on account of a hardening aging, there has been adopted method, in which a steel sheet coil quenched afterthe continuous appealing is successively subjected to a heat treatment (overaging treatment) at a temperature of 300 to 500C. in a continuous annealing furnace or a box annealing furnace. However, in case the continuous annealing is followed by the overaging treatment in a box annealing furnace, the productivity of the annealing is reduced and the original aim of carrying out the continuous annealing must be lost. However, even when carrying out the overaging treatment continuously in a continuous annealing furnace a sufficient softening of the steel material is still difficult to obtain, if the overaging treatment of an economic or practical scale (for instance, the overaging treatment for less than 10 minutes) is conducted.

As a result of numerous investigations made by the inventors of the present invention on chemical compositions of steel sheets for promoting the overaging phenomenon it was confirmed that, when a usual aluminum-killed low carbon steel contains an adequate amount of silicon, the overaging rate is quickened, whereby the steel material is made soft, even with an overaging treatment of a short time, and a cold-rolled steel sheet for press-forming can be produced.

It has been further confirmed by the same inventors of the present inventon that by subjecting the abovementioned aluminum-killed low carbon steel sheet to vacuum degassing treatment to reduce the C content to 0.001 to 0.009 percent there can be obtained a coldrolled steel sheet, in which the drawing workability and stretching workability are of the same degree as a titanium-added extremely low carbon steel sheet and the drawing workability is not influenced by the heating rate of the annealing.

Therefore, the object of the present invention is to produce a cold-rolled steel sheet having an excellent pressformability when subjected to a continuous annealing.

Another object of the present invention is to provide a cold-rolled steel sheet having an excellent press-formability by the application of an overaging treatment of a continuous annealing system.

A still another object of the present invention is to provide a cold-rolled steel having always an excellent drawing workability without being influenced by a rate of cooling subsequent to a hot-rolling and a coiling temperature by subjecting the said steel to a vacuum degassing treatment to reduce the C content to 0.001 to 0.009 percent.

Another object of the present invention shall be made clear by the following explanation with reference to an attached drawing.

The attached drawing shows influences of silicon contents on changes in properties of steel materials, when subjecting aluminum-killed low carbon steel sheets having different silicon contents to an overaging treatment of the continuous annealing.

The steel sheet of the present invention is a coldrolled steel sheet having an excellent press-formability, said steel consisting of less than 0.08 wt. percent carbon, 0.05 to 0.4 wt. percent manganese, 0.015 to 0.10 wt. percent silicon, 0.01 to 0.1 wt. percent acid-soluble aluminum, 0.003 to 0.015 wt. percent nitrogen and the rest being iron and unavoidable impurities.

At first, the components of the steel sheet of the present invention shall be explained.

The carbon content of the steel sheet of the present invention is the same as that of a cold-rolled low carbon steel of the SPCC class (cold-rolled steel sheet in general) or the SPCD class (cold-rolled steel sheet for drawing) prescribed by G 3141 of the 11s. it is not particularly necessary to reduce the carbon content for mitigating the hardening aging. However, in case properties of the same degree as of a titanium-added extremely low carbon steel sheet is required for the steel sheet of the present invention its carbon content is reduced to 0.001 to 0.009 wt. percent by subjecting the steel to a vacuum degassing treatment.

Manganese Mangan must be contained for the purpose of preventing a brittle rupture caused by sulfur at the time of hot-rolling. In view of the fact that an aluminum-killed steel prepared by melting in a converter contains usually sulfur in an amount of more than 0.005 percent, the manganese content must be more than 0.05 percent in order to fix the above-mentioned amount of sulfur. But, since an excessive amount of manganese brings about a deterioration of the workability of the steel, the upper limit thereof is made 0.4

percent.

Silicon is contained usually in an amount of less than 0.01 percent in a cold-rolled steel sheet refined in a steel making furnace prescribed in G 3141 of the HS. However, in the steel of the present invention the silicon content is necessary to be in an amount of more than 0.015 percent, preferably more than 0.02 percent, in order to obtain properties of a steel sheet for pressforming (of SPCD class of HS) by a continuous annealing comprising an overaging treatment, without being influenced by conditions of production steps (for instance, hot-rolling condition) prior to the cold-rolling.

However, when the silicon content exceeds 0.1 percent, the strength of the steel sheet is increased to a degree more than required, whereby the workability is deteriorated. Therefore, the upper limit thereof is made 0.1 percent.

In connection with the silicon content, as abovementioned, it is to note that there is a mutual relationship between the silicon content and the carbon content. It has been confirmed by the inventors of the present invention that in the case of the steel of the present invention containing silicon in a range of 0.015 to 0.1 percent it is preferable to reduce the carbon content to a level of less than 0.009 percent by subjecting the steel to a vacuum degassing treatment in order to liberate the 'r'-value, that is, an average value of the plastic strain ratio, which serves as an index of the drawability of the steel, from the dependency of the heating rate of annealing. However, if the silicon content is below the range as above-mentioned, the F-value can not be liberated from the dependency on the heating rate of annealing, even if the carbon content is in the range of from 0.001 to 0.009 percent, as specified by the present invention.

Further, also a phenomenon that, when the silicon content is more than 0.015 percent, the drawability is liberated from being influenced by a coiling temperature of annealing indicates that there is a relationship between the carbon content and silicon content.

Further, in order to achieve the object of the present invention it is necessary to contain an acid-soluble aluminum in an amount of more than 0.01 percent. However, the lower limit of the acid-soluble aluminum desirable to prevent the occurance of stretcher strain caused by oxides contained in the steel is defined to be 0.02 percent. On the other hand, if the acid-soluble aluminum exceeds 0.1 percent, the workability of the steel is deteriorated. Therefore, the upper limit thereof is made 0.1 percent.

In order to mitigate the hardening due to a hardening aging of cold-rolled steel sheet subjected to a continuous annealing treatment, that is, to quicken the overaging rate, the co-presence of three components, that is, silicon, aluminum and nitrogen, is more effective than a sole presence of silicon. For this purpose nitrogen is necessary to be contained in an amount of more than 0.003 percent. However, if exceeds 0.015 percent, the workability is deteriorated. Therefore, the upper limit thereof is made 0.015 percent.

As is evident from the foregoing, the prerequisite conditions for achieving the object of the present invention is the limitation of four basic chemical components: carbon, silicon, aluminum and nitrogen, for reasons asabove-mentioned.

The steel sheet of the present invention is manufactured by the steps of refining a molten steel in a steelmaking furnace such as converter, open hearth furnace, electric furnace, an ingot-making, a slab-making by blooming or a continuous casting and subsequent hot-rolling. The finishing temperature of the hot-rolling is preferable to be higher than 800C. A coiling temperature is not limited. That is, in the case of an aluminumkilled low carbon steel the drawing workability of the steel can not be obtained, unless the steel is heated to a grain coarsening point as a finishing temperature of hot-rolling and rapidly cooled after the hot-rolling, for reason that the drawability will be deteriorated on account of AlN precipitating at grain boundaries or coiled at a temperature near the A, transformation point.

On the contrary, in the present invention neither the finishing temperature of the hot-rolling nor coiling temperature has influence on the effects of components of the steel of the present invention for promoting the overaging. The thus obtained hot-rolled coil is then subjected to a cold-rolling after it was sealed. The reduction percentage of the cold-rolling must be more than 30 percent, preferably 60 to percent, from the view point of the drawing workability.

The cold-rolled coil is then annealed in a continuous annealing furnace with an adequate heating system.

The annealing temperature is higher than 680C, but lower than 900C., and the time required for the soaking is less than 10 minutes. The annealed coil is then subjected to an overaging treatment in a apparatus for overaging treatment under conditions of a temperature of 300 to 500C, and a time of less than minutes, preferably 2 to 5 minutes. Further, in the case of the steel of the present invention the rate of cooling from the annealing temperature to the overaging treatment EXAMPLE 1 Table 1 shows chemical components of steel materials of various kinds for explaining examples of the present invention.

These steel materials were cold-rolled to the sheet gauge of 0.8mm. through usual process steps of producing cold-rolled steel sheets under hot-rolling and cold rolling conditions as shown in Table 2. Thereafter,

the steel materials were annealed at 700C. for 3 min-' utes in a continuous annealing furnace after up to 700C. in a minute. After the annealing the steel materials were cooled down to 370C. by any of cooling rates shown in Table 3 and then were subjected to an overaging treatment at 430C. for 5 minutes. Further, each of the steel materials was subjected to a refining rolling with a reduction percentage of 1.5 percent. Mechanical properties of the cold-rolled steel sheets obtained through the above-mentioned process steps are shown in Table 4.

tents were subjected to a continuous annealing. Then, a part of them were subjected to the overaging treatment, while another part of them were not. The mechanical properties of the steels were investigated in comparison of both cases, and the results of the investigations are shown in Figure.

The composition of steel samples subjected to the investigations and the treatment conditions are as follows:

I. Composition C 0.04 0.07 percent Mn 0.18 0.30 percent S0]. A] 0.04 0.06 percent N 0.0045 0.0060 percent 2. Hot-rolling conditions Finishing temperature 890 10C.

Coiling temperature: 630 i l0"C.

3. Cold-rolling condition Reduction percentage 73 percent 4. Continuous annealing conditions Soaking condition 700C. X 90 sec.

Rate of heating-up 60 sec. till to 700C.

Rate of cooling Average 22C./sec.

Overaging treatment 350C. X 5 min.

5. Refining rolling 5.

Reduction percentage 1.5 percent As is seen from the attached drawing, if steel sheets contain silicon in an amount of more than 0.01 percent, a remarkable softening of the steel materials under the same overaging treatment conditions is obtained, whereby a press-formability is so strikingly improved that the material qualities obtained thereby are closely allied to those of SPCD class of the HS which are subjected to a box annealing.

Table 1 Chemical composition Coil Sol.

No. C Mn Si P S Al N 0 steel sheet A 0.05 0.15 0.032 0.02 0.01 0.053 0.0038 0.005 within the B 0.03 0.33 0.020 0.02 0.02 0.040 0.0048 0001; range or C 0.00 0.20 0.047 003 0.02 0.032 0.0068 0.007 the present D 0.05 0.23 0.017 0.01 0.01 0.0110 0.0059 0.004 invention E 0102 0.29 0.080 0.04 0.02 0.057 0.0045 0.006 F 0.06 0.19 0.023 0.02 0.01 0.045 0.0050 0.010 Steel sheet G 0.05 0.32 0.009 0.01 0.01 0.053 0.0049 0.005 out of the H 0.04 0.28 0.035 0.02 0.02 0.006 0.0029 0018 range of 1 0.05 0.31 0.010 0.02 0.02 0.002 0.0019 0.032 the present 1 0.07 0.32 0.024 0.01 0.02 0.045 0.0024 0.011 invention K 0.04 0.30 0.010 0.02 0.02 0.054 0.0058 0.006

Cold-rolled steel sheets containing acid-soluble alu- Table 2 minum, nitrogen and silicon in amounts within the ranges specified by the present invention are in general softer than those containing these components in Rolling record amounts out of the satd respective ranges and are ex- H cellent in all of the breaking elongation, n-value and r- C l f f 5?- O1 lnlS ing 01mg re uCIlOl'l value. Therefore, it 1s clear that the steel sheets of the N0 temperamre temperature percentage present tnventlon have very good press-formab1l1ty. At :7, the same time it is to note that the mechanical proper- A 890 560 73 ties of the steels of the present invention are little influ- B 880 560 73 enced by the coiling temperature of hot-rolling and the g 338 228 range of cooling after the annealing. E 900 680 73 Example 2 o 865 680 73 H 890 560 73 Wtth an atm of mvesttgattng the tnfluence of stltcon 880 680 73 on the effect of the overaging treatment aluminum- 338 228 killed low carbon steels having different silicon con- Table 3 Cooling condition down to 430C afterthe annealing at 700C Table 6 Hot-rolling conditions (C Cold- Cooling system Average cooling rate (C/sec.) C011 h ng Coiling reduction No. temperature temperature percentage 1 5.4 (Time required: 50 sec.) 11 22.5 (Time required: 12 sec.) 111 54.0 (Time required: 5 see.) A 890 560 70 B 880 530 70 Table 4 Mechanical properties Coil Cooling Yield point Tensile Elongateion n-value r-value Crystal No. system (kg/mm") strength ('70) grain No.

Cold-rolled A 1 22.5 33.2 44.0 0.236 1.40 9.0 steel Sheet B 11 21.3 32.5 44.3 0.240 1.42 8.4 within the C 111 22.0 33.2 43.8 0.236 1.45 8.8 range of D 1 20.8 32.0 44.8 0.241 1.46 8.5 the present E 11 23.0 34.0 44.0 0.237 1.47 8.3 invention F 111 19.5 31.2 45.7 0.240 1.51 8.0 Cold-rolled stcel G 111 23.7 35.3 41. 0.229 1.41 8.6 sheet out of H 111 23.4 34.8 42.0 0.230 1.23 9.4 the range of 1 111 27.3 36.5 40.1 0.225 1.08 10.2 the present .I 111 24.0 36.0 42.2 0.228 1.25 9.0 invention K 111 27.9 37.2 40.5 0.223 1.18 9.3

Example 3 C 890 620 74 D 870 570 76 Table 5 shows steel samples prepared by sub ecting E 850 570 70 the steels having chemical compositions shown in Ex- 2 888 228 32 amples 1 and 2 to a vacuum degassing treatment to re- Table 7 L Mechanical properties of annealed steel materials Mechanical properties Yield*4 Tensile Breaking Crystal Sample point strength elongan-value F-value grain No. (kg/mm) (kg/mm tion (71.) size No.

A 14.3 32.6 49 0.259 1.94 7.6 *1 B 15.0 32.0 49 0.250 1.95 8.0 C 15.1 32.0 49 0.255 1.80 8.0 D 15.3 32.5 47 0.254 2.00 6.9 *2 E 19.3 33.5 44 0.238 1.53 8.5 F 18.2 35.6 43 0.220 1.15 9.1 *3 G 12.8 31.8 53 0.291 2.40 8.1

1: Steel within the range of the present invention *2: Steel out of the range of the present invention *3: Ti-added extremely low carbon steel out of the range of the present invention *4: 0.2% proof stress Annealing system: Continuous annealing system 870C 4m in.

duce the carbon content to a range of from 0.001 to 0.009 percent.

These steels were hot-rolled and then cold-rolled under usual hot-rolling and cold-rolling conditions as 50 shown in Table 6 to obtain the gauge of 0.8 mm. thereby and thereafter were subjected to continuous annealing at 870C. for 4 minutes. The thus annealed steel sheets were than subjected to refining rolling with As is evident from the foregoing, particularly in reference to Examples 1 and 2, the fundamental idea of the present invention, which has for its purpose the provision of a steel sheet having an excellent press-formability by a continuous annealing method, is based on the discoveries that an excellent press-formability can be rendered to the steel sheet subjected to a continuous annealing by specifying main components of the steel,

a-reduction percentage of 1.0 percent. The mechanical 55 Particularly adding Silicon in an amount as is Specified properties of the steel sheets thus produced through the by the Present mventlon but press'formablhtxof above-mentioned process steps are as shown in Table S1961 can be more remflrkably lmproled SubJfmtmg 7 said steel to an overaging treatment, that IS, a kind of Table 5 Samp Sol.

No. C Mn Si P Al N O S A 0.007 0.15 0.032 0.02 0.043 0.0076 0.005 0.02 Al-klllcd. extremely low carbon B 0.005 0.33 0020 0.02 0.086 0.0103 0.008 0.02 stecl wtthtn the range of the C 0.009 0.30 0.065 0.02 0.097 0.0138 0.006 0.03 present invention D 0.004 0.10 0.145 0.01 0.027 0.0045 0.012 0.01 A1-ki1led steel out of thc E 0.013 0.28 0.030 0.02 0.043 0.0082 0.003 0.02 range of the present Invention F 0008 0.23 0.010 0.03 0.03 0.0056 0.005 0.02 Ti-addcd extremely low carbon Ti steel out of the range of the G 0.008 0.15 0.01 0.01 0.105 0.0045 0.003 0.03

present invention an artificial aging which is aimed at the softening of steel material.

Thus, the present invention has succeeded in economically overcoming the greatest problem, the steel material becoming hard due to a hardening aging, encountered in producing a cold-rolled low carbon steel sheet by a continuous annealing according to any conventional methods. The reason, why the overaging rate of the steel sheet of the present invention is quick, seems to lie in the fact that silicon contained in the steel will promote the precipitation of carbide. This is also assumed to be connected with the fact that the T-value of the steel of the present invention acquired after the continuous annealing is little influenced by a coiling temperature of the hot-rolling, because AlN precipitated during the hot-rolling will not take the simple form of AlN, but a compound precipitate of Al-N-C, consequently there would exist no great difference in size of the precipitate and distribution form thereof.

Further, Example 3 teaches that even without subjecting the steel of the present invention to the overaging treatment at the end of the continuous annealing treatment the press-formability may be improved, when the carbon content is reduced to a range of from 0.001 to 0.009 percent to soften the steel by subjecting the molten steel to a vacuum degassing treatment.

In this case it is further to note that the F-value may be also liberated from its dependency on a heating rate of annealing. In general, in the case of a steel contain ing carbon in an amount of more than 0.01 percent, the dependency of the F-value on the heating rate of annealing varies depending upon the carbon content of the steel, as is special for an aluminum-killed coldrolled steel sheet. Therefore, by reducing the carbon content of the steel of the present invention to less than 0.09 percent by subjecting the same to a vacuum degassing treatment, as above-mentioned, said dependency of the F-value disappears. This indicates that the recrystallization structure of the aluminum-killed cold-rolled steel sheet undergoes a change from a certain line of carbon content, while making the carbon content of 0.01 percent a boundary line, from which it is understood that the recrystallization structure of the aluminum-killed cold-rolled steel sheet having a carbon content of less than 0.01 percent is suited to an annealing process, in which rapid heating is to be conducted.

What is claimed is: 1. A process for preparing a cold-rolled steel sheet for a press forming, which process comprises heating a cold-rolled steel sheet consisting essentially of less than 0.08 wt. percent carbon 0.05 to 0.4 wt. percent manganese 0.015 to 0.10 wt. percent silicon 0.003 to 0.015 wt. percent nitrogen 0.01 to 0.10 wt. percent acid-soluble aluminum the remainder being iron and unavoidable impurities, to

a temperature in a range from 680 to 900C and holding the same at this temperature for less than 10 minutes, subsequently cooling the steel sheet from the said temperature with a cooling rate of 5 to 54C/sec. to the temperature of an overaging treatment, then subjecting the steel sheet to said overaging treatment, in which the steel sheet is held at a temperature of 300 to 500C for less than 10 minutes and thereafter subjecting the steel sheet to a continuous annealing treatment, in which the steel sheet is quenched to room temperature.

2. The process of claim 1 wherein the overaging treatment is for 2 to 5 minutes.

3. A process for preparing a cold-rolled steel sheet for press forming, which includes vacuum degassing, whereby prior to cold rolling, the initial carbon content of a molten steel of 0.02 to 0.08 wt. percent is reduced to range of 0.001 to 0.009 wt. percent, which process comprises heating a cold-rolled steel sheet consisting essentially of less than 0.08 wt. percent carbon 0.05 to 0.4 wt. percent manganese 0.015 to 0.10 wt. percent silicon 0.003 to 0.015 wt. percent nitrogen 0.01 to 0.10 wt. percent acid-soluble aluminum the remainder being iron and unavoidable impurities, to a temperature in a range from 680 to 900C and holding the same at this temperature for less than 10 minutes, subsequently cooling the steel sheet from the said temperature with a cooling rate of 5 to 54C/sec. to the temperature of an overaging treatment, then subjecting the steel sheet to said overaging treatment, in which the steel sheet is held at a temperature of 300 to 500C for less than 10 minutes and thereafter subjecting the steel sheet to a continuous annealing treatment, in which steel sheet is quenched to room temperature.

4. The product of the process of claim 1.

5. The product of the process of claim 3.

Claims

1. A PROCESS FOR PREPARING A COLD-ROLLED STEEL SHEET FOR A PRESS FORMING, WHICH PROCESS COMPRISES HEATING A COLD-ROLLED STEEL SHEET CONSISTING ESSENTIALLY OF LESS THAN 0.08 WT. PERCENT CARBON 0.05 TO 0.4 WT. PERCENT MANGANESE 0.025 TO 0.10 WT. PERCENT SILICON 0.003 TO 0.915 WT. PERCENT NITROGEN 0.01 TO 0.10 WT. PERCENT ACID-SOLUBLE ALUMINUM THE REMAINDER BEING IRN AND UNAVOIDABLE IMPURITIES, TO A TEMPERATURE IN A RANGE FROM 680* TO 900*C AND HOLDING THE SAME AT THIS TEMPERATURE FOR LESS THAN 10 MINUTES, SUBSEQUENTLY COOLING THE STEEL SHEET FROM THE SAID TEMPERATURE WITH A COOLDING RATE OF 5 TO 54*C/SEC. TO THE TEMPERATURE OF AN OVERAGING TREATMENT, THEN SUBJECTING THE STEEL SHEET TO SAID OVERAGING TREATMENT, IN WHICH THE STEEL SHEET IS HELD AT A TEMPERATUE OF 300 TO 500*C FOR LESS THAN 10 MINUTES AND THEREAFTER SUBJECTING THE STEEL SHEET TO A CONTINUOUS ANNEALING TREATMENT, IN WHICH THE STEEL SHEET IS QUENCHED TO ROOM TEMPERATURE.

3. A process for preparing a cold-rolled steel sheet for press forming, which includes vacuum degassing, whereby prior to cold rolling, the initial carbon content of a molten steel of 0.02 to 0.08 wt. percent is reduced to range of 0.001 to 0.009 wt. percent, which process comprises heating a cold-rolled steel sheet consisting essentially of less than 0.08 wt. percent carbon 0.05 to 0.4 wt. percent manganese 0.015 to 0.10 wt. percent silicon 0.003 to 0.015 wt. percent nitrogen 0.01 to 0.10 wt. percent acid-soluble aluminum the remainder being iron and unavoidable impurities, to a temperature in a range from 680* to 900*C and holding the same at this temperature for less than 10 minutes, subsequently cooling the steel sheet from the said temperature with a cooling rate of 5* to 54*C/sec. to the temperature of an overaging treatment, then subjecting the steel sheet to said overaging treatment, in which the steel sheet is held at a temperature of 300* to 500*C for less than 10 minutes and thereafter subjecting the steel sheet to a continuous annealing treatment, in which steel sheet is quenched to room temperature.

4. The product of the process of claim 1.

5. The product of the process of claim 3.