US3276917A

US3276917A - Process for producing cold-rolled steel sheets to be deep drawn

Info

Publication number: US3276917A
Application number: US238896A
Authority: US
Inventors: Matsukura Kameo; Shinagawa Yasuo
Original assignee: Yawata Iron and Steel Co Ltd
Current assignee: Yawata Iron and Steel Co Ltd
Priority date: 1961-12-02
Filing date: 1962-11-20
Publication date: 1966-10-04
Anticipated expiration: 1983-10-04
Also published as: GB1027158A

Description

Oct. 4, 1966 KAMEO MATSUKURA ETA!- 3,

PROCESS FOR PRODUCING COLD-ROLLED STEEL SHEETS TO BE DEEP DRAWN Filed Nov. 20, 1962 2 Sheets-Sheet 1 FIG. 1A FIG. 1 B

Normalized sheet Hot-rolled sheet 7.2 of grain size No. 9.6 of gram size No as left to cool In air after being heated at 950 C Kameo Matsukura Yasuo Shinagawa C 1966 KAMEO MATSUKURA ETAL 3,276,917

PROCESS FOR PRODUCING COLD-ROLLED STEEL SHEETS TO BE DEEP DRAWN 2 Sheets-Sheet 2 Filed .NOV. 20, 1962 FIG. 2A

g .m .m m w w a n 0.0- 6 m 6 D E F 6% FTS 6 6 h .ll I .70 MD. F F w hm Bp JAWS H V A I v V. w, I I w I I I I I I v I v I O O O O O 5 5 7 6 5 O O O O 5 O m 9 8 7 6 4 4 4 2 H w 9 5 5 4 4 oz 95 33 3 96 EE 5 35 Q0 5 cox-6:03 2 w E i m 32 0 225 E8.

30 4O 5O 7O Cold-rolling rate in INVENTORS KAMEO MATSUKURA YASUO SHINAGAWA BY M UM ATTORN United States Patent Japan Filed Nov. 20, 1962, Ser. No. 238,896 Claims priority, application Japan, Dec. 2, 1961 36/43,880 3 Claims. (Cl. 148-124) This invention relates to a processfor producing low carbon rimmed cold-rolled steel sheets to be cold-pressshaped for use in automobile bodies.

In the conventional process for producing the abovementioned steel sheets a slab made of ingot is hot-rolled into a thin strip with a continuous rolling mill after soaked in a heating furnace, said strip is then Water-cooled and coiled, is air-cooled and continuously acid-washed, is rolled with a continuous or reversible cold-rolling mill and is bright-annealed to be recrystallized so that any hard fibrous structure produced during the cold-rolling may be eliminated and cold-drawability may be improved. Further, the annealing for recrystallization is followed by a skin pass and a leveling, in order to prevent stretcher strains from being produced by the presence of yield point elongation in adjusting the shape of the steel sheet and in carrying out the pressing operation.

In the above mentioned conventional process, it is, however, very difiicult to obtain desirable optimum grain size and deep-drawability, though the steel sheet is kept for a considerably long time below the A transformation point in the annealing for recrystallization.

As regards a crystal grain size which is determined by the hot-rolling temperatures and contents of carbon, manganese and other accompanying elements contained in the steel, it is generally well known from experiences in press factories that the size number of Nos. 6.5 to 8.0 according to the standard classification of grain size issued by the A.S.T.M. are most suitable to a pressing work. (For instance, the grain size number of No. 7 has crystal grains amounting to 1024 in a cross-section of 1 mm. and an average diameter of 0.0355 mm.)

When the number is smaller than No. 6 (the crystal grains will become larger) the grains will be so coarse that the surface becomes rough after the press-shaping and presents a so-called orange peel skin state, thereby the product will reduce so much in its commercial value as not .to be endurable to uses.

011 the other hand, if the numberis larger than No. 8.0, the hardness and yield strength will become higher, the elongation and Erichsen value will reduce in general, the amount of the spring back will become .larger and the product will not be adapted to a deep-drawing work.

If an aluminum-killed steel deoxidized with aluminum, is used in this case a considerable degree of deep draw ability will be, of course, obtained with the same crystal grain size and the product will be nonageing on account of the production of so-called elongated grains based on the difference in the recrystal grain growing velocity depending on the precipitation of aluminum nitride in the annealing for recrystallization. However, because of such inherent defects of the aluminumekilled steel as the low yield, poor surface state and high price, it is not so much used today except for specific uses.

Patented Oct. 4, 1966 Further, it is also very difficult to obtain aluminumkilled steel sheet having regularly optimum grain size favorable to a pressing work and enduring against severe pressing Work.

There are numerous criteria for judging the pressshapability of cold-rolled steel sheets, but none of them has shown a perfect correltion with the rates of poor results.

The Lankford R; value recently proposed by Lankford and others is said to be one of the best criteria for judging press-shapability, which value is shown by the ratio of the strain in the width direction to the strain in the thickness direction after carrying out a tension work within the plasticity range of uniform elongation (usually an elongation of 20% is adopted) in a-tension test.

This value is known to have a close relationship with the preferred crystallographic orientation and to show a large number when the grains are arranged in a certain orientation. According to experiments, the R; value of Lankford and the deep-drawability are closely correlated particularly in drawing and composite shaping of drawing and stretching.

In general, the Lankiford R; value will become larger with the increase of the cold-rolling rate up to some value. But, the increase of the cold-rolling rate will be accompanied by the diminution of crystal grains on the other hand. That is to say, the yield strength, hardness will rise with it. Further, the yield point elongation will increase, producing unfavorable result in stretch forming works. Therefore, in order to improve the press-shapability of a cold-rolled sheet, it is necessary to obtain properly large crystal grains while elevating the R; value of Lankford showing a plastic anisotropy.

An object of the present invention is to provide a lowcarbon thin steel sheet having a high deep-drawability by applying a proper heat-treatment and cold-rolling work to a hot-rolled material sheet, so that the grain size of the final product may be adapted to a pressing Work, the rate of elongation, Erichsen value and R; value may be improved, a severe deep drawing work may be feasible even in the case of using a rimmed material and such defect as surface roughness may be eliminated.

The invention will now be explained with reference to the accompanying drawings in which:

FIG. 1A is a microphotograph showing the structure of hot rolled steel to be treated according to the present invention; I

FIG. 1B is a microphotograph of the structure of the.

steel of FIG. 1A; however, it has been'normalized above the A transformation point; and

FIGS. 2A-2E are graphs showing various test values for the steel treated according to the present invention.

The object of the present invention is attained by slabbing and hot-rolling an ingot as in an ordinary process so as to make it a hot-rolled coil, normalizing the coil at a temperature above the A point with or without prior cold-rolling it at a proper reduction rate, preferably below 50%, then rapidly cooling it, for example by quenching it, thereafter cold-rolling it at a reduction rate of about 40 to and then softening (annealing) for recrystallization.

According to the present invention, cold-rolled steel sheet having a proper crystal grain size, endurable to a severe deep-drawing work and having a high ductility can be obtained.

In the case of using a rimmed steel as the raw material in the present invention, nonageing cold-rolled steel sheets having a crystal grain size adapted to a pressing work, endurable to a severe deep-drawing work and having a high ductility can be obtained by positively decarburizing and denitriding the material by carrying out the softening annealing in an atmosphere containing wet hydrogen.

The microscopic structure of the hot-rolled coil to be treated according to the present invention generally corresponds to Nos. 9.511.0 in the standard classification of the A.S.T.M. (see FIGURE 1A). However, if the ferrite crystal grain coordination has taken place .by normalizing the coil above the A transformation point according to the present invention, grain size will become Nos. 7.0 to 7.5 in the A.S.T.M. classification as shown in FIG- URE 1B. (The chemical composition of the sample was then 0.09% C, 0.39% Mn 0.007% Si, 0.018% P, 0.017% S and 0.08% Cu.) In such case, it may be lightly coldrolled before being normalized. The cold-rolling rate in such case is determined by the relation between the thickness of the hot-rolled sheet and that of the product. The degree of growth of the crystal grains can be freely adjusted by varying the cold reduction rate, and the time during which the normalizing is carried out. Thus, when the hot-rolled material sheet is normalized in a temperature above the A; transformation point without or after being lightly cold-rolled, is rapidly cooled, for example by being quenched and is then cold-rolled and annealed, the crystal grains of the final product can be made to be adaptable to the work and the rate of elongation, Erich-' sen valueand R| value can be improved.

In the process of the present invention, the hot-rolled coil is normalized after being lightly cold-rolled depending on the circumstance,'is quenched and is then coldrolled at a reduction rate of about 40 to 80%. In the range of this reduction rate, the crystal grain size of the obtained cold-rolled steel sheet will be around No. 7.5, i.e. most adapted to the pressing work, wherein the elongation, Erichsen value and conical cup value (Fukui CCV) Will be critically improved and the RL value of Lankford will be also increased. This fact is shown in the comparative experiment diagrams shown in FIGS. 2A*E. These figures represent test values on the workability of a cold-rolled steel sheets produced by the conventional process and that of a cold-rolled steel sheet obtained according to the present invention. As evident from these diagrams, it is a peculiar phenomenon nnknown in the cold-rolled steel sheet by the conventional process. that the rate of elongation increases .with the rise of the cold reduction rate. I

. As shown by these comparative test values, it is evident that a cold-rolled steel sheet suitable to the deep-drawing maybe obtained by normalizing the hot-rolled material in a temperature range above the A point to adjust the grain, after lightly cold-rolled depending on the circumstance- An example of the process for producing cold-rolled steel sheets for deep-drawing according to the present invention is as follows:

(1) A rimmed steel slab is hot-rolled and is wound up i in the form of a coil and the coil is cooled an is then coldrolled at a reduction rate below 50% (2) Normalizing of the coil for a short time at a temperature above the A transformation point (in a temperature range of 10 to 100 C. above the A transformation point) and subsequent rapid cooling, for example by quenching.

(3) Acid pickling of the normalized and rapidly cooled coil.

(4) Cold-rolling at a reduction rate of 40 to 80%.

(5) Bright-annealing or decarburizing and denitriding annealing for recrystallization in an atmospheric gas containing wet hydrogen.

(6) Temper rolling.

(7) Leveling.

As the normalizing operation among the above mentioned steps is a comparatively quick quenching, it is, no doubt, desirable to be carried out in a continuous furnace, but is not always limited to be carried out in the continuous furnace. Even a batch furnace may also be used, if a quick quenching system, for example open coil system, will be adopted.

The atmosphere, in which the normalizing operation is carried out, is not limited to be of an inert gas. A molten salt, molten metal=or oxidizing gas may also be used. For the rapid cooling after the normalization, either air-cooled, molten saltor water may be used.

In the above mentioned steps, it is also possible to proceed from the first step, in which the strip is wound up at the temperature below 650 C., to the second step, without leaving it to cool down to the room temperature, but directly feeding it into a furnace, which is kept at a high temperature.

An electric cleaning step can be added between the fourth and fifth steps depending on the circumstance. All the a'bovementioned steps are carried out with the, strip in the form of a coil, but it is, of course, possible to use the material in theform of cut sheets for any of the latter half steps. 1

In the annealing for recrystallization, a batch type furnace or a continuous annealing furnace may be used. The annealing usually carried out is a bright-annealing in a protective atmosphere,- but is not always limited to that.

It can be carried out also in an oxidizing or reducing atmosphere. It is obvious from the nature of the present invention that decarburizingand denitriding actions may :be included in either case.

It is another feature of the present invention that a nonageing very low carbon steel sheet can be obtained by carrying out decarburization and denitrification by using an atmosphere containing wet hydrogen in normalizing annealing. In such case, a nonageing steel sheet can be made of a rimmed steel and the so-called deep-drawability will further improve to be above the values shown in FIGS. 2A-2E. Such extremely low carbon steelsheet can be also used for porcelain enameling.

The present invention shall be explained with reference to a concrete example.

Example.-A hot-rolled sheet having a composition shown in Table 1 and \made in the ordinary steps Otf steel making, slabbing and hot-rolling in a factory was rolled at :a reduction rate of up to 68.5% with a laboratory rolling mill. This steel sheet was normalized (air-cooled) at 950 C. for 2 hoursand was then rolled again by 30 to 80%. This cold-rolled sheet was annealed at 650 C. for 2 hours in an annealing atmosphere gas called DX (5.0% CO 10.5%' CO, 0.5% CH 12.5% H 0.8% H 0, the rest being N and of a dew point of 5 C.) and was subjected to furnace cooling.

The deep-drawability of the thus obtained product and that of a product obtained directly by ordinarily annealing the material (in a DX gas atmosphere) at 650 C. for 2 hours without primary cold rolling and normalizing but ground by surface grinding both surfaces of hot coil TABLE 1.COMPOSITION IN PERCENT OF THE HOT ROLLED MATERIAL SHEET 0 ISi Mn P18 Cu 0.09 t 0.007 0.39 i 0.018 I 0.017

TABLE 2.DEEP-DRAWABILITY [Cold-rolled rate in percent] Primary cold-reduction 68.5 53.5 55.5 44.5 27.0 12.5 Drawability Secondary cold reduction 4s. 7 45. (45. 7 47. 0 45. 7 45. 7 Elwgamn 111 Percent E50. 2 (51. (52. a 53. a 54. 2 53. 0 10. 45 10. a 10. 9. 95 9. 75 9. 05 Erlchsen Value mm i (11.95) (11.90) (11. 85) (11. 8 J 5. 15 5. 05 5. 10 5. 55 Value mm "i 55 g5 35 4 5. 30) 5. 10 4 5. 72 5 7 1.13 1. 07 0. 55 Rankford value 0.85 1.07 1.18 1.47 1.45 0.85 A.S.T.M. crystal grain size No g; g g: 8? $1? 3; g? 3; 3; 3

NOTE-The values in the parentheses are of the ordinarily annealed material.

What we claim is:

1. A process for reducing the thickness of a slab of rimmed steel to a cold-rolled steel sheet having superior deep drawability, consisting essentially of the steps of hot-rolling the slab of rimmed steel, normalizing the hotrolled steel at a temperature ranging from the A transformation point to 40 C. above the A transformation point for a time sufiicient to adjust the crystal grain size to 7.0 to 7.5 A.S.T.M., cold-rolling the normalized steel at a reduction rate of from 40 to 80%, and annealing the cold-rolled steel for recrystallization.

2. A process for reducing the thickness of a slab otf rimmed steel to a cold-rolled steel sheet having superior deep drawability, consisting essentially of steps of hotrolling the slab of rimmed steel, cold-rolling the hot-rolled steel at a reduction rate of less than 50%, normalizing the thus cold-rolled steel at a temperature ranging from the A transformation point to 40 C. above the A transformation point for a time sufiicient to adjust the crystal grain size to 7.0 to 7.5 A.S.T.M., cold-rolling the normalized steel at a reduction rate of from 40 to 80%, and annealing the cold-rolled steel for recrystallization.

3. In a process for producing from a slab of rimmed steel cold-rolled steel sheet having superior deep drawability, the steel having been hot-rolled from the slab of rimmed steel and, when cold rolled in addition to the hot rolling, having been cold-rolled at a reduction rate no greater than the steps consisting essentially of normalizing the rolled steel, said normalizing step being carried out at a temperature ranging from the A transformation point to 40 above the A transformation point for a time sufiicient to adjust the crystal grain size to 7.0 to 7.5 A.S.T.M., cold-rolling the normalized steel at a reduction rate of from 40 to and annealing the cold-rolled steel (for recrystallization.

References Cited by the Examiner UNITED STATES PATENTS 1,874,144- 8/1932 Tytus 14812.4 2,381,435 8/1945 Burns et al. 148-12 OTHER REFERENCES The Making, Shaping and Treating of Steel, US. Steel, Seventh Edition, 1957, page 813.

DAVID L. RECK, Primary Examiner.

H. F. SAITO, W. B. NOLL, Assistant Examiners.

Claims

1. A PROCESS FOR REDUCING THE THICKNESSOF A SLAB OF RIMMED STEEL TO A COOLD-ROLLED STEEL SHEET HAVING SUPERIOR DEEP DRAWABILITY, CONSISTING ESSENTIALLY OF THE STEPS OF HOT-ROLLING THE SLAB OF RIMMED STEEL, NORMALIZING THE HOTROLLED STEEL AT A TEMPERATURE RANGING FROM THE A3 TRANSFORMATION POINT TO 40*C. ABOVE THE A3 TRANSFORMATION POINT FOR A TIME SUFFICIENT TO ADJUST THE CRYSTAL GRAIN SIZE TO 7.0 TO 7.5 A.S.T.M., COLD-ROLLING THE NORMALIZED STEEL AT A REDUCTION RATE OF FROM 40 TO 80%, AND ANNEALING THE COLD-ROLLED STEEL FOR RECRYSTALLIZATION.