US3335036A

US3335036A - Deep drawing steel sheet and method for producing the same

Info

Publication number: US3335036A
Application number: US426667A
Authority: US
Inventors: Yoshida Hiroshi; Nakazato Yoshio
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1964-01-25
Filing date: 1965-01-19
Publication date: 1967-08-08
Anticipated expiration: 1984-08-08
Also published as: SE302471B; LU47843A1; DE1483253B2; BE658771A; DE1483253A1; GB1094745A; NL6500896A

Description

United States Patent 3,335,036 DEEP DRAWHNG STEEL SHEET AND METHOD FOR PRODUCING THE SAME Hiroshi Yoshida and Yoshio Nakazato, Chiba-shi, Japan, assignors to Kawasaki Steel Corporation, Fulriai-ku, Kobe-shi, Hyogo-ken, Japan, a corporation of Japan No Drawing. Filed Jan. 19, 1065, Ser. No. 426,667 Claims priority, application Japan, Jan. 25, 1964, 39/ 3,412 18 Claims. (Cl. 148-2) ABSTRACT OF THE DISCLOSURE A deep drawing rimmed steel sheet and method of manufacture wherein the sheet consists of up to .02% carbon, 0.20-O.60% manganese, 0.0030.050% selenium, the balance iron and incidental impurities and wherein the sheet is produced by subjecting a rimmed steel sheet containing 0.020.l0% carbon along With the requisite manganese and selenium to hot and cold rolling to produce the sheet and thereafter decarburizing to reduce the carbon content to less than 0.02%.

The present invention relates to a cold rolled steel sheet which has superior deep drawing qualities, excellent nonaging properties, and which is not subject to surface imperfections by fabrication and to a method for producmg same.

Heretofore, aluminum killed steels adapted for both deep drawability and non-aging have been extensively employed for use in automobile body sheet, various kinds of machine parts, electrical appliances, home appliances, furniture, etc. However, the aluminum killed steels are inferior to rimmed ones in the yield of ingots as well as in surface finish and their production costs are high. The conventional rimmed steels are of low cost and of good appearance but rimmed steels have not been utilized in the past for the above purposes because of their rather poor deep drawing as well as non-aging properties.

This invention contemplates to overcome the above disadvantages of rimmed steels, the principal object of this invention being to provide an improved rimmed steel sheet and a method of producing the same which is free from surface defects, of high yield as well as of low cost, and with excellent deep drawing as well as nonaging properties.

This invention is directed specifically to a deep drawing rimmed steel sheet having a composition consisting of up to 0.02% carbon, 0.20-0.60% manganese, 0.003- 0.050% selenium, the balance iron and incidental impurities, the physical properties of which are:

(1) A crystal grain size number of 8.010.0 according to A.S.T.M. standards (American Society for Testing Materials) (2) A C.C.V. (conical cup value) of less than 37.3 when the sheet thickness is 0.8 mm.; and,

(3) a (111) crystal aggregate structure.

In addition this steel may contain 0.010-0.040% phosplfatorus, the purpose for which will be described hereina er.

A method for producing this improved cold rolled steel sheet first comprises adding 0.0030.050% selenium to a molten steel containing 0.020.l0% carbon and 0.20- 0.60% manganese to produce a rimmed steel of the above specified composition. When phosphorus is also to be added both selenium and phosphorus are incorporated in the molten steel containing 0.020.10% carbon, 0.20- 0.60% manganese, and less than 0.010% phosphorus to produce a rimmed steel containing 0.0030.050% selenium and 0.010-0.040% phosphorus. After the additions are completed this steel is subjected to a series of hot and cold rolling procedures and then to a decarburizing anneal at a temperature of 600800 C. to reduce the carbon content to less than 0.02%.

According to a recent investigations there exists a close relation between deep drawability of steel sheet and crystal aggregate structure. It has been found that the crystal aggregate structure in which the (111) plane of the iron crystal is arranged in the orientation parallel to the rolling plane is the most favorable one for obtaining deep drawability in steel sheet.

However, a rimmed steel sheet of such crystal aggregate structure as the above has never been satisfactorily produced by the conventional method for obtaining a cold rolled steel sheet and, therefor, a good deep drawability has not been obtained in such sheets. It has been discovered that when the steel sheet is subjected to cold rolling and a decarburizing anneal, the content of carbon deleterious to the deep drawability of the steel sheet is reduced to a minimum with the (111) iron crystal aggregate structure being developed by the growth of crystal grains during decarburization. This (111) iron crystal aggregate structure will then be the main orientation of the aggregate structure of the cold rolled and annealed steel sheet, and the structure will develop a plastic anisotropy favorable for the deep drawing working process. Thus, this method can give a rimmed steel a deep drawing quality.

However, the above decarburizing anneal tends to develop the growth of extraordinary crystal grains, and a steel sheet having a coarse crystal grain will tend to produce surface defects when subjected to press Work. To inhibit surface defects the crystal grain size number should be in the range of 8.0-l0.0 as specified by the A.S.T.M.

In order to control the growth of crystal grains, we first considered if we could control the anneal hour so as to stop the decarburizing anneal prior to the occurrence of extraordinary growth of crystal grains but we found that such was not feasible for an actual operation since it maintains the carbon content of a steel sheet product at a relatively high level with a resultant considerable non-uniformity in product quality. Secondly, we considered if we could adopt a relatively low temperature for anneal and soak a steel product in it, but by this method, although we were effective to a certain degree in inhibiting the extraordinary growth of crystal grains, the decarburizing reaction did not proceed in a satisfactory manner to enhance deep drawability. It was then considered that the reaction in which an undesirable extra- 3 ordinary growth of crystal grains takes place is caused by the removal of carbon and its compounds which function to inhibit the migration of grain boundary, and therefore, thirdly, we decided to add another element which acts to inhibit the migration of grain boundary. Oxygen, selenium, and sulfur of the Group VII; of the Periodic Table are known to be such elements to provide the above desired action. Of these elements oxygen is the cause of non-metallic inclusions which deteriorate stretchability and sulfur forms a sulfide unfavorable for metal working. However selenium facilitates grain control in the course of decarburization, never deteriorates working, and gives an excellent deep drawability. Further, in comparison with the aluminum in the aluminum killed steep which imparts an ill effect to elongationworka-bility, selenium provides a good stretchability.

In view of our extensive study we have discovered that the carbon content of the steel ingot produced in the steel making step is required to be in the range of 0.020.10% to obtain the desired final product quality. If the carbon content of the steel is lower than the lower limit of the above range, the oxygen content thereof increases so much that the steel quality is deteriorated by the presence of non-metallic inclusions. If the carbon content is higher than the up er limit an uneconomical extended period of time is required for carrying out a decarburizing anneal described hereafter.

Manganese may be included in an amount of 0.20- 0.60%, which is a normal amount for conventional rimmed steel, Further, as pointed out above, phosphorus may be incorporated in the steel along with the selenium but where there is a phosphorus addition the molten steel should be fully dephosphorized to less than 0.010% phosphorus in the steel making step. Where a phosphorus addition is made the final steel product should contain between 0.010 and 0.040% of this element.

The above content of phosphorus is effective not only for the development of the (111) crystal grain aggregate structure in the steel sheet, but also to inhibit the decrease of tensile strength. Further it imparts a superior deep drawing quality to the steel sheet.

If the content of phosphorus is less than the lower limit, the effect of phosphorus is hardly recognized while if it is more than the upper limit, the stretchability .of sheet material is diminished.

The steel sheet with both added elements, phosphorus and selenium, has a better deep drawability than a sheet added with selenium only as described hereinafter.

It has further been discovered that a favorable effect of phosphorus on the deep drawability of steel sheet is noticeable only when it is added to the molten steel which has already been dephosphorized in the steel making step. Phosphorus originally present in the raw material will not so function. A theoretical explanation of the above phenomenon is not known but, as shown in the tables, a clear difference in deep drawability is recog nized between the steel with added selenium (Nos. 5-9) and those with both selenium and phosphorus (Nos. 13).

As pointed out above, a steel containing 0.020.10% carbon and O.200.60% manganese is produced by either converter, open hearth furnace or electric furnace and then either selenium or selenium and phosphorus are added to either ladle or mold to obtain a rimmed steel ingot. N0 deoxidizing treatment whatsoever is applied.

In this invention the amounts of other impurities than the specified elements should be reduced to as low quantities as possible.

The steel ingot thus produced is subjected to a series of conventional steps, such as, slabbing, hot rolling, pickling, and cold rolling. Then the cold rolled steel sheet is subjected to a decarburizing anneal to reduce the carbon content of the steel sheet to less than 0.02%, preferably less than 0.01%. With a considerable decrease of carbon the (111) crystal grain aggregate structure develops gradually whereby deep drawability and plastic anisotropy are improved. The type of anneal furnace is not particularly critical but an open coil type anneal is most desirable from the viewpoint of economy as well as product quality. An anneal temperature is preferred to be in the range of 600-800 C. If the anneal temperature is too low, the time for decarburization will adversely efiect deep drawability. If it is too high an extraordinary growth of crystal grain will take place so that it will be diflicult to obtain a product of uniform quality. Preferably the anneal temperature is in the range of 650750 C. in order to obtain superior deep drawability with the crystal grain under control.

The composition of the atmosphere gas for the anneal may be any of the known decarburizing compositions. Further a combination atmosphere may be utilized. Satisfactory atmospheres include ones consisting of hydrogen containing a small amount of water. Also the atmosphere may contain nitrogen which is desirable from an economic standpoint.

EXAMPLE follows:

Percent C 0.06 Si Tr. Mn 0.07 P 0.008 S 0.016

Then ferrophosphorus and ferromanganese alloys were added to the molten steel which had been poured into the ladle whereby a steel of the following composition was produced:

Percent C 0.08 Si Tr. Mn 0.34 P 0.020 S 0.014

Thereafter the molten steel was poured into an ingot mold while, at the same time, 670 g. metallic selenium was added and a 10 t. rimmed steel ingot was obtained. This steel ingot was heated in a soaking pit uniformly to about 1300 C. and then subjected to slabbing to produce a slab of mm. thickness. This slab was hot rolled in a hot strip mill to make a hot strip coil of 2.8 mm. in thickness. Then, the hot coil was pickled and cold rolled in a cold strip mill to obtain a cold strip coil of 0.8 mm. in thickness. Thereafter, the cold strip coil was subjected to a decarburizing anneal. The annealing furnace was an open coil type in which an atmosphere gas consisting of a mixture of hydrogen, nitrogen and steam (H :N ==3: 1) was employed. The cold strip coil was subjected to the decarburizing anneal for 20' hours at the temperature of 710 C. Lastly, the cold coil was subjected to a temperrolling with a slight (1.0%) reduction to produce a smooth surface thereon.

The following tables show chemical analysis (Table I) and physical properties (Table II) of steel products in which various amounts of added selenium and phosphorus and various anneal temperatures are illustrated. The steel sheets marked in Table I fall within the scope of this invention. For comparison, the properties of representative rimmed steel and aluminum killed steel are also listed in these tables.

TABLE I.GHEMIOAL ANALYSIS OF TEST STEELS Composition (Percent) No. Steel Si S Al N Se Mn P Commercial Rimmed SteeL... .043 Tr .34 .011 .020 Tr .0018 Al-killed Steel .040 Tr .35 .008 .016 .035 .0065 De-C, De-N Rimmed Stee .006 Tr .34 .008 .016 TI .0008 De-C, De-N Rimmed Steel. .015 Tr .33 .007 .018 Tr .0010 Se-contg. De C, De-N Rim. St .006 Tr .36 .009 .020 Tr .0006 .005 Se-con. De-C, De-N Rim. St .008 Tr .36 .009 .015 Tr .0008 .010 Se-con. De C, De-N Rim. St .006 Tr 34 .010 016 Tr .0009 030 Se-con. De-C, De-N Rim. St .007 Tr .35 .008 .017 Tr .0009 .003 Se-con. De-C, De-N Rim. St .006 Tr .38 .010 .018 Tr .0009 .028 P-, Se-con. De-C, De-N Rim. St .006 Tr .34 .020 .019 Tr .0007 .005 P-, Se-con. De-C, De-N Rim. St 007 Tr .36 .020 018 Tr .0006 .005 P-, Se-con. De-C, De-N Rim. Sh.-. .007 Tr .33 .015 .020 Tr .0009 .025 P-, Se-con. De-C, De-N Rim. St .006 Tr .35 .018 .017 Tr .0008 .003

TABLE II.PHYSIOAL PROPERTIES OF STEEL'PRODUCTS A.S.T.M. Y.P. T.S. El Y.E. C.C.V Er. No. Grain (Kg/mm?) (Kg/mini (Percent) (Percent) (111111.) (mm.) R value Size No.

1 Drawn through.

In Table II the crystal grain size specified by A.S.T.M. 40

is obtained by summing up the number of crystal grains per square inch by means of a 100 magnification microphotograph.

Tensile strength test is conducted by the use of J.I.S. (Japanese Industrial Standard) No. 5 test specimen.

Erichsen test is a cupping test, using a tool with a spherical end of 20 mm. diameter to deform the test specimen which is held between annular jaws of 27 mm. internal diameter. The test sheet, which is 3% inches square, is first clamped between the jaws to measure the thickness; the jaws are then moved apart by 0.05 mm. and clamped in that position to allow metal to be drawn into the cup as the test progresses. The tool is pressed into the metal until a fracture appears in the cup, and the depth of the cup at fracture is taken as a measure of the ductility of the metal.

With reference to the direct test method for the deep dra'wability of steel sheet, the conical cup test has been adopted. This test is called Fukuis Cup Test, the details of which are specified in J.I.S. Z2249. When this conical test is conducted on the steel sheet of 0.8 mm. in thickness, a test specimen of a circular disk form is deep drawn until it is broken by employing a conical die having a die opening angle of 60 and the average value of the maximum and minimum rim diameters of the fractured test specimen are determined as a conical cup value (C.C.V.). Therefore, it follows that the smaller C.C.V., the better deep drawability. For proper drawability it has been determined that the C.C.V. of the steel sheet of this invention as applied to a sheet thickness of 0.8 mm. is 37.30 or less but C.C.V. depends on sheet thickness, the diameter of a test specimen and the size of a die. Thus when the conical cup test is applied as specified in J.I.S. Z-2249 the steel sheets of this invention have the following 'values'in relation to the sheet thickness:

C.C.V. of the sheet of this invention 0.6 Less than 26.40

Sheet thickness, mm.:

R value shows whether deep drawability is good or not and can'be obtained by measuring the deformation of a tensile test specimen in the thickness direction and that of the same specimen in the width direction. The larger R value the smaller the deformation of sheet in the tensile deformation which shows greater drawability.

In the tables N0. 1 refers to a known rimmed steel in which the crystal grain is very fine, but its C.C.V. is 38.48

and its R value 1.14 which shows a poor drawability. No. 2 refers to a commercial aluminum stabilized steel which has a good deep drawability for its fine crystal grain size, and in addition, a good aging property, but

which is of low yield as Well as of poorer surface appearance than that of the rimmed steel of No. 1. N0. 3 refers to a commercial rimmed steel which has been subjected to the decarbnrizing anneal and its deep drawability is more improved than that of No. 1 but its crystal grain is noticeably coarsened. No. 4 refers to the same steel as No. 3 but subjected to a lower anneal temperature, up to 680 C., so as to prevent the crystal grain size from coarsening but, owing to the insufficient anneal, inferior deep drawability results. Nos. 5-13 refer to the steel sheet of this invention. Nos. 5, 6 and 7 refer to the rimmed steel sheet containing 0.0030.050% selenium subjected to the decarbnrizing anneal at the temperature of 710 C., in which the crystal grain size is sufficiently fine, and the deep drawability is rather good, its C.C.V. being about 37. No. 8 refers to an improved steel sheet subjected to the anneal temperature of 690 C. in which an appropriate crystal grain size is developed by the addition of 0.003% selenium. No. 9 refers to a steel sheet of this invention in which the anneal temperature of 740 C. which is somewhat high but coarsening of crystal grain is prevented by adding 0.028% which is relatively high. Nos. 10, 11 and 12 refer to the rimmed steel containing 0.0100.040% phosphorus in addition to selenium and wherein the steel is subjected to a decarburizing anneal at the temperature of 710 C., in which the crystal grain is suitably fine, and its deep drawability is more improved than those of Nos. 5, 6 and 7 containing selenium only, and further, in which the decrease of tensile strength is inhibited. No. 13 refers to the steel sheet contaning 0.003% selenium and 0.018% phosphorus subjected to the anneal at a temperature of 690 C. in which an 8.6 A.S.T.M. crystal grain size is obtained due to the slight decrease of selenium content.

Elongation (EL) and stretchability (EL) of the steel sheet in accordance with the present invention are the same or better than those of the aluminum killed steel.

The test results of yield point-elongation (Y.E.), one of the criteria for non-aging, show that the steel sheet of this invention has a sufficiently low Y.E. value, which is evidenced by the excellent non-aging property. It is believed that this is caused by the removal of nitrogen from the steel sheet by the action of hydrogen in the course of the decarburizing anneal step. Accordingly, the removal of carbon as well as nitrogen from the steel sheet of this invention is advantageous for making the steel sheet non-aging. To this end, it is desirable to increase the hydrogen partial pressure and decrease the nitrogen partial pressure in the annealing atmosphere.

In the example described (No. with the atmosphere gas consisting of the ratio H :N =3:1, the resultant steel sheet had a nitrogen content to about 00006-00009 Various changes and modifications of this invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.

We claim:

1. A method for producing a deep drawing steel sheet which comprises forming a rimmed steel ingot containing 0.020.l0% carbon, 0.200.60% manganese, and 0.0030.050% selenium, subjecting said rimmed steel ingot to hot and cold rolling to produce a thin steel sheet, and then decarburizing said sheet by anneal at a temperature of 600-800" C. to reduce the carbon content to less than 0.02%.

2. A method according to claim 1 wherein the decarburizing anneal atmosphere has a low nitrogen partial pressure whereby the nitrogen content of the steel is reduced.

3. A method according to claim 1 wherein after the final anneal the steel sheet is temper-rolled to produce a smooth surface.

4. A method according to claim 2 wherein the anneal is of the open coil type.

5. A method according to claim 1 wherein the decarburizing atmosphere is a mixture of hydrogen, nitrogen and steam.

6. A method according to claim 1 wherein the carbon content is reduced by the anneal to less than 0.01%.

7. A method according to claim 1 wherein the decarburizing anneal is effective at a temperature of between 650 and 750 C.

8. A method for producing a deep drawing steel sheet which comprises providing a rimmed steel ingot containing 0.0030.050% selenium and 0.010.040% phosphorus by adding selenium and phosphorus to a molten rimmed steel containing 0.020.10% carbon, 0.20 0.60% managnese, and up to 0.010% phosphorus, subjecting said rimmed steel ingot to hot rolling and then to cold rolling to produce a thin steel sheet, and then decarburizing said sheet by an anneal at a temperature of 600-800 C. to reduce the carbon content to less than 0.02%.

9. A method according to claim 8 wherein the decarburizing anneal atmosphere has a low nitrogen partial pressure whereby the nitrogen content of the steel is reduced.

10. A method according to claim 8 wherein after the anneal the steel sheet is temper-rolled to produce a smooth surface.

11. A method according to claim 9 wherein the anneal is of the open coil type.

12. A method according to claim 8 wherein the decarburizing atmosphere is a mixture of hydrogen, nitrogen and steam.

13. A method according to claim 8 wherein the carbon content is reduced by the anneal to less than 0.01%.

14. A method according to claim 8 wherein the decarburizing anneal is efiected at a temperature between 650 and 750 C.

15. A deep drawing rimmed steel sheet consisting of up to 0.02% carbon, 0.200.60% manganese, 0.003- 0.050% selenium, the balance iron and incidental impurities, said steel sheet characterized by having an A.S.T.M. crystal grain size number of 8.010.0, a C.C.V. of less than 37.3 for 0.8 mm. thickness and containing predominantly a (111) crystal aggregate structure.

16. A deep drawing sheet in accordance with claim 15 wherein the carbon content is less than 0.01

17. A deep drawing rimmed steel sheet consisting of up to 0.02% carbon, 0.200.60% manganese, 0.003- 0.050% selenium, 0.010-0.040% phosphorus, the balance iron and incidental impurities, said steel sheet being characterized by having an A.S.T.M. crystal grain size number of 80-100, a C.C.V. of less than 37.3 for 0.8 mm. thickness and containing predominantly a (111) crystal aggregate structure.

18. A deep drawing sheet in accordance with claim 17 wherein the carbon content is less than 0.01%.

References Cited UNITED STATES PATENTS 2,009,713 7/1935 Palmer l23 2,009,714 7/1935 Palmer 75-123 2,258,604 10/1941 Gagnebin 75-123 2,316,948 4/1943 Gagnebin 75123 3,239,388 3/1966 Sasaki l4812 CHARLES N. LOVELL, Primary Examiner.

Claims

8. A METHOD FOR PRODUCING A DEEP DRAWING STEEL SHEET WHICH COMPRISES PROVIDING A RIMMED STEEL INGOT CONTAINING 0.003-0.050% SELENIUM AND 0.01-0.040% PHOSPHORUS BY ADDING SELENIUM AND PHOSPHORUS TO A MOLTEN RIMMED STEEL CONTAINING 0.02-0.10% CARBON, 0.200.60% MANAGNESE, AND UP TO 0.010% PHOSPHORUS, SUBJECTING SAID RIMMED STEEL INGOT TO HOT ROLLING AND THEN TO COLD ROLLING TO PRODUCE A THIN STEEL SHEET, AND THEN DECARBURIZING SAID SHEET BY AN ANNEAL AT A TEMPERATURE OF 600-800*C. TO REDUCE THE CARBON CONTENT TO LESS THAN 0.02%.