US3404047A

US3404047A - Method for producing deep-drawing low-carbon steel sheet

Info

Publication number: US3404047A
Application number: US515232A
Authority: US
Inventors: Stephen R Goodman; Hu Hsun
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1965-12-20
Filing date: 1965-12-20
Publication date: 1968-10-01
Anticipated expiration: 1985-10-01
Also published as: GB1101110A; FR1505832A; DE1508455B2; DE1508455A1; NL6617890A; NL154272B

Description

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- r 1 3,404,047 METHOD FOR PRODUCING DEEP-DRAWING LOW-CARBON STEEL SHEET I Stephen R. Goodman, Monroeville, andHsun Hu, Franklin Township, Westmoreland County, Pa., assignors to United States Steel Corporation, a corporation of Delaware.

No Drawing. Filed Dec. 20, 1965, Ser. No. 515,232

, {5 Claims. (Cl. 148-121) This invention relates to low-carbon steels having improved drawability. More particularly, the invention is directedto a method of producing low-carbon sheet steel with improved deep-drawing characteristics and high yield strength. Y It is well known that aluminum killed steels have excellent drawability; 'Aluminum killed steels, referred to as,,fSK grade steel, are characterized by flattened or paricake shap'ed ferrite grains which are crystallographically oriented to provide good drawability. Such grains are developed in the final sheet product by a properly controlled box annealing process during which selective growth of'the favorably oriented grains is effected by the. "aluminum 'nitride precipitate. Although the exact mechanism of the process is 'not fully known, the phenomenon that a critically dispersed second phase can markedly affect the recrystallization and grain growth is frequently observed, and in some cases, successfully employed in metallurgical applications.

However, aluminum-killed steels are relatively expensive due not only to the cost of the alloy, but also to the low yield from ingots and high conditioning costs. It is, thereforefdesirable to develop a new method for producing deep-drawing sheets of relatively less expensive'low-carbon steels. R p

' The drawability'of'sheet material can be evaluated by simple tension tests. When a strip specimen is pulled to a greater length; its width and thickness are decreased. The plastie strain ratio 'can'vserve as an indication of the degree of mechanical anisotropy of the material. This ratio is referredto as the R value and is defined as the ratio. ,of percent change in width (e the width strlainyto the. percent change in thickness (e the thickness strain), i.e. 5 V

t ne-raw Law the. width and length, respectively, of 'the, gauge section, and the'subscripts i and 1 refer tolthe' initial and'final' measurements (before and after straining): of these dimensions. This expression is based on the'fas'surnptibnthat the'volur'ne of the gauge section re inainsfconstan't during testing and it eliminates the direct'measurementjof the thickness which owing to its s'm'all' valu e in: a sheet material'yields less accurate resultsl'f The R 'value,"is therefore, a useful parameterfor indicatin'g thedegree of mechanical anisotropy of a given materi alQForfan' isotropic sheet, the R value is equal to one. If is'les's'jthan" one, the sheet thins unduly and is, thereforeimdesirable for drawing operations. For deep drawing, it is preferable to have R values equal to or gre ate-rj than about 11.5 however, for some applications, materialwith lower R valuesmaybe satisfactory.

To obtain "an, 'ayerageR value, tensile tests are conducted: on. severaljspecirnenstaken at various angles, iisuallyfatid," 45fandf 90 to the rolling direction. An average R value" of the sheet can then be obtained as follows:

Patented Oct. 1, 1968 'ice The difference among the individual R values indicates the earing tendency of the sheet in the drawing operation. The larger the difference, the stronger is the tendency for caring.

It has been found that the crystallographic orientation of the grains, and not the grain shape, is primarily responsible for the drawing properties. We have discovered that the drawability and the R value can be correlated with the crystallographic texture of the sheet. Good drawability and high R values are associated with the socalled cube-on-corner or the (111) texture, i.e. the (111) planes are parallel to the plane of the sheet. Poor drawability and low R values are associated with the cube-on-face texture. The cube-on-edge or the texture has intermediate drawing properties. Thus, for good drawing properties, the amount of the (111) texture should be high, whereas that of the (100) texture should be low. For a specific crystallographic plane, the R value varies also with the directions lying in the plane. Therefore, the ideal texture for optimum drawability is 111) fiber texture with the sheet plane normal as the fiber axis.

The crystallographic texture of a specimen is normally determined by the construction of complete pole figures from X-ray intensity measurements; however, for detection of small variation in the texture, a direct comparison of two pole figures cannot reveal the detailed differences quantitatively. Accordingly, we have found it best to measure the integrated peak intensities of several refiections from the plane of the sheet and express them in units of corresponding peak intensities of a random specimen. The numerical values of these relative intensities so obtained are directly proportional to the pole densities of a specific plane lying parallel to the plane of the sheet. Since the drawability of a sheet depends on the relative population of specific crystallographic planes in the plane of the sheet, this technique is very useful. The intensities of five different reflections, i.e. (110), (200), (112), (310) and (222) are measured. The intensity of the (222) reflection which is the second order reflection of the (111) therefore represents the amount of (111). texture. Similarly, the intensityof the (200) reflection represents the amount of the (100) texture, respectively. These two textures have, therefore, effects on drawability as do their counterparts. The correlation between R values and texture has been found to be very consistent in actual test results.

We have found a method of producing low-carbon sheet steel of good drawability without sacrificing yield strength which involves a combination of steps applied to low-carbon steels having initially greater than 0.02% carbon. Our method produces a crystallographic texture with a high degree of (111) orientation and a lesser quantity of 100) orientation. Thus, low-carbon steel processed according to the invention has a very favorable average R value indicating good drawability. In addition, however, we have found that by controlling the final carbon content and precluding undue decarburization, a high yield strength may be maintained and a low-carbon steel having both improved drawability and high strength can be produced. According to the invention, a low carbon, hot rolled plate having more than about 0.02% carbon is cold rolled to from 50% to 85% reduction into sheet gauge. The cold-rolled sheet is then annealed at normal annealing temperature in the range from about 1025 F. to'about 1550 F. (below the transformation temperature) for more than 10 hours in an atmosphere containing dry hydrogen. The term dry hydrogen as used herein refers tohydrogen having a dew point less than 30 F. The sheet steel is annealed at the stated temperature for the stated time until the carbon content is in the range from about 0.004 to about 0.02% and then TABLE I.CHEMICAL COMPOSITION OF STEELS C Mn Si S P A1501 N O It should be noted that compositions A and B had a desired carbon content above 0.02% whereas composi tion C had a lower initial carbon content. Each of these steels was hot rolled in a commercial mill under the following conditions.

TABLE II.HOT ROLLING CONDITIONS Temperature, F. Finish Thick- Steel ness (in.)

Enter Finish Coil The microstructure of the hot-rolled plate from these steels consisted of equiaxed grains of about #8 ASTM grain sizes. In compositions A and B, small colonies of fine pearlite were found to exist between the grains and at three grain junctions and carbide plates were present at the grain boundaries. In composition C, pearliteicolonies were rare; however, a few thin carbide plates existed at some of the grain boundaries.

Crystallographic examinations (the results of which are reported in Table III below) of the hot rolled plate showed only minor differences between the three steels, except that the (110) intensity of composition C steel was very high and the (222) intensity was lower than in either of compositions A or B. The texture of compositions A and B were practically the same.

TABLE III.RELATIVE INTENSITIES OF SELECTED X RAY REFLECTIONS FROM HOT-ROLLED PLATE, STEELS A, B, AND C A B C In aluminum-killed steels, crystallographic texture is controlled by precipitation of aluminum nitride. In lowcarbon steels treated according to the invention, the crystallographic texture is controlled utilizing the cementite normally present. This may be accomplished by various annealing treatments such as the solution and tempering treatments prior to cold rolling, or decarburization during recrystallization anneal to eliect the development of desired textures. It'has been found that excellent R values can be obtained in low-carbon steels having initial carbon contents of more than about 0.02%, such as compositions A and B, by proper annealing in a dry hydrogen atmosphere whereby the carbon content is reduced to within the range of from 0.004 to 0.02% carbon. It has also been found, as will be shown hereinafter, that the treatment did not produce satisfactory results for composition C because the initial carbon content was already below the 0.02% limit.

Hot-rolled plate samples of compositions A, B and C were cold rolled to 70% reduction in thickness from 0.096 to 0.029 inch for compositions ,A and B, and from 0.086 to 0.026 for composition C. Thesarnples'weredhn annealed at a temperature of 1320 F. in dry hydrogen having a dew point of approximately 90 F. The specimens were held at annealing temperature for 20 hours after which they were allowed to cool in the furnace. The texture of the samples together with that of a typical aluminum-killed (SK grade) deep-drawing steel are shown in Table-IV; TABLE Iv.-RELATIvE INTENs'ITIEs, OF SELECTED RAY REFLECTIONS FROM DRY HYDRO GEN ANNEAL-E Low-CARBON sTEELs AND A TYPICAL SK STEEL A I B C It can be seen that the unfavorable texture components, i.e. (200) and of compositions A and B are sub; stantially lower than those of the SK grade, whereas the most favorable texture component (222) is higher than that of the SK grade. On the other hand, the (110) and (222) components of composition C are equal ,to" those of the SK grade, but the (200) and (310) components are appreciably higher. The (112) component of all three low-carbon steels are lower than that of the SK steel.

The R values indicating relative drawability and yield strength and grain size, as determined from duplicate samples of these steels which were also tested are shown in Table V. Corresponding data for the SK grade steel are also listed for comparison. 7 TABLE V.THE R VALUES, YIELD STRENGTH, AND

GRAIN SIZE OF DRY HYDROGEN ANNEALED LOW- CARBoN sTEELs As COMPARED WITH THOSE OF A TYPICAL SK STEEL v si; 5 1:73; 1 1.31 3 2.22; 1.64; 23.2 'as Grain size As can be seen, the R values of compositions A and B are equal or superior to those of the SKLgrad'e steel and the R values for composition C are notas satisfactory. The grains in all three carbon steels, in contr'a'stt'o the SK grade steels, are equiaxed. I

The extent of decarburization dependsto some extent on'the flow rate of the hydrogen-containing atmosphere and the surface exposure of the specimen. With'a' flow rate of approximately 6080 ccf/minute, the carbon content may be reduced to as low as 0.004%, if care is exercised in the placement of. the sheets. If the sheets are placed loosely in contact with each other, the carbon content can be maintained at about0.016%, Extensive testing indicates that consistently good texture, high :R values and high yield strength are. always obtained iflthe carbon content in the annealed strip is reduced' jto below 0.02%. On the other hand, we have found that if'the' ii'nal carbon content is greater than 0.02%, the texture and the R values of the annealed strip ,ar'einvariably poor,' The results of numerous tes'tsindicatethat during'thei'i'di'y hydrogen anneal, the desired crystallographic texture is developed through the influence of the iron carbide p'recipitate on the growth characteristics of the grains,

The effect of heating rate and soaking tirn efon" plastic strain ratio is shown in'Tablc vL 'These results showftliat at a constant soaking time theave'ra'ge'R yalues decrease slightly with increasing heating rate and with' a constant heating rate, R decreases with decreasing fs'oak i ng time". The remaining carbon content correlates withthe Rvalues and shows that poor R values 'are associated "with high carbon content, i.e. carbon contents g'reater thari0.02% The remaining carbon ,contents indicate also that the effective decarburization occurs mainly during the soaking period of the annealing treatment.

TABLE VL-EFFECT OF HEATING RATE OR SOAKING TIME ON THE AVERAGE PLASTIC STRAIN RATIO, R, AND THE REMAINING CARBON CONTENTS Heating r ate Soaking time 0, percent (time to 1,320" F.) (Hr. at 1,320 F.)

A B A B To illustrate the importance of using dry hydrogen, a series of tests were conducted which differed only in that in some dry hydrogen was employed and in others wet hydrogen was used. Although, as is known, wet hydrogen is more effective for decarburization than dry hydrogen, we have found that excessive decarburization and grain growth results with the use of wet hydrogen even though acceptable R values may be obtained. Table VII reports the results of four tests of compositions A and B, two of which were conducted with the annealing performed in dry hydrogen and two with the annealing in wet hydrogen. The results of these tests are reproduced in Table VII.

TABLE VII Dry Hydrogen Wet Hydrogen A B A B R 1. 99 1. 59 1. 73 1. Y.S. (K psi.) 17. 8 23. 2 10.0 9. 8 Grain size (ASTM N 0.) 6. 0 7. 0 6-1 7. 0 Carbon Content 0. 007 0. 004 0015 0015. 0014 As can be seen, the nitrogen content has been lowered significantly. Since unfixed nitrogen is a principal cause of strain aging of low-carbon steels, the undesirable effects of agingon mechanical properties can be minmized by this treatment.

TABLE VIIL-CHEMICAL COMPOSITION OF STEEL AFTER DRY HYDROGEN ANNEAL Steel C Si S The strain aging index of dry hydrogen annealed products is shown below in Table IX.

TABLE IX.- Strain aging index of dry hydrogen annealed steels (prestrained 8%, aged 4 hrs. at 212 F., and retested) Percent increase in yield stress (Strain Aging Index):

The percent increase in yield stress when the specimen is prestrained 5% minimum or beyond discontinuous yielding, aged 4 hours at 212 F., then retested.

The above values compare favorably to the strain aging index of conventional box-annealed rimmed steels, which usually ranges between 20 to 25% It is apparent from the above that certain changes and modifications may be made without departing from the invention. It is seen, however, that it is essential for the successful practice according to the invention for the inital carbon content of the steels treated to be above 0.02% and, further, that the carbon be reduced during the dry hydrogen anneal to from between 0.016 to 0.004%. The effect of this treatment is to impart improved drawability without sacrificing yield strength. It is also shown that too extensive decarburization results in excessive grain growth which impairs the drawability and yield strength of the sheet steel. Moreover, the product is relatively nonaging as Well as deep drawable due to the fact that the dry hydrogen anneal also reduces the nitrogen content considerably, i.e., to about 0.001%.

We claim:

1. A method of producing low-carbon sheet steel of improved drawability and high yield strength which comprises cold rolling hot-rolled plate of lowcarbon steel having more than 0.02% carbon to from 50 to 85% reduction into sheet gauge, annealing said cold-rolled sheet at a temperature in the range of from about 1025 F. to about 1550 F. for more than ten hours in an atmosphere containing dry hydrogen having a dew point less than 30 F. to result in a carbon content of from 0.004 to less than 0.02% and cooling the annealed sheet.

2. A method according to claim 1 wherein said hotrolled sheet is cold rolled to from to reduction.

3. A method according to claim 1 wherein said coldrolled sheet is annealed at a temperature in the range of 1200 F. to 1400" F.

4. A method according to claim 3 wherein said coldrolled sheet is annealed for more than 15 hours.

5. A method according to claim 4 wherein said coldrolled sheet is annealed for about 20 hours.

References Cited UNITED STATES PATENTS 2,360,868 10/ 1944 Gensamer 148-16 3,239,388 3/1966 Sasaki 148-12.1 3,239,389 3/1966 Yoshida 148--12.1 3,244,565 4/1966 Mayer et al 14812.1 3,262,821 7/1966 Yoshida 14812.1 3,281,286 10/1966 Shimizu et a1. 14816 3,348,980 10/1967 Enrietto l48-l2.1

OTHER REFERENCES Low et al., vol. 158, AIME Transactions, Iron and Steel Division, p. 209 et seq.

HYLAND B IZOT, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

Claims

1. A METHOD OF PRODUCING LOW-CARBON SHEET STEEL OF IMPROVED DRAWABILITY AND HIGH YIELD STRENGTH WHICH COMPRISES COLD ROLLING HOT-ROLLED PLATE OF LOW-CARBON STEEL HAVING MORE THAN 0.02% CARBON TO FROM 50 TO 85% REDUCTION INTO SHEET GAUGE, ANNEALING SAID COLD-ROLLED SHEET AT A TEMPERATURE IN THE RANGE OF FROM ABOUT 1025*F. TO ABOUT 1550*F. FOR MORE THAN TEN HOURS IN AN ATMOSPHERE CONTANING DRY HYDROGEN HAVING A DEW POINT LESS THAN -30*F. TO RESULT IN A CARBON CONTENT OF FROM 0.004 TO LESS THAN 0.02% AND COOLING THE ANNEALED SHEET.