US3897280A

US3897280A - Method for manufacturing a steel sheet and product obtained thereby

Info

Publication number: US3897280A
Application number: US423821A
Authority: US
Inventors: Hisashi Gondo; Hiroshi Takechi; Tsuyoshi Kawanc; Hiroaki Masui
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1972-12-23
Filing date: 1973-12-11
Publication date: 1975-07-29
Anticipated expiration: 1992-07-29
Also published as: BR7310000D0; IT1000382B; DE2362658C3; DE2362658A1; GB1450131A; DE2362658B2

Abstract

A killed steel having a particular composition is heated, subjected to hot rolling such that the finishing temperature is 750*C or less, and subjected to recrystallization treatment. A steel sheet thus obtained has excellent press formability.

Description

United States Patent Gondo et al.

[451 July 29, 1975 METHOD FOR MANUFACTURING A STEEL SHEET AND PRODUCT OBTAINED TI-IEREBY Inventors: Hisashi Gondo; I'Iiroshi Takechi;

Tsuyoshi Kawano, all of Kisarazu; I-Iiroaki Masui, Kimitsu, all of Japan Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Dec. 11, 1973 Appl. No.: 423,821

Foreign Application Priority Data Dec. 23, 1972 Japan 47-128853 Dec. 23, 1972 Japan 47-128854 US. Cl 148/12 C Int. Cl. C21D 9/48 Field of Search 148/12 C Primary ExaminerW. Stallard Attorney, Agent, or Firm-Watson Leavenworth Kelton & Taggart [57] ABSTRACT A killed steel having a particular composition is heated, subjected to hot rolling such that the finishing temperature is 750C or less, and subjected to recrystallization treatment. A steel sheet thus obtained has excellent press formability.

14 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to methods for manufacturing steel sheet, and more particularly. to methods for manufacturing steel sheet demonstrating excellent deep drawing properties.

2. Description of the Prior Art One of the most important properties of a steel sheet for use in complex press formability is a capability for deep drawing or deep-drawability. This capability becomes better as the number of crystallographic planes {111} parallel to the surface of the steel sheet is increased or the number of planes {100} is decreased, and can be estimated by means of Rankford value or Conical cup value.

In an attempt to obtain a steel sheet having deep-drawability, there has been proposed a method for cold-rolling a hot-rolled steel sheet under a suitable reduction rate and thereafter annealing the same above the recrystallization temperature.

It is a common practice at present to effect a hotrolling above A point or usually above 860C for a steel sheet to be used for deep drawing'ln this case, however, the deep-drawability is extremely inferior to that of a cold-rolled steel sheet since the axial density of the plane {11 1} parallel to the surface of the steel sheet is as low as about 1 and it is arranged under substantially random conditions from the crystallographic viewpoint. In a special case, a hot-rolled steel sheet having a low yield point may be manufactured by lowering the finishing temperature down to about 800C. In this case, the steel sheet becomes soft but the axial density of the plane {100} parallel to its surface is remarkably increased and thus the deep-drawability of the sheet becomes so low that it can not be used as a steel sheet for deep drawing. Accordingly, as far as deep drawing is concerned, cold-rolled steel sheet which can be manufactured by means of the first-mentioned method is suitable. The disadvantage of cold-rolled steel lies, however, in its high production cost due to complicated manufacturing steps as compared with the manufacturing steps used for making hot-rolled steel sheet.

Recently, there has been conducted a study for improving the deep-drawability by effecting, after rolling at warm temperatures, a treatment for recrystallization without cold rolling. When it is desired to obtain excellent deep-drawability by means of rolling a lowcarbon steel at warm temperatures, it is inevitable, according to the result of the inventors study, to bring the finish temperature down to 550C or less, otherwise the object can not be accomplished.

Rolling at such low temperatures will not only greatly reduce commercial productivity extremely but will also result in increase in rolling load. In view of this, it is desirable that evenwhen rolling at warm temperatures is effected, the rolling temperature should be capable of being increased to the highest level possible yet providing a steel sheet having excellent deep-drawability.

SUMMARY OF THE INVENTION As the result of various studies made by the inventors, they have succeeded in manufacturing a steel sheet having excellent deep-drawability by limiting the composition of the steel and the conditions for its hot rolling to a material of a particular range and effecting the hot rolling followed by recrystallization treatment without a step of cold rolling.

It is therefore an object of the invention to provide a method for manufacturing a steel sheet having excellent press formability by means of hot-rolling at low temperatures followed by recrystallization without a step of cold-rolling after the hot rolling.

According to the invention, there is provided a method for manufacturing a steel sheet having excel.- lent press formability which comprises heating a killed steel composed of not more than 0.10% C, 0.05% to 1.0% Mn, 0.005% to 0.15% A1, at least one member of the group consisting of Ti, Nb and Zr, and the rest Fe and unavoidable impurities, the Ti%/4 Nb%/7.8 Zr%/7.6 being more than the C% in the steel, subjecting the same to hot rolling such that the finishing temperature is 750C or less, and thereafter effecting a treatment for recrystallization. Also provided is a method according to the above method in which said killed steel further contains at least one member selected from the group consisting of not more than 0.15% P, not more than 0.10% W, not more than 0.10% Mo, not more than 0.3% Cr,.not more than 0.3% Cu, and not more than 0.4% Si.

Still further is provided a steel sheet having excellent press formability produced by the above mentioned method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS sheet having a thickness of 2.7 mm was manufactured under the conditions shown in Table 2. The conical cup value tested thereon is also shownin Table 2.

Table 1 Chemical composition of steel materials C Si Mn P s N 0 Al T1 4 7'8 Table 2 Conical 'cup value (mm) ol'stccl sheets Condition of manufacture Conical cup (mm) Finishing Heat treatment Temperature after rolling A B Conventional 900C None (650C.coiling) 85.4 mm 86.5 mm method 820C None (700C coiling) 87.5 88.5 750C 85.0 89.0

700% 80.3 88.1 This Box annealing This invention 600C (700C X 4 hr) 76.4 imention 871 Remarks:

Thickn s", 2.7 mm

('onicul.cup blank diameter; 108 mm .As seen from Table 2, deep-drawability decreases as soon as the finishing temperature, which is above 900C decrease from 900C which is the usual finishing but deep drawability increases as the finishing temperature is further decreased. It is then obvious that the B steel, or control, has deep-drawability which becomes equal or superior to that of steel finished at 900C when the finishing temperature is reduced to about 550C or less. Contrary to this, the A steel of the invention, when subjected to hot rolling such that the finishing temperature is 750C or less, and then to annealing, shows deep-drawability which is superior to that of steel finished at 900C and which is extremely superior to that of the B steel at the same finishing temperature.

The subsequent study revealed that in order to manu facture a steel sheet having such excellent deep-drawability, it is necessary to satisfy the conditions shown below.

One of such conditions is that the chemical composition of the steel material requires such combination of elements as to enhance the recrystallization temperature indicated for that of the cold rolled material. At the same time it is necessary not only to enhance the recrystallization temperature but also to form a deposit or precipitate at hot or warm temperature rolling which is suitable for developing the axial density of the plane {1 l 1} parallel to a surface of the steel sheet during the recrystallization treatment.

For the purpose of accomplishing the above, a composition of the steel according to the instant invention should be that C is not more than 0.10%; Mn is 0.05 to 1.0%; A1 is 0.005 to 0.15%, and that when Ti, Nb or Zr is to be added singly, Ti%/C% 4, Nb%/C% 7.8 or Zr%/C% 7.6. is to be satisfied respectively. If the Ti, Nb or Zr are added in combination, the characteristic properties of the product becomes more stabilized. When they are added in such combination, Ti%/4 l- Nb%/7.8 l'- Zr%/7.6 C% in the steel must be satisfied. If necessary, not more than 0.15% P, not more than 0.10% W, not more than 0.10% Mo, not more than 03% Cr, not more than 0.3% Cu, and not more than 0.4% Si should be added singly or in combination. As for the content of C, it should be up to 0.10%. If it is more than 0.10%, it will not only deteriorate the deep-drawability of steel sheet, but will also increase the amount of Ti, Nb or Zr to be added, which gives rise to an increase in the cost. Moreover, the increase of C content will result in elevation of the heating temperature necessary for solubilizing TiC, NbC' or ZrC which extremely lowers the productivity of hot rolling. For these reasons the content of C should preferably be 0.03% or less, if the strength of the product is not stressed very much. Deep drawability is further enhanced by satisfying C 0.02% by means of vacuum degassing treatment and the like.

Mn is necessary in an amount of at least 0.05%, and preferably at least 0.1% for the purpose of deoxidation and prevention of hot brittleness. However, deep-drawability is lowered when the amount of Mn is more than 1 .0%. If the steel sheet is not stressed, it should preferably be 0.5% or less in view of the deep-drawability.

Al is necessary in an amount of at least 0.005%, and preferably at least 0.01%, for deoxidation. If it is, however, more than 0.15%, it deteriorates the cleanliness of the steel and degrades the surface condition of the steel.

P serves to enhance the anti-weather property as well as deep-drawability but it should be added in an amount of 0.15% or less, since an amount more than 0.15% will harden the steel.

W and Mo will promote deep-drawability. The amount of W and Mo to be added should be not more than 0.10% since an amount more than 0.10% will add substantially nothingto the effect thereof.

Cr and Cu will promote the anti-weather property. The amount of Cr and Cu to be added should be not more than 0.3% since an amount more than 0.3% will add substantially nothing to the effect thereof.

Si to be added should be not more than 0.4%. If it is more than 0.4%. it will impart an adverse effect upon the ductile property of the material.

In addition to the above limitation for each component of the steel, another condition to consider in carrying out the method of the instant the invention is the control of the rolling conditions. In order to effectively generate finely-divided deposits or precipitates which are considered to be necessary for enhancing the recrystallization temperature and for forming an aggregative structure desirable for deepadrawability in the recrystallization treatment after hot rolling, that is, in order to allow the TiC in case of Ti-added steel, or the NbC in case of Nb-added steel, etc. to effectively deposit or precipitate, or in order to allow the cold strain to remain in the rolled material, it is necessary to keep the finishing temperature at 750C or less. If it is more than 750C, the deep-drawability of the material subjected to hot rolling, followed by recrystallization treatment. is inferior to that of the hot rolled material subjected to finish rolling at a temperature more than A;, point which has been a common practice. The finishing temperature will promote the deep-drawability as it decreases as shown in Table 2. It is thus preferable to keep the finishing temperature as low as possible. However, a finishing temperature below 400C is not practicable since it will increase the deformation resistance of the mill to such an extent that the rolling power must be made greater, and in addition it will fail to provide a better deep drawing. In addition to the control or limitation of the finishing temperature, when the total reduction rate below 800C is made greater, preferably made 40% or more of the thickness of the material at 800C, the deep-drawability is further promoted. In conventional continuous finish rolling, the limitation of the finishing temperature to 750C or less will give a steel sheet having better deep-drawability than the conventional case in which the rolling is finished at more than A point.

In this invention, after the above rolling is over, the recrystallization treatment is effected so as to release the strain accumulated in the rolled material as well as develop the axial density of the planes {1 1 l} which are parallel to the surface of the steel and which are desirable for the deep-drawability. This recrystallization treatment is conducted by heating at temperatures ranging from the recrystallization temperature up to A point, or by such suitable methods as box annealing, open coil annealing, continuous annealing, etc. When the heating is effected below the recrystallization temperature, not only is the strain can not fully be released but also that an aggregative structure desirable for deep drawing can not be developed. Moreover, when the annealing is effected above A point, the structure becomes substantially random from the crystallographic 6 viewpoint, which results in lowering of deep-drawability.

EXAMPLE A steel material having a chemical composition shown in Table .3 was melted in a converter, from which a slab was manufactured by the conventional steps. The slab was heated at about 1,200C, then subjected to hot rolling in a continuous hot rolling mill at three levels of finishing temperatures, 550C, 650C and 740C, followed by water cooling and then, coiling. Thereafter the steel was subjected to a box annealing at 700C for 4 hours or to a continuous annealing at 850C for 5 minutes. The conical cup value and the axial density of-the planes {1 11} and {100} parallel to the surface with respect to'the above-treated steel sheet and the same steel sheet differing only by having subjected'to hot rolling at the conventional finishing temperature, 900C, are shown in Table 4 and Table 5, respectively. The thickness of the sheet was 2.7 mm in both cases.

As is obvious from Table 4, the steel sheet of the invention has the high axial density of the planes {1 l l} and excellent deep-drawability as compared with that of the prior art, said deep-drawability being comparable to that of cold rolled steel sheet.

Table 3 Chemical composition in Example C Si Mn P S p O A1 Ti Control C 0.062 0.01 0.26 0.01 1 0.013 0.0024 0.030 0005 steel Steel D 0.008 0.02 0.27 0.009 0.009 0.0051 0.005 0.026 0.1 l of this E 0.007 0.03 0.20 0.019 0.010 0.0062 0.006 0.018 invention F 0.008 0.02 0.32 0.012 0.007 0.0052 0.006 0.024

O 0.015 0.03 0.19 0.015 0.013 0.0049 0.005 0.015 0.06 H 0.008 0.02 0.82 0.017 0.012 0.0043 0.005 0.074 0.18 1 0.010 0.02 0.17 0.013 0.012 0.0041 0.005 0.032 0.12 J 0.007 0.02 0.21 0.074 0.008 0.0037 0.006 0.029 0.07 K 0.01 l 0.03 0.24 0.019 0.006 0.0044 0.006 0.038 0.10 L 0.006 0.01 0.23 0.015 0.011 0.0051 0.005 0.043 0.04 M 0.008 0.18 0.30 0.12 0.008 0.0047 0.005 0.091 0.04 N 0.010 0.35 0.74 0.010 0.010 0.0036 0.008 0.034 0.16

Table 3a Nb 2 Ti Nh Zr 7 r T 6 W Mo Cr Cu Control ,c 0oo1 steel Steel D I 0.028 of, this E 0.10 0.013 invention F 0.12 0.016

' I O 0.05 0.08 0.032 H 0.02 0.04 0.070 l 0.01 0.02 0.034 0.05

L 0.02 0.08 0.023 0.23 0.17 M 0.02 0.10 0.026 N 0.04 0.04 0.050 0.08 0.07 (H4 0.26

Table 4 Conical cup value in Example Condition of manufacture Conical cup value (mm) Fishing Heat treat- Control temperament after steel Steel of this invention ture rolling C D E F G H l .1 K L M N Control 900C None (Coiled 86.3 85.6 85.5 85.7 85.5 86.1 85.4 85.6 85.8 85.7 85.8 86.2 method at 650C) Table 4 Continued Conical cup value in Example Condition of manufacture Conical cup value (mm) Fishing Heat treat- Control l temperament after steel Steel of this 1nver1t1on ture rolling C D 1. F (l H l .l K L M N Method 650C Box annealing 87.6 77.2 77.0 77.1 76.9 78.3 76.3 75.9 76.8 77.0 76.1 77.9 of this 550C 700C X 4 hr. 86.4 75.0 74.3 74.8 74.2 76.4 74.1 73.9 74.0 75.2 74.2 75.6 inven- 740C Continuous 89.1 85.4 85.2 85.3 85.1 85.4 84.7 84.7 85.1 85.0 85.1 86.4 tion 650C annealing 88.1 76.8 77.7 77.7 76.7 77.3 76.2 75.7 75.9 76.8 75.9 77.2 550C 850C X 5 min. 86.3 74.0 74.7 75.5 74.0 75.6 73.9 73.4 74.0 75.1 73.7 75.8

Thickness: 2.7 mm (onicnl cup blank diz1.; 108 mm Table 5 Strength of X-ray reflection in Example Strength of X-ray reflection {l l 1} axial density 100} axial density] Finishing Control tempera- Heat treatment steel Steel of this Invention ture after rolling C D. 1 E, J F, K G, L

Control 900C None (Coiled at 1.0/ 1.1 1.1 1.2 1.0/ 1.0 1.0/ 1.1 1.2 1.] method 650C) '1 740C 1.5/3.0 2.0/1.3 2.1/1.3 Method 650C Box annealing 2.0 1.9 7.0/ 1.0 6.9 0.9 of this 550C 700C X 4 hr. 2.6/ 1.6 7.5 1.0 7.8/ 1.0 inven- 740C Continuous annealing 1.7 3.3 2.1 1.2 2.3 1.3

tion 650C 850C X 5 min. 2.1 1.8 7.4/ 1.1 7.2/ 1.0 550C 2.5/1.4 7.4/l.0 7.6/1.1

'1 not measured What is claimed is:

1. A method for manufacturing a steel sheet having an excellent press formability which comprises heating 21 killed steel composed of not more than 0.10% C, 0.05% to 1.0% Mn, 0.005% to 0.15% A1, at least one member of the group consisting of Ti, Nb and Zr, and the rest Fe and unavoidable impurities, the Ti%/4 -l- Nb%/7.8 Zr%/7.6 being more than the C in the steel, subjecting the same to hot rolling such that the finishing temperature is 750C or less, and thereafter effecting a treatment for recrystallization.

2. A method according to claim 1 in which said killed steel further contains at least one member selected from the group consisting of not more than 0.15% P, not more than 0.10% W, not more than 0.10% Mo, not more than 0.3% Cr, not more than 0.3% Cu and not more than 0.4% Si.

3. A method according to claim 1 in which the total reduction rate below 800C is at least 40% of the thickness of the material at 800C.

4. A method according to claim 1 in which C is no greater than 0.03%; Mn is 0.1% to 0.5%; and Al is 0.01% to 0.15%.

5. A method according to claim 1 in which the finishing temperature is 400C to 750C.

6. A method according to claim 1 in which the recrystallization treatment is conducted by heating at temperatures ranging from the recrystallization temperature up to A point.

7. A method according to claim 1 in which the recrystallization treatment is conducted by box annealing, open coil annealing or continuous annealing.

8. A method according to claim 2 in which the total reduction rate below 800C is at least 40% of the thickness of the material at 800C.

9. A method according to claim 2 in which C is no greater than 0.03%; Mn is 0.1% to 0.5%, and Al is 0.01% to 0.15%.

10. A method according to claim 2 in which the finishing temperature is 400C to 750C.

11. A method according to claim 2 in which the recrystallization treatment is conducted by heating at temperatures ranging from the recrystallization temperature up to A point.

12. A method according to claim 2 in which the recrystallization treatment is conducted by box annealing. open coil annealing or continuous annealing.

13. A method according to claim 1 in which the ratio of 111 axial density axial density is within the range from about 2.0/1.3 to about 7.8/1.0.

14. A method according to claim 2 in which the ratio of l l l axial density {100} axial density is within the range from about 2.0/1.3 to about 7.8/1.0.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,897,280 Dated July 29, 1975 Inventor(s) Hisashi Gondo, et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 5, lines 23 and 24 should read perature, not only is the strain not fully released but also an aggregative structure desirable for deep-.

Col. 6, line 18, after "having" insert --been- Table 5, the second line thereof should read Strength of X-ray reflection lll} axial density/ Col. 8, line 52, "111" should read {111} Col. 8, line 55,- "111" should read {111} Signed and Sealed this sixteenth D a y 0 f March 19 76 [SEAL] A ttes t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ofPatems and Trademarks

Claims

1. A METHOD FOR MANUFACTURING A STEEL SHEET HAVING AN EXCELLENT PRESS FORMABILITY WHICH COMPRISES HEATING A KILLED STEEL COMPOSED OF NOT MORE THAN 0.10% C, 0.05% TO 1.0% MN, 0.005% TO 0.15% AL, AT LEAST ONE MEMBER OF THE GRUP CONSISTING OF TI, NB AND ZR, AND THE REST FE AND UNAVOIDABLE IMPURITIES, THE TI%/4 + NB%/7.8 + ZR%/7.6 BEING MORE THAN THE C % IN THE STEEL, SUBJECTING THE SAME TO HOT ROLLING SUCH THAT THE FINISHING TEMPERATURE IS 750*C OR LESS, AND THEREAFTER EFFECTING A TREATMENT FOR RECRYSTALLIZATION.

3. A method according to claim 1 in which the total reduction rate below 800*C is at least 40% of the thickness of the material at 800*C.

5. A method according to claim 1 in which the finishing temperature is 400*C to 750*C.

6. A method according to claim 1 in which the recrystallization treatment is conducted by heating at temperatures ranging from the recrystallization temperature up to A3 point.

8. A method according to claim 2 in which the total reduction rate below 800*C is at least 40% of the thickness of the material at 800*C.

10. A method according to claim 2 in which the finishing temperature is 400*C to 750*C.

11. A method according to claim 2 in which the recrystallization treatment is conducted by heating at temperatures ranging from the recrystallization temperature up to A3 point.

12. A method according to claim 2 in which the recrystallization treatment is conducted by box annealing, open coil annealing or continuous annealing.

13. A method according to claim 1 in which the ratio of (111) axial density (100) axial density is within the range from about 2.0/1.3 to about 7.8/1.0.

14. A method according to claim 2 in which the ratio of (111) axial density (100) axial density is within the range from about 2.0/1.3 to about 7.8/1.0.