AU592481B2 - Steel strip or sheet for di cans and production method thereof - Google Patents

Description

AUSTRALIA

I

S

v A -592481 PATENTS ACT 1952 F 1 Form COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: *8upa e aon a pnr o, Lapsed: S Published: r. Priority: *P.m P- ,lUM 0 UOarP t oaortiuoaj toumnoop sql Related Art: 4 4t TO BE COMPLETED BY APPLICANT Name of Applicant: i NIPPON STEEL CORPORATION Address of Applicant: 2-6-3, OTE-MACHI

CHIYODA-KU

TOKYO

JAPAN

Actual Inventor: Address for Service: CLEMENT HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Complete Specification for the invention entitled: STEEL STRIP OR SHEET FOR DI CANS AND PRODUCTION METHOD THEREOF The following statement is a full description of this invention including the best method of performing it known to me:i I- Background of the Invention: Field of the Invention: The present invention relates to a surface treated steel sheet or strip (herein called simply steel sheet) suitable for production of drawn and ironed (DI) cans, which is soft enough and excellent in stretching flange formability after the DI working and also is hardenable during paint coating baking after the DI working so as to provide improved pressure resistance strength for DI cans.

*1 Description of the Related Art: Surface treated steel sheets, such as tin-plate It and tin-free steel sheet (TFS), have recently been used in increasing amounts for production of food cans, air-sol cans and easy-open cans. These surface treated steel sheets for the above applications have been often subjected to multiple-step drawing or DI working so that they must have excellent workability in addition to excellent corrosion resistance.

Surface treated steel sheets for DI working must meet the requirements that they have excellent workability, free from biting by the tool, require less working energy, provide excellent stretching flange formability after the DI working, and show a high pressure resistance strength as finished cans.

Conventionally, as the surface treated steel sheets for the DI working, chiefly box-annealed steel 2 k I I I I i sheets, such as B-containing Al-killed steel sheets disclosed in Japanese Laid-Open Patent Application Sho 53-48913, and Cu-containing low carbon steel sheets disclosed in Japanese Patent Publication Sho 52-16965 have been used mainly because the box-annealing can give better elongation and drawability, and thus has been considered to be better applicable for steel sheets for the DI working.

For the DI working, the excellent stretching flange formability is considered to be very important and it is necessary to reduce the reject ratio in this connection to less than several tens ppm. For these reasons the box-annealed steel sheets having excellent elongation, r value and less carbon in solid solution have been selected for the DI working.

Regarding the softness, indicated by the "temper degrees", the steel sheet for the DI working must have a temper degree ranging from T-1 to T-3, and these soft materials have conventionally produced by the box-annealing.

The temper degree is expressed in terms of the Rockwell superficial hardness (HR 30 T) with 46 to 52; T-2: 50 to 56; T-3: 54 to 60 r1=4: 58 to 64; 62 to 68,' and T-6: 67 to 73 in the order from soft to hard tempers.

However the box-annealing has disadvantages that the shape of the annealed sheets is irregular and the r chanical properties are not uniform in the 3 1 lengthwise direction of the strip, the productivity is low and the production cost is high as compared with a continuous annealing process.

For overcoming the disadvantages of the box-annealing, proposals have been made for production of soft steel sheets for surface treatment of T-3 grade by continuous annealing as disclosed in Japanese Patent Publication Sho 61-11290 in which Al-killed steels with lowered carbon and nitrogen are subjected to overaging 4 in continuous annealing. This proposal, however, fails o,6* 0 to assure consistent production of tin-plate sheets in S o a commercial production line.

S* Meanwhile, many efforts have been made to thin the wall of the DI cans and strong demands are increasingly made for improved pressure resistance strength of the steel sheets.

The pressure resistance strength of the can r body is determined by (sheet thickness) 2 x (strength) and it is necessary to increase the strength of the sheet in order to thin the wall of the can body. In this connection, the box-annealing material is generally soft and is not applicable for the purpose of thinning the can wall. Therefore, for improving the strength of the sheet material, it is necessary to add strengthening elements and to provide a relatively high alloyed composition. These measures have danger that the biting is caused during the DI working and the required working energy increases.

4 In recent years, many trials have been made for producing steel sheets for DI working by continuous annealing, but have not been successful to prevent the small cracks in the flange formation during the DI working, and fails to completely prevent the biting.

The present inventors have conducted many studies on various properties of the surface treated steel sheets required for the DI working and found that the continuous annealing is more suitable than the o box-annealing from the considerations of the practical o o utilities of the DI cans.

0 4 Summary of the Invention: One of the objects of the present invention is to provide a continuously annealed steel sheet for surface treatment having a temper degree ranging from T-1 to T-3 suitable for the DI working.

Another cbject of the present invention is to provide a steel sheet for DI cans having a tensile strength not higher than 42 kgf/mm 2 and JIS grain size number ranging from 8.5 to 11.5, excellent in stretching flange formability after the DI working and is hardenable during the paint baking after the DI working to give a high pressure resistance of DI cans.

The steel sheet for surface treatments having a temper degree ranging from T-I to T-3 for Di working is produced by a method which comprises hot rolling a low carbon Al-killed steel slab containing 0.004 to 0.06% carbon, 0.05 to 0.60% manganese, not more than 0.02% phosphorus, 0.005 to 0.100% acid soluble aluminum, not more than 0.0070% nitrogen with the balance being iron and unavoidable impurities, coiling the hot rolled strip at a temperature ranging from 600 to 710*C, cold rolling the strip, subjecting the strip to recrystallizing annealing at a temperature not lower than the recrystallizing temperature but not higher than 850 0

C

for 5 seconds to 3 minutes, then cooling the strip at 00 a cooling rate of from 5 to 250 0 C/second, and subjecting j strip to an overaging treatment at a temperature of from 300 to 500 0 C for 30 to 180 seconds., The method may be modified by that the Al-killed slab is cooled to a temperature not higher than Ar 3 transformation temperature and then heated to a temperature or lower as below; =6875 (3.865 log CAI% 0.0151]) 250 Meanwhile the steel sheet sheet excellent in stretching flange formability after the DI working and hardenable to give a high degree of pressure resistance to the final cons can be provided by the steol sheet according to the present invention which has the same composition as defined abovo and is subjected to continuous annealing so as to give a tonuilo strength not higher than 42 kgf/mmn 2 and a grain size number not less than 8.5 to not larger than 11.5 accord~ing to 318 standard.

The above continuously annealed stoel sheet w.

have been found to be excellent in stretching flange formability after the DI working despite the carbon in Ssolid solution as compared with the box-annealed material and further found that the cans made of the above continuously annealed material increases the strength during the paint baking after the DI working so that the pressure resistance strength of the can increases accordingly (this property is called hereinafter BH property).

Thus it has been found by the present inventors that by using the continuously annealed material as mentioned above, which is softer than the box-annealed material, better DI workability can be obtained because S* the material is softer and a higher pressure resistance strength of the cans can be obtained after the paint baking. This means if the same strength of material is used, a higher pressure resistance strength can be obtained by the continuously annealed material than the box-annealed material.

The continuous annealing for obtaining the above steel material can be done in any way so far the specific tensile strength and grain size number defined above can be obtained.

Detailed Description of the Inventions Explanation will be made on the reasons for the limitations of various alloying elements in the steel composition according to the present invention, Carbon generally hardens the steel and for -7-

I

this reason its upper limit is set at 0.06%. Lower carbon contents are effective to soften the steel, but carbon contents less than 0.004% will unduly reduce the solid solution carbon so that the BH property cannot be obtained. Therefore the lower limit is set at 0.004%.

Manganese is required to be present in amounts not less than 0.05% in order to prevent hot embrittlement caused by sulfur which is an unavoidable impurity, but manganese contents more than 0.60% harden the steel just as the excessive carbon contents, resulting in failure to provide the desired properties. A preferable range of manganese content is from 0.10 to 0.30%.

S* Aluminum is requires as deoxidizer for reducing S* oxide inclusions which are harmful to the workability of the steel, and also necessary for suppressing through o its fixation of nitrogen, the hardening of the steel 4 9 due to the strain aging during the surface treatments.

this purpose, at least 0.005% acid soluble alunmnum is required. However, more than 0.100% acid soluble aluminum will harden the steel, hence increasing possibility of surface defects. A preferable range for the acid soluble aluminum is from 0.015 to 0.06%.

Both phosphorus and nitrogen markedly harden the steel, and by lowering these elements, remarkable softening effects unconceivable by the conventional arts can be obtained. For providing the steel sheets having a tensile strength not higher than 42 kgf/mm 2 the upper limit of phosphorus is set at 0.02% arnd the upper limit 8of nitrogen is set at 0.0070%. A preferable range for nitrogen is no more than 0.0030%.

Sulfur tends to form inclusions in the steel, which in turn cause surface defects of the surface treated steel sheet and induce crackings of the sheet during the DI working. Therefore, it is desired to maintain the sulfur content not more than 0.015%.

To the basic steel composition defined above, boron and chromium may be added for consistent production of the soft steel sheet.

aO* For production of surface treatment substrates having a temper degree of T-1 for DI working, the steel 0 composition should desirably contain 0.01 to 0.03% carbon; 94 o* 0 0.05 to 0.20% manganese; 0.04 to 0.07% acid soluble aluminum; not more than 0.01% phosphorus; and not more 0* than 0.0025% nitrogen.

Explanation will be made on the hot rolling conditions applicable to the steel sheet according to the present invention.

o In principle, by specifying the coiling 9 temperature which is the primary factor and the slab heating temperature which is the secondary factor in the hot rolling, it is possible to obtain a continuously annealed soft steel sheet for surface treatments, suitable for the DI working and having favorable BH property.

If the coiling temperature is too low, the stretching flange formability deteriorates and the hardness increases. For this reason, the coiling temperature should be not lower than 600 0 C. On the other hand, an excessively high coiling temperature will cause difficulties in acid pickling of the steel strip. For this purpose the coiling temperature should be not higher than 710 0

C.

Fig. 1 illustrates the relation between the hot rolling temperature (6875 3.865 log [AI% 0.15]) 2501 and both the hardness and D workability of a tin-plate having the following steel composition: C: 0.0015 to 0.025% 0 Mn: 0.15 to 0.25% P% 0.006 to 0.010% Al: 0.03 to 0.08% N: not more than 0.004% The production conditions in Fig. 1 are as below.

[lot rolling finishing temperature 870 to 910 0

C

likt rolling coiling temperature 550 to 7100C Cold rolling reduction: 87 to 91% Annealing condition 700"C x 30 seconds 4006C x 60 seconds (primary cooling rate: to Temper rolling a Plating i electrolytic tin plating In Fig. 1, the hardness was measured with the surface hardness (1it OT) after the electrolytic tin 10 plating, and the DI workability was measured by subjecting plated steel sheets to DIt working using a laboratory DI machine and measuring the flange stretching rate with an expansion tube D 0

D

0 1) x 100%.

As understood from Fig. 1, when the coiling temperature is in the range from 600 to 710 0 C, the hot rolling temperature will satisfy the condition: (T)OC 6875 (3.865 log [Al% 0.015)) 250, and the hardness is lower, so that steel sheets having a temper degree fromn T-1 to T-3 and suitable to the DI working can be obtained.

2 illustrate the relation between the hot rolling temperature (T)OC, (6875 (3.865 log CAI% 0.015)) 2501 and both the hardness of a tin plate having the composition set forth below and the pressure resistance and the flange stretching rate of D1 cans produced from the tin-plate.

C: 0.01 to 0.06% Mn: 0.10 to 0.40% Pi 0.006 to 0.020% Ali 0.01 to 0.07% N: 0.0015 to 0.0070% The production conditions in Fig. 2 are as belowt Hot rolling finishi~ng temperature :870 to 9106C Hot rolling coiling temperature 1 600 to 7106C Cold rolling reductions 87 to 91W 11 Annealing condition (600 to 800 0 C) x (30 to 180 seconds) (Primary cooling ratet to 40 0 C/second Temper rolling Plating electrolytic tin-plating In Fig. 2, the hardness was measured with the surface hardness (HR 3 0T) after the electrolytic tin plating, and the pressure resistance and the flange stretching rate are measured using DI cans prepared in a laboratory.

In Fig. 2, the marks plotted at the positions correspone~±ng to (T)OC =50, (T)OC 100, and =150 include only the data of the materials which showed a temper degree from T-1 to T-2 under the annealing condition varying in a wide range.

Also in Fig. 2, similar data of comparative box-annealed materials having a temper degree from T-1.

to T-2 were plotted by the marks As understood from data shown in Fig. 2, the steel sheet for surface treatments according to the present invention, as compared with the comparative box-annealed materials, is softer by about a half temper degrou under the condition of as tin-plated, and hence is more easier to perform the DI worki.ng. Also as the BNf effect is considerably large in the paint baking process after the DX working, the steel sheet according to the present invention can provide a similar or even 12 better pressure strength of the DI cans as compared with the comparative box-annealed materials. It is noted also the flange stretching rate obtainable by the present steel sheet is better than that obtainable by the comparative materials.

It is also understood that the highest grade of surface treated steel sheet having a maximum softness and excellent ctretching flange formability very suitable for the DI working can be obtained when the hot rolling j temperature satisfies the condition:

(T)

0 C 6875 (3.865 log EAL% 0.0151) 250. This formula has been sought for through experiments on the Al contents and the heating temperature and found to I show a significant correlation in connection with the relation between the hot rolling conditions and the hardn~ess of the tin-plate and the DI workability and 1~ the pressure resistance strength of the DI cans.

For production of soft sheets for surface treatments having a temper degree ranging from T-1 to T-3, the heat history prior to the hot rolling is irrespective.

Thus the cast steel slab may be directly hot rolled while being hold at a temperature not lower than the Ar 3 transformation tomporo or the slab may be once cooled to a temperature not higher than the Ar 3 transformation temperature and then again heated and rolled. However, for production of the, sheets having a tamper degree of T-1 to 'i1-2, the steel slab should 13

M

TOC i 6875 (3.865 log [Al% 0.015)) 250 Explanation will be made on the annealing conditions applicable to the steel sheet according to the present invention.

The annealing is done by continuous annealing instead of box-annealing. The annealing cycle comprises a short time recrystallization annealing at a temperature ranging from the recrystallization temperature to 8500C for 5 seconds to 3 minutes, preferably in a temperature range from 680 tL 7200C, a cooling step with a cooling rate ranging from 5SC to 250'C/second, and overaging at a temperature ranging from 300 to 500C for 30 to 180 seconds. The cooling rate should be maintained in the above range because an excessive cooling rate will produce an excessive amount of saturated solid solution carbon present before the overaging treatment, which i excessively promotes the progress of the overaging, thus of: carbon after the overaging and honc failing to obtain the desired aa property. On the other hand if the cooling rate is excessively slow, the hardness of the sto l sheet increases despite a better B property being obtained.

egarding the ovragining temperature, lower overaging temperature will more slow the diffusion rate of carbon requirin a longer time for the overaging of carbon requirin a longer time for the oeeaging 14

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treatment. Therefore the lower limit of the overaging temperature is set at 300°C. On the other hand, as the overaging temperature increases, the solid solution limit of carbon after the overaging increases to harden the steel. Therefore, the upper limit is set at 500 0

C.

Regarding the overaging time, an excessively short time will cause shortness of the overaging and harden the steel, while an excessively longer time will markedly reduce the amount of solid solution carbon after the overaging treatment, resulting in failure to obtain the o desired BH property. A preferable overaging condition *is (350 to 450 0 C) x (60 to 120 seconds).

~Regarding the temper degree, a harder temper degree will cause biting during the DI working and increase the energy required for the working. Therefore 49 the temper degree may be in the range from T-1 to T-3.

S4 4 As stated hereinbefore, excellent DI workability of the steel sheet as well as excellent stretching flange formability and BH property of the DI cans can be provided o* by the continuously annealed steel sheet having the specific composition as define before and have a tensile strength not higher than 42 kg/mm 2 and a grain size number ranging from 8.5 to 11.5, irrespective of the j continuous annealing conditions.

Description will be made on the tensile strength of the sheet with reference to Fig. 3.

When the tensile strength is too large, the forming load and forming energy during in the DI working 15 r increase so that the working becomes hard to perform, and much biting is likely to occur. Therefore the upper limit of the tensile strength is set at 42 kgf/mm 2 A preferable tensile strength is not higher than kgf/mm 2 Fig. 3 illustrates the relation between the tensile strength of the sheet for DI working, prepared in an experimental pilot line by vacuum melting of a steel composition containing 0.0040 to 0.080% carbon, 0.15 to 0.60% manganese, 0.006 .o 0.030% phosphorus, *IA 0.005 to 0.070% aluminum, and not more than 0.0070% nitrogen, and the total forming energy in the DI test forming machine and the pressure strength of DI cans coated with paint and baked.

As understood from Fig. 3, when the tensile t strength exceeds 42 kgf/mm the total forming energy remarkably increases, causing much biting and hence great difficulty in the DI working. In order to maintain the St total forming energy at a low level consistently, the I tensile strength should preferably be maintained at a 2 i. level not more than 40 kgf/mm 2 an6 the yield point should be maintained at a level not more than 36 kgf/mn 2 more preferably the tensile strength should be maintained at a level not more than 37 kgf/mm 2 and the yield point should be maintained at a level not more than 30 kgf/mm 2 Regarding the pressure resistance strength, this strength increases as the tensile strength increases, but in the case of the contituously annealed material, 16 as compared with a comparative box-annealed material, 2 shows a pressure resistance strength about 1 to 2 kgf/mm higher than that obtainable by the box-annealed material for the same tensile strength. For assuring such BH property, it is desirable that the continuously annealed shept contains at least 5.0 ppm of solid solution carbon.

I Hereinbelow description will be made on the grain size of the steel sheet according to the present invention with reference to Fig. 4.

Fig. 4 illustrates the relation between the JIS grain size numbers of the surface treated steel sheets 994* for DI working prepared in an experimental pilot line by vacuum melting a steel composition containing 0.0044 9 to 0.076% carbon, 0.16 to 0.57% manganese, 0.008 to 0.030% phosphorus, 0.007 to 0.080% aluminum, and 0.0020 to 0.0070% nitrogen, and the working rate until the rupture occurs in the flange stretching after the DI working tie and the pressure rsistance strength of DI cans in connection with the annealing methods.

Regarding the stretching flange formability, it has been confirmed that sheets having a stretching flange formability not lower than 9.0% as measured in the laboratory tests by the present inventors can satisfy it the demand of commercial users.

As understood from Fig. 4, the stretching flange formability is more improved as the grain becomes finer (as the grain size number increases), and for assuring the stretching flange formability of 9.0% or more for 17 i i I"~"ai 45 00 0 400 04,06 *o v .0 o~ 0 00 040410 *44 0 a a4 00 64 0 1.

the continuously annealed material, the grain size number must be 8.5 or larger. Quite unexpectedly, it has been found that the continuously annealed material shows a better stretching flange formability than the box-annealed material. On the other hand, as the grain becomes finer, while the stretching flange formability and the pressure resistance strength increase as the grains become finer, the total forming energy in the DI working remarkably increases as the grain size number exceeds 11.5, resulting in occurrence of biting and difficulties in the DI working. Therefore, tho grain size number is specified in the range from 8.5 to 11.5. A preferable range is from 9.0 to 11.0.

For the production of the steel sheets having the specific tensile strength and the grain size number as defined above, the continuous annealing conditions as described hereinbefore can be applied. However it is desirable that the steel slab is cooled to a temperature lower than the Ar 3 transformation temperature and again heated to a temperature not higher than 1150°C.

Detailed Description of the preferred Embodiments: The present invention will be better understood from the following description of the preferred embodiments of the present invention.

1 Comparative data of the steel sheets according to the present invention are shown in Table 1. The sttel compositions shown in the table were melted in a convertor 18 r( il 11~ U~^ els ft ft ft ft ft U fteq ft f ft ft.

ft ft ft fte ft ft f f and continuously annealed and the slabs thus obtained were hot rolled to 3.0 mm thick under the conditions shown in the table, acid pickled, then cold rolled to 0.32 mm thick and annealed under the conditions shown in the table and then terper rolled by 1.0% and electrolytically tin plated. The hardness (HR30T) of the sheets as electrolytically tin plated and the pressure resistance strength and stretching flange formability after DI working are also shown in the table.

It is clearly shown by the comparative data that the coils Nos. 1 to 12 produced according to the present invention can provide a surface treated steel sheet having excellent DI workability and pressure resistance strength and having a temper degree ranging from T-1 to T-3. The comparative coils Nos. 13 to produied outside the scope of the present invention show a lower pressure resistance strength and a lower stretching flange formability despite the high hardness.

Similar comparative data are shown in Table 2.

The steel compositions shown in the table were melted in a convertor, and continuously cast. The steel slabs thus obtained were hot rolled to 3.0 mm thick, acid pickled, then cold rolled to 0.32 mm thick, annealed under the conditions shown in the table, temper rolled by 1.0% and electrolytically tin plated. Also the grain sizes and tensile strengths of the tin plated steel sheets are shown.

As understood from the data shown in Table 2, 19 the steel sheets according to the present invention require less total forming energy, are free from biting and show a satisfactory pressure resistance strength and a very high degree of stretching flange formability.

While the comparative steel sheets (No. 13 to No. 15), though the total forming energy is small and the biting is not observed, are significantly low in the stretching flange formability and the pressure resistance strength. The box-annealed sheet (No. is outside of the scope of the present Invention with respect to the grain size number so that the total forming energy is too large and the biting is observed. The S comparative continuously annealed sheet (No. 16), though S the total forming energy is small and no biting is observed, is significantly low in the stretching flange formability due to Its coarse grain size outside the scope of the present invention. The comparative continuously annealed sheets (No. 17 and No. 18), though high in the pressure resistance strength and excellent in the stretching flange formability, require a large total forming energy and susceptible to much biting.

The steel shoet according to the present inve~ntion shown improved pressure resistance strength due to the S311 effect, but also shows increased strength in the vertical direction of the can body, an compared with the box-annealed material for the same material strength. Also an the steel sheot according to the present invention has excellent stretching flange 20

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Ii

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formability not only the reject ratio due to failure in the simple flange forming is low, but also the sheet can stand against more severe working.

II

it t V ft t 21 Table 1 Chemical Composition (wt%) C Mn P AL N Hot Rolling Conditions T* FT CT (oC) (OC) Annealing Conditions Soak-Soaking ing Temp.Time (sec) Cooling Rate OA OA Temp. Time (sec) Pressure Hard- sre Resisness tance Strength HR30T (kgf/cm 2

SFF

(X)

rI 0.022 0.03 0.018 0.045 0.031 0.027 0.022 0.03 0.018 0.045 0.055 0.51 0.16 0.18 0.21 0.17 0.25 0.24 0.16 0.18 0.21 0.17 0.25 0.24 0.004 0.005 0.008 0.004 0.006 0.039 0.005 0.008 0.012 0.011 0.015 0.06 0.055 0.046 0.021 0.078 0.058 0.06 0.055 0.046 0.021 0.078 0.058 0.0015 0.0018 0.0022 0.0014 0.0036 0.0066 0.0015 0.0018 0.0022 0.0014 0.0036 0.0066 1128 1120 1103 1045 1154 1125 1128 1120 1103 1045 1154 1125 1040 1020 1060 990 1020 1050 1040 1020 1060 1030 1020 1180 900 890 860 820 910 920 900 890 860 820 910 920 680 700 710 690 630 650 680 700 710 690 630 650 710 690 740 740 680 720 710 690 740 710 680 720 50 25 75 100 10 50 50 25 75 100 10 50 27 31 145 238 35 41 27 31 145 238 35 41 400 430 350 400 400 400 400 430 350 400 400 400 7.8 7.6 7.5 8.0 7.6 7.5 7.6 7.7 7.5 8.7 8.6 8.8 11.8 11.6 11.3 12.0 11.6 11.3 11.6 11.6 11.3 10.2 4.2 S13 0.061 0.18 0.015 0.055 0.0056 1120 1020 890 650 670 Box-Annealing 51 7.1 10.2 14 0.066 0.21 0.013 0.046 0.0033 1103 1240 930 500 660 Box-Annealing 50 6.5 9.3 1 15 0.097 0.17 0.011 0.046 0.0042 1103 1250 920 560 650 Box-Annealing 51 7.2 9.8 T* 6875/(3.865-log[A£% 0.015]) 250 Hot Rolling Heating Temperature FT Hot Rolling Finishing Temperature CT Hot Rolling Coiling Temperature OA Temperature Overaging OA Time Overaging Time Temperature SFF Stretching Flange Formability _1 Table 2 Chemical Composition (wt%) Annealing Conditions C Si Mn P S At N *t 00 0.0092 0.0096 0.0042 0.0206 0.0052 0.0220 0.0380 0.0214 0.0320 0.0340 0.0510 0.0412 0.013 0.016 0.012 0.016 0.011 0.013 0.016 0.022 0.021 0.023 0.020 0.022 0.16 0.16 0.17 0.18 0.21 0.17 0.25 0.24 0.17 0.17 0.25 0.25 0.004 0.004 0.005 0.005 0.008 0.008 0.009 0.015 0.006 0.009 0.012 0.016 0.004 0.008 0.005 0.007 0.011 0.009 0.004 0.008 0.014 0.013 0.011 0.012 0.060 0.060 0.046 0.055 0.046 0.043 0.078 0.058 0.028 0.023 0.021 0.021 0.0015 0.0015 0.0017 0.0018 0.0022 0.0021 0.0022 0.0024 0.0027 0.0026 0.0028 0.0036 Soaking Temp.

c) 710 710 740 720 740 690 710 690 710 720 680 650 27 27 145 31 3/5 31 35 41 35 238 238 Soak- Cooling ing Time Rate (scc)(°C/s) OA OA Temp. Time (sec) 4OO 400 350 430 350 430 400 4OO 400 400 400 400 13 0.0531 0.020 0.18 0.015 0.018 0.055 0.0058 670 Box-Annealing (a 14 0.0580 0.021 0.21 0.013 0.023 0.046 0.0033 660 Box-Annealing 15 0.1124 0.023 0.35 0.027 0.015 0.046 0.0065 650 Box-Annealing 16 0.0028 0.018 0.16 0.005 0.017 0.060 0.0015 7/.0 100 27 400 240 17 0.0622 0.016 0.41 0.022 0.011 0.055 0,0028 6/0 25 31 430 0 18 0.0610 0.019 0.33 0.026 0.014 0.046 0.0032 690 75 11.5 350 180

I

0 (to be cont'd) 23 -q 4

?A

Table 2 (cont'd) ~9 o 2 0 q~J 0

J~

0 4.9 ~*t 0..

4 o 02 *4 0 9 4,, *0 P 0.2 0 44* 9.

9 0 P9 0 2 .4 9~ 0 C' 4 4 *4 JIS Tensile Total Pressure Stretching No. Grain Strength Forming BiigRegistance Flange size Energy Strength Formability (kgf/cm') (kjf /cmI) M% 1 9.4 33.6 62147 no 7.9 10.3 2 9.5 33.9 6174 no 8.0 10.3 3 8.6 30.5 5868 no 7.5 4 9.7 34.8 6254 no 8.1 10.5 0

.H

4J 5 9.0 32.0 6001 no 7.7 10.1 Z3 6 9.8 35.1 6280 no 8.1 7 10.2 36.7 6427 no 8.3 10.7 8 48 24n 811.

9 9.7 34.8 6254 no 8.1 10.5 10 9.6 34.2 6201 no 8.0 10.4 11 11.0 40.0 6720 no 8.7 11.1 12 10.8 39.3 6653 no 8.6 11.0 S13 9.7 34.3 6254 no 6.6 9.2 4J' 14 9.3 33.3 6121 no 6.4 is1 12.0 43.7 7053 some 7.7 8 4Ji 16 8.2 28.9 5721 no 7.3 8.7 S17 12.0 43.7 7053 sowcc 9.2 3.1.1 0 18 1.2.6 44 .4 71,20 some 9.3 11.4 24

Claims (2)

1. A method for producing a steel sheet for surface treatment suitable for DI working, which comprises hot rolling a low carbon Al-killed steel slab containing 0.004 to 0.06% carbon, 0.05 to 0.60% manganese, not more than 0.02% phosphorus, 0.005 to 0.100% acid soluble aluminum, not more than 0.0070% nitrogen with the balance being iron and unavoidable impurities, coiling the hot rolled strip at a temperature ranging from 600 to 710 C, cold rolling the strip, subjecting the strip to :rcrystallizing annealing at a temperature not lower than the recrystallizing temperature but not higher than 850 C for seconds to 3 minutes, then cooling the strip at a cooling rate of from 5 to 250 0 C/second, and subjecting the strip to an I, overaging treatment at a temperature of from 300 to 5000C for 30 to 180 seconds.

2. A method according to claim 1, wherein said Al-killed steel slab is cooled to a temperature not higher then Ar 3 transformation temperature and then heated to a temperature not Sv,, higher than defined belowt 6875 (3.865 log [Al% 0.0151]) 250 before the hot rolling. DATED THIS 27TH DAY OF OCTOBER, 1989. I NPPON STEEL COAPORATION By its Patent Attorneyst GRITVITH HACK CO.. Follows Institute of Patent Attorneys of Australia.