Development of high-formability high-strength galvannealed steel sheet of tensile strength 440 MPa grade with excellent formability

ELSEVIER

JSAE Review 17 (1996) 313-318

Development of high-formability high-strength galvannealed steel sheet of tensile strength 440 MPa grade with excellent formability Susumu Okada a, Kei Sakata a, Makoto Imanaka a, Kazuaki Kyono a, Toshitake Hanazawa b a Technical Research Laboratories. Kawasaki Steel Corporation, Kawasaki-dori 1, Mizushima, Kurashiki-shi, Okayama, 712 Japan b Mizushima Works. Kawasaki Steel Corporation, Kawasaki-dori 1. Mizushirna, Kurashiki-shi, Okayama. 712 Japan

Received 2 October 1995

Abstract Molybdenum and carbon were chosen as strengthening alloy elements for the production of high strength steel sheet with high press formability and excellent galvannealing properties. The strengthening effect of molybdenum addition was 6 MPa/0.1%, while that of carbon addition was 8.5 MPa/10ppm. The decrease in formability, especially elongation due to molybdenum (0.2%/0.1%) and the r value due to carbon (almost 0/10ppm), was small. Steel containing carbon of 50-100ppm exhibited the highest formability with coiling temperature at 600°C. By applying these results, the high strength galvannealed sheet steel of TS 440 MPa grade with elongation of 35% and an r value of 1.5 was newly developed. This steel also exhibited good galvannealing properties.

1. Introduction In recent years, a high-strength cold-rolled steel sheet has been generally used to reduce the weight of automotive bodies. The formability of high-strength steels is improved by lowering the carbon content to approximately 20ppm and adding strengthening elements such as P, Si, Mn, etc., which have little effect on press formability and also offer cost advantages [1]. However, the production method described above has some problems when it is applied to corrosion resistant coatings especially galvannealing, which are frequently required in high-strength steels. Specifically, it is known that Si tends to form a surface oxide which can cause incomplete galvanizing [2]. The addition of P is limited, because P retards the alloying reaction of the zinc layer in galvannealed products [3], leading to the deterioration of surface quality. It is difficult to obtain a strengthening effect and secure good press formability at the same time by using only Mn, which possesses a relatively weak strengthening capability. For these reasons, it has been considered difficult to manufacture deep drawable galvannealed steel sheets of the T S 440 MPa class and over. In this article, we focused on Mo and C as the strengthening element, to develop high-formability high-strength galvannealed steel sheet of TS 440 MPa grade, and ob-

served the mechanism of strengthening without negative effects on zinc coating ability.

2. Experimental method The chemical composition of the steels used is shown in Table 1. The basic chemical composition was a semi ultra low carbon steel with C of 50-70ppm. An interstitial atom free (IF) material can be obtained by adding Ti and Nb in amounts greater than the carbon equivalent to stabilize solute C as Ti- or Nb-carbides. The effect of Mo addition was investigated with Group 1 in Table 1, the effect of C was investigated with Groups 2 through 4, and the effect of the coiling temperatures ( C T ) was investigated with Group 5. All the samples were vacuum melted steels which were rough hot rolled to a thickness of 30 ram, heated to 1250°C, and then hot rolled to a thickness of approximately 4.2 mm at finishing temperatures of 870°C or higher. The effect of C T was simulated by reheating to the equivalent temperature. Hot-rolled sheet without this simulation is equivalent to material processed by low coiling temperature at 500°C or under. Cold rolling was conducted at a reduction of 77%. Continuous annealing condition at 770-860°C was simulated using an alumina fluidized bath furnace. The forma-

0389-4304/96/$15.00 © 1996 Society of AutomotiveEngineers of Japan, Inc. and Elsevier Science B.V. All rights reserved PIl S0389-4304(96)0001 4-8

JSAE9631560

314

S. Okada et al./JSAE Review 17 (1996)313-318

Table 1 Chemical composition of steels Steel Group A B Group C D Group E F Group G H I Group J

A

C Si Mn P AI Ti (ppm) (%) (9'0) (%) (%) (%)

Nb (%)

Mo B (Ti* + N b ) / C (%) (ppm) (atomic)

1 70 70

0.1 0.6 0.05 0.05 0.050 0 0.1 0.6 0 . 0 5 0 . 0 4 0 . 0 5 0 0

0 11 0.5 8

1.49 1.49

26 48

0.1 1.6 0.05 0.05 0.042 0 0.1 1.6 0.05 0.05 0.043 0

0.2 0.2

7 9

2.39 1.40

43 70

0.1 1.6 0.06 0.04 0.028 0.016 0.2 0.1 1.6 0.05 0.05 0,043 0.027 0.2

7 7

1.37 1.43

47 70 90

0.1 1.6 0 . 0 6 0 . 0 4 0 . 0 3 0 0 . 0 4 9 0 . 2 0.1 1.6 0.06 0.04 0.033 0.081 0.2 0.1 1.6 0 . 0 6 0 . 0 5 0 . 0 4 4 0 . 1 0 0 0 . 2

5 4 2

2.41 2.32 2.38

53

0.i

4

2.10

2

3

4

*~v10-1 t~

0 10"a

o

o

"6 .~ 10 .3 o

I

Mild Steel

L

I

Steel A Steel B (0% Mo) (0.5% Mo)

Fig. 2. Effect of Mo on zinc hot galvanizing property of cold-rolled high-strength steel sheet.

5 1.6 0 . 0 6 0 . 0 5 0 . 0 4 7 0 . 0 1 6 0 . 2

Ti* = T i - ( 4 8 / 3 2 ) S - ( 4 8 / 1 4 ) N

bility test of the annealed sheet was conducted using JIS5 tensile test pieces.

3. Results

3.1. Effects of Mo addition Figure 1 shows the effect of 0.5% Mo addition on the strength and formability of the Group 1 steels relative to 500

" ~ 450 (3.

,oo

o-------o

o------o

Steel A (0%Mo)

45

I

I

4O

Irn

35

30 1.8

1.6 o

"~ 1.4 i ~-

1.2 No CT simulations 1.0 750

, 800

850

900

Annealing temperature (°C) X=(XL+XT+2xXDy4,

L: longitudinal din~tion T: transverse direction D: diagonal direction

Fig. 1. Effect of Mo on strength and formability of cold-rolled highstrength steel sheet.

annealing temperature. Although the strengthening effect of Mo addition tends to decrease as the annealing temperature increases, the tensile strength of the sample with Mo is still higher than that without Mo by 30-40 MPa at the annealing temperature of 800-850°C. Meanwhile, both elongation (El) and the Rankford value (r value), which are indexes of formability, deteriorate slightly with increased Mo addition by 1-1.5% of El and 0.2 of r value at the annealing temperature of 800850°C. The marked drop in the formability of Mo-containing steels at low temperature annealing of 770°C is clue to the result of incomplete recrystallization. It was therefore considered that Mo addition either raises the recrystallization temperature or delays recrystallization. Mo is known to be a solid solution hardening element, but it is also considered to produce additional grain-refining strengthening by retarding the grain growth of the annealed sample. This is presumed not only from the delay in recrystallization described above, but also from the fact that the annealing temperature dependence of strength and formability shown in Mo bearing steel is stronger than that in Mo free steel, as shown in Fig. 1. The effect of Mo addition on the hot dip galvanizing property of the Group 1 steels is shown in Fig. 2. The experiment was conducted with an apparatus in which the annealing furnace and the hot dip galvanizing pot were directly linked. The results were evaluated from the rate of bare area in galvanizing after annealing in a 20% hydrogen atmosphere (dew point, - 20°C) at 830°C. The experimental conditions may be more severe than those applied in the factory, as evidenced by the occurrence of a small amount of bare area even with mild steel (0.003%C0.1%Mn-Ti-Nb; shown in Fig. 2), which can normally be galvanized on the factory with no problem. From the results of this experiment, Mo bearing steels do not show any increase in the rate of bare area and possess a good wettability, approaching that of mild steel. An analysis of the material surface by glow discharge spectrometry showed no surface concentration of Mo. For this reason, it

S. Okada et al./JSAE Review 17 (1996) 313-318 480

O A

" ~ 460 el ¢,o 1--

{Ti IF) Group2 (Ti-Nb IF} Group 3 (Nb-Ti IF)

460

./l[" ~.~

Group I

315

~ '420 sv e direction CT : 600"C CAL : 830~850°C

420 /

36

A v

v

34

45

~

o~

Transverse direction

t--

32

W 30

4O

1.6

D

I

i

I

I

w 35

1.4

30

I

I

I

--~ 1.2 > I'--1. 0

I

1.6 "~ i~...

~

0

1.4

__

__

i A

0.8

•

it-------

I

A.C. 550 (equivalent to below 500°C)

1.9

I

I

I

600

650

700

750

Coiling Temperature (°C) 1.0

i

0

20

i

i

i

40

60

80

Fig. 5. Effect of coiling temperature on mechanical properties of coldrolled high-strength steel sheet.

100

C content (ppm) Fig. 3. Effect of C on mechanical properties of cold-rolled high-strength steel sheet.

was concluded that Me did not form surface oxides which negatively affect galvanizing property, leading to a good galvannealing property. 3.2. Effect of C content The properties of the annealed sheets (CT = 600°C) of steels from Groups 2 through 4 were arranged in terms of C content in Fig. 3. The mechanical properties of the steels were almost entirely dependent on the C content in spite of the different balance of carbide forming elements in each group, Group 2 comprising Ti IF steels, Group 3, Ti-Nb (Ti-rich) IF steels, and Group 4, Nb-Ti (Nb-rich) IF steels. In the range of C content from 20 to 100ppm, the tensile strength shows an increase of approximately 0.85

I x.._~.

".

~,. . . . .

<.~,.

%;-

-

.

.

•

~

MPa/ppm C, and elongation decreases as strength rises. The rate of decrease in elongation is approximately 1.5% per TS 10 MPa, and that rate is not so large compared with the amount of strengthening. On the other hand, the r value shows no tendency of decreasing as the C content increases. Therefore, C appears to be a possible strengthening element in the semi ultra low carbon region (C = 50100ppm). Figure 4 shows the optical microstructures of steels G (43ppm C) and I (90ppm C). The crystal grain size of the 90ppm C steel is finer than that of the 43ppm C steel. 3.3. Effect of hot rolling conditions The effect of CT is shown in Fig. 5 for the Group 5 steel (J). The elongation and r values of the steels without coiling simulation (equivalent to low CT) are low, but formability is significantly improved when coiling simulation is conducted. However, in coiling simulation at 700°C, the r value falls lower than that shown at 600°C. The CT dependence of tensile strength is slight. Accordingly, it appears that the optimum CT for obtaining good strength and formability is in the vicinity of 600°C.

•

4. Discussions ~ )

.....

s'-;

"

,<-~ /> P'~: -:,.~'i~7

(a) Steel G (43ppm C)

(b) Steel I (90ppm C)

Fig. 4. Optical micrographs of semi ultra low carbon IF cold-rolled steels (coiling temperature, 600°C).

4.1. Evaluation of Me, C as strengthening elements Figure 6 shows a comparison of the effects of Me, C, and other elements [4] on the strength and formability. Although this is not a strict comparison, since the base materials are different, the deterioration in both elongation

S. Okada et al./JSAE Review 17 (1996) 313-318

316

by Mo and r value by C are small. Considering the fact that neither Mo nor C has a negative effect on the hot dip galvannealing process, it is clear that Mo and C are promising strengthening elements for high-formability GA steel sheets.

Transverse direction 6

n <

4 2 620

4.2. Effect of CT on formability

•

I

I

I

I

'

'

'

n~600~ The improvement of the formability of hot-rolled strip by optimum coiling simulation is considered to be as follows. In ultra low carbon mild steels, decreases in CT hardly affect the interstitial atom free situation. However, in high-tensile steels, the precipitation of carbides tends to be delayed due to heavy addition of the solute elements for strengthening, and it is possible that the solute C will not be sufficiently reduced in low CT. In particular, this tendency appeared to be strong in the semi ultra low

-120 -100 8O ATS(MPa)

Mo

6O 40 2O

I

I

I

I

L

-12

-10

-8

-6

-4

Cr

I "~

j

-2

A El(%) (a) T S - E 1 b a l a n c e

120 ZxTS(MPa) SicI100

Mn Ni

.

60

~580 560 540

A.C. 550 (equivalentto below500°C)

'

'

'

600

650

700

750

Coiling Temperature (°C) Fig. 7. Effect of coiling temperature on aging index and tensile strength of semi ultra low carbon hot-rolled steel sheet.

carbon steels. Figure 7 shows the CT dependence of strength for the hot-rolled steel sheets of steel J (Group 5) and their aging index (A/), which indicates the content of solute C. The A1 of steel without coiling simulation (equivalent to low CT) is high for IF steel, but the AI decreases at the condition of coiling simulation of 600700°C, and the reduction in solute C which is necessary for securing good formability is obtained. On the other hand, the r value obtained by coiling simulation at 700°C is lower than that at 600°C. Large phosphides (FeTiP, etc.) were observed in the steel obtained by coiling simulation at 700°C. It was proposed that this kind of large precipitate negatively influences the cold-rolled and recrystallized textures [5], so that the r value showed the above coiling temperature dependence. Based on the reduction in the tensile strength of the hot rolled sheet, it is also considered possible that a reduction in accumulated strain due to the recovery phenomenon reduces the driving force needed to achieve the {111} texture during annealing.

4.3. Mechanism of strengthening by C

\ i i

-0.4

i

i

-0.2

2

I

i

0 Zx-F

0.2

~

I 0.4

(b) T S - r b a l a n c e

base : 0.003C-0.035Ti-0.005N'b-0.0OIB Si, Mn, Cu, Ni : 1.0% addition P : 0.1% addition B : 0.01% addition

SRT : 1250"C (4) FDT : _~900~C cold reduction : 78% annealing temp. : 850"C

Mo : 1.0% addition (based on Fig.l) C : 0.01% addition (based on Fig.3) Fig. 6. E f f e c t s o f s t r e n g t h e n i n g e l e m e n t s on f o r m a b i l i t y o f c o l d - r o l l e d steel sheet.

In these experiments, the strength of the materials was increased by raising the C content without deteriorating the r value. This minimal deterioration in the r value and the low AI of hot-rolled steels obtained by coiling simulation at 600-700°C (see Fig. 7) makes it difficult to attribute this strengthening effect to solute C. Rather, the fact that the strength of the hot-rolled sheets had its maximum value in these experiments in the vicinity of 600°C, as shown in Fig. 7, suggests precipitation hardening due to the presence of carbides in the hot rolled sheet. Accordingly, the mechanism of hardening by C is considered to be mainly attributable to distributed hardening due to carbides which have precipitated during hot rolling, and

317

S. Okada et a l . / J S A E Review 17 (1996) 313-318

Table 2 Chemical composition of steel for mill production

60

C Si Mn P (ppm) (%) (%) (%)

50

100

0.l

AI (%)

Ti (%)

Nb (%)

Mo B (Ti* + N b ) / C (%) (ppm) (atomic)

1.0 0.07 0.05 0.060 0.01 0.2

10

1.4

40

inq 30

grain refinement hardening (see Fig. 4) due to the suppression of recrystallized grain growth by these carbides.

2.5

2.0

5. Mill trial manufacture

-'~ >

5.1. Chemical composition design

1.5

1.0

Mill trial production of TS 440MPa class GA steel sheet for deep drawing applications was carried out based on the experimental results in the laboratory. Because it was expected to be difficult to obtain an interstitial atom free situation due to the increased C content, C was set at 100ppm. The Si and P levels were held to 0.1% and 0.07% respectively in consideration of galvannealing treatment. Mn was also set to 1.0% in order to keep the high tensile strength and to avoid deteriorating the r value. The predicted strength shortfall was then covered with a 0.2% Mo addition, as shown in Table 2.

0.5 200

I 300

1

1

400

500

600

TS (UPa) Fig. 8. Tensile strength-formability balances of conventional cold-rolled steel sheets and newly developed steel.

6. Conclusions

5.2. Results of trial production The production conditions of the hot dip galvannealed steel sheet and the strength and press formability obtained is shown in Table 3. Manufacturing was performed at Kawasaki Steel Corporation's Mizushima Works CGL (continuous annealing/hot dip galvanizing line). As shown in Table 3, the target material properties were substantially achieved, and there were no problems with the quality of the galvanized coating (wettability, alloying property, powdering). The r value showed an apparently low value in a tensile test with the coating intact due to the restraining effect of the coating, but was measured at 1.5 after the coating was peeled off. Figure 8 shows a comparison of the strength and press formability of the trial-manufactured steel with a conventional steel. The material properties of the new steel lie on an extension of the strength-formability balance line for conventional high r value type steel shed'S-.'

Table 3 Process conditions and mechanical properties of manufactured steel CT

Ts

TS

El

(°C)

Thickness (mm)

YS

(°C)

(MPa)

(MPa)

(%)

r value [stripped]

650

830

0.7

285

435

34

1.4 [1.5]

CT: Coiling temperature; Ts: Soaking (annealing) temperature

The effects of Mo and C on the press formability of high-strength cold-rolled steel sheets were investigated. The results are summarized below. (1) The addition of Mo has a strengthening effect in the same order as that of Si (6 MPa/0.1%) and causes only slight deterioration of press formability, with no loss of zinc coating ability. (2) Increasing the C content within the range of 20 to 100ppm has a strengthening effect of about 8.5 M P a / p p m , and at a coiling temperature of about 600°C, C causes only slight deterioration in formability, especially in the r value even at 100ppm. (3) In semi ultra low carbon steels with C additions of 50-100ppm, the highest level of formability is obtained at a coiling temperature in the vicinity of 600°C. (4) The formability of semi ultra low carbon steels is not improved with low temperature coiling at 500°C or less due to residual solute C in the hot rolled sheet; on the other hand, the r value deteriorates with high temperature coiling at 700°C or over, apparently due to the precipitation of large phosphides. (5) Based on the above results, the high-strength hot-dip galvannealed steel sheet with a steel composition of 100ppm C-0.1% S i - l . 0 % Mn-0.07% P-0.06% Ti-0.01% Nb-0.2% Mo-10ppm B was developed, and achieved 440 MPa of TS, 35% of El, and 1.4 of r value (1.5 after peeling the coating).

318

S. Okada et al./JSAE Review 17 (1996) 313-318

References [1] Matsudo, K., 74th and 75th Nishiyama Kinen Koza (Nishiyama Memorial Lectures, in Japanese) [Iron and Steel Institute of Japan]. [2] Hirose, Y., et al., Wetting Characteristics of Silicon Containing Steel with Molten Zinc (in Japanese with English summary), Tetsu-toHagane, 68 (1982), p. 665.

r

[3] Nakayama, Sl015. [4] Tosaka, A., [5] B r u n e t al., TX (1982),

M., et al., Tetsu-to-Hagane (in Japanese), 66 (1982), et al., Unpublished work. Metallurgy of Continuous-Annealed Sheet Steel, Dallas, p. 173 [AIME].

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