STRUCTURAL STEELS

EFFECT OF DE OXIDAT ION ON THE FRACTURE

OF H IGH-MANGANESE STEEL

G. A. Charushn ikova , Ya . E . Go l 'dshte in , and Yu. G. Razumov

UDC 669.046.558:669.$5W4-194

At the present t ime research is being conducted on the propert ies of Fe -Mn alloys both here and abroad. Most of this work concerns alloys containing no more than 3-4% Mn or no less than 12% Mn. Al- loys containing 5-11% Mn have been studied very l itt le. The main reason for this is the long-statading opinion that ra is ing the manganese concentrat ion over 3% makes the steel exceedingly br i t t le [1-2]. It was shown in [3] that, depending on the deoxidizing conditions, the toughness of this type of steel may vary within wide l imits .

Here we present the resul ts from an e lec t ron-microscop ic investigation of the effect of deoxidizing with aluminum and titanium on the f racture of low-carbon (C< 0.10%) manganese steel with 8% Mn.

an~ kg-m/cm 2

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TABLE i Composition, %

I ~n [ S ] [ P ~klno m nlr~ , Tino m Tir~ ~

i S i

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C

=: . - _= ~ . . . . . .

1 0.11 2 o11o 3 0,07 4 0,08 5 0,03 6 0;08 70,05 8 0,08 9 0,05

0,24 0,16 0,17 0,24 0,21 0,25 0,36 0,30 0,33

7,35 Io.o25t 0,021 8,89 i 0.024 0,014 7,18 0025 0,013 7,74 0.026 0,012 8,55 10.0241 0,014 8,52 [0,029 0,014 6,84 i0.025 0,014 8,448'27 0,029__ 0,013__

0,10 0,10 0,10

070 0,15

0,008

0,061 0,080 0,030

0,130 0,15

070 0,15 0,20 0,40 0,15

N

- - 0,0080

- - 0,0102 0,05 0,0084 0,07 0,0111 0,07 0,0074 0,20 0,0075 0,11 0,018

Fig. 2. Effect of temper ing temperature on the type of f rac ture of 8% Mn s tee l (heat 2)quenched f rom 820~ (x 18,000). a) Not tempered; b) tempered at 200~ e) 475~ d) 600~

The s tee ls invest igated were laboratory heats mel ted in a 50-kg induction furnace. The chemica l compos i t ion of the exper imenta l heats is given in Tab le 1.

F igure 1 shows the effect of deoxidat ion with a luminum (a) and t i tan ium (b) on the impact toughness of s tee l 08G8 in re la t ion to the temper ing temperature fol lowing quenching. It should be noted that tem- per ing at 550-625~ is in the in terer i t i ca l range, which is due to the lower ing of the c r i t i ca l points by man- ganese and the wide range of coex is tence of a - and ? -phases . The s t ructure of the s tee l a f ter heating at these temperatures is a fine mixture of fe r r i te and austen i te highly a l loyed with manganese as wel l as a smal l amount of ~--phase [3]. Thus, the te rm "h igh- temperature temper ing" is only a convent ion in this case.

As can be seen from Fig. 1, the steel not deoxidized with aluminum or titanium (curve 1) as well as the steel containing a negligible amount of residual aluminum (0.008%, curve 2) is characterized by a very low impact toughness up to a tempering temperature of 525~ it increases above this temperature.

The addition of aluminum or titanium substantially increases the impact toughness after low (200- 250~ and high-temperature tempering. After tempering at 300-475~ there is a sharp decrease of the impact toughness, which is due to temper brittleness. The impact toughness also decreases after tem- pering above 630-650~ when the structure becomes single-phase (martensite) in place of double-phase (~+7).

The best results (highest impact toughness) were obtained for the heats containing 0.05-0.08% alu- minum or titanium.

The electron-microscopic investigation of the fracture surfaces of quenched samples and also sam- ples tempered at 200, 475, and 600~ after quenching showed the effect of deoxidizing on the type of frac- ture. Carbon replicas were used; the replicas were separated electrolytically in a 5% solution of 1-12SO 4 in alcohol.

The quenched samples without aluminum or titanium fractured by shearing (Fig. 2a). Ductile compo- nents appeared in the fracture of steels with ->0.05% Al or Ti.

Tempering at 200~ resulted in completely ductile fracture of the steels with ->0.05% A1 or Ti, but only ductile components (Fig. 2b) in the fracture of the steel without A1 or Ti.

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Fig. 3. A luminum (a) and t i tan ium n i t r ides (b, c) in 8% Mn s tee l , a, b) x18,000; c) x25,000.

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a Testing temperature b

F ig. 4

a n, kg -m/cm 2 Z ~ .....

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# -780 -7~0 -ZOO -50 -20 0 ~163

Testing temperature

F ig. 5

F ig. 4. Cold br i t t leness of 8% Mn s tee l in re la t ion to deox id iz ing condit ions with a luminum (a) and t i tan ium (b). The heat numbers a re given on the curves .

F ig . 5. Ef fect of e lec t ros lag remel t ing on the cold br i t t leness and an isot ropy of 8% Mn s tee l . 1) ESH; 2) open melt ing. ) Longitudinal samples ; - - - ) t rans - verse samples .

With temper ing at 475~ the character of the f rac ture changes - it becomes in tergranu lar (F ig. 2c); the deoxid iz ing condit ions have no effect on the f rac ture except that the gra in s i ze is reduced in the s tee ls deox id ized with t i tan ium.

With temper ing at 600~ the f rac ture is duct i le (Fig. 2d).

It was shown in [3] that the sharp increase in the impact toughness resu l t ing f rom the addit ion of a lu- minum or titaniL~m to manganese s tee l is due to the high aff in i ty of these e lements for n i t rogen. Combining n i t rogen into s tab le n t t r ides , these e lements substant ia l l y neut ra l i ze its harmfu l effect as an e lement b lock- ing d i s locat ions . This is conf i rmed by our resu l t s : in the s tee l deox id ized with a luminum one observes a luminum n i t r ides , usua l ly located in the bottom of d imples (F ig. 3a); t i tan ium n i t r ides are observed in the s tee l deox id ized with t i tan ium (Fig. 3b and c).

In the s tee ls tempered at 475~ hard ly any inc lus ions are observed af ter any of the deoxid iz ing pro - cedures . Ev ident ly this is due to the fact that the inc lus ions are located in the body of the gra ins and do not affect f rac ture along the gra in boundar ies .

The presence of a la rge quantity of coarse t i tan ium n i t r ides , usua l ly observed at t i tan ium concent ra - t ions over 0.05-0.07%, is undes i rab le , s ince it lowers the impact toughness. In heat 7, character i zed by the h ighest impact toughness (Fig. 1) and lowest cold br i t t leness thresho ld (]~ig. 4), the amount of t i tan ium

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nitride is smal l and the inclusions are of smal l size. Thus, to obtain a high impact toughness it is im- portant not only to combine the nitrogen but also use melting and crystal l iz ing procedures ensuring smal l s izes and even distribution of the nitr ides through the bulk of the ingot. This can be achieved by electro- slag remelt ing (ESR).

As Fig. 5 shows, ESR substantial ly increases the impact toughness of steel of this type (0.07% C, 8.16% Mn, 0.29% Si, 0.36% Mo, 0.05% Ti, 0.003% Aires). Also, ESR sharply reduces the anisotropy of the steel.

Interacting with the elements of the alloy, titanium combines not only nitrogen but also carbon into stable carbides and carbonitr ides, which may also increase the impact toughness.

CONCLUSIONS

i. The toughness of low-carbon (C < 0.10%) h igh-manganese (.6-9%) steel depends not only on the heat t reatment but also the melt ing and deoxidizing conditions.

2. The opt imal heat treatment, ensur ing the highest impact toughness, is temper ing in the inter- critical range, wh ich r'esults in a structure consisting of a fine mixture of ferrite and austenite highly al- loyed with manganese as well as a smal l amount of e -phase.

3. To obtain a high impact toughness, and particularly a low cold brittleness threshold, it is neces- sary to neutralize the harmfu l effect of the elevated nitrogen concentrat ion (up to 0.018%) in such steels by adding ni tr ide- forming e lements - a luminum (0.04-0.07%) or t itanium (0.04-0.07%).

4. To obtain a high impact toughness it is important not only to combine the nitrogen into stable ni- trides but also to use melt ing and crystallizing procedures ensur ing finely d ispersed and evenly distributed nitrides. This can be achieved by electroslag remelt ing.

1.

2. 3.

L ITERATURE C ITED

V. D. Sadovskii and N. P. Chuprakova, in: Transact ions of the Institute of Metal Physics and Metal- lurgy, No. 6 [in Russian], Izd. UFAN SSSR (1945). W. Rees, B. Hopkins, and H. Tipler, J . Iron Steel Inst., 169 (1951). Ya. E. Gol'dshtein and G. A. Charushnikova, Izv. Akad. Nauk SSSR, Metallurgiya i Gornoe Delo, No. 4 (1963).

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