STRUCTURAL STEELS

MICROALLOYING OF MEDIUM-CARBON PEARLITIC-FERRITIC STEEL

E. K. Krokhina, N. M. Fonshtein, and A. A. Petrunenkov

UDC 669.14.018.298.3

The theory and practice of microalloying with strong carbonitride forming elements (V, Nb, Ti) was developed extensively in the 1960s and 1970s in view of the development of tech- nology for controlled rolling of low-carbon low-alloy steels [i].

The effect of these elements on the structure and properties of medium-carbon steels has notyet been studied systematically. At the same time, this question is important in view of estimating the possibility for using these steels in the hot-rolled condition (without finishing heat treatment) [2].

Direct use of experience applied to low-carbon microalloyed steels for medium-carbon steels is impossible. First, the structure of medium-carbon steels is pearlitic-ferritic, and possible changes in the properties of pearlite, and not ferrite as in pearlitic steels [3], start to play a governing role. Second, with an increase in carbon content there is a change in carbonitride phase solubility and austenite stability.

In the present work a study was made of the effect of microalloying additions of vanadium, niobium, and titanium (individually or together) on the formation of structure and properties for medium-carbon chromium-manganese steel after hot working. Steel 30 KhG was used as a base.

Steel was melted in an open induction furnace from clean charge materials, forged at I150C into blanks 50 mm thick, which were then given hot working by a controlled schedule: heating to 1200C, deformation with E = 70% in four passes at 900C. Mechanical tests were carried out on longitudinal specimens.

Microalloying elements (V, Nb, Ti), which form carbonitride precipitates during hot working and subsequent cooling, have a considerable effect on structure formation for chromium- manganese steel. Addition of these elements causes a change in the rate of austenite grain growth during blank heating, recrystallization parameters for deformed austenite, the 7 + m-transformation temperature, and also it makes a contribution to dispersion hardening [3, 4].

Quantitative evaluation of steel microstructure drawing on regression analysis made it possible to reveal the formation mechanism for its final properties in relation to micro- alloying additions. Microstructural parameters were determined: the volume fraction of pearlite (Vp), average peariite grain size (D D is average chord), structural component micro- hardness, i.e., pearlite and ferrite (Hp and Hf), which made it possible to estimate indirectly the contribution of dispersion hardening and dispersion of pearlite to property formation.

Alloying of steel with vanadium leads to a reduction in the 7 ~ e-transformation temper- ature and formation of more dispersed pearlite. These processes, and also dispersion hard- ening, promote an increase in strength and an increase in steel ductility. The reduction in ductility is due to hardening, which is insufficiently compensated due to an increase in structural refinement (pearlite grains) [5] (Fig. la).

Niobium dissolved in austenite retards the 7 + e-transformation, which promotes an increase in the volume fraction of pearlite. At the same time, precipitation of niobium carbonitride Nb(C, N) in the 7-region promotes decomposition of austenite, which has a greater amount of structural imperfection than austenite without niobium. This leads to a reduction in Ar I and formation of more ductile and less hard eutectoid. As a result of this the depen- dence of microstructural parameters and mechanical properties on niobium content is extreme in nature (Fig. Ib). Thus, on the whole microalloying with niobium makes it possible simul- taneously to increase steel strength and ductility.

I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy, Moscow. Trans- lated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 20-24, July, 1987.

504 0026-0673/87/0708-0504512.50 1988 Plenum Publishing Corporation

0 ~lm6~ 45 Cloe5 J/cm 2 gf' ool--y-2 T, ---J-Jgo ]+3o

2~a~. 2 0 0 ~ fooc ,oo ~~___~

o, o2 0,06 o,~o o, t4 o,18 %v

Vp% mp,~m

s5 ",, L t 50 7 ! . .~o- -g - I t 45 ~! ~ ...T._P I I

o " ' c F" -o I ~o, zs, J/cm 2 f t2o Tso,% I . cto 25 650 ' 90 ]+5o 80o 17, "~A~-~,~----

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. . . . 90 ].30 do 1.o 30 ~- lO

o, o2 o, oo o, to o, f4 o, r sgor f b e

Fig. i. Dependence of microstructure parameters and mechanical properties of medium carbon steel on microalloying element content: vanadium (a) niobium (b), and titanium (c)" V ) volume fraction of pearlite; Dp) average pearlite grain size; Hp) pearlite microhardness; Hf) ferrite microhardness.

With a content in the steel up to 0.06% Ti (with constant nitrogen concentration) only almost insoluble nitride phase may form in its structure as a result of the high affinity of titanium for nitrogen [5]. Fine (up to 0.i Dm) titanium nitride, which forms with a con- tent of not more than 0.03% Ti in the steel, is most effective for restraining austenite grain growth. A further increase in titanium content leads to formation of coarse primary nitrides with a size of more than 1Dm. They serve as centers for forming nuclei which facilitate recrystallization and austenite decomposition, With a content of 0.06% Ti, apart from nitrides there is formation of titanium carbonitrides and carbides whose precipitation range embraces all of the temperature range for hot working. These particles effectively slow down austenite grain growth. A change in the ratio of the three types of precipitate forming with titanium governs the nonuniform dependence of pearlite hardness and failure characteristics on titanium content (Fig. ic).

It has been established that microalloying elements (with combined addition of them) in contrast to low pearlitic steels do not affect additively the properties and microstructural parameters of medium-carbon steel [5]. This is indicated by the presence of reaction terms in the regression equations obtained* describing the change in pearlite zone size Dp (Dm),

pearlite contentVp (%), oo.2 and of (N/mm2), a i (J/cm2), andTs0 (C) on carbon content (0.2-0.6%), manganese (up to 2%), chromium (up to i%) and microadditions of niobium and vanadium:

~p =40[C]+ 125[Cr]+ 10[Mn]--4[Vl--22[Nb];

A Vp = 170[CI-k-71 [Cr]+40[Mn] --16[V]+ 132[Nb] q- 125[V]. [Nb]--150[V]2--1126[Nb]2;

Aoo,~=272[C]-6175[Cr]+ 115[Mn] -61459[V]+621 [Nb]-- --3060[V][Nb] --4815[V]2--2929[Nb]~;

Aof=735[C]+272[Cr]-k- 171[bin] +563[V]+ 1197[Nb]--7764[Nb]2; Aax=220[C]-t-33[bin]--300[V] Jr 1600[V]~-k-300[Nb]; ATso= 143[C]--20[bin]+ 103[V]--221[Nbl--971[V][Nbl.

Analysis of the results of studying the composition of carbonitride phase precipitates showed that the nonadditive effect of microalloying elements in V-Nb and Nb--Ti steels is connected with formation of complex carbonitrides, and in V-Ti steels due to the fact that titanium removes nitrogen from vanadium.

Analysis of regresssion equations describing the effect of vanadium and niobium makes it possible to conclude that interaction of these elements leads to a reduction in hardening intensity and a greater increase in impact strength and cold brittleness. Less than in vanadium steel, strengthening with dispersed particles of vanadium carbonitride V(C, N) is

*The adequacy of equations for experimental relationship was checked by the Fisher criterion with a significance level not worse than 5%.

505

n,% 20.

1 I 10

20

10

0 1 2 4 8 16 32Dp. ,~

Fig. 2: Fig. 3

Fig. 2. Distribution histograms for pearlite grain size in medium-carbon steels containing 0.14% and 0.043% Nb separately (a) and together (b), (n is number of cases).

Fig. 3. A particle of complex carbonitride (NbTi) (C, N) in a fracture of steel type 30KhGBT. 2000.

due to transfer of part of the vanadium into a complex carbonitride (Nb, V)(C, N). This is confirmed by x-ray diffraction and x-ray microanalysis data.

Complex carbonitride (V, Nb)(C, N) has a wider precipitation temperature range than vanadium and niobium carbonitrides V(C, N) and Nb(C, N). In addition, complex compounds have a lower tendency towards coalescence [I, 6]. As aresult of this fine particles of carbo- nitride (V, Nb)(C, N) have a capacity to retard effectively austenite grain growth and re- crystallization processes. This leads to formation of austernite with a greater number of structural imperfections, i.e., potential areas for forming s-phase nuclei during 7 + ~- transformation. As a result of this decomposition of austenite a uniform finely-dispersed pearlitic-ferritic structure forms in the steel. This is indicated by a reduction and shift to the left of pearlite grain size distribution histograms in steel with 0.14% V and 0.043% Nb compared with those for steels containing these additions individually (Fig. 2).

An increase in dispersion and uniformity of the final steel structure with microadditions of vanadium and niobium causes an increase in its toughness and cold brittleness with a simultaneous increase in strength. Results of the combined effect of vanadium and niobium depend not only on their concentration, but also on the nitrogen content which governs the chemical composition and solubility in the steel of the carbonitride phase. An increase in nitrogen content in the steel promotes enrichment in nitrogen in th carbonitride formed. This is confirmed by data for the reduction in crystal lattice spacing of carbonitride phase from 0.4456 to 0.4438 nm with an increase in nitrogen content from 0.0035 to 0.018%.

Precipitation of these particles over a wider temperature range promotes more effective retardation of grain growth and austenite recrystallization. As a result of this equal mechanical properties of steels are achieved with the lower content of microalloying additions. For example, in steel containing 0.10% V an increase in the nitrogen content from 0.009 to 0.018% equates to an increase in niobium content from 0.046 to 0.14%.

Addition of titanium to steel with niobium suggests a reduction in the effective amount of niobium necessary for forming optimum properties. This effect is explained by the fact that in steel with niobium and titanium there is formation of coarse (more than 1 ~m) complex carbonitrides (Ti, Nb) (C, N) based on titanium (Fig. 3) which have a deleterious effect on material strength and ductility.

No complex carbontride V-Ti phase was detected in steel microalloyed with vanadium and titanium by phase chemical, x-ray diffraction and x-ray microanalysis. The reduction observed

506

in strength with a simultaneous increase in ductility as a result of adding titanium to vanadium-containing steel is due to a reduction in the effect of dispersion hardening by vanadium carbonitride V(C, N) particles. This occurs as a result of binding of nitrogen by titanium, and thus impoverishment of vanadium carbonitride in nitrogen. After binding all of the nitrogen it is theoretically possible to form vanadium carbide, although it is known that during cooling in air (Vcool = 2-5 K/sec) this carbide does not manage to precipitate [7]. Therefore, in practice a reduction in nitrogen content in the vanadium carbonitride V(C, N) composition leads to a reduction in volume fraction of dispersed precipitates. A change in the amount of nitrogen in steel containing vanadium and titanium may affect its properties by reacting on the volume of precipitates only with an amount of titanium insufficient for binding nitrogen.

CONCLUSIONS

i. With combined addition to steel type 30 KhG of microalloying additives (V, Nb, Ti) therefore is a unidirectional, but not additive, effect of these components on material properties, which is connected with formation of complex carbonitrides, redistribution of nitrogen between precipitating carbonitride phases, and a change in the temperature range for their precipitation.

2. An increase in nitrogen content to 0.02% in V-Nb steel increase the effectiveness of microalloying additions providing expansion of the temperature range for the dispersed particle precipitation.

LITERATURE CITED

i. Yu. I. Matrosov, A G. Nasibov, and I. N. Golikov, "Properties of low-pearlite steels with vanadium and niobium after controlled rolling," Metalloved. Term. Obrab Met., No. I, 27-34 (1974).

2. Yu. D. Yashin, A. Ao Petrunenkov, N. V. Bogdanova, et al. Use of High-Strength Steels with Controlled Forging for Automobile Components (Review) [in Russian], Avtomob. Prom., Tolyatti (1986).

3. F. B. Pickering, Physical Metallography and Development of Steels [Russian translation] Metall~irgiya, Moscow (1982).

4. M. I. Gol'dshtein and V. M. Farber, Dispersion Hardening of Steel [in Russian], Metal- lurgiya, Moscow (1979).

5. Yu. I. Matrosov, "Complex microalloying of low-pearlitic steels given controlled rolling," Metalloved. Term. Obrab. Met., No. 3, 10-17 (1986).

6. V. G. Cheremnykh, R. Sh. Shklyar, M. I. Gol'dshtein, and S. N. Chirkova, "Composition and structure of vanadium and niobium carbonitride phases in low-carbon steel," Fiz. Met~ Metalloved., 38, No. 3, 541-547 (1974).

7. T. Siwecki, A. Sandberg, W. Roberts, and R. Lagneborg, "The influence of processing route and nitrogen content on microstructural development and precipitation hardening in vanadium-microalloyed HSLA steels," Thermomechanical Processing of Microalloyed Austenite, Pittsburgh (1981).

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