Microstructural Considerations Limiting the Mechanical Properties of HSLA Steel
Mikrostrukturne omejitve mehanskih lastnosti HSLA jekel
L. Parilak1, IMR SAS, Košice, Slovakia
Prejem rokopisa - received: 1996-10-01; sprejem za objavo - accepted for publication: 1996-11-04
The influence of chemical composition, grain and subgrain size, and precipitation on yield strength, transition temperature and the work strengthening exponent was analyzed for HSLA (high strength, low alioy) steel. The relationships are quantified and transferred to graphic charts - nomogram for steel with polygonal as well as non polygonal microstructure. The limits of mechanical properties (the highest combinations of yield strength and transition temperature) were quantified for the polygonal HSLA microstructure.
Key words: microstructure, mechanical properties, HSLA steels
Vpliv kemijske sestave, velikosti zrn in podzrn ter izločanja na mejo plastičnosti, prehodno temperaturo žilavosti in keficienta deformacijske utrditve je bil analiziran za HSLA jekla. Odvisnosti so kvantificirane in zapisane v grafih - nomogramih za jekla s poligonalno in acirkularno mikrostrukturo. Mejne mehanske lastnosti (kombinacije največje meje plastičnosti in prehodne temperature žilavosti) so bile kvantificirane za poligonalno mikrostrukturo.
Ključne besede: HSLA jekla, mikrostrukture, mehanske lastnosti
1	Introduction
At the development of nevv steel types the key problem is to understand the influence of chemical composition and obtainable parameters of microstructure on strength. plasticity and brittle fracture resistance. It is es-sential to obtain the quantitative description of the rela-tion, and the description should be based on the knowl-edge concerning the nature of the mechanical properties in question. This way a valuable information can be obtained for the production technology, first for the prime chemical composition and heat treatment. In the presented work descriptions of correlations between chemical composition and parameters of microstructure on one side, and yield strength, work strengthening exponent, and transition temperature on the other, are compiled. They are quantified enabling direct application in engi-neering. In the second part of the work the limits of mechanical properties - combinations of strength, plasticity and brittle fracture resistance, are shown for the polygo-nal microstructure.
2	Microstructural essence of mechanical and fracture properties of microalloyed steels
Investigated were lovv-carbon microalloyed steels based on Ti, V, Nb, with eventual addition of Mo, in po-lygonal and non-polygonal microstructures. Introductory studies were devoted to the kinetics of precipitation of carbides, nitrides or carbonitrides of microalloying elements from the viewpoint of its intensity and effective-ness. Furthermore, investigated were also questions of
' Ass. Prof. Dr. L udovii PARILAK Institute of Materials Research SAS Watsonova 47. 04353 Košice. Slovakia
laws of interphase precipitation and precipitation in austenite and ferrite. The main objeetive was to gain the knovvledge of laws of the effect of precipitation states on strength as well as, plastic and brittle fracture properties. Analyses were carried out on several hundreds of struc-tural states in the state after rolling at hot rolling mili in VSZ JSC Košice, or in the state after thermal processing. Main attention was paid to the yield point, work strengthening exponent and transition temperature of noteh toughness.
2.1 Yield point
The analyses were based on the assumption of an ad-ditive character of individual strengthening contributions to the yield point Re and the follovving relationship was proposed for the studies set of steels:
Rc = Rpn+Rin+R0+Rsg+Rs+Rpr+Rp+Rd (1)
where Rpn - is the contribution of lattice frietion stress; Rin - contribution to strengthening on account of interstitially dissolved atoms of additives; Rd - contribution of dislocation strengthening; Rg - strengthening contribution resulting from the size of grains; Rsg - contribution resulting from the effect of subgrains; Rpr -pearlitic contribution; Rs - substitution contribution; Rp -precipitation contribution.
Their quantitative expression is based on relations comprised in
Analyses provided a quantitative expression of the substitution effect of manganese RMn and confirmation of the effect of silicon and pearlite on strengthening contributions (Rsi, Rpr).
In addition to that a quantitative effect of polygonaI ferrite grains d, or formations delimited by large angle boundaries (dF) in non-polygonal microstructures was
described. The quantitative expression of subgrain strengthening with intensity Rsg = Gb-dsG"1 = 0.1 dsG"1 (the size of a subgrain dsG in mm) in a very good agree-ment with the Landford-Cohen relation, was used. The interchangeability of Rg and Rd was demonstrated, with Rd representing a contribution of transformation or of "geometrically inevitable" dislocations.
The analyses of the influence of precipitation on pre-cipitation strengthening, employing ali available theo-retical models, were carried out. These analyses resulted in a quantitative relation for precipitation strengthening:
Rp = k^"2	(2)
where A. is the average planary interparticle distance of precipitates. The physical interpretation of this relation is follovving: Precipitation strengtheningis inversely pro-portional to the mean size of a free sliding area, corresponding to one precipitate (obstacle) standing in the way of the moving dislocation. The strengthening inten-sity constant kpR acquires a force dimension and can represent a mean value of force interaction phenomena between dislocations and precipitations, leading to a critical stress for the passing of dislocations trough ob-stacles. Its value kpR = 76.8 ■ 10"8 N is of the order corresponding to the size of an interaction of an edge dislocation vvith an elastic field of a particle F = 10"7 N).
The quantitative behaviour of a thermaly dependent constituent of the yield point (R*) was determined in the range -196 to +20°C, together vvith parameters Ct, B, ap-pearing in the relation:
R* = C, ■ exp(-T/B)	(3)
yield point B (relation (3)). The surface-plastic energy y, shear modulus of elasticity G, parameters ky, kf and the mode of stressing q are connected to the values A and B in the relation
A = Bln(^--kf)	(6)
ATj is the shift of transition temperature and depends from the struetural parameter t or eventually from the chemical composition.
The positive effect of grain refining on an improve-ment of brittle fracture resistance has been demonstrated and a direct relationship of its intensity and a thermal change of the yield point has been observed. A good agreement of the parameter B in relations (3) and (5) was deteeted. An embrittlement effect of pearlite and silicon has been demonstrated.
The influence of precipitation on the shift of transition temperature vvas demonstrated to follovv the relation
ATp = kpT-X"2	(7)
An estimate of the barrier effect of grain boundaries against propagation of cleavage cracks (k = 55 Nmm~3/2) vvas provided together vvith a value of surface plastic en-ergy at T k (y = 10"2 Nmm"1)- In case of polygonal micro-struetures analyses did not exclude a positive effect of manganese on the improvement of brittle fracture resistance and the quantitative expression corresponded to results of Pickering. In case of non-polygonal microstruc-tures an absence of significant effect of subgrains on transition temperature changes vvas observed.
2.2 Transition temperature
Our analyses vvere based on CottrelFs energetic bal-ance of cohesion of tough/brittle transition and Petch's condition of equality of the yield point Re and fracture stress Rfr for determination of the transition temperature of brittleness Tk. Contrary to Petch's formulation, we have assumed a general interaction betvveen individual parameters of microstructure and chemical composition and the frietion stress Rofr, appearing in the relation for fracture stress
Rpk = R0FR+kf ■ d"1'2	(4)
vvhich resulted in the development of a corresponding model and analytical formulation. The kf parameter rep-resents a barrier effect of grain boundaries direeted against the propagation of cracks aeross boundaries of grains.
The performed analyses provided the follovving relation for the transition temperature
TK05) = A-B-ln(d-U2) + 5;AT1	(5)
(i)
vvhere A is the so-called threshold value of brittleness, dependent on the intensity of the thermal change of
2.3 Complex relations
In our previous vvorks1,2,3 vve presented simplified relations for the evaluation of the influence of microstructure on yield strength Re, transition temperature T35 and vvork strengthening exponent n. For a polygonal microstructure it is expressed as:
Re = R0 + Rm„ + AR	(8)
T35 = A - B ■ ln(d-"2) + C • AR	(9)
n = a + —	(10)
AR
vvhere Rg = 15 • d~1/2 is the strengthening by ferrite grain size d (mm); RMn = 50 ■ XMn is the strengthening share of manganese XMn (%); AR is the part of embrittlement caused by strengthening, for microalloyed steel in-cluding mainly precipitation strengthening Rp, and also the influence of strengthening by silicon content Rsi, pearlite content Rpr, Peierls-Nabarro stress Rpn, and by interstitial strengthening Rin (AR = Rp + Rsi + Rpr + Rpn + Rin); A = 147°C, B = 110°C, C = 0.4°C/MPa is an embrittlement constant, a, b are regression coeffi-cients. For non polygonal microstructure similar relation vvere derived:
Re = R0 + RSG + RMn +AR	(11)
T35 = A - B • ln(d-"2) + C • AR	(12)
where Rg = 19 ■ d"1/2; A = 143°C; B = 100°C, C = 0.4 °C/MPa while Rsg = 0.1 ■ dsc"1 is the strengthening contribution of the subgrain size dsG (mm).
The graphic interpretation of the relations is shown in Fig.l for the polygonal microstructure (eq. 8-10) and in Fig.2 for the non-polygonal one (eq. 11-12).
It is important to note that the yield strength is con-trolled by a set of strengthening contributions with different influences on the brittle fracture resistance. The embrittlement from strengthening AR is resulting for every 100 MPa of strengthening a 40°C shift of the transition temperature into the wrong direction, causing the worsening of plastic properties, too, as shown by the work strengthening exponent. There is an influence of manganese content and subgrain size on strengthening too, though their influence on the transition temperature is not significant. Practically only one microstructural parameter is known, the increasing of the yield point to-
gether vvith the increase of brittle fracture resistance. It is the ferrite grain size d, or described more generally the size of the microstructural object limited by large angled borders. It is of prime importance to constitute the chemical composition and microstructure in the way to obtain first this microstructural parameter in the quality reflecting the desired complex of properties. The relations given in the work are simplifted theoretical descrip-tions with coefficients calculated by regression analysis made on more than 300 microstructure types of steel produced in ironvvorks VSŽ, a.s. Košice, Slovakia.
3 Limits of polygonal microstructures
We decided to define the limits of the complex of mechanical properties for a steel vvith polygonal microstructure. With this aim the HSLA steel, vvith yield strength from 420 to 700 MPa vvere evaluated. The basic features of the evaluation are shovvn in the graphic chart in Fig.3, vvhich was calculated for a 1% Mn content. The straight lines are representing the yield strength Re. The nomo-
Re (MPa 1
Figure 1: A eomplex nomogram for relation betvveen microstructural parameters and mechanical properties of HSLA steels vvith polygonal microstructure
Figure 2: A complex nomogram for relation betvveen microstructural parameters and mechanical properties of HSLA steels vvith non-polygonal microstructure
Figure 3: Microstructural considerations limiting mechanical properties of HSLA steels with polygonal microstructure
gram shovvs the possible combination of embrittlement AR and ferrite grain size d, necessary to obtain the selected yield point. The transition temperature T35 and the work strengthening exponent n are shown also.
In Tab. 1 the combinations of embrittlement AR and ferrite grain size d in grades according to ASTM are shown, vvhich are necessary for a steel vvith the desired combination of yield strength Re and transition temperature T35.
Table 1: Required ferrite grain size d and embrittlement by strengthening AR necessary for the combination of properties Rc and
T35
T35(°C)	0		-20	-40	-60
Re	d AR	d	AR	d AR	d AR
(MPa)	(MPa)		(MPa)	(MPa)	(MPa)
420	10 230	11-10 220		11 190	11-12 170
490	11-10 275	11	260	12-11 230	12-13 210
560	11-12 320	12	290	12-13 270	13 250
630	12-13 370	13	340	13-14 320	14 290
700	13-14 420	14	400	14 350	? ?
In ali cases a fine ferrite grain is required. Knovving the manufacturing technology and the limits of the vvide strips hot rolling mili the production of steel vvith ferrite
grain size under grade 14 cannot be experted. To obtain the grade 13 is very difficult, grade 12 is demanding, vvhile the more coarse grains are currently obtained. Consequently, Tab. 1 vvas simplified to Tab. 2 vvhich shovv that the elaboration of polygonal steel vvith the yield strength Re = 700 MPa and the transition temperature T35 under -40°C, is not be reliable. It is also not re-alistic to desire expert a limit of elasticity Re = 630 MPa vvith the transition temperature T35 better than -60°C.
Table 2: Limits of the polygonal microstructure for different combinations of Re and T35
Rc	T35 (°C)
(MPa)_0_-20_;40_-60
420	1 1 1 1
490	1 1 2 2
560	1 2 2 3
630	2 3 3 4
700	3 _3_4_4
The possibilities are denoted: 1 - realistic, 2 - demandig,
3	- very difficult, 4 - fiction.
In Fig. 3 it can be also seen, that for the mentioned Re and T35 values the ductility is very lovv, the vvork strengthening exponent in the range 0.10 to 0.16 (for the lovver strength) because for high Re values the embrittlement by AR is necessarily high, degrading the ductility and brittle fracture resistance.
4	Conclusion
Starting from theoretical relations the influence of chemical composition and parameters of the microstructure on strength, transition temperature and vvork strengthening exponent vvere investigated. The results are compiled and the limit combinations of strength, plastic properties and resistance to brittle fracture for HSLA steel vvith polygonal microstructure are calculat
5	Acknovvledgment
The vvork is supported by Project No. 2/1106/96 of the Slovak Scientific Grant Agency - VEGA.ed.
6	References
'L'. Parilak, M. Šlesar. B. Štefan: Structural Prediction of Mechanical Properties of HSLA Steels. In.: Proc. of Microalloying 88, ASMI, USA, 198B, 559
2B. Štefan: Fyzikalna metalurgia a vyvoj konštrukčnych zvaritetl'nych oceli. Doktorska dizertačnd praca, UEM SAV Košice, 1990 3L'. Parilak: Štrukturna podstata mechanickych a lomovych vlastnosti materialov. In.: Predikce mechanickych vlastnosti' kovovych materialu na zaklade strukturnlch charakteristik. l.dil. Nove Mesto na Morave, 11.-14.5.1993 Brno, P MSVTS VU 070 1993, 125