Lieng

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: Pre-quenching prior to intercritical annealing quenching and partitioning (Q-P) process was proposed to enhance the volume fraction of retained austenite and the mechanical properties of a low-carbon Si-Mn steel.The intercritical austenite exhibited a lath morphology due to the martensitic microstructure maintained prior to intercritical annealing.Consequently,the alloy element enrichment of intercritical austenite,in which the alloy element was aggregated at the austenitic boundaries and further diffused inside,improved the stability of intercritical austenite and decreased the Ms of it.As a result,the fraction of retained austenite in steel was increased,which improved the mechanical properties of the experimental Q-P steel.

Key words: pre-quenching and quenching and partitioning; intercritical annealing; lath intercritical austenite; retained austenite; mechanical properties

1 Introduction

The amount and stability of retained austenite at room temperature are of critical importance to the design of quenching and partitioning (Q-P) steels.The simultaneous improvement of strength and ducti-lity observed for Q-P steels is due to the high work hardening rate resulting from the strain-induced mar-tensitic transformation of retained austenite[1-3].Chemical composition strongly influences the stabi-lity of metastable retained austenite at room tempera-ture[4-7].Reducing its grain size increases the stabi-lity of retained austenite by suppressing martensitic transformation[8-10].Also,the martensitic transfor-mation can efficiently enhance the stability of sur-rounding austenite due to the three-dimensional hydrostatic pressure from the expanded volume.

The excellent combination of high strength and good ductility of Q-P steels with low carbon content (approximately 0.2%) has been widely investi-gated.These steels have shown a 15% maximum volume fraction of retained austenite at room temperature[11-14].The retained austenite in Q-P steels appears as block and film-like shape.The block one generally trans-forms to martensite at a small strain and minimally contributes to the TRIP effect[15].Therefore,the key point for low-carbon Q-P steels is how to decrease the volume fraction of block retained austenite.In this work,pre-quenching (quenched from complete austenitizing temperature to room temperature) and subsequent intercritical annealing are performed prior to quenching and partitioning.The chemical com-position,grain size,and shape of retained austenite are also analyzed to investigate the influence of this novel heat treatment on the mechanical properties of Q-P steels.

2 Experimental procedure

Table 1 presents the chemical composition of the experimental steel that is mainly derived from the role of alloying elements and in consideration of produc-tion cost[14].A casting ingot weighing 50 kg was manu-factured by vacuum induction melting,homogenized at 1 200 ℃ for 2 h,and hot-forged to slabs with a width of 100 mm and a thickness of 30 mm.These bars were soaked at 1 200 ℃ for 2 h and then hot-rolled to sheets with a thickness of 4 mm after eight rolling passes with the starting and finishing temperatures of 1 150 ℃ and 850 ℃,respectively.After pickled in 10% hydrochloric acid,these sheets were cold-rolled to strips with a thickness of 1.2 mm.

Table 1 Chemical composition and critical temperatures of the investigated steel

For the design of appropriate heat treatment,the critical temperaturesAc1,Ac3,andMsof the experimental steel was first obtained by dilatometer tests as listed in Table 1.The specimens were divided into three groups.The first group (named Q-P1) was austenitized at 910 ℃ for 3 min,and then quenched to 220 ℃.Subsequently,these quenched specimens were held at 400 ℃ for 100 s,finally quenched again to room temperature.The second group (named Q-P2) was austenitized at 850 ℃ for 3min,then quenched to 220 ℃ and parti-tioned at 400 ℃ for 100 s,and quenched again to room temperature.The third group (named Q&Q-P) was treated in the same way as the second group but underwent pre-quenching from 910 ℃ to room tem-perature first.Rectangular tensile specimens (width 12.5 mm,thickness 1.2 mm,and gauge length 25 mm) were prepared along the rolling direction after heat treatment.Tensile tests were performed on a SANSCMT-5000 tensile machine at room tem-perature with a strain rate of 2×10-3s-1.The micro-structure was analyzed by scanning electron microscopy (SEM) and electron backscattered dif-fraction (EBSD).The volume fraction of retained austenite was measured via X-ray diffraction (XRD) with Cu Kα radiation operated at 40 kV and 100 mA in the scanning range of 40° -120° at a scanning rate of 2°/min.The diffraction lines of (200)α,(211)α,(200)γ,(220)γ,and (311)γwere employed to determine the value of retained austenite[16-17].The positions of the maximum diffraction peaks of austenite were used to deter-mine the lattice constant of austeniteaγ.This para-meter is necessary to calculate the carbon concentra-tion in the retained austenite[18].Work hardening beha-viors were analyzed using the Hollomon equation[19].

3 Result and discussion

3.1 Microstructural observations

Fig.1 shows the representative microstructure of steels under the three kinds of heat treatment.As illustrated in Fig.1(a),the typical microstructure of the Q-P1 sample has an almost fully martensitic microstructure,and the original austenite grain boundaries are still visible.Fig.1(b) displays that,after intercritical annealing (Q-P2),the micro-structure consists of the equiaxed grains of inter-critical ferrite surrounded by the martensite formed by block original austenite grains.As shown in Fig.1(c),the Q&Q-P sample exhibits a lath microstructure consisting of original lath austenite separated by elongated intercritical ferrite.The amount and size of original block austenite had declined.The lath microstructure was formed by the fine martensite obtained from pre-quenching.This arrangement led to the increase in nucleation sites for austenite dur-ing intercritical annealing and the formation of small austenite grains.The lath ferrite also origi-nated from martensite laths,which underwent car-bon depletion and fast recovery during intercritical annealing.

Fig.1 Typical SEM micrographs of the samples after Q-P1,Q-P2,and Q&Q-P treatments

Fig.2 shows the band-contrast map of the experimental steels.As shown in Fig.2(a),the retained austenite in Q-P1 sample exhibits an almost film-like morphology and is distributed on the grain boundaries of original austenite or between martensite laths.For the Q-P2 sample,the retained austenite shows a block-like morphology and is distributed on the interface of austenite and ferrite.The volume fraction of retained austenite in the Q-P2 sample is larger than that in the Q-P1 sample.Alloy elements,such as C and Mn,diffuse and redistribute between ferrite and austenite[20].In this work,the alloy elements first aggregate at the grain boundaries of the austenite and then diffuse inside.Therefore,before the equilibrium state between ferrite and austenite is obtained,the concentration of alloy elements at the austenite boundaries is always higher than that in the inner regions.As a result,the austenite near the boundaries is more stable than that in the inner regions,and the retained austenite is evident on the interface.

Fig.2 Band-contrast map showing the retained austenite (blue block) in the experimental steel by EBSD analysis

The retained austenite in the Q&Q-P sample is smaller and more uniformly distributed than that in the Q-P2 sample.Some of the retained austenite in the Q&Q-P sample exhibits a film-like morpho-logy,and that in the Q-P2 sample displays an al-most block-like morphology.Owing to pre-quench-ing,the original austenite of the Q&Q-P sample is smaller than that of the Q-P2 sample and exhibits a lath morphology,which further leads to high stability and a large amount of untransformed au-stenite after quenching.The presence of film retain-ed austenite confirms that martensite transfor-mation during quenching mechanically stabilizes the adjacent film austenite between lath martensite.This effect can be attributed to the three-dimensional hydrostatic pressure,which is one of the reasons for retained austenite stabilized at room temperature.

3.2 Mechanical properties

Fig.3 shows the mechanical properties and work hardening exponents of each sample,and Table 2 lists their mechanical properties.The Q-P1 sample exhibits the highest ultimate tensile strength but the lowest elongation due to its almost fully martensitic microstructure and insufficient retained austenite fraction.This sample also possesses the lowest instantaneous work hardening exponents because of its minimal amount of retained austenite,leading to insufficient work hardening from austenite transfor-ming to martensite[21].The Q-P2 sample has the lowest strength among the three samples,a medium elongation,and higher work hardening exponents than the Q-P1 sample due to its relatively high volume fraction of retained austenite.Compared with that in retained austenite fraction,the increase in elongation was more impressive.The latter is partly attributed to ferrite acting as a soft constituent that deforms first during strain and delays mar-tensite deformation.Pre-quenching endowed excel-lent mechanical properties for the Q&Q-P sample.Owing to the increased fraction of retained au-stenite,the uniform deformation was prolonged,and the local necking was strongly suppressed.Work hardening from martensitic transformation was also maintained at a high level until breakage.

Table 2 Mechanical properties,measured volume fraction of retained austenite by XRD tests,and calculated carbon content of retained austenite in the investigated steel

4 Conclusions

In this research,the microstructure resulting from different kinds of heat treatment was investigated by SEM,TEM,XRD,and EBSD.Due to Mn and C distribution during intercritical annealing,the austenite in the samples has achieved high stability.As a result,the volume fraction of retained austenite increases and the mechanical performance is impro-ved.Maintaining a fully martensitic microstructure prior to intercritical annealing encouraged the inter-critical austenite to exhibit a lath shape due to the morphology of the martensitic microstructure.This feature further increases the volume fraction of retained austenite and significantly enhances the mechanical properties of Q-P steels.