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1)Technology Center,Ma’anshan Iron and Steel Co.,Ltd.,Ma’anshan 243003,China;

2)Technology Center,Baowu Group Masteel Rail Transit Materials Technology Co.,Ltd.,Ma’anshan 243003,China

Abstract：To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite (γ) on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope (OM),a scanning electron microscope (SEM),a transmission electron microscope (TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above Ac3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N) second phase semicoherent with the matrix in the original γ grain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N) and ferrite (α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes the α nucleation point,causing granular and ellipsoidal intragranular ferrite (IGF,with an average size of 4-6 μm) to nucleate in the original γ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.

Key words：vanadium microalloyed; railway wheel; strength and toughness match; low undercooling in austenite; intragranular ferrite; second phase; step-cooling process

1 Introduction

Railway wheels with ferrite (α)/pearlite (P) microstructure have been widely used all over the world because of their excellent wear resistance,thermal stability,and machinability.As the key running part of a railway train,the wheel is borne by a complex thermal-mechanical load in service.Improving the physical quality of the wheel is the only way to ensure safe train operation,and the core objective is to ensure the strength and toughness of wheel steels.Improving the strength of the wheel by adjusting the composition and process is not dif-ficult.The development of vanadium-containing wheels,in particular,has significantly improved the yield ratio.However,there is a competitive rel-ationship between toughness and strength.The pur-suit of high strength and toughness match has always been the focus of material research.As an important index to determine the ability to resist crack instability propagation,fracture toughness reflects the comprehensive properties and process con-trol level of a wheel.From the perspective of fatigue and fracture mechanics[1-2],improving the fracture toughness of wheel steels is one of the most effec-tive measures to prevent brittle cracking on the rim (i.e.,rim crack fatigue).The calculation research of wheel fatigue and stress concentration[3]also shows that improving fracture toughness effectively hinders crack propagation.

The effect of microstructure on the fracture toughness of wheel steels has been studied[1,4-7].Soft α phase plays an active role in hindering crack propagation.People have always paid attention to vanadium’s contribution to precipitation streng-thening in steel in the past.In recent years,the development of intragranular ferrite (IGF) techno-logy in the case of low-carbon vanadium micro-alloyed steels has provided a new idea to improve fracture toughness.A previous study[8]shows that the V(C,N) precipitated in γ promotes IGF nuc-leation and refines α grains significantly.Further,IGFs improve the resistance to crack propagation.

However,currently,there are few studies on IGF nucleation in medium-high carbon α/P steels,and the specific role of IGF remains unclear and needs further research.In this paper,the effect of small cooling pretreatment in the γ phase zone (i.e.,low undercooling) on the microstructure,second phase,and strength/toughness match of a commercial van-adium-containing wheel was investigated,and the nucleation mechanism and effect of IGF were analyzed.Based on this,a new heat-treatment step-cooling (SC) process[9]that can promote the forma-tion of IGF was developed.

2 Material and methods

The major chemical composition of the experi-mental steel is as follows:w(C)=0.530 0%,w(Si)=0.280 0%,w(Mn)=0.750 0%,w(V)=0.080 0%,w(N)=0.005 4%,and the residual is Fe and inevi-table impurities.According to Thermo-Calc soft-ware calculations,the critical temperatureAc3of the experimental steel is about 781 ℃.The experi-mental raw material was prepared by melting,cast-ing,annealing,and forging.Several round samples with the size ofφ7 mm×3 mm were taken and heat treated with a high-temperature laser confocal micro-scope (model VL2000DX-SVF17SP) according to Table 1,and the nucleation of IGF was observed in-situ.

Table 1 Heat-treatment process parameters for the laser confocal experiment

The Vickers hardness of the sample was measured after the experiment.The morphology,number,and size of IGF were observed and counted by OM (model Axio Imager M2m) and SEM (model Fei Quanta 450).The volume fraction of proeutectoid α was quantified using OM,and the P interlamellar spacing was measured using SEM.The morpho-logy,number,size,and composition of pre-preci-pitated particles and the phase interface relationship between particle and matrix were analyzed by TEM (model Fei G2 60-300).

On the basis of the above research results,a finite element simulation of the new heat-treatment cool-ing process was carried out to determine the optimi-zed weak cooling time.Then the process practice and verification were carried out on the physical wheel in the production field,and samples were taken for fracture toughness and static tensile tests.

3 Results and discussion

3.1 IGF nucleation

The in-situ observation of sample Exp.810 is shown in Figs.1(a)-(c).Granular and ellipsoidal IGFs appear in the original γ matrix after phase transformation,indicating that the low undercooling pretreatment in the γ phase zone provides condi-tions for intragranular nucleation of α.

The hardness of the experimental samples is shown in Fig.1(d).Compared with the sample Ref.,the hardness of samples Exp.850,Exp.830,and Exp.810 is almost not reduced,while the hardness of sample Exp.790 is reduced by about 5%.It shows that the low undercooling pretreat-ment above 800 ℃ after austenitization has little effect on the final hardness,but the sacrifice of strength increases when the undercooling temper-ature is below 800 ℃.

Fig.1 In-situ IGF nucleation observation

3.2 Microstructure

Figs.2(a)-(e) depict microstructuring experi-mental samples.Consistent with the sample Ref.,after low undercooling pretreatment,the final micro-structure of the sample is mainly also P and pro-eutectoid α with an intermittent network along the grain boundary (GB).The P interlamellar spacing of the five groups of samples is 131±7 nm,and the volume fraction of proeutectoid α is 10.6%±0.8%.The quantitative microstructure results of the five groups of samples are close,showing the contri-bution of phase transformation strengthening to strength is not much different from each other.How-ever,distinct from the sample Ref.,ellipsoidal and IGFs precipitate in the original γ of the under-cooling pretreatment sample,which is randomly distributed in the matrix,and the IGF number increases with a decreasing undercooling temper-ature.The SEM morphology and energy spectrum of IGF are shown in Fig.2(c).The interface between IGF and P is obvious,flat,and smooth,and IGF nucleates in the second phase with large size.Figs.2(f)-(i) show the quantity and size statistics of IGF in the microstructure of the four groups of samples after undercooling pretreatment.The size of IGF is 2-12 μm,and the average size is concentrated at 4-6 μm.Different undercooling temperatures have little effect on the IGF size.

Fig.2 Microstructure of the experimental samples

The essence of low undercooling pretreatment is to change the weight distribution of vanadium in precipitation strengthening contribution and coope-rate with the control of α morphology distribution,which provides a new way to improve the strength and toughness match of vanadium-containing wheel steels.In fact,the heat treatment of the physical wheel is heated by the temperature-stepping annular heating furnace,which is difficult to realize with the low undercooling pretreatment,but it can be considered to implement the SC in the quenching cooling stage to realize a similar effect.

3.3 New heat-treatment cooling process and practice

As shown in Fig.3,a new type of SC follows the tread spray cooling mode with water.But different from the traditional continuous cooling (CC),the tread is sprayed with a weak flow (1/5-1/4 of the strong cooling flow) in SC for a short time,which produces an undercooling inside the rim and keeps the temperature still above the critical temperatureAc3,ensuring the strengthening of the matrix in the subsequent strong cooling stage.

Fig.3 Description of wheel structure and finite element model of heat-treatment cooling

As shown in Fig.4,the finite element calculation shows that the temperature inside the rim is 800-870 ℃ after 80 s of weak cooling.The undercooling degree is appropriate,and the temperature condition for IGF formation is met.

Fig.4 Temperature field of the rim after 80 s weak cooling in SC

The verification test was performed in the produ-ction field.The mechanical properties of the physical wheel are shown in Table 2.In a wide austenitizing heating temperature range,the SC process increases the average and minimum fracture toughness values by more than 10.9% and 18.5%,respectively,and reduces fracture toughness fluctuation by more than 23.1%.The six specific fracture toughness values equally distributed along the wheel circumference are shown in Fig.5.It is noted that the SC process also slightly improves the tensile strength of the tread sub-surface.This can be explained by the precooling effect,which is produced by short-time weak cooling,and the effective cooling rate in this area is increased in the strong cooling stage[10].

Fig.5 Circumferential fracture toughness distribution of the physical wheel with different heat-treatment conditions

Table 2 Mechanical properties of the physical wheel

3.4 Discussion

During the low undercooling pretreatment,solid-solved vanadium pre-precipitates inevitably from the original γ due to the decrease in solubility and exists in the form of a second phase.As shown in Fig.6,the vanadium and nitrogen are concentratedat the short rod-shaped pre-precipitate particle,while the carbon is partially enriched at the particle,and there is no manganese dilution around the particle.Therefore,it is determined that this type of particle is V(C,N),which also proves that V(C,N) also precipitates in the original γ without solute dilution.

Fig.6 STEM analysis of the second phase pre-precipitated in original γ of the sample obtained by brine bath chilling after undercooling pretreatment at 830 ℃

In the absence of MnS or AlN large inclusions,the nucleation of V(C,N) in the γ grain interior (GI) or at the GB depends on the competition between them.Although the GB is the preferential nucleation site of the second phase,the GB nuclea-tion mechanism is not dominant,and it transfers gradually to GI nucleation.Fig.7(a) shows that the particle size of V(C,N) pre-precipitated in GI is 20-90 nm,with an average size of 56.7 nm,with particles sized 40-70 nm,accounting for approxi-mately 70% of the total.The particle size is relatively large,and thus the contribution to precipitation strengthening is limited.Fig.7(b) depicts a high-resolution image of pre-precipitated V(C,N).It can be seen that the fringe contrast on the matrix side is relatively uniform,while the fringe contrast of the second phase is not uniform,showing that there is a stress field caused by dislocations and point defects at the phase interface.As shown in Fig.7(c),a part of the lattice at the transition interface between the pre-precipitated V(C,N) and matrix is shrunk or expanded,resulting in the formation of mismatch dislocation (indicated by the arrow),demonstrating that the pre-precipitated V(C,N) has a semicoherent relationship with the matrix and indicating that the pre-precipitated V(C,N) in original γ has little contribution to the strength.

Fig.7 Fine analysis of the pre-precipitated V(C,N) in original γ of the sample obtained by brine bath chilling after undercooling pretreatment at 830 ℃

For vanadium-containing wheel steels of α/P micro-structure,the strengthening mechanisms mainly include P transformation strengthening,α grain strengthening,dislocation strengthening,and precipitation streng-thening.With the consistent austenitizing and cool-ing transformation conditions,the contributions of the first three strengthening mechanisms to the final strength of the experimental steel are similar,but due to the low undercooling,a small amount of solid-solved vanadium pre-precipitates from the matrix.The pre-precipitated amount increases with the increase of undercooling,which reduces the content of solid-solved vanadium in the matrix and reduces the number of nano-scale V(C,N) precipitated streng-thening phases in the final microstructure.This is why the strength of the experimental steel decreases after undercooling pretreatment below 800 ℃.Therefore,in order to obtain an optimized strength and toughness match,the undercooling temperature should be reason-ably controlled.

In the subsequent cooling transformation stage,the undercooled γ transforms to α/P,and the GB remains the main site for α nucleation.In the Baker-Nutting orientation relationship (i.e.,{100}α-Fe∥{100}V(C,N)and <011>α-Fe∥<010>V(C,N)),the habit plane lattice mismatch between α and V(C,N) is much lower than that between α and γ,according to the lattice matching principle;therefore,the pre-precipitated V(C,N) becomes the secondary site for α nucleation,effectively inducing the hetero-geneous nucleation of α in GI.The lower the undercooling temperature,the greater the number of pre-precipitated V(C,N),the greater the nucleation sites of IGF,and the greater the number of IGF in the final microstructure.But under the same cooling phase transformation condition (such as 2 K/s cool-ing to a temperature below 50 ℃ in this study),the diffusion rate of solute atoms at the γ/α interface is the same,and the growth condition of nucleated IGF is the same,resulting in a small difference in the final IGF size.Hence,the undercooling temperature has no obvious effect on the IGF size.

Phase α is a type of plastic phase with con-siderably lower strength than that of cementite (θ).The cleavage crack of α/P steel initiates from the θ layer or α/θ layer interface,propagates to the adjacent α layer,and continues to travel in the α layer.After a short distance from the main propagation direction,the crack encounters various interfaces and then deflects and bends many times to continue to propagate along the relatively weak interface[11].The stress at the crack tip is relaxed when the crack arrives in the α zone,plastic passivation occurs,and the propagation path is bent and forked.When the volume fraction of α is high,the crack propagation path tends to bypass the hard P and passes through the softer α to obtain a higher fracture toughness.With the formation of IGFs,the volume fraction of total α is increased,further improving the resistance encountered in the process of crack propagation.The existence of IGF also increases the phase interface of the microstructure,and the probability of encountering an interface during the process of crack propagation increases,resulting in a more complicated deflection and more energy consumption,and resulting in higher fracture toughness at the macro level.

4 Conclusions

(1) The low undercooling of vanadium-contain-ing wheel steels is produced via precooling in the γ phase zone,and solid-solved vanadium is pre-precipitated in the GI to form V(C,N) with a large size and semicoherent with the matrix.In the subsequent cooling phase transformation stage,the pre-precipitated V(C,N) becomes the catalyst for the nucleation of α and induces IGF formation.

(2) The presence of IGF increases the phase interface of the microstructure,causing complex deflections to occur many times during crack propagation,which consumes more energy,resulting in higher fracture toughness at the macro level.

(3) The lower the undercooling temperature,the greater the number of IGFs,though the average IGF size,which is 4-6 μm,slightly varies.Low under-cooling pretreatment has little effect on the volume fraction of proeutectoid α and P interlamellar spacing,but it decreases the amount of precipitated strengthening phase in the final microstructure.Therefore,the undercooling temperature should be controlled reasonably.

(4) In a wide range of heating temperatures,the average and minimum fracture toughness of the physical wheel are increased by the new step-cooling process by more than 10.9% and 18.5%,respectively,and the fluctuation of fracture tough-ness is reduced by more than 23.1%.An ideal bal-ance between strength and toughness is obtained.