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

Abstract: In this study,the effect of vanadium on the microstructure and properties of the metastable austenitic stainless steel AISI 301LN was investigated.Results of the study show that the addition of vanadium can refine grains and increase the strength of AISI 301LN by solution treatment.After 60% cold-rolling reduction,the microstructure of the steel was composed of work-hardened austenite bands and deformation-induced ɑ′ martensite.Considerable work-hardening and phase transformation strengthening occurred.After cold rolling and subsequent annealing,the deformation-induced ɑ′ martensite was reversed into fine-grained austenite.The work-hardened austenite bands underwent recrystallization;however,the structure of the recrystallized austenite grains was coarser than that of the reversed ones.Simultaneously,the strength of the experimental steels decreased with the increase in annealing temperature.The pinning effect of precipitates of vanadium inhibited the growth of austenite grains.Thus,the desirable combination of strength and ductility was obtained by grain refinement.

Key words: microalloy; grain refinement; strength; austenite stainless steel

1 Introduction

Grain refinement has attracted increasing attention as an effective method for improving material strength without reducing plasticity and tough-ness[1-10].In particular,the production of ultrafine-grained metastable austenitic stainless steel through the advanced thermomechanically controlled process is considered the most promising method for indus-trial lines.For example,TOMIMURA et al.[11]suc-cessfully obtained ultrafine-grained austenitic stain-less steel with a grain size of less than 1 μm.The core process included cold rolling with 90% reduction and subsequent annealing at 900 K for 600 s (holding time).Achieving 90% cold reduc-tion and low-temperature annealing for 600 s is a challenging task in an industrial line.Microalloying has been widely adopted in industrial applications for refining grains and improving strength.The pur-pose of the present study is to understand the effect of vanadium on the microstructure and properties of the metastable austenitic stainless steel AISI 301LN.

2 Experimental methods and procedures

Three kinds of steels with different vanadium con-tents were melted in a 50-kg vacuum induction fur-nace.Based on the chemical composition of AISI 301LN,different vanadium contents were added.The specific chemical composition is given in Table 1.

Steel ingots were forged and hot-rolled into a 3.5-mm-thick sample for cold rolling.Multipass cold rolling was performed in a four-high rolling mill with oil lubrication.Thickness reductions from 5% to 60% were carried out at room temperature.During cold rolling,each pass reduction was 2%,and the total reduction was 60%,with subsequent short-term annealing at 1 073 K,1 173 K,and 1 273 K for 60 s (holding time).The process is shown in Fig.1.

Table 1 Chemical composition of the experimental steels %

Fig.1 Schematic of the experimental steel processing

The samples were mounted and ground with 400#,800#,and 1 200#sandpapers.Next,they were electrolyzed using 10% oxalic acid solution.Then,the surfaces of the samples were cleaned using pure ethanol and dried.The OLYMPUS-GX71 microscope was used to obtain optical metallography images.Electron backscatter diffraction was used to investigate the evolution of the microstructure of AISI 301LN.The diffraction patterns were collected using a field emission gun-scanning electron microscope oper-ating at 20 kV and the HKL-F400 system (Oxford Instruments).Analysis was performed on the longi-tudinal section after standard mechanical and elec-tron polishing.The electron polishing was con-ducted in an electrolytic bath containing 8% perchloric

acid in ethanol at 25 V and ～298 K.

Mechanical properties were measured by tensile tests using the MTS810.10 material testing system with the application of 5 000-kg tensile force.

3 Results

3.1 Microstructure and properties of the experimental steels after solution treatment

The metallographic structure of the experimental steels with different vanadium contents after the solution treatment is shown in Fig.2.It can be seen that the grains are gradually refined with the increase in vanadium content from 0 to 0.6% after the solution treatment.The mechanical properties of the experimental steels are given in Table 2.When the vanadium content increased from 0 to 0.6%,yield and tensile strength increased from 280 MPa and 640 MPa to 330 MPa and 716 MPa,respective-ly.However,no significant decrease in elongation was detected.

Fig.2 Annealing microstructure of the experimental steels with different vanadium contents

Table 2 Mechanical properties of the experimental steels with different vanadium contents after solution treatment

3.2 Microstructure and properties of experimental steels after cold rolling and subsequent annealing

Microstructure of the experimental steels 1#,2#,and 3#was work-hardened austenite bands and deformation-induced ɑ′ martensite after cold rolling (Fig.3).After 60% cold reduction,the volume fractions of the ɑ′ martensite content in the experimental steels 1#,2#,and 3#were 28%,34%,and 43%,respectively.The microstructure of the experimental steel 1#after annealing at 1 073 K,1 173 K,and 1 273 K is shown in Fig.4.The deformation-induced ɑ′ martensite was completely transformed into fine austenite.The work-hardened austenite was recrystallized after annealing.The grain size of the reversed austenite was finer than that of the recrystallized one.The grain size increased with the increase in annealing temperature (1.19 μm,3.20 μm,and 12.57 μm).The micro-structure of the experimental steels 2#and 3#after annealing at 1 073 K,1 173 K,and 1 273 K is shown in Fig.5.The microstructure was composed of work-hardened austenite bands and reversed austenite after annealing at 1 073 K.This means that the recrystal-lization of work-hardened austenite bands did not occur completely.When the annealing temperature increased from 1 173 K to 1 273 K,the average grain size of the experimental steels 2#and 3#increased from 4.0 μm and 3.0 μm to 7.6 μm and 4.0 μm,respecti-vely.As shown in Figs.5(c) and (d),the recrys-tallization completed at 1 173 K during annealing.The grains were not uniform.In particular,the austenite grains obtained from the deformation-induced ɑ′ martensite had a fine structure,whereas the austenite grains formed by recrystallization were coarse.Figs.5(e) and (f) show the austenite grains at 1 273 K during annealing.The grain size of steel 2#was coarser than that of steel 3#.

Fig.3 Microstructure after cold rolling

Fig.5 Microstructure of the experimental steels 2# and 3# at different annealing temperatures after cold rolling

The mechanical properties of the experimental steels after annealing at 1 073 K,1 173 K,and 1 273 K are shown in Fig.6.Because of the work-hardened and deformation-induced ɑ′ martensite,both the yield strength and tensile strength of the experimental steels were very high under the cold-rolling condition.The tensile strength of steels 1#,2#,and 3#reached 1 206 MPa,1 288 MPa,and 1 306 MPa,respectively.After annealing at 1 073 K,the strength of the experi-mental steels decreased rapidly.The tensile strength of steels 1#,2#,and 3#reached 897 MPa,1 014 MPa,and 1 084 MPa,respectively.After annealing at 1 173 K,the strength of the experimental steels decreased continuously.The tensile strength of steels 1#,2#,and 3#reached 806 MPa,789 MPa,and 837 MPa,respectively.After annealing at 1 273 K,the strength decreased slowly.Notably,steel 3#exhibited a good combination of strength and ductility.The yield strength,tensile strength,and elongation at break were 427 MPa,797 MPa,and 48%,respectively.This indicates an obvious strengthening effect due to fine grains.

4 Discussion

4.1 Effect of vanadium on austenite stability

Austenite stability is determined from chemical composition and deformation parameters.Md30/50is a common temperature for evaluating austenite stabil-ity under plastic deformation;it refers to the tem-perature at which 50% of strain-induced martensite is produced after 30% true deformation under ten-sion and can be estimated using the equation proposed by POULON-QUINTIN et al.[12]:

(1)

With the values taken from Table 1,theMd30/50values of steels 1#,2#,and 3#are 12 ℃,23 ℃,and 31 ℃,respectively.In fact,the actualMd30/50values of steels 2#and 3#are higher than the calculated values.The precipitates of V(C,N) reduce the contents of carbon and nitrogen.The highMd30/50results in the high volume fraction of ɑ′ martensite.Therefore,the ɑ′ martensite volume fraction of steel 3#is the highest after 60% cold reduction.

Fig.6 Effect of annealing temperature on the properties of the experimental steels

4.2 Effect of vanadium on austenite grain growth

The grain size decreased with the increase in vanadium content after the solution treatment owing to the precipitates of V(C,N) hindering grain growth.However,the grain growth after annealing for the ex-perimental steels 1#,2#,and 3#varied with temper-ature.At 1 073 K during annealing,the precipitates of V(C,N) increased the recrystallization temperature significantly and retarded recrystallization.Steel 1#without vanadium could be fully recrystallized,whereas steel 2#with 0.291% vanadium and steel 3#with 0.599% vanadium did not recrystallize,and the microstructure was composed of a large amount of uncrystallized austenite and a small amount of the reversed one.At 1 173 K during annealing,the recrystallized grains of steel 1#grew and uncrys-tallized austenite of steels 2#and 3#disappeared.The recrystallization at this temperature was closely related to the precipitates.According to the calcula-tion results of Thermo-Calc (Fig.7),the V(C,N) precipitation temperatures for steels 2#and 3#were 1 392 K and 1 476 K,respectively.The V(C,N) precipitation volume fractions for steels 2#and 3#were 0.40% and 0.68%,respectively.Accord-ingly,the grain size at the abovementioned tem-peratures was very small.At 1 273 K during anneal-ing,the grain boundary migration rate of steel 1#accelerated grain growth.The V(C,N) precip-itation volume fractions for steels 2#and 3#reduced to 0.28% and 0.52%,respectively.However,the precipitates were still able to reduce grain boundary migration and prevent rapid grain growth.Steel 3#had the finest grains owing to the largest volume fraction of V(C,N) precipitates.

Fig.7 Thermo-Calc calculation results of steels 2# and 3#

5 Conclusions

In this study,the effect of vanadium on the micro-structure and properties of metastable austenitic stainless steel AISI 301LN was investigated.The main conclusions are as follows.

(1) When vanadium content increases from 0 to 0.6%,the grain size during the solution treatment decreases and yield strength increases from 280 MPa to 330 MPa.

(2) After 60% cold rolling,the microstructure is composed of work-hardened austenite bands and deformation-induced ɑ′ martensite.The volume fraction of ɑ′ martensite increases with the increase in vanadium content,which reduces austenite stability.

(3) After cold rolling and subsequent annealing,deformation-induced ɑ′ martensite is reversed into fine-grained austenite.Work-hardened austenite bands undergo recrystallization.Notably,recrys-tallized grains are coarser than reversed ones.The strength decreases with the increase in annealing temperature.At 1 273 K during annealing,V(C,N) precipitates inhibit the growth of austenite grains.The result exhibits a good combination of strength and ductility owing to grain refinement strength-ening.