Tensile Properties and Stretch-Flangeability of TRIP Steels Produced by Quenching and Partitioning (Q&P) Process with Different Fractions of Constituent Phases

Abstract

The influence of microstructure on tensile properties and stretch-flangeability of TRIP steels with tensile strengths higher than 1.2 GPa has been investigated under various Quenching and Partitioning (Q&P) conditions. As lowering the quenching stop temperature, Q_T, below Ms temperature in the range of 340°C to 280°C, volume fractions of tempered martensite and retained austenite increased and volume fractions of bainite and fresh martensite decreased in the final microstructure. The higher the Q_T temperature in the range of 280°C to 330°C, the more the relative proportion of untransformed austenite at the end of the partitioning step was transformed into fresh martensite. The microstructural characteristics of fresh martensite and retained austenite under different Q_T conditions were analyzed by EBSD. The fresh martensite phase was identified by a new method applying the threshold values of both Image Quality (IQ) and Kernel Average Misorientation (KAM). It is suggested that the decrease in the HER (Hole Expansion Ratio) value with increasing Q_T temperature is due to the increase in the size and the volume fraction of fresh martensite particle.

The mechanical properties of Q&P steels were evaluated before and after tempering at 200°C for 1 hour. Under conditions where the initial volume fraction of fresh martensite before tempering was higher, tensile elongation and HER values were improved by tempering. Tensile elongation was increased with the volume fraction of retained austenite. Lower HER values were obtained with higher volume fractions of fresh martensite, regardless of tempering.

1. Introduction

TRIP (Transformation-Induced Plasticity) steels are classified as AHSS (Advanced High Strength Steel), where low-temperature transformed microstructures such as bainite and martensite are actively used to obtain better combinations of strength and ductility. TRIP steels were introduced as automotive material through intensive work by Japanese steel companies in the 1980s. In recent years, as the demand from the automotive industry for lightweighting to improve fuel efficiency has increased, so called Giga-steels with tensile strengths of 980 MPa and 1180 MPa have been commercialized for the cold forming of automotive structural parts. In the development of these ultrahigh strength steels with competitive formability, the studies of Sugimoto et al.^1,2,3,4) regarding TRIP-aided high strength steels and the Q&P (Quenching-and-Partitioning) process suggested by Speer et al.⁵⁾ have been utilized extensively.

The cold formability of automotive steels can be assessed by the tensile elongation and the HER (Hole Expansion Ratio) value. Tensile ductility can be enhanced by the TRIP effect of retained austenite introduced by the addition of Si and/or Al. Tensile elongation of TRIP steels is known to be dependent on the stability and amount of retained austenite.^3,6) The HER test representing stretch-flangeability measures the change in diameter of the punched hole, which is expanded by a conical punch until a through-thickness failure occurs. The stretch-flangeability of DP (Dual Phase)^7,8,9,10) and TRIP-assisted steels^11,12,13,14) have been reported to be determined by the volume fraction, morphology, and the hardness difference of constituting phases. In this work, the influence of microstructure on tensile properties and stretch-flangeability of TRIP steels with tensile strengths higher than 1.2 GPa has been investigated. The TRIP steels were produced by Q&P process which introduces tempered martensite and bainite as the main microstructural constituents. The volume fractions of tempered martensite, bainite, and fresh martensite were determined by dilatometric analysis. The volume fraction of retained austenite was measured by XRD (X-Ray Diffraction).

Phase quantification methods based on EBSD (Electron Back Scatter Diffraction) technique have been applied to the microstructural analysis of multiphase steels. Phase identification using Image Quality (IQ) has been successfully applied to DP steel consisting of ferrite and martensite phases.^15,16) From the distribution of IQ values, symmetric peaks representing each microstructural phase were separated using a Gaussian function. For the distinction between bainitic ferrite and ferrite in the TRIP steels, Kernel Average Misorientation (KAM) calculated from crystallographic orientation was used.¹⁷⁾ However, the separation between low-temperature transformed microstructures such as tempered martensite, bainite and fresh martensite has not been extensively investigated. In this work, a new method of phase identification based on EBSD analysis has been proposed to distinguish constituent phases transformed at low-temperature. In the method, both IQ and KAM values were used to identify fresh martensite phase and to measure the size of fresh martensite and retained austenite.

2. Experimental Procedure

The steel investigated was a 1.0 mm thick cold rolled steel containing 0.18C-1.5Si-2.6Mn-0.05Al (wt.%). The reduction ratio of cold rolling from hot rolled strip was about 47%. Dilatometry specimens of 2.0 mm × 10.0 mm × 1.0 mm size were taken from the cold rolled sheet parallel to the rolling direction. JIS5 tensile specimens were taken parallel to the transverse direction. The area dimensions of the HER specimens cut from the cold rolled sheet were 120 × 120 mm². A hole with a diameter of 10 mm was made in the center of the HER specimen by punching with a clearance of 12%. The HER test was performed according to ISO 16630 standard using a 60° conical punch moving at a rate of 8 mm/min. The punch was stopped when a through-thickness crack was formed. The hole expansion ratio was calculated using the following equation where D_f and D_o are the final and original hole diameters, respectively.   

$$\text{HER(\%)}=\frac{D_f-D_0}{D_0}\times 100$$

The schematic Q&P heat treatment schedules used in this study are shown in Fig. 1. The specimens were soaked at 850°C for 150 seconds for full austenitization. After cooling to 600°C at a rate of 5°C/s, the specimens were more rapidly cooled at a rate of 10°C/s to temperatures between 280°C and 340°C. In this work the rapid cooling stop temperature will be referred to as quenching temperature, Q_T. After reaching Q_T, the specimens were reheated at a rate of 25°C/s to 425°C and then held for 600 seconds, which is a partitioning step of Q&P heat treatment.

Fig. 1.

Schematic heat cycles used in this study. (a) full thermal history (b) enlarged from the rectangular portion of Fig. 1(a) to show Q&P conditions. (Online version in color.)

The volume fraction of retained austenite was determined by XRD analysis using Cu-K_α radiation on the basis of the integrated intensity of (200)_α, (211)_α, (200)_γ, (220)_γ and (311)_γ diffraction peaks.¹⁸⁾ Substituting the a_γ value (×10⁻¹⁰ m) measured from (220)_γ diffraction peak into the following equation,¹⁹⁾ carbon concentration of the retained austenite (C_γ, mass%) was calculated.   

$${\alpha }_{\gamma }=3.5467+0.0467C_{\gamma }$$

The microstructure of Q&P steel obtained by different heat treatment conditions was analyzed by electron backscatter diffraction technique. The heat-treated samples were cut into sections containing ND (Normal Direction) and RD (Rolling Direction) and then polished using standard metallographic methods. To obtain a mirror-like surface finish for EBSD work, electrolytic polishing at 50 V for about 20 s in 12% perchloric acid + 90% acetic acid solution was performed. Measurements were performed at a quarter thickness position with a scan area of 60(ND) × 60(RD) μm² and a step size of 0.07 μm. TSL-OIM software (ver. 7.3) was used for EBSD analysis. A clean-up procedure was conducted to remove the non-indexed points with low CI (Confidence Index) values.

3. Results and Discussion

3.1. Microstructural Evolution during Q&P Process

During cooling to Q_T temperatures, some amount of athermal martensite transformation occurs since all the Q_T temperatures are below the Ms (Martensite start) temperature. The Ms temperature of the steel used in this study was determined to be 368°C from dilatometry analysis by applying a 5% transformation criterion. Since the annealing effect will be provided during the partitioning step, the athermal martensite formed during rapid cooling can be referred to as tempered martensite (TM). During the partitioning heat treatment, part of the austenite is transformed into bainitic ferrite since the precipitation of bainitic cementite is restricted by Si addition. Some of the untransformed austenite, which is relatively unstable at the end of the partitioning step, is transformed to “fresh” martensite (FM) during final cooling. Some of the untransformed austenite is stable at room temperature, which is referred to as retained austenite (RA). The microstructures at each heat cycle condition were observed by SEM as shown in Fig. 2. Mainly lath-like microstructures are observed at lower Q_T temperatures of 280°C and 300°C (Figs. 2(a) and 2(b), respectively). It can be seen that the volume fractions of fresh martensite (indicated by arrows) increase as Q_T temperature increases (Fig. 2(a) through Fig. 2(e)). Figure 2 also shows the presence of precipitation in FM phase at Q_T temperatures above 320°C.

Fig. 2.

Typical microstructure of the steels obtained by different Q_T conditions in this study. (fresh martensite phases are marked by arrows). (a) 280°C Q_T (b) 300°C Q_T (c) 320°C Q_T (d) 330°C Q_T (e) 340°C Q_T.

The volume fractions of the microstructural constituents, excluding the retained austenite, were determined by dilatometry analysis applying a lever rule based on two straight lines representing the thermal expansion of austenite and ferrite, as shown in Fig. 3(a). An example of determining phase volume fractions by dilatometry analysis is shown in Fig. 3(b). The volume fraction of retained austenite was determined by XRD analysis. The volume fractions of TM (tempered martensite), B (bainite), FM (fresh martensite), and RA (retained austenite) phases with the changes in the Q_T temperatures from 280°C to 340°C are shown in Table 1 and Fig. 4. The untransformed austenite volume fraction after the quenching step is obtained by subtracting the volume fraction of TM from 100%. The untransformed austenite volume fraction after the partitioning step is obtained by summing the volume fraction of FM and RA. Carbon enrichment in RA was calculated from the lattice parameter estimated from (220)_γ XRD peak.

Fig. 3.

An example of dilatometry analysis. (a) Changes in dilatation along the heat cycle. (b) Evolution of microstructural phases during cooling. (Online version in color.)

Table 1. Volume fractions of the constituent phases in Q&P steels.

Q&P conditions		Phase volume fractions (%)				Untransformed γ fractions (%)		Cγ from (220) XRD peak (wt.%)
QT	Partitioning	TM	B	FM	RA	After quenching	After partitioning
280°C	425°C for 600 sec	76.4	6.9	7.4	9.3	23.6	16.7	1.32
300°C		70.6	13.2	10.0	6.2	29.4	16.2	1.15
320°C		65.5	18.5	12.7	3.4	34.5	16.1	1.11
330°C		58.3	25.7	14.0	2.0	41.7	16.0	1.02
340°C		36.1	38.0	24.8	1.1	63.9	25.9	1.00

Fig. 4.

Changes in the volume fractions of the constituent phases with Q_T. (Online version in color.)

With decreasing Q_T temperature, the volume fraction of TM (originally athermal martensite) increases due to the increased undercooling below the Ms temperature. In Fig. 5, the volume fractions of athermal martensite measured by dilatometry were expressed by a Koistinen-Marburger²⁰⁾ type equation shown below. The values of α and n were determined by a regression analysis to be 0.025 and 0.929.   

$$f_{\alpha }=1-exp\left[-\alpha {\left(M_s-Q_T\right)}^n\right]$$

Fig. 5.

Comparison of the measured and calculated volume fractions of athermal martensite with various Q_T temperatures.

The changes in the bainite volume fractions with partitioning heat treatment at different Q_T range from 280°C to 340°C are shown in Fig. 6. The bainite volume fraction was nearly saturated within 600 seconds of the partitioning holding time except for Q_T 340°C condition. In the Q_T temperature range from 280°C to 330°C, the volume fractions of untransformed austenite at the end of the partitioning step are similarly about 16% to 17% as shown in Table 1. On the other hand, the higher the Q_T, the more the relative proportion of untransformed austenite was transformed into FM. To investigate the microstructural changes of RA and FM with Q_T temperature, the following EBSD analysis was performed.

Fig. 6.

Isothermal bainite transformation kinetics at 425°C.

From the IQ (Image Quality) distribution, independent symmetric peaks were separated using a Gaussian function as in the exemplary analysis shown in Fig. 7(a). Since the IQ value reflects the lattice defect of the material, the peak with the lowest IQ values can be considered to have originated from FM. In this study, the intersection of the lowest IQ peak and the adjacent IQ peak was considered a threshold for identifying fresh martensite from softer phases such as tempered martensite or bainite. A phase map was drawn as shown in Fig. 7(b) by dividing the black area corresponding to the lowest IQ peak and the remaining blue area corresponding to softer phases. It can be seen that the black area is mainly composed of interphase boundaries of FM particles.

Fig. 7.

Results of EBSD analysis on the IQ (image quality) distribution. (a) Three independent symmetric peaks separated by a Gaussian function. (b) The phase map divided by the black area corresponding to the lowest IQ peak and the remaining blue area corresponding to the other IQ peaks.

The distribution of KAM (Kernel Average Misorientation) values of a cropped FM particle was analyzed as shown in Fig. 8(a). The 3rd nearest neighbors up to 3° misorientation were used for the calculation of KAM. The misorientation angle profile showed a normal type distribution at KAM values less than 2.8°. When KAM value is equal to or greater than 2.8°, an abrupt increase in the KAM number density was observed. In this work, a threshold KAM value of 2.5°, which best represents the FM region, was determined. However, as shown in Fig. 8(b), the FM region close to the interphase boundary is not well identified with the 2.5° KAM threshold alone. To overcome this discrepancy, FM in this study was determined by combining two different contributions of KAM and IQ thresholds. The region that satisfies either KAM or IQ thresholds was identified as FM, as shown in Figs. 9(a) and 9(b). The sizes of FM and RA measured by EBSD analysis are shown in Fig. 10. It can be seen that the size of FM increases with increasing Q_T. Considering that the size of FM measured by EBSD reflects the size of the parent austenite and the stability of austenite decreases as the size increases, the relative increase in the volume fraction of FM compared to RA with increasing Q_T can be explained. At 340°C Q_T condition, the size of FM was especially larger compared to other Q_T conditions. It is considered that the bainite transformation kinetics at 425°C is not fast enough to be saturated within 600 seconds, when the volume fraction of athermal martensite formed by quenching to 340°C is as low as 36.1%. However, the increase in the size of the untransformed austenite at the end of the partitioning step due to the increase in Q_T in the range of 280°C to 330°C needs to be explained. Since the sum of the volume fractions of tempered martensite and bainite is similar in this Q_T range, the grain refinement effect of TM can be considered to be stronger than that of bainite.

Fig. 8.

EBSD analysis using KAM (Kernel average misorientation) values for a cropped fresh martensite particle. (a) The KAM distribution within a cropped FM particle and (b) Result of identification of a fresh martensite particle using a 2.5° KAM threshold.

Fig. 9.

EBSD analysis results of a Q&P steel in this study. (a) IQ map and (b) Spatial distribution map of fresh martensite and retained austenite obtained by applying IQ and KAM thresholds.

Fig. 10.

Sizes of fresh martensite and retained austenite particles analyzed by EBSD analysis. (Online version in color.)

3.2. Mechanical Properties

The changes in the mechanical properties with Q_T temperatures between 280°C and 340°C are shown in Table 2. Yield phenomena were not observed in any tensile tests. A 0.2% offset strain criterion was used to determine yield strength. The stress-strain curves obtained by tensile tests are shown in Fig. 11. The decrease in yield strength with increasing Q_T from 280°C to 320°C can be explained by the reduction of the volume fraction of TM, which has higher yield strength than FM.^{21,22,23,24,25)} The introduction of a small amount of FM, which provides mobile dislocation in surrounding softer phases, will also decrease yield strength. However, as the volume fraction of FM further increases as Q_T increases from 320°C to 340°C, the yield strength increases due to the high hardness of FM. Since the hardness of FM is the highest among all the constituting phases, the increase in tensile strength with Q_T temperature can be attributed to the increase in the volume fraction of FM. As the Q_T increases, the elongation in the tensile test decreases due to a decrease in the RA volume fraction and an increase in the tensile strength of the steel.

Table 2. Mechanical properties of Q&P steels investigated in this study.

Q&P conditions		Mechanical properties
QT	Partitioning	YS	TS	EL	u-EL	HER
280°C	425°C for 600 sec	964	1298	13.5	10.1	37.5
300°C		719	1344	10.9	8.5	25.3
320°C		685	1400	9.8	7.4	20.8
330°C		733	1465	8.6	6.4	16.8
340°C		843	1532	7.9	5.6	12.9

Fig. 11.

Stress-strain curves of Q&P steels. (Online version in color.)

The HER values decrease as the Q_T temperature increases as shown in Fig. 12. When punching is performed to create a hole for the HER test, the RA within the punched area undergoes large deformation and can be transformed into martensite.¹³⁾ As a result of punching, it is considered that the micro-voids will be formed along the interface of the newly transformed martensite particles, as well as along the interface of the already existing FM particles. Since the particle sizes of FM are larger than those of RA as shown in Fig. 10, it is considered that the detrimental effect of FM on HER will be greater than that of RA. In addition, it should be pointed out that only part of the austenite will be transformed into martensite during punching depending on the degree of strain imposed and the stability of the retained austenite. It can be concluded that the decrease in the HER value with increasing Q_T temperature is due to the increase in the size and the volume fraction of FM particle. The lowest HER value was obtained at 340°C Q_T condition, where the average size of FM was the largest and the volume fraction of FM was the highest.

Fig. 12.

Changes in HER values before and after tempering at various quenching temperatures. (Online version in color.)

To investigate the effect of tempering on the properties of RA and FM, additional tempering at 200°C for 1 hour was performed on Q&P heat treated steels. The microstructure of tempered steels are shown in Fig. 13. The microstructural changes in TM and B phases were not noticeable, whereas the size and volume fraction of carbides in FM were seen to be increased by tempering. The mechanical properties of 200°C tempered Q&P steels with Q_T temperatures from 280°C to 340°C are shown in Table 3. The volume fractions of RA of the tempered steels measured by XRD are also presented in Table 3. Figure 14 represents stress-strain curves of Q&P steels after 200°C tempering. By comparing the mechanical properties of Q&P steels before and after tempering, it can be noticed that the yield strengths were increased and the tensile strength were decreased due to tempering. It has been reported that yield strength of low carbon martensitic steel increased by tempering.^{21,22,23,24,25)} Song et al.²³⁾ explained that both a reduction of glissile non-screw type dislocation density by static recovery and a dislocation pinning by fine carbides formed on dislocations resulted in an increase of elastic limit at the early stage of uniaxial tensile test. In this study, it is though that increase in yield strength after tempering could also be attributed to the reduction of dislocation mobility. The changes in dislocation configuration by recovery in TM and B Phases are thought to be a main cause of dislocation mobility reduction. The decrease in tensile strength can be attributed to softening of FM by 200°C tempering. And the loss of tensile strength by tempering is proportional to the initial amount of FM Phases in the microstructures before tempering. The total and uniform elongations were increased by tempering, and the increments were particularly greater at higher Q_T temperatures of 330°C and 340°C. From Tables 1 and 3, it can be seen that the volume fraction of RA at Q_T temperatures of 330°C and 340°C increased significantly by tempering. It is considered that BCC to FCC transformation has occurred in the carbon enriched region within the fresh martensite particles due to the uneven carbon distribution in the parent austenite formed during the partitioning step. Austenite reversion from martensite through the partitioning of alloying elements has been reported in a high Cr-containing steel²⁶⁾ and a medium Mn steel.²⁷⁾ In this study, it is noticeable that austenite reversion takes place in a low alloy steel through carbon partitioning at a relatively low tempering temperature of 200°C. Meanwhile, tempering has a favorable influence on HER in most cases (Fig. 12), which is possibly led by softening of FM and carbon enrichment into retained austenite.

Fig. 13.

Typical microstructure of Q&P steels after being tempered at 200°C for 1 hr. (fresh martensite phases are marked by arrows). (a) 280°C Q_T (b) 300°C Q_T (c) 320°C Q_T (d) 330°C Q_T (e) 340°C Q_T.

Table 3. Mechanical properties of Q&P steels after being tempered at 200°C for 1 hr.

Heat treatment conditions			RA (%)	Cγ (wt.%)	Mechanical properties
QT	Partitioning	Tempering			YS	TS	EL	u-EL	HER
280°C	425°C for 600 sec	200°C for 3600 sec	8.8	1.17	1005	1291	14.2	10.1	33.6
300°C			6.7	1.26	905	1296	13.5	9.7	28.2
320°C			4.7	1.17	783	1337	12.2	9.1	22.8
330°C			4.0	1.07	825	1382	11.8	8.0	20.6
340°C			4.2	1.04	917	1411	10.0	7.0	17.3

Fig. 14.

Stress-strain curves of Q&P steels after being tempered at 200°C for 1 hr. (Online version in color.)

The relationships between the HER value and the volume fraction of FM, RA, and the sum of FM and RA are shown in Figs. 15(a)–15(c) by using all the data obtained before and after tempering. The volume fraction of FM after tempering was obtained by subtracting the RA volume fraction measured by XRD from the untransformed austenite volume fraction at the end of the partitioning step. Figure 15(a) shows that lower HER values are obtained with higher volume fractions of FM, regardless of tempering. Figure 15(b) suggests that HER value increases with the volume fraction of RA, however, this trend is considered to be a result of similar volume fraction of untransformed austenite at the end of the partitioning step for Q_T temperatures from 280°C to 330°C, since the detrimental effect of FM is overwhelming. It can also be pointed out that the stability of RA, which is beneficial to stretch-flangeability, will be higher when the resultant FM volume fraction is low. Figure 15(c) shows two different groups of untransformed austenite volume fraction at the end of the partitioning step in this study, which is approximately 16 to 17% and 26%, respectively. Large scatters in the HER values at a similar untransformed austenite volume fraction at the end of the partitioning step of about 16 to 17% can be seen in Fig. 15(c), which represents the different relative contribution of fresh martensite and retained austenite on HER. Due to the decrease in FM volume fraction by tempering, the HER values of the Q&P steels with Q_T temperatures of 330°C and 340°C were increased after tempering. The tensile elongation of the steels tested in this study increased with the volume fraction of retained austenite as shown in Fig. 15(d). The relationship of tensile elongation and retained austenite was similar regardless of tempering.

Fig. 15.

Relationships of mechanical properties and the volume fractions of the constituent phases. The changes in HER according to the volume fractions of (a) FM, (b) RA, (c) FM plus RA and (d) the changes in tensile elongation according to the volume fraction of RA. (Online version in color.)

4. Conclusion

The influence of microstructure on tensile properties and stretch-flangeability of TRIP steels produced by Q&P process and additional tempering has been investigated, and conclusions are drawn as follows.

• As lowering the Q_T temperatures, volume fractions of tempered martensite and retained austenite were increased and volume fractions of bainite and fresh martensite were decreased.

• A new method of phase identification using EBSD data of both IQ and KAM values was proposed to distinguish phases transformed at low-temperature in Q&P steels. The region that satisfies either KAM or IQ thresholds was identified as fresh martensite. A threshold IQ value was determined as the intersection of the lowest IQ peak and the adjacent IQ peak. A threshold KAM value of 2.5°, which best represents the FM region, was determined.

• From EBSD analysis, it was found out that the size of fresh martensite particles increased as Q_T increased. The size of fresh martensite particles was larger than that of retained austenite particles in all Q_T conditions.

• In all steels tested, tensile elongation was increased with the volume fraction of retained austenite. Lower HER values were obtained with higher volume fractions of fresh martensite, regardless of tempering.

• The more detrimental effect of fresh martensite on HER compared to retained austenite is considered due to the larger size of fresh martensite as measured by EBSD analysis.

• After tempering at 200°C for 1 hour, the tensile elongation and HER values were improved in most cases, and the increase was higher when the initial volume fraction of fresh martensite before tempering was higher.

Acknowledgements

The authors gratefully acknowledge the support of the POSCO Technical Research Laboratories.

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