Effect of prestrain and baking on the secondary deformation properties of cold-r

,

1)State Key Laboratory of Metal Matrix Composites,School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai 200240,China; 2)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China; 3)State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel), Shanghai 201999,China

Abstract: Two experimental steels with tensile strength above 980 MPa were prepared to investigate the effect of prestrain and baking on their mechanical and fracture behaviors.The experimental results reveal that,for both experimental steels,with increases in the prestrain level,the bake hardening value increases before reaching a maximum point,and then decreases with further increases in the prestrain level.The results of a “bending-baking-secondary bending” test indicate that the secondary bendability deteriorates at a high level of prestrain.The yield strength of the experimental steels was found to increase and the elongation to decrease after high levels of prestrain and bake hardening.Fracture morphology images indicate that a high prestrain level is associated with shallow dimples and more and larger local cleavage areas.

Key words: ultra-high-strength steel; bake hardening; prestrain

1 Introduction

The bake hardening value (BH value) is an important performance indicator in bake-hardened steel.For example,for steel plates with yield strength above 200 MPa,the yield strength can be increased by 30-50 MPa by bake hardening during the baking process,which is very effective in improving the dent resistance of thin steel plates.

Cold-rolled ultra-high-strength steels (such as dual-phase steels) have bake-hardened properties.Due to the high strength of ultra-high-strength steels,their BH values have been given less attention.Most standards for dual-phase steels have no requirements regarding BH values.The standards that do have requirements for the BH value of the dual-phase steel generally require a BH2value no less than 30 MPa.The research results obtained regarding the hardening properties of dual-phase steels are also relatively consistent[1- 4],and can be summarized as follows:① With increases in the prestrain level,the BH value of the steel increases consistently until it reaches an inflection point,after which the BH value decreases with increases in the prestrain level.② As the baking temperature increases and the baking time is extended,the BH value of the cold-rolled dual-phase steel increases.③ The higher the strength of the dual-phase steel,the higher its BH value.

Generally,the BH value of the dual-phase steel with strength above 780 MPa easily meets the require-ment that the BH2value be no less than 30 MPa.Therefore,this has no significance in the study of the BH values of dual-phase steels.Research on the bake hardening of dual-phase steels focuses on the effects of the prestrain and baking parameters on the BH value.

Although the BH value is not important for the ultra-high-strength steel,work hardening and baking are necessary aspects of steel-plate processing.The changes that occur in the mechanical properties of the ultra-high-strength steel after its formation and baking merit further research.In this study,two ultra-high-strength steels with strength above 980 and 1 400 MPa were taken as research objects,and the effects of prestrain and baking on the mechanical behavior and fracture modes of these steels with dif-ferent martensite contents were studied.The results will provide significant guidance for the application of ultra-high-strength steels.

2 Experiments and methods

The test materials were two kinds of industrially produced ultra-high-strength cold-rolled steel plates,the mechanical properties of which are shown in Table 1.

Table 1 Mechanical properties of the experimental steels

The composition of the two test steels is based on ferrite and martensite.Test steel 1#has relatively low yield and tensile strength and contains more ferrite,whereas test steel 2#has relatively high yield and tensile strength and contains less ferrite.Fig.1 shows the microstructures of the two experimental steels.

In the pre-tensioning and tensile tests,JIS 5#plate-shaped tensile test specimens were used with a gauge length of 50 mm and the length direction of the test specimens perpendicular to the rolling direction.An Instron tensile tester was used to perform pre-tensioning and to test the mechanical properties before and after baking.Baking was performed in an electric heating oven at 170 ℃ for 20 min.

In addition to the tensile test,the baked samples were also subjected to a bending test.In this test,the test steels were first bent at 90° on a steel-plate bending tester,and then baked at 170 ℃ for 20 min or left unbaked,and then the steel plates were further bent 30° to realize a final bending angle of 60°.The variable parameter during this bending process is the inside corner radiusr.The size of the bending specimens was 100 mm×50 mm,and the bending ridge line was parallel to the rolling direction.The microstructures of the specimens were observed with a ZEISS EVOMA25 scanning electron microscope after treatment with 4% nitric acid and alcohol.

3 Results and discussion

Fig.2 shows the BH value curves of the two test steels under different prestrain levels.It can be seen that as the prestrain level increases,the BH values of the two experimental steels also increase.When the highest BH value is reached,further increases in the prestrain level cause the BH value of the two experimental steels to decrease.In the absence of prestrain,the BH values of the two experimental steels are basically 0,which indicates that prestrain is a prerequisite for promoting bake hardening.Due to the different strength levels of the two test steels,the positions of the highest points of the BH values are different.For test steel 1#with lower strength,the maximum BH value occurs when the prestrain level is 2.0%,whereas the maximum BH value of test steel 2#with higher strength occurs when the prestrain level is 1.0%.The maximum BH value of test steel 2#with higher strength is much higher than that of test steel 1#with lower strength.

Fig.3 shows the changes in elongation of the two test steels after baking at different levels of prestrain.It can be seen that the elongation of the two steel sheets did not decrease after baking without prestrain.However,with increases in the prestrain level,the elongation of the two test steels after baking decreased rapidly,whereby the greater the prestrain,the greater the decrease in elongation after baking.After 3.0% prestrain and baking of test steel 1#and 2.0% prestrain and baking of test steel 2#,the elongation of the two experimental steels was very small.

Fig.3Effectofprestrainonelongationofthetwoteststeelsafterbaking

Fig.4 shows the change trends in the yield and tensile strength of the two test steels after baking under different levels of prestrain.It can be seen that,corresponding to the lesser elongation in Fig.3,the yield strength of test steel 2#is very close to its tensile strength under a large amount of prestrain,the yield strength of the steel plate after baking increases rapidly,and the yield ratio of the steel plate is high.In contrast,under a small amount of prestrain,the yield ratio of the steel plate after baking is low,and the elongation of the two test steels decreases particularly slowly.A comparison of test steels 1#and 2#shows that test steel 1#with low strength gradually increases in strength with increases in the prestrain level,and when the prestrain is above 2.0%,the yield ratio of test steel 1#increases significantly.The yield strength of test steel 2#with high strength increases in strength more rapidly with increases in the prestrain level,and at 1.0% prestrain,its yield strength becomes very close to its tensile strength.

The main method used to form martensitic steel sheets is bending,with the maximum bending angle generally being 90°.Understanding the bending limit can facilitate the correct application of these materials.Fig.5 shows the quality of the bending ridge surface of the samples after 90° bending tests.

Given the concern about the low deformation ability of steel plates after extensive prestrain and baking,the steel plates were subjected to a bending-baking-secondary bending test.Table 2 shows the results of the curved ridge surface of the secondary-curved samples.

Fig.4Effectsofprestrainonyieldandtensilestrengthofthetwosteelsafterbaking

Table 2 The results of the curved ridge surface of the secondary-curved samples

After being bent to 90° for an inner-bend radiusr=2 mm and then further bending to 60° for an inner-bend radiusr=1 mm,test steel 1#exhibited no cracking (Fig.5(a)).However,the final bending streak of the baked sample was more obvious (Fig.5(b)),which indicates that baking has an adverse effect on its secondary bending performance.After being bent to 90° for an inner-bend radiusr=2 mm and then further bending to 60° for an inner-bend radiusr=2 mm,test steel 2#with high strength showed no cracking in the unbaked group,but the baked samples exhibited severe cracks (Figs.5(c) and (d)).This indicates that at higher prestrain levels,baking has a severe adverse effect on the secondary bending properties of higher strength steels.

The results obtained after prestrain and baking on the mechanical and fracture behaviors of the test steels reveal that after high levels of prestrain and baking,the yield ratio of the ultra-high-strength steel increases greatly,the elongation decreases significantly,and the ability to further deform is also reduced,especially in the test steel plate with high strength.To further investigate the cause of this phenomenon,the stress-strain curves of the two test steels after baking at different prestrain levels are plotted (Fig.6).It can be seen from Fig.6 that for both test steels,when the prestrain level reaches a certain point,the shape of the stress-strain curve changes.The outstanding feature of the stress-strain curves of the two test steel plates is the elimination of the work hardening effect,whereby the highest stress point of the steel plate occurs at the end of the elastic section and the subsequent stress gradually decreases with increases in the strain level.With respect to test steel 1#with low strength,the inflection point of the stress-strain curve occurs at a prestrain level less than 2%.For test steel 2#with high strength,the inflection point of the stress-strain curve occurs at a prestrain level of 1%.

When the prestrain levels of the two test steels are high,no work hardening is evident during the stretching process after baking,and the probability of the sample breaking outside the gauge range is very high.Figs.7 and 8 show the stress-strain cur-ves of the test steels after baking when they break within and outside the gauge range.These curves indicate that the deformability of the steel sheet is very low,and the location of the fracture occurs unstably.

Fig.6Theoriginalstress-straincurvesandthestress-straincurvesoftheteststeelsheetsafterdifferentprestrainlevelsandbaking

Fig.7Stress-straincurvesofteststeel1#after3.0%prestrainandbaking

Fig.9 shows images of the fracture morphology of test steel 1#under different prestrain conditions,in which it can be seen that when the prestrain level of the test steel was 0.5%,the fracture of the ten-sile sample was basically dimple-like with local cleavage micro-regions.The dimples near the sur-face of the steel plate were smaller and shallower,whereas those in the middle of the steel plate were deeper,larger,and tear-shaped.At 3.0% prestrain,the fracture of the sample was still dominated by dimples,but the dimples had become shallower and more cleavage micro-regions had appeared.

Fig.8Stress-straincurvesofteststeel2#after2.0%prestrainandbaking

Fig.10 shows the fracture morphology of test steel 2#under different prestrain levels.It can be seen that when the prestrain level was 0.5%,the fracture of the tensile sample was basically dimple-like with slightly more cleavage micro-regions in the edge area of the fracture.At 2.0% prestrain,the dimples near the surface were extremely shallow,which indicates that the plasticity of the test steel had become very low.In the central area of the fracture,it can be seen that the cleavage micro-regions were larger and wider.

When ultra-high-strength steel sheets are used in the manufacture of automotive parts,forming and baking procedures are required.During the process of vehicle service,components with passive safety-protec-tion functions must be able to withstand further deformation.The prestrain-baking-re-deformation test results of the two test steels reveal that at high levels of prestrain,the yield strength of the steel plates after baking increases greatly,but the plasticity and work hardening performance of the steel plates decrease.Therefore,slight deformation of the steel plate during the forming stage is beneficial for maintaining good deformation ability after baking.This phenomenon is closely related to the strength of the steel plate.

4 Conclusions

(1) The BH values of two types of test steels with tensile strength above 980 MPa were found to increase with increases in the prestrain level.When the maxi-mum BH value is reached,further increases in the pre-strain level will result in decreases in the BH value.Without prestrain,the BH value is essentially zero.

(2) The results of the bending-baking-secondary bending test reveal that under a high prestrain level,baking has an adverse effect on the secondary bendability,and that the higher the strength of the test steel,the lower its secondary bending performance.

(3) When the ultra-high-strength steel is sub-jected to a high level of prestrain and baking,its yield strength ratio increases greatly,and the elongation rate decreases significantly.When the prestrain level reaches a certain point,the shape of the steel’s stress-strain curve changes,and the work hardening effect disappears,with the highest level of steel-plate stress occurring at the end of the elastic section,and subsequent stress decreases with increases in the strain level.

(4) For ultra-high-strength steels,under different prestrain circumstances,the tensile fractures that occur after baking are very diverse.Higher levels of prestrain correspond to shallower fracture dimples and more local cleavage micro-regions.