Experimental study on Ti+Nb bearing ultra-low carbon bake hardening sheet steel hot-rolled in the ferrite region

A Ti＋Nb bearing ultra-low carbon bake hardening sheet steel hot-rolled in the conventional austenite region and in the ferrite region with lubrication was experimentally studied. Subsequent cold rolling and continuous annealing processes were also conducted. The results show that microstructures of ultra-low carbon bake hardening hot strips at room temperature are basically irregular polygonal ferrites. The yield strength, ultimate tensile strength, n value, and r value of the No.2 specimen hot-rolled in the ferrite region with lubrication are 243 MPa, 364 MPa, 0.29, and 1.74, respectively, which are similar to those of the No.1 specimen hot-roiled in the conventional austenite region. The elongation rate and bake hardening value of No.2 specimen are 51% and 49.4 MPa, respectively, which are greater than those of No. 1 specimen. The No.2 specimen hot-rolled in the ferrite region with lubrication exhibits good mechanical properties and relatively excellent baking hardening performance. Therefore, the hot rolling experiment of Ti＋Nb bearing ultra-low carbon bake hardening steel in the ferrite region with lubrication is feasible and can be considered in the future industrial trial production.

Bake hardening behavior of TRIP and DP steels

The bake hardening （BH） behavior of transformation-induced plasticity （TRIP） and dual-phase （DP） steels after different prestrains was studied. The experimental results indicate that TRIP steel exhibits good BH ability while DP steel does not, and prestrain displays a strong effect on the BH values of both steels. The comparison of microstructures of the two steels showed that the hard second phase in the matrix might be harmful to the BH ability. For deformed specimens, baking resulted in a loss of uniform elongation, but there was no obvious decrease in uniform elongation for unprestrained specimens.

Effect of Overaging on Solute Distributions and Bake Hardening Phenomenon in Bake Hardening Steels

Specimens of two different kinds of bake hardening steels （BH-Mn and BH-P） were prepared and treated with different annealing processes （water quenching and overaging）. A novel technique of three dimensional atom probe was used to investigate solute distributions in these steels. The results indicate that C concentration decreases, whereas V increases during overaging in both bake hardening steels. The conclusion that no vanadium carbides pre- cipitate during the overaging is therefore originally obtained by microanalysis in bake hardening steels. Moreover, bake hardening values of all the specimens were tested by tensile experiments with 2 0/~ pre-deformation. However, those of overaged specimens were further measured with higher levels of pre-deformation because no bake hardening phenomenon was present at 2% pre-deformation. As the pre-deformation increases from 2% to 6% and 8%, both overaged steels show bake hardening values, and the value data are almost the same.

Effects of Pre-strain and Temperature on Bake Hardening of TWIP900CR Steel

The effects of pre-strain and baking temperature on bake hardening behaviour of TWIP900CR steel were investi- gated. The results reveal that the bake hardening process contributes to an increase in yield strength up to 65 MPa at the baking temperature of 200 ℃. The difference in yield strength between baking temperatures of 170 and 200 ℃ is al- most insignificant. It is clearly observed that baking at a high temperature does not result in a significant increase in yield strength. For a reasonable bake hardening, a good combination of pre strain and baking temperature is necessa- ry. Besides, the toughness of the material is found to decrease with increasing pre-strain.

Effect of Continuous Annealing on Microstructure and Bake Hardening Behavior of Low Carbon Steel

The influences of the heating rate, annealing temperature and soaking time during continuous annealing on the microstructure evolution and bake hardening behavior of the low carbon steel were investigated. After 2% pres- training, bake treatment （180 ℃ × 20 min） was carried out to measure BH （bake hardening） values. The results showed that, with the increase of the annealing temperature, the grain size increased except for annealing in the in- tercritical region. Furthermore, the BH value increased and the mean grain size reduced with increasing the heating rate from 5 to 80 ℃/s. The BH value was very low before complete recrystallization. After a significant decrease from 630 to 720 ℃ , the BH value enhanced when the steel was annealed at 750 ℃. With the increase of the soaking time from 20 to 100 s, the BH value reduced linearly due to the segregation of solute C atoms.

Influence of Pre-strain on the Mechanical Properties of A6111-T4P Sheet with Bake Hardening

A6111 is an aluminum alloy, which exhibits good formability and excellent bake hardening property. This study aimed to reveal the influence of strain path, pre-strain orientation （c0 as well as pre-strain level on the mechanical properties of A6111-T4P sheet under bake treatment through uniaxial tension test. （0-5）% pre-strain, 150-170 ℃ bake temperature and 20-30 min bake time were considered in the study by referring to the actual production process. The results show that both pre-strain level and strain path play significant roles in improving the material properties. In the condition that tensile orientation （β） parallel with pre-strain orientation （β= a）, the yield strength can be remarkably improved, and much higher parameter of n in Hockett-Sherby model can be obtained than those when tensile orientation non-parallel with pre-strain orientation （a≠ β）. In addition, when the pre-strain level, paint bake process were settled and β = a, the curves obtained in five tension orientations are similar in the plastic deformation stage.

Effects of Combined Pre-straining and Pre-aging on Natural Aging and Bakehardening Response of an Al-Mg-Si Alloy

For a series of Al-0.34／%Mg-1.05／%Si-0.08／%Cu alloys, the effects of pre-treatment on natural aging, bake hardening response and plasticity were investigated using Vickers hardness test and tensile test. Differential scanning calorimetry （DSC） analysis was conducted to reveal corresponding precipitation mechanisms. The results showed that pre-straining and pre-aging alone couldn＇t completely suppress natural aging and improve bake hardening response simultaneously. The sensitivity of the pre-straining and pre-aging on the mechanical properties was evaluated. It was found that pre-aging was the main factor which greatly improved bake hardening response and pre-treatment had a weak influence on plasticity in pre-treated samples. DSC analysis showed that the dissolution trough of clusters disappeared in pre-treated samples, both β”and β＇ precipitation peak shifted to lower temperature and were reduced in pre-strained and pre-aged samples. It was concluded that the formation of Mg/Si clusters was replaced by the accelerated precipitation ofβ” andβ＇ phases, which caused the suppression of natural aging and the improvement of bake hardening response （BHR）.

Pre-treatment to Improve the Bake-hardening Response in the Naturally Aged Al-Mg-Si Alloy

This paper describes pre-treatment methods to improve the bake-hardening response of naturally aged AA6022 （Al-0.6Mg-1.0Si）, which is an alloy used in automotive body panels. These methods are used to accelerate the artificial aging process, which proceeds 30-day period of natural aging. The precipitation behavior of the sheets of this aluminum alloy was analyzed by differential scanning calorimetry （DSC） and electrical conductivity measurements, and subsequently confirmed by microstructure observations by transmission electron microscopy （TEM）. Tensile tests and microhardness measurements were performed to determine the mechanical properties of the samples. Under simulated paint-baking conditions （30 min at 170℃）, the microhardness and the yield strength （YS） of the pre-strained ＋ pre-aged samples were found to be I13 HV and 225 MPa, respectively. These values are considerably higher than those of pre-aged samples, and they did not decrease significantly during the initial stage of artificial aging.

Bake-Hardening Response of High Martensite Dual-Phase Steel with Different Morphologies and Volume Fractions

Bake-hardening behaviour of carbon steel with different martensite morphologies and volume fraction was investigated. The specimens with fibrous and bulky martensite were prestrained in tension by 4%. After this, they were unloaded and bake hardened at 180 ℃ for 10-160 min. It was found that dual-phase steel samples which were bake hardened at 180 ℃ for 20 rain showed an increase in the yield stress （YS） and ultimate tensile stress （UTS） but a decrease in ductility. Further increase in the bake-hardening time of 80 or 160 min has reduced the YS and UTS, but increased the ductility. △σ （increase in stress due to bake hardening）, YS and UTS values are higher for the microstructure containing fibrous martensite compared to the microstructure-containing bulky martensite. It was also observed that at a given baking temperature Aa, YS and UTS increased by volume of martensite.

Effect of prestrain and baking on the secondary deformation properties of cold-rolled ultra-high-strength steel sheets

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.

Evolution of appearance profile of hot-dip galvanized car-body steel sheet during deformation process

In recent years, the waterborne free intermediate coating process has been widely used in the automotive industry. Because the baking times and coating thickness are decreased, the surface covering capability of the painting process is reduced, which directly affects the appearance quality（ long-and short-wave values） of the body paint. Thus, there are correspondingly higher requirements for the white body surface profile prior to painting. The surface profile of the white body is mainly affected by the plate material, the surface profile, and the deformation process. So,the change rule for the surface profile during deformation of the steel plate is a key factor in coating appearance optimization. In this paper, we first analyze the typical deformation of the outer cover of a car body. Then ,we examine the change tendency of the surface profile of steel plates with respect to different deformation rates, specifically for a steel plate comprising a hot-dip galvanized bake-hardened steel sheet. Based on our analysis of the influence of the deformation on the coating appearance,we selected 3% ,5% ,and 8% deformation rates in this research. We found the roughness （Ra） value in the typical deformation range （3% -8% ） of the car body to exhibit a decreasing trend at first and then an increasing trend. The Ra value of the 8% deformation is not more than the original plate test value. When the Pc value of the original plate is in the lower range （ about 60）, it exhibits a slight increasing trend in the deformation process （3 % -8 % ）. And when the Pc value of the original plate is in the higher range （ about 120 ）, it exhibits no increasing trend in the deformation process （ 3% -8% ）. In contrast,the waviness （WCA） value in the car body＇s typical deformation range （3%-8%） shows a significant growth trend.

Effect of intercritical annealing temperature and cooling rate on the microstructure and mechanical properties of low-carbon aluminum killed steel

In this study, the intercritical annealing process for a typical low-carbon aluminum killed steel is investigated. A cold-rolled sheet was annealed at intercfitical temperatures ranging from 730 ℃ to 770 ℃ and then cooled in air or water. The annealed steel was then baked at 210 ℃, and its mechanical properties and microstructures were analyzed in detail. It is shown that after the air-cooling process,the strength of steel decreased and ductility increased with an increase in the annealing temperature. However, after water-cooling, the strength and ductility both increased with the increase of annealing temperature. These results are attributed to the property- optimization of the steel.

Study of Bake Hardenability and Strengthening Mechanism of Hot Rolled Low Carbon Steel

The controlled rolling and controlled cooling,bake hardening experiments have been carried out for the test steel,mechanical property test of the sample and microstructure analysis have been made by tensile testing machine,optical microscope,TEM and X-ray diffractometer.The results show that fine and uniform microstructures can achieve much higher BH and BHT values;With the increase of pre-strain there is a trend of increase first and then decrease in the BH and BHT values;bake hardening mechanism mainly consists of Cottrell atmosphere strengthening,second phase strengthening,dislocation strengthening.

Microstructure and Properties of Ti and Ti+Nb Ultra-Low-Carbon Bake Hardened Steels

Hot rolling, cold rolling and continuous annealing processes of Ti bearing and Ti＋ Nb stabilized ultra-low-carbon bake hardened steels were experimentally studied. The microstructure and texture evolution, as well as the morphology, size and distribution of second phase precipitates during hot rolling, cold rolling and continuous annealing were also analyzed. The results showed that the size of NbC precipitates in Ti＋Nb stabilized ultra-low-carbon bake hardened steel was smaller than that of TiC precipitates in Ti bearing ultra-low-carbon bake hardened steel, which made the average grain size of Ti＋ Nb stabilized ultra-low-carbon bake hardened steel finer than that of Ti bearing ultra-low-carbon bake hardened steel; for the yield strength, the former was higher than the latter; but for the r value which reflects the deep-drawing performance, the former was lower than the latter.

Strain Limit of Extra Galvannealed Interstitial-Free and Bake Hardened Steel Sheets Under Different Stress Conditions

The formability of bake hardened steel （thickness 0.82 mm）, and the extra galvannealed IF steel （thickness 0.82 mm） have been studied. The suitability of the above steels for forming applications has been critically examined. The microstructure, tensile properties, and formability parameters of the above sheet metals were determined. The manufacturing process of the steels and the significance with reference to its formability were studied.

Resistance Spot Weldability of Dissimilar Materials: BH180-AISI304L Steels and BH180-IF7123 Steels

In this study, resistance spot weldability of 180 grade bake hardening steel （BH180）, 7123 grade interstitial free steel （IF7123） and 304 grade austenitic stainless steel （AISI304L） with each other was investigated. In the joining process, electrode pressure and weld current were kept constant and six different weld time were chosen. Microstructure, microhardness, tensile-shear properties and fracture types of resistance spot welded joints were examined. In order to characterize the metallurgical structure of the welded joint, the microstructural profile was developed, and the relationship between mechanical properties and microstructure was determined. The change of weld time, nugget diameter, the HAZ （heat affected zone） width and the electrode immersion depth were also investigated. Welded joints were examined by SEM （scanning electron microscopy） images of fracture surface. As a result of the experiment, it was determined that with increasing weld time, tensile shear load bearing capacity （TLBC） increased with weld time up to 25 cycle and two types of tearing occurred. It was also determined that while the failure occurred from IF side at the BHIS0＋IF7123 joint, it occurred from the BH180 side at the BHIS0＋AISI304L joint.

Effects of Rolling Parameters on Texture and Formability of High Strength Ultra-Low Carbon BH Steel

The effects of hot rolling and cold rolling parameters on texture and r （ plastic strain ratio ） value of high strength ultra low carbon bake hardening （ ULC-BH ） steels are studied with orientation distribution function （ ODF ） structural analysis method.After hot rolling , the high strength ULC-BH steel sheet has weakγ-fiber with uniform orientation distributions , and weakα-fiber , of which { 445 } component forms a high intensity peak at coiling temperature of 750℃.After cold rolling , both { 111 } -{ 111 } intensity on theγ-fiber and{ 111 -}{ 112 } intensity on theα-fiber enhanced.As a result of substitutional solute elements Mn and P being added to the steel , strong { 112 } deformation texture component is observed onα-fiber , especially at 80%cold rolling reduction , and this leads to the strong { 111 } recrystallization texture after annealing.The increase of cold rolling reduction shifts the maximum intensity on theγ-fiber from { 111 } to { 111 } .After annea-ling , a very strongγ-fiber is obtained , with intensity peak at { 111 } component when cold rolling reduction reaches 80%.Increasing coiling temperature and cold rolling reduction improveγ-fiber intensity and r value , resulting in good deep drawability.

Development of 600MPa High Aluminum Cold Rolled Dual Phase Steel and Its Application

An original 600MPa high aluminum dual phase steel had been successfully developed in this paper.The microstructure analysis showed that the microstructures of the steel mainly consist of ferrite and 16% martensite which distributed around the ferrite grain boundaries.TEM analysis demonstrated that there exist lots of high density dislocations in ferrite as well as an obvious twin-structure and some ribbon structures in martensite.The tensile test and the bake-hardening test showed that the steel delivers an excellent mechanical properties of R p0.2 =350-390MPa,R m =620-660MPa,R p0.2 /R m =0.55-0.65,A 80mm =25-29%,n=0.18-0.21 and an excellent bake-hardening property of BH 2 =50-60MPa as well as an excellent of work-hardening (WH) property of WH=120MPa。The spot-welding test showed that this dual phase steel has a good welding adaptability.Own to the excellent formability,the steel has been successfully used in the production of some auto parts.