Ferritic rolling to produce soft deep‐drawable thin hot strips

In a conventional production of deep‐drawable sheets of steel, a hot rolling in austenite and a cold rolling at room temperature together with a subsequent recrystallization annealing are applied with a cold strip as a final product. As a cost saving replacement for this, a thin‐gauge hot strip with a required deep‐drawability can be employed. A conventionally (in austenite) hot rolled strip is hardly applicable for this because of its unsatisfactory texture as well as because of the technical difficulties to produce hot strips with sufficiently small thicknesses. As a promising realization of cost saving thin‐gauge deep‐drawable hot strips of steel, a warm rolling can be applied with reduced finish rolling temperatures. In this practice the finish rolling is shifted down into the temperature region of ferrite. In the present work extensive laboratory tests on IF‐ and ELC‐steel were carried out by using the hot deformation simulator Wumsi. By the measurements of the texture development as well as by the computing of r‐values, the process parameters of ferritic rolling and a subsequent direct recrystallization in the coil of these steels could be optimized.     

摩擦对IF钢铁素体区热轧和退火织构的影响

采用X射线衍射仪分析IF钢铁素体区热轧织构以及退火织构的演化,在实验室热轧机上进行了IF钢的铁素体区热轧,研究了摩擦对IF钢铁素体区热轧、退火织构的影响。结果表明：无润滑轧制时,钢板表层形成强高斯织构组分{110}〈001〉,弱γ纤维织构,导致再结晶织构中高斯组分强度高,γ纤维织构强度低;润滑轧制时,钢板表层高斯织构组分强度降低,{100}〈011〉、γ纤维织构强度提高,退火后γ纤维织构强度提高。钢板中心受摩擦作用影响较小,轧制过程中发展为较强的α和γ纤维织构,退火后γ纤维织构成为主要织构组分。     

Effect of rolling temperature on the deformation and recrystallization textures of warm-rolled steels

Warm-rolling trials were carried out on three interstitial-free (IF) steels (stabilized with either niobium or titanium), an extralow-carbon (ELC) steel, and an experimental low-carbon chromium (LC Cr) material at temperatures between 440 °C and 850 °C. The influence of rolling temperature on their as-rolled microstructures and deformation and recrystallization textures was investigated. Also, the effect of coiling simulation and degree of rolling reduction on the r values of some of these materials was examined. In-grain shear bands were evident in all as-rolled microstructures, but their sensitivity to deformation temperature varied between steels. Shear bands of moderate intensity were formed in the IF steels across all temperatures. In the ELC material, intense shear bands were formed at low rolling temperatures, but at higher temperatures, this intensity was drastically reduced. The development of shear bands at the higher rolling temperatures was significantly enhanced by alloying with chromium. The deformation textures produced were typical of rolled ferrite materials. The intensity of this texture increased markedly with temperature for the ELC grade. Conversely, the intensity of the recrystallization texture decreased with increasing temperature. The addition of chromium was found to strengthen the {111} component and, hence, the formability. The sharpness of both the deformation and recrystallization textures of the IF steels was relatively unaffected by rolling temperature. These differences are attributed to the intensity and frequency of shear-band formation and the dynamic strain-aging (DSA) behaviors of the various materials.     

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

冷轧变形量对Ti+P-IF钢微观结构的影响

采用金相组织观察、TEN分析和织构测试等手段,对Ti＋P—IF钢在不同冷轧变形量下再结晶组织和织构的变化规律进行了研究。结果表明,随着冷轧变形量的增加,Ti＋P—IF钢再结晶组织中晶粒尺寸逐渐减小,第二项粒子呈方向性趋势排列,织构组分由高斯织构和旋转立方织构向γ织构转变。     

铁素体区热轧高温卷取条件下IF钢的织构特征

 研究了铁素体区热轧高温卷取条件下，2种不同成分的IF钢的织构特征。结果表明，高温卷取后，普通Ti IF钢发生了完全再结晶，中心面和1/4面上形成了很强的<111>∥ND再结晶织构；高强Ti IF钢大部分晶粒仍处于轧制状态，只有极少部分晶粒发生了再结晶，中心面和1/4面上形成的织构组分以<110>∥RD为主，<111>∥ND再结晶织构较弱。织构分析表明，2种钢的表面织构均较弱，普通Ti IF钢表面上<001>∥ND组分占优，高强Ti IF钢表面上<110>∥ND组分强度最高。     

Texture and microstructure development during intercritical rolling of low-carbon steels

Laboratory rolling trials have been performed to investigate the development of microstructure and crystallographic texture during and after intercritical rolling. The finishing temperature was varied over a wide range, and samples were taken just prior to the last pass, after quenching following the last pass, after air cooling and coiling, and after accelerated cooling and coiling. Cooling of the samples to the entry temperature for the last pass does not influence the texture of the sample, nor do higher cooling rates after austenitic finishing within the range of cooling rates in this study, although it may cause a refinement of the ferrite grains. Recrystallization after intercritical rolling leads to a decrease in texture intensity. In the case of recrystallization of low-carbon steels, the nucleation mechanism is strain-induced boundary migration (SIBM), which leads to unfavorable textures for deep drawing. In the case of recrystallization of interstitial-free (IF) steel after ferritic rolling, the nucleation mechanism shifts from the SIBM mechanism at high finishing temperatures to subgrain coalescence at (SGC) low finishing temperatures. The latter mechanism leads to more favorable textures for deep-drawing applications. Transformation-induced (TI) nucleation explains the occurrence of a sudden increase in ferrite grain size after high-temperature intercritical deformation of low-carbon (LC) steels.     

Ferritic rolling with additional annealing to produce a deep‐drawable ultra‐thin‐gauge hot strip

In order to produce ultra‐thin hot strips by ferritic rolling, that exhibit a good deep‐drawability, finishing temperatures must be reduced, so that an additional recrystallization annealing of hot strips after coiling becomes necessary. In the present work laboratory investigations were carried out on a commercial IF and ELC steel by using the hot deformation simulator Wumsi. By means of texture measurement and the calculation of r‐values the process parameters of the ferritic rolling and of a subsequent annealing were optimized. Additionally to a conventional annealing of cold coils a new energy saving processing route with direct charging of warm coils was tested by the laboratory simulation. Particularly good deep‐drawabilty were achieved by this technology.     

Effect of Deformation on Continuous Cooling Phase Transformation Behaviors of 780 MPa Nb‐Ti Ultra‐High Strength Steel

For the purpose of achieving the reasonable rolling technology of 780 MPa hot‐rolled Nb‐Ti combined ultra‐high strength steel, the effect of deformation and microalloy elements Nb and Ti on phase transformation behaviors was investigated by thermal simulation experiment. The results indicated: the deformation promoted ferritic transformation; due to the carbon content of the experimental steel was lower (<0.12% wt), the deformation indirectly impacted perlitic transformation through promoting ferritic transformation; the effect of the deformation on bainitic transformation was subject to condition whether proeutectoid ferrite precipitated before bainitic transformation. At low cooling rate of 0.5 °C/s, Nb and Ti promote transformation process γ → α, but that not good for refining the ferrite grain; at high cooling rate of 25 °C/s, Nb and Ti to a certain extent promote bainitic transformation. The recrystallization stop temperature of experimental steel was greater than 1000 °C, and phase transformation point A_r3 was 764 °C. In order to obtain the fully bainite microstructure in the practical rolling process, the cooling rate should be controlled above 15 °C/s, the start finish rolling temperature between 950–980 °C, the finishing temperature between 830–850 °C, the coiling temperature between 450–550 °C.     

卷取温度对Ti-IF钢第二相粒子及晶粒尺寸的影响

采用热模拟技术，研究了卷取温度对Ti-IF钢第二相粒子析出行为及晶粒尺寸的影响，结果表明，卷取温度的变化，对TiN,TiS和Ti4C2S2粒子的影响不大，但对TiC粒子的影响较大，高温卷取有利于TiC粒子的析出，长大，高温卷取有利于铁素体晶料珠长大，碳含量较高，会析出较多的TiC粒子，对退火织构的发展不利。     

Ti-IF钢多道次变形静态再结晶研究

在热模拟实验机上进行了Ti—IF钢多道次铁素体区变形实验，采用后插法确定了静态再结晶分数。在实验的基础上建立了超低碳Ti—IF钢多道次铁素体变形时静态再结晶动力学模型。研究结果表明，建立的超低碳Ti—IF钢多道次铁素体变形静态再结晶动力学模型与实验结果吻合。     

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

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

Development of {111} transformation texture in interstitial-free steels

The evolution of the transformation texture in two Ti₄C₂S₂-stabilized interstitial-free (IF) steels (Ti-and Ti/Nb-) was studied as a function of different thermomechanical processing (TMP) parameters using orientation distribution function (ODF) analysis. The ensuing transformation texture, largely independent of the steel composition and TMP path of the kind used in these experiments, is strongly related to the combination of initial austenite grain size and the amount of deformation applied in the early rolling passes. It is proposed that the formation of crystallographic heterogeneities in austenite induces the gamma-fiber texture development around the {111} components in the ferrite.     

Ti-IF钢热镀锌钢板力学性能控制

在设计生产DQ级以下产品的热镀锌机组上进行了Ti-IF钢热镀锌试验,研究了RTH带钢温度和光整对Ti-IF钢热镀锌板组织、织构和力学性能的影响.试验结果表明,以超低碳Ti-IF钢为基础,保证合适的过剩钛,采用合适的工艺制度,可以生产力学性能优良的正常锌花热镀锌钢板,其屈服强度在150 MPa左右,r、n平均值分别达到2.5和0.25.但是,经光整后,屈服强度明显提高,n值明显降低,且成品晶粒尺寸较粗时,光整后的n值较低.根据试验结果,提出了根据热镀锌板表面状态和用户使用条件采用不同的工艺控制技术路线来保证Ti-IF热镀锌板力学性能的策略.     

深冲用钢DC04热轧原料的开发与试制

为了适应市场汽车业及家电业对超低碳钢深冲性能的需求,拓展包钢产品的品种,提升包钢产品的质量,为后续投产的冷轧生产线开发试制超深冲用钢做技术储备,包钢稀土钢板材厂对深冲用钢DC04热轧原料进行开发与试制,该钢种通过加入合金元素Ti控制间隙原子含量,使钢质纯净,采用高温终轧与卷取的工艺,控制碳化物的析出与固溶,得到低屈服强度、低屈强比的深冲原料,满足下游工序要求。     

Factors influencing planar anisotropy of batch-annealed cold strip

Based on small titanium contents, cold-rolled and batch-annealed sheet can be produced with a quasi-isotropic forming behaviour. Using the chemical composition of deep-drawing steel, the influence of low titanium contents on the deformation and recrystallization textures has been investigated. Whereas the deformation texture was not affected by the addition of titanium, the recrystallization textures showed clear differences. For example, the volume fractions of orientations corresponding to the deformation texture were increased by the addition of titanium. The reczystallization kinetics is slowed down by the addition of titanium. To achieve a low planar anisotropy, the delay of recrystallization due to Ti-alloying has to match the driving force for recrystallization, i. e. the degree of deformation. Furthermore, a low coiling temperature after hot-rolling and a Ti/N-ratio of > 4.0 are required to produce deep-drawing steel with a low planar anisotropy.     

IF钢铁素体区热轧工艺参数对深冲性能的影响

 采用润滑轧制,在不同压下率、终轧温度及退火制度下,研究了IF钢铁素体区热轧时,不同工艺参数对IF钢深冲性能的影响。结果表明：在实验范围内,随压下率增加,r值提高;终轧温度下降,r值略微增加;高温退火可使变形铁素体充分再结晶,从而提高r值。ODF织构分析结果与检测结果是一致的。     

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 weak γ-fiber with uniform orientation distributions, and weak α-fiber, of which {445}<110> component forms a high intensity peak at coiling temperature of 750 ℃. After cold rolling, both {111}<110>-{111}<112> intensity on the γ-fiber and {111}-{112}<110> intensity on the α-fiber enhanced. As a result of substitutional solute elements Mn and P being added to the steel, strong {112}<110> deformation texture component is observed on α-fiber, especially at 80% cold rolling reduction, and this leads to the strong {111}<112> recrystallization texture after annealing. The increase of cold rolling reduction shifts the maximum intensity on the γ-fiber from {111}<112> to {111}<113>. After annealing, a very strong γ-fiber is obtained, with intensity peak at {111}<112> component when cold rolling reduction reaches 80%. Increasing coiling temperature and cold rolling reduction improve γ-fiber intensity and r value, resulting in good deep drawability.     

Austenite Recrystallization and Controlled Rolling of Low Carbon Steels

The dynamic recrystallization and static recrystallization in a low carbon steel were investigated through single-pass and double-pass experiments. The results indicate that as the deformation temperature increases and the strain rate decreases, the shape of the stress-strain curve is changed from dynamic recovery shape to dynamic recrystallization shape. The austenite could not recrystallize within a few seconds after deformation at temperature below 900 ℃. According to the change in microstructure during deformation, the controlled rolling of low carbon steel can be divided into four stages： dynamic recrystallization, dynamic recovery, strain-induced ferrite transformation, and rolling in two-phase region. According to the microstructure after deformation, the controlled rolling of low carbon steel can be divided into five regions： non-recrystallized austenite, partly-recrystallized austenite, fully-recrystallized austenite, austenite to ferrite transformation, and dual phase.     

磷、钛对含磷IF高强钢织构的影响

借助电子背散射衍射(EBSD)技术,以罩退生产的冷轧Ti-IF钢及含磷Ti-IF高强钢为目标,分析不同磷、钛合金质量分数对产品特征织构的影响。结果表明,磷元素虽然有利于γ取向线上{111}〈112〉织构的增加,但也增加了组分强度差,不利于塑性应变比r值,并且磷元素对{111}织构发展的促进作用取决于钢中过剰钛的质量分数,过剩钛质量分数过高会促进FeTiP二相粒子的析出,从而阻碍{111}取向再结晶晶粒的长大,弱化{111}面织构的强度。研究结果对该材料合金成分的调整起到了指导作用。为了保证所生产的含磷IF高强钢获得一定的强度,同时兼备良好的冲压性能,应降低IF钢中的钛质量分数,适当加入铌以弥补因钛减少对间隙原子固定产生的影响。