Influence of Gas Atmosphere Dew Point on the Selective Oxidation and the Reactive Wetting During Hot Dip Galvanizing of CMnSi TRIP Steel

The selective oxidation and reactive wetting of intercritically annealed Si-bearing CMnSi transformation-induced plasticity steels were investigated by high-resolution transmission electron microscopy. In a N₂ + 10 pct H₂ gas atmosphere with a dew point (DP) ranging from 213 K to 278 K (?60 °C to 5 °C), a continuous layer of selective oxides was formed on the surface. Annealing in a higher DP gas atmosphere resulted in a thinner layer of external oxidation and a greater depth of internal oxidation. The hot dipping was carried out in a Zn bath containing 0.22 mass pct Al, and the bath temperature was 733 K (460 °C). Coarse and discontinuous Fe₂Al_5?x Zn _x grains and Fe-Zn intermetallics (?? and ??) were observed at the steel/coating interface after the hot dip galvanizing (HDG) of panels were annealed in a low DP atmosphere 213 K (?60 °C). The Fe-Zn intermetallics were formed both in areas where the Fe₂Al_5?x Zn _x inhibition layer had not been formed and on top of non-stoichiometric Fe-Al-Zn crystals. Poor wetting was observed on panels annealed in a low DP atmosphere because of the formation of thick film-type oxides on the surface. After annealing in higher DP gas atmospheres, i.e., 263 K and 278 K (?10 °C and 5 °C), a continuous and fine-grained Fe₂Al_5?x Zn _x layer was formed. No Fe-Zn intermetallics were formed. The small grain size of the inhibition layer was attributed to the nucleation of the Fe₂Al_5?x Zn _x grains on small ferrite sub-surface grains and the presence of granular surface oxides. A high DP atmosphere can therefore significantly contribute to the decrease of Zn-coating defects on CMnSi TRIP steels processed in HDG lines.     

Selective Oxidation and Reactive Wetting during Galvanizing of a CMnAl TRIP-Assisted Steel

A transformation induced plasticity (TRIP)-assisted steel with 0.2 pct C, 1.5 pct Mn, and 1.5 pct Al was successfully galvanized using a thermal cycle previously shown to produce an excellent combination of strength and ductility. The steel surface chemistry and oxide morphology were determined as a function of process atmosphere oxygen partial pressure. For the 220 K (–53 °C) dew point (dp) + 20 pct H₂ atmosphere, the oxide morphology was a mixture of films and nodules. For the 243 K (–30 °C) dp + 5 pct H₂ atmosphere, nodules of MnO were found primarily at grain boundaries. For the 278 K (+5 °C) dp + 5 pct H₂ atmosphere, nodules of metallic Fe were found on the surface as a result of alloy element internal oxidation. The steel surface chemistry and oxide morphology were then related to the reactive wetting behavior during continuous hot dip galvanizing. Good wetting was obtained using the two lower oxygen partial pressure process atmospheres [220 K dp and 243 K dp (–53 °C dp and –30 °C dp)]. An increase in the number of bare spots was observed when using the higher oxygen partial pressure process atmosphere (+5 °C dp) due to the increased thickness of localized oxide films.     

Mechanical Property and Microstructural Characterization of C-Mn-Al-Si Hot Dip Galvanizing TRIP Steel

 Mechanical properties and microstructure in high strength hot dip galvanizing TRIP steel were investigated by optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD), dilatometry and mechanical testing. On the heat treatment process of different intercritical annealing (IA) temperatures, isothermal bainitic transformation (IBT) temperatures and IBT time, this steel shows excellent mechanical properties with tensile strength over 780 MPa and elongation more than 22%. IBT time is a crucial factor in determining the mechanical properties as it confirms the bainite transformation process, as well as the microstructure of the steel. The microstructure of the hot dip galvanizing TRIP steel consisted of ferrite, bainite, retained austenite and martensite during the short IBT time. The contents of ferrite, bainite, retained austenite and martensite with different IBT time were calculated. The results showed that when IBT time increased from 20 to 60 s, the volume of bainite increased from 14.31% to 16.95% and the volume of retained austenite increased from 13.64% to 16.28%; meanwhile, the volume of martensite decreased from 7.18% to 1.89%. Both the transformation induced plasticity of retained austenite and the hardening of martensite are effective, especially, the latter plays a dominant role in the steel containing 7.18% martensite which shows similar strength characteristics as dual-phase steel, but a better elongation. When martensite volume decreases to 1.89%, the steel shows typical mechanical properties of TRIP, as so small amount of martensite has no obvious effect on the mechanical properties.     

On the Transition of Internal to External Selective Oxidation on CMnSi TRIP Steel

The selective oxidation of a CMnSi transformation-induced plasticity (TRIP) steel during intercritical annealing (IA) in a N₂ + 10 pct H₂ gas atmosphere with a dew point (DP) in the range from 213 K to 278 K (?60 °C to +5 °C) was investigated by transmission electron microscopy. The decarburization during IA resulted in a fully ferritic matrix at the TRIP steel surface. Annealing in high DP gas atmospheres resulted in a reduction of the oxide layer thickness at the surface and an increase of the depth of the subsurface internal oxidation. The experimental results were compared to the calculations of the DP for the transition from internal to external oxidation based on the Wagner model. The evolution of the surface oxide composition during annealing was analyzed thermodynamically by means of the chemical potential diagram for the surface oxides. In the high DP atmosphere conditions, mainly, Mn-rich xMnO·SiO₂ (1 < x < 2) oxides were formed at the surface, while Si-rich xMnO·SiO₂ (x < 1) oxides were formed by internal oxidation. The use of a high DP gas atmosphere is therefore advantageous to induce internal selective oxidation and reduce the amount of surface oxides. It also leads to the formation of Mn-rich xMnO·SiO₂ (1 < x < 2) oxides.     

连续热镀锌工艺对IF钢力学性能的影响

对影响热镀锌IF钢力学性能的主要工艺参数进行了研究，其中包括段退火温度、带钢速度、退火张力和拉矫延伸率等。结果表明，影响IF钢热镀锌板力学性能的主要因素是退火温度制度、退火张力和拉矫延伸率。     

热镀锌液中Al含量及工艺对TRIP钢表面质量的影响

通过对TRIP590钢进行不同Al含量锌液热镀锌对比试验,研究了锌液中不同Al含量及热镀锌工艺对TRIP钢镀层性能的影响。试验结果表明,镀锌液中加入5%Al,均热退火温度800℃,可明显提高TRIP钢的镀层质量,改善热镀TRIP钢的抗腐蚀性能。     

添加Ti元素对热浸镀锌钢板的影响

采用SEM和金相显微镜研究了在纯锌浴中加Ti元素对含硅钢热浸镀锌的影响.结果表明,随着锌浴中Ti含量的增加,Ti对ζ相生长的抑制作用增强,合金层厚度逐渐减薄,Ti能有效地抑制含硅活性钢镀层的超厚生长.当锌浴中Ti元素含量大于0.05%时,镀层中出现Γ2粒子.锌浴中添加0.03%～0.05%Ti时,能完全抑制含0.09%Si钢在常规热浸镀锌时的圣德林效应.但对于含0.36%Si钢,钛的作用不明显.     

本钢特深冲用热镀锌板研制

结合本钢现有生产技术装备水平,阐述了EDDQ特深冲热镀锌板的生产工艺特点,分析了所采用的工艺制度对产品性能的影响,总结了本钢特深冲热镀锌板的开发研制过程.     

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N₂+10 pct H₂ gas atmosphere with low DPs [213 K and 243 K (?60 °C and ?30 °C)], the coating layer consisted of δ (FeZn₁₀) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (?10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe₃Zn₁₀) phase crystals, and a thin layer of Г ₁ (Fe₁₁Zn₄₀) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.     

新型带钢连续热镀锌机组的发展

由于热镀锌及其合金化产品具有优良的耐蚀性能及成本较低等优点，因而在汽车和家电等行业得到了广泛的应用。近年来，国内相继建成多条热镀锌线，还有多条在建和拟建。在此简要介绍了新型带钢连续热镀锌机组设备(包括焊机、清洗设备、连续退火炉、锌锅设备、合金化炉及平整机等)的发展情况。     

热镀锌钢板漏镀原因及解决方案

对改良森吉米尔法镀锌线热镀锌钢板产生漏镀现象的原因进行了详细分析,通过控制无氧化段的氧化、还原炉和喷冷段的露点及正确使用耐火材料等措施,有效控制了热镀锌钢板漏镀缺陷,使钢板漏镀率从1.3%降低到0.3%     

析出硬化型热强钢中合金元素的作用机制

采用X射线衍射和透射电镜等手段,对高温下析出硬化型热强钢中合金元素的再分配行为及其作用机制进行了分析和探讨,并和用余氏理论分析了该钢的价电子结构,阐述了高温下钴的作用机制。结果表明：在700℃的高温下,钢中的合金元素会逐渐析出并形成碳化物等析出相;在700℃下保持600h后,钢中全部的钼、钨、碳及部分的铬、微量的钴和钒等脱溶析出,形成μ相、Layes相、MC、M23C6和M6C等析出相,产生析出强化作用。而大部分的钴、铬和少量的钒保留在基体中起固溶强化作用。价电子结构计算结果表明,钴增强了基体晶格的键合力,同时也提高了回火抗力。     

热轧和退火后汽车用热镀锌440 MPa级高强IF钢析出物分析

 通过TEM和EDS对汽车用热镀锌440MPa级高强IF钢热轧和不同退火工艺条件下的析出物的形貌、分布和成分特征进行研究,并分析了退火工艺参数对析出物的影响。结果表明,热轧和退火得到的析出物主要有TiN、NbC、TiC、Fe(Ti+Nb)P等。退火后的析出物数量都明显多于热轧板中的析出物数量,并且随着退火温度的升高和保温时间的延长,原来粗大析出物细化、分解,而原来细小析出物得以粗化、长大。     

TRIP钢中合金元素的作用和处理工艺的研究进展

 为了给TRIP钢的试制提供参考,对各种合金元素在TRIP钢中的作用进行了描述,并介绍了热轧和冷轧TRIP钢的处理工艺。认为TRIP钢的研究、生产、应用与双相钢相似,能生产双相钢的生产线即可生产TRIP钢。为生产符合我国国情的TRIP钢,应加强微合金元素钒、钛在TRIP钢中作用的基础研究。     

Hot Dip Galvanizing Simulation of Interstitial Free Steel by Liquid Zinc Spin Coater: Influence of Dew Point on Surface Chemistry and Wettability

In order to establish the relationship between surface chemistry and wettability as a function of dew point, an attempt has been made to simulate the hot‐dip galvanizing process with an ‘in‐house’ built Liquid Zinc Spin Coater. Interstitial free (IF) steel was annealed at 820°C in N₂‐5%H₂ gas atmospheres with dew points of ‐79°C, ‐29°C and 0°C, respectively. The wettability tests were conducted at 470°C at low dew point of ‐79°C. Surface analyses prior to wetting were carried out by using X‐ray photoelectron spectroscopy (XPS) and Field Emission Scanning Electron Microscopy (FE‐SEM). As expected, external oxidation of Al was observed only at the low dew point. With increasing dew point the oxidation of Cr and Si becomes internal. The formation of manganese silicates was observed at all dew points. While sulphur was detected on the specimen surface after all annealing conditions, the segregation of P starts to be significant at dew point 0°C by forming Mn‐phosphates. Despite the surface oxides, specimens annealed at all dew points are in the wetting regime by liquid zinc. Investigations on the steel/zinc interface of IF steel by using the liquid zinc spin coater were successful.     

新设备新工艺在热镀锌机组上的应用

汽车工业是21世纪的支柱产业之一.随着中国加入WTO和世界经济一体化速度的加快,国内汽车市场成为国际竞争舞台,作为高级轿车外板主要原材料之一的热镀锌板以其良好的性价比越来越受到汽车行业的青睐.通过宝钢现有热镀锌机组与国外同类热镀锌机组的对比,简要介绍一些新设备、新工艺在现代热镀锌机组上的应用,主要包括前清洗、退火炉、镀锌段和平整段.     

Influence of Alloying Elements Corrosion Resistance of Cold on Mechanical Properties and Rolled C-Mn-Si TRIP Steels

 The rust layer plays an important role in the corrosion of steel in chlorinated environments. Salt spray, potentiodynamic polarization curve and tensile test were conducted in laboratory for the specimens after two-stage heat treatment. The influence of the alloying elements on mechanical properties and corrosion resistance of three kinds of steels was investigated by observing the microstructure and the morphologies of rust layer. The results show that the highest value (29%) of total elongation for steel A is obtained. The mechanical property of the specimen for steel C exhibits best strength ductility balance (21384 MPa·%) because of the presence of the multiphase microstructures after a two-stage heat treatment and the addition of the alloying elements. The corrosion products are known to be a complex mixture of Fe3O4, Fe2O3 and α-FeOOH for steel C. The presence of the alloying elements results in the formation of compact and dense rust layers in steel B and C. Passive film protects the substrate of TRIP (transformation induced plasticity) steel containing a complex mix of multiphase. Superior corrosion performance is exhibited for steel C with low alloying contents due to the enrichment of alloying elements within the rust layers.