摘要
成功地研制和开发出了一种600MPa级高铝系冷轧双相钢，采用以Al代Si的成分设计思路，实现了同一种成分的钢，在炼钢、热轧、冷轧前工序相同的前提下，既可以通过连续退火生产线得到连退板，又可以由连续热镀锌线生产出热镀锌板。两种工艺下钢板的屈服强度均为360～390MPa，抗拉强度均达到600～640MPa，伸长率为24%～29%，烘烤硬化值（BH）达到65MPa以上，加工硬化指数（n）大于0.16，同一成分的两个品种均完全符合国标GB/T20564.2—2006及通用标准GMW3399中对该强度级别冷轧双相钢力学性能的要求。对试验以及生产试制的双相钢进行了系统的试验研究，包括不同工艺的退火试验、力学性能测试、金相组织观察、透射电子显微镜（TEM）组织结构分析、扫描电子显微镜（SEM）下热镀锌板表面及抑制层形态与能谱分析、EBSD不同相的统计分析等，揭示了新钢种组织转变的行为机理；对热镀锌板锌层与抑制层进行了系统分析，揭示了影响锌层表面质量与附着性能的关键因素。
新钢种采用了C-Mn-Al-Mo-Si系成分，将Si元素含量降低至0.10%以下、Al元素含量提高至0.9%，并适当调整了Mn、Mo等成分。Si元素显著提高钢的强度并“净化”铁素体，排出铁素体中的碳从而提高延性；然而Si容易在钢板表面氧化富集，影响热镀锌板的表面质量；Al元素与Si元素的作用相似，但对于热镀锌板的表面质量的影响不如Si恶劣，新成分体系的双相钢既适合热镀锌板的生产，又适合连续退火板的生产。
在热浸镀模拟试验机（HDPS）上对试验钢进行了系列的退火试验，通过力学性能检验分别确定了连续退火板与连续热镀锌板的最佳生产工艺，并找出了退火工艺参数对试验钢组织与力学性能的影响规律：
对于连续退火板，随退火温度的升高，马氏体岛的尺寸长大，抗拉强度下降，最佳退火温度为820℃。随缓冷末端温度的降低，屈服强度降低，最佳缓冷末端温度为630℃。随时效温度的升高，马氏体岛内回火碳化物数量增多、尺寸长大，抗拉强度下降，最佳时效温度为300℃。300℃下进行时效处理，随时效时间的延长试验钢屈服强度与抗拉强度变化不明显，但伸长率逐渐提高，工业生产试制时，时效段时间约350～460S，生产的双相钢伸长率达到24%以上。
热镀锌退火工艺制度与连续退火存在区别，生产线上炉内加热段长度不一致，实际工艺参数对双相钢的影响不同，退火温度对热镀锌双相钢的微观组织与力学性能影响较小，快冷末端温度是影响组织与性能的关键因素。快冷末端温度从470℃升高至520℃时，试验钢的抗拉强度升高，低于500℃时，抗拉强度接近600MPa的下限，将快冷段末温度控制在500±5℃范围是保证热镀锌双相钢力学性能、锌层质量满足要求的关键。
结合扫描电镜与透射电镜对热镀锌双相钢的抑制层进行了分析研究，抑制层中Fe2Al5颗粒的致密度是影响锌层附着性能的关键因素，锌液中的Al与钢板表面富集的MnO充分反应生成Al的氧化物颗粒时，MnO的表面富集不会对热镀锌板的表面质量产生影响；而形成薄膜状MnO，且不能充分被锌液中Al元素还原时，锌层附着性能变差。影响锌层附着性能的因素主要是钢板表面氧化富集状态、锌锅中Al、Fe等元素含量，双相钢的生产要求带钢表面清洗状态较好，炉内还原气氛中H2浓度达到5%以上，锌液中Al元素含量在0.19～0.20%。
生产试制的双相钢经过上海汇众汽车零件配套厂等汽车制造公司的使用，制作成汽车车门内板（B柱）、门槛内板等零件，连续退火板与热镀锌板的使用性能均达到了该强度级别双相钢的性能要求。
A600MPa grade high Al cold rolling dual phase steel was developed successfully, this steel canproduced not only by continuous annealing line, but also by continuous hot dip galvanized line. With both ofthe two processes, the steels was got well mechanical properties with yield strength from360to390MPa,tensile strength from600to640MPa, elongation rate is above24%, and the BH value is above65MPa, all theproperties agreement the standard GB/T20564.2-2006and the standard GMW3399.The laboratory and pilottest steels were systematically invested, including continuous annealing test, mechanical properties test,microstructures analysis by optic microscopy(OM), transmission electron microscopy(TEM) and scanningelectron microscopy(SEM), respectively. The morphology of the surface and inhibition layer of thegalvanized steel was studied, and the different phases of ferrite and island martensite were analyzed by bothmicroscopy and EBSD, the mechanics of the microstructure transformation of the new steel was revealed.The inhibition layer was studied by both SEM and TEM, influences of the quality and the adhesion of thesurface were figured out.
The composition of the new steel was mainly composed of C, Mn, Al, Mo and Si, which was based onthe600MPa grade dual phase steel’s composition of ThyssenKrupp Steel Company, and some elementscontent were optimized, Si was decreased from0.50%to0.10%, and Al was increased from0.5%to0.9%,meanwhile, the contents of Mn and Mo were fine adjusted. It is well known that Si and Al were used for solidsolution strengthening and suppress the carbide formation during cooling from annealing temperature,however, the segregation and selective oxidation of the alloy element Si, Mn, and some other elements duringannealing process deteriorate the surface quality of the hot dip galvanized steels, Al have the same effect withSi to improve the steel’s strength, and the selected oxidation effect is not so bad as Si, the new compositiondual phase steel was employed as continuous annealing sheet, what’s more, it also fit for produce hot dipgalvanized steel.
Serials annealing tests were carried out on HDPS hot dip galvanizing simulator, the most suitableproduction technology was got for continuous annealing sheet and continuous hot dip galvanized steel aftermechanical test, and the influence of annealing process parameters on the mechanical properties andmicrostructure of the steel was found out.
For the continuous annealing steel, with the increase of the annealing temperature, the size of islandmartensite grown, meanwhile, the tensile strength decreased; the best annealing temperature of this steel was820℃. Along with the decrease of the slow cooling temperature, the yield strength of the steel decreased, andthe best slow cooling temperature was630℃. With the aging temperature increased, the amount of temperingcarbide in the island martensite grown, the size of which increased, and the tensile strength of the steeldecreased accordingly, the best aging temperature is300℃. When aging treatment at300℃, with the agingtime expand, the change of yield and tensile strength was not obviously, however, the elongation increasedgradually. The aging time was about350s to460s when pilot produced, following this production process theelongation of the test dual phase steel can reach to above24%.
There are differences between hot dip galvanizing process and continuous annealing produce process,one is the structure length of the annealing furnace, and the other is the processing parameter. Annealingtemperature made a little influence on microstructure and mechanical properties of ho dip galvanized dualphase steel, rapid cooling temperature was the critical factor which influences the microstructure andmechanical properties of the steel. With the rapid cooling temperature increased from470℃to520℃, thetensile strength increased gradually, when this temperature was below500℃, the tensile strength of the steelhardly reach600MPa, rapid cooling temperature must be controlled in the range from495℃to505℃, whichwas the main factor to get a good mechanical properties of the galvanized dual phase steel.
The inhibition layer of hot dip galvanized steel was analyzed by both SEM and TEM, the result showedthat the density of the particles in the inhibition layer is direct influence the adhesion of Zn coating layer.Manganese oxides on the surface of the sheet were reduced by Al in the Zn bath according to thealuminothermic reduction reaction, which formed Al2O3particles, and then were trapped in the inhibitionlayer during the formation and growth of Fe2Al5grains. When the manganese oxides were thin film shape, itis hardly to react with Al in the Zn bath, an this film inhibit the nucleation of Fe2Al5phase at the interfaceof the iron in contact and the liquid zinc, so the adhesion of Zn coating become worth.. Several factorsinfluenced the adhesion of hot dip galvanized steel such as the selected oxidation of the steel surface duringannealing process, the atmosphere state of the annealing furnace, the content of Al and Fe in the Zn bath, andetc. For the production of dual phase steel, it is necessary that the surface of the sheet should be cleaned tothe greatest extent, the content of H2in the annealing furnace kept above5%, and the content of Al in the Znbath was controlled in0.19～0.22%.
Both of the continuous annealing and the continuous hot dip galvanized sheet were tested by automobilefactories, there are made into automobile parts such as the B-column and internal plates of the doorsill, theperformance test and user’s feedback show that the products meet the requirement of600MPa grade dualphase steel.
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