Hot strip processing of low carbon enameling grades in the range of 600 to 700 °C

The processing of continuously cast low carbon steel grades for enameling usually requires a high coiling temperature. Low-temperature coiling, however, is required to ensure deep drawability. A steel composition characterized by low AIN leads to a material combining both good formability and enamelability ratio for hot strip coiling temperatures in the range of 600 to 700 °C. The influence of the coiling temperature on the mechanical properties, cementite morphology, H permeation, and enamel adhesion was studied in detail.     

High Temperature Oxidation of Micro-alloyed Steel and Its Scale Adhesion

The present work investigated the high temperature oxidation behaviour of the micro-alloyed steel and the adhesion of thermal oxide scale to its steel substrate. Oxidation testing was conducted at 815 °C in oxygen without and with 17.9% v/v water vapour. The oxidation kinetics in the two atmospheres were parabolic with similar rate constants, i.e. 1.13 × 10^?9 and 1.17 × 10^?9 g² cm^?4 s^?1 for the sample oxidised in oxygen without and with water vapour, respectively. The XRD peaks for wustite, magnetite, Ti-doped magnetite and titanium carbide were detected for the sample oxidised in oxygen. For the sample oxidised in the humidified atmosphere, Ti-doped magnetite was dominantly observed, additionally with titanium carbide. A tensile testing machine equipped with an optical lens was used to monitor scale failure during straining. For the sample oxidised for 1 min, the strain initiating the first spallation of the steel oxidised in the humidified oxygen was 1.74 ± 0.14%. This strain was higher than the strain initiating the first spallation of the steel oxidised in oxygen which was 1.00 ± 0.04%, indicating the improved adhesion of scale formed in the atmosphere containing water vapour. Mechanisms of water vapour effect on scale adhesion are discussed.     

Adhesion of Thermal Oxide Scales on Hot-Rolled Conventional and Recycled Steels

The mechanical adhesion of thermally-formed oxide scales formed on industrial hot-rolled low carbon steel strips produced through the blast-furnace route (conventional steel) or the electric-arc-furnace route (recycled steel) was studied. A new macro-tensile test was compared to a micro-tensile test previously used. It was observed that spallation of scales during straining increased with increasing the tensile strain rate. A higher strain rate resulted in a lower strain inducing the first spallation. As a result, the mechanical adhesion energy of scales actually formed on the recycled steel was in the range 300–700 J m^?2. Comparison at the same strain rate of the conventional and recycled steels showed higher scale adhesion for the recycled steel due to the presence of high amounts of interfacial silica.     

Oxidation of Low-Carbon,Low-Silicon Mild Steel at 450–900°C Under Conditions Relevant to Hot-Strip Processing

The oxidation behavior of a low-carbon, low-silicon mild steel was investigated in ambient air at 450–900°C to simulate steel strip oxidation during finishing hot rolling and coiling. Oxide scales developed at 880–900°C for a very short time (12 sec) had a structure similar to that formed on pure iron, but with a greater thickness ratio between the magnetite and wüstite layers. However, the scale structure after oxidation for a longer period (200 sec) at 900°C deviated significantly from that reported for pure iron. This difference was attributed to the loss of scale–steel adhesion at some locations. Oxide scales formed in the range of 580–700°C after oxidation for more than 2 hr also differed from those reported for pure iron. The scale structures were irregular, comprising mainly hematite and magnetite with no or very little wüstite, while the thickness ratio of these two layers differed considerably at different locations. The scale formed at 450–560°C was relatively uniform with a two-layered (hematite and magnetite) structure; however, the thickness ratio of these two scale layers varied for different oxidation temperatures and different oxidation durations. It was also found that limited oxygen supply (zero air flow) improved the scale–steel adhesion, and substantially reduced the relative thickness of the hematite layer. Continuous-cooling experiments proved that significant growth of the hematite layer, as well as the entire scale layer, may occur if the steel is cooled slowly through the temperature range 600–660°C, and even more significantly through the range 660–720°C.     

Some Effects of Hot Strip Mill Rolling Temperatures on Properties of Low Carbon Sheet Coils

The phase changes occurring in low carbon steel during hot strip mill rolling are shown to be metallurgically significant when related to commonly used temperature control points, particularly finishing and coiling temperatures. In combination, these temperatures are shown to have an important influence on the level and uniformity of hardness, grain size, and carbide characteristics of the finished hot and cold rolled sheets.     

Oxide-Scale Structures Formed on Commercial Hot-Rolled Steel Strip and Their Formation Mechanisms

The oxide-scale structure developed on commercial hot-rolled steel strip at the mid-coil position was examined. The initial oxide scale after rolling and cooling on the run-out table had a three-layer (hematite, magnetite, and wustite) structure; the thickness was found to be a function of the finishing temperature. From this initial structure, various final scale structures developed after coiling, depending on the coiling temperature, oxygen availability, and cooling rate. For relatively low coiling temperatures (e.g., at 520°C), the final scale structure comprised an inner magnetite/iron mixture layer, an outer magnetite layer, and, at regions away from the center, a very thin outermost hematite layer. For higher coiling temperatures (e.g., in the range of 610 to 720°C), a two-layer hematite/magnetite structure was observed at the edge regions, whereas at the center regions, these two layers were absent and the entire scale layer comprised a mixture of the wustite-transformation products, i.e., a mixture of proeutectoid magnetite, magnetite+iron eutectoid, and a certain amount of retained wustite. At regions between the edges and the center, the oxide structures were similar to those developed at low coiling temperatures (<570°C), i.e., an inner layer comprising a mixture of the wustite-transformation products, an intermediate magnetite layer and at regions near the edges, an outermost hematite layer. In addition, two distinct structures were observed on strips with a coiling temperature of 720°C. One structure comprised a very thick hematite layer (3–5 m) formed near the edges (within 10–20 mm from the edges), while the other structure comprised a substantial amount of retained wustite formed at the center regions. The formation mechanisms of various oxide scale structures are discussed.     

深冲用SPHC钢开裂原因分析及改进措施

针对某厂生产的SPHC热轧酸洗卷在制作压缩机壳体过程中出现了制耳甚至开裂的现象，对其产生原因进行了研究。结果表明，SPHC低碳钢的Ar3温度较高，而终轧温度偏低，导致其在两相区轧制且带钢长度和宽度方向温度不均，使SPHC带钢产生混晶或粗晶组织，这是产生深冲开裂和制耳的主要原因。为此，提出了工艺改进措施，如提高加热温度、减少除鳞水；增加中间坯厚度、提高穿带和轧制速度、加盖保温罩、采用热卷取箱等，以保证薄规格SPHC带钢的终轧温度不小于910 ℃且改善带钢温度均匀性。生产实践表明，采用改进措施后，显著提高了薄规格SPHC带钢深冲性能，开裂率由30%降低至3‰。     

深冲用SPHC钢开裂原因分析及改进措施

针对某厂生产的SPHC热轧酸洗卷在制作压缩机壳体过程中出现了制耳甚至开裂的现象,对其产生原因进行了研究。结果表明,SPHC低碳钢的Ar3温度较高,而终轧温度偏低,导致其在两相区轧制且带钢长度和宽度方向温度不均,使SPHC带钢产生混晶或粗晶组织,这是产生深冲开裂和制耳的主要原因。为此,提出了工艺改进措施,如提高加热温度、减少除鳞水;增加中间坯厚度、提高穿带和轧制速度、加盖保温罩、采用热卷取箱等,以保证薄规格SPHC带钢的终轧温度不小于910 ℃且改善带钢温度均匀性。生产实践表明,采用改进措施后,显著提高了薄规格SPHC带钢深冲性能,开裂率由30%降低至3‰。     

轧制工艺对800 MPa复相钢组织性能的影响

随着汽车行业的发展,先进高强钢的研究与应用越来越广泛。设计了低C,以Cr、Mn、Si为基本元素,复合添加Ti、Nb、V、Mo等元素的复相(CP)钢化学成分;通过控轧控冷工艺,充分发挥了马氏体和贝氏体相变强化及合金元素的析出强化、细晶强化的复合作用,成功获得了屈服强度大于680 MPa,抗拉强度大于780 MPa,伸长率大于10％的热轧CP钢。研究了不同终轧温度、卷取温度下钢板的组织形貌和析出物大小对其力学性能和扩孔性能的影响,得到了最佳终轧温度为890 ℃,卷取温度为490 ℃。在此工艺下,试制钢板的组织形貌和析出物大小得到了良好的配合,其扩孔率达到47%,扩孔性能最优。     

基于氧化特性分析的IF钢连退麻点缺陷产生机理

首钢现行工艺下IF钢热卷铁皮厚度达到10 μm以上,边部铁皮略薄,中部铁皮较厚,随着卷取温度的升高,铁皮厚度增加且对应酸洗时间延长;不同卷取温度下的热卷酸洗后均发现小麻坑缺陷。本文利用差热分析手段研究了IF钢的高温氧化机理,发现IF钢抗氧化性低,随温度升高铁皮增厚明显,精轧区间以FeO铁皮结构为主,在1 150 ℃左右发生明显的内氧化,界面形成大颗粒氧化质点。综合分析得出：IF钢带钢连退后麻点缺陷产生的主要机理为热卷的铁皮较厚,热轧过程压入钢板表面所致。为此,提出了控制措施,即降低热轧过程温度,改变层冷模式,加大精轧用水量,提高精轧轧制速度,降低冷轧酸洗速度等,有效减少了麻点缺陷的发生概率。     

Small Punch Tests at Oxide Scales Surface of Structural Steel and Low Silicon Steel

This work is focused on the study of the behavior of oxide scale layers grown on two types of steel: plain carbon steel and silicon-alloyed steel. The small punch test method was chosen to study the mechanical properties of oxide scale layers. Several specimens were prepared from both materials and exposed to 1,000 °C for different oxidation times to obtain the desired scale layers. When the oxidation process was complete, oxide layers of varying thickness, composed mainly of hematite and magnetite, were created. The specimens were used for testing at temperatures between 600 and 900 °C.     

控冷工艺对低碳钢珠光体转变及力学性能的影响

生产低碳钢带钢,采用层流冷却后段冷却且卷取温度低于空冷的Ar1时,由于抑制了先共析铁素体的析出,成品带钢组织中获得了远超平衡含量的退化伪珠光体组织。本试验研究了层流冷却后段冷却下不同卷取温度对带钢显微组织、力学性能的影响。结果表明：卷取温度在600 ℃以上时,卷取温度越低,带钢的强度越高;卷取温度为540 ℃时,带钢组织中渗碳体在晶界偏聚程度较高,导致力学性能恶化。     

Influence of Polishing-Induced Surface Hardening on the Adhesion of Oxide Scales Grown on a Ferritic Stainless Steel

The influence of surface preparation on the stress and adhesion of oxide scales formed on the ferritic stainless steel AISI 441 was studied. Steel coupons were surface-finished to different degrees of surface roughness from 400-grit SiC through to 1-micron diamond, and were also electropolished to remove the work hardened surface. Initial metal roughness was measured by optical profilometry. Oxidation was carried out at 800 °C under synthetic air for 100 h. Oxide residual stress was derived from the Raman shift of the main chromia line, and adhesion of oxide scales was quantitatively obtained using forced spallation by tensile straining. The results show that surface hardening is the most influential factor on adhesion, with the high dislocation-containing mirror-polished samples exhibiting the lowest adhesion energy (~4 J m^?2), and the electropolished samples with non-mechanically affected surface exhibiting the highest adhesion energy (17 J m^?2). Recrystallisation of the subsurface zone during heating to the oxidation temperature is thought to be the most influential factor reducing scale adhesion.     

热轧温度对Q345E力学性能的影响

探讨了热轧过程中的几个关键温度对武钢热轧Q345E钢力学性能的影响,以期能对该钢种的生产过程有指导作用。结果表明,在试验范围内,随着加热温度的提高,钢的强度提高;随着终轧温度、卷取温度的提高,钢的强度降低而伸长率提高。     

中碳钢盘条的高温氧化行为研究

针对中碳钢盘条的高温氧化问题,利用热重分析仪对45钢和40Cr钢盘条的高温氧化行为进行了试验研究;采用场发射电子探针表征了氧化铁皮厚度及截面微观形貌,对氧化铁皮形貌演变规律以及合金元素在氧化铁皮与基体界面处的分布规律进行了研究分析。结果表明:45钢和40Cr钢盘条的氧化增重曲线在1 050~1 250 ℃范围内遵循抛物线规律,当氧化条件相同时,相比于常规低碳钢,其氧化激活能较高,抗氧化性能更好;氧化铁皮呈典型的3层结构,从外到里分别为Fe₂O₃、Fe₃O₄及FeO,并且在氧化铁皮与基体界面处存在合金元素富集层;45钢盘条在高温氧化时,Cr元素分布不明显,Si元素在氧化铁皮与基体界面处有少量富集,Mn元素在氧化铁皮中均匀分布;40Cr钢盘条在氧化铁皮与基体界面处不仅有富Si层,还明显存在一层均匀完整的富Cr层,由于合金元素富集层阻碍了Fe²⁺向外扩散,提高了盘条的高温抗氧化性能。     

Multiresponse Optimization of Mechanical Properties and Formability of Hot Rolled Microalloyed Steels

In this study, a 2³ factorial design analysis was conducted to screen the significant factors influencing the mechanical properties and formability of Nb-microalloyed steel sheets in hot-rolling process and for optimizing these factors to predict maximum yield strength (YS), ultimate tensile strength (UTS), elongation (El), and dome height (DH), simultaneously. For this purpose, two levels for the major hot-rolling process parameters of roughing temperature (RT), finishing temperature (FT), and coiling temperature (CT) were chosen; eight experiments for each response were conducted. From the analysis of variance, the most important parameters affecting the YS, UTS, El, and DH were determined, and satisfactory prediction regression models were derived. It was observed that optimal condition of factors for the best combination of response variables was obtained at 1150 °C of RT, 836 °C of FT, and 604 °C of CT. Findings of this research show that the obtained predicted values were close to the experimental values indicating suitability of the models.     

卷取温度对不锈钢带钢表面氧化铁皮的影响

分析了氧化铁皮结构、性质、厚度与不锈钢带钢卷取温度之间的关系，概述了改善氧化铁皮分布状态、厚度和成分控制的基本方法。经过实际生产应用，提高酸洗工序的产出率约0.5%，降低酸洗不足重工量约 1 000 t/月。     

高品质热轧双相钢的开发

为了提高热轧双相钢的品质,研究了化学成分、轧制工艺、冷却工艺和不同季节水温等参数对热轧双相钢组织和性能的影响。结果表明,无Si成分设计显著提高了热轧双相钢的表面质量;较低的终轧温度和中间保温温度有利于获得更为细小的铁素体组织和弥散的马氏体组织;低的卷取温度（280 ℃）可以获得铁素体+马氏体双相组织;冷却水水温的降低显著提高马氏体含量并提高双相钢的强度。基于上述研究,邯钢实现了系列热轧双相钢的稳定生产,双相钢制作的汽车车轮性能良好。     

700MPa级高强汽车用钢热轧工艺研究与应用

利用Gleeble-3500热-力模拟试验机并通过实验室热机轧制,研究了加热温度、终轧温度、轧后冷却速率、卷取温度及卷取后保温时间对T700钢显微组织、析出物和性能的影响规律,并在此基础上经工业试制,开发出满足用户要求的抗拉强度700MPa级高强汽车用钢板。研究表明:在设定的化学成分条件下,较适宜的加热温度为1 240~1 270℃,终轧温度为860~890℃,卷取温度为550~600℃。     

热轧双相钢显微组织和力学性能

以热轧Si-Mn系双相钢为研究对象,在实验室通过控制轧制和控制冷却实验,研究了变形工艺参数对高强热轧双相钢显微组织和力学性能的影响.研究表明,具有高密度位错亚晶结构的马氏体形貌和分布对双相钢的力学性能有很大影响,通过控制卷取温度、冷却速度和精轧温度,可以得到不同的微观组织形貌和力学性能的热轧双相钢.