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Key Technology to Reduce the Residual Stress Levels of High Strength Steel

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Submitted By DINGWENHONG
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降低高强钢残余应力水平的关键技术

中冶南方工程技术有限公司 武汉 430223 丁文红
摘要:针对传统热轧平整工艺无法满足高强钢精整加工需求这一技术难题,开发出带压力增益调节功能的高强钢矫直平整工艺(Leveler and Temper passing Process with Dual Servo Valve 以下简称LTP-DSV)。LTP-DSV工艺利用矫直机降低高强钢的残余应力水平;利用双伺服变增益轧制力控制技术提升高强钢的性能稳定性;借助平整加工改善高强钢的表面质量。在保证高强钢性能的前提下充分降低产品的残余应力水平。实际生产数据表明:LTP-DS工艺能全面提升高强钢的性能及质量。
关键词:高强钢、残余应力、矫直、平整
中图分类号:(TG335.5) 文献标识码:A 文章编号:

Key technology to reduce the residual stress levels of high strength steel

Ding Wenhong
WISDRI Engineering & Research Incorporation Limited WuHan 430223 China
Abstract:Since traditional hot-rolled temper passing process can not meet the technical problems of high strength steel needs, leveler and temper passing process with dual servo valve (LTP-DSV) has been developed in this paper. Leveler has been used to reduce the residual stress. Temper mill with dual servo valve has been used to get high strength steel products with stable property, lower the quality defects of the products surface. Practical application demonstrates that the LTP-DSV process can effectively improve the quality and property of high-strength steel.
Key words: high strength steel, residual stress, leveler, temper passing

引言

采用高强钢减少材料用量,降低环境影响,是现代工业的一项重要研究内容。在汽车制造领域,运用高强度钢板减轻车身重量,已成为该领域的重要技术发展方向[1] ,与此相适应,钢铁行业将高强钢加工技术作为新的研究热点,通过合金强化以及再结晶过程控制,不断提高材料强度,优化使用性能[2],使高强钢的应用领域不断拓宽。 各类高强钢中,热轧高强钢具有显著的成本优势,发展更为迅猛,占客车、货车用钢量的60%-70%[3];在桥梁、机械制造领域,热轧高强钢的用量也在持续增加。然而,与普通热轧产品相比,热轧高强钢具有更高的残余应力水平。在后续使用过程中,高强钢内部的残余应力逐步释放,造成材料变形,尺寸、形状精度降低。 针对上述问题,笔者对热轧高强钢板内部的残余应力分布进行了研究,提出采用矫直平整工艺,降低具有板织构特征的热轧高强钢板残余应力水平的工艺方法,在此基础上进行了实验验证研究。实验结果表明:矫直平整工艺能够显著降低具有板织构特征的热轧高强钢的残余应力水平。

传统热轧平整工艺存在的技术问题

为了提升热轧产品的表面质量、平直度以及厚度精度,传统上采用热轧平整工艺对热轧产品进行精整加工。对于普通热轧产品而言,由于产品内部的残余应力水平相对较低,热轧平整工艺可以满足产品的后续加工、使用要求。但是,由于热轧高强钢产品内部的残余应力水平相对较高,平整加工后残余应力水平依然较高,无法满足后续使用要求。为解决这一问题,需要对热轧产品内部的残余应力分布以及平整工艺对普通热轧产品残余应力的改善机理进行研究。

1 平整工艺的机理及降低残余应力的能力限制

1 带钢内部的板织构特征

经过热轧加工后的钢板,其内部形成轧制板织构,致使组织内部的残余应力分布具有如图1所示的确定的方向性。这种与轧制方向平行的应力显著大于其他方向上的应力值,是造成热轧板残余应力高以及板形质量差的主要原因。 导致热轧板内部形成图1所示的应力分布的原因主要有两个,一是为了消除疏松、孔隙等铸造缺陷,热轧通常采用大压缩比进行轧制;二是由于热轧板表面温度高,芯部温度低,造成轧制过程中表层与内部具有不同的流动性特征。因此,在热轧板中存在显著的具有板织构特征的残余应力。改变这种板织构应力分布,是降低产品残余应力水平的唯一途径。
[pic]
图1:热轧后沿带钢厚度方向的内应力分布 实际生产表明:平整工艺能够在一定程度上降低热轧产品的残余应力,进而改善板形,这主要是基于摩擦机制和延伸率机制[4]。

2 作用机制及其能力瓶颈

在轧制、平整过程中,由于轧辊与带钢之间的摩擦力的作用,使带钢在厚度方向上产生不均匀变形,加工后在带钢内部形成残余应力。由于平整过程产生的残余应力与轧制过程产生的残余应力方向相反,因此,借助平整工序,可使轧制过程产生的宏观级别的残余应力得到降低,进而改善板形。其作用机理如图2所示。
[pic]
图2:平整机降低带钢残余应力的摩擦机制 从上述摩擦机制可以看出:平整工艺降低残余应力的能力受两个因素影响,一是带钢与轧辊间的摩擦系数,二是平整过程中所采用的轧制力。而钢板表面的粗糙度由产品表面状态要求确定,轧制力由平整产品的性能决定,因此,对于既有表面粗糙度要求又有性能要求的产品,平整工艺就存在一个能力瓶颈。因此,传统平整工艺无法满足热轧高强钢的加工需求。机制不难推断,受产品表面的粗糙度要求的制约,在平整过程中,借助摩擦机制降低高强钢残余应力水平的能力有限。 延伸率机制是通过在带钢轧制方向上施加一定的延伸量,使带钢变形的不均匀程度得到改善,因而降低带钢内部的残余应力。由于高强钢对产品的性能有着严格的要求,为保证其性能,需要在加工过程中限制其延伸率水平。 因此,对于残余应力水平相对较高,且性能敏感度也很高的热轧高强钢产品来说,传统热轧平整工艺无法满足产品降低残余应力的需求。

2 平整工艺的性能缺陷

从高强钢的强化机制看,加工过程必须对高强钢的性能进行严格控制。因此,作为降低高强钢残余应力的工艺必须兼具降低残余应力与控制材料性能双重功能。普通平整工艺,在材料性能控制方面存在系统缺陷,将其应用于高强钢加工,必须进行性能控制技术的提升。 普通平整工艺采用恒延伸率方式控制材料性能。在延伸率闭环控制模式下,当实际延伸率与设定延伸率之间出现偏差时,采用调节轧制力的方式使延伸率保持恒定。因此轧制力动态控制精度的高低直接决定了平整加工后带钢的性能稳定性。平整机的伺服控制系统采用流量伺服阀调节平整机的轧制压力。在负载情况下,流量伺服阀的零位压力增益较大,因此,实施动态轧制力调节时,较小的电流输入会造成很大的压力变化量,降低轧制力的动态控制精度,造成产品的性能稳定性不高。传统平整机的伺服控制系统无法满足热轧高强钢的性能控制需要。

针对热轧高强钢的矫直平整工艺

如前所述,降低热轧高强钢的残余应力水平的关键是减小钢板上下表面与中部的应力差,而增加钢板上下表面的变形量,是降低具有轧制板织构特征热轧产品的最直接手段。同时,作为高强钢的加工工艺,还必须能够准确控制材料的性能。综合上述两项要求,我们研制出具有高精度性能控制能力的高强钢矫直平整工艺(Leveler and Temper passing Process 以下简称LTP)。

1 矫直降低残余应力的机制

如前所述:降低板织构的残余应力水平的核心是减小带钢表层与中部的变形差,而弯曲矫直工艺的变形特点正好满足了这一需求。图3为辊式矫直以及带钢进行矫直加工时的变形特点。矫直过程中,带钢在沿辊子表面弯曲,在带钢厚度方向上,其伸长量存在差异:带钢外表面伸长量大,中部区域伸长量小。而且,通过调整矫直辊辊径或者压入深度,就可以改变带钢表层与带钢中部的变形差,有效降低轧制后带钢内部的残余应力水平。
[pic] [pic] a:矫直过程的辊缝设定 b:矫直过程的材料变形
图3:带钢矫直过程分析
对高强钢而言,仅降低钢板内部的残余应力的不能满足其使用性能要求的。而传统的辊式矫直在带钢性能控制上又缺乏手段,因此,我们开发出如图4所示的矫直平整工艺。

2 矫直平整工艺设计

具有伺服控制功能的平整机可以通过调节钢板的延伸率控制材料的性能,结合矫直工艺在改变轧制板织构应力分布上的特点,就可以满足高强钢加工的需求,这就是LTP工艺。利用矫直机降低产品的残余应力,借助平整机的动态调节,保证产品的性能要求。
[pic]
图4:用于热轧高强钢精整加工的矫直平整工艺
1-开卷机 2-入口转向夹送辊 3-矫直机 4-平整机 5-出口转向夹送辊 6-带钢 7-卷取机 热轧高强钢矫直平整机组主要包括开卷机、入口转向夹送辊、矫直机、平整机、出口转向夹送辊、卷取机等关键设备,并在入口转向夹送辊、出口转向夹送辊处配置延伸率控制编码器。 在热轧高强钢精整加工过程中,通过矫直机充分降低轧制后高强钢内部的残余应力水平;利用入口转向夹送辊、出口转向夹送辊的延伸率检测以及平整机的实时动态调整功能控制加工后高强钢产品的性能;借助平整机的平整加工提升热轧高强钢产品的表面质量。因此,LTP工艺既具有强的残余应力控制能力、稳定的产品性能控制手段,又具有优异的产品表面质量改善功能。 平整机利用伺服控制系统可以实时调控材料的性能,但在性能控制精度上还无法满足高强钢的性能控制需求。针对高强钢的特点,还需要进一步改进平整机的性能控制能力。

3 提升高强钢性能稳定性的变增益伺服控制系统

平整机采用轧制力调节方式控制材料性能,实现轧制力动态调节控制的技术手段为借助流量伺服阀对轧制力实施控制。由于流量伺服阀在负载情况下零位的压力增益较大,轧制力的动态控制精度低,无法满足高性能高强钢产品的性能稳定性需求。针对这一问题,我们研制出:变增益轧制力控制技术。 1. 双伺服变增益轧制力控制技术 为了降低轧制力控制系统在负载状态下的压力增益,采用大、小两个伺服阀的并联组合替代传统的伺服阀,如图5所示。在正常工作时,用小流量伺服阀进行轧制力动态控制,用大流量伺服阀调整系统的压力-流量系数,降低系统的压力增益。在靠辊压下以及快开等工艺操作过程中,两伺服阀同时投入,满足各种工况的工艺需求。这就是变增益轧制力控制方法。将其应用于LTP工艺,形成满足高强钢性能需求的矫直平整工艺(Leveler and Temper passing Process with Dual Servo Valve 以下简称LTP-DSV)。
[pic]
图5:双伺服变增益轧制力控制技术 2. LTP-DSV的性能模拟 图6为系统启动过程中,双伺服变增益轧制力控制系统与传统单伺服阀轧制力控制系统控制腔压力曲线变化情况的比对。实线对应双伺服变增益轧制力控制系统,虚线对应传统单伺服轧制力控制系统。
[pic]
图6:系统启动时控制腔的压力曲线对比
(实线对应双伺服变增益轧制力控制系统;虚线对应传统轧制力控制系统)
图7为给系统施加阶跃响应信号时,双伺服变增益轧制力控制系统与传统单伺服轧制力控制系统控制腔压力曲线的比对。实线对应双伺服变增益轧制力控制系统,虚线对应传统单伺服轧制力控制系统。
[pic]
图7:向系统施加阶跃响应信号时控制腔压力曲线比对
(实线对应双伺服变增益轧制力控制系统;虚线对应传统轧制力控制系统)
因此,采用双伺服变增益轧制力控制系统可以有效提高轧制力的动态控制精度,进而提升延伸率的控制精度以及加工后带钢的性能。实际生产数据也证明了这一点。采用双伺服变增益轧制力控制技术后,延伸率的波动范围由±0.05%降低为±0.03%。

LTP-DSV工艺的实际应用

为验证LTP-DSV工艺的实际应用效果,我们对LTP-DSV工艺与传统热轧平整工艺进行了对比性试验。比较了两种工艺对不同强度级别、不同厚度规格带钢平直度的改善情况。并在此基础上分析了两种工艺对产品残余应力水平的控制能力。 采用宽度为1500mm,厚度分别为3.4mm、4.5mm、6.35mm、12.7mm的10号钢、50号钢、80号钢进行试验。将同一批次原料分为两组,一组采用传统热轧平整工艺进行加工,另一组采用LTP工艺进行加工。对加工后的带钢进行分条,测量分条后最长带钢与最短带钢间的长度差ΔL,计算加工后的平直度。由于带钢的平直度是带钢内部残余应力水平的反应,因此可以通过平直度的改善情况分析残余应力的改变。平直度计算公式如(1)所示。 [pic] (1) 式中L为最短带钢的长度,单位mm。

1 对不同强度级别带钢残余应力改善情况比较

第一组试验采用宽度为1500mm,厚度为3.4mm的带钢,比较两种工艺对不同强度级别带钢残余应力水平的降低能力。图8为两种工艺加工后产品平直度的对比数据,图9为两种工艺加工后同种带钢的平直度之比,比较两种工艺对不同强度级别产品的作用效果。
[pic]
图8:两种工艺对不同强度级别带钢平直度的改善能力
[pic]
图9:两种工艺加工后同种带钢平直度的比值 从图8可以看出:对于各种强度级别的带钢,LTP工艺均比传统平整工艺具有更强的残余应力改善能力。从图9可以看出,与普碳钢相比,LTP工艺对高强钢具有更强的残余应力改善能力。这主要是由于高强度带钢具有更高的抗表面不均匀屈服能力。

2 对不同厚度规格带钢残余应力改善情况比较

第二组实验采用宽度为1500mm的50号钢,比较两种工艺对不同厚度规格带钢残余应力水平的降低能力。试验结果如图10、图11所示。
[pic]
图10:两种工艺对不同厚度规格带钢平直度的改善能力
[pic]
图11:两种工艺加工后同厚度规格带钢平直度的比值 从图10可以看出:对于各种厚度规格的带钢,LTP工艺均比传统平整工艺具有更强的残余应力改善能力。从图11可以看出:LTP工艺对厚规格带钢残余应力的改善能力强于对薄规格的改善能力。造成这种现象的原因是:为了保证试验数据的可比性,在实验过程中对各种厚度规格的带钢采用了相同的压入深度。在实际生产中可以根据厚度调节上矫直辊的压入深度以得到满意的效果。

结论

充分降低产品的残余应力水平以及提升产品的性能稳定性是生产热轧高强钢产品的两个关键要素。笔者研制出带压力增益调节功能的矫直平整工艺(LTP-DSV)解决了热轧高强钢生产的技术难题。LTP-DSV工艺通过矫直机降低产品在轧制过程中形成的残余内应力;依托平整机改善产品的表面质量;利用精确控制矫直平整工艺段的总延伸率,保证产品加工后的性能稳定性。实际生产数据表明: 1) LTP-DSV工艺较传统热轧平整工艺具有更强的产品残余应力以及性能稳定性的控制能力。 2) 对于不同强度级别、不同厚度规格的带钢,采用LTP-DSV工艺加工后的残余应力水平均大幅低于采用传统热轧平整工艺加工后的残余应力水平。 3) LTP-DSV工艺对高强度级别带钢残余应力的控制能力优于对普通热轧产品残余应力的控制能力。

参考文献
[1] 马鸣图. 我国汽车钢板研究与应用进展[J]. 钢铁. 2001(8): 64-69.
[2] 中国产业竞争情报网. 汽车用钢产品需求特点及生产技术发展趋势分析[Z]. 20091.
[3] 丁文红. 平整机的板形改善机制及其应用[J]. 轧钢. 2012(1): 26-28.

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