喷射成形新型热作模具钢的组织与性能研究

发布时间：2018-01-15 11:24

本文关键词：喷射成形新型热作模具钢的组织与性能研究　出处：《北京科技大学》2015年博士论文　论文类型：学位论文

【摘要】：H13钢是世界范围内仍广泛使用的热作模具钢,但等向性差和600℃以上热强度不足等问题一直影响其使用寿命和应用范围。等向性差是由传统铸造的低冷速所引起的粗大一次碳化物与合金元素偏析造成的,而热强度不足则主要与合金成分及碳化物种类、数量、分布有关。H13钢含有较多的Cr(-5.0wt%),回火时主要析出热稳定性较低的M7C3型碳化物,该碳化物在600℃以上高温回火时容易聚集长大,从而对高温性能不利。鉴于此,本文采用喷射成形工艺来制备新型热作模具钢,利用其均匀细小的组织优势来改善传统铸造H13钢的等向性,并通过调整合金成分来解决高温强度不足的问题,同时研究了喷射成形工艺与合金元素对合金组织性能的作用规律,主要得出以下结论：对H13钢的合金成分进行了改进,并利用喷射成形技术制备出了一系列高热强性兼具韧性的喷射成形新型热作模具钢。在冲击韧性与H13钢相当的情况下,喷射成形新型热作模具钢的回火抗性、高温强度、热稳定性、高温磨损抗性、热疲劳抗性都明显高于H13钢。尤其是高温强度和热稳定性,其中700℃下的高温抗拉强度平均提高了200MPa左右；而在660℃保温12小时后的硬度更是由H13钢的26.5HRC提高到了33HRC以上。喷射成形新型热作模具钢中最具代表性的合金成分为：Fe-0.37C-0.8Si-0.3Mn-3.0Cr-2.4Mo-1.5V-0.04Nb(wt%),该合金具有最好的高温强度和高温磨损抗性,700℃下高温抗拉强度达到530MPa,比H13钢提高了240MPa,400℃下的高温磨损体积降低到H13钢的1/7左右。确定了适合新型热作模具钢和H13钢的最佳喷射成形工艺参数：浇注温度1600℃～1625℃,雾化压力0.45MPa-0.50MPa。在此工艺下,喷射成形新型热作模具钢与H13钢的沉积态组织为均匀细小的等轴晶组织,而非铸态H13钢中的粗大枝晶组织,并且晶粒细化到20-30μm,消除了粗大一次碳化物,明显改善了碳化物与合金元素偏析,从而有助于提高等向性。探索出了适合喷射成形新型热作模具钢的最佳热处理工艺：淬火温度1040℃-1070℃,回火温度620℃-640℃,保证其具有优异的综合性能。研究发现喷射成形新型热作模具钢的高温强度比H13钢明显提高的原因主要与回火组织中马氏体的回复程度及碳化物的强化效果有关。H13钢在650℃回火时,马氏体已回复完全,基体强度明显下降,并在原马氏体板条界和晶界上析出了较多的120nm左右的Cr7C3和M6C型碳化物,第二相强化效果降低；而喷射成型新型热作模具钢经650℃回火后,基体依然为板条马氏体,板条内较高的位错密度确保了基体较高的强度,同时板条内析出的大量细小弥散的薄片状VC(片长～25nm,片厚～2.5nm)在起到弥散强化作用的同时还钉扎位错,推迟了马氏体的回复,进一步提高了组织稳定性。在合金元素作用规律方面,发现将H13钢中的Cr含量降到3.0wt%,可明显抑制回火时Cr7C3的析出,促进VC析出,从而明显提高回火抗性和700℃的高温强度。增加Mo含量可进一步抑制Cr7C3的析出,促进VC析出并增加其稳定性,进一步增加700℃的高温强度。W和Mo的作用基本类似,虽然W提高700℃高温强度的能力不如Mo,但其提高热稳定性的能力高于Mo,其中添加0.6wt%W的7号合金在660℃保温12h后的硬度最高,仍然保持在37HRC。这是因为添加的W可进到回火二次析出的碳化物中,由于W远低于Mo的扩散速率明显降低了碳化物在高温回火时的粗化速度,从而提高了热稳定性。增加V含量可促进VC析出并增加其稳定性,同时抑制Cr7C3和M6C析出,从而提高700℃的高温强度并抑制淬火时的晶粒长大。V和Mo复合增加不仅抑制Cr7C3的形成,促进VC析出并增加其稳定性,提高700℃的高温强度,而且还增加了淬火时未溶碳化物的含量,从而提高高温磨损抗性,但韧性有所降低。添加0.04wt%Nb就可抑制淬火时的晶粒长大,但当Nb含量增加到0.12wt%时会导致合金沉积态组织中出现粗大一次VC和NbC,加剧合金元素偏析,从而对强韧性不利。综上所述,本论文利用喷射成形技术成功制备出高热强性兼具韧性的新型热作模具钢,明显改善了碳化物与合金元素的偏析并且极大的提高了高温强度,为开发高性能热作模具钢提供了一条新的途径。同时对合金元素Cr、Mo、W、V、Nb作用规律的认识也可为高性能热作模具钢的合金设计提供一定的理论参考。
[Abstract]:H13 steel is widely used in the world are still hot die steel, but other issues to the poor 600 degrees above and lack of hot strength has to affect its service life and application range. To the poor is caused by conventional casting and low cooling rate of coarse primary carbides and segregation of alloy elements caused. The lack of hot strength of species composition and quantity distribution of carbide alloy,.H13 steel containing more Cr (-5.0wt%), M7C3 carbides during tempering stability of the main precipitation in the low heat, the carbide in high temperature above 600 DEG tempering easily gathered long, and on the high temperature performance disadvantage. In view of this, this paper the spray forming process to produce new hot working die steel, with its fine structure to improve the traditional advantages of H13 cast steel to high temperature, and to solve the problem of insufficient strength by adjusting the composition of the alloy were also studied. The effect of spray forming technology and alloying elements on the microstructure and properties of the alloy is mainly drawn as follows:
The alloy composition of H13 steel was improved, and the use of spray forming technology to prepare a series of thermal strength with toughness of spray forming new hot working die steel. The impact toughness of H13 steel and equivalent, spray tempering resistance, forming a new hot working die steel high temperature strength, thermal stability, high temperature the wear resistance and thermal fatigue resistance were significantly higher than that of H13 steel. Especially high temperature strength and thermal stability, the high temperature tensile strength at 700 DEG C increased by about 200MPa; while in the 660 DEG C for 12 hours after the hardness is made of H13 steel 26.5HRC increased to more than 33HRC. Spray forming alloy thermal model composition of die steel in the most representative is: Fe-0.37C-0.8Si-0.3Mn-3.0Cr-2.4Mo-1.5V-0.04Nb (wt%), the alloy has the best high temperature strength and high temperature wear resistance, high tensile strength under 700 DEG C to 530MPa, than H13 steel increased At 240MPa, the high temperature wear volume at 400 C is reduced to about 1/7 of H13 steel.
The optimum spray forming process parameters for the new hot die steel and H13 steel pouring temperature 1600 to 1625 DEG C, atomization pressure of 0.45MPa-0.50MPa. in this process, the microstructure of spray deposition forming new hot working die steel and H13 steel for the uniform and fine equiaxed crystals, rather than casting coarse dendrite microstructure in H13 steel, and the grain refinement to 20-30 m, eliminating the coarse primary carbides, carbides and improve the segregation of alloy elements, which is helpful to improve the other. To explore optimal thermal spray forming model for hot die steel process. The quenching temperature is 1040 DEG -1070 DEG, tempering the temperature of 620 DEG -640 DEG, which has excellent comprehensive performance.
The study found that recovery degree and the carbide causes high strength die steel thermal spray forming was better than the H13 steel and the main body in the tempering on strengthening effect of.H13 steel in 650 DEG C during tempering of martensite has been restored completely, the matrix strength decreased significantly, and the original martensite lath boundaries and grain boundaries. The precipitation of Cr7C3 and M6C carbides more about 120nm, second phase strengthening effect is reduced; and the injection molding new hot working die steel after tempering at 650 C, the matrix is lath martensite lath in high dislocation density matrix to ensure high strength, at the same time a large number of dispersed precipitation of thin slab VC (a ~ 25nm, ~ 2.5nm in thickness) to strengthen the role of diffusion and dislocation pinning, delayed martensite recovery, further improve the stability of structure.
The effect of alloy elements, it is found that Cr content in H13 steel can be reduced to 3.0wt%, the precipitation of Cr7C3 obviously inhibited the tempering, promote the precipitation of VC, can significantly improve the strength at high temperature tempering resistance and 700 DEG C. Precipitation increased the content of Mo can further inhibit Cr7C3, promote VC analysis and further increase the stability. The increase of high temperature of 700 DEG C strength.W and the role of Mo is similar, although the W improve high temperature strength of 700 DEG C ability than Mo, but its ability to improve the thermal stability of Mo is higher than that of 0.6wt%W, the hardness of alloy 7 at 660 DEG C for 12h after the high, remains at 37HRC. this is because the added W can enter into the two times tempering precipitated carbides, because W is far lower than the diffusion rate of Mo significantly decreased the coarsening of carbides during tempering at high speed, so as to improve the thermal stability.
The increase of V content can promote the precipitation of VC and increase its stability, and inhibition of Cr7C3 and M6C precipitation, the formation of grain so as to improve the high temperature strength of 700 DEG C and inhibit the quenching growth of.V and Mo composite increase not only inhibit Cr7C3 and promote the precipitation of VC and increase its stability, improve the strength at high temperature of 700 DEG C, but also increased the content of when quenching undissolved carbide, so as to improve the high temperature wear resistance, but the toughness decreased. Adding 0.04wt%Nb can inhibit the grain growth during quenching, but when the content of Nb increased to 0.12wt% will be a rough VC and NbC resulted in the microstructure of alloy deposition, thus increasing the segregation of alloying elements on the strength and toughness of disadvantage.
In summary, spray forming technology successfully prepared a new heat thermal strength with toughness of die steel in this paper, significantly improve the segregation of carbide and alloy elements and greatly improves the high temperature strength, provides a new way for the development of high performance hot die steel. At the same time, alloy elements Cr, Mo W, V, Nb, understanding the effect of can also provide a theoretical reference for the high performance of hot work die steel alloy design.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

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