Research on Hardenability in High-purity Steels

本文研究了除碳外不含其他合金元素的高纯碳钢中P、S等杂质元素对淬透性和等温转变行为的影响。4种P、S含量不同的0.35%碳钢（如：P、S）（10 ppmP + 10 ppmS, 70 ppmP + 10 ppmS， 410 ppmP + 10 ppmS和10 ppmP + 100 ppmS）真空熔化并用于Jominy试验，连续转化试验和等温转化试验。得到的主要结果如下：(1)随着样品磷含量的增加，连接距离增大。连接距离随P含量的增加呈抛物线趋势，即连接距离- P含量曲线的斜率随P含量的增加而减小。(2)与马氏体相变竞争并控制淬透性的扩散相变是820 K左右的珠光体相变。(3) P和S的加入明显延缓了奥氏体向珠光体的等温转变动力学。(4)P和S的加入显著降低了823 K时珠光体集落的生长速率，但对成核速率几乎没有影响。结果表明，在0.35%高纯碳钢中加入P、S等杂质元素可提高淬透性，其原因是珠光体转变的生长速度减慢，而珠光体转变与马氏体转变相竞争。

The present study has been undertaken to make clear the effect of impurity elements such as P and S on hardenabilty and isothermal transformation behavior in high-purity carbon steels without other alloying elements excepted for carbon. Four 0.35% carbon steels with different level of P and S contents (i,e., 10 ppmP + 10 ppmS, 70 ppmP + 10 ppmS, 410 ppmP + 10 ppmS, and 10 ppmP + 100 ppmS) were vacuum-melted and used for the Jominy tests, the continuous transformation tests, and the isothermal transformation tests. The main results obtained are as follows.(1) The Jominy distance increased with increase in the P content of specimen. The tendency of increase in the Jominy distance against the P content was parabolic, namely, the slope of the Jominy distance-the P content curve decreased with increase in the P content.(2) The diffusional transformation which competes with martensite transformation and controlls the hardenabilty in the present steels was the pearlite transformation at around 820 K.(3) The isothermal transformation kinetics from austenite to pearlite was markedly retarded by adding the impurity element of P as well as S.(4) The addition of the impurity elements of P and S strongly reduced the growth rate of the pearlite colony at 823 K but scarcely affected the nucleation rate.It can be concluded from these results that the hardenabilty increases by adding the impurity elements of P and S to high-purity 0.35% carbon steel and this increase in the hardenability is contributed to the decrease in the growth rate of the pearlite transformation which competes with the martensite transformation.