1.绪言 在STX-21项目中,本研究是以400MPa级钢为基础开发可焊接的800MPa级高强度钢为目的,讨论了低碳钢组织控制中添加P的效果。 在低碳钢中,由于P对低温韧性产生极差的影响,因此在精炼过程中为去除P而化费了许多精力。另一方面,研究人员目前正在研讨1μm以下的铁素体晶粒的微细化。虽然通过添加0.1mass％左右的P使延性/脆性转移温度提高40K,但是由于铁素体晶粒粒径一旦微细化,转移温度将大幅度下降,所以由于P的缘故所产生的脆化问题,期待通过结晶晶粒的微细化来加以解决。进一步而言,P 1. INTRODUCTION In the STX-21 project, the purpose of this study was to develop a weldable 800MPa grade high-strength steel based on 400MPa grade steel. The effect of adding P to the microstructure control of mild steel was discussed. In mild steel, since P has an extremely poor influence on the low-temperature toughness, much energy has been spent in the refining process to remove P. On the other hand, researchers are currently investigating the refinement of ferrite grains below 1 μm. Although the ductile / brittle transition temperature is increased by 40 K by adding about 0.1 mass% of P, the ferritic crystal grain size becomes finer and the transfer temperature greatly decreases. Therefore, due to the embrittlement problem caused by P, It is expected to be solved by the miniaturization of crystal grains. Further,