Table 1.

The chemical compositions of the four types of experimental steels with different niobium contents (wt.%).

Steel	C	Si	Mn	P	S	Nb	Ni	Cr	Mo	Ti	Cu
No.8	0.047	0.20	1.59	0.009	0.006	0.035	0.615	0.413	0.249	0.015	0.29
No.9	0.05	0.21	1.55	0.008	0.006	0.069	0.596	0.410	0.252	0.014	0.30
No.10	0.05	0.19	1.54	0.008	0.007	0.084	0.600	0.430	0.240	0.011	0.343
No.11	0.05	0.197	1.56	0.009	0.007	0.107	0.582	0.450	0.241	0.013	0.336