Niobium-containing steel for axles of motor train unit and heat treatment process of niobium-containing steel

The invention relates to niobium-containing steel for axles of a motor train unit and a heat treatment process of the niobium-containing steel. The niobium-containing steel is prepared from the following components in percentage by weight: 0.24 to 0.30 percent of C, 0.20 to 0.40 percent of Si, 0.70 to 1.00 percent of Mn, 0.90 to 1.20 percent of Cr, 0.70 to 1.30 percent of Ni, 0.20 to 0.30 percent of Mo, 0.10 to 0.60 percent of Cu, 0.01 to 0.04 percent of Zr, 0.015 to 0.050 percent of Nb, 0.001 to 0.005 percent of Ca, smaller than or equal to 0.010 percent of P, smaller than or equal to 0.008 percent of S, 0.0040 to 0.0060 percent of [N], smaller than or equal to 0.015 percent of T[O], 0.015 to 0.045 percent of Als, and the balance being Fe and inevitable impurities. The structure of the steel is tempered sorbate plus a little part of bainite, wherein the content of the tempered sorbate on an axle near surface is 100 percent, and the content of the tempered sorbate at one second radius of an axle is 75 to 85 p