20 tensile tests for Chinese pressure vessel steel Q345R were carried out at room temperature and dimensionless stress -strain curves were obtained,and fitted by Ramberg-Osgood equation.A lower limits dimensionless curve was obtained base on discussing their variance .A curve of external pressure stress factor B was given with the stability safety factor(m=3.0).The allowable external pressures calcu-lated by the proposed curve will be the same as those from the current standard GB 150—2011 in linear elastic stage,but in the nonlinear stage,they are about 1.2 times of GB 150—2011 results and are similar to the results from ASME Ⅷ-1.A new external pressure stress factor B curve with (variable m) was dis-cussed and it reflected the idea of variable stability safety factor .In linear elastic stage,the calculation re-sults are the same as those from the standard GB 150—2011,but in nonlinear stage,the calculated allow-able external pressure will be significantly higher than those from current standard GB 150—2011,and it is similar to the results from ASME Ⅷ-2 and EN 13445 standards.% 对国产Q345R材料的20个试样进行了常温拉伸试验，得到了相关的无因次修约曲线，并采用Ramberg－Osgood方程进行拟合。在讨论其离散性的基础上，得到了一条包络下限的无因次修约曲线。在考虑了稳定安全系数m＝3．0的基础上，得到外压应力系数B曲线（m＝3．0）。用该曲线进行设计时，在线弹性阶段，计算结果与GB 150—2011相同；而在非线性阶段，较GB 150—2011所得，许用外压高1．2倍左右，与ASME Ⅷ－1相近。同时，探讨了一种新型变m外压应力系数B曲线，体现了变稳定安全系数的设计思想，在线弹性阶段，计算结果与GB 150—2011相同，在非线性阶段，计算所得的许用外压明显高于GB 150—2011结果，与ASME Ⅷ－2和EN 13445所得相当。
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