Effect of Stepwise Cooling on the Microstructure and Mechanical Properties of a 1300 MPa‐Grade High‐Strength Steel for Engineering Machinery

Abstract

This study investigated a 1300 MPa‐grade engineering‐machinery steel processed by four stepwise cooling routes with intermediate cooling temperatures of 800 or 850 °C and final cooling transition temperatures of 500 or 650 °C. The cooling route markedly affected the bainite/martensite transformation and microstructure. The 800–500 sample, corresponding to intermediate and final cooling transition temperatures of 800 and 500 °C, respectively, showed the lowest bainite fraction and highest martensite fraction. It achieved the highest ultimate tensile strength, yield strength, and total elongation. This sample also exhibited the smallest average grain size, highest KAM value and GND density, and finest martensite laths. Misorientation analysis and representative prior‐austenite grain reconstruction showed Kurdjumov–Sachs‐related variants and packet/block interfaces in all samples. The 800–500 sample had weaker variant selection and a more uniform distribution of packet/block‐related crystallographic units, indicating more uniform crystallographic subdivision. The improved strength–ductility balance is mainly attributed to the higher martensite fraction, refined microstructure, enhanced dislocation‐related strengthening, and more uniform crystallographic subdivision.