Ferrite low-temperature steel forging and low-temperature cryogenic preparation method thereof

Abstract

The invention provides a ferrite low-temperature steel forging and a low-temperature cryogenic preparation method thereof, and belongs to the technical field of alloy machining. The preparation methodcomprises the following steps that a casting blank is subjected to homogenizing annealing and then sequentially subjected to quenching, first-time tempering, stepped cryogenic treatment and second-time tempering, and the ferrite low-temperature steel forging is obtained. Due to stepped segmented cryogenic treatment, blocky retained austenite can be prevented from appearing in a final-state structure, the adverse effect of the blocky retained austenite on toughness is eliminated, the risks of cracking and deformation of low-temperature steel are avoided, and formed martensite with enriched solute atoms has the advantages on nucleation and final form distribution of reverse-transformation austenite in the second-time tempering process. The result of the embodiment shows that the yield strength of the low-temperature steel forging prepared through the method is 560-790 MPa, the tensile strength is 750-860 MPa, the elongation is 17-26%, and the -196 DEG C V-shaped notch impact energy is 225-270J.

Description

technical field [0001] The invention relates to the technical field of alloy processing, in particular to a ferritic low-temperature steel forging and a low-temperature cryogenic preparation method thereof. Background technique [0002] Large-scale castings and forgings are widely used in nuclear power, marine equipment, large-scale low-temperature wind tunnels and other fields, and their production technology is the embodiment of the country's manufacturing capacity. As a thick plate material for liquefied natural gas (LNG) storage vessels or transport ships, low-temperature ferritic 9Ni steel has been extensively and systematically studied, and its toughening mechanism has been basically clarified, that is, reverse transformed austenite as a tough phase mainly through Three aspects to improve low temperature (-196°C) toughness: (1) Adding additional ferrite / austenite interface to increase the number of crack transitions or propagation energy; (2) Transformation-induced pla...

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Problems solved by technology

But as a large-scale low-temperature steel forging, due to its large cross-section, there are new problems in the "controlling" and "controlling" aspects of forging and heat treatment: 1. When the minimum critical cooling rate of the forging core is possible, theoretically Obtain tempered martensite or bainite structure with multi-level structure to ensure the impact toughness of the quenched + tempered state at -196°C; II. To avoid the appearance of a large number of irregular retained austenite or M-A islands , to control the reverse transformation of austenite in the original austenite grain boundary or martensite lath interface uniform precipitation in strips, in addition to the introduction of high temperature tempering in the critical zone, cryogenic treatment is also required to ensure that the massive retained austenite as much as possible Decompose completely before final tempering

In the prior art, 9Ni steel low-temperature forgings are mainly processed by liquid nitrogen cryogenic cooling, but the energy consumption of liquid nitrogen cryogenic cooling is high, which greatly increases the production cost, and at the same time, the temperature of liquid nitrogen cryogenic cooling is low (-196 ℃~-150℃), and the uncontrollable continuous cooling makes the treated 9Ni steel low-temperature forgings have a tendency to crack, which poses hidden dangers to the safety of related components

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