Method for deriving cooling time when quenching steel material, method for quenching steel material, and method for quenching and tempering steel material

Abstract

Provided are a method for deriving a cooling time when quenching a steel material, a method for quenching a steel material and a method for quenching and tempering a steel material. The method for deriving a cooling time comprises: step A of cooling a test specimen heated to a cooling initiation temperature Ts under a cooling condition C and measuring the cooling time t until the temperature of a center portion of the test specimen reaches a cooling termination temperature Te; step B of determining a function f which is defined by t=f(V/S) and which is satisfied by data including the volume V, the surface area S and the cooling time t of the test specimen obtained in step A; and step C of substituting the volume V1 and the surface area S1 of the shape F1 of a steel material to be actually quenched in the relational expression t=f(V/S), and calculating a cooling time t1. The method for quenching a steel material comprises: actually heating the steel material to be quenched to the cooling initiation temperature Ts; cooling only for the cooling time t1 under the cooling condition C; and then stopping cooling under the cooling condition C. The method for quenching and tempering a steel material comprises tempering the steel material quenched by this method.

1 . A method for deriving a cooling time length in advance of quenching of a steel material, the cooling time length being from a start of the cooling of the steel material heated at a cooling start temperature Ts under a cooling condition C until the cooling is stopped, the method comprising:
 a first step of preparing n pieces of specimens, wherein n is a natural number of two or more, made of a same steel type A as the steel material to be quenched and having different volumes V and different surface areas S with each other, and then making a hole at each of the specimens for inserting a temperature sensor at a central portion of the specimen;   a second step of inserting the temperature sensor in the hole of each specimen, and subjecting each of the n specimens to a test, wherein the test comprises:
 heating the specimen and maintain at a cooling start temperature Ts; 
 starting cooling of the specimen under the same cooling condition C; and 
 measuring a cooling time length t from the start of the cooling until the temperature at the central portion of the specimen measured by the temperature sensor reaches a cooling stop temperature Te; 
   a third step of determining a function f satisfying Relational Expression 1: t=f (V/S), from n sets of data obtained in the second step, each set of data including the volume V, the surface area S and the cooling time length t; and   a fourth step of substituting a volume V 1  and a surface area S 1  of a shape F 1  of the steel material to be quenched in the Relational Expression 1 to calculate a cooling time length t 1 .   
     
     
         2 . A method for quenching a steel material using the cooling time length t 1  derived by the method according to  claim 1 , comprising
 cooling the steel material heated at the cooling start temperature Ts for the cooling time length t 1  under the cooling condition C, and   then stopping the cooling.   
     
     
         3 . The method according to  claim 2 , wherein the cooling includes multiple cooling stages having different cooling conditions with each other, and the function f for each cooling stage is determined for deriving the cooling time length of each cooling stage. 
     
     
         4 . A method for quenching and tempering a steel material, comprising tempering the steel material quenched by the method according to  claim 2 . 
     
     
         5 . The method according to  claim 4 , wherein the cooling in the quenching includes multiple cooling stages having different cooling conditions with each other, and the function f for each cooling stage is determined for deriving the cooling time length of each cooling stage.