Manufacturing method of high-strength and high-toughness thin steel and heat treatment apparatus

Abstract

To provide a technique suitable for elevating strength and toughness of a thin low-carbon steel. By performing rapid heating and rapid cooling to a thin low-carbon steel which is an ordinary steel with a thickness of 1.2 mm or less, a steel where a microstructure becomes a duplex grain size structure mixed with crystal grains having different grain diameters, which is not homogeneous, preferably, hard phase structures are contained in addition to the duplex grain size structure is obtained, and a high-strength and high-toughness thin low-carbon steel is obtained. Further, by performing a heat treatment process involving rapid heating and rapid cooling multiple times, a duplex grain size structure of crystal grains with smaller grain diameters or a hard phase structure contained therein is obtained, so that a thin low-carbon steel with higher strength and higher toughness is obtained.

1 . A method for manufacturing a high-strength and high-toughness thin steel by heat-treating a steel raw material, comprising:
 using a thin low-carbon steel which is an ordinary steel worked to have a thickness of 1.2 mm or less as the steel raw material which is a reception material of heat treatment;   a first process of rapidly cooling the thin low-carbon steel after rapid heating thereof to obtain a martensite structure; and   a second process of rapidly cooling the thin low-carbon steel which have been subjected to the first process after rapidly reheating the same to a temperature lower than a temperature at a rapid heating time in the first process, wherein   the first process and the second process are implemented while the thin low-carbon steel is being relatively moved to each heating section and each cooling section which perform rapid heating and rapid cooling treatments in the first process and the second process;   the first process includes a step of rapidly heating the thin low-carbon steel up to a temperature of 1000° C. or higher at a rate of 300° C./second or more and a step of rapidly cooling the thin low-carbon steel at a rate of 300° C./second or more after the thin low-carbon steel is held at a temperature of 900° C. or higher within ten seconds; and   the second process includes a step of rapidly heating the thin low-carbon steel up to a temperature of 700° C. or higher at a rate of 300° C./second or more after the cooling in the first process and a step of rapidly cooling the thin low-carbon steel at a rate of 300° C./second or more after the thin low-carbon steel is held at a temperature of 600° C. or higher within ten seconds.   
     
     
         2 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein the rapid heating in the first process and the rapid heating in the second process are implemented by high-frequency induction heating. 
     
     
         3 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein the rapid heating in the first process and the rapid heating in the second process are implemented by laser heating. 
     
     
         4 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein the treatments in the first process and the second process are performed multiple times. 
     
     
         5 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein C content of the thin low-carbon steel is in a range of 0.01 to 0.12% by mass % and the rest thereof is composed of iron and inevitable impurities. 
     
     
         6 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein
 rapid heating is performed up to a temperature in a range of 1000° C. to 1250° C. at the rapid heating step in the first process and   rapid heating is performed up to a temperature in a range of 750° C. to 1050° C. at the rapid heating step in the second process.   
     
     
         7 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein a holding time before the rapid cooling after the rapid heating in the first process is set within five seconds, and the holding time before the rapid cooling after the rapid heating in the second process is set within five seconds. 
     
     
         8 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein a heat treatment apparatus which treats the thin low-carbon steel is provided with a first heating section which performs the rapid heating treatment in the first process, a first cooling section which performs the rapid cooling treatment in the first process, a second heating section which performs the rapid heating treatment in the second process, and a second cooling section which performs the rapid cooling treatment in the second process, and
 the thin low-carbon steel is sequentially treated in the first heating section, the first cooling section, the second heating section, and the second cooling section.   
     
     
         9 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 8 , wherein the first heating section and the second heating section are composed of one heating section having a predetermined length extending in a moving direction, and the cooling treatment in the first process and the cooling treatment in the second process can be performed to the thin low-carbon steel which is a target to be treated from opposite faces thereof. 
     
     
         10 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein, when the thin low-carbon steel is pipe-shaped, treatment is performed while the thin low-carbon steel is being rotated. 
     
     
         11 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein the steel raw material which is a reception material of heat treatment is a thin low-carbon steel with a thickness of 1.0 mm or less. 
     
     
         12 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein the steel raw material which is a reception material of heat treatment is a thin low-carbon steel with a thickness of 0.8 mm or less 
     
     
         13 . The method for manufacturing a high-strength and high-toughness thin steel according to  claim 1 , wherein the steel raw material which is a reception material of heat treatment is a thin low-carbon steel with a thickness of 0.5 mm or less. 
     
     
         14 . A heat treatment apparatus which uses a thin low-carbon steel which is an ordinary steel which has been worked to have a thickness of 1.2 mm or less as the steel raw material which is a reception material of heat treatment and which is used to manufacture a high-strength and high-toughness thin steel by a first process of rapidly cooling the thin low-carbon steel after rapid heating thereof to obtain a martensite structure and a second process of rapidly cooling the thin low-carbon steel which have been subjected to the first process after rapidly reheating the same to a temperature lower than a temperature at a rapid heating time in the first process, wherein
 a work supporting section which supports the thin low-carbon steel which is a target to be treated, a first heating section which performs the rapid heating treatment in the first process, a first cooling section which performs the rapid cooling treatment in the first process, a second heating section which performs the rapid heating treatment in the second process, and a second cooling section which performs the rapid cooling treatment in the second process are sequentially arranged; and   the first heating section, the first cooling section, the second heating section, and the second cooling section are provided so as to be movable relative to the work supporting section.   
     
     
         15 . The heat treatment apparatus according to  claim 14 , wherein one heating section having a predetermined length extending in a moving direction, the first cooling section arranged on the side opposite to the heating section via the work, and the second cooling section arranged on the same side as the heating section or on the same side as the first cooling section via the work to be separated from the heating section or the first cooling section by a predetermined distance rearward in the moving direction are provided, and the heating section has such a length that a vicinity of a front portion thereof corresponds to the first cooling section and a vicinity of a rear portion thereof extends rearward in the moving direction beyond the first cooling section; and
 the heating section is configured to have two functions such that the vicinity of the front portion of the heating section has a function of the first heating section which performs the rapid heating in the first process and the vicinity of the rear portion of the heating section has a function of the second heating section which performs the rapid heating in the second process.   
     
     
         16 . The heat treatment apparatus according to  claim 15 , wherein the second cooling section is disposed on the same side as the heating section. 
     
     
         17 . The heat treatment apparatus according to  claim 14 , wherein the work supporting section is rotatably provided in a supporting state of the thin low-carbon steel. 
     
     
         18 . The heat treatment apparatus according to  claim 14 , wherein each of the heating sections includes a coil performing high-frequency induction heating. 
     
     
         19 . The heat treatment apparatus according to  claim 14 , wherein each of the heating sections is provided with a laser performing laser heating.