Process for producing brake piston

Abstract

A brake piston having required mechanical strength is produced through fewer steps than in conventional processes. An aluminum alloy containing Si and Mg, wherein the aluminum alloy has a Si concentration (C Si , mass %) and a Mg concentration (C Mg mass %) falling within a range surrounded by four points A (0.75, 1.25), B (1.4, 1), C (1.4, 0.6), and D (0.75, 0.85) in terms of a relationship thereof (C Si , C Mg ), and further has a Cu concentration of 0.07 to 0.9 mass %, a Mn concentration of 0.1 to 0.9 mass %, a Ti concentration of 0.005 to 0.15 mass %, a Cr concentration of 0.2 mass % or less, and a Fe concentration of 0.5 mass or less %, the balance being A1 and inevitable impurities is cast to obtain a rod-shaped ingot. The rod-shaped ingot, without being subjected to a homogenization treatment, is straightened within 3 (three) days after the casting. The straightened rod-shaped ingot is cut into a given thickness to obtain a forging raw material. This forging raw material, without being subjected to annealing, is cold-forged at a reduction rate of 25 to 90% within 7 (seven) days after the casting of the rod-shaped ingot into a cup shape, which is subjected to age-hardening without being subjected to a solution heat treatment.

1 . A production method of a brake piston, comprising the steps of:
 casting a rod-shaped ingot of aluminum alloy containing Si and Mg, wherein the aluminum alloy has a Si concentration (C Si  mass %) and a Mg concentration (C Mg  mass %) falling within a range surrounded by four points A (0.75, 1.25), B (1.4, 1), C (1.4 0.6), and D (0.75, 0.85) in terms of a relationship thereof (C Si , C Mg ), and further has a Cu concentration of 0.07 to 0.9 mass %, a Mn concentration of 0.1 to 0.9 mass %, a Ti concentration of 0.005 to 0.15 mass %, a Cr concentration of 0.2 mass % or less, and a Fe concentration of 0.5 mass % or less, the balance being A1 and inevitable impurities;   straightening the rod-shaped ingot within 3 (three) days after the casting without being subjected to a homogenization treatment;   cutting the straightened rod-shaped ingot into a given thickness to obtain a forging raw material;   cold-forging the forging raw material at a reduction rate of 25 to 90% within 7 (seven) days after the casting of the rod-shaped ingot without being subjected to annealing to form a cup-shaped brake piston; and   age-hardening the formed cup-shaped brake piston without being subjected to a solution heat treatment.   
     
     
         2 . The production method of a brake piston as recited in  claim 1 , wherein the rod-shaped ingot is continuously cast at a casting rate of 200 m/min or more. 
     
     
         3 . The production method of a brake piston as recited in  claim 1 ,
 wherein a Rockwell hardness of the continuously cast rod-shaped ingot naturally aged for 30 days after the continuous casting is defined as a reference hardness, and   wherein the straightening of the rod-shaped ingot is performed within a period of time during which a relative hardness of the rod-shaped ingot given by the following equation is 0.9 or less:
   [the relative hardness of the rod-shaped ingot]=[Rockwell hardness of the rod-shaped ingot]/[the reference hardness]. 
   
     
     
         4 . The production method of a brake piston as recited in  claim 1 ,
 wherein a Rockwell hardness of a continuously cast rod-shaped ingot naturally aged for 30 days after the continuous casting is defined as a reference hardness, and   wherein the straightening of the rod-shaped ingot is performed within a period of time during which a relative hardness of the rod-shaped ingot given by the following equation is 0.95 or less:
   [the relative hardness of the rod-shaped ingot]=[Rockwell hardness of the rod-shaped ingot]/[the reference hardness]. 
   
     
     
         5 . The production method of a brake piston as recited in  claim 2 ,
 wherein a Rockwell hardness of the continuously cast rod-shaped ingot naturally aged for 30 days after the continuous casting is defined as a reference hardness, and   wherein the straightening of the rod-shaped ingot is performed within a period of time during which a relative hardness of the rod-shaped ingot given by the following equation is 0.9 or less:
   [the relative hardness of the rod-shaped ingot]=[Rockwell hardness of the rod-shaped ingot]/[the reference hardness]. 
   
     
     
         6 . The production method of a brake piston as recited in  claim 2 ,
 wherein a Rockwell hardness of a continuously cast rod-shaped ingot naturally aged for 30 days after the continuous casting is defined as a reference hardness, and   wherein the straightening of the rod-shaped ingot is performed within a period of time during which a relative hardness of the rod-shaped ingot given by the following equation is 0.95 or less:
   [the relative hardness of the rod-shaped ingot]=[Rockwell hardness of the rod-shaped ingot]/[the reference hardness].