Method for producing low-carbon cold rolled steel sheet having excellent cold working properties and an apparatus for continuous treatment thereof

Abstract

In the production of low-carbon steel, particularly low-carbon steel sheet containing less than 0.25% of Mn, improvements in the method and apparatus, which comprises hot rolling a steel having a controlled range content of Mn, O and S contents, then coiling thus hot rolled steel at temperatures between 600°-800° C., further cold rolling the hot rolled plate down to a prescribed thickness, annealing and overaging the steel in a continuous furnace, rapidly cooling the steel near room temperatures and skin pass rolling, leveling, and recoiling the sheet. Thus produced steel sheet is very useful for applications such as pressed automobile body parts which require good workability particularly good drawability.

What is claimed is: 
     
       1. A process for producing a cold-rolled low-carbon steel sheet having excellent cold-working properties, comprising the steps of .Iadd.adjusting the amounts of Mn, O, S in a low-carbon steel containing less than 0.25% Mn so as to satisfy the following relation: ##EQU4## hot rolling the steel, coiling the hot rolled steel sheet at temperatures between 600° to 800° C., cold rolling the hot rolled steel sheet, .Iaddend.feeding .[.a.]. .Iadd.said .Iaddend.low-carbon steel sheet which has been reduced to the product gage .[.after having been hot and cold rolled in the conventional way.]. continuously through a continuous annealing furnace consisting of heating, soaking and overaging chambers, heating the steel sheet in the heating chamber to a temperature in the range of 700° to 900°, feeding the thus heated steel sheet directly from the heating chamber into and through the soaking chamber so that the sheet is in the soaking chamber at a temperature in the above range for a time required for recrystallization and growth of crystal grain in the steel sheet, rapidly cooling the steel sheet coming from the soaking chamber to an overaging temperature at a rate so that the carbon contained in the steel sheet is in a super-saturated solid solution before overaging, feeding the thus cooled .[.and.].  steel sheet directly into and through the overaging chamber so that the sheet is in the overaging chamber for a time not exceeding 8 minutes for precipitating carbon contained in the steel sheet and fixing it as a carbide so as to give the steel excellent cold working property, cooling the thus overaged steel sheet coming from the overaging chamber down to less than 50° C. and directly feeding the steel sheet through a skin pass mill to subject it to temper rolling and then directly feeding the temper rolled steel sheet to a leveller for leveling it before it is recoiled. 
     
     
       2. A process according to claim 1 in which the low-carbon steel sheet contains less than 0.20% Mn. 
     
     
       3. A process for producing a low-carbon steel sheet having excellent cold working properties which comprises subjecting a low-carbon steel sheet which has been hot rolled and cold rolled in an ordinary way continuously to the steps of recrystallization annealing by continuously passing the steel sheet through a heating chamber for heating the cold rolled steel sheet to a temperature in the range of 700° C. to 900° C. within 2 minutes and passing the steel sheet through a soaking chamber for maintaining the sheet in the above temperature range for less than two minutes, primary cooling to an overaging temperature by continuously passing the steel sheet through a primary cooling chamber for rapidly cooling the sheet from the above temperature range to an overaging temperature at a rate of 5° to 30° C./sec., overaging at the overaging temperature by continuously passing the steel sheet through an overaging chamber for maintaining the sheet between 300° to 450° C. for up to eight minutes, secondary cooling to a temperature below 50° C. by passing the steel sheet through a secondary cooling chamber for cooling the sheet from the above overaging temperature to below 50° C. within 2 minutes, skin pass rolling and leveling. 
     
     
       4. A process as claimed in claim 3 in which the primary cooling step comprises jetting cooling medium into said primary cooling chamber for rapid cooling of the sheet from a temperature range of 700° to 900° C. to a temperature range of 300° to 450° C. at a rate of less than 20° C./sec. 
     
     
       5. A process as claimed in claim 3 in which the secondary cooling step comprises jetting cooling gas into a portion of said secondary cooling chamber for cooling the sheet from the overaging temperature to 100° C. and refrigerating the remainder of said secondary cooling chamber for further cooling the sheet below 100° C. 
     
     
       6. A process for producing a low-carbon steel sheet, which comprises adjusting the amounts of Mn, O, S in a low-carbon steel containing less than 0.25% Mn so as to satisfy the following relation: ##EQU5## hot rolling the steel, coiling the hot rolled steel sheet at temperatures between 600° to 800° C., cold rolling the hot rolled steel sheet, and subjecting the thus cold rolled steel sheet continuously to the steps of recrystallization annealing, primary cooling to an overaging temperature, overaging at the overaging temperature, .Iadd.and .Iaddend.secondary cooling to a temperature below 50° C., skin pass rolling and leveling. 
     
     
       7. A process according to claim 6 in which the coiling temperature of the hot rolled steel sheet is between 675° and 800° C. .Iadd. 8. A process for producing a cold-rolled low carbon steel sheet having excellent cold-working properties, comprising the steps of adjusting the amounts of Mn, O, S in a low-carbon steel containing less than 0.25% Mn so as to satisfy the following relation: ##EQU6## hot rolling the steel, coiling the hot rolled steel sheet at temperatures between 600° to 800° C., cold rolling the hot rolled steel sheet, feeding said low-carbon steel sheet which has been reduced to the product gage continuously through a continuous annealing furnace consisting of heating, soaking and overaging chambers, heating the steel sheet in the heating chamber to a temperature in the range of 700° to 900°, feeding the thus heated steel sheet directly from the heating chamber into and through the soaking chamber so that the sheet is in the soaking chamber at a temperature in the above range for a time required for recrystallization and growth of crystal grain in the steel sheet, rapidly cooling the steel sheet coming from the soaking chamber to an overaging temperature at a rate so that the carbon contained in the steel sheet is in a super-saturated solid solution before overaging, feeding the thus cooled steel sheet directly into and through the overaging chamber so that the sheet is in the overaging chamber for a time not exceeding 8 minutes for precipitating carbon contained in the steel sheet and fixing it as a carbide so as to give the steel excellent cold working property, cooling the thus overaged steel sheet coming from the overaging chamber down to less that 50° C., and skin pass rolling and leveling. .Iaddend.