Case Hardening Steel Tube Having Improved Workability and a Process for its Manufacture

Abstract

A case hardening steel tube which has a hardness of 72-80 HRB and which gives a carburized layer with a high strength and high wear resistance and adequate resistance to impact fracture when it is formed into a final product by working and subsequent carburizing and quenching under relatively mild conditions is manufactured by forming a tube from a steel having a steel composition comprising, in mass percent, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: at most 0.02%, S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: less than 0.006%, and a remainder essentially of Fe, then subjecting the resulting steel tube to normalizing by soaking at a temperature of 880-980° C. followed by cooling at a cooling rate of at most 70° C. per minute, carrying out cold working of the normalized steel tube, and then annealing the cold worked steel tube at a temperature of 700-820° C.

1 . A process for manufacturing a case hardening steel tube characterized in that the process comprises producing a tube from a steel having a steel composition comprising, in mass percent, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: at most 0.02%, S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: less than 0.006%, and a remainder essentially of Fe, subjecting the resulting steel tube to normalizing by holding at a temperature of 880-980° C. and then cooling at a cooling rate of at most 70° C. per minute in a temperature range of 880-400° C., performing cold working of the normalized steel tube, and then annealing the cold worked steel tube at a temperature of 700-820° C. 
   
   
       2 . A process as set forth in  claim 1  wherein the steel composition contains at least one element selected from the following (1) and (2):
 (1) in mass percent, Ni: 0.3-4.0%   (2) in mass percent, at least one element selected from Ti: 0.01-0.3%, Nb: 0.01-0.3%, V: 0.01-0.3%, and Zr: 0.01-0.3%.   
   
   
       3 . A process as set forth in  claim 1  or  claim 2  wherein the B content in the steel composition is B: 0.0005-0.003%. 
   
   
       4 . A cold worked, case hardening steel tube characterized by having a steel composition comprising, in mass percent, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: at most 0.02%, S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: less than 0.006%, and a remainder essentially of Fe, and by having a steel structure selected from (a) a mixed structure of ferrite+pearlite+spheroidized cementite and (b) a mixed structure of ferrite+spheroidized cementite. 
   
   
       5 . A case hardening steel tube as set forth in  claim 4  wherein the steel composition contains at least one element selected from the following (1) and (2):
 (1) in mass percent, Ni: 0.3-4.0%   (2) in mass percent, at least one element selected from Ti: 0.01-0.3%, Nb: 0.01-0.3%, V: 0.01-0.3%, and Zr: 0.01-0.3%.   
   
   
       6 . A case hardening steel tube as set forth in  claim 4  or  claim 5  wherein the B content in the steel composition is B: 0.0005-0.003%. 
   
   
       7 . A case hardening steel tube as set forth in  claim 4  or  claim 5  wherein the tube has a Rockwell B scale hardness (HRB) of 72-80.