Method of forming low-carbon, high toughness, steel plates for railroad tank cars

Abstract

A method of manufacturing a normalized steel alloy plate for a railroad tank car is provided. The steel alloy plate includes a steel alloy including in wt. %: C: 0.1-0.15; Mn: 1.0-1.65; Si: 0.15-0.40; Al: 0.015-0.06; Mo: 0.1-0.3; Ni: 0.1-0.25; Nb: 0.015-0.045; Ti: up to 0.02; Cr: up to 0.22; V: up to 0.08; Cu: up to 0.35; P: max 0.025; S: max 0.015; and N: 0.004-0.01. The alloy plate is normalized for at least 30 minutes at 900° C. The normalized alloy plate may have a tensile strength of at least 560 MPa; a yield strength of at least 345 MPa; a total elongation of at least 22%; a CVN impact toughness of at least 135.5 J at −34.4° C.; a CVN impact toughness of at least 122 J at −45.5° C. The normalized alloy plate may have a ferrite-bainite microstructure, with 10% or less pearlite. The alloy normalized alloy plate may have an absence of any banded ferrite-pearlite/martensite structure.

What is claimed is: 
     
         1 . A method of manufacturing a normalized steel alloy plate for a railroad tank car comprising
 providing an alloy plate comprising a steel alloy including in wt. %: C: 0.1-0.15; Mn: 1.0-1.65; Si: 0.15-0.40; Al: 0.015-0.06; Mo: 0.1-0.3; Ni: 0.1-0.25; Nb: 0.015-0.045; Ti: up to 0.02; Cr: up to 0.22; V: up to 0.08; Cu: up to 0.35; P: max 0.025; S: max 0.015; and N: 0.004-0.01, a balance being Fe;   normalizing the alloy plate for at least 30 minutes at 900 degrees C.;   the normalized alloy plate having a tensile strength of at least 560 MPa;   the normalized alloy plate having a yield strength of at least 345 MPa;   the normalized alloy plate having a total elongation of at least 22%;   the normalized alloy plate having a CVN impact toughness of at least 135.5 J at −34.4 degrees C.;   the normalized alloy plate having a CVN impact toughness of at least 122 J at −45.5 degrees C.; and   the normalized alloy plate having a ferrite-bainite microstructure with 10% or less pearlite.   
     
     
         2 . A method of manufacturing a railroad tank car, comprising:
 manufacturing a normalized steel alloy plate according to claim  1 ; and   manufacturing a railroad tank car including said normalized steel alloy plate.   
     
     
         3 . The method as recited in  claim 1  wherein said ferrite-bainite microstructure has 5% or less pearlite. 
     
     
         4 . The method as recited in  claim 1  wherein said ferrite-bainite microstructure has 1% or less pearlite. 
     
     
         5 . The method as recited in  claim 1  wherein:
 the normalized alloy plate has a tensile strength of at least 575 MPa; 
 the normalized alloy plate has a yield strength of at least 425 MPa; and 
 the normalized alloy plate has a total elongation of at least 33%. 
 
     
     
         6 . The method as recited in  claim 4  wherein:
 the normalized alloy plate has a CVN impact toughness of at least 176.2 J at −34.4 degrees C.; and 
 the normalized alloy plate has a CVN impact toughness of at least 203 J at −45.5 degrees C. 
 
     
     
         7 . The method as recited in  claim 1  wherein:
 the normalized alloy plate has a tensile strength of at least 580 MPa; 
 the normalized alloy plate has a yield strength of at least 460 MPa; and 
 the normalized alloy plate has a total elongation of at least 33%. 
 
     
     
         8 . The method as recited in  claim 6  wherein:
 the normalized alloy plate has a CVN impact toughness of at least 156 J at −34.4 degrees C.; and 
 the normalized alloy plate has a CVN impact toughness of at least 128.8 J at −45.55 degrees C. 
 
     
     
         9 . The method as recited in  claim 1  further comprising subjecting the normalized alloy plate to a post weld heat treatment of 30 to 60 mins at 600 to 650 degrees C. 
     
     
         10 . The method as recited in  claim 1  wherein the steel alloy includes, by weight Mn: 1.3-1.5. 
     
     
         11 . Th method as recited in  claim 1  wherein the tensile strength is 560>=TS>=695, MPa. 
     
     
         12 . The method as recited in  claim 1  wherein the steel alloy includes, by weight, 0.032-0.045 Nb. 
     
     
         13 . Th method as recited in  claim 1  wherein the normalized alloy plate has a tensile strength of at least 580 MPa; a yield strength of at least 460 MPa; an elongation of at least 33%; a CVN impact toughness of at least 156 J at −34.4 degrees C.; and a CVN impact toughness of at least 128.8 J at −45.5 degrees C. 
     
     
         14 . The method as recited in  claim 1  wherein the steel alloy includes 0.045 wt. % Nb. 
     
     
         15 . The method as recited in  claim 1  wherein the normalized alloy plate is 22 mm thick, and the ferrite-bainite microstructure consists of ferrite, bainite and optionally 10% or less pearlite. 
     
     
         16 . The method as recited in  claim 14  wherein the tensile strength is 559>=TS>=580. 
     
     
         17 . The method as recited in  claim 14  wherein the normalizing includes normalizing the alloy plate for 30 to 60 minutes at 900 degrees C. 
     
     
         18 . The method as recited in  claim 14  wherein the normalizing includes normalizing the alloy plate for 30 minutes at 900 degrees C. 
     
     
         19 . The method as recited in  claim 11  wherein the tensile strength is 559>=TS>=580. 
     
     
         20 . The method as recited in  claim 17  further comprising subjecting the normalized alloy plate to a post weld heat treatment of 30 to 60 mins at 600 to 650 degrees C. 
     
     
         21 . The method of  claim 1 , wherein the ferrite-bainite microstructure lacks any banded ferrite-pearlite/martensite structure.