Method of hot forming a steel blank and the hot formed part

Abstract

A method of hot forming a steel blank into an article the method including the following steps: d) cooling a heated steel blank to form an article during hot forming, starting at a starting temperature T 2 above Ar3; to an interrupt temperature T 3 in the range of 400-550° C. at a cooling rate V 2 of at least 25° C./s; e) immediately further cooling the article from the interrupt temperature T 3 to ambient temperature at a cooling rate V 3 of 0.2-10° C./s, wherein the interrupt temperature T 3 and cooling rate V 3 are selected such that the article thus obtained has multiphase microstructure including by volume fraction: 55-90% of bainitic ferrite 5-15% of retained austenite 5-30% martensite.

1 . Method of hot forming a steel blank into an article, the method comprising the following steps:
 d) cooling a heated steel blank to form an article during hot forming, starting at a starting temperature T 2  above Ar3 to an interrupt temperature T 3  in the range of 400-550° C. at a cooling rate V 2  of at least 25° C./s,   wherein the blank has the following composition in weight %:
 C: 0.15-0.45 
 Si: 0.6-2.5 
 Mn: 1.0-b  3 . 0   
 Al: 0-1.5 
 Mo: 0-0.5 
 Cr: 0-1.0 
 P: 0.001-0.05 
 S: <0.03 
 Ca: <0.003 
 Ti: <0.1 
 V: <0.1 
   the balance being Fe and inevitable impurities,   wherein Si+Al=1.2-2.5%;   e) without holding the blank for a predetermined time at a temperature of T 3  immediately further cooling the article from the interrupt temperature T 3  to ambient temperature at a cooling rate V 3  of 0.2-10° C./s,   wherein the interrupt temperature T 3  and cooling rate V 3  are selected using the relation that the higher T 3  is the lower V 3  is, such that the article thus obtained has a multiphase microstructure comprising by volume fraction:
 55-90% of bainitic ferrite 
 5-15% of retained austenite 
 5-30% martensite. 
   
     
     
         2 . Method according to  claim 1 , wherein the blank is produced from a steel strip or sheet. 
     
     
         3 . Method according to  claim 1 , wherein Mn+Cr≦3% and C+⅓ Mo≦0.45%. 
     
     
         4 . Method according to  claim 1 , wherein the composition comprises, in weight %:
 C: 0.2-0.4 and/or   Si: 0.8-2.0 and/or   Mn: 1.5-2.5 and/or   Mo: 0.05-0.5 and/or   Cr: 0.05-1.0 and/or   P: 0.005-0.05 and/or   Ca: 0.0003-0.003.   
     
     
         5 . Method according to  claim 1 , wherein the composition comprises, in weight %:
 C: 0.2-0.35 and/or   Si: 1.2-1.8 and/or   Mn: 1.7-b  2 . 4 .   
     
     
         6 . Method according to  claim 1 , wherein the bainitic ferrite is essentially carbide free and the retained austenite is carbon enriched. 
     
     
         7 . Method according to  claim 1 , wherein the grains of bainitic ferrite have a length of at most 15 μm and a thickness of at most 0.3 μm. 
     
     
         8 . Method according to  claim 1 , further comprising—prior to the hot forming step d)—the steps of
 a) heating the steel blank to an austenitizing temperature T 1  above Ac3; 
 b) soaking the steel blank in said range; 
 c) optionally transferring the heated and soaked blank to a hot forming facility. 
 
     
     
         9 . Method according to  claim 8 , wherein step a) is performed in a continuous annealing facility, or a hot forming facility, or a salt bath or equivalent. 
     
     
         10 . Method according  claim 1 , wherein step d) is performed in a hot forming facility. 
     
     
         11 . Method according to  claim 1 , wherein step e) is performed outside a hot forming facility in air. 
     
     
         12 . Steel article hot formed according to the method of  claim 1 , wherein the steel has a microstructure comprising by volume fraction:
 55-90% of bainitic ferrite   5-15% of retained austenite   5-30% martensite, and   
       wherein the steel article has the following composition in weight %:
 C: 0.15-0.45 
 Si: 0.6-2.5 
 Mn: 1.0-3.0 
 Al: 0-1.5 
 Mo: 0-0.5 
 Cr: 0-1.0 
 P: 0.001-0.05 
 S: <0.03 
 Ca: <0.003 
 Ti: <0.1 
 V: <0.1 
 
       the balance being Fe and inevitable impurities, 
       wherein Si+Al=1.2-2.5%. 
     
     
         13 . Steel article according to  claim 12 , wherein the steel has an Ultimate Tensile Strength of at least 1400 MPa, and/or a total elongation of at least 8%. 
     
     
         14 . Steel article according to  claim 12 , wherein Mn+Cr≦3% and C+⅓ Mo≦0.45%. 
     
     
         15 . Steel article according to  claim 12 , wherein, expressed in weight %:
 C: 0.2-0.4 and/or   Si: 0.8-2.0 and/or   Mn: 1.5-2.5 and/or   Mo: 0.05-0.5 and/or   Cr: 0.05-1.0 and/or   P: 0.005-0.05 and/or   Ca: 0.0003-0.003.   
     
     
         16 . Method according to  claim 1 , further comprising—prior to the hot forming step d)—the steps of
 a) heating the steel blank to an austenitizing temperature T 1  above Ac3, in the range of Ac3+20° C.-Ac3+60° C., at a heating rate of 10-25° C./s; 
 b) soaking the steel blank in said range, preferably during a soaking time of 1-5 minutes; 
 c) optionally transferring the heated and soaked blank to a hot forming facility. 
 
     
     
         17 . Steel article according to  claim 12 , wherein the steel has an Ultimate Tensile Strength of at least 1500 MPa and/or at least 10% 
     
     
         18 . Steel article according to  claim 12 , wherein the steel has an Ultimate Tensile Strength of at least 1600 MPa and/or a total elongation of at least 12%. 
     
     
         19 . Steel article according to  claim 12 , wherein the steel has an Ultimate Tensile Strength of at least 1400 MPa, advantageously at least 1700 MPa and/or a total elongation of at least 14%. 
     
     
         20 . Steel article according to  claim 12 , wherein, expressed in weight %:
 C: 0.2-0.35 and/or   Si: 1.2-1.8 and/or   Mn: 1.7-2.4 and/or   Mo: 0.05-0.5 and/or   Cr: 0.05-1.0 and/or   P: 0.005-0.05 and/or   Ca: 0.0003-0.003.