Methods to improve the toughness of press hardening steel

Abstract

Methods include pressing and quenching a heated blank in a die to form the shaped steel object. A first portion of the heated blank is selectively cooled at a first cooling rate and a second portion of the heated blank selectively cooled at a lower second cooling rate. The shaped steel object has an alloy with wt. % of chromium at ≥about 0.5 to ≤about 6; carbon at ≥about 0.01 to ≤about 0.5; manganese at ≥about 0 to ≤about 3; silicon at ≥about 0.5 to ≤about 2; nitrogen at ≥0 to ≤about 0.01; nickel at ≥0 to ≤about 5; copper at ≥0 to ≤about 5; molybdenum at ≥0 to ≤about 5; vanadium at ≥0 to ≤about 1%; niobium at ≥0 to ≤about 0.1; and a balance being iron.

What is claimed is: 
     
         1 . A method of selectively quenching at least one region of a shaped steel object, the method comprising:
 pressing and quenching a heated blank in a die to form the shaped steel object, the pressing and quenching including:
 selectively cooling a first portion of a heated blank at a first cooling rate, and 
 selectively cooling a second portion of the heated blank at a second cooling rate, the first cooling rate being less than the second cooling rate, the shaped steel object comprising an alloy composition comprising: 
 chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. %; 
 carbon (C) at a concentration of greater than or equal to about 0.01 wt. % to less than or equal to about 0.5 wt. %; 
 manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %; 
 silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %; 
 nitrogen (N) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %; 
 nickel (Ni) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 copper (Cu) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %; 
 niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.1 wt. %; and 
 a balance of the alloy composition being iron. 
   
     
     
         2 . The method of  claim 1 , wherein after the selectively cooling the first portion and selectively cooling the second portion, the first portion has a greater ductility than the second portion. 
     
     
         3 . The method of  claim 1 , wherein after the selectively cooling the first portion, the first portion has a bending angle greater than or equal to 90°. 
     
     
         4 . The method of  claim 1 , wherein the alloy composition further comprises at least one of nickel, molybdenum, copper, niobium, vanadium, or titanium. 
     
     
         5 . The method of  claim 1 , wherein the die includes a first shell having a first surface region corresponding to the first region of the heated blank and a second surface region corresponding to the second region of the heated blank, the first surface region of the first shell comprising a first material with a lower thermal conductivity than a second material of the second surface region. 
     
     
         6 . The method of  claim 1 , wherein the die includes a first shell having a first region configured to interface with the first portion of the heated blank and a second region configured to interface with the second portion of the heated blank, the first region of the first shell comprising a first plurality of cooling channels and the second region comprising a second plurality of cooling channels, wherein the first plurality of cooling channels is distinct from the second plurality of cooling channels. 
     
     
         7 . The method of  claim 1 , wherein the die includes a first shell having a first surface region configured to interface with the first portion of the heated blank and a second surface region configured to interface with the second region of the heated blank, the first surface region having a first surface roughness that is lower than a second surface roughness of the second surface region. 
     
     
         8 . The method of  claim 1 , wherein the die includes a first shell having a first region configured to interface with the first portion of the heated blank and a second region configured to interface with the second portion of the heated blank, the first region of the first shell configured to have a first contact pressure with the first portion of the heated blank that is greater than a second contact pressure of the second region with the second portion of the heated blank. 
     
     
         9 . The method of  claim 1 , wherein the die includes a first shell having a first region configured to interface with the first portion of the heated blank and a second region configured to interface with the second portion of the heated blank, the first region of the first shell has a first die gap and the second region has a second die gap that is distinct from the first die gap. 
     
     
         10 . The method of  claim 1 , wherein
 after the selectively cooling the first portion of the heated blank, the first portion has a microstructure comprising greater than or equal to 0.1% by volume to less than or equal to 12% by volume retained austenite in a matrix of martensite, and   the second portion of the cooled heated blank has a microstructure comprising greater than or equal to 0.1% by volume to less than or equal to 5% by volume retained austenite in a matrix of martensite.   
     
     
         11 . The method of  claim 1 , wherein the first cooling rate is greater than or equal to 20K/s to less than or equal to 60K/s. 
     
     
         12 . A method of selectively quenching at least one region of a shaped steel object, the method comprising:
 pressing and quenching a heated blank disposed in a die to form the shaped steel object, the pressing and quenching including selectively cooling the heated blank at a first cooling rate less than 60 K/s,
 the shaped steel object comprises an alloy composition comprising: 
 chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. %; 
 carbon (C) at a concentration of greater than or equal to about 0.01 wt. % to less than or equal to about 0.5 wt. %; 
 manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %; 
 silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %; 
 nitrogen (N) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %; 
 nickel (Ni) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 copper (Cu) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %; 
 niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.1 wt. %; and 
 a balance of the alloy composition being iron. 
   
     
     
         13 . The method of  claim 12 , wherein after the selectively cooling the heated blank, the press hardened blank has a microstructure comprising greater than or equal to 0.1% by volume to less than or equal to 12% by volume retained austenite in a matrix of martensite. 
     
     
         14 . The method of  claim 12 , wherein the alloy composition further comprises at least one of nickel, molybdenum, copper, niobium, vanadium, or titanium. 
     
     
         15 . The method of  claim 12 , wherein pressing and quenching occurs for greater than or equal to 6 seconds to less than or equal to 10 seconds. 
     
     
         16 . The method of  claim 12 , wherein a die contact pressure for the heated blank is greater than or equal to 0.5 MPa to less than or equal to 4 MPa. 
     
     
         17 . A method of selectively quenching at least one region of a shaped steel object, the method comprising:
 pressing and quenching a heated blank in a die for greater than or equal to 6 seconds to less than or equal to 10 seconds having a die contact pressure of greater than or equal to 0.5 MPa to less than or equal to 4 MPa to form the shaped steel object, the pressing and quenching including:
 selectively cooling a first portion of a heated blank at a first cooling rate of greater than or equal to about 20K/s to less than or equal to about 60K/s, and 
 selectively cooling a second portion of the heated blank at a second cooling rate, the first cooling rate being less than the second cooling rate, the shaped steel object comprising an alloy composition comprising: 
 chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. %; 
 carbon (C) at a concentration of greater than or equal to about 0.01 wt. % to less than or equal to about 0.5 wt. %; 
 manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %; 
 silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %; 
 nitrogen (N) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %; 
 nickel (Ni) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 copper (Cu) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %; 
 vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %; 
 niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.1 wt. %; and 
 a balance of the alloy composition being iron. 
   
     
     
         18 . The method of  claim 17 , wherein after the selectively cooling the first portion and selectively cooling the second portion, the first portion has a greater ductility than the second portion. 
     
     
         19 . The method of  claim 17 , wherein after the selectively cooling the first portion, the first portion has a bending angle greater than or equal to 90°. 
     
     
         20 . The method of  claim 17 , after the selectively cooling the first portion of the heated blank, the first portion has a microstructure comprising greater than or equal to 0.1% by volume to less than or equal to 12% by volume retained austenite in a matrix of martensite, and
 the second portion of the cooled heated blank has a microstructure comprising greater than or equal to 0.1% by volume to less than or equal to 5% by volume retained austenite in a matrix of martensite.