Zinc-coated hot formed high strength steel part with through-thickness gradient microstructure

Abstract

Methods of strengthening surface regions of high-strength transformation induced plasticity (TRIP) steel are provided. The method may comprise shot peening at least one region of an exposed surface of a hot-formed press-hardened component comprising a high-strength steel. Prior to shot peening, the component has a microstructure comprising ≥ about 5% by volume retained austenite in a matrix of martensite. The shot peening is conducted at a temperature of ≤ about 150° C. and forms at least one hardened surface region comprising ≤ about 2% by volume austenite. The TRIP steel may be zinc-coated and having a surface coating comprising zinc and substantially free of liquid metal embrittlement (LME). Zinc-coated hot-formed press-hardened components, including automotive components, formed from such methods are also provided.

What is claimed is: 
     
       1. A method of strengthening regions of a high-strength steel, the method comprising:
 shot peening at least one region of an exposed surface of a hot-formed press-hardened component comprising a high-strength transformation induced plasticity (TRIP) steel selected from the group consisting of: 
 a Mn-TRIP steel comprising:
 manganese (Mn) at greater than or equal to about 4% by weight of the total steel composition, and 
 carbon (C) at greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight, and 
 
 a delta-TRIP steel comprising:
 aluminum (Al) at greater than or equal to about 3% by weight to less than or equal to about 6% by weight of the total steel composition; 
 manganese at greater than or equal to about 0.1% by weight to less than or equal to about 1% by weight of the total steel composition; and 
 carbon at greater than or equal to about 0.3% by weight to less than or equal to about 0.5% by weight, 
 
 wherein the hot-formed press-hardened component has a microstructure comprising greater than or equal to about 5% by volume retained austenite in a matrix of martensite and wherein the shot peening is conducted while the exposed surface is at a temperature of less than or equal to about 150° C. and the shot peening forms at least one hardened surface region comprising less than or equal to about 1% by volume austenite. 
 
     
     
       2. The method of  claim 1 , wherein the shot peening creates a gradient microstructure from the exposed surface to a center region of the hot-formed press-hardened component. 
     
     
       3. The method of  claim 1 , wherein the shot peening is conducted while the exposed surface is at a temperature of less than or equal to about 100° C. 
     
     
       4. The method of  claim 1 , wherein the shot peening is conducted with shot peen media selected from the group consisting of: solid carbon dioxide particles, steel shot, and combinations thereof. 
     
     
       5. The method of  claim 1 , wherein the hot-formed press-hardened component has a first thickness and the at least one hardened surface region has a second thickness, wherein the second thickness is greater than 0.1% of the thickness of the first thickness and less than or equal to about 20% of the second thickness. 
     
     
       6. The method of  claim 1 , wherein the high-strength transformation induced plasticity (TRIP) steel is a Mn-TRIP steel comprising:
 manganese (Mn) at greater than or equal to about 4% by weight to less than or equal to about 12% by weight of the total steel composition, 
 carbon (C) at greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight of the total steel composition; 
 silicon (Si) at greater than or equal to about 0.1% by weight to less than or equal to about 0.5% by weight of the total steel composition; 
 chromium (Cr) at greater than 0% to less than or equal to about 1% by weight of the total steel composition; 
 titanium (Ti) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 aluminum (Al) at greater than 0% to less than or equal to about 0.1% by weight of the total steel composition; 
 phosphorus (P) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 sulfur (S) at greater than 0% to less than or equal to about 0.05% by weight of the total steel composition; 
 one or more impurities cumulatively present at greater than or equal to 0% to less than or equal to about 0.5% by weight of the total steel composition; and 
 a balance iron. 
 
     
     
       7. The method of  claim 1 , wherein the high-strength transformation induced plasticity (TRIP) steel is a delta-TRIP steel comprising:
 aluminum (Al) at greater than or equal to about 3% by weight to less than or equal to about 6% by weight of the total steel composition; 
 manganese at greater than or equal to about 0.1% by weight to less than or equal to about 1% by weight of the total steel composition; 
 carbon at greater than or equal to about 0.3% by weight to less than or equal to about 0.5% by weight of the total steel composition; 
 silicon (Si) at greater than or equal to about 0.1% by weight to less than or equal to about 0.5% by weight of the total steel composition; 
 chromium (Cr) at greater than 0% to less than or equal to about 1% by weight of the total steel composition; 
 titanium (Ti) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 phosphorus (P) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 sulfur (S) at greater than 0% to less than or equal to about 0.05% by weight of the total steel composition; 
 one or more impurities cumulatively present at greater than or equal to 0% to less than or equal to about 0.5% by weight of the total steel composition; and 
 a balance iron. 
 
     
     
       8. The method of  claim 1 , prior to the shot peening further comprising: heating a blank comprising a high-strength TRIP steel to a temperature of less than or equal to about 782° C. to austenitize the high-strength TRIP steel; and press hardening the blank in at least one die to press and quench the blank to form the hot-formed press-hardened component. 
     
     
       9. The method of  claim 1 , wherein the at least one hardened surface region comprises less than 0.5% by volume austenite. 
     
     
       10. The method of  claim 1 , wherein the exposed surface includes a first surface and a second opposite surface of the hot-formed press-hardened component, wherein the at least one hardened surface region cover the first surface and the second surface. 
     
     
       11. A method of strengthening regions of a zinc-coated high-strength steel, the method comprising:
 shot peening at least one region of an exposed surface of a zinc-coated hot-formed press-hardened component comprising a high-strength transformation induced plasticity (TRIP) steel having a surface coating comprising zinc, the steel selected from the group consisting of: 
 a Mn-TRIP steel comprising:
 manganese (Mn) at greater than or equal to about 4% by weight of the total steel composition, and 
 carbon (C) at greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight, and 
 
 a delta-TRIP steel comprising:
 aluminum (Al) at greater than or equal to about 3% by weight to less than or equal to about 6% by weight of the total steel composition; 
 manganese at greater than or equal to about 0.1% by weight to less than or equal to about 1% by weight of the total steel composition; and 
 carbon at greater than or equal to about 0.3% by weight to less than or equal to about 0.5% by weight, 
 
 wherein the zinc-coated hot-formed press-hardened component has a microstructure comprising greater than or equal to about 5% by volume retained austenite in a matrix of martensite, wherein the shot peening is conducted while the exposed surface is at a temperature of less than or equal to about 150° C. and the shot peening forms at least one hardened surface region comprising less than or equal to about 1% by volume austenite and wherein after the shot peening, the component is substantially free of liquid metal embrittlement (LME). 
 
     
     
       12. The method of  claim 11 , wherein the shot peening seals microcracks in the surface coating of the exposed surface of the zinc-coated hot-formed press-hardened component. 
     
     
       13. The method of  claim 11 , wherein the shot peening creates a gradient microstructure from the exposed surface to a center region of the zinc-coated hot-formed press-hardened component. 
     
     
       14. The method of  claim 11 , wherein the shot peening is conducted with shot peen media selected from the group consisting of: solid carbon dioxide particles, steel shot, and combinations thereof. 
     
     
       15. The method of  claim 11 , wherein the zinc-coated hot-formed press-hardened component has a first thickness and the at least one hardened surface region has a second thickness, wherein the second thickness is greater than 0.1% of the thickness of the first thickness and less than or equal to about 20% of the second thickness. 
     
     
       16. The method of  claim 11 , wherein the high-strength transformation induced plasticity (TRIP) steel is:
 (i) a Mn-TRIP steel comprising:
 manganese (Mn) at greater than or equal to about 4% by weight to less than or equal to about 12% by weight of the total steel composition, and 
 carbon (C) at greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight of the total steel composition; 
 silicon (Si) at greater than or equal to about 0.1% by weight to less than or equal to about 0.5% by weight of the total steel composition; 
 chromium (Cr) at greater than 0% to less than or equal to about 1% by weight of the total steel composition; 
 titanium (Ti) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 aluminum (Al) at greater than 0% to less than or equal to about 0.1% by weight of the total steel composition; 
 phosphorus (P) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 sulfur (S) at greater than 0% to less than or equal to about 0.05% by weight of the total steel composition; 
 one or more impurities cumulatively present at greater than or equal to 0% to less than or equal to about 0.5% by weight of the total steel composition; and 
 a balance iron; or 
 
 (ii) a delta-TRIP steel comprising:
 aluminum (Al) at greater than or equal to about 3% by weight to less than or equal to about 6% by weight of the total steel composition; 
 manganese at greater than or equal to about 0.1% by weight to less than or equal to about 1% by weight of the total steel composition; 
 carbon at greater than or equal to about 0.3% by weight to less than or equal to about 0.5% by weight of the total steel composition; 
 silicon (Si) at greater than or equal to about 0.1% by weight to less than or equal to about 0.5% by weight of the total steel composition; 
 chromium (Cr) at greater than 0% to less than or equal to about 1% by weight of the total steel composition; 
 titanium (Ti) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 phosphorus (P) at greater than 0% to less than or equal to about 0.2% by weight of the total steel composition; 
 sulfur (S) at greater than 0% to less than or equal to about 0.05% by weight of the total steel composition; 
 one or more impurities cumulatively present at greater than or equal to 0% to less than or equal to about 0.5% by weight of the total steel composition; and 
 a balance iron. 
 
 
     
     
       17. The method of  claim 11 , wherein the at least one hardened surface region comprises less than 0.5% by volume austenite.