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EP-3847288-B1 - METHOD FOR IMPROVING THE FORMABILITY OF STEEL BLANKS

EP3847288B1EP 3847288 B1EP3847288 B1EP 3847288B1EP-3847288-B1

Inventors

  • GAIED, Sadok

Dates

Publication Date
20260506
Application Date
20190830

Claims (15)

  1. A method to improve the formability of a steel blank (1), said steel blank (1) having a microstructure containing at least 5% martensite in area percentage, and possibly some ferrite, bainite and residual austenite and having an ultimate tensile strength of at least 500MPa and having a zinc based metallic coating (14) on at least a portion of an upper face (2) and/or a lower face (4), wherein a heat-treatment operation is performed on the steel blank (1) by directing a thermal energy Q provided by at least one heat source (16) on at least part of a peripheral thickness (6) of said steel blank (1) to form a peripheral heated portion (18) and a heat-treated volume (22), wherein the temperature of said heat-treated volume (22) is comprised within the range of 400°C to 1500°C, wherein the entire volume of the steel blank (1) stays solid during the heat-treatment operation, and wherein the thickness of the metallic coating (14) after heat treatment in the area covering the heat-treated volume (22) decreases by less than 30% as compared to the thickness of the metallic coating (14) in the areas not covering the heat-treated volume (22).
  2. Method according to claim 1, wherein the depth D of the heat-treated volume (22) is comprised within the range of 0.5 mm to 50.0 mm.
  3. Method according to claim 1 or 2, wherein the duration of the heat treatment is comprised between 1 millisecond and 10 minutes.
  4. Method according to any of claims 1 to 3, wherein several steel blanks (1) are piled in a stack of blanks (12) and are heat-treated as a batch by at least one heat source (16).
  5. Method according to any of claims 1 to 3, wherein each steel blank (1) is heat-treated individually.
  6. Method according to any one of claims 1 to 5, wherein the heat source (16) is moving and the steel blank (1) is static.
  7. Method according to any one of claims 1 to 6, wherein the heat source (16) is static and the steel blank (1) is being moved in front of said heat source (16).
  8. Method according to any one of claims 1 to 5, wherein the heat source (16) and the steel blank (1) are both static.
  9. Method according to any one of claims 1 to 8, wherein the heat source (16) is directed toward the same peripheral heated portion (18) at least twice.
  10. Method according to any one of claims 1 to 9, wherein the entire surface forming the peripheral thickness (6) is heat-treated by at least one heat source (16).
  11. Method according to any of the claims 1 to 10, wherein the heat source (16) is a Laser emitting a defocused Laser beam and mounted on an industrial robot (28).
  12. Method according to any of the claims 1 to 11, wherein the heat sources (16) is an array of static infrared tubes, in front of which the steel blanks (1) are moved.
  13. A steel blank (1) that can be obtained according to any one of the claims 1 to 12.
  14. Steel blank (1) according to claim 13, wherein the steel blank (1) is covered by a metallic coating (14) on at least part of its upper and/or lower face 2 and 4, and the thickness of said metallic coating (14) in the area covering the heat-treated zone (22) is less than 30% below the thickness of said metallic coating (14) in the areas not covering the heat-treated zone (22).
  15. Steel blank (1) according to any claim 13 or 14, wherein the hole expansion ratio measured in the heat-treated volume (22) is at least 50% higher than the hole expansion ratio measured in the steel blank (1) outside of the heat-treated volume (22).

Description

BACKGROUND The invention deals with a method for improving the formability of High Strength Steels (HSS). HSS have an ultimate tensile strength above 500MPa and contain at least 5% martensite, the remainder consisting of a combination of other phases such as ferrite, bainite or residual austenite. Because they offer the possibility to improve the fuel efficiency and crashworthiness of vehicles, their use in the automotive industry, among others, is steadily increasing. HSS are shaped into parts by forming a steel blank, for example by stamping or bending or roll forming said steel blank. Because of the very high hardness gradient between the different phases they contain, HSS are particularly sensitive to the formation of cracks during forming. More particularly, HSS are very sensitive to the formation of cracks which are initiated on the edge of the blank and which can then propagate inside the final part, rendering the part unfit for use. Indeed, the cut edge of a blank combines two critical mechanical conditions, which make them particularly sensitive to crack formation during deformation. The first condition is the strain distribution on the cut edges, which is a plane strain distribution, considered the most critical for forming and hence the most susceptible to crack formation. The second condition is the hardening of the edge induced by the process of cutting the blank before forming it. For example, when using mechanical cutting, the action of cutting is actually a combination of shearing and of tearing of the steel material, which induces a significant amount of internal stresses and therefore of work hardening of the material at the edge and the periphery of the blank. The steel material in this area has thus already lost part of its ductility and part of its ability to accommodate the stresses induced by the forming step through deformation instead of through crack formation. For these reasons, the material on the edge and the periphery of a steel blank is particularly prone to crack formation during the forming step. The sensitivity of a steel blank toward the formation of cracks on the edge during forming can be measured by the hole expansion test, which is defined by the ISO 16630:2017 standard testing method. Said test measures a hole expansion ratio, which is the ratio between the diameter of a hole deformed by a punch at the onset of cracks on the edge of the hole during deformation to the initial diameter of said hole before deformation. When faced with edge crack issues on a part, the part manufacturer can choose to improve the quality of the edge of the cut blanks, to tune the clearance of the cutting tool, to modify the part design, to modify the blank shape or to change the forming process. It is however not always industrially feasible to make these changes and in any case the edge crack problem can remain after these options have been explored. The only remaining possibility is then to mechanically brush the side of the blanks in the area where crack formation occurs. By relieving the tensions on the edge and periphery of the parts induced by the blank cutting process, mechanical brushing can indeed solve edge crack problems. It however introduces an expensive post treatment step after the blanking operation. An alternative is to locally modify the properties of the steel in the area of the blanks where cracks occur after forming. Several inventions have been published providing methods to locally modify the properties of steel using a heat source. For example, US2015075678 describes a method to improve the formability of steel blanks by irradiating the surface with a Laser beam. JP0987737 describes a method to locally soften high strength steel blanks by heating the surface of the steel using an arc or a laser beam. Similar methods for heat treating coated sheets blanks for automotive industry are disclosed in US-A 2017/008059, WO-A 2013 013 305 and G. Bergweiler et.al." Improvement of formability and performance of coated ultra-high-strength steels by local heat treatment using a high power diode laser", Proceedings: 2nd International Conference super-high-strength steels, 1 January 2010 , Milan, Italy ISBN: 978-88-85298-79-8. However, there are several limitations associated with heat treating the surface of steel blanks. First of all, the process has low productivity because it can only be performed one blank at a time. Furthermore, when treating metallic coated steel blanks, the high temperature reached at the surface of the blanks will result in significant or total evaporation of the coating. The treated blanks will not benefit from the functions normally ensured by the coating, e.g. corrosion protection or paintability. To solve these problems, a first object of the invention is disclosed in claim 1. It involvesheat treating the blank on at least part of its thickness using a at least one heat source applied to the thickness of the blank, which heats the steel at a temperatur