METHOD FOR PROCESSING ADVANCED HIGH STRENGTH STEEL

Abstract

A method of manufacturing an energy absorbing component for a vehicle is provided. The method includes heating a bainitic GENS steel material which has a microstructure including ferrite and bainite to a temperature above the Ac3 temperature to convert a portion of the ferrite and bainite to austenite. The method further includes forming while cooling the heated steel blank into a component in a temperature controlled steel die. During the cooling step, the steel material is cooled to a temperature below the Ms temperature to form retained austenite. A portion of the austenite transforms to martensite and bainite during the forming and cooling step. The method can further include heating the component to a temperature above the Ms temperature after the forming and cooling step to increase energy absorption characteristics. During a crash event, the strain imposed on the component converts retained austenite present in the component to martensite.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This PCT International Patent Application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 63/026,230 filed on May 18, 2020, titled “Method For Processing Advanced High Strength Steel,” the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processing steel, a method of manufacturing a component formed of steel, and components formed of steel, such as energy absorbing components for vehicle applications.

2. Related Art

This section provides background information related to the present disclosure which is not necessarily prior art.

Energy absorbing components, such as structural components for vehicle applications, are oftentimes formed of steel. Energy absorption is the product of strength and ductility, and manufacturability requires good formability and weldability. Thus, the energy absorbing components formed of steel preferably have a good combination of strength, ductility, weldability, and formability.

There are several types of steel materials used to manufacture energy absorbing components. Many traditional energy absorbing components are formed of a steel material referred to as boron steel. A traditional process of forming a component from boron steel includes heating a sheet formed of the boron steel to a defined elevated temperature and for a time period that enables the formation of a face-centered cubic crystallographic phase referred to as austenite. The austenitic steel sheet is then transferred to a temperature-controlled steel die. A hydraulic press forms the component and achieves a desired profile. The hydraulic press applies the force required to form the desired profile and controls the rate of heat transfer, to achieve the desired cooling rate. The cooling rate and alloy composition of the boron steel causes a phase transformation of the low strength austenite to either a high strength martensite phase or pearlitic microstructure. The critical cooling rate is based on the alloy composition. The combination of alloy composition and cooling rates imposed by conventional hot stamp processing of boron steel does not result in retained austenite.

Emerging energy absorbing components are currently comprised of bainitic quenched and partitioned steels referred to as bainitic GEN3 steels. The GEN3 steels are a commercially available series of advanced high strength steel (AHSS) which have a high strength and ductility, which is associated with the bainitic microstructure. There are various grades of GEN3 steels, based on alloy composition and thermal processing. The transformation of austenite to bainite in the steel is typically accomplished at the rolling mill and is referred to as a quench and partition heat treatment. The components formed of the GEN3 steels are formed at room temperature.

There are advantages and disadvantages associated with the boron steel and the GEN3 steels described above.

For example, the hot stamped boron steel exhibits a higher energy absorption characteristic than the GEN3 steel. The forming tonnage required to form the boron steel at an elevated temperature is lower than that required for the GEN3 steel at room temperature. In addition, the cost of a boron steel sheet is less than a GEN3 steel sheet.

However, post-formed hot stamped boron steel has a relatively low ductility, which limits commercial applications to crash-formed strength-based bending applications, which do not include flange design features. The flange design features increase design efficiency and facilitate attachment to other components. The post-formed strength and ductility characteristics of components formed of the hot stamped boron steel necessitate the use of lasers to trim the stamped components. The processing and manufacturing costs of the hot stamped boron steel components are high due to the capital costs, operating costs, and floor space allocation associated with blank preheat furnaces, hydraulic presses, and laser trim equipment typically used to manufacture the components. The manufacturing and processing costs are greater than those associated with the GEN3 steels, due to the increased capital and operating cost associated with inline solution heat treat of the boron steel sheet prior to the forming operation, use a hydraulic press capable of stopping at the bottom to achieve the required transformation cooling rate, and the laser-based trim processes required to trim the stamped components formed of the boron steel. In addition, the post-formed microstructure of the conventional hot stamped boron steel typically includes martensite, but does not include retained austenite. Thus, the boron steel components lack a post-forming work hardening response associated with the transformation of retained austenite in the post-formed matrix.

As indicated above, the quenched and partitioned GEN3 steels, comprised of a combination of bainite and retained austenite, have improved formability and ductility relative to martensitic hot stamped steel enabling the ability to form flange features to increase the design efficiency of the component. The quenched and partitioned GEN3 steels also have the advantage of reduced processing costs, relative to the hot stamped boron steel. The reduced processing costs are typically associated with processing at room temperature, reduced cycle time related to the use of a higher speed mechanical press, avoidance of dwell time associated with transformation cooling, and feasible secondary operations (restrike, trim, flange and pierce) which are performed in-line to the forming operation.

However, the GEN3 steels are typically more costly than the boron steel. The post-formed dimensional repeatability of the GEN3 steel stamped components is low relative to the hot stamped boron steel and other high strength steel alloys stamped at room temperature. The reduced dimensional repeatability is related to spring back. The post-formed total energy absorption characteristics of the GEN3 steel is lower relative to boron steel. The strength of the GEN3 steel during the forming operation is high relative to the hot stamped boron steel, which limits the size (area) or number of GEN3 steel parts formed for a given press tonnage. Increased press tonnage is required relative to the hot stamped boron steel. In addition, bainitic GEN3 steel does not exhibit a work hardening characteristic due to the lack of retained austenite after the forming operation.

A commercially available series of GEN3 steel is an austenitic advanced high strength steel (AHSS) referred to as austenitic GEN3 TRIP steel. TRIP steels leverage the strength and ductility associated with the transformation to austenite to martensite (known as the TRIP effect) to enhance formability and strength characteristics, as a result of strain imposed during the forming process.

There is a continuing desire to further develop and refine steels used to form energy absorbing components, such as those used in vehicle applications. Objectives include increasing product design efficiency by enabling the capability to form flange features; avoiding cost by enabling inline restriking, trimming, flanging, and piercing operations; avoiding cost associated with secondary laser processing operations; improving post-formed dimensional repeatability, associated with springback of GEN3 steel stamped components; avoiding cost by cycle time reduction; avoiding capital cost associated with use of a mechanical press compared to a hydraulic press; avoiding cost by cycle time reduction, specifically by eliminating cooling rate-dependent transformation events; reducing capital equipment, operational costs, and floorspace required by preheat ovens, presses, and secondary trim operations; and increasing energy absorption associated with TRIP enabled work hardening and plasticity characteristics during a strain induced crash event.

SUMMARY

One aspect of the invention provides a method for processing steel material, such as material used to form an energy absorbing component for a vehicle. The method comprises heating a steel material to a temperature above an upper critical temperature (Ac3) of the steel material. The steel material has a microstructure which includes ferrite and bainite, and the heating step includes converting a portion of the ferrite and bainite to austenite. The method further includes forming the steel material into a component after the steel material is heated to the temperature above the upper critical temperature (Ac3) of the steel material. The steel material is cooled during the forming step, and a portion of the austenite transforms to martensite and bainite during the forming step.

Another aspect of the invention provides a component formed of the steel material, for example an energy absorbing component for a vehicle. The steel material includes iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel material. The steel material also includes bainite and martensite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an energy absorbing component formed of a steel material according to an example embodiment;

FIG. 2 illustrates a quench and partition process wherein a steel material is heated above the Ac3 temperature of the steel material, die quenched in a heated die to a temperature between the M_s and M_f temperature of the steel material, and then heated to an elevated temperature to increase energy absorption.

FIG. 3 illustrates a quench and temper process wherein a steel material is heated above the Ac3 temperature of the steel material, die quenched in a steel die to a temperature below the M_s and M_f temperatures of the steel material, and reheated to an elevated temperature to increase energy absorption.

FIG. 4 is a table showing ultimate tensile strength (TS), yield strength (YS), and elongation (E) for a steel material in an as-received condition and steel materials processed according to example embodiments.

FIG. 5 is a graph of phase distribution and temperature for a steel material according to an example embodiment.

DETAILED DESCRIPTION

One or more of the above objectives are achieved by embodiments of the invention. In general, the subject embodiments are directed to a method for processing advanced high strength steel (AHSS). However, the example embodiments are only provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and/or well-known technologies are not described in detail.

According to example embodiments, the method includes processing of the advanced high strength steel (AHSS) referred to as bainitic GEN3 steel. Bainitic GEN3 steel typically comprises iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel material. According to a specific example, the composition of the steel material, demonstrated at lab scale, comprises iron in an amount of 96.03 wt. %, carbon in an amount of 0.22 wt. %, manganese in an amount of 2.35 wt. %, silicon in an amount of 0.6 wt. %, and aluminum in an amount of 0.8 wt. %, based on the total weight of the steel. The microstructure of the steel includes bainite, typically in an amount of at least 75 vol. %, based on the total volume of the steel material. The remainder of the steel material includes ferrite. The process begins with a blank formed of the steel material, which is typically in the form of a sheet. The following example embodiments will refer to the steel sheet, however, the steel material could comprise other shapes.

The bainitic steel material is heated to a temperature above the upper critical temperature (Ac3) of the steel material. The Ac3 temperature is defined at the temperate at which the ferrite and bainite phases of the steel material transform to austenite. Thus, during the heating step, a portion of the ferrite and bainite transform to austenite. Typically, for the GEN3 steel material, the temperature above the Ac3 temperature ranges from 850° C. to 900° C. The Ac3 temperature for the bainitic GEN3 steel disclosed above is 850 ° C. However, the Ac3 temperature varies by composition, and Ac3 kinetics are slow. Heating above the Ac3 temperature reduces the time required to achieve a microstructure which is 100% austenite.

The specific fraction of ferrite, bainite and austenite in the steel material after the heating step is dependent on a phase equilibrium at temperature for the specific composition of the steel material. The fraction of ferrite, bainite and austenite in the steel sheet is also dependent on the temperature history of the steel sheet prior to forming and the specific composition of the steel material.

Next, the steel sheet, which was previously heat treated to a temperature above the Ac3 temperature, typically 850° C. to 900° C., is formed into a component 10 having a desired shape. The forming step is preferably conducted in a temperature controlled steel die using a forming press, preferably a mechanical press. The method also includes cooling the steel material during and possibly after the forming step. The temperature of the steel die ranges from 100° C. to 360° C. while forming the steel material into the desired shape. The temperature of the steel material itself during the forming step ranges from 900° C. to a temperature ranging between 100° C. to 360° C.

A high fraction percentage of the austenite is transformed to martensite and bainite during the forming process, as a result of the rate of heat transfer imposed by the forming process. The transformation of the austenite to a combination of bainite and/or martensite during the forming step reduces the forming tonnage required, improves formability, reduces dimensional variance by improving dimensional repeatability associated with spring back, and increases the strength of the formed component 10. An example of the component 10 is shown in FIG. 1. According to this example, the component 10 is a B-pillar between a passenger and driver door of a vehicle.

As indicated above, during and possibly after the forming step, the method preferably includes cooling the steel material and/or shaped component in the die, for example by quenching. The steel material and/or component is cooled to a temperature below the M_s temperature. After cooling to a temperature below the M_s temperature, the method preferably includes heating or tempering the component to a temperature above the M_s temperature in the die. The M_s temperature is the temperature at which the formation of martensite in the steel material begins, and the M_f temperature is the temperature at which the formation of martensite in the steel material finishes. Regulating the temperature of the die during and after the forming step controls the amount of martensite, bainite, and retained austenite in the component and thus is able to tailor the energy absorption, weldability, and/or deformation characteristics in specific regions of the component.

The cooling step typically includes forming retained austenite in the component. The retained austenite is maintained in a matrix of bainite and martensite. For example, greater than 0 and up to 15 volume % of the austenite present in the steel material prior to the forming step may be retained in the matrix of bainite and martensite after the cooling step. The percentage of retained austenite in the post-formed steel sheet is dependent on the temperature of the form die, cooling rate, strain imposed during the forming process and the specific steel composition.

The amount of retained austenite present in the component after forming is the result of diffusion-related transformation kinetics relative to the martensite start temperature (M_s) and martensite finish (M_s) temperature range. The M_s temperature for the steel composition disclosed above is approximately 350° C. to 360° C. and the M_f temperature is approximately 135° C. to 145° C. The percentage of retained austenite in the component ranges from 0% to 15% based on stability of the austenite during cooling determined by the cooling rate below the M_s temperature. Austenite stability and the relative percentage of bainite versus martensite present in the formed component is determined by the cooling rate below the M_s temperature which is influenced by the temperature of the steel die used to form the component.

The method can further including heating or tempering the steel component after the cooling and forming steps.

According to one embodiment, the temperature of the steel component in the steel die is controlled and is kept at temperature between the M_s and M_f temperatures of the steel material after the forming step, and then the steel component is heated to a temperature above the M_s temperature for a defined period of time. FIG. 2 illustrates a quench and partition process wherein the steel material is heated above the Ac3 temperature, die quenched in a heated die to a temperature between the M_s and M_f temperatures, which are specific to the steel material composition, and then heated to an elevated temperature to increase energy absorption.

According to another embodiment, the temperature of the steel component is controlled and kept at a temperature below the M_f temperature prior to heating the steel component to a temperature above the M_s temperature for a defined period of time. FIG. 3 illustrates a quench and temper process wherein the steel material is heated above the Ac3 temperature, die quenched in a steel die to a temperature below the M_s and M_f temperatures, which are specific to the steel material composition, and reheated to an elevated temperature to increase energy absorption. The cooling and reheating steps are conducted to increase energy absorbing properties of the steel component. Various other heating, tempering, quenching, partitioning, and/or austenitizing steps can be conducted on the steel component after the forming step to increase energy absorbing properties of the steel component.

The composition of the steel material of the finished component still includes iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel material.

According to an example embodiment, the method includes heating the steel material to a temperature above the Ac3 temperature, preferably to a temperature of 900° C. The steel material is then cooled during the forming process in a steel die, preferably controlled to a temperature of 100° C. to 350° C. The cooling rate of the steel material below the M_s temperature is greater than 10° C./second, preferably 50° C./second. The formed component is then reheated to a temperature above the M_s temperature, preferably to a temperature range of 360° C. to 400° C.

FIG. 4 is a table showing the ultimate tensile strength (TS), yield strength (YS), and elongation (E) for a steel component in the as-received condition; and steel components which have been austenized, quenched and partitioned; austenized and quenched; and austenized, quenched, and tempered according to example embodiments. FIG. 5 includes a graph of phase distribution and temperature for a steel material according to an example embodiment.

The process can further include restriking, trimming, flanging, and/or piercing operations on the finished formed steel component. If the finished formed component is used in a vehicle application and includes a fraction percentage of retained austenite, then during a possible crash event the formed component is subjected to strain which transforms some of the retained austenite to martensite. The transformation of the retained austenite to martensite during the crash event increases strength and energy absorption characteristic of the component.

As indicated above, the process and finished component formed by the process described above provides numerous advantages. The transformation of austenite to a combination of martensite, bainite and/or retained austenite addresses the need to improve dimensional repeatability, formability, forming tonnage requirements, and energy absorption characteristics, relative to GEN3 bainitic steel formed at room temperature. The transformation of austenite to a combination of martensite, bainite and retained austenite also addresses the need to reduce manufacturing costs, enables use of a mechanical press, and increases design efficiency relative to hot stamped boron steel components. The transformation of retained austenite to martensite during a strain event imposed during a crash addresses the need to improve energy absorption characteristics relative to GEN3 bainitic steel.

The steel component of the present disclosure also provides enhanced formability due to the presence of retained austenite and transformation of the austenite to martensite during the forming process. The dimensional characteristics associated with the steel component are also enhanced due to the presence of the retained austenite and the transformation of the austenite to martensite during the forming process. The post-formed energy absorption characteristics of the steel component are greater than GEN3 boron steel due to the transformation of a portion of austenite to martensite during the forming event and the transformation of the retained austenite to martensite during a crash event. The cost associated with the manufacture of the steel component is less than boron steel due to reduced heating requirements and use of lower cost trimming methods. The design efficiency of the steel component is greater than hot stamped boron steel due to the ability to form flange features.

It should be appreciated that the foregoing description of the embodiments has been provided for purposes of illustration. In other words, the subject disclosure it is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varies in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of disclosure.

Claims

1. A method for processing steel material, comprising the steps of:

heating a steel material to a temperature above an upper critical temperature (Ac3) of the steel material, and the steel material having a microstructure which includes ferrite and bainite;

the heating step including converting a portion of the ferrite and bainite to austenite;

forming the steel material into a component after the steel material is heated to the temperature above the upper critical temperature (Ac3); and

cooling the steel material during the forming step, wherein a portion of the austenite transforms to martensite and bainite during the forming step.

2. The method of claim 1, wherein the heating step includes heating the steel material to a temperature greater than 850° C.

3. The method of claim 1, wherein the steel material includes iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel material.

4. The method of claim 1, wherein the microstructure of the steel material, prior to the heating step, includes bainite in an amount of at least 75 vol. %, based on the total volume of the steel material.

5. The method of claim 1, wherein the forming step is conducted in a steel die while the steel die is at a temperature of 100° C. to 360° C.

6. The method of claim 1, wherein the cooling step includes cooling the steel material from the temperature above the Ac3 temperature to a temperature below a martensite start (Ms) temperature of the material.

7. The method of claim 6, wherein the Ac3 temperature is greater than 850° C. and the temperature below the Ms temperature is from 100° C. to 350° C.

8. The method of claim 1, wherein the cooling step includes forming retained austenite, wherein the retained austenite is maintained in a matrix of the bainite and martensite

9. The method of claim 8, wherein up to 15% of the austenite present in the steel material prior to the forming step is maintained as the retained austenite in the matrix of the bainite and martensite.

10. The method of claim 1, wherein the cooling is conducted at a rate of less than 50° C./second.

11. The method of claim 1 including heating the component to a temperature above the Ms temperature of the steel material after the forming and cooling step.

12. The method of claim 1 including restriking, trimming, flanging, and/or piercing component after the forming step.

13. The method of claim 1, wherein the forming step is conducted in a die, and the method further includes regulating the temperature of the die during and/or after the forming step to control the amount of martensite, bainite, and retained austenite in the component and thus tailor the energy absorption, weldability, and/or deformation characteristics in specific regions of the component.

14. The method of claim 1, wherein the forming step includes shaping the steel material into the component having the shape of an energy absorbing component for a vehicle.

15. A component, comprising:

a steel material including iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel; and

the steel material including bainite and martensite.

16. The method of claim 1, wherein before the heating step, the microstructure of the steel includes bainite in an amount of at least 75 vol. %, based on the total volume of the steel material, and a remainder of the microstructure includes ferrite.

17. The method of claim 1, wherein the component is a B-pillar.

18. The method of claim 1, wherein the cooling is conducted at a rate of greater than 10° C./second.

19. The component of claim 15, wherein the steel material includes retained austenite in a matrix of bainite and martensite.

20. The component of claim 15, wherein the component is a B-pillar.