EP3473735A1

EP3473735A1 - Treatment process for obtaining graded performance and member thereof

Info

Publication number: EP3473735A1
Application number: EP16905967.2A
Authority: EP
Inventors: Hongliang Yi; Pengju DU
Original assignee: Easyforming Steel Technology Co Ltd
Current assignee: Easyforming Steel Technology Co Ltd
Priority date: 2016-06-20
Filing date: 2016-07-20
Publication date: 2019-04-24
Anticipated expiration: 2036-07-20
Also published as: EP3473735C0; EP3473735B1; EP3473735A4

Abstract

The present invention relates to a treatment process for obtaining gradient properties, comprising the steps of: A. preparing a blank, dividing the blank into a region to be formed as a hard zone, and a region to be formed as a soft zone; B. heating the region to be formed as a hard zone to 720 °C or more, such that its microstructure is transformed into austenite; C. stamping the entire blank and cooling it by any cooling method after the stamping; D. subjecting the hard zone of a component obtained after the stamping to carbon partitioning treatment to diffuse carbon from martensite to austenite; keeping a temperature of the region to be formed as the soft zone lower than 720 °C in step B, or adding a step E after the step D, wherein the region forming the soft zone is heated to 600 to 720 °C and soaked for 0.5 to 60 minutes. The invention also relates to a formed component made by the above-described treatment process. The treatment process of the invention has a simple die and good process reliability, and can realize a tensile strength of 900-1500 MPa of the soft zone and an elongation of more than 15%, which is far superior to the prior art.

Description

TECHNICAL FIELD

The present invention relates to a treatment process for obtaining gradient properties. Specifically, the invention relates to a process for realizing gradient properties on the same component, wherein the stamping process, the heat treatment method and the mechanical properties of the hard zone are different from the stamping process and the heat treatment method of the soft zone thereof. The invention also relates to components made via the treatment process, which has both a hard zone and a soft zone with different mechanical properties.

BACKGROUND

Energy saving and emission reduction are urgent problems to be solved in the automotive field, and automobile lightweighting is one of the effective means and ways to achieve energy saving and emission reduction. Automotive lightweighting can be realized by reasonable designs and advanced forming methods. The use of high-strength steel can ensure the safety of automobile while achieving lightweighting. However, high-strength steel has low stretch flangeability and a low hole expansion ratio, so there are problems such as easy cracking during stamping and increased springback of the parts after the stamping.
In order to solve the problem of forming high-strength steel, a forming method called hot stamping or hot forming for manufacturing a vehicle component having a strength of 1000 MPa or more has been commercialized. However, although the hot formed component has high strength, its elongation is very low. In the actual automobile collision process, it is not only required that the safety component has high strength to effectively resist the intrusion of an collision object, but ductility and toughness of the whole or a part of the component are required to ensure high collision energy absorption. The traditional hot forming steel 22MnB5 can hardly achieve both high strength and high ductility in a single hot forming process. Therefore, in order to solve this problem, the industry has developed tailored properties technologies so that a single part is made up of two different regions with high strength and high elongation respectively, such as a B-pillar, with high strength and good intrusion resistance at the upper end and lower strength and better elongation at the lower end for energy absorption. The tailored properties are usually achieved through two approaches: the tailor welded blank technology and the segmented/gradient strengthening technology.
The general practice of the tailor welded blank technology is to use laser welding in some locations to weld two steel sheets of different compositions and different thicknesses together and obtain different properties after treatment. Dissimilar materials and different thicknesses make the welding difficult and increase production procedures and the potential for weak welds or damage.
The segmented/gradient strengthening technology generally controls the cooling rate of a part during the hot stamping process, thus obtaining different structures to achieve different properties. It mainly comprises controlling the thermal conductivity of the die, including active cooling ( CN102212742 A ), and passive cooling ( CN104831020A , CN103521581A , CN103409613A ). In CN102212742A , by designing a flexibility controllable hot stamping die with cooling water channels, different regions of the part are designed with different water flow speeds to achieve different cooling rates. In CN104831020A , through the water channel design in the die, the temperature is different with the distance that the cooling water flows in the die, and a non-uniform temperature field is formed in the die to realize gradient control in the stamping process. Both CN103521581A and CN103409613A achieve property gradients by changing the cooling rate of the part by a thermal barrier coating on the die.
WO2006/038868A1 describes a method for achieving gradient properties which controls the cooling rate by controlling the air gaps formed by the grooves between the die and the part to be stamped. US2013/0048160A1 pre-cools a region of the part to be softened between austenitizing and hot stamping of the steel sheet to obtain a soft phase structure, and then conducts hot stamping to realize gradient properties of the part.
CN101861265A describes a B-pillar for a vehicle and a method of manufacturing the same. The purpose thereof is to design a B-pillar with gradient properties, with a soft zone of at least 30 mm long near the lower end (the fixation part) of the B-pillar, which is manufactured by controlling the cooling rate of the soft zone.
CN103878237A discloses a method for processing a hot formed part from high-strength steel, which performs localized annealing on a uniform component after the hot forming to realize gradient properties of the part. The method requires designing a special induction heating coil which has a high annealing temperature of 600 to 1000 °C, preferably 800 °C, and is air cooled to 100 to 500 °C after the annealing.
In summary, the methods in the various reference patents described above are mainly characterized in that after the steel sheet is austenitized and hot formed, the structural property of the soft zone is obtained by controlling the phase transformation thereof in the cooling process, the cooling rates of different parts are controlled to obtain different strengths, or a soft zone is obtained by designing the die cooling or by grooves or pre-cooling treatment. Significant disadvantages are the need to change the original die design, poor process stability and short die life, and it is difficult for the the soft zone on the basis of a 22MnB5 material to break through the 15% elongation.

SUMMARY

The present invention relates to a process for realizing gradient properties on the same part, wherein the stamping process, the heat treatment method and the mechanical properties of the hard zone are different from the stamping process and the heat treatment method of the soft zone thereof. The invention also relates to components having both a hard zone and a soft zone, wherein the hard zone can ensure small collision deformation due to high strength, and the soft zone can guarantee collision energy absorption due to high elongation.
According to a preferred embodiment of the present invention, there is provided a treatment process for obtaining gradient properties, characterized by, comprising the steps of: A. preparing a blank, dividing the blank into a region to be formed as a hard zone, and a region to be formed as a soft zone; B. heating the region to be formed as the hard zone to 720 °C or more, such that its microstructure is transformed into austenite, and meanwhile maintaining the temperature of the region to be formed as the soft zone below 720 °C; C. stamping the entire blank and cooling it by any cooling method after the stamping; D. conducting carbon partitioning treatment on the hard zone of components obtained after the stamping to diffuse carbon from martensite to austenite.
According to another preferred embodiment of the present invention, there is provided a treatment process for obtaining gradient properties, characterized by, comprising the steps of: A. preparing a blank, dividing the blank into a region to be formed as a hard zone, and a region to be formed as a soft zone; B. heating the region to be formed as the hard zone to 720 °C or more, such that its microstructure is transformed into austenite, and meanwhile maintaining the temperature of the region to be formed as the soft zone below 720 °C; C. stamping the entire blank and cooling it by any cooling method after the stamping; D. conducting carbon partitioning treatment on the hard zone of components obtained after the stamping to diffuse carbon from martensite to austenite.
According to still another preferred embodiment of the present invention, there is provided a component having gradient properties, characterized in that the component is made by the treatment processes of the preferred embodiments described above.
In one solution of the invention, the heating processes of the hard zone and the soft zone are different (the soft zone is not fully austenitic-heated), and the stamping process is the same for the soft zone and the hard zone, so that no die modification is required and the process is reliable. Another solution of the present invention is to first form a formed component by a stamping process, and then separately heat treat the soft zone, which has no influence on the stamping die or the process. It is worth emphasizing that it can be ensured that the soft zone elongation obtained by the two solutions of the present invention is greater than 15%, preferably 25 to 35%, which is far superior to the state of the art.
It should be noted that in the present description, the components with gradient properties include, but are not limited to, a B-pillar, an A-pillar, a front longitudinal beam, and the like of an automobile. "Hard zone" refers to the quench hardened region with high strength on the part, and "soft zone" refers to the region with low strength and high elongation. Taking the automotive B-pillar as an example, the hard zone is a region of the upper end that needs to prevent collision intrusion, and the soft zone is a region of the lower end that needs to absorb collision energy.
The microstructure of the soft zone includes: by area, from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide. The mechanical properties of the soft zone are: a tensile strength of 900-1500 MPa, and an elongation greater than 15%.
The microstructure of the hard zone includes, by area, from 3% to 23% of retained austenite, less than 2% of carbide, and the remainder being martensite. The mechanical properties of the hard zone are: a yield strength greater than 1200 MPa, a tensile strength greater than 1600 MPa, and an elongation greater than 10%.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a process route view in accordance with a first embodiment of the present invention;
FIG. 2 is a process route view in accordance with a second embodiment of the present invention;
FIG. 3 is a process route view in accordance with a third embodiment of the present invention;
FIG. 4 is a process route view in accordance with a fourth embodiment of the present invention;
FIG. 5 is a schematic illustration of an automobive B-pillar containing a hard zone and a soft zone.

It should be noted that the solid lines in the drawings represent the process paths that must be experienced, while the dashed lines represent the optional process paths.

DETAILED DESCRIPTION

The process routes of the present invention will be described in more detail below with reference to exemplary embodiments. The following embodiments are intended to illustrate the exemplary process routes of the present invention, and those skilled in the art should be aware that the present invention is not limited to these embodiments.
According to the present invention, first, a steel material for stamping can be provided, which includes in weight percent 0.22 to 0.48% of C, 5 to 9.5% of Mn, 0.2 to 3.0% of Si+Al, and the remainder being Fe and unavoidable impurities, wherein the steel material is one of hot-rolled coil (sheet), hot-rolled pickled coil (sheet), cold-rolled annealed coil (sheet), and cold-rolled coated coil (sheet). The soft zone portion is obtained by the heat treatment wherein it is heated before the stamping (solution 1, wherein the soft zone temperature is controlled during the austenitizing heating of the hard zone) or after the hot stamping (solution 2, wherein the soft zone is heated separately) to 600 to 780 °C and then soaked (wherein the temperature can be selected from 680, 700, 720 and 750 °C, etc.) for 0.5 to 60 minutes (wherein the time can be selected from 1, 3, 5, 10, 20, 30, 40 and 50, etc.). The changes of manganese, carbon and other alloying elements in the austenite upon annealing equilibrium of the two-phase zone are computed with thermodynamics softwares. With the composition design and the process selection, the soft zone obtains retained austenite with a volume fraction of 30% to 60%, a martensite (ferrite) structure with a volume fraction of 40% to 70%, and carbide of less than 3%. the carbon content and manganese content as originally designed in the alloy are 0.22% and 5% or more respectively, and in the heating process of the soft zone, carbon and manganese are diffused and enriched into the austenite,. Since the carbon content in the retained austenite finally measured under the above heating process is 0.5% or more, the manganese content is more than 7%, and the austenite grain size is less than 2 µm or the austenite lath thickness is less than 1 µm, compared with the prior art manganese steel, it is not necessary to achieve or be close to the carbon and manganese content in austenite in thermodynamic equilibrium to form stable retained austenite. During the deformation of the steel sheet, the retained austenite itself has high deformability and toughness, and the martensite transformation and/or deformation twinning in the retained austenite are/is beneficial to improving the energy absorption and the elongation of the steel sheet.
The steel material of the present invention is based on a high carbon and medium manganese composition, the carbon content being between 0.22 and 0.48%, preferably between 0.25 and 0.45%, and the manganese content being between 5 and 9.5%, preferably between 6 and 8%. Both carbon and manganese are austenite stabilizing elements, which can strongly reduce the austenitizing temperature of steel and the martensite transformation start temperature. During the annealing heat treatment, a structure of alternative austenite/ferrite laths is formed, and the carbon and manganese are partitioned into the austenite to stabilize the austenite to below room temperature. The retained austenite itself has high deformability and toughness, and can also undergo a TRIP effect during the deformation to gradually transform into martensite, which improves the strength and ductility of the steel material. In particular, the steel material of the invention has an optimized composition design and annealing process, wherein the retained austenite has high carbon and manganese, part of the austenite has high stacking fault energy, and deformation twinning is produced during the deformation, which can further improve the work hardening rate and meanwhile improve the material strength and ductility. When the contents of carbon and manganese are low, in order to obtain more austenite, the preferable annealing temperature needs to be increased, resulting in a decrease in the contents of carbon and manganese in the austenite and coarse grains, thus leading to poor austenite stability and a decrease in the toughness of the steel during the deformation. When the carbon content is high, a hyper-eutectoid structure may be formed. In the above heating process, a large number of coarse carbides are easily formed to deteriorate the mechanical properties of the steel, and a further increase of the carbon content deteriorates the toughness of the hard zone. The applicant finds that when the Mn content is controlled between 5 to 9.5%, and the carbon content is controlled between 0.22 and 0.48%, good strength and plasticity can be obtained.
According to a preferred embodiment of the present invention, the steel material further comprises at least one of the following elements: Cr: 0.001% to 5%; Mo: 0.001% to 2.0%; W: 0.001% to 2.0%; Ti: 0.0001% ∼0.4%; Nb: 0.0001% to 0.4%; Zr: 0.0001% to 0.4%; V: 0.0001% to 0.4%; Cu: 0.0005% to 2%; Ni: 0.0005% to 3.0%; B: 0.0001% to 0.005 %. By combination of at least one of these elements with the above-mentioned basic compositions, the combination of ultra-high strength and toughness of the stamped component can be further ensured, so that the mechanical properties thereof can be reached: a yield strength of 0.5 to 1.2 GPa, a tensile strength of 1.0 to 1.5 GPa, and a product of strength and elongation (tensile strength × elongation) of 25 GPa % or more.
According to a preferred embodiment of the invention, the steel material comprises a hot-rolled steel sheet, a cold-rolled steel sheet, or a steel sheet with a coating. The steel sheet with a coating may be a zinc-coated steel sheet which is a hot-rolled steel sheet or a cold-rolled steel sheet on which a metal zinc layer is formed. The zinc-coated steel sheet includes one selected from the group consisting of hot-dip galvanizing (GI), galvannealing (GA), and zinc plating or zinc-iron plating (GE). The steel sheet with a coating may be a hot-rolled steel sheet or a cold-rolled steel sheet on which an aluminum-silicon layer is formed, or a steel sheet with an organic coating or a steel sheet with other alloyed coatings.
Several preferred process routes of the gradient property treatment process of a steel material in the present invention will be described in detail below, which can achieve gradient properties on a single part. Of course, it should be understood by those skilled in the art that the process route of the present invention is not limited to the specific process routes described below.

Process route 1:

First, a blank such as a steel sheet, a steel coil, or a blanked sheet or a preformed component is prepared. The blank may be, for example, a blank having the composition and properties of the above steel material.
Then, as shown in FIG. 1, the entire blank is annealed, wherein the steel sheet and the steel coil can be heat treated in a continuous annealing production line or a continuous annealing coating production line of a steel plant. For example, the entire blank can be heated to 600-720 °C, soaked for 0.5-60 minutes, and then cooled by any means (such as gas cooling, air cooling in a continuous annealing production line, or cooling in a hot stamping die or air cooling) to a temperature of -100 °C or more, preferably to room temperature. After the annealing treatment, the microstructure of the blank includes, by area: from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide, wherein the retained austenite includes, by weight percent, 7% or more of Mn and 0.5% or more of carbon. After the annealing treatment, the blank has a tensile strength of 900 to 1500 MPa and an elongation of 15% or more, so that the blank has good formability at room temperature.
Then, the region to be formed as the hard zone is heated to 720 to 850 °C to transform its microstructure into austenite, while ensuring that the region to be formed as the soft zone is at a temperature lower than 720 °C in the process (for example, the region to be formed as the soft zone is not heated but maintained at room temperature or is heated to 650 °C). In this process, for example, the hard zone may first be inductively heated to, for example, 650 °C, while the soft zone is not heated (maintained at room temperature or rising to a lower temperature due to heat conduction during heating of the hard zone), and then the entire blank is placed in a furnace at a temperature of, for example, 780 °C to be heated. For the hard zone, the heating time required for heating from 650 °C to 780 °C is short, such as 40 seconds, and then the temperature is maintained for 20 seconds, for instance, to achieve homogenization of the austenite structure, so the entire blank is in the furnace at 780 °C for one minute, for example. In this one minute, the soft zone cannot be heated to 780 °C in the furnace at 780 °C due to the its low temperature upon entry into the furnace. In practice, it is only required that its temperature be controlled at 720 °C or less. It should be noted that a conventional hot formed steel material (for example, 22MnB5) cannot achieve homogenization of the austenite structure within a heating time of one minute because its austenitizing temperature is much higher than that of the material involved in the present invention.
Then, the entire blank is subjected to stamping. After the full austenite region is stamping formed, the hard zone can be cooled to 150 to 260 °C below its martensite transformation start temperature point (Ms) by any cooling method (for example, in-die cooling or air cooling). After the stamping, the soft zone can be cooled by a cooling method that is the same as that of the hard zone, such as in-die cooling or air cooling. The microstructure of the soft zone includes, by area: from 30% to 60% of austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide.
Then, the hard zone of the component obtained after the stamping (i. e., the formed component) is subjected to carbon partitioning treatment; for example, heating the hard zone to 160 to 450 °C and maintaining the temperature for 1 to 10000 seconds to cause carbon to diffuse from supersaturated martensite to austenite, so that the austenite is enriched in carbon, thereby greatly improving the stability of the austenite and increasing its residual amount at room temperature. Preferably, a phase transformation from martensite to austenite occurs, thereby increasing the residual austenite content and improving the mechanical properties. In addition, for the soft zone, the same carbon partitioning treatment can be performed with the hard zone, or the carbon partitioning treatment is not performed (that is, the hard zone is separately subjected to carbon partitioning treatment). Regardless of whether or not carbon partitioning treatment is carried out, the mechanical properties of the soft zone of the formed component can reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more. However, in order to simplify the operation, it is preferable to carry out carbon partitioning treatment on the entire formed component.

Process route 2:

First, a blank such as a steel sheet, a steel coil, or a blanked sheet or a preformed component is prepared. The blank may be, for example, a blank having the composition and properties of the above steel material.
Then, as shown in FIG. 2, the regions of the blank to be formed as the soft zone and the hard zone are simultaneously heated and soaked for 0.5 to 60 minutes, wherein the region to be formed as the soft zone is heated to and kept at a temperature of 600 to 720 °C, and the region to be formed as the hard zone is heated to and kept at a temperature of 720 to 850 °C to transform its microstructure into austenite. During the heating and soaking, the microstructure of the soft zone includes, by area: from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide, wherein the retained austenite includes, by weight percent, 7% or more of Mn and 0.5% or more of carbon, while the hard zone contains a full austenitic structure and less than 3% of the carbide. In this process, for example, the hard zone can first be inductively heated to, for instance, 650 °C, while the soft zone is heated by induction to, for instance, 500 °C, and then the entire blank is placed in a furnace at a temperature of, for example, 780 °C to be heated. For the hard zone, the heating time required for heating from 650 °C to 780 °C is short, such as 40 seconds, and then the temperature is maintained for 20 seconds, for instance, to achieve homogenization of the austenite structure, so the entire blank is in the furnace at 780 °C for one minute, for example. In this one minute, the soft zone cannot be heated to 780 °C in the furnace at 780 °C due to the its low temperature upon entry into the furnace. In practice, it is only required that its temperature be controlled at 720 °C or less.
Then, the entire blank is subjected to stamping, and after the stamping, is cooled by any cooling method (in-die cooling or air cooling). The hard zone is cooled to 150 to 260 °C below its martensite transformation start temperature point, and the soft zone is cooled to any temperature of -50 °C or more. Preferably, in order to facilitate the implementation of the stamping and cooling process, the formed component as a whole is cooled by the cooling method of the hard zone, and the microstructure and properties of the soft zone are also satisfactory.
Then, the hard zone of the formed component is subjected to carbon partitioning treatment; for example, heating the hard zone to 160 to 450 °C and maintaining the temperature for 1 to 10000 seconds to cause carbon to diffuse from supersaturated martensite to austenite, so that the austenite is enriched in carbon, thereby greatly improving the stability of the austenite and increasing its residual amount at room temperature. Preferably, a phase transformation from martensite to austenite occurs, thereby increasing the residual austenite content and improving the mechanical properties. In addition, for the soft zone, the same carbon partitioning treatment can be performed with the hard zone, or the carbon partitioning treatment is not performed (that is, the hard zone is separately subjected to carbon partitioning treatment). Regardless of whether or not carbon partitioning treatment is carried out, the mechanical properties of the soft zone of the formed component can reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more. However, in order to simplify the operation, it is preferable to carry out carbon partitioning treatment on the entire formed component.

Process route 3:

First, a blank such as a steel sheet, a steel coil, or a blanked sheet or a preformed component is prepared. The blank may be, for example, a blank having the composition and properties of the above steel material.
Then, as shown in FIG. 3, the entire blank is heated to 720 to 850 °C and soaked for 0.5 to 60 min so that it has a full austenite structure and less than 3% of carbide.
Then, the entire blank is subjected to stamping, and after the stamping, is cooled by any cooling method (in-die cooling or air cooling). During the cooling, the region to be formed as the hard zone is cooled to 150 to 260 °C below its martensite transformation start temperature point, and the soft zone is cooled to any temperature from -100 °C to 600 °C. Preferably, in order to facilitate the implementation of the stamping and cooling process, the formed component as a whole is cooled by the cooling method of the hard zone, and the microstructure and properties of the soft zone are also satisfactory.
Then, the region to be formed as a hard zone of the formed component obtained after the stamping is subjected to carbon partitioning treatment; for example, heating the hard zone to 160 to 450 °C and maintaining the temperature for 1 to 10000 seconds to cause carbon to diffuse from supersaturated martensite to austenite, so that the austenite is enriched in carbon, thereby greatly improving the stability of the austenite and increasing its residual amount at room temperature. Preferably, a phase transformation from martensite to austenite occurs, thereby increasing the residual austenite content and improving the mechanical properties. In addition, for the soft zone, the same carbon partitioning treatment can be performed with the hard zone, or the carbon partitioning treatment is not performed (that is, the hard zone is separately subjected to carbon partitioning treatment). Regardless of whether or not carbon partitioning treatment is carried out, the mechanical properties of the soft zone of the formed component can reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more. However, in order to simplify the operation, it is preferable to carry out carbon partitioning treatment on the entire formed component.
Then, as shown in FIG. 3, the soft zone is separately heat treated again, heated to 600 to 720 °C, soaked for 0.5 to 60 minutes, and then cooled to room temperature in any manner (for example, air cooling). After the heat treatment, the microstructure of the steel material includes, by area: from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide, wherein the retained austenite includes, by weight percent, 7% or more of Mn and 0.5% or more of carbon, and its mechanical properties reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more.

Process route 4:

First, a blank such as a steel sheet, a steel coil, or a blanked sheet or a preformed component is prepared. The blank may be, for example, a blank having the composition and properties of the above steel material.
Then, as shown in FIG. 4, the entire blank is heated to 720 to 800 °C and soaked for 0.5 to 60 min so that it has a full austenite structure and less than 3% of carbide.
Then, the entire blank is subjected to stamping, and after the stamping, is cooled by any cooling method (in-die cooling or air cooling). During the cooling, the region to be formed as the hard zone is cooled to 150 to 260 °C below its martensite transformation start temperature point, and the soft zone is cooled to any temperature of -100 °C or more. Preferably, in order to facilitate the implementation of the stamping and cooling process, the formed component as a whole is cooled by the cooling method of the hard zone, and the microstructure and properties of the soft zone are also satisfactory.
Then, as shown in FIG. 4, the soft zone is separately heat treated again; for instance, it is heated to 600 to 720 °C, soaked for 0.5 to 60 minutes, and then cooled to room temperature in any manner (for example, air cooling). After the heat treatment, the microstructure of the steel material includes, by area: from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide, wherein the retained austenite includes, by weight percent, 7% or more of Mn and 0.5% or more of carbon, and its mechanical properties reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more.
Then, the region to be formed as the hard zone of the formed component obtained after the stamping is subjected to carbon partitioning treatment; for example, heating the hard zone to 160 to 450 °C and maintaining the temperature for 1 to 10000 seconds to cause carbon to diffuse from supersaturated martensite to austenite, so that the austenite is enriched in carbon, thereby greatly improving the stability of the austenite and increasing its residual amount at room temperature. Preferably, a phase transformation from martensite to austenite occurs, thereby increasing the residual austenite content and improving the mechanical properties. In addition, for the soft zone, the same carbon partitioning treatment can be performed with the hard zone, or the carbon partitioning treatment is not performed (that is, the hard zone is separately subjected to carbon partitioning treatment). Regardless of whether or not carbon partitioning treatment is carried out, the mechanical properties of the soft zone of the formed component can reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more. However, in order to simplify the operation, it is preferable to carry out carbon partitioning treatment on the entire formed component.
It should be noted that in the separate heat treatment process of the soft zone, for example, the soft zone may be heated by means of flame heating, induction heating, laser heating, or the like, or the whole compnent enters the heating furnace to be treated by different heating temperatures in the soft and hard zones. For example, thermal radiation resistant baffles are placed above and below the hard zone, a heat-insulating material wraps the hard zone, or a heat-insulating coating is applied on the hard zone.
According to the embodiments of the present invention, the heating processes of the hard zone and the soft zone are different (the soft zone is not fully austenitic-heated), but the stamping process is the same for the soft zone and the hard zone, so that no die modification is required and the process is reliable.
The above described process routes of the present invention can be used to fabricate any component that requires gradient properties including, but not limited to, a B-pillar, an A-pillar, a front longitudinal beam, and the like of an automobile. FIG. 5 is a schematic view of an automobile B-pillar comprising a hard zone and a soft zone made by the above-described process of the present invention, wherein the hard zone is a region of the upper end that needs to prevent collision intrusion, and the soft zone is a region of the lower end that needs to absorb collision energy.
According to the embodiments of the present invention, the microstructure of the soft zone of the formed component includes, by area: from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide. The mechanical properties of the soft zone are: a tensile strength of 900-1500 MPa, and an elongation greater than 15%. The microstructure of the hard zone includes, by area, from 3% to 23% of retained austenite, from 0 to 2% of carbide, and the remainder being martensite. The mechanical properties of the hard zone are: a yield strength greater than 1200 MPa, a tensile strength greater than 1600 MPa, and an elongation greater than 10%.
The preferred embodiments of the present invention have been described above, but it should be understood by those skilled in the art that any possible variation or substitution made without departing from the concept of the present invention falls into the scope of protection of the invention.

Claims

A treatment process for obtaining gradient properties, characterized by, comprising the steps of:
A. preparing a blank, dividing the blank into a region to be formed as a hard zone and a region to be formed as a soft zone;

B. heating the region to be formed as the hard zone to 720 °C or more, such that its microstructure is transformed into austenite, and meanwhile maintaining a temperature of the region to be formed as the soft zone below 720 °C;

C. stamping the entire blank and cooling it by any cooling method after the stamping;

D. conducting carbon partitioning treatment on the hard zone of a component obtained after the stamping to diffuse carbon from martensite to austenite.
The treatment process for obtaining gradient properties according to claim 1, characterized in that, after step A and before step B, the method further comprises the steps of conducting annealing treatment of the entire blank, that is, heating the entire blank to 600 to 720 °C, soaking for 0.5 to 60 minutes, and then cooling it to room temperature by any cooling method.
The treatment process for obtaining gradient properties according to claim 2, characterized in that, in step B, the temperature of the region to be formed as the soft region is maintained at 720 °C or less.
The treatment process for obtaining gradient properties according to claim 1, characterized in that, in step B, the regions of the blank to be formed as the soft zone and the hard zone are simultaneously heated and soaked for 0.5 to 60 minutes, wherein the region to be formed as the soft zone is heated to and kept at a temperature of 600 to 720 °C, and the region to be formed as the hard zone is heated and kept at a temperature of 720 to 850 °C.
The treatment process for obtaining gradient properties according to any one of claims 1 to 4, characterized in that, in step C, the hard zone is cooled to 150 to 260 °C below its martensite transformation start temperature point, and the soft zone is cooled to any temperature of -100 °C or more.
The treatment process for obtaining gradient properties according to any one of claims 1 to 4, characterized in that, in step D, the hard zone is heated to 160 to 450 °C and soaked for 1 to 10000 seconds.
The treatment process for obtaining gradient properties according to any one of claims 1 to 4, characterized in that, in step D, the soft zone is treated by the same carbon partitioning treatment conducted in the hard zone, or is not treated.
The treatment process for obtaining gradient properties according to any one of claims 1 to 4, characterized in that, a microstructure of the soft zone of the component obtained after the stamping includes, by area, from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide, wherein mechanical properties of the soft zone are: a tensile strength of 900-1500 MPa, and an elongation greater than 15%; a microstructure of the hard zone includes, by area, from 3% to 23% of retained austenite, from 0 to 2% of carbide, and the remainder being martensite, and mechanical properties of the hard zone are: a yield strength greater than 1200 MPa, a tensile strength greater than 1600 MPa, and an elongation greater than 10%.
A treatment process for obtaining gradient properties, characterized by, comprising the steps of:
A. preparing a blank, dividing the blank into a region to be formed as a hard zone, and a region to be formed as a soft zone;

B. heating the entire blank to 720 to 800 °C and maintaining the temperature for 0.5 to 60 min so that its structure transforms into austenite;

C. stamping the entire blank and cooling it by any cooling method after the stamping;

D. conducting carbon partitioning treatment on the hard zone of the component obtained after the stamping to diffuse carbon from martensite to austenite.
The treatment process for obtaining gradient properties according to claim 9, characterized in that, in step C, the region to be formed as the hard zone is cooled to 150 to 260 °C below its martensite transformation start temperature point, and the soft zone is cooled to any temperature of -100 °C or more.
The treatment process for obtaining gradient properties according to any one of claims 9-10, characterized in that, before or after step D, heat treatment of the soft zone is conducted separately, that is, it is heated to 600 to 720 °C, soaked for 0.5 to 60 minutes, and then cooled to room temperature by any cooling method.
The treatment process for obtaining gradient properties according to claim 11, characterized in that, after heat treatment of the soft zone is conducted separately, a microstructure of the blank or the formed component includes, by area, from 30% to 60% of retained austenite, from 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbide, wherein the retained austenite includes, by weight percent, 7% or more of Mn and 0.5% or more of carbon, and its mechanical properties reach a tensile strength of 900 to 1500 MPa and an elongation of 15% or more.
A component with gradient properties, characterized in that, the component is made by the treatment process of any one of claims 1-12.
The component according to claim 13, characterized in that, the component comprises a B-pillar, an A-pillar, and a front longitudinal beam of an automobile.
The component according to any one of claims 13 to 14, characterized in that, the microstructure of the soft zone of the component comprises, by area: 30% to 60% of retained austenite, 40% to 70% of martensite or ferrite with a body-centered cubic crystal structure, and less than 3% of carbides, and the mechanical properties of the soft zone are: a tensile strength of 900-1500MPa, and an elongation greater than 15%; the microstructure of the hard zone includes, by area: 3% to 23% of retained austenite, 0 to 2% of carbides, and the remainder being martensite, and the mechanical properties of the hard zone are: a yield strength greater than 1200 MPa, a tensile strength greater than 1600 MPa, and an elongation greater than 10%.