High-strength press-hardened steel parts and their manufacturing method

Abstract

The present invention provides press-hardened steel parts that possess high mechanical properties, including a tensile strength (TS) of 1000 MPa or more, a uniform elongation loss (ΔUEI) of 25% or less in the spot-welded area, and a bending angle of 55° or more. [Solution] A press-hardened steel part is provided, having a composition in weight percent of C: 0.2~0.34%, Mn: 0.50~1.24%, Si: 0.5~2%, P≦0.020%, S≦0.010%, N≦0.010%, and optionally containing one or more of the following elements: Al≦0.2%, Cr≦0.8%, Nb≦0.06%, Ti≦0.06%, B≦0.005%, Mo≦0.35%, with the remainder of the composition being iron and unavoidable impurities resulting from smelting. This press-hardened part has a microstructure in which tempered martensite accounts for 95% or more of the surface fraction, and bainite, austenite, and ferrite totaling 5% or less.

Description

[Technical Field]

[0001] This invention relates to high-strength press-hardened steel parts with excellent bendability and weldability. [Background technology]

[0002] High-strength press-hardened parts can be used as structural elements in automobiles for intrusion prevention or energy absorption functions.

[0003] For this type of application, it is desirable to manufacture steel components that combine high mechanical strength, high impact resistance, and good corrosion resistance.

[0004] Another major challenge in the automotive industry is reducing vehicle weight to improve fuel efficiency from an environmental protection perspective, without neglecting safety requirements.

[0005] This weight reduction can be achieved, in particular, by using steel components with tempered martensite or bainite / martensite microstructures.

[0006] These types of parts can be welded, and automotive manufacturers stipulate that welded joints should not constitute the weakest part of a welded steel component.

[0007] In fact, if spot welds are present on structural components of the vehicle body, strain can concentrate in the softened heat-affected zone (HAZ), potentially leading to failure during a collision. [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, the object of the present invention is to solve the above problems and provide a press-hardened steel part having high mechanical properties with a tensile strength TS of 1000 MPa or more, a uniform elongation loss ΔUEl of 25% or less in the spot-welded area, and a bending angle of 55° or more.

[0009] Preferably, the press-hardened steel part according to the present invention has a fracture strain of 0.50 or more.

[0010] Preferably, the press-hardened steel part according to the present invention has a yield strength YS of 980 MPa or more.

Means for Solving the Problems

[0011] The object of the present invention is achieved by providing a steel part according to claim 1. The steel part can also include any of the features of claims 2 to 4. Another object is achieved by providing a method according to claim 5. Another object is achieved by providing a method according to any of claims 6 to 8.

Embodiments for Carrying Out the Invention

[0012] Here, the present invention will be described in detail and illustrated by way of example without introducing any limitations.

[0013] Next, the composition of the steel of the present invention will be described, and the content is expressed in weight percent.

[0014] According to the present invention, the carbon content is 0.2% to 0.34% in order to ensure satisfactory strength. If the carbon exceeds 0.34%, the fracture strain and bending angle of the steel sheet do not reach the target values. Also, the weldability of the steel sheet may decrease. When the carbon content is less than 0.2%, the tensile strength and yield strength do not reach the target values.

[0015] The manganese content is 0.50% to 1.24%. When the addition exceeds 1.24%, the risk of center segregation increases, impairing the bendability and possibly reducing the fracture strain. When it is less than 0.50%, the hardenability of the steel sheet decreases, and the tensile strength and yield strength do not reach the target values.

[0016] The silicon content is 0.5% to 2%. Silicon is an element involved in the hardening in the solid solution. Silicon is added to limit the formation of carbides and ensure a high level of tensile strength. When it exceeds 2%, silicon oxide is formed on the surface, impairing the coating property of the steel. Also, there is a risk of deterioration in the weldability of the steel sheet. Preferably, the silicon content is 0.5% to 1.8%. More preferably, the silicon content is 0.6% to 1.8%, and even more preferably 0.6% to 1.6%.

[0017] Several elements can be added optionally.

[0018] Since the aluminum content is an element very effective for deoxidizing the steel in the liquid phase during refining, it can be added optionally up to 0.2%. Preferably, the aluminum content is 0.1% or less. More preferably, the aluminum content is 0.06% or less.

[0019] Optionally, the chromium content can be added up to 0.8% to improve the hardening in the solid solution. The chromium content is 0.8% or less to limit the processing problems and cost. Preferably, the chromium content is 0.6% or less.

[0020] The niobium content can be added optionally up to 0.06% for the refinement of the prior austenite grain size and the improvement of the ductility of the steel. With an addition exceeding 0.06%, the risk of formation of carbides such as NbC or Nb(C,N) increases, impairing the bendability.

[0021] The titanium content can be added optionally up to 0.06% to protect boron from the formation of BN. Preferably, the titanium content is higher than 0.01%.

[0022] The boron content can be added optionally up to 0.005%. Boron improves the hardenability of the steel. The boron content is 0.005% or less to avoid the risk of breaking the slab during continuous casting.

[0023] Molybdenum can be optionally added up to 0.35%. Similar to boron, molybdenum improves the hardenability of steel. To limit costs, the molybdenum content is 0.35% or less.

[0024] The remainder of the steel's composition consists of iron and impurities resulting from smelting. In this respect, P, S, and N are considered residual elements that are at least unavoidable impurities. Their content is 0.020% or less for P, 0.010% or less for S, and 0.010% or less for N.

[0025] Next, the microstructure of the press-hardened steel part according to the present invention will be described.

[0026] Press-hardened steel parts have a microstructure containing tempered martensite at a surface fraction of 95% or more. This tempered martensite is present in the steel parts at temperatures between 390°C and 510°C. temp The holding time t is included in 1 second to 1000 seconds. temp It forms while being heated.

[0027] Some bainite, ferrite, and austenite may be present at will, and their total proportion is less than 5% of the surface fraction.

[0028] Preferably, the microstructure of the press-hardened steel part is made entirely from tempered martensite.

[0029] The press-hardened steel parts according to the present invention can be manufactured by any suitable manufacturing method, and a person skilled in the art can specify such method. However, it is preferable to use the method according to the present invention, which includes the following steps.

[0030] A steel sheet having the composition according to the present invention is prepared and cut into a predetermined shape to obtain a steel blank.

[0031] The steel blank is manufactured at temperatures within the range of 810°C to 960°C, preferably 850°C to 950°C, and more preferably 880°C to 950°C.HF To obtain a heated steel blank that is heated up to and has a fully austenite fine structure, a holding time t included in 5 seconds to 1200 seconds HF during which the said T HF is maintained at the temperature. The heated steel blank is transferred to a forming press and hot forming to obtain a steel component.

[0032] Next, the steel component is quenched until it reaches a temperature of 200°C or lower.

[0033] The steel component is reheated to a temperature T included in 390°C to 510°C, and a holding time t included in 1 second to 1000 seconds temp during which the said temperature T temp is maintained to obtain a tempered steel component and ensure temperature uniformity on all steel components. temp When exceeding 510°C, the tensile strength of the steel component decreases. When less than 390°C, the uniform elongation loss ΔUEl in the spot welding area exceeds 25%. Then, the tempered steel component is cooled to room temperature.

[0034] For each tempered product, the HAZ susceptibility is evaluated by the uniform elongation loss of the JIS tensile test piece with welding compared to the reference without welding. The uniform elongation loss ΔUEl is calculated as follows.

[0035] The uniform elongation UEl of the steel is measured according to JIS standard Z2241 for the tensile test piece. A welding spot is made on the tensile test piece centered on the deformation area of the test piece. The uniform elongation UEl

[0036] of this welded tensile test piece is measured according to JIS standard Z2241. w is measured according to JIS standard Z2241.

[0037] The uniform elongation loss ΔUEl is obtained by the following formula. ΔUEl = [(UEl - UEl w ) / UEl] * 100

[0038] In a first preferred embodiment of the present invention, the steel sheet prepared for manufacturing a steel component is manufactured by the following sequential process.

[0039] A steel slab having the above composition is cast, and the temperature range T is within 1100℃ to 1300℃. reheat After reheating, the steel is hot-rolled at a finishing hot-rolling temperature within the range of 800°C to 950°C to obtain a hot-rolled steel sheet.

[0040] Next, the hot-rolled steel sheet was heated to a temperature lower than 670°C T coil Wind it up.

[0041] Hot-rolled steel sheets can be optionally pickled to remove oxidation.

[0042] Hot-rolled steel sheets are produced at temperatures within the range of 500°C to 750°C, with the temperature range being selected as appropriate. HBA It is heated to a certain temperature, and the holding time is 300 seconds to 50 hours. HBA During the period, the aforementioned T HBA It can be maintained at a certain temperature.

[0043] Next, the steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction ratio is preferably between 20% and 80%. If it is less than 20%, recrystallization during subsequent heat treatment is undesirable, which may impair the ductility of the steel sheet. If it exceeds 80%, there is a risk of edge cracking during cold rolling.

[0044] Cold-rolled steel sheets are annealed at an optional temperature range T, which falls within the range of 650°C to 900°C. A Annealing until 10 seconds to 1200 seconds, with a holding time of t A During the period of the temperature T A The process is maintained to obtain an annealed steel sheet, which reduces the tensile strength and makes the steel easier to cut. Finally, the steel sheet is cooled to room temperature.

[0045] Preferably, the annealed steel sheet is coated with an aluminum coating or an aluminum alloy coating, or a zinc coating or a zinc alloy coating, before it cools to room temperature.

[0046] In a second embodiment of the present invention, the steel sheet prepared for manufacturing a steel component is manufactured by the following sequential process.

[0047] A steel slab having the composition according to the present invention is cast, and the temperature T within 1100℃ to 1300℃ is cast. reheat After reheating, the steel is hot-rolled at a finishing hot-rolling temperature within the range of 800°C to 950°C to obtain a hot-rolled steel sheet.

[0048] Next, the hot-rolled steel sheet was heated to a temperature lower than 670°C T coil Wind it up.

[0049] Hot-rolled steel sheets can be selectively pickled to remove oxidation. HBA Heat until heated, then hold for 300 seconds to 50 hours. HBA During the T HBA It can be maintained at a certain temperature.

[0050] Subsequently, the material is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction ratio is preferably 20% to 80%. If it is less than 20%, recrystallization during subsequent heat treatment is undesirable, which may impair the ductility of the steel sheet. If it exceeds 80%, there is a risk of edge cracking during cold rolling.

[0051] Cold-rolled steel sheets are annealed at an arbitrary temperature range T between 500°C and 750°C. A Annealing is performed until 300 seconds to 50 hours, with a holding time of t. A During the period of the temperature T A The process is maintained to obtain an annealed steel sheet, which reduces the tensile strength and makes the steel easier to cut. Finally, the steel sheet is cooled to room temperature.

[0052] The press-hardened steel part according to the present invention has a tensile strength TS of 1000 MPa or more, a uniform elongation loss ΔUEl of 25% or less in the spot-welded area, and a bending angle of 55° or more.

[0053] In a preferred embodiment of the present invention, the press-hardened steel part has a yield strength YS of 980 MPa or more.

[0054] In another preferred embodiment, the press-hardened steel part according to the present invention has a fracture strain of 0.50 or more.

[0055] The present invention will be explained by the following examples, but these are by no means limiting. [Examples]

[0056] The eight grades whose compositions are summarized in Table 1 were cast into semi-finished products, and then processed into steel plates and then steel parts according to the process parameters summarized in Table 2.

[0057] The tested compositions are summarized in the table below, with elemental content expressed as weight percentage.

[0058] [Table 1]

[0059] The cast steel semi-finished product was reheated at 1250°C, hot-rolled at a finish hot-rolling temperature within 800-950°C, coiled at 580°C, and cold-rolled with a reduction ratio of 58%. Subsequently, the steel sheet was cold-rolled at a temperature of 790°C T A Heat until done, then hold for 180 seconds. A During the period of the temperature T A Maintain it.

[0060] This steel plate was cut into a predetermined shape to obtain a steel blank. Next, the steel blank was heated to a temperature T HF Hold time t for 120 seconds HF After heating, the blank was transferred to a forming press. The heated blank was hot-formed in the forming press to obtain steel parts, which were then hardened in a mold until the temperature reached 80°C.

[0061] Next, the steel parts are heated to a temperature of 390°C to 510°C T tempReheat until done, then hold for 1 to 1000 seconds. temp During the T temp The temperature was maintained and then cooled to room temperature.

[0062] [Table 2]

[0063] The steel components were analyzed, and the corresponding microstructures and properties were summarized in Tables 3 and 4, respectively.

[0064] [Table 3]

[0065] The surface fraction is determined by the following method: A specimen is cut from a press-hardened steel part, polished, and etched with a known reagent, such as Nital reagent, to reveal the microstructure. Then, sections are examined using an optical or scanning electron microscope, for example, a scanning electron microscope with a field emission gun ("FEG-SEM") with a magnification of over 5000x coupled to an electron backscatter diffraction (EBSD) apparatus. Tempered martensite can be distinguished from martensite because it has a lower dislocation density compared to martensite.

[0066] TS and YS are measured according to ISO standard ISO6892-1.

[0067] The bending angle was determined for press-hardened parts according to the VDA238-100 bending standard (normalized to a thickness of 1.5 mm).

[0068] The term "fracture strain" refers to the fracture strain criteria defined by Pascal Dietsch et al. in "Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration" in Metallurgical Research Technology, Vol. 114, No. 6, 2017. Fracture strain is the equivalent strain in the material at the deformation point when the critical bending angle is reached. Fracture strain values ​​are determined under the plane strain condition, which is the most severe condition from the perspective of vehicle crashes, and are obtained by finite element analysis.

[0069] [Table 4]

[0070] Weld spots are made on the tensile test specimen, centered on the deformation region. The corresponding uniform elongation loss ΔUEl of the resistance spot weld is summarized in Table 5.

[0071] [Table 5]

[0072] As a result of the specific compositions and process parameters used, the embodiments according to the present invention, namely Examples 1-7, are unique in that they exhibit a combination of high mechanical properties with a TS higher than 1000 MPa, a bending angle of 55° or more, and a uniform elongation loss of less than 25%. Furthermore, Examples 1-7 have a fracture strain higher than 0.50.

[0073] The tempering temperatures applied to the steel parts in trials 8 and 9 were too low to limit the detrimental effects of HAZ softening on uniform elongation, as indicated by the uniform elongation loss of more than 25%.

[0074] Furthermore, compared to trial 2, which has the same steel composition, the lower tempering temperature in trial 8 results in higher uniform elongation loss and lower fracture strain than in trial 2.

[0075] The steel parts in trial 10 were not tempered, which means a uniform elongation loss higher than 25%.

[0076] In trial 11, the carbon content of the steel component was too high to achieve the target fracture strain and flexural values.

Claims

1. Press-hardened steel parts, in weight percentage, C: 0.2-0.34% Mn: 0.50-1.24% Si: 0.5-2% P ≤ 0.020% S ≤ 0.010% N ≤ 0.010% It includes, optionally, the following elements in weight percentage: Al ≤ 0.2% Cr ≤ 0.8% Nb ≤ 0.06% Ti ≤ 0.06% B ≤ 0.005% Mo ≤ 0.35% The composition has one or more of the following, the remainder of the composition is iron and unavoidable impurities resulting from smelting, and in terms of surface fraction, - 95% or more tempered martensite, and - Bainite, austenite, and ferrite making up 5% or less of the total. Press-hardened steel parts made from steel having a microstructure including the following.

2. The press-hardened steel part according to claim 1, having a tensile strength TS of 1000 MPa or more, a uniform elongation loss ΔUEl of 25% or less in the spot-welded area, and a bending angle of 55° or more.

3. A press-hardened steel part according to claim 1 or 2, having a fracture strain of 0.50 or more.

4. A press-hardened steel part according to any one of claims 1 to 3, having a yield strength YS of 980 MPa or more.

5. A method for manufacturing press-hardened steel parts, comprising the following series of steps: - A step of preparing a steel sheet having the composition described in claim 1, - A step of cutting the steel plate into a predetermined shape to obtain a steel blank, - The steel blank is subjected to a temperature T within the range of 810°C to 960°C. ＨＦ Heat until heated, and hold for 5 to 1200 seconds. ＨＦ During the T ＨＦ The steps include maintaining a temperature and obtaining a heated steel blank, - Step of transferring the heated blank to a forming press, - A step of obtaining a steel part by hot forming the heated blank in a forming press, - A step of heat-treating the steel parts in a mold until they reach a temperature of 200°C or lower. - Temperature T included in 390°C to 510°C ｔｅｍｐ The steel parts are reheated until the holding time t is included in 1 second to 1000 seconds. ｔｅｍｐ During the T ｔｅｍｐ A step of maintaining a temperature to obtain tempered steel parts, - A step of cooling the tempered steel part to room temperature. A method for manufacturing press-hardened steel parts, including the method described above.

6. A method for manufacturing a press-hardened steel part according to claim 5, wherein the steel sheet is prepared in the following sequential steps: - A step of casting steel to obtain a slab, wherein the steel has the composition described in claim 1. - Temperature T corresponding to 1100°C to 1300°C ｒｅｈｅａｔ The step of reheating the slab, - The reheated slab is hot-rolled at a finishing hot-rolling temperature within 800°C to 950°C to obtain a hot-rolled steel sheet. - The hot-rolled steel sheet is wound at a temperature T of less than 670°C. ｃｏｉｌ The step of winding up the steel plate and obtaining the wound-up steel plate, - Optionally, a step of pickling the wound steel sheet, - Optionally, heating the hot-rolled steel sheet to a temperature T included in the range of 500°C to 750°C ＨＢＡ and maintaining it at the temperature T for a holding time t included in the range of 300 seconds to 50 hours ＨＢＡ during which ＨＢＡ step - The step of cold-rolling the steel sheet to obtain a cold-rolled steel sheet, - Optionally, the cold-rolled steel sheet is annealed at an annealing temperature T between 650°C and 900°C. Ａ Heat until heated, and hold for 10 to 1200 seconds. Ａ During the time the steel plate is heated to the temperature T Ａ The step of maintaining and obtaining an annealed steel sheet, - A step of cooling the steel plate to room temperature. A method for manufacturing press-hardened steel parts.

7. A method for manufacturing a press-hardened steel part according to claim 5, wherein the steel sheet is subjected to the following continuous steps: - A step of casting steel to obtain a slab, wherein the steel has the composition described in claim 1. - Temperature T corresponding to 1100°C to 1300°C ｒｅｈｅａｔ The step of reheating the slab, - The reheated slab is hot-rolled at a finish hot-rolling temperature within 800°C to 950°C to obtain a hot-rolled steel sheet. - The hot-rolled steel sheet is wound at a temperature T of less than 670°C. ｃｏｉｌ The step of winding up the steel plate and obtaining the wound-up steel plate, - Optionally, a step of pickling the wound steel sheet, - Optionally, the hot-rolled steel sheet is subjected to a temperature T within the range of 500°C to 750°C. ＨＢＡ Heat until heated, and the holding time is 300 seconds to 50 hours. ＨＢＡ During the T ＨＢＡ Steps to maintain temperature, - The step of cold-rolling the steel sheet to obtain a cold-rolled steel sheet, - Optionally, the cold-rolled steel sheet is annealed at an annealing temperature T within the range of 500°C to 750°C. Ａ Heat until heated, and the holding time is 300 seconds to 50 hours. Ａ During the time the steel plate is heated to the temperature T Ａ The step of maintaining and obtaining an annealed steel sheet, - A step of cooling the steel plate to room temperature. A method for manufacturing press-hardened steel parts.

8. The method for manufacturing a press-hardened steel part according to claim 6, wherein the annealed steel sheet is coated with an aluminum coating or an aluminum alloy coating, or a zinc coating or a zinc alloy coating.

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