High-strength cold-rolled steel sheet for automotive use having excellent overall formability and bending properties

Abstract

The invention relates to a high strength cold rolled steel sheet having a composition consisting of the following elements in wt%: C 0.08-0.14 Mn 2.5-3.0 Si 0.7-1.1 Cr 0.05-0.4 optionally Al ≤ 0.2 Nb ≤ 0.1 Mo ≤ 0.1 V ≤ 0.1 Ti ≤ 0.1 Ca ≤ 0.05 Cu ≤ 0.1 Ni ≤ 0.2 B ≤ 0.005 balance Fe except impurities. The steel has a yield ratio ≤ 0.72 and a bendability (Ri / t) ≤ 2.0. The invention also relates to a method of manufacturing the steel sheet and to a motor vehicle structural part comprising the steel sheet.

Description

Technical Field

[0001] This invention relates to high-strength steel sheets suitable for use in motor vehicles. In particular, this invention relates to cold-rolled steel sheets having a tensile strength of at least 980 MPa and exhibiting excellent global formability and excellent bending properties. Background Technology

[0002] Increased strength is a prerequisite for lightweight construction for a wide variety of applications, especially in the automotive industry, where reduced vehicle weight leads to lower fuel consumption.

[0003] Motor vehicle body parts are typically stamped from sheet steel to form complex structural components made of thin sheet metal. However, such parts cannot be produced from conventional high-strength steel due to its extremely low formability for complex structures. For this reason, transformation-induced plasticity (TRIP) steel has attracted considerable attention in recent years, particularly for use in automotive body structural parts.

[0004] TRIP steel possesses a multiphase microstructure, including a metastable retained austenitic phase capable of generating the TRIP effect. When the steel is deformed, the austenite transforms into martensite, resulting in significant work hardening. This hardening effect resists necking and delays failure during sheet forming operations. The microstructure of TRIP steel can significantly alter its mechanical properties.

[0005] WO2018 / 09090A1 discloses a high-strength TBF steel with a high yield ratio (local formability), excellent resistance to edge cracking, and high porosity.

[0006] While these steels disclose several attractive properties, there is still a need for 980MPa steel sheets with improved performance characteristics in terms of overall formability and bending properties. In particular, this material will be used for B-pillar hinges, roof rails, door panels, or similar parts in motor vehicles. Summary of the Invention

[0007] This invention relates to high-strength (TBF) steel sheets with tensile strength of 980-1180 MPa, excellent overall formability, and excellent bending properties. Furthermore, the steel sheets should be industrially producible in a continuous annealing line (CAL). The object of this invention is to provide steel compositions that can be machined into complex structural components, where both yield ratio and bending properties, which affect overall formability, are important. They are particularly suitable for B-pillar hinges, roof rails, door panels, or similar parts in motor vehicles. Attached Figure Description

[0008] Figure 1 This is a schematic diagram in which the bending property Ri / t is plotted against the yield ratio. Detailed Implementation

[0009] The present invention is described in the claims.

[0010] The steel plate has a composition consisting of the following alloying elements (in weight percent):

[0011] C 0.08-0.14

[0012] Mn 2.5-3.0

[0013] Si 0.7-1.1

[0014] Cr 0.05-0.4

[0015] Optional

[0016] Al≤0.2

[0017] Nb≤0.1

[0018] Mo≤0.1

[0019] V≤0.1

[0020] Ti≤0.1

[0021] Ca≤0.05

[0022] Cu≤0.1

[0023] Ni≤0.2

[0024] B≤0.005

[0025] The balance consists of iron and impurities.

[0026] The following briefly explains the importance of individual elements and their interactions with each other, as well as the limitations on the chemical composition of the claimed alloy. Throughout this specification, all percentages of the chemical composition of the steel are given in weight % (wt.%). The amount of the hard phase is given in volume % (vol.%). Upper and lower limits for individual elements can be freely combined within the limits set forth in the claims. For all values ​​given in this application, the arithmetic precision of the numerical values ​​can be increased to one or two decimal places. Thus, a value given as, for example, 0.1% can also be expressed as 0.10% or 0.100%.

[0027] C: 0.08-0.14%

[0028] Carbon (C) stabilizes austenite and is important for obtaining sufficient carbon within the retained austenite phase. C is also crucial for achieving the desired strength level. Generally, an increase of approximately 100 MPa in tensile strength can be expected for every 0.1% C. A tensile strength of 980 MPa is difficult to obtain when C is below 0.08%. Weldability is impaired if C exceeds 0.14%. The upper limit is 0.13% or 0.12%, and the lower limit is 0.09% or 0.10%. The preferred range is 0.09–0.12%.

[0029] Preferably, the carbon equivalent CE L (=C+Si / 50+Mn / 25+P / 2+Cr / 25) should be in the range of 0.20-0.30.

[0030] Mn: 2.5-3.0%

[0031] Manganese is a solid solution strengthening element; it works by lowering M. s Temperature is used to stabilize austenite and prevent the formation of ferrite and pearlite during cooling. Furthermore, Mn lowers the austenite content. c3 Temperature is also important for austenite stability. At contents below 2.5%, it may be difficult to obtain the desired amount of retained austenite and tensile strength of 980 MPa, and the austenitizing temperature may be too high for conventional industrial annealing lines. Furthermore, at lower contents, the formation of polygonal ferrite may be difficult to avoid. However, if the Mn content exceeds 3.0%, segregation problems may occur because Mn accumulates in the liquid phase and causes banding, leading to potential deterioration in workability. Therefore, the upper limit can be 3.0%, 2.9%, 2.8%, or 2.7%, and the lower limit can be 2.5% or 2.6%.

[0032] Si: 0.7-1.1%

[0033] Si acts as a solid solution strengthening element and is important for ensuring the strength of thin steel sheets. Si inhibits cementite precipitation and is crucial for austenite stabilization. However, if the content is too high, excessive silica will form on the strip surface, which can lead to caking on the rolls in CAL and surface defects on the steel sheets produced subsequently. Therefore, the upper limit is 1.1%, and it can be limited to 1.05%, 1.0%, or 0.95%. The lower limit can be 0.75% or 0.80%. The preferred range is 0.7-1.0%.

[0034] Cr: 0.05-0.4%

[0035] Cr effectively increases the strength of steel plates. Cr is an element that forms ferrite and inhibits the formation of pearlite and bainite. As the Cr content increases, A... c3 Temperature and M s The temperature decreases only slightly. Cr leads to an increase in the amount of stabilized retained austenite. The amount of Cr is limited to 0.4%. The upper limit can be 0.35%, 0.30%, or 0.25%. The lower limit can be 0.10% or 0.15%. The preferred range is 0.1-0.3%.

[0036] Al: ≤0.2%

[0037] Al promotes ferrite formation and is also commonly used as a deoxidizer. M s Temperature increases with increasing Al content. Another drawback of Al is that it causes A... c3 The rapid increase in temperature makes austenitization of steel in CAL more difficult. For these reasons, the Al content is preferably limited to less than 0.2%, more preferably less than 0.1%, and most preferably less than 0.06%.

[0038] Nb: ≤0.1%

[0039] Due to its influence on grain size, nitrogen (Nb) is commonly used in low-alloy steels to improve strength and toughness. Nb improves the balance between strength and elongation by refining the matrix microstructure and the retained austenite phase resulting from NbC precipitation. Steels may contain ≤0.1% Nb. According to the present invention, the intentional addition of Nb is not required. Therefore, the upper limit can be limited to ≤0.03%. The upper limit can be further limited to 0.01% or 0.005%.

[0040] Mo≤0.1%

[0041] Molybdenum can be added to improve strength. It can further enhance the benefits of NbC precipitates by reducing carbide coarsening kinetics. According to the invention, the intentional addition of Mo is not required. Therefore, the upper limit can be limited to ≤0.03%. The upper limit can be further limited to 0.02% or 0.01%.

[0042] V: ≤0.1%

[0043] The function of V is similar to that of Nb in that it contributes to precipitation hardening and grain refinement. Steel may contain ≤0.1% V. Upper limits may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. According to the invention, the intentional addition of V is not required. Therefore, the upper limit may be limited to ≤0.01%.

[0044] Ti: ≤0.1%

[0045] Ti is commonly used in low-alloy steels to improve strength and toughness because it influences grain size by forming carbides, nitrides, or carbonitrides. In particular, Ti is a strong nitride-forming element and can be used to bind nitrogen in steel. However, this effect tends to saturate above 0.1%. Upper limits can be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. According to the present invention, the intentional addition of Ti is not required. Therefore, the upper limit can be limited to ≤0.005%.

[0046] Ca≤0.05

[0047] Ca can be used to modify non-metallic inclusions. The upper limit is 0.05%, and it can be set to 0.04%, 0.03%, or 0.01%. According to the present invention, the intentional addition of Ca is not required. Therefore, the upper limit can be limited to ≤0.004%.

[0048] Cu: ≤0.1%

[0049] Cu is an undesirable impurity element, and its concentration is limited to ≤0.1% through careful selection of the waste materials used. The upper limit can be limited to ≤0.06%.

[0050] Ni: ≤0.2%

[0051] Ni is an undesirable impurity element, and its concentration is limited to ≤0.2% through careful selection of the waste materials used. The upper limit can be limited to ≤0.08%.

[0052] B: ≤0.005%

[0053] B is an undesirable impurity element, and its content is limited to ≤0.005% through careful selection of the scrap used. B increases hardness but may come at the cost of reduced flexibility, making it undesirable in the steels currently proposed. B can further complicate scrap recycling, and its addition can also worsen machinability. Therefore, according to the invention, the intentional addition of B is not desired. Thus, the upper limit is limited to ≤0.0006%.

[0054] Steel may contain other impurity elements present in normal amounts. However, it is preferred to limit the amounts of P, S, As, Zr, and Sn to the following optional maximum contents:

[0055] P: ≤0.02%

[0056] S: ≤0.005%

[0057] As≤0.010%

[0058] Zr≤0.005%

[0059] Sn≤0.015%

[0060] It is also preferable to control the nitrogen content within the following range:

[0061] N: ≤0.015%, preferably 0.003-0.008%

[0062] Within this range, stable nitrogen fixation can be achieved.

[0063] Oxygen and hydrogen can be further limited to

[0064] O: ≤0.0003

[0065] H: ≤0.0020

[0066] The high-strength TRIP-assisted bainitic ferrite (TBF) steel sheet of the present invention has a microstructure mainly composed of residual austenitic inclusions embedded in the matrix.

[0067] The microstructure composition is expressed below as volume % (vol.%).

[0068] This steel contains a bainitic ferrite (BF) matrix. Therefore, the amount of bainitic ferrite is typically ≥50%. The microstructure may also contain tempered martensite (TM). The BF and TM components may be difficult to distinguish from each other. Therefore, the total content of the two components can be limited to 70-90%. The amount is typically in the range of 75-85%.

[0069] Martensite can exist in the final microstructure because, depending on its stability, some austenite may transform into martensite during cooling at the end of the overaging step. Martensite can be present in amounts of ≤15%, preferably ≤10%. The amount is typically in the range of 5-10%. These untempered martensite particles are usually in close contact with the retained austenite particles, and therefore they are often referred to as martensite-austenite (MA) particles.

[0070] Retained austenite is a prerequisite for obtaining the desired TRIP effect. Therefore, the amount of retained austenite should be in the range of 2-20%, preferably 5-15%. The amount of retained austenite is measured by means of the saturation magnetization method described in detail in Proc. Int. Conf., TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, pp. 61-64.

[0071] Polygonal ferrite (PF) is not a desired microstructure component and is therefore limited to ≤10%, preferably ≤5%, ≤3%, or ≤1%. Most preferably, the steel does not contain PF.

[0072] The mechanical properties of the steel to be protected are important, and it should meet at least one of the following requirements:

[0073] Tensile strength (R) m 980-1180MPa

[0074] Yield strength (R) p0.2 580-750MPa

[0075] Total elongation (A) 50 ≥11%

[0076] Yield ratio (R) p0.2 / R m )≤0.72

[0077] Flexibility (Ri / t) ≤ 2

[0078] Preferably, all of these requirements are met simultaneously.

[0079] Tensile strength (R) m The upper limit can be further limited to 1160, 1140, 1120, or 1100 MPa. The lower limit can be further limited to 990 or 1000 MPa.

[0080] Yield strength (R) p0.2 The upper limit can be further limited to 740, 730, 720, 710, 700, 890, 680, 670, or 660 MPa. The preferred range is 580-700 MPa.

[0081] Yield ratio (R) p0.2 / R m The upper limit can be further limited to 0.71, 0.70, 0.69, 0.68, 0.67, 0.66, or 0.65. The lower limit can be 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, or 0.58.

[0082] R m R p0.2 Value and total elongation (A) 50 According to Japanese Industrial Standard JIS Z 2241:2011, samples are obtained along the transverse direction of the strip.

[0083] The upper limit of the flexural strength Ri / t may be further limited to 1.9, 1.8, 1.7, 1.6 or 1.5. Preferably Ri / t ≤ 1.7, more preferably ≤ 1.5. The lower limit of Ri / t may be 0.5, 0.6, 0.7, 0.8, 0.9 or 1.0.

[0084] Bending performance is evaluated by the ratio of the ultimate bending radius (Ri) (defined as the minimum bending radius without cracking) to the sheet thickness (t). For this purpose, a 90° V-block is used to bend the steel sheet according to JIS Z2248. The sample size is 35 × 100 mm. The bending value Ri / t is obtained by dividing the ultimate bending radius (in mm) by the thickness (in mm).

[0085] The mechanical properties of the steel plate of the present invention can be largely adjusted through alloy composition and microstructure. The microstructure can be adjusted by heat treatment in CAL, particularly by the isothermal treatment temperature in the over-aging step.

[0086] The proposed steel can be produced by preparing conventional metallurgical steel slabs through converter melting and secondary metallurgy using the composition described above. The slabs are then hot-rolled into hot-rolled strip within the austenitic range. Preferably, the slabs are rolled entirely within the austenitic range by reheating them to a temperature of 1000°C to 1280°C, with a final hot-rolling temperature greater than or equal to 850°C, to obtain hot-rolled steel strip. The hot-rolled strip is then coiled at a coiling temperature in the range of 500-650°C. Optionally, the coiled strip is subjected to a scale removal process, such as pickling. Subsequently, the coiled strip is batch-annealed at a temperature in the range of 500-650°C, preferably 550-650°C, for a duration of 5-30 hours. Subsequently, the annealed steel strip is cold-rolled at a reduction rate of 35% to 90%, preferably about 40% to 60%. The cold-rolled steel strip is further processed in a continuous annealing line (CAL).

[0087] The annealing cycle in CAL includes heating to a temperature of 800-890°C, preferably 840-860°C, soaking for 80-180 seconds, preferably 100-140 seconds, slow gas cooling at a rate of 5-15°C / second to a temperature of 700-750°C, rapid gas cooling at a rate of 20-60°C / second, preferably 30-50°C / second to an overaging temperature of 405-460°C, holding for 150 to 1000 seconds, and then cooling to room temperature. The overaging temperature can be upper limited to 450, 440, 430, or 420°C. The lower limit can be 405, 406, 407, 408, 409, or 410°C. The preferred range of overaging temperature is 405-420°C.

[0088] Example

[0089] Steels I1-I3 and reference steels R1-R3 are produced using conventional metallurgical techniques involving converter smelting and secondary metallurgy. The composition is shown in Table 1; other elements are present only as impurities and are below the minimum levels specified in this specification.

[0090] Table 1 discloses the composition of the steel plates inspected.

[0091] I1 0.114 0.95 2.76 0.181 0.047 I2 0.105 0.83 2.65 0.194 0.049 I3 0.105 0.82 2.65 0.198 0.042 R1 0.106 0.84 2.67 0.197 0.048 R2 0.118 0.94 2.77 0.17 0.051 R3 0.112 0.93 2.7 0.169 0.046

[0092] Table 1. Composition of the steel plates inspected.

[0093] Steel alloy slabs are produced in a continuous casting machine. The slabs are then reheated and hot-rolled to the thicknesses shown in Table 2. The final hot rolling temperature is approximately 900°C, and the coiling temperature is approximately 550°C. The hot-rolled strip is pickled and annealed in batches at approximately 620-625°C for 10 hours to reduce the tensile strength of the hot-rolled strip, thereby reducing the cold rolling force. Subsequently, the strip is cold-rolled in a five-stand cold rolling mill to a final thickness of approximately 1.4 mm (I1, I3, R1, R2) or 1 mm (I2, R3), and finally subjected to continuous annealing.

[0094] Table 2 discloses the hot and cold rolling parameters. Batch annealing is performed for approximately 10 hours between the hot and cold rolling steps.

[0095]

[0096] Table 2. Hot and cold rolling parameters.

[0097] The annealing cycle includes heating to a temperature of approximately 850°C, soaking for approximately 120 seconds, slow gas cooling at a rate of approximately 10°C / second to a temperature of approximately 720°C, rapid gas cooling at a rate of approximately 40°C / second to an over-aging temperature above 405°C (for embodiments of the invention) and approximately 390-395°C (for non-inventive embodiments), isothermal holding at the over-aging temperature, and finally cooling to ambient temperature.

[0098] Details of the processing in CAL are given in Table 3. Different overaging temperatures affect the yield strength and bending properties of the steel, as shown in Table 4.

[0099]

[0100] Table 3. Parameters processed in CAL.

[0101] The materials produced according to the present invention have been found to have excellent mechanical properties, as shown in Table 4.

[0102] The tensile strength of all steels is in the range of 980-1180 MPa. The total elongation of all steels exceeds 11%.

[0103] The steels I1-I3 of this invention have a yield strength of less than 750 MPa. In particular, it can be noted that all embodiments of this invention disclose a combination of flexural strength (Ri / t) less than 2.0 and a yield ratio less than 0.72. The highest Ri / t is 1.5, and the highest yield ratio is 0.64. Reference steels R1-R3 do not meet the requirements for yield strength, and the combination of yield ratio and flexural strength.

[0104] Figure 1 This is a schematic diagram in which the flexural strength Ri / t is plotted against the yield ratio. Examples I1-I3 are marked as "protected areas" within the boundary claimed in claim 1, while references R1-R3 are outside of this area.

[0105]

[0106] Table 4. Mechanical properties.

[0107] R m and R p0.2 The values ​​are derived according to European standard EN 10002 Part 1, where samples are taken along the longitudinal direction of the strip. For samples taken along the transverse direction of the strip, the elongation (A) is... 50 This is derived from Japanese Industrial Standard JIS Z 2241:2011.

[0108] Ri / t is determined according to JIS Z2248 in a V-bending test. A sample of the produced strip (35 × 100 mm) is subjected to a V-bending test to determine the limiting bending radius (Ri). The sample is examined visually and under a 25x magnifying optical microscope to detect the presence of cracks. Ri / t is determined by dividing the limiting bending radius (Ri) by the thickness (t) of the cold-rolled strip. Ri is the maximum radius at which the material does not show cracks after three bending tests.

[0109] Industrial applicability

[0110] The material of this invention can be widely used in high-strength structural parts in motor vehicles. High-strength steel sheets are particularly well-suited for producing parts with high requirements for overall formability and bending resistance. They are especially suitable for B-pillar hinges, roof longitudinal beams, or door panels in motor vehicles.

Claims

1. High-strength cold-rolled steel sheet, which has the following characteristics: a) Composition, wherein the composition consists of the following elements (in weight %): C0.08-0.14 Mn2.5-3.0 Si0.7-1.1 Cr0.05-0.4 Optional Al≤0.2 Nb≤0.1 Mo≤0.1 V≤0.1 Ti≤0.1 Ca≤0.05 Cu≤0.1 Ni≤0.2 B≤0.005 The remaining Fe after removing impurities, b) Multiphase microstructure, wherein the multiphase microstructure comprises (volume %): Residual austenite 2-20 Martensite ≤15 bainitic ferrite and tempered martensite 50-90 Polygonal ferrite ≤10, c) Tensile strength (R) m 980-1180 MPa Yield strength (R) p0.2 580-750 MPa Yield ratio (R) p0.2 / R m ≤0.72 d) For a sample with dimensions of 35 × 100 mm, the bending property is determined by a 90° V-bending test value Ri / t ≤ 2.0, where Ri is the bending radius in mm and t is the thickness of the steel plate in mm. The high-strength cold-rolled steel sheet is manufactured by a method comprising the following steps: a) Provide a steel slab having the aforementioned composition. b) Hot-rolling the slab into hot-rolled strip within the austenitic region; c) The hot-rolled strip is wound at a winding temperature in the range of 500-650°C; d) Optionally, perform a descaling process on the coiled steel strip; e) The duration of batch annealing at a temperature in the range of 500-650℃ for 5-30 hours; f) Cold rolling the annealed steel strip at a reduction rate of 35% to 90%; g) Heat the strip to a temperature in the range of 800°C to 890°C in a continuous annealing line and homogenize it for 80-180 seconds. h) Slowly cool the strip to 700-750°C at a rate of 5-15°C / second, then rapidly cool the strip to an over-aging temperature of 405-460°C at a rate of 20-60°C / second, and hold for 150 to 1000 seconds. i) Cool to room temperature.

2. The high-strength cold-rolled steel sheet according to claim 1, wherein the microstructure satisfies at least one of the following requirements (in volume %): Retained austenite 5-15 Martensite 5-10 Bainitic ferrite and tempered martensite 70-90 Polygonal ferrite ≤ 5.

3. The high-strength cold-rolled steel sheet according to claim 2, wherein the microstructure meets all the requirements: Residual austenite 5-15 Martensite 5-10 Bainitic ferrite and tempered martensite 70-90 Polygonal ferrite ≤ 5.

4. The high-strength cold-rolled steel sheet according to any one of claims 1-3, wherein the composition satisfies the following (in weight %): C0.09-0.12 Mn 2.5-2.9 Si0.7-1.0 Cr0.1-0.3 Al 0.005-0.1 optional Nb≤0.1 Mo≤0.1 V≤0.1 Ti≤0.1 Ca≤0.05 Cu≤0.1 Ni≤0.2 B≤0.005 The remaining Fe after removing impurities.

5. The high-strength cold-rolled steel sheet according to any one of the preceding claims, wherein the mechanical properties satisfy at least one of the following requirements: Tensile strength (R) m 1000-1100 MPa Yield strength (R) p0.2 580-700 MPa Total elongation (A) 50 ≥11% Yield ratio (R) p0.2 / R m 0.50-0.70 Bending resistance (Ri / t) ≤ 1.

7.

6. The high-strength cold-rolled steel sheet according to any one of the preceding claims, wherein the mechanical properties meet the following requirements: Tensile strength (R) m 1000-1100 MPa Yield strength (R) p0.2 580-700 MPa Total elongation (A) 50 ≥11% Yield ratio (R) p0.2 / R m 0.50-0.70 Bending resistance (Ri / t) ≤ 1.

5.

7. The high-strength cold-rolled steel sheet according to any one of the preceding claims, wherein the thickness of the cold-rolled sheet is 0.9-1.6 mm.

8. The high-strength cold-rolled steel sheet according to any one of the preceding claims, wherein at least one element selected from the group consisting of Nb, Mo, V, Ti and Ca exists only as an impurity.

9. The high-strength cold-rolled steel sheet according to claim 8, wherein all elements in the group consisting of Nb, Mo, V, Ti and Ca exist only as impurities.

10. A method for manufacturing a cold-rolled steel sheet according to any one of claims 1-9, comprising the following steps: a) Providing a steel slab having the composition according to any one of the preceding claims, b) Hot-rolling the slab into hot-rolled strip within the austenitic region; c) The hot-rolled strip is wound at a winding temperature in the range of 500-650°C; d) Optionally, perform a descaling process on the coiled steel strip; e) The duration of batch annealing at a temperature in the range of 500-650℃ for 5-30 hours; f) Cold rolling the annealed steel strip at a reduction rate of 35% to 90%; g) Heat the strip to a temperature in the range of 800°C to 890°C in a continuous annealing line and homogenize it for 80-180 seconds. h) Slowly cool the strip to 700-750°C at a rate of 5-15°C / second, then rapidly cool the strip to an over-aging temperature of 405-460°C at a rate of 20-60°C / second, and hold for 150 to 1000 seconds. i) Cool to room temperature.

11. A motor vehicle structural part comprising a high-strength cold-rolled steel sheet according to any one of claims 1-9.

12. The motor vehicle structural component according to claim 11, wherein the structural component is a B-pillar hinge, a roof longitudinal beam, or a door panel of a motor vehicle.

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