A production method for 750MPa grade cold-rolled complex phase steel

Abstract

The present invention relates to the technical field of cold-rolled complex phase steel, and specifically to a production method of 750MPa grade cold-rolled complex phase steel, comprising the following steps: refining smelted molten iron, hot-rolling and coiling, and pickling to obtain a complex phase steel chilled coil; subjecting the complex phase steel chilled coil to continuous annealing treatment to obtain a continuously annealed cold-rolled complex phase steel, or subjecting the complex phase steel chilled coil to galvanizing treatment to obtain a galvanized cold-rolled complex phase steel. The 750MPa grade cold-rolled complex phase steel product prepared by the present invention has good stamping forming performance, and at the same time has excellent hole expansion, flanging, and bending performance, and has excellent comprehensive performance. It can make up for the shortcomings of existing 780MPa grade cold-rolled dual-phase steel and complex phase steel, and achieve replacement. This product can greatly meet market demand and is widely used in the manufacture of automotive parts.

Description

Technical Field

[0001] The present invention relates to the technical field of cold-rolled complex phase steel, and in particular to a production method of 750MPa grade cold-rolled complex phase steel. Background Art

[0002] Lightweighting, energy conservation, and emission reduction are the general trends in the automotive industry. In recent years, the proportion of cold-rolled high-strength steel in automotive parts manufacturing has gradually increased, and demand for high-strength cold-rolled duplex and complex-phase steels has been on the rise. Existing 780MPa-grade cold-rolled duplex steels have a microstructure of ferrite + martensite, a yield strength of 420-500MPa, a low yield strength ratio (less than 0.6), and an elongation of 15%-20%. This allows for good stamping properties, but its hole expansion ratio is generally less than 40%, making it unsuitable for hole expansion, folding, and bending parts. Existing 780MPa-grade cold-rolled complex-phase steels have a microstructure of ferrite + martensite + bainite, and an elongation of only 11%-16%. Their stamping properties are inferior to those of 780MPa-grade cold-rolled duplex steels, with actual yield strengths generally ranging from 580-700MPa. Their yield strength is high, exceeding 0.7, while their hole expansion and bending properties are superior to those of 780MPa-grade cold-rolled duplex steels.

[0003] The design of automotive parts is becoming increasingly sophisticated and complex, with increasingly stringent requirements for formability. Meanwhile, stamping, hole expansion, folding, and bending often occur simultaneously during the part forming process. As analyzed above, currently available 780MPa-grade cold-rolled duplex steel and 780MPa-grade complex phase steel cannot combine excellent stamping formability with excellent hole expansion, folding, and bending performance. For example, CN113444972A provides a low-cost 600MPa-grade hot-dip galvanized complex phase steel sheet and its preparation method. This invention addresses the problem of poor mechanical properties of existing 600MPa-grade hot-dip galvanized complex phase sheet through reasonable composition and process design. However, its technical solution cannot achieve the production of 750MPa-grade complex phase steel, and the patent cannot be used for the production of the same-grade continuously annealed complex phase steel. Publication No. CN105950984A provides a 650MPa-grade hot-rolled complex phase steel and its preparation method. However, its production technology solution cannot achieve the production of cold-rolled complex phase steel. Summary of the Invention

[0004] In response to the technical problem that 780MPa-level cold-rolled dual-phase steel and complex-phase steel cannot achieve both stamping, hole expansion, flanging, and bending properties, the present invention provides a production method for 750MPa-level cold-rolled complex-phase steel. The resulting cold-rolled complex-phase steel has both good stamping performance and excellent hole expansion, flanging, and bending properties, and is more applicable.

[0005] The present invention provides a production method of 750MPa grade cold-rolled complex phase steel, comprising the following steps: refining smelted molten iron, hot-rolling and coiling, and pickling to obtain a complex phase steel chilled coil; continuously annealing the complex phase steel chilled coil to obtain a continuously annealed cold-rolled complex phase steel, or galvanizing the complex phase steel chilled coil to obtain a galvanized cold-rolled complex phase steel.

[0006] Furthermore, after the molten iron is refined, the ingot includes the following chemical components in weight percentage: C 0.095% to 0.130%, Si 0.55% to 0.75%, Mn 2.1% to 2.3%, Alt 0.065% to 0.085%, Cr 0.10% to 0.25%, S≤0.015%, P≤0.025%, Ti≤0.005%, Nb≤0.005%, and the balance is Fe.

[0007] The main component design principles of the present invention are as follows:

[0008] The C element determines the austenite content during the annealing process and affects the amount of martensite formed during cooling. Too low a C content can easily lead to insufficient strength of the finished product. Too high a C content increases the amount of martensite during the rapid cooling process, affecting the stamping effect of the material. In order to obtain a certain amount of bainite during the rapid cooling process, the C content of the present invention is designed to be 0.095% to 0.130%.

[0009] The Si element plays a certain role in solid solution strengthening in steel. Too high Si content affects the surface quality of high-strength steel, easily leading to an increase in hot-rolled surface hardening scale, causing surface degradation of the finished product. At the same time, an increase in Si content will enhance the hardenability of austenite and increase the strength of the material. The Si content of the present invention is designed to be 0.55% to 0.75%;

[0010] Mn has the same effect as C and Si. Mn has a greater influence on austenite stability and hardenability. Excessive Mn content increases the hardness and strength of the material, but damages the plasticity and reduces the elongation of the material. The Mn content of the present invention is designed to be 2.1% to 2.3%;

[0011] Al is also a solid solution strengthening element, which refines the grain size, reduces the austenite phase area, increases the martensite transformation temperature, and reduces the production of retained austenite during the quenching process. At the same time, as a deoxidizer, Al can increase the oxidation resistance of steel and improve processing performance. The Al content of the present invention is designed to be 0.065% to 0.085%;

[0012] The Cr element reduces the critical cooling rate of steel, improves hardenability, and has certain anti-oxidation and anti-wear effects. The Cr element can make the structure uniform. If the Cr content is too high, the elongation of the material will be reduced. In the present invention, bainite is obtained in the structure, so the hardenability of the steel should not be too high. This steel does not need to add a sufficient amount of Cr. The Cr content of the present invention is designed to be 0.10% to 0.25%.

[0013] Furthermore, the mechanical properties of the cold-rolled complex phase steel are as follows: thickness 0.5-2.2 mm, yield strength 480-530 MPa, tensile strength 760-800 MPa, yield strength ratio 0.60-0.70, elongation ≥23.0%, and hole expansion rate 65%-80%; the structure of the cold-rolled complex phase steel is 60%-50% ferrite + 20%-30% martensite + 16%-10% bainite + 3-6% retained austenite.

[0014] Furthermore, the process of molten iron refining is to adopt LF+RH refining, wherein the RH refining vacuum degree is ≤145pa, the ladle soft blowing time is ≥15min, the length of the continuous casting billet is 6~9m, and the head and tail billet peeling depth is 4.5mm.

[0015] Furthermore, the hot rolling coiling process is as follows: the hot rolled slab is heated to 1260-1280°C, the heating time in the furnace is 260-300 minutes, and the hot charging time in the furnace is 200-240 minutes;

[0016] The rough rolling outlet temperature is 1100-1120℃; the slab thickness after rough rolling is d, and finishing rolling is performed after rough rolling;

[0017] When 2.4≤d<3.5mm, the finishing rolling inlet temperature is 1040~1070℃, and the finishing rolling outlet temperature is 860~880℃;

[0018] When 3.5mm≤d<5.0mm, the finishing rolling inlet temperature is 1030~1050℃, and the finishing rolling outlet temperature is 840~860℃;

[0019] The finishing rolling adopts edge heating compensation; the laminar cooling mode is intensive cooling of the head;

[0020] The coiling temperature is 580-600°C, and the coiling tension is 35-50 MPa.

[0021] Further, the process of pickling comprises the following steps:

[0022] (1) Welding: Laser welding is used with a welding speed of 2.5 to 5.5 m / min, a laser power of 7 to 15 kW, a laser welding head pressure of 20 to 35 bar, and a preheating power of 3 to 7 kW.

[0023] (2) Acid rolling: acid concentration is 170-200 g / L, rinsing water conductivity is 10-20 μs / cm; finished steel thickness is M;

[0024] When M is 0.60-1.00 mm, the hot rolling finishing thickness is 2.3 mm, and the corresponding cold rolling reduction is 57%-74%.

[0025] When M is 1.01-1.60 mm, the hot rolling finishing thickness is 3.5 mm, and the corresponding cold rolling reduction is 54%-71%;

[0026] When M is 1.61-1.80 mm, the hot rolling finishing thickness is 4.5 mm, and the corresponding cold rolling reduction is 60%-64%;

[0027] When M is 1.81-2.20 mm, the hot rolling finishing thickness is 5.0 mm, and the corresponding cold rolling reduction is 56%-64%;

[0028] The obtained complex phase steel chilled coil has a roughness greater than 0.7 μm and a reflectivity greater than 70%.

[0029] Furthermore, the continuous annealing process includes the following steps:

[0030] (1) The preheating section temperature of the continuous annealing furnace is 200-300°C, the soaking section temperature is 780-795°C, the slow cooling outlet temperature is 670-690°C, the fast cooling outlet temperature is 315-325°C, the aging outlet temperature is 285-300°C, and the final cooling temperature is ≤180°C;

[0031] (2) When the thickness of the finished steel is 0.5-1.8 mm, the annealing belt speed is 100-130 m / min; when the thickness of the finished steel is 1.81-2.2 mm, the annealing belt speed is 90-120 m / min;

[0032] (3) When the thickness of finished steel is 0.5-1.8 mm, two-stage bellows cooling is adopted in the rapid cooling stage, with the power of the first stage bellows at 80%-95%; when the thickness of finished steel is 1.81-2.2 mm, three-stage bellows cooling is adopted in the rapid cooling stage, with the power of the first stage bellows at 50%-65%; the bellows are 70-90 mm away from the strip surface;

[0033] (4) When the thickness of the finished steel is 0.5-1.8 mm, the flattening reduction rate is 0.6%-0.8%; when the thickness of the finished steel is 1.81-2.2 mm, the flattening reduction rate is 0.4%-0.6%.

[0034] Furthermore, the galvanizing treatment includes the following steps:

[0035] (1) The preheating temperature of the continuous galvanizing annealing furnace is 260-280°C, the soaking section temperature is 795-815°C, the slow cooling outlet temperature is 680-695°C, the rapid cooling outlet temperature is 300-320°C, the induction heating temperature after the rapid cooling section is 420-450°C, the zinc pot temperature is 450-470°C, and the final cooling temperature is ≤160°C;

[0036] (2) When the thickness of the finished steel is 0.5-1.6 mm, the strip speed is 90-140 m / min; when the thickness of the finished steel is 1.61-2.2 mm, the strip speed is 85-120 m / min;

[0037] (3) Rapid cooling stage: When the thickness of finished steel is 0.5-1.6 mm, two-stage bellows cooling is adopted, and the power of the first stage bellows is 80-95%; when the thickness of finished steel is 1.61-2.2 mm, three-stage bellows cooling is adopted, and the power of the first stage bellows is 50%-65%; the bellows are 80-90 mm away from the strip surface;

[0038] (4) When the thickness of the finished steel is 0.5-1.6 mm, the skin-pass reduction rate is 0.5%-0.6%; when the thickness of the finished steel is 1.61-2.2 mm, the skin-pass reduction rate is 0.7%-0.8%.

[0039] The principle of the present invention is as follows: through reasonable composition and process design, the present invention produces a cold-rolled complex phase steel product with a structure consisting of ferrite + martensite + a small amount of bainite + very small amount of retained austenite, wherein the main phases are ferrite + martensite to ensure strength; bainite is used to transition the strength of ferrite and martensite, improve the material's stamping performance, and improve the hole expansion and bending properties; retained austenite increases the material's ductility during the stamping process and improves the material's plasticity.

[0040] The beneficial effects of the present invention are:

[0041] (1) The 750 MPa grade cold-rolled complex phase steel prepared by the present invention has a thickness range of 0.5 to 2.2 mm and a microstructure of 60% to 50% ferrite + 20% to 30% martensite + 16% to 10% bainite + 3 to 6% retained austenite. The resulting finished product has a yield strength range of 480 to 530 MPa, a tensile strength of 760 to 800 MPa, a yield strength ratio of 0.60 to 0.70, an elongation of ≥23.0%, and a hole expansion ratio of 65 to 80%.

[0042] (2) The 750 MPa grade cold-rolled complex phase steel product prepared by the present invention has good stamping forming performance, and at the same time has excellent hole expansion, flanging and bending performance, and excellent comprehensive performance. It can make up for the shortcomings of existing 780 MPa grade cold-rolled dual-phase steel and complex phase steel and achieve replacement. This product can greatly meet market demand and is widely used in automobile parts manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0044] Figure 1 This is a scanning microstructure diagram of the finished steel of Example 1-A according to a specific embodiment of the present invention.

[0045] Figure 2 This is a scanning microstructure diagram of the finished steel of Example 2-D according to a specific embodiment of the present invention. DETAILED DESCRIPTION

[0046] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0047] A method for producing 750MPa grade continuously annealed cold-rolled complex phase steel, comprising Example 1-A and Example 2-A;

[0048] A method for producing 750MPa grade galvanized cold-rolled complex phase steel, comprising Example 1-D and Example 2-D;

[0049] The production method of complex phase steel chilled coil is common to both Example 1-A and Example 1-D;

[0050] Example 2-A and Example 2-D share the production method of complex phase steel chilled coil.

[0051] The production method of common complex phase steel chilled coil specifically comprises the following steps:

[0052] (1) The molten iron after smelting was refined by LF+RH refining, wherein the RH refining vacuum was 100 Pa, the ladle soft blowing time was 20 min, the continuous casting billet length was 8 m, the head and tail billet peeling depth was 4.5 mm, and the billets of each embodiment included the chemical components in weight percentages shown in Table 1.

[0053] Table 1 Chemical composition of the refining slabs in each embodiment

[0054]

[0055] (2) Hot rolling and coiling: The temperature parameters of each section are controlled by a hot rolling two-stage temperature control system. The ingot is heated, rough rolled, finished rolled, and coiled in sequence to obtain a hot rolled coil. The finishing rolling adopts edge heating compensation, and the laminar cooling is sparse cooling in the entire section. The specific control parameters of each embodiment are shown in Table 2 below.

[0056] Table 2 Specific control parameters for hot rolling coiling in various embodiments

[0057]

[0058] (3) Pickling: including welding and pickling, pickling including pickling and continuous cold rolling, to obtain a common complex phase steel chilled coil. The control parameters of each embodiment are shown in Table 3.

[0059] Table 3 Pickling control parameters of each embodiment

[0060]

[0061]

[0062] The common complex phase steel chilled coils prepared by the above method were used to produce the continuously annealed cold-rolled complex phase steels of Examples 1-A and 2-A, and the galvanized cold-rolled complex phase steels of Examples 1-D and 2-D. The common complex phase steel chilled coils were annealed to obtain the continuously annealed cold-rolled complex phase steels of Examples 1-A and 2-A, and the common complex phase steel chilled coils were galvanized to obtain the galvanized cold-rolled complex phase steels of Examples 1-D and 2-D.

[0063] The method for obtaining the continuously annealed cold-rolled complex phase steel of Example 1-A and Example 2-A by annealing treatment is as follows: a common complex phase steel chilled hard coil is sequentially passed through the preheating section, heating section, soaking section, slow cooling section, rapid cooling section, aging section, and final cooling section of an annealing furnace at a certain speed, and is flattened to finally obtain a continuously annealed cold-rolled complex phase steel product. The specific parameters are shown in Table 4 below.

[0064] Table 4 Annealing process parameters

[0065]

[0066]

[0067] The method for obtaining the galvanized cold-rolled complex phase steel of Example 1-D and Example 2-D by galvanizing is as follows: a common complex phase steel chilled coil passes through the annealing furnace preheating section, heating section, soaking section, slow cooling section, rapid cooling section, induction heating section, entering the zinc pot section, final cooling section, and smoothing section at a certain speed, and finally cools to obtain the cold-rolled galvanized complex phase steel. The specific parameters are shown in Table 5.

[0068] Table 5 Specific process parameters of galvanizing treatment

[0069]

[0070]

[0071] The physical property test data of the 750 MPa grade cold-rolled complex phase steel produced in various embodiments of the present invention are shown in Table 6.

[0072] Table 6 Physical property test data of 750MPa grade cold-rolled complex phase steel obtained in Example

[0073]

[0074] Among them, such as Figure 1 、 2 As shown, the finished product structure of Example 1-A is 56% ferrite + 24% martensite + 16% bainite + 3% retained austenite; the finished product structure of Example 2-D is 53% ferrite + 27% martensite + 13% bainite + 6% retained austenite.

[0075] Although the present invention has been described in detail with reference to the accompanying drawings and in conjunction with preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be easily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.

Claims

1. A method for producing 750MPa grade cold-rolled complex phase steel, characterized in that: The method comprises the following steps: refining, hot-rolling and pickling molten iron after smelting to obtain a complex phase steel chilled coil; continuously annealing the complex phase steel chilled coil to obtain a continuously annealed cold-rolled complex phase steel, or galvanizing the complex phase steel chilled coil to obtain a galvanized cold-rolled complex phase steel; the mechanical properties of the cold-rolled complex phase steel are as follows: thickness 0.5-2.2 mm, yield strength 480-530 MPa, tensile strength 760-800 MPa, yield strength ratio 0.60-0.70, elongation ≥23.0%, and hole expansion ratio 65%-80%; the structure of the cold-rolled complex phase steel is 50%-60% ferrite + 20%-30% martensite + 10%-16% bainite + 3%-6% retained austenite; The slab after molten iron refining comprises the following chemical components in weight percentage: C 0.095% to 0.130%, Si 0.55% to 0.75%, Mn 2.1% to 2.3%, Alt 0.065% to 0.085%, Cr 0.10% to 0.25%, S≤0.015%, P≤0.025%, Ti≤0.005%, Nb≤0.005%, and the balance is Fe; The continuous annealing process includes the following steps: (1) The preheating section temperature of the continuous annealing furnace is 200-300°C, the soaking section temperature is 780-795°C, the slow cooling outlet temperature is 670-690°C, the fast cooling outlet temperature is 315-325°C, the aging outlet temperature is 285-300°C, and the final cooling temperature is ≤180°C; (2) When the thickness of the finished steel is 0.5-1.8 mm, the annealing belt speed is 100-130 m / min; when the thickness of the finished steel is 1.81-2.2 mm, the annealing belt speed is 90-120 m / min; (3) When the thickness of finished steel is 0.5-1.8 mm, two-stage bellows cooling is adopted in the rapid cooling stage, and the power of the first-stage bellows is 80%-95%; when the thickness of finished steel is 1.81-2.2 mm, three-stage bellows cooling is adopted in the rapid cooling stage, and the power of the first-stage bellows is 50%-65%; the bellows are 70-90 mm away from the strip surface; (4) When the thickness of the finished steel is 0.5-1.8 mm, the flattening reduction rate is 0.6%-0.8%; when the thickness of the finished steel is 1.81-2.2 mm, the flattening reduction rate is 0.4%-0.6%; The galvanizing process includes the following steps: (1) The preheating temperature of the continuous galvanizing annealing furnace is 260~280℃, the soaking section temperature is 795~815℃, the slow cooling outlet temperature is 680~695℃, the rapid cooling outlet temperature is 300~320℃, the induction heating temperature after the rapid cooling section is 420~450℃, the zinc pot temperature is 450~470℃, and the final cooling temperature is ≤160℃; (2) When the thickness of the finished steel is 0.5-1.6 mm, the strip speed is 90-140 m / min; when the thickness of the finished steel is 1.61-2.2 mm, the strip speed is 85-120 m / min; (3) In the rapid cooling stage, when the thickness of the finished steel is 0.5-1.6 mm, two-stage bellows cooling is adopted, and the power of the first-stage bellows is 80-95%; when the thickness of the finished steel is 1.61-2.2 mm, three-stage bellows cooling is adopted, and the power of the first-stage bellows is 50%-65%; the bellows are 80-90 mm away from the strip surface; (4) When the thickness of the finished steel is 0.5-1.6 mm, the skin-pass reduction rate is 0.5%-0.6%; when the thickness of the finished steel is 1.61-2.2 mm, the skin-pass reduction rate is 0.7%-0.8%.

2. The method for producing 750 MPa grade cold-rolled complex phase steel according to claim 1, wherein: The process of molten iron refining is LF+RH refining, where the RH refining vacuum degree is ≤145pa, the ladle soft blowing time is ≥15min, the continuous casting billet length is 6~9m, and the head and tail billet peeling depth is 4.5mm.

3. The method for producing 750MPa grade cold-rolled complex phase steel according to claim 1, characterized in that: The hot rolling coiling process is as follows: the hot rolled slab is heated to 1260-1280°C, the heating time in the furnace is 260-300 minutes, and the hot charging time in the furnace is 200-240 minutes; The rough rolling outlet temperature is 1100-1120℃; the slab thickness after rough rolling is d, and finishing rolling is performed after rough rolling; When 2.4≤d<3.5mm, the finishing rolling inlet temperature is 1040~1070℃, and the finishing rolling outlet temperature is 860~880℃; When 3.5mm≤d<5.0mm, the finishing rolling inlet temperature is 1030~1050℃, and the finishing rolling outlet temperature is 840~860℃; The finishing rolling adopts edge heating compensation; the laminar cooling mode is intensive cooling of the head; The coiling temperature is 580~600℃ and the coiling tension is 35~50MPa.

4. The method for producing 750MPa grade cold-rolled complex phase steel according to claim 1, wherein: The process of pickling includes the following steps: (1) Welding: Laser welding is used with a welding speed of 2.5-5.5 m / min, a laser power of 7-15 kW, a laser welding head pressure of 20-35 bar, and a preheating power of 3-7 kW; (2) Acid rolling: acid concentration is 170-200 g / L, rinsing water conductivity is 10-20 μs / cm; finished steel thickness is M; When M is 0.60-1.00mm, the hot-rolled finishing thickness is 2.3mm, and the corresponding cold-rolled reduction rate is 57%-74%; when M is 1.01-1.60mm, the hot-rolled finishing thickness is 3.5mm, and the corresponding cold-rolled reduction rate is 54%-71%; When M is 1.61-1.80 mm, the hot rolling finishing thickness is 4.5 mm, and the corresponding cold rolling reduction is 60%-64%; When M is 1.81-2.20 mm, the hot rolling finishing thickness is 5.0 mm, and the corresponding cold rolling reduction is 56%-64%; The roughness of the obtained complex phase steel chilled coil is greater than 0.7 μm and the reflectivity is greater than 70%.

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