CN113186469B - 780 MPa-grade hot rolled steel plate for hydraulic bulging and preparation method thereof - Google Patents
780 MPa-grade hot rolled steel plate for hydraulic bulging and preparation method thereof Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
- B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
- B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Abstract
The invention discloses a hot rolled steel plate for 780 MPa-level hydraulic bulging and a preparation method thereof, belonging to the technical field of steel material engineering. The hot rolled steel plate for 780 MPa-level hydraulic bulging comprises, by weight, 0.02-0.04% of C, 0.05-0.10% of Si, 1.7-2.00% of Mn, 0.040-0.050% of Nb, 0.070-0.080% of Ti, 0.030-0.060% of V, 0.20-0.30% of Mo, less than or equal to 0.02% of P, less than or equal to 0.003% of S, less than or equal to 0.0040% of N, 0.010-0.050% of Als, 0.0020-0.0050% of Ca and the balance of Fe and inevitable impurities. The invention adopts a micro/low C-Nb-Ti-Mo microalloying component system and obtains the microstructure of ultrafine grained acicular ferrite and a nano precipitated phase by a controlled rolling and controlled cooling process technology. The mechanical properties of the steel plate provided by the invention meet the following requirements: the yield strength is more than or equal to 650MPa, the tensile strength is more than or equal to 780MPa, the elongation is more than or equal to 21%, the n value is more than or equal to 0.10, the steel is qualified when the cold bending test d of 180 degrees is 1.5a, the grain size is more than or equal to 12 grade, and the problem that the forming performance of the traditional 780MPa grade high-strength steel is poor can be effectively solved.
Description
Technical Field
The invention belongs to the technical field of steel material engineering, and particularly relates to a hot rolled steel plate for 780 MPa-level hydraulic bulging and a preparation method thereof.
Background
With the rapid development of the automobile industry, automobiles bring convenience to people's lives and bring many problems, and safety and environmental protection are the most central topics. To solve these problems, weight reduction of the vehicle body has been carried out. In addition to the optimized design of the vehicle body structure and the adoption of high-strength materials, the optimized part structure, such as 'replacing solid with space', the variable cross-section design and the like, can not only reduce the mass, but also fully utilize the strength and the rigidity of the materials. The hydroforming technology is an advanced manufacturing technology developed to meet this requirement. The hydraulic bulging process can be used for forming complex parts, improving the quality of the parts, reducing the forming procedures and reducing the processing cost, so that the hydraulic bulging process is widely applied to the fields of automobiles, aerospace, aviation and the like.
The conventional hot rolled steel plate adopts a conventional converter smelting process, which comprises the steps of molten iron desulfurization, converter smelting combined blowing, deoxidation, alloying, Al wire feeding on a small platform behind a furnace, LF refining heating, RH vacuum treatment, continuous casting to obtain a plate blank, reheating the plate blank, high-pressure water descaling, rough rolling, coiling by a hot coil box, finish rolling, laminar cooling, coiling, packaging and warehousing. With the development of the automobile industry and the increasing market competition, structural steel for automobiles is gradually developing toward low cost and high performance. At present, the steel for frames and bodies of main truck enterprises is generally improved to 600MPa level, and the high-strength steel of 700 MPa level and 800MPa level is gradually popularized.
2005-12-21-st patent CN1711142A discloses a special-shaped blank tube for hydraulic bulging processing and a processing device, a method and a processed product thereof, 2012-1-18-st patent CN102319826A discloses a steel tube bulging forming method for a large automobile axle housing, 2012-1-18-st patent CN102319827A discloses a steel tube bulging forming method for a small automobile axle housing, 2012-1-18-th patent CN102319834A discloses a steel tube bulging forming method for an engineering vehicle axle housing, 2013-5-8-th patent CN103084460A discloses an integral forming method for a light and medium truck weldless axle housing. The above patents all relate to production methods of hydraulic bulging processes, and all relate to special steel for hydraulic bulging.
Patent CN106868281A of 2017, 6 and 20 discloses ultra-fine grained ferrite low-temperature bainite dual-phase steel and a preparation method thereof. By adopting a micro/low C-high Si-Ni-Cr-W alloying technical route, the slab required by the steel rolling needs to be subjected to the working procedures of annealing, hot rolling, quenching, tempering and the like, and the rolling production working procedure is complex after the slab with the tempered troostite structure is prepared, so that the industrial mass production is difficult.
Patent CN105779874A No. 7/20 in 2016 discloses a Cr-Nb 780 MPa-grade hot-rolled dual-phase steel and a production method thereof, which comprises the following components in percentage by weight: 0.06 to 0.09 percent of C, less than or equal to 0.20 percent of Si, 1.50 to 1.90 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.004 percent of S, 0.020 to 0.050 percent of Als, 0.30 to 0.49 percent of Cr, and 0.015 to 0.035 percent of Nb. The steel related to the invention adopts a low C-Cr-Nb alloying technical route, and adopts a three-stage air cooling process, the coiling temperature is below 250 ℃, and the obtained steel is a ferrite and martensite dual-phase structure. When the dual-phase steel is used for producing pipe fittings such as torsion beams and the like by a hydraulic bulging process, a high-frequency welding process is adopted in the middle pipe manufacturing process, the softening phenomenon of a heat affected zone is obvious, the problem of severe necking of the heat affected zone in the hydraulic bulging process can be caused, and the problem of instant failure exists in the torsional fatigue testing process, so that the use requirement of parts with high fatigue performance can not be met.
At present, the high-strength steel produced by the hydraulic bulging process is still conventional high-strength steel, and the forming performance of the high-strength steel cannot completely meet the hydraulic bulging production process. Therefore, it is necessary to develop a steel material having high strength, high elongation, high n value and high fatigue property, in view of the required characteristics of the high-strength steel for hydroforming.
Disclosure of Invention
The invention aims to solve the technical problem that the existing 780 MPa-grade high-strength steel has poor forming performance.
The technical scheme adopted by the invention for solving the technical problems is as follows: the 780 MPa-grade hot rolled steel plate for hydraulic bulging comprises the following chemical components in percentage by weight: 0.02-0.04% of C, 0.05-0.10% of Si, 1.7-2.00% of Mn, 0.040-0.050% of Nb, 0.070-0.080% of Ti0.030-0.060% of V, 0.20-0.30% of Mo, less than or equal to 0.02% of P, less than or equal to 0.003% of S, less than or equal to 0.0040% of N, 0.010-0.050% of Als0.0020-0.0050% of Ca, and the balance of Fe and inevitable impurities.
The microstructure of the 780MPa grade hot rolled steel sheet for hydraulic bulging is composed of quasi-polygonal ferrite, acicular ferrite and a trace amount of pearlite.
The yield strength of the hot rolled steel plate for 780 MPa-grade hydraulic bulging is more than or equal to 650MPa, the tensile strength is more than or equal to 780MPa, the elongation is more than or equal to 21%, the n value is more than or equal to 0.10, the d-1.5 a pass of a 180-degree cold bending test is qualified, and the grain size is more than or equal to 12 grade.
The preparation method of the hot rolled steel plate for 780 MPa-level hydraulic bulging comprises the following steps:
a. preparing a steel billet according to the chemical components of the hot rolled steel plate for 780 MPa-level hydraulic bulging;
b. rough rolling: carrying out 5-7 times of rough rolling on the heated steel billet to obtain an intermediate billet, wherein the deformation of each time of the first 2 times is controlled to be more than or equal to 18 percent, and the deformation of each time of the rest times is controlled to be more than or equal to 20 percent;
c. finish rolling: the initial rolling temperature is 1020-;
d. laminar cooling: and c, performing ultra-fast cooling on the steel plate obtained in the step c to 580-640 ℃ in the front section, wherein the cooling speed is more than 60 ℃/s.
In the step a, the thickness of the billet is 200-250mm, and the length is 9-10 m.
In the step b, the slab is heated to 1180-.
In the step b, the thickness of the intermediate blank is 35-45 mm.
And c, performing finish rolling for 6-7 times.
The invention has the beneficial effects that: the invention adopts a micro/low C-Nb-Ti-Mo microalloyed component system, utilizes Mo to improve the stability of austenite, inhibits dislocation disappearance at high temperature, increases the bit density and prevents the nucleation position of Nb (C, N); meanwhile, Mo is eccentrically polymerized to the interface of Nb (C, N) and ferrite, so that the diffusion of Nb atoms from a ferrite matrix to Nb (C, N) can be prevented, the growth of a precipitated phase under the high-temperature condition is inhibited, the purpose of refining the precipitated phase is achieved, and more nanoscale precipitated phases are obtained; in addition, Mo can improve the solid solubility of microalloy elements Nb, V and Ti in austenite, promote the precipitation of microalloy carbonitride in ferrite, improve the stability of the microalloy carbonitride, improve the high-temperature performance of steel, delay austenite recrystallization and enlarge the rolling process window of an austenite non-recrystallization region.
The steel grade of the invention utilizes micro/low C to avoid forming a large amount of pearlite and cementite, and improves the toughness and plasticity of the material; nb is utilized to control the size of austenite grain size and refine grains; a large amount of finely dispersed nano carbides are precipitated in ferrite by adding high content of Ti, so that the dispersion precipitation strengthening effect is achieved; the low content of N is utilized to avoid generating a large amount of micron-sized liquated TiN particles in steel making; ca is adopted to improve the appearance of non-metallic inclusions of sulfides in the steel, thereby improving the fatigue performance of the material.
The method controls the rough rolling times and the single-pass deformation to refine the austenite grain size, and controls the thickness of the intermediate blank after rough rolling to reduce the adverse effect on the finish rolling deformation; austenite grain refinement is promoted through finish rolling, nucleation cores can be provided for subsequent ferrite phase transformation, and ferrite structure refinement of finished products is promoted; meanwhile, the generation of mixed crystal texture is inhibited by controlling the initial rolling temperature and the final rolling temperature of the finish rolling.
When the thickness of the steel product is 2-8mm, the steel plate can be fully cooled along the thickness direction by adopting the cooling rate of more than 60 ℃/s, and the steel plate is deformed into fine ferrite grains under a larger supercooling degree, so that the strength and the toughness of the steel product are improved, meanwhile, the final cooling temperature is set to be 580-640 ℃, so that a larger driving force can be provided for the precipitation of Ti, and the precipitation of Ti (C, N) and (Nb, Ti) (C, N) is facilitated.
The invention adopts a micro/low C-Nb-Ti-Mo microalloyed component system and combines a controlled rolling and controlled cooling process to obtain the microstructure of ultrafine grained acicular ferrite and a nano precipitated phase, has high strength, high elongation, high n value and high fatigue performance, and shows good toughness and plasticity and hydraulic bulging forming performance, which are difficult to achieve by ferrite and pearlite steel with the same strength level. The mechanical properties of the steel plate provided by the invention meet the following requirements: the yield strength is more than or equal to 650MPa, the tensile strength is more than or equal to 780MPa, the elongation is more than or equal to 21 percent, the n value is more than or equal to 0.10, the product is qualified after a 180-degree cold bending test d is 1.5a, and the grain size is more than or equal to 12 grade, so that the product can be widely applied to manufacturing automobile chassis parts, exhaust manifolds of exhaust systems, exhaust pipes, lower arms of chassis systems, front shafts, rear shafts and the like. The preparation method is simple, has strong technological adaptability, and is suitable for common hot continuous rolling production lines and continuous casting and rolling production lines; the preparation method provided by the invention has good process stability, and the steel produced by the preparation method has good mechanical property stability.
Drawings
FIG. 1 shows the microstructure of a steel grade according to example 3 of the present invention.
FIG. 2 shows the microstructure of a steel grade according to comparative example 1 of the present invention.
Detailed Description
The technical solution of the present invention can be specifically implemented as follows.
The 780 MPa-grade hot rolled steel plate for hydraulic bulging comprises the following chemical components in percentage by weight: 0.02-0.04% of C, 0.05-0.10% of Si, 1.7-2.00% of Mn, 0.040-0.050% of Nb, 0.070-0.080% of Ti, 0.030-0.060% of V, 0.20-0.30% of Mo0.20, less than or equal to 0.02% of P, less than or equal to 0.003% of S, less than or equal to 0.0040% of N, 0.010-0.050% of Als, 0.0020-0.0050% of Ca, and the balance of Fe and inevitable impurities.
The preparation method of the hot rolled steel plate for 780 MPa-level hydraulic bulging comprises the following steps:
a. preparing a steel billet according to the chemical components of the hot rolled steel plate for 780 MPa-level hydraulic bulging;
b. rough rolling: carrying out 5-7 times of rough rolling on the heated steel billet to obtain an intermediate billet, wherein the deformation of each time of the first 2 times is controlled to be more than or equal to 18 percent, and the deformation of each time of the rest times is controlled to be more than or equal to 20 percent;
c. finish rolling: the initial rolling temperature is 1020-;
d. laminar cooling: and c, performing ultra-fast cooling on the steel plate obtained in the step c to 580-640 ℃ in the front section, wherein the cooling speed is more than 60 ℃/s.
In the step a, a conventional converter smelting process is adopted, molten iron desulphurization → converter smelting combined blowing → deoxidation, alloying → a small platform at the back of the converter for Al wire feeding → LF refining heating → RH vacuum treatment → continuous casting is carried out to obtain a billet, and the billet comprises the following chemical components in percentage by weight: 0.02-0.04% of C, 0.05-0.10% of Si, 1.7-2.00% of Mn, 0.040-0.050% of Nb, 0.070-0.080% of Ti0.030-0.060% of V, 0.20-0.30% of Mo, less than or equal to 0.02% of P, less than or equal to 0.003% of S, less than or equal to 0.0040% of N, 0.010-0.050% of Als0.0020-0.0050% of Ca, and the balance of Fe and inevitable impurities.
In order to obtain enough accumulated deformation amount of an austenite non-recrystallization region in the rough rolling stage, increase the grain-crystal boundary area and the deformation zone area of the austenite unit volume and achieve the effect of refining the austenite grain size, and simultaneously, in order to prevent the tail from rolling to be broken due to overlarge deformation resistance in rolling caused by overlarge tail temperature drop in finish rolling caused by overlarge length of a steel blank after being rolled into an intermediate blank thickness, the billet thickness is preferably 200-250mm and the billet length is preferably 9-10m in the step a.
The subsequent processing technology of the billet obtained in the step a comprises the following steps: heating a steel billet → descaling under high pressure → rough rolling → coiling in a hot coil box → finishing rolling → laminar cooling → coiling → packaging and warehousing.
In order to fully dissolve alloy elements, eliminate segregation in the steel billet and simultaneously avoid the condition that the original austenite grains of the steel billet are coarse due to too long furnace time and are inherited into the finished steel, it is preferable that in the step b, the plate blank is heated to 1180-1220 ℃, the heating time is 180-300min, and the soaking period time is more than or equal to 40 min.
In order to obtain a sufficiently large accumulated deformation amount of the unrecrystallized region of austenite in the finish rolling stage and to increase the grain-crystal boundary area per unit volume of austenite and the area of the deformed strip, it is preferable that the thickness of the intermediate billet is 35 to 45mm in the above step b and the finish rolling is performed in 6 to 7 passes in the step c.
When the finish rolling initial rolling temperature is lower, the rolling load is too heavy when the thin gauge steel is rolled, and when the finish rolling initial rolling temperature is higher, the thin gauge steel enters an incomplete recrystallization area to generate a mixed crystal structure; in the step c, the initial rolling temperature is preferably 1020-.
The laminar cooling speed and the final cooling temperature both influence the phase transformation process, when the cooling speed is lower or the final cooling temperature is higher, coarse pro-eutectoid ferrite and large-size pearlite structures are easily formed, even a large amount of cementite distributed along grain boundaries and in a gridding manner is generated, the mechanical property and the forming property of finished steel are influenced, and in addition, when the final cooling temperature is more than 600 ℃, a larger driving force can be provided for Ti precipitation, and Ti (C, N) and (Nb, Ti) (C, N) precipitation is facilitated. It is therefore preferred that in step d above, the cooling rate is > 60 ℃/s and the final cooling temperature is 580-.
The microstructure of the hot rolled steel plate for 780 MPa-grade hydraulic bulging consists of quasi-polygonal ferrite, acicular ferrite and trace pearlite, the yield strength is more than or equal to 650MPa, the tensile strength is more than or equal to 780MPa, the elongation is more than or equal to 21%, the n value is more than or equal to 0.10, the steel plate is qualified when the steel plate is subjected to 180-degree cold bending test d is 1.5a, and the grain size is more than or equal to 12 grade.
The technical solution and effects of the present invention will be further described below by way of practical examples.
Examples
This example provides 5 sets of hot rolled steel sheets for 780MPa class hydroforming prepared by the preparation method of the present invention, as in examples 1, 2, 3, 4 and 5, and the hot rolled steel sheets for 780MPa class hydroforming in examples 1, 2, 3, 4 and 5 have the same composition design, as in the examples in table 1. The present invention provides two sets of comparative examples, such as comparative examples 1 and 2, with comparative examples 1 and 2 being of the same chemical composition, as shown in table 1 for the comparative examples.
TABLE 1 chemical compositions of examples and comparative examples
The production of hot rolled steel sheets for 780 MPa-grade hydroforming of examples 1, 2, 3, 4 and 5 included the steps of:
a. adopting a conventional converter smelting process, namely molten iron desulphurization → converter smelting combined blowing → deoxidation, alloying → Al wire feeding on a small platform behind the converter → LF refining heating → RH vacuum treatment → billet is obtained after continuous casting, wherein the thickness of the billet is 200-;
b. b, heating the steel billet obtained in the step a, descaling by high-pressure water, and carrying out rough rolling on the steel billet for 5-7 times to obtain an intermediate billet with the thickness specification of 35-45mm, wherein the deformation of each time in the first 2 times is more than or equal to 18%, and the deformation of each time in the rest times is more than or equal to 20%;
c. carrying out finish rolling on the intermediate blank for 6-7 times to obtain a steel plate with the thickness of 2-8mm, wherein the initial rolling temperature is 1020-1080 ℃, and the final rolling temperature is 850-900 ℃;
d. and carrying out laminar cooling after rolling, wherein an ultra-fast cooling laminar cooling process is adopted, the cooling speed is more than 60 ℃/s, and the final cooling temperature is 580-640 ℃.
The rough rolling process parameters of examples 1, 2, 3, 4 and 5 are controlled as shown in table 2, and the remaining process parameters are controlled as shown in table 3.
Comparative examples 1 and 2 were prepared by the same method as in the example, but the control values of part of the hot rolling process were not within the control range of the hot rolling process of the present invention, the parameters of the rough rolling process of comparative examples 1 and 2 were as shown in Table 2, and the parameters of the remaining processes were as shown in Table 3.
TABLE 2 roughing process parameter control
TABLE 3 Hot Rolling Process control values
FIG. 1 is a microstructure of a steel grade of example 3, from which it can be seen that a limit structure of a steel grade prepared according to the present invention is composed of quasi-polygonal ferrite, acicular ferrite and trace pearlite, and the structure is fine and uniform; FIG. 2 is a microstructure of a steel grade according to comparative example 1 of the present invention, and it can be seen from the figure that the composition of the steel grade prepared according to the comparative example is quasi-polygonal ferrite + pearlite, and the problems of mixed crystals and structure segregation exist in the structure.
The mechanical properties of the steel coils of examples 1, 2, 3, 4 and 5 and comparative examples 1 and 2 were measured, and the results are shown in table 4.
TABLE 4 mechanical properties of the steel coil
As can be seen from Table 4, the elongation and n values of examples 1, 2, 3, 4 and 5 are superior to those of comparative examples 1 and 2, which shows that the 780MPa class hot rolled steel sheet for hydroforming according to the present invention has good formability and formability. The 780 MPa-grade hot rolled steel plate for hydraulic bulging has the advantages of high strength, high elongation, high n value and high fatigue performance, the yield strength is larger than or equal to 650MPa, the tensile strength is larger than or equal to 780MPa, the elongation is larger than or equal to 21%, the n value is larger than or equal to 0.10, the cold bending test d at 180 degrees is 1.5a qualified, and the grain size is larger than or equal to 12 grade.
Claims (7)
- The 1.780 MPa-grade hot rolled steel plate for hydraulic bulging is characterized by comprising the following chemical components in percentage by weight: 0.02-0.04% of C, 0.05-0.10% of Si, 1.7-2.00% of Mn, 0.040-0.050% of Nb, 0.070-0.080% of Ti, 0.030-0.060% of V, 0.20-0.30% of Mo, less than or equal to 0.02% of P, less than or equal to 0.003% of S, less than or equal to 0.0040% of N, 0.010-0.050% of Als, 0.0020-0.0050% of Ca, and the balance of Fe and inevitable impurities;the microstructure of the steel plate consists of quasi-polygonal ferrite, acicular ferrite and trace pearlite.
- 2. The hot-rolled steel sheet for 780 MPa-grade hydroforming according to claim 1, characterized in that: the yield strength is more than or equal to 650MPa, the tensile strength is more than or equal to 780MPa, the elongation is more than or equal to 21 percent, the n value is more than or equal to 0.10, the d is qualified after 180-degree cold bending test (1.5 a), and the grain size is more than or equal to 12 grade.
- The preparation method of the hot rolled steel plate for 3.780 MPa-level hydraulic bulging is characterized by comprising the following steps of:a. preparing a steel billet: preparing a steel billet by adopting a conventional converter smelting method, wherein the components of the steel billet are controlled to be 0.02-0.04% of C, 0.05-0.10% of Si, 1.7-2.00% of Mn, 0.040-0.050% of Nb, 0.070-0.080% of Ti, 0.030-0.060% of V, 0.20-0.30% of Mo, less than or equal to 0.02% of P, less than or equal to 0.003% of S, less than or equal to 0.0040% of N, 0.010-0.050% of Als, 0.0020-0.0050% of Ca, and the balance of Fe and inevitable impurities;b. rough rolling: carrying out 5-7 times of rough rolling on the heated steel billet to obtain an intermediate billet, wherein the deformation of each time of the first 2 times is controlled to be more than or equal to 18 percent, and the deformation of each time of the rest times is controlled to be more than or equal to 20 percent;c. finish rolling: the initial rolling temperature is 1020-;d. laminar cooling: and c, performing ultra-fast cooling on the steel plate obtained in the step c to 580-640 ℃ in the front section, wherein the cooling speed is more than 60 ℃/s.
- 4. The method for manufacturing the hot rolled steel sheet for 780 MPa-level hydroforming according to claim 3, characterized in that: in the step a, the thickness of the billet is 200-250mm, and the length is 9-10 m.
- 5. The method for manufacturing the hot rolled steel sheet for 780 MPa-level hydroforming according to claim 3, characterized in that: in the step b, the billet is heated to 1180-1220 ℃, the heating time is 180-300min, and the soaking time is more than or equal to 40 min.
- 6. The method for manufacturing the hot rolled steel sheet for 780 MPa-level hydroforming according to claim 3, characterized in that: in the step b, the thickness of the intermediate blank is 35-45 mm.
- 7. The method for manufacturing the hot rolled steel sheet for 780 MPa-level hydroforming according to claim 3, characterized in that: and c, performing finish rolling for 6-7 times.
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