CN113846272B

CN113846272B - A kind of hot rolling preparation method of 1700MPa grade high Cr-Si thin gauge hot forming steel

Info

Publication number: CN113846272B
Application number: CN202111428574.5A
Authority: CN
Inventors: 徐伟; 王鲁宁; 胡军; 杨得草; 苗隽; 柴志松
Original assignee: Bengang Steel Plates Co Ltd; Northeastern University China
Current assignee: Bengang Steel Plates Co Ltd; Northeastern University China
Priority date: 2021-11-29
Filing date: 2021-11-29
Publication date: 2022-03-01
Anticipated expiration: 2041-11-29
Also published as: CN113846272A

Abstract

The invention belongs to the technical field of metal processing, and in particular relates to a hot rolling preparation method of 1700 MPa grade high Cr-Si thin-gauge hot forming steel. Aiming at the problems in the prior art that the steel sheet for hot forming needs to be prepared by adding cold rolling, and the hot forming needs to be coated, the present invention provides a low-cost 1700MPa grade high Cr-Si alloyed hot forming steel without coating, and A hot-rolled preparation method of thin-gauge electroless hot-formed steel with this composition is proposed. The hot-formed steel of the invention has a simple preparation process and low cost, and the prepared hot-formed steel has a uniform structure, excellent mechanical properties and high-temperature oxidation resistance.

Description

Hot rolling preparation method of 1700MPa grade high Cr-Si thin-specification hot forming steel

Technical Field

The invention belongs to the technical field of metal processing, and particularly relates to a hot rolling preparation method of 1700MPa grade high Cr-Si thin-specification hot forming steel.

Background

Safety, environmental protection and energy conservation are the subjects of development of the current automobile manufacturing industry, and the automobile body manufactured by adopting the high-strength steel plate can effectively reduce the weight of the automobile body and the oil consumption, can also improve the safety and the comfort of the automobile, and is the best way for realizing the light weight of the automobile body and improving the collision safety. Along with the improvement of the strength of the advanced high-strength steel for the automobile, the plastic deformation range is narrowed, the stamping force is increased, the part forming resilience is serious, the size and shape stability is poor, the formability and the weldability of the material are adversely affected, the conventional cold stamping forming is very difficult, and the processing difficulty and the manufacturing cost of automobile manufacturing enterprises are improved. The hot forming technique can solve the problems of cold forming springback and the like of the high-strength steel by converting cold formability deteriorated at room temperature of the ultrahigh-strength steel into hot workability in a good high-temperature state. According to the data reported by the market information consortium of passenger cars, the number of the passenger cars sold in 2018 all the year round domestically is 2352.9 ten thousand, the steel consumption of each white car body is about 700kg, the weight of the hot forming parts accounts for about 15%, and the consumption of the hot forming steel is up to 240 ten thousand tons in total.

The most commercially available hot-formed steels of stable production and technology are Mn-B series steel sheets of 1500MPa (22 MnB 5) grade and 1800MPa (30 MnB 5) grade. The application proportion of the hot forming steel on the European and American car shapes is higher and can generally reach 10 to 20 percent. The 2015 Volvo XC90Gen I and XC90Gen II automobile bodies adopt a plurality of hot formed parts, and respectively account for 7 percent and 38 percent of the total mass of the white automobile bodies. Research shows that the weight of the automobile body can be reduced by more than 20 percent by using the hot forming steel of 1500MPa (tensile strength) level to replace the steel of 800MPa level; if the tensile strength reaches 2000MPa, the weight can be further reduced by about 13 percent.

The hot forming technology solves the problems of stamping cracking, serious resilience, poor dimensional accuracy and the like of complex parts, and the stamped parts have the advantages of high strength, high dimensional accuracy, good forming quality, good weldability, avoidance of the resilience problem of cold forming of high-strength steel and the like, so that the hot formed parts are increasingly emphasized in light-weight automobile materials. And the use of the hot forming technology in the automobile field can effectively ensure that the light weight is realized and simultaneously good collision performance is kept. According to the classification of steel plate coatings, the current commercial hot forming steels are mainly divided into Al-Si coating steels and non-coating steels, wherein the Al-Si coating has the main function of preventing the surface of the steel plate from decarbonization and oxidation in the austenitizing process, and the coating can isolate a steel plate matrix from the external environment, so that the steel plate has a certain anticorrosion function, but the Al-Si coating also blocks the diffusion of hydrogen to cause coating cracking and hydrogen embrittlement, and roll sticking can occur in the hot forming process to cause the service life of a roller table to be reduced, so that the production cost is increased. The surface of the bare plate is easy to form a black oxide layer in the hot stamping process, the subsequent welding quality and the welding strength are seriously influenced, shot blasting treatment is needed, but the shot blasting is easy to deform and has poor corrosion resistance. In order to ensure the precision of the door ring, the shot blasting process needs to be continuously optimized subsequently, and the mass production precision and consistency of the door ring are ensured.

Due to the continuous rising of the automobile yield in China, the requirements of light weight, energy conservation and emission reduction and strict automobile safety regulations, a wide market is provided for hot stamping products, and huge business opportunities are provided for hot stamping steel, hot stamping equipment and the like. However, the problems of insufficient toughness and potential hydrogen-induced delayed fracture of hot-stamped parts are not negligible.

The existing thin steel plates for hot forming are produced by adding a cold rolling procedure after hot rolling, and the hot forming steel of 1500MPa or above has high alloy content and high hardenability, so that the rolling force in the cold rolling process is large and even exceeds the upper limit of a cold rolling unit, and the problems of edge cracking, poor plate shape and the like of the steel plates are often caused by adopting a complex procedure of intermediate annealing and secondary cold rolling, so that the production cost is greatly improved, and the production efficiency and the product percent of pass are reduced. If the direct hot-rolling thin-specification hot-forming steel with independent intellectual property rights can be developed without a cold rolling process, the method has the advantages of low cost and high efficiency, and the quality and the qualification rate of products are greatly improved. The alloy component design and controlled rolling and controlled cooling process for preparing the hot forming steel by the hot cooling method are important for the final structure performance after hot forming.

Disclosure of Invention

Aiming at the problems in the prior art, the invention provides 1700 MPa-grade high Cr-Si alloyed coating-free hot forming steel with low cost and provides a hot rolling preparation method of thin-specification coating-free hot forming steel with the components.

The 1700MPa grade high Cr-Si coating-free hot forming steel has good high temperature oxidation resistance, no oxide scale or only extremely thin oxide scale is formed on the surface during high temperature forming, so on one hand, hot forming can be carried out without coating, the coating material cost is saved, the welding problem of an Al-Si coating steel plate and the problems of hydrogen embrittlement and coating cracking can be avoided, on the other hand, shot blasting treatment is not needed to be carried out after hot forming like other bare plate hot forming steel, and the dimensional precision of parts can be effectively ensured after hot forming. In addition, the coating-free hot forming steel has good toughness, can also omit the cold rolling procedure necessary for the preparation of the existing hot forming steel, has simple process and greatly reduces the cost.

The technical scheme of the method is as follows:

a1700 MPa-grade hot-rolled coating-free hot-forming steel with high Cr-Si alloying is different from a 22MnB5 hot-forming steel alloy component system of a traditional Al-Si coating, and the used high Cr-Si alloying hot-forming steel comprises the following chemical components in percentage by mass: c: 0.15 to 0.35%, Mn: 0.8-3.2%, Si: 0.8-2.8%, S: < 0.01%, P: < 0.015%, Al: 0.01-0.05%, Cr: 1.5-3.9%, Nb: 0.01-0.05%, V: 0.01-0.05%, Ti: 0.01-0.03%, Cu: 0.05-0.15%, and the balance of Fe and other inevitable impurities.

The chemical composition adopts high Cr-Si alloying steel and a small amount of microalloying elements are compositely added. C is an austenite stabilizing element, and C and a microalloying element are combined to play a role in precipitation strengthening, but the welding performance is deteriorated due to the excessively high content of C, so that the mass percent of carbon is 0.15-0.35%. Mn can obviously increase hardenability, but the high manganese content improves carbon equivalent, deteriorates welding performance, and reduces high-temperature oxidation resistance in the hot forming process, so the mass percent of manganese is 0.8-3.2%. The Cr can remarkably improve hardenability and refine quenched martensite laths, the high-temperature oxidation resistance of the hot forming process can be greatly improved, and the mass percent of the adopted Cr is 1.5-3.59% in consideration of the properties of 1700MPa hot forming steel and the alloy cost. Si can play a role in solid solution strengthening, the formation of coarse carbides can be effectively inhibited by the Si, the high-temperature oxidation resistance of the Si is enhanced, and the brittleness is caused by excessive Si, so that the mass percent of the silicon is 0.8-2.8%. Nb inhibits austenite recrystallization in the controlled rolling process, high-temperature austenite grains can be obviously refined, the effect of hot rolling replacing a cold rolling process is realized, and the surface quality of the continuous casting billet is deteriorated due to the overhigh Nb content, so that the mass percent of the adopted niobium is 0.01-0.05%. V can refine the quenched martensite lath, a precipitated phase of V and Nb can play a role in precipitation strengthening, and the precipitated phase can improve the hydrogen embrittlement resistance, so that the mass percent of vanadium is 0.01-0.05%. The Ti has the function of refining original austenite grains, and trace Ti fixes N atoms to form a precipitated phase and inhibits the abnormal growth of the austenite grains in a welding coarse grain heat affected zone, so that the mass percent of the adopted titanium is 0.01-0.03%. The Cu can improve the corrosion resistance, and the excessively high Cu causes brittleness, so that the mass percent of the adopted copper is 0.05-0.15%. The Al is mainly used for deoxidizing and refining grains, and the structural property uniformity of the steel is improved to a certain extent, so that the mass percent of the aluminum is 0.01-0.05%; s, P is impurity element in the test steel, and should be controlled in a certain range.

The invention also provides a hot rolling preparation method of the 1700 MPa-grade high Cr-Si thin-specification hot forming steel, which is characterized in that the steel continuous casting billet with the components is prepared into a hot rolling steel plate and further into the hot forming steel by hot rolling, and the hot rolling preparation method comprises the following steps:

step 1: heating a steel continuous casting billet in a heating furnace to 1150-1300 ℃, preserving heat for 1-2.5 h, removing furnace-generated iron scale through primary descaling, and carrying out rough rolling and finish rolling: the initial rolling temperature of rough rolling is 1100-1300 ℃, the rough rolling is carried out for about 6 passes, the total reduction rate is 80-90%, and the thickness of the obtained rough rolling intermediate blank is 30-45 mm; removing secondary oxide scales before finish rolling, performing finish rolling for about 7 passes to obtain a steel strip with the thickness of 1.5-4.5 mm, wherein the start rolling temperature and the finish rolling temperature of the finish rolling are respectively 1000-1090 ℃ and 880-950 ℃;

step 2: and after finishing the finish rolling, cooling the steel strip to 550-750 ℃ with water at a cooling rate of 5-30 ℃/s, wherein the temperature range (550-750 ℃) is also used as the initial coiling temperature, coiling the steel strip at the initial coiling temperature, and then cooling the steel strip to room temperature in air to obtain the hot rolled steel plate. The hot rolled steel sheet after hot rolling and coiling is a hot rolled steel sheet for hot forming of a 1700MPa grade high Cr-Si thin gauge (hereinafter referred to simply as "hot rolled steel sheet" or "hot rolled steel sheet"), and has a microstructure of ferrite, pearlite, bainite, and a small amount of carbide.

The hot-rolled supplied steel plate for hot forming with thin specification (the thin specification of the invention specifically refers to the thickness of 1.5-4.5 mm) is prepared by adopting the mode of hot-replacing-cold, the tissue inheritance effect of the initial tissue refinement formed in the controlled rolling and controlled cooling process on the hot forming process is mainly relied on, the combination of controlled rolling and Nb microalloying is adopted to realize the regulation and control of rolled original austenite grains, and then the water cooling rate is controlled, so that the formation of initial iron oxide scales in the air cooling process is avoided, the generation of coarse high-temperature ferrite is inhibited, and meanwhile, a large number of nucleation points can be provided for the tissue phase change in the subsequent coiling process. And the proper coiling temperature is selected, so that the problem that the plate shape after uncoiling and cutting is influenced due to high residual stress caused by the formation of a large amount of martensite during low-temperature coiling is avoided. The initial coiling temperature range can be regulated and controlled to obtain ferrite, pearlite and bainite tissues with different proportions and carbide combinations with certain content. Therefore, the hot rolled steel plate for 1700MPa grade high Cr-Si thin specification hot forming is prepared in a hot strip cooling mode by regulating and controlling the process parameters such as the thickness of the intermediate blank, the rolling and cooling control process parameters, the coiling system and the like, and the cold rolling process required by other types of steel plates for hot forming is omitted.

And step 3: and then carrying out hot stamping forming on the hot-rolled goods supply plate for 1700MPa grade high Cr-Si thin specification hot forming, thus obtaining the 1700MPa grade high Cr-Si thin specification plating-free hot forming steel.

The invention also provides a specific process for hot stamping in the step 3, which mainly comprises the following steps:

step 3.1: uncoiling and straightening the hot-rolled material supply plate, putting the hot-rolled material supply plate into a heating furnace at 880-980 ℃, keeping the temperature in the furnace for 2-15 min, wherein the atmosphere in the furnace is air, nitrogen or nitrogen plus methane;

step 3.2: the transfer process comprises the following steps: transferring the steel plate to a hot stamping forming press in the air, wherein the transferring process is 5-18 s, and the temperature after transferring is 720-860 ℃;

step 3.3: hot stamping forming treatment: and (3) carrying out hot stamping forming on the steel plate in a die with a cooling system inside, keeping the pressure at 3-25 MPa, and then quenching and cooling to below 200 ℃ (martensite finish temperature) at a die cooling speed of 15-200 ℃/s to obtain a 1700MPa grade high Cr-Si thin-specification hot forming steel product to be prepared. The microstructure of the hot-formed steel after die quenching is martensite and 1-8% of residual austenite, the yield strength is 1080-1410 MPa, the tensile strength is 1565-1790 MPa, the elongation is 8.3-13.1%, the bending angle can reach 40-67 degrees, and the thickness of the surface iron scale is 0.2-0.9 mu m.

The following baking step may also be added after step 3.3:

step 3.4: and (3) baking:

and (3) placing the steel plate into a heat treatment furnace at 170 ℃ for heat preservation for 20min, improving the yield strength, slightly reducing the tensile strength, and simultaneously improving the bending angle and the elongation.

The hot-rolled steel sheet according to the present invention has a thin scale of 0.2 to 0.9 μm, which is a hot-formed steel scale produced by hot forming by the above-described method. The hot rolled steel sheet is not required to be subjected to a plating treatment before hot forming, the bare steel sheet can be directly subjected to hot press forming, and the bare steel sheet after hot forming is not required to be subjected to a shot blasting descaling step.

In addition, before step 3.1, the following bell annealing (cover annealing) process of the hot rolled steel sheet may be added:

performing cover annealing on the hot rolled steel plate, wherein a hydrogen atmosphere is adopted in the cover annealing process, the temperature is heated from room temperature to 245-400 ℃, and the average heating speed is 120-200 ℃/h; then, heating to a heat preservation target temperature of 650-770 ℃, wherein the average heating speed is 42-48 ℃/h; preserving the heat for 8-12 h at the heat preservation target temperature; cooling the mixture to 300-500 ℃ along with the furnace from the heat preservation target temperature; then quickly cooling to the tapping temperature of 100 ℃, wherein the average cooling speed is 25-42 ℃/h; and discharging the product when the temperature is cooled to 100 ℃.

The hood-type annealing has the following advantages:

1. the utilization rate of the steel plate is high, and the total yield is high. Due to the fact that the through width and the head and tail performance fluctuation of the hot rolled steel plate (also called as a steel coil or strip steel) are large after coiling, uncoiling buckling is caused, cover annealing greatly improves the uniformity of the whole hot rolled steel plate, material loss of the edge and the head and tail is reduced, and the blanking utilization rate of the hot rolled steel plate and the total yield of the hot formed steel plate are improved.

2. The preparation process is simple and easy to realize industrialization. The common cold-rolled high-strength steel production line is provided with a hood-type annealing furnace, the production line is not required to be transformed and upgraded, the process flow is simple to control and operate, and the industrial production is easy to realize;

3. the high temperature hydrogen reduction of the hood annealing produces a decarburized layer of a certain thickness to increase the bending angle of the hot formed steel. Therefore, by introducing the cover annealing process and regulating and controlling the cover annealing process parameters, the comprehensive mechanical property and the process stability of the hot rolled steel plate and the finally manufactured hot forming steel can be enhanced.

After the cover annealing process is added, the finally prepared hot forming steel has high structural property uniformity. The comprehensive mechanical property fluctuation of the steel coil after the cover retreats in the width direction, the head, the tail and the coil core are subjected to thermal forming is very small; the yield strength of the edge of the steel coil before the cover retreats is 800-1000 MPa, the tensile strength is 1000-1500 MPa, and the elongation is 7.5-15.5%; the yield strength of the middle part is 500-600 MPa, the tensile strength is 600-800 MPa, and the elongation is 10.5-25.5%. The yield strength of the edge of the steel coil after the steel coil is withdrawn is 450-700 MPa, the tensile strength is 550-800 MPa, and the elongation is 9.5-22.5%; the yield strength of the middle part is 420-600 MPa, the tensile strength is 530-800 MPa, and the elongation is 10.5-27.5%. After the annealing treatment, the strength of the edge part of the hot rolled steel plate is greatly reduced and is basically equivalent to that of the middle part, and the performance of the hot rolled steel plate in the width direction is uniform.

4. The hot forming steel has excellent bending properties. Covering and removing the surface of the steel plate for a long time under the condition of high-temperature hydrogen to form a decarburized layer, so that the bending property of the steel plate after hot forming is improved, and the bending angle is improved and can reach more than 60 degrees. In addition, the pre-oxidation layer formed on the surface in the covering and annealing process enhances the high-temperature oxidation resistance of the steel plate in the hot forming process, and the thickness of the surface oxidized iron sheet after hot forming of the hot rolled steel plate subjected to covering and annealing is only about 0.2-0.9 μm.

5. Greatly improves the welding performance in the acid washing threading process. Before hot-forming the hot-rolled steel plate, the iron scale may need to be pickled, and the key link of pickling is the welding process of strip penetration, when the head, the tail and the edge of the steel coil are too high in strength due to fast cooling rate and high hardenability, and huge internal stress is generated, the laser tailor-welding process is difficult to implement, so the cover-annealing process reduces the internal stress of the steel plate and improves the welding performance of the pickling strip penetration process.

The hot rolled steel sheet surface has scale, and therefore the hot rolled steel sheet of the invention needs to be subjected to scale removal after the uncoiling and straightening in step 3.1, and before heating and subsequent hot forming, and preferably, the scale thickness is removed to not more than 2 μm before hot forming. Because Cr and Si elements are added into chemical components, the iron oxide scale on the surface of the steel plate is more compact, the iron oxide scale is more tightly combined with a matrix, and the difficulty in removing the iron oxide scale of the hot-rolled steel plate after hot rolling is higher. Therefore, the invention also provides a scale removing process combining a chemical method and a physical method, which can completely meet the control requirement of subsequent hot forming on the thickness of the scale on the premise of no Al-Si coating. The iron scale removing process comprises the following steps:

(1) and (3) withdrawal and straightening treatment: because the steel contains higher Cr and Si contents, the iron scale is more compact and more closely combined with a matrix than other steel types, aiming at the steel type, the pulling and straightening process of the step 3.1 before pickling is adopted, and the pulling and straightening elongation is higher to achieve the scale breaking effect, so that conditions are provided for effectively removing the iron scale by the subsequent hydrochloric acid pickling, and the pulling and straightening elongation is set to be 0.5-2.0% according to different thicknesses of the hot rolled steel plate by combining equipment conditions.

(2) Acid washing with hydrochloric acid:

the hydrochloric acid pickling is divided into three sections,

a first stage: the concentration of the hydrochloric acid is 60 g/L-150 g/L, and the temperature of the acid liquor is 60 ℃ to 95 ℃;

and a second stage: the concentration of the hydrochloric acid is 80 g/L-180 g/L, and the temperature of the acid liquor is 60 ℃ to 95 ℃;

a third stage: the concentration of the hydrochloric acid is 100 g/L-200 g/L, and the temperature of the acid liquid is 60 ℃ to 95 ℃.

The concentration of iron ions in the three sections of acid tanks is required to be not more than 150 g/L;

the speed of the hydrochloric acid pickling process is 15-120 m/min, and rinsing and drying are carried out after the hydrochloric acid pickling process is carried out; after the primary pickling, the oxide scales on the surface of the hot-rolled steel plate are effectively removed, but the thickness of the oxide scales on the part of the hot-rolled steel plate is between 0.2 and 6 mu m, namely after the primary pickling, the thickness of the oxide scales on the surface of the hot-rolled steel plate does not completely meet the requirement of the subsequent hot forming on the thickness of the oxide scales not more than 2 mu m.

(3) EPS (Eco Pickled Surface ecological cleaning Surface) treatment: in the hydrochloric acid pickling process, the strip steel can be coiled again, so before EPS treatment, the strip steel is firstly pulled and corrected again to achieve the effects of further improving the plate shape and breaking scales, the pulling and correcting elongation rate is 0.2-2.0%, and the EPS treatment process speed is 10-60 m/min.

In order to further effectively remove the iron scale on the surface of the hot rolled steel plate, the iron scale removing process of the steel grade is additionally provided with an EPS cleaning procedure on the basis of the conventional hydrochloric acid pickling process, and the EPS working principle is that special media (steel grit, water and a water-based antirust agent) are sprayed on the surface of the steel plate to be treated in an EPS unit, so that the aim of effectively removing the iron scale on the surface of the steel plate is fulfilled. The water-based antirust agent has a lubricating effect on steel grit passing through a pipeline, can remove floating foam generated in the treatment process, and can be attached to the surface of a treated steel plate to form an antirust protective film so as to prevent the steel plate from being rusted.

The EPS cleaning process speed is determined according to the specific situation of iron scale residues on the surface of the hot-rolled steel plate after being washed by hydrochloric acid, the surface of the hot-rolled steel plate after being processed by EPS has uniform color, and the thickness of the iron scale meets the requirement of subsequent hot forming on the thickness of the iron scale not more than 2 mu m.

Compared with the prior art, the invention has the advantages that:

1. the cost is low. On one hand, expensive alloy elements such as Ni, Mo and the like are not added in the steel composition design, so that the cost is reduced from the source, on the other hand, the process flow is simplified, cold rolling and plating procedures are not needed, the energy of intermediate links is saved, and the cost is reduced in the process.

2. The preparation process is simple and easy to realize industrialization. The production process flow comprises simple hot rolling, coiling, cover annealing, acid washing and hot forming, compared with the original production process, the complex cold rolling annealing treatment and coating working procedures are omitted, the process flow is simple to control and operate, the industrial production is easy to realize, compared with the existing Al-Si coating hot forming steel, the heat transfer is fast, the production efficiency is high, the problems of part surface damage and roller way cleaning after the Al-Si coating is stained with a roller are not needed to be considered, the effects of high attribute and surface quality improvement can be obtained by regulating and controlling the atmosphere of a heating furnace and the hot stamping pressure, the industrial production is easy to realize, the pollution of the roller way of hot forming equipment is also prevented, and the hydrogen embrittlement sensitivity is reduced.

3. The hot-formed steel has a uniform composition structure. The hot rolling method is adopted for rolling, a banded structure which is usually generated in cold rolling is avoided, the composite microalloyed precipitated phase can refine original austenite grains and play a role in precipitation strengthening, and Cr remarkably improves the hardenability of a hot-formed steel plate to obtain a uniform and fine martensite structure.

4. The hot forming steel has excellent comprehensive performance. The hot forming steel has excellent plasticity, the prior austenite grain size is small, uniform lath martensite and a certain content of retained austenite are obtained after hot forming die quenching, and the plasticity is obviously improved due to the tissue uniformity and the TRIP effect of the retained austenite; the yield strength after thermoforming is 1080-1410 MPa, the tensile strength is 1565-1790 MPa, the elongation is 8.3-13.1%, and the bending angle can reach more than 60 degrees.

5. The hot-rolled steel sheet for hot forming has excellent high-temperature oxidation resistance. The thickness of the iron scale on the surface of the bare plate after hot forming is 0.2-0.9 μm.

Drawings

FIG. 1 shows the SEM structure of a hot-rolled steel sheet after hot rolling and coiling in example 1.

FIG. 2 shows an SEM structure of a hot rolled steel sheet in example 2 after hot forming.

FIG. 3 is a SEM cross-sectional scale morphology of a hot rolled steel sheet of example 2 after hot forming.

FIG. 4 shows the cross-sectional scale appearance of the hot-rolled steel sheet in example 3 after pickling with hydrochloric acid and EPS cleaning.

Detailed Description

The rolling and cooling control equipment adopted in the embodiment of the invention is a hot rolled strip steel production line of the steel;

the hot forming equipment is a hot stamping forming press;

in the embodiment of the invention, an Ultra 55 field emission scanning electron microscope of Zeiss company is adopted for observing the shapes of the microstructures and the section scale.

Example 1

The hot rolled steel plate with the thickness of 1.5mm and the thickness of 1700MPa grade high Cr-Si thin specification for hot forming is prepared by the following process steps:

(1) controlled rolling and controlled cooling treatment

The alloy blank comprises the following chemical components in percentage by weight: c: 0.30%, Mn: 0.8%, Si: 1.8%, S: 0.005%, P: 0.008%, Al: 0.01%, Cr: 3.0%, Nb: 0.03%, V: 0.01%, Ti: 0.03%, Cu: 0.05%, the balance being Fe and other unavoidable impurities; heating the continuous casting billets containing the components to 1300 ℃ along with a furnace, and preserving heat for 2.5 hours; removing furnace-generated oxide scales through primary descaling, wherein the initial rolling temperature of rough rolling is 1170 ℃, the total rolling reduction rate of the rough rolling is 87%, and the thickness of the intermediate blank is 30.55mm through 6 times of rough rolling; removing secondary iron scale before finish rolling, carrying out finish rolling on the steel strip with the thickness of 1.5mm by 7 times, wherein the reduction rate of each time of finish rolling is 52.58%, 45.80%, 38.87%, 33.62%, 28.39%, 19.98% and 14.81%, the initial rolling temperature and the final rolling temperature of the finish rolling are 1085 ℃ and 880 ℃ respectively, and cooling the steel strip to the coiling temperature at the cooling rate of 30 ℃/s after the finish rolling.

(2) Coiling treatment

The initial coiling temperature of the steel strip is 680 ℃, then the steel strip is slowly cooled to room temperature by air, and the microstructure of the hot rolled steel plate prepared after coiling is ferrite, pearlite, bainite and a small amount of carbide. As shown in fig. 1.

Example 2

After uncoiling and straightening, the hot rolled steel sheet obtained in example 1 was mechanically removed of scale, heated to 930 ℃, held for 350s, then quickly transferred to a hot stamping die within 10s for hot stamping forming under a pressure of 20MPa, and then quenched to 200 ℃ or less at a cooling rate of 150 ℃/s to obtain a hot formed steel with a structure of martensite and a small amount of retained austenite, as shown in fig. 2.

Mechanical property experiments are carried out on the hot forming steel, and finally the yield strength 1390MPa, the tensile strength 1750MPa, the elongation 8.5 percent and the bending angle 65 degrees of the hot forming steel are obtained. The thickness of the scale on the surface of the bare hot-rolled steel sheet after hot forming was about 0.9 μm, and shot blasting was not required as shown in FIG. 3.

Example 3

The hot-rolled steel sheet obtained in example 1 was uncoiled and pulled out, and the set range of the pulling-out elongation was 1.0%;

hydrochloric acid concentration in the first-stage acid tank: 120 g/L +/-20 g/L, acid liquor temperature of 80 +/-10 ℃,

hydrochloric acid concentration in the second-stage acid tank: 150 g/L +/-20 g/L, acid liquor temperature of 80 +/-10 ℃,

hydrochloric acid concentration in the third stage acid tank: 190 g/L +/-20 g/L, and the temperature of the acid liquor is 80 +/-10 ℃;

the concentration of iron ions is required to be not more than 100 g/L;

the process speed is 60 m/min.

After pickling with hydrochloric acid, the iron scales on most parts of the surfaces of the edges and the middle parts of the hot-rolled steel plate are removed, the thickness of the iron scales on the respective edge parts is 0.7-1 μm, and the thickness of the iron scales on the respective middle parts is 0.7-4 μm.

(2) EPS cleaning

2 EPS units are put into;

withdrawal and straightening elongation: 1.0 percent

The process speed is as follows: 50 m/min;

after EPS cleaning, as shown in figure 4, the thickness of the iron scale on the surface of the hot rolled steel plate is 0-1 μm, so that the requirement of subsequent hot forming on the thickness of the iron scale not more than 2 μm is met, and the surface color is uniform.

The hot rolled steel sheet, from which the scale was removed after the pickling plus EPS cleaning, was heated, kept warm, hot press formed and annealed in the same manner as in example 2.

Compared with the embodiment 2, the mode of removing the scale has simpler and more time-saving operation flow. Because more Cr and Si elements are added into the steel, the oxide skin on the surface of the hot-rolled strip steel is more compact, the oxide skin is more tightly combined with a matrix, the difficulty in removing the oxide skin after hot rolling is higher, and the requirement of subsequent hot forming on the thickness of the oxide skin not more than 2 mu m can not be met only by adopting a single procedure of hydrochloric acid pickling or EPS cleaning. After the hydrochloric acid pickling and EPS cleaning treatment, the requirement of subsequent hot forming on the oxide skin thickness not more than 2 mu m can be met. And through EPS treatment, the EPS plate coiled material with different surface roughness (different Ra values and lower Rz values) can be produced according to customer requirements on the premise of ensuring effective removal of the iron scale, and the corrosion resistance of the steel plate after EPS treatment is improved.

Example 4

Basically the same as example 2 except that the hot rolled steel sheet (or steel coil) was subjected to a hood type annealing treatment before uncoiling and hot forming.

The bell-type annealing process adopts hydrogen atmosphere, the temperature is heated from room temperature to 245 ℃, and the heating speed is 123 ℃/h; heating from 245 ℃ to 680 ℃ at a heating speed of 42.9 ℃/h; keeping the temperature at 680 ℃ for 9 hours; furnace cooling from 680 deg.C to 300 deg.C; then cooling to the tapping temperature of 100 ℃, wherein the cooling speed is 26.7 ℃/h; and discharging the product when the temperature is cooled to 100 ℃.

Performance comparison before and after cover withdrawal:

the yield strength of the edge of the steel coil before the cover retreating is 852MPa, the tensile strength is 1149MPa, and the elongation is 14.9 percent; the yield strength of the middle part is 523MPa, the tensile strength is 645MPa, and the elongation is 23.7 percent;

the yield strength of the edge of the steel coil after the cover is retreated is 677MPa, the tensile strength is 729MPa, and the elongation is 19.5%; the yield strength of the middle part is 506MPa, the tensile strength is 603MPa, and the elongation is 24.5 percent.

Therefore, after the 680 ℃ cover annealing treatment, the strength of the edge part of the hot-rolled steel plate is greatly reduced, the performance gap with the middle part is reduced, and the performance uniformity of the hot-rolled steel plate in the width passing direction is improved.

Example 5

The process for preparing the hot-rolled thin-specification plating-free hot-formed steel with the thickness of 2.5mm and the high Cr-Si alloying level of 1700MPa comprises the following steps:

(1) controlled rolling and controlled cooling treatment

The alloy blank comprises the following chemical components in percentage by weight: c: 0.15%, Mn: 3.0%, Si: 0.8%, S: 0.006%, P: 0.009%, Al: 0.05%, Cr: 1.5%, Nb: 0.01%, V: 0.03%, Ti: 0.02%, Cu: 0.10 percent of Fe and other inevitable impurities, heating the continuous casting billet with the components to 1285 ℃ along with a furnace, and preserving heat for 2 hours; removing furnace-generated oxide scales through primary descaling, wherein the initial rolling temperature of rough rolling is 1108 ℃, the total rolling reduction rate of the rough rolling is 86%, and the thickness of the intermediate blank is 32.5mm through 6 times of rough rolling; removing secondary iron scale before finish rolling, and then carrying out finish rolling on the steel strip with the thickness of 2.5mm by 7 times, wherein the reduction rate of each time of finish rolling is respectively 42.5%, 44%, 36.2%, 30.3%, 26.4%, 21.4% and 11.6%, the initial rolling temperature and the final rolling temperature of the finish rolling are respectively 1068 ℃ and 925 ℃, and after the finish rolling is finished, water cooling is carried out at the cooling rate of 30 ℃/s to the coiling temperature.

(2) Coiling treatment

The initial coiling temperature of the steel strip is 725 ℃, then the steel strip is slowly cooled to room temperature by air, and the microstructure of the hot rolled steel plate prepared after coiling is ferrite, pearlite, bainite and a small amount of carbide.

Example 6

The hot-rolled steel sheet obtained in example 5 was uncoiled and pulled and straightened, and the set range of the pull-straightening elongation was 1.5%;

hydrochloric acid concentration in the first-stage acid tank: 120 g/L +/-10 g/L, the temperature of the acid liquor is 80 +/-10 ℃,

hydrochloric acid concentration in the second-stage acid tank: 150 g/L +/-10 g/L, the temperature of the acid liquor is 80 +/-10 ℃,

hydrochloric acid concentration in the third stage acid tank: 190 g/L +/-10 g/L, and the temperature of the acid liquor is 80 +/-10 ℃;

the concentration of iron ions is required to be not more than 100 g/L;

the process speed is 30 m/min.

After hydrochloric acid pickling and SEM analysis, the iron scales on the surface parts of the edges and the middle parts of the hot-rolled steel plate are completely removed, the thickness of the iron scales on the individual edge parts is 0.7-1 mu m, and the thickness of the iron scales on the individual middle parts is 0.7-4 mu m.

(2) EPS cleaning

2 EPS units are put into; withdrawal and straightening elongation: 0.5 percent; the process speed is as follows: 25 m/min;

after EPS cleaning, the oxide scales on the surface of the hot-rolled steel plate are basically removed, the requirement of subsequent hot forming on the thickness of the oxide scales not more than 2 mu m is completely met, and the surface color is uniform.

Heating the hot rolled steel plate with the oxide scale removed by acid pickling and EPS cleaning to 950 ℃, preserving heat for 6min, then quickly transferring to a hot stamping die within 15s, keeping the pressure at 5MPa, and quenching to below 200 ℃ at a cooling speed of 40-120 ℃/s to obtain martensite and a small amount of residual austenite.

Mechanical property experiments are carried out on the hot forming steel, and the yield strength 1250MPa, the tensile strength 1630MPa, the elongation 9.8 percent and the bending angle 55 degrees of the hot forming steel are finally obtained. The thickness of the iron scale on the surface of the bare plate after hot forming is 0.6 μm.

Example 7

The process for preparing the hot-rolled thin-specification plating-free hot-formed steel with the thickness of 3.5mm and the high Cr-Si alloying level of 1700MPa comprises the following steps:

(1) controlled rolling and controlled cooling treatment

The alloy blank comprises the following chemical components in percentage by weight: c: 0.21%, Mn: 2.2%, Si: 2.6%, S: 0.008%, P: 0.007%, Al: 0.05%, Cr: 1.5%, Nb: 0.01%, V: 0.04, Ti: 0.02, Cu: 0.09, and the balance of Fe and other inevitable impurities, heating the continuous casting billet with the components to 1285 ℃ along with a furnace, and preserving heat for 2 hours; removing furnace-generated oxide scales through primary descaling, wherein the initial rolling temperature of rough rolling is 1108 ℃, the total rolling reduction rate of the rough rolling is 86%, and the thickness of the intermediate blank is 32.5mm through 6 times of rough rolling; removing secondary iron scale before finish rolling, and then carrying out finish rolling on the steel strip with the thickness of 3.5mm by 7 times, wherein the reduction rate of each time of finish rolling is respectively 42.5%, 44%, 36.2%, 30.3%, 26.4%, 21.4% and 11.6%, the initial rolling temperature and the final rolling temperature of the finish rolling are respectively 1068 ℃ and 925 ℃, and after the finish rolling is finished, water cooling is carried out at the cooling rate of 30.3 ℃/s to the coiling temperature.

(2) Coiling treatment

The initial coiling temperature is 725 ℃, then the steel plate is slowly cooled to room temperature by air, and the microstructure of the hot rolled steel plate prepared after coiling is ferrite, pearlite, bainite and a small amount of carbide.

Example 8

The hot-rolled steel sheet or steel coil obtained in example 7 was subjected to a hood-type annealing treatment:

the cover annealing process adopts hydrogen atmosphere, the temperature is heated from room temperature to 350 ℃, and the heating speed is 165 ℃/h; heating from 350 ℃ to 650 ℃ of heat preservation temperature at a heating speed of 42.9 ℃/h; keeping the temperature at 650 ℃ for 8 h; furnace cooling from 650 ℃ to 430 ℃; then cooling to the tapping temperature of 100 ℃, wherein the cooling speed is 36.7 ℃/h; and discharging the product when the temperature is cooled to 100 ℃.

Performance comparison before and after cover withdrawal:

the yield strength of the edge of the steel coil before the cover retreats is 952MPa, the tensile strength is 1279MPa, and the elongation is 14.1%; the yield strength of the middle part is 563MPa, the tensile strength is 832MPa, and the elongation is 18.7 percent;

the yield strength of the edge of the steel coil after the cover is retreated is 677MPa, the tensile strength is 729MPa, and the elongation is 20.4%; the yield strength of the middle part is 553MPa, the tensile strength is 663MPa, and the elongation is 24.2 percent.

Therefore, after the 650 ℃ cover annealing treatment, the strength of the edge part of the hot rolled steel plate is greatly reduced and is close to the performance of the middle part, and the performance of the hot rolled steel plate in the width direction becomes more uniform.

And heating the hot rolled plate after the cover is removed to 940 ℃, preserving heat for 6min, transferring the hot rolled plate into a hot stamping die within 12s, maintaining the pressure at 25MPa, and quenching the hot rolled plate to below 200 ℃ at the cooling speed of 40-120 ℃/s to obtain martensite and a small amount of residual austenite. Mechanical property experiments are carried out on the hot forming steel, and the yield strength of the hot forming steel is 1210MPa, the tensile strength of the hot forming steel is 1590MPa, and the elongation of the hot forming steel is 11.9%. The thickness of the iron scale on the surface of the bare plate after hot forming is 0.82 μm.

Claims

1. a hot rolling preparation method of 1700MPa grade high Cr-Si thin gauge hot forming steel, is characterized in that, comprises the following steps:

Step 1: Heat the steel continuous casting billet to 1150~1300℃ in the heating furnace and keep it for 1~2.5h, after one descaling, remove the furnace pig oxide scale, and carry out rough rolling and finishing rolling: the rough rolling temperature is 1100 ~1300℃, the total reduction rate of rough rolling is 80~90%, and the thickness of rough rolling intermediate billet is 30mm~45mm; before finishing rolling, the secondary iron oxide scale is removed, and finishing rolling is made into a steel strip with a thickness of 1.5~4.5mm. The rolling start temperature and final rolling temperature are 1000~1090℃ and 880~950℃ respectively;

The chemical composition of the steel continuous casting billet is by mass percentage: C: 0.15~0.35%, Mn: 0.8~3.2%, Si: 0.8~2.8%, S: <0.01%, P: <0.015%, Al: 0.01~ 0.05%, Cr: 1.5~3.9%, Nb: 0.01~0.05%, V: 0.01~0.05%, Ti: 0.01~0.03%, Cu: 0.05~0.15%, the balance is Fe and other inevitable impurities;

Step 2: After finishing rolling, water-cooled at a cooling rate of 5~30°C/s to an initial coiling temperature of 550~750°C, coiled the steel strip at the initial coiling temperature, and then air-cooled to room temperature to obtain 1700MPa high Cr - Hot-rolled steel sheets for Si thin gauge hot forming;

Step 3: hot-stamping the hot-rolled steel sheet to obtain 1700MPa-grade high Cr-Si thin-gauge hot-formed steel;

The hot stamping forming includes the following steps:

Step 3.1: After the hot-rolled steel sheet is uncoiled and straightened, it is put into a heating furnace at 880-980 ° C, and the atmosphere in the furnace is air, nitrogen or nitrogen plus methane, and then kept in the furnace for 2-15 minutes;

Step 3.2: Transfer process: transfer the steel plate to a hot stamping forming press in air, the transfer process is 5~18s, and the temperature after transfer is 720~860℃;

Step 3.3: Hot stamping processing: The steel plate is hot stamped in a mold with a cooling system inside, and is in a pressure-holding state with a pressure of 3~25MPa, and then quenched to martensite at a cooling rate of 15~200℃/s. below temperature.

2 . The hot rolling preparation method of 1700 MPa grade high Cr-Si thin-gauge hot forming steel according to claim 1 , wherein the rough rolling is 6 passes; the finishing rolling is 7 passes. 3 .

3. The hot-rolled preparation method of 1700MPa grade high Cr-Si thin-gauge hot-formed steel according to claim 1, characterized in that, after the step 3.3, the following step 3.4 is further included:

Baking process: Put the steel plate into a heat treatment furnace at 170°C for 20 minutes.

4. The hot-rolled preparation method of 1700MPa grade high Cr-Si thin-gauge hot-formed steel according to claim 1, characterized in that, before the step 3, it further comprises a hood annealing step of the hot-rolled steel sheet:

The hood annealing process adopts a hydrogen atmosphere, so that the temperature is heated from room temperature to 245 ° C ~ 400 ° C, the average heating rate is 120 ~ 200 ° C / hour; and then heated to the target temperature of 650 ° C ~ 770 ° C, the average heating rate is 42~48℃/hour; keep warm for 8~12 hours at the target holding temperature; cool down from the target temperature holding temperature to 300℃~500℃; then cool down to the release temperature of 100℃, with an average cooling rate of 25~42℃/hour; Baked at 100°C.

5. The hot-rolled preparation method of 1700MPa grade high Cr-Si thin-gauge hot-formed steel according to claim 1, characterized in that, after performing the uncoiling and tension leveling of the step 3.1, it also comprises: The step of descaling treatment is carried out.

6. The hot-rolled preparation method of 1700MPa grade high Cr-Si thin-gauge hot-formed steel according to claim 5, characterized in that, in step 3.1, the tensile straightening elongation is set to 0.5-2.0%, and the deoxidation The method of iron sheeting includes the following steps:

Hydrochloric acid pickling:

The hydrochloric acid pickling is divided into three stages,

The first stage: the concentration of hydrochloric acid is 60 ~ 150 g/L, and the temperature of the acid solution is 60 ℃ ~ 95 ℃;

The second stage: the concentration of hydrochloric acid is 80~180 g/L, and the temperature of the acid solution is 60℃~95℃;

The third stage: the concentration of hydrochloric acid is 100~200 g/L, and the temperature of the acid solution is 60℃~95℃;

The iron ion concentration of the above-mentioned three-stage acid tank is not more than 150 g/L;

The process speed of hydrochloric acid pickling is 15 ~ 120m/min, after hydrochloric acid pickling, rinsing and drying are carried out;

EPS treatment: Tensile straightening is carried out before EPS treatment, the elongation of pull straightening is 0.2~2.0%, and the process speed of EPS treatment is 10~60m/min.

7. A 1700MPa grade high Cr-Si thin gauge hot forming steel, characterized in that, prepared according to the hot rolling preparation method of the 1700MPa grade high Cr-Si thin gauge hot forming steel described in any one of claims 1 to 6 .