A free-cutting high-strength specific-ductility cold-rolled dual-phase steel and a method for manufacturing the same

Abstract

The application discloses an easy-to-cut high-strength plastic-product cold-rolled dual-phase steel and a preparation method thereof, and belongs to the technical field of dual-phase steel plates. The steel is prepared from the following components in percentage by weight: C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: 0.012% or less, Cu: 0.20% or less, Alt: 0.60%-0.80%, S: 0.020%-0.035%, Nb: 0.020%-0.035, Ti: 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm 0.04%-0.06%, 15ppm

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of dual-phase steel plate, more particularly, to an easy-cutting high-strength plastic product cold-rolled dual-phase steel and a preparation method thereof. BACKGROUND

[0002] Commercial vehicles are important transportation tools in the development of the national economy. In recent years, with the rapid development of logistics, higher demands have been put forward for the loading capacity, safety and economy of commercial vehicles. High-strength dual-phase steel is the first generation of high-strength steel developed earliest, which has excellent formability and welding performance, and is currently the most widely used. However, its strength and plasticity is relatively low, which cannot meet the stamping forming demand of complex parts and commercial steel plates. At the same time, with the increase of carrying weight, it cannot meet the safety demand of commercial vehicle steel plates. The strength and plasticity product of TWIP steel is relatively high, which can reach 50 GPa%, but the process, application and cost cannot be applied to commercial vehicles in large quantities. Due to the stamping processing of dual-phase steel, higher demand for cutting performance is put forward. Better cutting performance can not only improve production efficiency and reduce cost, but also has better forming capacity without affecting the strength and plasticity product.

[0003] Due to the high demand for the strength of dual-phase steel, the demand for martensite content in dual-phase steel increases. At the same time, the increase of strength brings difficulties in stamping and processing. In the production process, the water quenching process commonly used in modern continuous annealing line can speed up the production rhythm, but too high cooling speed can easily cause deformation. Increasing the alloy content can improve the strength, but the cost increases at the same time, which brings difficulties to rolling, reduces the weldability, formability and shape freezing of the product. In summary, the technical demand for easy-cutting high-strength plastic product cold-rolled dual-phase steel and preparation method is increasing. Through searching, there are few technical solutions for easy cutting, rolling, high strength and plastic product, and high-performance preparation process and annealing process of existing grades of cold-rolled dual-phase steel.

[0004] Patent CN114540717A A kind of calcium-tellurium synergistic gear steel and its preparation method and application.Calcium-tellurium synergistic gear steel, with mass percentage calculation, include: C 0.40%-0.43%, Si 0.23%-0.28%, Mn 0.75%-0.85%, S 0.01%-0.035%, P 0.015%-0.020%, Cr 1.05%-1.15%, Mo 0.19%-0.23%, Ca 0.001%-0.007%, Te 0.005%-0.07%, the rest is Fe and inevitable impurity element.Calcium-tellurium synergistic gear steel preparation method: raw material is smelted by converter, LF refining, RH refining and continuous casting, to obtain the calcium-tellurium synergistic gear steel.The calcium-tellurium synergistic gear steel provided in the present application limits the deformation of manganese sulfide, increases the cutting lubricity.But the patent is only suitable for the gear steel with small deformation, and is not suitable for the high-strength plate material with large deformation.

[0005] Patent CN107287504A discloses a sulfur and tellurium containing medium carbon free cutting non-quenched and tempered steel and a production process method thereof.The main components of the non-quenched and tempered steel are as follows: C: 0.35-0.50%, Si: 0.10-0.80%, Mn: 0.75-1.60%, P≤0.03%, S: 0.02-0.08%, Mo: 0-0.08%, Nb: 0-0.025%, V: 0.02-0.12%, N: 0.012-0.016%, Te: 0.02-0.08%, Ca: 0.001-0.003%, and the balance is iron and inevitable impurities, wherein the ratio of tellurium / sulfur is 0.25-2.5.The free cutting non-quenched and tempered steel of the present application is treated by adding tellurium to modify the inclusions in the steel, so that the cutting performance, fatigue resistance and other properties of the non-quenched and tempered steel are significantly improved, and the environment is not polluted.But the patent is only suitable for non-quenched and tempered steel without heat treatment, not suitable for high-strength steel requiring annealing, and cannot solve the problem of high-strength steel. SUMMARY

[0006] 1. Problem to be solved

[0007] In view of the problem of poor cutting performance caused by insufficient high-strength steel strength-plasticity product, the present application provides a high-strength steel with high strength-plasticity product and easy cutting performance.

[0008] The present application also discloses a preparation method for preparing the above-mentioned steel, which is annealed by adopting flash cooling and slow cooling annealing process.

[0009] 2. Technical scheme

[0010] In order to solve the above-mentioned problem, the technical scheme adopted by the present application is as follows:

[0011] The application discloses a chemical composition ratio (in percentage by weight) of an easy-to-cut high-strength plastic-product cold-rolled dual-phase steel, which is C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: <=0.012%, Cu <=0.20%, Alt: 0.60%-0.80%, S: 0.020%-0.035%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 15ppm <=Te <=30ppm, 10ppm <=Ca <=15ppm, T.O: <=15ppm, [N]: <=60ppm, and the rest is Fe and inevitable impurity elements, and meanwhile, through pilot steel smelting and calculation of thermodynamic solid solubility product, the following composition ratio needs to be met after fitting: 0.20% <=D m = Al-4.12* [N]-2.25* (Ti+Nb)-1.21* C <=0.35%, 2.5% <=Fn=(2.8*Te+1.5*Ca) / (0.1*S) <=3.5%.

[0012] C: the most basic, effective and economical strengthening element in steel, the carbon content directly affects the martensite transformation point, and the enrichment of carbon in austenite during the heat treatment process will cause the Ms point temperature to decrease. Carbon can stabilize austenite, and if the carbon content is too low, the steel plate strength is insufficient, and there is not enough C atom to enrich in the residual austenite during the partition process, so that the stability of the obtained residual austenite is insufficient. However, too high carbon content will be unfavorable to the forming performance and welding performance, therefore, generally, the higher the carbon content of steel is not better. In the application, the C content is controlled in the lower range of 0.18%-0.22%.

[0013] Si: Si is a deoxidizer, can be solid solution strengthening to improve the hardness of steel, and is a ferrite forming element, can hinder the precipitation of cementite and play a role in stabilizing austenite; on the other hand, too high silicon content will cause selective oxidation of the surface of the steel plate during annealing, and produce iron oxide skin during heating. When the iron oxide skin is pressed into the surface of the plate during hot rolling, the surface quality of the plate will be reduced, causing welding difficulty, hot galvanizing difficulty and poor surface coating of the material. Therefore, the Si content is controlled in the range of 0.10%-0.15% in the application.

[0014] Mn: Mn can expand the austenite phase region, and stabilize the austenite structure, improve the hardenability of the steel, but Mn can be dissolved in ferrite, improve the hardness and strength of ferrite and austenite in steel, and Mn can improve the stability of austenite structure, another effect of adding Mn element in low carbon steel is to make the proeutectoid ferrite precipitation line right shift, so that the amount of ferrite precipitation is less during annealing cooling, to ensure the residual austenite content in the final microstructure. But too much Mn can easily cause segregation, banded structure is poor, and also reduce the plasticity of the steel, the toughness of the steel is bad when hot rolling. The content of Mn is controlled in 2.00%-2.30%.

[0015] Cr: is to improve the strength of the steel, and the carbide is precipitated during continuous annealing to improve the stability of the grain boundary, but too high Cr content not only increases the cost, but also the carbide will be crystallized, which affects the strength and plasticity of the steel. Therefore, the content of the present application is controlled in 0.40-0.60%.

[0016] Nb: precipitation strengthening element, which plays a role in refining the grain, is beneficial to obtain uniform and fine finished product structure, and is good for improving the strength and elongation of the finished product. At the same time, by adding trace Nb element to refine the annealing austenite grain, the Ms temperature is reduced, so that the steel of the present application can complete the distribution process at a lower temperature, which is more conducive to the realization on the conventional continuous annealing line. The content of Nb in the present application is controlled in 0.025%-0.035% by weight.

[0017] Ti and [N]: the addition of trace Ti in the steel can improve the strength of the steel, play the role of precipitation strengthening and grain refinement, improve the toughness of the welding heat affected zone, and improve the welding performance of the material. At the same time, Ti delays the bainite phase transition, promotes the precipitation of ferrite and pearlite, and improves the toughness. But too high Ti can produce liquid TiN, and the sharp angle can cause fatigue failure, so the content of Ti in the present application is controlled in 0.04%-0.06% by weight, and [N]≤60ppm.

[0018] Al: Al is an effective deoxidizer, and can form AlN to refine the grain, aluminum is a strong stable element of ferrite, one of the ways of carbide formation is to obtain by eutectoid reaction decomposition of austenite, and the increase of Al and C content can promote the precipitation of κ carbide. On the other hand, the addition of Al element can increase the Ms temperature; the refinement of austenite grain size can significantly reduce the Ms temperature, and improve the stability of austenite. But too high Al can cause secondary oxidation to form inclusions, which can cause the decline of service performance, and too high Al can cause nozzle blockage, so the addition of Al in the steelmaking process needs to be adjusted, and certain Ca treatment is needed, and the content of Al should be controlled in 0.60%-0.80%.

[0019] Ca: Ca element can be combined with sulfur element, nucleate around hard inclusions such as Al2O3, form CaS and other inclusions, improve cutting performance, and Ca can avoid large inclusions in the nozzle Wall, increase the castability. But too much Ca will deteriorate D class and DS class inclusions, so the content of Ca element should be controlled in 10-15ppm.

[0020] Te: Te element can modify Ca sulfide, through the spheroidization of S and Ca composite inclusions, optimize the morphology of the second class of sulfide, and Te hardens the sulfide inclusions, so that the sulfide is not easy to deform, and the length-diameter ratio is reduced. But too much Te element yield is reduced, and the cost yield and Te modification effect are considered, and the content of Te should be controlled in 15-30ppm.

[0021] P and S: Sulfur is easy to form MnS inclusions with manganese in steel, which makes the steel produce thermal embrittlement, but adding a small amount of S can significantly improve the cutting performance of gear steel without affecting the product performance, and MnS has the effect of refining grains; P is an element with strong segregation tendency, which increases the cold brittleness of steel, reduces the plasticity, and is harmful to the uniformity of product organization and performance. Control P: ≤0.015%, S: 0.020%-0.035%.

[0022] T.O and [H]: T.O forms oxide inclusions in steel, and the content of T.O is controlled to be less than or equal to 15ppm.

[0023] The present application utilizes D m The chemical composition is constrained, so that the strength and plasticity product is improved, TiN is avoided, Al element is an economical strength improving element, and Al can be added as a deoxidizer. AlN also has the effect of grain boundary pinning, which can also be added to prevent grain coarsening. Previous studies have shown that free Al in austenite can delay the transformation of austenite to ferrite, and the reason is related to the distribution of Al near the ferrite-austenite transformation interface. At the same time, Nb and Ti have the effect of refining grains, but the formation of bainite and martensite is inhibited at the same time, which reduces the strength, so the chemical composition should be constrained by Dm.

[0024] Compared with the prior art, the component aspect adds sulfur element to improve the deformation capacity and easy cutting capacity of the plate, and in order to reduce the aspect ratio of sulfide and avoid the deterioration of the second type of sulfide, Ca and Te composite modification treatment is added during continuous casting, so that the continuous casting can be smoothly cast, and the Ca and Te composite modification treatment is added. The aspect ratio of the sulfide in the prepared steel is ≤13, and the residual austenite content is ≤4%. Through the Ca and Te composite modification treatment, the micro-alloying is used as the formation core of the sulfide, the generation of the second type of sulfide is greatly reduced, the sulfide is in an ellipsoidal shape, the aspect ratio of the sulfide after large deformation is ≤13, the residual austenite content is reduced by 4.5% or less after micro-alloying and high Al content, and the soft phase residual austenite is reduced, so that the high strength steel is greatly improved in strength and plasticity, and the easy cutting capacity is improved.

[0025] Meanwhile, for the component design of the high strength and plasticity composite steel, high Al composite steel is introduced to improve the content of martensite and bainite, and appropriate micro-alloying is used to ensure high strength and improve plasticity, and the chemical component ratio meets the constraint of the relationship formula of Dm, Ca, Te and S on the component.

[0026] The production method of the above-mentioned dual-phase steel includes the steps of smelting, continuous casting, hot rolling, pickling and cold rolling, continuous annealing and finished product. Specifically, the production method comprises the following steps:

[0027] 1) Smelting and continuous casting: suitable for converter, electric furnace and induction furnace smelting, adopting continuous casting production of casting blank, using electromagnetic stirring and dynamic soft reduction device during pouring process to reduce composition segregation in solidification process;

[0028] 2) Hot continuous rolling of casting blank or ingot: the heating temperature of the casting blank is 1200-1260℃, the holding time is 2-3 hours, 5-7 passes of rough rolling mill are carried out, and the hot rolling is carried out to 30-50mm intermediate blank, 5-7 passes of hot continuous rolling mill are carried out, the final rolling temperature is 870-910℃, and after rolling to the target thickness, the coiling is carried out at 540-600℃ to form a steel coil;

[0029] 3) Pickling and cold rolling: after the hot rolled strip is pickled in a hydrochloric acid tank to remove the surface iron oxide scale, cold continuous rolling or cold rolling is carried out, the cold rolling reduction is 50-70%, and the rolling is carried out to the target thickness;

[0030] 4) continuous annealing: the pickled steel plate is annealed by using a continuous annealing production line, wherein the annealing process adopts slow heating + flash cooling process, the soaking temperature is 790-810 DEG C for 2h, the furnace is cooled to 680-700 DEG C for flash cooling, the flash cooling speed is > 30 DEG C / s, and the flash cooling is to 350-400 DEG C for 4h and then air cooling. The annealing process temperature is used to make Nb and Ti precipitate, and the solid solution Al is greatly increased, the microstructure is refined, and the content of residual austenite is reduced. The precipitates of Nb and Ti can be used as the core of sulfide formation, and under the combined action of Ca and Te, the sulfide is precipitated, the aspect ratio of the sulfide is reduced, and the strength and plasticity product of the high-strength steel is improved.

[0031] 3. Beneficial effects

[0032] Compared with the prior art, the beneficial effects of the application are:

[0033] The application adopts a specific composition and a reasonable preparation method to produce a high-strength and high-plasticity steel, and utilizes a specific annealing process to regulate the microstructure, so as to meet the market demand. The product has high strength and plasticity, good machinability, a tensile strength R m ≥1100MPa, a yield strength R p0.2 ≥700MPa, A80≥20%, a strength and plasticity product ≥22GPa, an aspect ratio of sulfide ≤13, a minimum bending core radius of longitudinal 180 DEG cold bending ≤1.0t, t is the thickness of the steel plate, and a hole expansion rate ≥40%. BRIEF DESCRIPTION OF DRAWINGS

[0034] The technical solutions of the application will be further described in detail below in combination with the drawings and examples, but it should be known that these drawings are only designed for the purpose of explanation, and therefore, they are not limited to the scope of the application. In addition, unless specifically indicated, these drawings are only intended to conceptually illustrate the structural configuration described herein, and are not necessarily drawn to scale.

[0035] Figure 1 The metallographic morphology of example 3 is shown in the figure.

[0036] Figure 2 The electron microscope graph of example 3 is shown in the figure. DETAILED DESCRIPTION

[0037] The following detailed description of example embodiments of the application references the drawings, which form a part thereof, and in which are shown by way of illustration example embodiments in which the application can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the application, and it is to be understood that other embodiments can be utilized and that changes can be made without departing from the spirit and scope of the present application. The following detailed description is, therefore, not to be taken in a limiting sense, as the scope of the application is defined by the appended claims.

[0038] The chemical composition and production process parameters of the present application are shown in Table 1 and Table 2.

[0039] The chemical composition of the easy-to-cut high-strength plastic product steel of the present application is as follows (in percentage by weight): C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: ≤0.012%, Cu ≤0.20%, Alt: 0.60%-0.80%, S: 0.020%-0.035%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 15 ppm ≤Te ≤30 ppm, 10 ppm ≤Ca ≤15 ppm, T.O: ≤15 ppm, [N]: ≤60 ppm, the rest being Fe and unavoidable impurity elements, while satisfying the chemical composition ratio 0.20% ≤D m = Al-4.12x[N]-2.25x(Ti+Nb)-1.21xC ≤0.35%, 2.5% ≤Fn=(2.8xTe+1.5xCa) / (0.1xS) ≤3.5%.

[0040] Table 1 Chemical composition of the present application and comparative examples (units: Te, Ca, [N], [T.O] are ppm, the others are wt%)

[0041]

[0042]

[0043] Table 2 Rolling process and annealing process of the present application and comparative examples, the present application uses flash cooling process, the comparative examples use ordinary annealing process.

[0044] Table 2 Rolling process and annealing process of the present application and comparative examples, the present application uses flash cooling process, the comparative examples use ordinary annealing process.

[0045]

[0046] Table 3 is the mechanical properties and strength plastic product of the examples and the comparative examples of the application, from table 3, it can be seen that the mechanical properties and strength plastic product of the dual phase steel prepared by the process of the application are greatly improved. By micro alloying and high Al regulating martensite and bainite and residual austenite, combined with annealing process, high strength plastic product index is achieved. Among them, Nb and Ti can refine the martensite and bainite half strip, improve the strength and plasticity, and the reduction of soft phase residual austenite, the strength does not decrease, and the strength plastic product is improved.

[0047] Table 3 is the mechanical properties and strength plastic product of the examples and the comparative examples of the application, from table 3, it can be seen that the mechanical properties and strength plastic product of the dual phase steel prepared by the process of the application are greatly improved. By micro alloying and high Al regulating martensite and bainite and residual austenite, combined with annealing process, high strength plastic product index is achieved. Among them, Nb and Ti can refine the martensite and bainite half strip, improve the strength and plasticity, and the reduction of soft phase residual austenite, the strength does not decrease, and the strength plastic product is improved.

[0048]

[0049]

[0050] Table 4 is the microstructure and hole expansion rate of the examples and the comparative examples of the application. The high strength steel of the examples has low sulfide aspect ratio, and the second type of sulfide is not easy to gather, which improves the easy cutting performance, and the austenite content is low, the strength is not decreased, and the hole expansion rate is greatly improved. By regulating the morphology of sulfide in the application, the hole expansion rate of high strength steel is greatly improved, and the effect of solid solution Al is improved due to the increase of martensite content, the strength is improved, and the effect of composite micro alloying is to refine the lath and reduce the aspect ratio of sulfide as the core of sulfide formation, thereby improving the strength plastic product.

[0051] Table 4 is the microstructure and hole expansion rate of the examples and the comparative examples of the application. The high strength steel of the examples has low sulfide aspect ratio, and the second type of sulfide is not easy to gather, which improves the easy cutting performance, and the austenite content is low, the strength is not decreased, and the hole expansion rate is greatly improved. By regulating the morphology of sulfide in the application, the hole expansion rate of high strength steel is greatly improved, and the effect of solid solution Al is improved due to the increase of martensite content, the strength is improved, and the effect of composite micro alloying is to refine the lath and reduce the aspect ratio of sulfide as the core of sulfide formation, thereby improving the strength plastic product.

[0052] Examples Sulphide aspect ratio Residual austenite content Hole expansion ratio / % Example 1 12.5 3.5 42.5 Example 2 12.3 4.2 41.7 Example 3 11.5 3.7 43.2 Example 4 12.2 3.2 44.5 Example 5 11.7 3.8 45.5 Comparative Example 1 20.8 9.5 40.5 Comparative Example 2 18.7 10.2 42.3

[0053] The application adopts high Al chemical composition, uses flash annealing process, introduces martensite organization, improves strength, and adopts micro alloying, refines organization and residual austenite, reduces residual austenite content, increases plasticity. By adding S element, the easy cutting performance is increased, the Ca-Te composite modification treatment is used, the sulfide morphology is optimized, and the hole expansion rate is more than 40%. Through the implementation of the application, the strength plastic product of the steel is greatly improved, which meets the demand of light weight, increases the continuous casting furnace number, reduces the cost, and the optimized sulfide star vein is beneficial to the improvement of the strength plastic product, and further improves the easy cutting performance.

Claims

1. A free-machining, high-strength, high-ductility cold-rolled dual-phase steel, characterized in that, Its weight percentages are: C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: ≤0.012%, Cu≤0.20%, Alt: 0.60%-0.80%, S: 0.020%-0.035%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 15ppm≤Te≤30ppm, 10ppm≤Ca≤15ppm, TO: ≤15ppm, [N]: ≤60ppm, the remainder being Fe and unavoidable impurity elements, of which 0.20%≤D m =Al-4.12×[N]-2.25×(Ti+Nb)-1.21*C≤0.35%, 2.5%≤Fn=(2.8*Te+1.5*Ca) / (0.1*S)≤3.5%; The steel produced has a sulfide aspect ratio ≤13 and a retained austenite content ≤4.5%; The tensile strength R of the product obtained m ≥1100MPa, yield strength R p0.2 ≥700MPa, A 80 ≥20%, strength-plasticity volume ≥22GPa; The minimum bending radius for longitudinal 180° cold bending is ≤1.0t, where t is the thickness of the steel plate, and the hole expansion rate is ≥40%.

2. A method for preparing the free-machining, high-strength, high-ductility cold-rolled dual-phase steel according to claim 1, characterized in that, The process includes the following steps: smelting → continuous casting → hot rolling → pickling and cold rolling → continuous annealing → finished product.

3. The preparation method according to claim 2, characterized in that, In the smelting and continuous casting process, converters, electric furnaces and induction furnaces are used for smelting, and continuous casting is used to produce billets. Electromagnetic stirring and dynamic light reduction devices are used during the casting process.

4. The preparation method according to claim 2, characterized in that, In the hot rolling process: the billet is heated to 1200-1260℃ and held for 2-3 hours. It is then rolled in 5-7 passes by a roughing mill to a 30-50mm intermediate billet. Finally, it is rolled in 5-7 passes by a hot continuous rolling mill at a final rolling temperature of 870-910℃. After reaching the target thickness, it is coiled into a steel coil at a temperature of 540-600℃.

5. The preparation method according to claim 2, characterized in that, In the pickling and cold rolling process, the hot-rolled strip is pickled in a hydrochloric acid bath to remove the surface iron oxide scale, and then cold-rolled with a cold rolling reduction rate of 50%-70%.

6. The preparation method according to claim 2, characterized in that, In the continuous annealing step, the pickled and rolled steel sheet is annealed using a continuous annealing production line.

7. The preparation method according to claim 6, characterized in that, The annealing process adopts a slow heating + flash cooling process. The soaking temperature is 790-810℃ and held for 2 hours. The furnace is cooled to 680-700℃ and then flash-cooled at a rate of >30℃ / s. The temperature is then flash-cooled to 350-400℃ and held for 4 hours before air cooling.

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