1000MPa-grade bainite dual-phase steel with excellent forming performance

Abstract

The invention discloses 1000MPa-grade bainite dual-phase steel with excellent forming performance. The dual-phase steel comprises the following chemical components in percentage by weight: 0.10 to 0.13 percent of C, 1.20 to 1.30 percent of Si, 2.25 to 2.55 percent of Mn, 0.21 to 0.26 percent of Cr, 0.015 to 0.025 percent of Nb, 0.016 to 0.024 percent of Ti and the balance of Fe. N is less than or equal to 0.0030%; p is smaller than or equal to 0.010%, S is smaller than or equal to 0.01%, and the balance is Fe and unavoidable impurity elements. The production method comprises the steps of plate blank heating, rolling and continuous annealing treatment. The microstructure of the bainite dual-phase steel is uniform ferrite and bainite, and the bainite dual-phase steel is good in obdurability matching and excellent in forming performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel rolling, and particularly relates to a 1000 MPa grade bainitic dual-phase steel with excellent formability. Background Art

[0002] DP1180 dual-phase steel is a high-strength cold-rolled steel, which is widely used in the automotive industry due to its unique microstructure and excellent mechanical properties. However, the traditional DP1180 dual-phase steel has poor formability, and its elongation is only about 6 - 7%, indicating low plasticity and difficulty in withstanding large deformations. Moreover, due to the tensile strength of DP1180 dual-phase steel being as high as over 1180 MPa, a large rolling force is required during the rolling process, which makes it prone to problems such as edge wave and peeling during temper rolling, affecting product quality.

[0003] The poor formability and plasticity of DP1180 dual-phase steel are mainly due to the relatively high content of martensite in its microstructure, which leads to strain incoordination during plastic deformation, thus limiting its elongation. Adding Nb and Ti elements to the steel can effectively improve the above problems. The presence of Nb helps to refine grains and inhibit grain growth, while the presence of Ti enhances the hardness and strength of the material by forming TiN precipitates. This composite addition method can balance strength, plasticity-toughness, and weldability, thereby improving the overall performance of the steel. Summary of the Invention

[0004] The object of the present invention is to provide a 1000 MPa grade bainitic dual-phase steel with excellent formability, having a uniform ferrite and bainite structure, good strength-toughness matching, and excellent formability.

[0005] To achieve the above object, the technical solution adopted by the present invention is: A 1000 MPa grade bainitic dual-phase steel with excellent formability, by weight percentage, its chemical composition is as follows: C: 0.10 - 0.13%, Si: 1.20 - 1.30%, Mn: 2.25 - 2.55%, Cr: 0.21 - 0.26%, Nb: 0.015 - 0.025%, Ti: 0.016 - 0.024%; N ≤ 0.0030%; P ≤ 0.010%, S ≤ 0.01%, and the balance is Fe and inevitable impurity elements.

[0006] The mechanism of action of Nb, Ti, and Si elements in the dual-phase steel of the present invention is as follows: Nb is a strong carbide-forming element that can combine with C and N in steel to form Nb carbides and nitrides. After adding a trace amount of Nb element to DP1180 dual-phase steel, nano-sized fine NbC phases will precipitate in the steel. These NbC particles are pinned at the austenite grain boundaries, effectively inhibiting the growth of grains, thereby refining the grain structure. This refining effect not only increases the hardness but also enhances the toughness of the material. In addition, the precipitates of Nb can also pin dislocations, increasing the resistance to dislocation migration and further improving the strength of the material. In this invention, Nb: 0.015 - 0.025%.

[0007] The addition of Ti can enhance the mechanical properties of steel. This is mainly attributed to the precipitation strengthening effect of Ti combining with C element to form fine TiC particles. These fine TiC particles are evenly distributed at the grain boundaries, which can effectively prevent the growth of grains, thus achieving the effect of grain refinement. The presence of TiC particles not only improves the wear resistance of the steel but also enhances its strength and toughness. However, when the Ti content is too high, it may lead to grain coarsening and an increase in the size of precipitates, thereby reducing the toughness of the material. In this invention, Ti: 0.016 - 0.024%.

[0008] Si can exist in ferrite or austenite in the form of solid solution, and improve the strength and hardness of steel through solid solution strengthening. Increasing the content of Si element in DP1180 dual-phase steel can refine the martensite structure, increase the effective grain boundary area of ferrite and martensite, thereby improving the anti-dislocation movement ability of the steel. In this invention, Si: 1.20 - 1.30%.

[0009] Furthermore, for the 1000MPa grade bainitic dual-phase steel with excellent formability described in this invention, its microstructure is uniform ferrite and bainite.

[0010] Furthermore, for the 1000MPa grade bainitic dual-phase steel with excellent formability described in this invention, its yield strength Rel≥700MPa, tensile strength Rm≥1200MPa, and elongation after fracture A≥17%.

[0011] Furthermore, the production method of the 1000MPa grade bainitic dual-phase steel with excellent formability described in this invention includes slab heating, rolling, and continuous annealing treatment.

[0012] Furthermore, for the slab heating described in this invention, the slab is heated in a heating furnace. The furnace gas temperature in the preheating section is 850 - 950°C, the furnace gas temperature in the heating section is 1100 - 1200°C, the furnace gas temperature in the soaking section is 1200 - 1300°C, and the holding time of the steel billet in the furnace is 1 - 3h.

[0013] Furthermore, in the rolling process of the present invention, the heated slab is descaled under high pressure and then rough rolled. The rough rolling finishing temperature is 1100 - 1200 °C. Then it is finish rolled. The finish rolling entry temperature is 900 - 1000 °C, and the finish rolling temperature is 800 - 850 °C; the coiling temperature is 650 - 700 °C.

[0014] Furthermore, in the continuous annealing treatment of the present invention, the annealing temperature is 750 - 850 °C, the holding time is 2 - 4 min, and the cooling rate is 30 - 50 °C / s.

[0015] The invention principle and beneficial technical effects of the present invention are as follows: By adding a certain amount of Nb and Ti elements to the DP1180 dual-phase steel, the present invention improves the formability and plasticity, and increases the Si element content on the basis of the traditional DP1180 dual-phase steel, increasing the ferrite volume fraction in the steel, refining the grains, thereby improving the strength and toughness of the steel. At the same time, combined with the continuous annealing process, on the basis of ensuring the strength and hardness of the DP1180 dual-phase steel, a dual-phase steel with good strength-toughness matching, uniform internal and external structures and containing bainite structure is obtained. Brief Description of the Drawings

[0016] Figure 1 It is the micro-structure of the bainite dual-phase steel in Example 1. Detailed Description of the Invention

[0017] The technical solution of the present invention will be further described in detail below with reference to the embodiments. Example 1

[0018] The chemical composition of the bainite dual-phase steel in this example is calculated by weight percentage as follows: C: 0.10%, Si: 1.20%, Mn: 2.25%, Cr: 0.21%, Nb: 0.015%, Ti: 0.016%; N: 0.0022%; P: 0.007%, S: 0.0031%, and the balance is Fe and unavoidable impurity elements. Its production method is as follows: The slab is obtained by the processes of hot metal pretreatment - converter smelting - vacuum smelting - continuous casting; The slab is heated in a heating furnace. The furnace gas temperature in the preheating section is 850 °C, the furnace gas temperature in the heating section is 1100 °C, the furnace gas temperature in the soaking section is 1200 °C, and the holding time is 60 min; The heated slab is descaled under high pressure and then rough rolled. The rough rolling exit temperature is 1100 °C. Then it is finish rolled. The finish rolling entry temperature is 900 °C, the finish rolling temperature is 800 °C, the coiling temperature is 650 °C, and then continuous annealing treatment is carried out. The annealing temperature is 750 °C, the holding time is 2 min, and the cooling rate is 30 °C / s. Thus obtained.

[0019] The micro-structure of the bainite dual-phase steel in this example is asFigure 1 as shown; from Figure 1 it can be seen that its microstructure is uniform ferrite and bainite. The microstructures of the bainitic dual-phase steels in the remaining examples are the same as those in Example 1 and will not be provided one by one hereafter. Example 2

[0020] The chemical composition of the bainitic dual-phase steel in this example is as follows by weight percentage: C: 0.11%, Si: 1.24%, Mn: 2.35%, Cr: 0.23%, Nb: 0.02%, Ti: 0.019%; N: 0.0023%; P: 0.008%, S: 0.0034%, and the balance is Fe and inevitable impurity elements; its production method is as follows: A slab is obtained by using the processes of hot metal pretreatment - converter smelting - vacuum smelting - continuous casting; The slab is heated in a heating furnace, the furnace gas temperature in the preheating section is 900°C, the furnace gas temperature in the heating section is 1150°C, the furnace gas temperature in the soaking section is 1250°C, and the holding time is 120 min; The heated slab is descaled by high pressure and then rough rolled, the rough rolling outlet temperature is 1150°C, then finish rolled, the finish rolling inlet temperature is 950°C, the final rolling temperature is 825°C, the coiling temperature is 670°C, and then continuous annealing treatment is carried out, the annealing temperature is 800°C, the holding time is 3 min, and the cooling rate is 40°C / s. Example 3

[0021] The chemical composition of the bainitic dual-phase steel in this example is as follows by weight percentage: C: 0.13%, Si: 1.30%, Mn: 2.55%, Cr: 0.26%, Nb: 0.025%, Ti: 0.024%; N: 0.0029%; P: 0.009%, S: 0.0033%, and the balance is Fe and inevitable impurity elements; its production method is as follows: A slab is obtained by using the processes of hot metal pretreatment - converter smelting - vacuum smelting - continuous casting; The slab is heated in a heating furnace, the furnace gas temperature in the preheating section is 950°C, the furnace gas temperature in the heating section is 1200°C, the furnace gas temperature in the soaking section is 1300°C, and the holding time is 180 min; The heated slab is descaled by high pressure and then rough rolled, the rough rolling outlet temperature is 1200°C, then finish rolled, the finish rolling inlet temperature is 1000°C, the final rolling temperature is 850°C, the coiling temperature is 700°C, and then continuous annealing treatment is carried out, the annealing temperature is 850°C, the holding time is 4 min, and the cooling rate is 50°C / s. Example 4

[0022] The chemical composition of the bainitic dual-phase steel in this embodiment is as follows by weight percentage: C: 0.12%, Si: 1.28%, Mn: 2.45%, Cr: 0.25%, Nb: 0.023%, Ti: 0.023%; N: 0.0025%; P: 0.008%, S: 0.004%, and the balance is Fe and inevitable impurity elements; its production method is as follows: A slab is obtained by using the process of hot metal pretreatment - converter smelting - vacuum smelting - continuous casting; The slab is heated in a heating furnace. The furnace gas temperature in the preheating section is 920 °C, the furnace gas temperature in the heating section is 1180 °C, the furnace gas temperature in the soaking section is 1270 °C, and the holding time is 150 min; After the heated slab is descaled by high pressure, rough rolling is carried out. The rough rolling exit temperature is 1180 °C, then finish rolling is carried out. The finish rolling inlet temperature is 1080 °C, the finish rolling temperature is 950 °C, the coiling temperature is 675 °C, and then continuous annealing treatment is carried out. The annealing temperature is 750 °C, the holding time is 3 min, and the cooling rate is 40 °C / s.

[0023] Comparative Example 1

[0024] This comparative example is a traditional DP1180 dual-phase steel. The chemical composition is as follows by weight percentage: C: 0.11%, Si: 0.52%, Mn: 2.35%, Cr: 0.24%, N 0.0026%; P 0.007%, S 0.0031%, and the balance is Fe and inevitable impurity elements; its production method is as follows: A slab is obtained by using the process of hot metal pretreatment - converter smelting - vacuum smelting - continuous casting; The slab is heated in a heating furnace. The furnace gas temperature in the preheating section is 900 °C, the furnace gas temperature in the heating section is 1150 °C, the furnace gas temperature in the soaking section is 1250 °C, and the holding time is 120 min; After the heated slab is descaled by high pressure, rough rolling is carried out. The rough rolling exit temperature is 1150 °C, then finish rolling is carried out. The finish rolling inlet temperature is 1100 °C, the finish rolling temperature is 900 °C, the coiling temperature is 670 °C, and then continuous annealing treatment is carried out. The annealing temperature is 760 °C, the holding time is 4 min, and the cooling rate is 45 °C / s.

[0025] According to GB / T228.1 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", the mechanical properties of the dual-phase steels in each example and Comparative Example 1 are detected, and the detection results are shown in Table 1.

[0026] Table 1 Mechanical properties of the dual-phase steels in each example and Comparative Example 1 .

Claims

1. A 1000MPa grade bainitic dual-phase steel with excellent formability, characterized in that: The chemical composition and weight percentage of the bainitic dual-phase steel are: C: 0.10-0.13%, Si: 1.20-1.30%, Mn: 2.25-2.55%, Cr: 0.21-0.26%, Nb: 0.015-0.025%, Ti: 0.016-0.024%; N≤0.0030%; P≤0.010%, S≤0.01%, and the balance is Fe and unavoidable impurity elements.

2. The 1000MPa grade bainite dual-phase steel with excellent formability according to claim 1, characterized in that: The microstructure of the bainite dual-phase steel is uniform ferrite and bainite.

3. The 1000MPa grade bainite dual-phase steel with excellent formability according to claim 1, characterized in that: The bainite dual-phase steel has a yield strength Rel≥700MPa, a tensile strength Rm≥1200MPa, and an elongation after fracture A≥17%.

4. The 1000MPa grade bainite dual-phase steel with excellent formability according to claim 1, characterized in that: The production method includes billet heating, rolling and continuous annealing treatment.

5. The 1000MPa grade bainite dual-phase steel with excellent formability according to claim 4, characterized in that: The steel billet is heated in a regenerative heating furnace, with a furnace gas temperature of 850-950°C in the preheating section, 1100-1200°C in the heating section, 1200-1300°C in the soaking section, and the steel billet is kept in the furnace for 1-3 hours.

6. The 1000MPa grade bainite dual-phase steel with excellent formability according to claim 4, characterized in that: The rolling is performed by descaling the heated slab under high pressure and then rough rolling, the rough rolling final rolling temperature is 1100-1200°C, and then finishing rolling is performed, the finishing rolling inlet temperature is 900-1000°C, the final rolling temperature is 800-850°C; the coiling temperature is 650-700°C.

7. The 1000MPa grade bainite dual-phase steel with excellent formability according to claim 4, characterized in that: The continuous annealing treatment has an annealing temperature of 750-850°C, a holding time of 2-4 minutes, and a cooling rate of 30-50°C / s.

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