CN106319375A - Alloy structural steel cold-rolled sheet for stamping and preparation method thereof - Google Patents

Abstract

The invention discloses a cold-rolled alloy structural steel plate and a preparation method thereof, C: 0.08-0.55, Si: 0.17-1.30, Mn: 0.30-1.80, P: <0.035, S: <0.035, Cr: 0.40-1.40, Mo : 0.15~0.30, the rest is Fe and unavoidable impurities. The heating temperature of the billet in hot rolling is 1170-1270°C, the finishing temperature of hot rolling is 850-950°C, the coiling temperature is 700-750°C; the reduction rate of cold rolling is 25%-55%, and the annealing temperature is Ac1-( 10-30) °C, the holding time is 8-14 hours, air-cooled when the heat preservation temperature is below 400 °C; the flat elongation rate is 0.5%-1.2%.

Description

A cold-rolled alloy structural steel plate for stamping and its preparation method

technical field

The invention belongs to the technical field of steel manufacturing, and specifically designs a cold-rolled alloy structural steel plate suitable for stamping and a production method thereof.

Background technique

Stamping is a production method that uses external force to plastically process materials. It has many advantages such as convenient operation, high production efficiency, high workpiece size and shape accuracy, good surface quality during processing, and stable product quality. Stamping parts are widely used in many industrial production and daily life related fields such as machinery, automobiles, electronics, instruments, household appliances, bicycles, office equipment, living utensils, etc.

The carbon content of the alloy structural steel ranges from 0.08 to 0.55wt%. It is a steel that adds a certain amount of alloying elements (Cr, Mo, etc.) to the carbon steel to achieve the required performance. The role of alloying elements makes it have higher tensile strength and yield strength ratio, higher toughness and fatigue strength after proper heat treatment.

The use of stamping technology to process alloy structural parts can improve production efficiency, broaden the application of this steel type, and apply it to structural parts with harsh service conditions, easy to wear and corrosion, and heavy loads at a relatively low cost. Such as pressure vessels, petrochemical, engineering machinery, bridges, shipbuilding and other steel structures.

The currently commonly used high-strength cold-rolled sheet is micro-alloyed by adding alloying elements such as V, Ti, and Nb to the steel, and beneficial precipitates are generated by controlling the rolling process, thereby preparing cold-rolled sheets with high strength and good toughness. For example: the invention patent of publication number CN 101684533A discloses a high-strength cold-rolled sheet with excellent formability. The high-strength phosphorus-containing steel sheet of the invention has the characteristics of ultra-low carbon, micro-alloying, and pure steel. Adding a certain amount of V, Nb, and Ti alloys to the steel makes the steel have high plasticity. The corresponding rolling and annealing process is adopted to make the performance index of the steel meet the requirements of high-strength automobile stamping parts. Since the chemical composition of the cold-rolled sheet contains V, Nb, and Ti alloy elements, the production cost thereof is increased.

Alloy structural steel is currently mostly used in the production of parts in the form of cast and forged materials, and less in the form of cold-rolled plates. At present, there is no relevant report on the preparation of alloy structural steel cold-rolled sheets.

Contents of the invention

The technical problem to be solved by the present invention is to provide a cold-rolled alloy structural steel plate and its preparation method, so as to solve the problem that the hard and brittle martensitic structure is easily formed in the current hot-rolling process, so that the rolling cracks during the cold-rolling process, Affecting its formability and other issues, after component design and reasonable annealing treatment, it has low yield strength and hardness, suitable for stamping production, suitable for large-scale production operations, high product strength, good toughness and stamping performance.

To achieve the above object, the present invention takes the following technical solutions:

A cold-rolled alloy structural steel sheet for stamping, characterized in that: the chemical composition is: C: 0.08-0.55, Si: 0.17-1.30, Mn: 0.30-1.80, P: <0.035, S: <0.035, Cr: 0.40-1.40, Mo: 0.15-0.30, and the rest are Fe and unavoidable impurities. The invention is conventional alloy structural steel with simple chemical composition, no addition of special alloy elements and low production cost.

The design principle of composition and content of each element in this invention is as follows:

C element is the basic element in steel, which can ensure the hardness, strength and toughness of steel, and make the material after heat treatment have good strength and toughness matching. C element below 0.08% will not guarantee the mechanical properties of the steel, and if the content is too high, it will affect its subsequent welding and processing. Therefore, the chemical content of 0.08-0.55% is used to ensure the good comprehensive mechanical properties of the steel.

Si element is used as reducing agent and deoxidizing agent in steelmaking process. The lower limit is set at 0.17% in view of the capacity of steelmaking equipment. In alloy structural steel, silicon can not only increase the hardenability of the steel, but also increase the tempering resistance of the steel after quenching. Considering the existence of other hardenability elements in alloy structural steel, the lower limit is determined to be 0.37%.

Mn element is an indispensable deoxidizer and desulfurizer in the steelmaking process, and its lower limit is set at 0.3% in consideration of the steelmaking equipment capacity. Mn element can increase the hardenability of steel and improve its cutting performance to a certain extent. When 0.7% to 1.8% of Mn is added to carbon steel, it not only has sufficient toughness (under proper heat treatment conditions), but also has high strength and hardness, so the upper limit is set at 1.8%.

S is a harmful element in steel, which exists in steel in the form of FeS, and forms a low-melting point compound with Fe, which causes hot embrittlement of steel, reduces the ductility and toughness of steel, and has an adverse effect on high-strength steel. Therefore, the S element content should be as little as possible, but considering the refining process load, the upper limit of the S element is limited to 0.035%.

The precipitation of P element in the grain boundary can reduce the toughness of steel, increase its cold brittleness, and cause the deterioration of welding performance and cold bending performance. Therefore, its content should be as small as possible. However, in order to completely remove the P element contained in the steel, higher requirements will be placed on the smelting process, so it is enough to control the P content to be less than 0.035%.

Cr element can significantly improve the strength, hardness and wear resistance of steel grades, improve the oxidation resistance of steel, and increase the corrosion resistance. But it will damage the plasticity and toughness while improving the hardenability of steel. Especially when the Cr element is too high, the brittle transition temperature of the steel will be significantly increased, and the temper brittleness of the steel will be promoted. Considering the comprehensive performance requirements, the content of the Cr element is determined to be 0.40-1.40%.

Mo is a precious alloying element that can refine grains. Therefore, the role of this element in steel is to improve hardenability and heat strength and prevent temper brittleness. Because molybdenum increases the thermal strength of steel, when the content of molybdenum is high, it will also increase the difficulty of hot working. Considering economic factors, the content is determined to be 0.15-0.30%.

The invention discloses a method for preparing a cold-rolled alloy structural steel sheet for stamping, which includes molten steel smelting, continuous casting, hot rolling, cold rolling, annealing, and leveling, and is characterized in that: the heating temperature of the steel billet in the hot rolling is 1170-1270°C , the hot rolling finishing temperature is 850~950℃, the coiling temperature is 700~750℃; the cold rolling reduction rate is 25%~55%, the annealing temperature is Ac1-(10~30)℃, and the holding time is 8~ 14h, heat preservation to below 400 ℃ and air cooling; the flat elongation rate is 0.5% to 1.2%.

The product specifications are cold-rolled coils with a thickness of 3.5mm-1.0mm and a width of 900mm-1250mm.

Process design reason of the present invention:

Alloy structural steel contains more Cr and Mo alloying elements, so that the steel plate forms a hard and brittle martensitic structure during cooling, which causes rolling cracks during cold rolling and affects its yield. Therefore, the appropriate hot rolling production process conditions should be selected, especially the final rolling temperature, coiling temperature, etc., to obtain suitable microstructure and properties, so as to improve the finished pass rate of alloy structural steel cold rolled plates. At the same time, the alloy structural steel chilled plate after cold rolling should choose a suitable spheroidizing annealing process to obtain a uniform spheroidizing structure, which is conducive to subsequent cutting and stamping processing.

(1) Heating temperature

If the heating temperature of the billet is too high or the time is too long, it can cause defects such as austenite grain growth, overheating, overburning, increase in scale and decarburization; while the heating temperature is too low, it can hinder the full solid solution of alloying elements and the formation of austenite. Homogenization of bulk grains. Therefore, the heating temperature of the hot-rolled steel slab in the present invention is selected to be 1170-1270°C.

(2) Finishing temperature

Finishing temperature is a key parameter affecting the microstructure and properties of steel during hot deformation. During final rolling in the high temperature zone, the grains are basically recrystallized austenite, and the final structure is relatively uniform. When the finish rolling temperature is lowered, the number of unrecrystallized austenite increases, and the defects such as dislocations in the deformed austenite increase, which makes the structure after phase transformation more refined, but not very uniform, and will produce banded structure. In order to prevent the occurrence of cold-rolled cracking of alloy structural steel, it is necessary to reduce the hardness of the hot-rolled state, so a higher final rolling temperature is selected; and the final rolling in the high temperature zone can prevent the cooling rate from being too fast after thermal deformation, resulting in a larger hardness martensite. Considering the equipment capability and the high temperature higher than 1000°C, it is easy to thicken the oxide scale of the steel plate, so the final rolling temperature of the present invention is selected as 850-950°C.

(3) coiling temperature

A considerable part of the transformation from austenite to ferrite is completed during slow cooling at the coiling temperature. As the coiling temperature decreases, the ferrite grains gradually become finer, the number of acicular ferrite gradually increases, and the lamellar spacing of pearlite also gradually decreases, and upper bainite also appears when the temperature is lower. For alloy structural steel, when the coiling temperature is low, the temperature will drop too quickly from the final rolling process to the coiling process, resulting in the appearance of cold and brittle martensite structure, which will affect its cold rolling production. Therefore the present invention selects higher coiling temperature. Considering the requirements of equipment capacity, the coiling temperature should be lower than 750°C, and the hot-rolled steel coil is prone to collapse when it is lower than 700°C, so the coiling temperature is selected as 700-750°C.

(4) Spheroidizing annealing

Spheroidizing annealing can spheroidize flaky pearlite, thereby reducing hardness and improving cutting performance. And the structure is uniform after spheroidization, which is extremely beneficial to improve the performance of heat treatment process, for example, it can reduce the loss of deformation and cracking during quenching and heating. After quenching and tempering, excellent comprehensive mechanical properties are obtained (when the original structure is spherical, when the strength and hardness are the same after quenching and tempering, the plasticity and toughness are better, and it has higher contact fatigue strength and longer life).

Spheroidizing annealing methods include: (1) slow cooling after heating to a temperature just above Ac1; (2) method of maintaining for a long time at a temperature just below Ac1; (3) at just above and just below Ac1 temperature, repeated heating/cooling method. Wherein, adopt the method for bell-type annealing in the present invention, consider that the heating and cooling rate of large-scale steel coil is slow, in order to avoid the inhomogeneity of overall structure performance, select method (2) to carry out constant temperature spheroidizing annealing. If the spheroidizing annealing temperature is too low or the holding time is too short, undissolved flaky carbides will increase, and a uniform spherical structure cannot be obtained. However, if the spheroidizing annealing temperature is too high or the holding time is too long, there will be too little residual carbide in the pearlite, and a uniform spherical structure cannot be obtained. Therefore, the annealing temperature of the spheroidizing annealing in the present invention is selected as Ac1-(10-30)°C (Ac1 temperature can be measured by a phase change instrument), the holding time is 8-14h, and the temperature is kept below 400°C with a cover and air-cooled.

Since the alloy structural steel is relatively hard, the present invention controls the cold rolling reduction rate at 25% to 50%, and the flat elongation rate at 0.5% to 1.2%, which is beneficial to obtain better performance.

Beneficial effects of the present invention:

By controlling the hot-rolling production process conditions of alloy structural steel, ferrite and pearlite structures with uniform crystal grains are obtained, and the occurrence of cold-rolling cracking is prevented. And through the spheroidizing annealing treatment of the cold-rolled sheet, the alloy structural steel cold-rolled annealed sheet with tensile strength Rm≥600MPa, elongation A50mm≥20%, and spheroidizing grade 5-6 is obtained. The product has good plasticity, toughness, and excellent cutting performance, which is beneficial for users to carry out cutting, stamping processing and subsequent heat treatment.

Description of drawings

The metallographic structure diagram after the cold-rolled annealing of Fig. 1 embodiment 4;

detailed description

The present invention will be further described below by way of examples.

Alloy structural steels having the chemical composition in Table 1 were continuously casted. The Ac1 temperatures of the steels in Examples 1 to 4 were measured by a LINSEIS L78RITA phase transformation instrument to be 740°C, 750°C, 760°C, and 760°C, respectively. Hot rolling and cold rolling were carried out under the conditions shown in Table 2 and Table 3, and spheroidizing annealing was carried out using the bell annealing method under the conditions shown in Table 3, and then air cooling was carried out after keeping the temperature at 400°C with a cover. The mechanical properties of cold-rolled annealed sheets with thicknesses of 1.8, 2.0, 2.45, and 2.95 mm are shown in Table 4. Fig. 1 is a metallographic structure diagram of Example 4 after cold rolling and annealing. It can be seen that spherical pearlite is evenly distributed on the ferrite matrix, and the spheroidization grade is the highest grade 6.

Table 1 Chemical composition (wt, %) of the embodiment of the present invention

Table 2 hot rolling process parameters of the embodiment of the present invention

Example number Thickness (mm) Heating temperature (℃) Finishing temperature (℃) Coiling temperature (℃) 1 3.3 1280 925 715 2 3.5 1250 920 720 3 3.95 1250 920 680 4 4.45 1250 910 690

Table 3 cold rolling and spheroidizing annealing process parameters of the embodiment of the present invention

Table 4 Performance of the steel plate of the embodiment of the present invention

Claims (2)

1. A cold-rolled alloy structural steel sheet for stamping, characterized in that: the chemical composition is: C: 0.08-0.55, Si: 0.17-1.30, Mn: 0.30-1.80, P: <0.035, S: < 0.035, Cr: 0.40-1.40, Mo: 0.15-0.30, and the rest are Fe and unavoidable impurities. 2. A method for preparing a cold-rolled alloy structural steel sheet for stamping according to claim 1, comprising molten steel smelting, continuous casting, hot rolling, cold rolling, annealing, and smoothing, characterized in that: the hot-rolled The heating temperature of the medium billet is 1170-1270°C, the finishing temperature of hot rolling is 850-950°C, the coiling temperature is 700-750°C; the reduction rate of cold rolling is 25%-55%, and the annealing temperature is Ac1-(10- 30) °C, the holding time is 8 to 14 hours, and air cooling when the holding temperature is below 400 °C; the flat elongation rate is 0.5% to 1.2%.