CN102899562B - Complex phase-type high-strength low-carbon bainitic medium and heavy steel plate and production method thereof - Google Patents

Abstract

The invention discloses a complex-phase low-carbon bainite high-strength medium-thick steel plate and a production method thereof, which belong to the technical field of high-strength medium-thick plate steel. The chemical composition of the steel plate is C: 0.04-0.1% by weight, Si: 0.3-0.55%, Mn: 1.5-2.0%, P≤0.015%, S≤0.015%, Al: 0.02-0.05%, Nb: 0.05-0.10 %, Ti: 0.05-0.10%, Nb+Ti≤0.15%, the balance is Fe and unavoidable inclusions. The steel does not need to add Ni, Mo, Cu and other precious metal elements, and does not need heat treatment after rolling. It adopts controlled rolling and UFC process system to obtain refined bainite and 1%-5% MA structure, which can be successfully and stably produced with excellent Strong and tough cost-effective complex-phase high-strength medium-thick steel plate, thickness specification 16-40mm, yield strength Rp0.2 550-620Mpa, Rm 670-750Mpa, -20℃ V-notch Charpy impact energy ≥ 40J, suitable for engineering Machinery and coal machinery industry and other fields.

Description

A kind of complex-phase low-carbon bainite high-strength medium-thick steel plate and its production method

technical field

The invention belongs to the technical field of high-strength medium-thick plate steel, and in particular relates to a complex-phase low-carbon bainite high-strength medium-thick steel plate and a production method thereof, especially a method without adding precious metal elements such as Ni, Mo, and Cu, It is a multi-phase low-carbon bainitic high-strength steel produced by controlled rolling + ultra-rapid cooling process, and the structure is bainite (B) + (1-5%) MA. This production method is suitable for wide and thick plate production lines equipped with UFC system.

Background technique

Low-carbon bainitic steel is a kind of high-strength, high-toughness, multi-purpose steel, with high yield strength, tensile strength, good elongation performance, cold bending performance, welding performance and impact resistance, etc., the main application In various fields such as coal machinery, construction machinery, ship boards, bridges, etc.

Prior to the present invention, the invention of Patent No. 201110027503.4 provided a high-performance low-carbon bainite steel and its production method, adopting the microalloying design of Cr-Mo-Nb-Nb-Ti, adopting two-stage controlled rolling, and then Accelerated cooling was performed, followed by air cooling. The disadvantage is that 0.1%-0.5% Mo element is used to hinder the nucleation and growth process of proeutectoid ferrite and inhibit the transformation of ferrite. However, Mo element is a precious metal and is expensive, which is not good for controlling the cost of the alloy.

Patent No. 200810030404.X discloses a high-strength micro-alloyed low-carbon bainitic steel and its production method, adopting the micro-alloying design of Ni+Cr+Mo+Nb+V+Ti, and adopting tempering heat treatment. The disadvantages are (1) the use of 0.1-0.15% Ni, 0.1-0.15% Mo and 0.0008-0.002% B, Ni and Mo are both precious metals, which increases the cost of the alloy; (2) tempering treatment after rolling not only increases the process cost, but also increase the production cycle.

Patent 200810022600.2 A low-carbon bainite steel for high-strength and low-temperature use and its production process, adopts the micro-alloying design of Nb+Mo+Ti, and adopts high-temperature tempering treatment after rolling. The disadvantage is also the use of 0.1-0.5%Mo and tempering treatment, respectively increase the alloy cost and process cost of the steel plate, and increase the production cycle.

Contents of the invention

The object of the present invention is to provide a kind of multi-phase low-carbon bainite high-strength medium-thick steel plate and its production method, which does not use precious metal elements and heat treatment methods, reduces the alloy cost and process cost of bainite steel plate, and shortens the production process. Period, adding appropriate amount of microalloying elements, adopting controlled rolling + UFC cooling method, using bainite + (1-5%) MA to produce multi-phase low carbon bainite high strength steel.

The multi-phase low-carbon bainite high-strength medium-thick steel plate designed by the present invention has a chemical composition of C: 0.04-0.1%, Si: 0.3-0.55%, Mn: 1.5-2.0%, P≤0.015 %, S≤0.015%, Al: 0.02-0.05%, Nb: 0.05-0.10%, Ti: 0.05-0.10%, Nb+Ti≤0.15%, the balance is Fe and unavoidable inclusions. The organization is bainite (B) + (1-5%) MA.

The present invention can add Nb and Ti elements on the basis of C-Mn steel, and Nb+Ti≤0.15%; the steel plate thickness specification is 16mm-40mm.

In the composition design selected among the present invention, the effect of each element is as follows:

C: The choice is 0.04-0.1%. The carbon content has an effect on the strength, toughness and weldability of steel. The necessary carbon content plays the role of solid solution strengthening, interacts with the added Nb and Ti elements, precipitates microalloy carbides, and plays the role of inhibiting recrystallization and precipitation strengthening; at a lower carbon content, the lath bainite The cementite is fine, does not show continuous distribution, and has good toughness; more importantly, the lower carbon content can ensure that the steel plate has good welding performance.

Silicon: choose 0.3-0.5%. Si is an essential element for deoxidation in steelmaking, and it increases the strength of steel in the form of solid solution strengthening; if the content is too low, the deoxidation effect is not good, and if the content is too high, the toughness will be reduced and the weldability will be poor.

Manganese: The choice is 1.5-2.5%. Mn is the most important element for solid solution strengthening and improving the tensile strength of the steel plate. It has a greater effect on promoting the transformation of bainite, and the cost is low. In the present invention, Mn is used as the main alloy element.

Aluminum: choose Al: 0.02-0.05%. Al is a deoxidizing element, forming AlN effective silicon refinement grains, similar to Si, if the content is insufficient, the deoxidation effect is very poor, and if it is too high, the toughness will be affected.

Nb: 0.05-0.10% is selected. Nb can effectively inhibit austenite recrystallization, increase the recrystallization temperature, expand the range of non-recrystallization zone, and provide guarantee for the final microstructure refinement by controlled rolling and increasing reduction in the finishing rolling stage. Among them, the fine carbonitrides of solid-solution niobium play a role of pinning and dragging on the austenite grain boundaries and sub-crystals. During the cooling process, part of the solid-solution niobium can precipitate carbonitrides in the bainite, which plays a role in precipitation strengthening. role.

Ti: The choice is 0.05-0.10%. In addition to fixing nitrogen, titanium can also prevent grain growth during heating, rolling and welding, and improve the toughness of the base metal and the heat-affected zone of welding. In addition, the solid-solution titanium in the steel can be precipitated in the form of carbonitrides during the cooling process, which can prevent grain growth and disperse precipitation strengthening.

Phosphorus and sulfur: choose ≤0.015%. Both P and S are harmful elements that endanger the toughness of the steel plate.

The manufacturing process of the present invention comprises smelting, controlled rolling and controlled cooling; the technical parameters controlled in the process are as follows:

(1) Smelting: Smelting in a vacuum induction furnace and pouring into billets;

(2) Controlled rolling:

The heating temperature is set at 1180~1250℃, and the heating time is 3.5-4.5h, so that the billet is austenitized and carbonitrides are dissolved, and the temperature is not too high to cause coarse austenite grains;

Rolling adopts two-stage controlled rolling technology:

The first stage is the rough rolling stage, the rolling start temperature is 1140°C±30°C, the pass reduction rate is 20-35%, and it should be completed in 4-5 passes as far as possible to achieve the full refinement of austenite grains; The thickness to be warmed in the middle of the piece is controlled by 2 to 3 times the thickness of the finished product;

The second stage is the finish rolling stage: the starting rolling temperature in the austenite non-recrystallized area is 880°C-900°C, the total rolling reduction rate is 50%-65%, and the final rolling temperature is controlled at 800-850°C;

The rolling temperature in the austenite non-recrystallized region is 880°C-900°C, and the amount of deformation should be increased as much as possible, so that the austenite grains are fully broken before the phase transformation, and the dislocation density and substructure are fully introduced, which is the microstructure and structure after the phase transformation. The carbide precipitation of Nb and Ti provides more nucleation points, and the structure and precipitates are uniform and fine after phase transformation, which plays a good role in fine grain strengthening and precipitation strengthening.

(3) Adopt UFC (controlled cooling method, the final cooling temperature is controlled at 500-600°C, and the cooling speed is controlled at 30-40°C/s.

(4) After ultra-rapid cooling, the steel plate is air-cooled to room temperature.

The present invention adopts UFC (Ultra Fast Cooling) technology. The ultra-fast cooling technology is an important technical measure to improve the performance of rolled steel products, and it is also one of the core links of the controlled cooling process of the present invention. By accelerating the cooling rate after rolling, not only the growth of grains can be suppressed, but also the ultra-fine ferrite structure or bainite structure required for high strength and high toughness can be obtained. The steel plate is cooled to 500°C-600°C at a cooling rate of 20-40°C/s, and air-cooled after exiting the water. Under this ultra-rapid cooling technology, the austenite undergoes bainite transformation and forms a fine bainite lath structure , the slats are small, the cementite is small and intermittently dispersed, and (1-5%) MA is small in size and dispersed, which plays a very good role in strengthening.

The present invention introduces (1-5%) MA tissue. The MA structure has high hardness and high strength, but the traditional MA structure has a great influence on the toughness, and the coarse MA structure reduces the low temperature impact toughness of the steel plate. However, in the present invention, the MA content is controlled at 1-5% by adopting the microstructure ultra-fine technology and ultra-rapid cooling technology, so that the MA microstructure not only exerts a high-strength effect, but also exerts the positive effect of grain refinement on strength and toughness .

The advantages of the present invention are:

The invention adopts high Nb and high Ti alloy design, does not use precious metal elements such as Ni, Mo and Cu, has low alloy cost, saves social resources, and satisfies the concept of green environmental protection design.

The invention adopts controlled rolling + UFC (ultra-fast cooling) technology, does not use heat treatment technologies such as quenching and tempering, and has low production cost and short production cycle.

According to the chemical composition and production method provided by the present invention, it is possible to successfully and stably produce cost-effective high-strength steel with excellent strength and toughness, thickness specification 16-40mm, yield strength Rp0.2550-620Mpa, Rm670-750Mpa, -20°C V type Notched Charpy impact energy ≥ 40J, suitable for construction machinery, coal machinery industry, ship plates, bridges and other fields.

Description of drawings

Figure 1 is a photo of 16mm medium-thick steel plate structure bainite + MA.

Figure 2 is a photo of 40mm medium-thick steel plate structure bainite + MA.

Figure 3 is a 1.33% MA photo of a 16mm medium-thick steel plate.

Figure 4 is a 2.63% MA photo of a 40mm medium-thick steel plate.

Detailed ways:

Implementation example 1:

This implementation example is the production process of the multi-phase low-carbon bainite high-strength medium-thick steel plate with a specification of 16mm, and its composition weight percentage is shown in Table 1:

Table 1 Chemical composition

element C Si mn P S al Nb Ti Fe % by weight 0.090 0.5 1.84 0.014 0.007 0.025 0.081 0.066 margin

The production process is carried out according to the following procedures:

(1) Smelting: Smelting in a 50kg vacuum induction furnace and casting into ingots of 120mm*120mm*300mm.

(2) Controlled rolling and UFC (Ultra Fast Cooling)

Heating temperature: The billet heating temperature in this test is set at 1210°C, and the heating time is 4h.

Controlled rolling: Two-stage controlled rolling is adopted. The starting rolling temperature in the rough rolling stage is about 1140°C, the reduction rate of the four passes is greater than 20%, and the maximum pass reduction reaches 31%. The rolling start temperature is 900°C, and the final rolling temperature is 820°C.

UFC (Ultra Fast Cooling): Start cooling temperature is 805°C; final cooling temperature is 535°C, cooling speed is 38.6°C/s:

Implementation example 2:

This implementation example is the production process of the multi-phase low-carbon bainite high-strength medium-thick steel plate with a specification of 40mm, and its composition weight percentage is shown in Table 2:

Table 2 Chemical Composition

element C Si mn P S al Nb Ti Fe % by weight 0.083 0.48 1.75 0.012 0.008 0.043 0.09 0.051 margin

The production process is carried out according to the following procedures:

(1) Smelting: Smelting in a 50kg vacuum induction furnace and casting into ingots of 120mm*120mm*300mm.

(2) Controlled rolling and UFC (Ultra Fast Cooling)

Heating temperature: The billet heating temperature in this test is set at 1210°C, and the heating time is 4h.

Controlled rolling: Two-stage controlled rolling is adopted. The starting temperature of the rough rolling stage is about 1105°C, and the reduction rate of the three passes is greater than 20%; the thickness of the rolled piece to be warmed in the middle is controlled by 80mm twice the thickness of the finished product; the starting temperature of the finish rolling stage is 850°C, and the final rolling temperature 820°C.

UFC (Ultra Fast Cooling): The starting cooling temperature is 805°C, the final cooling temperature is 565°C, and the cooling speed is 34.2°C/s:

Table 3 is the mechanical properties of implementation example 1 and example 2

The mechanical property of table 3 example 1 and example 2

Figures 1 to 4 are photos of the multiphase structure bainite + MA and MA of 16mm and 40mm medium-thick steel plates.

Claims (3)

1. A multi-phase low-carbon bainite high-strength medium-thick steel plate, characterized in that: the chemical composition of the steel plate is C: 0.04-0.1%, Si: 0.3-0.55%, Mn: 1.5-2.0% by weight percentage , P≤0.015%, S≤0.015%, Al: 0.02-0.05%, Nb: 0.05-0.10%, Ti: 0.05-0.10%, Nb+Ti≤0.15%, the balance is Fe and inevitable inclusions; structure For bainite + (1-5%) MA; The steel plate thickness specification is 16mm-40mm. the 2. The multi-phase low-carbon bainite high-strength medium-thick steel plate according to claim 1, characterized in that Nb and Ti elements are added on the basis of C-Mn steel, and Nb+Ti≤0.22%. the 3. A production method for producing a complex-phase low-carbon bainite high-strength medium-thick steel plate as claimed in claim 1, the technique includes smelting, controlled rolling and controlled cooling; it is characterized in that: the technical parameters controlled in the technique are: (1) Smelting: smelting in a vacuum induction furnace and pouring into billets; (2) Controlled rolling: The billet heating temperature is 1180-1250 °C, and the heating time is 3.5-4.5 hours, so that the cast billet is austenitized and carbonitrides are dissolved, and the temperature is not too high to cause coarse austenite grains; Two-stage controlled rolling is adopted: The first stage is the rough rolling stage, the rolling temperature is 1110℃～1170℃, the pass reduction rate is 20-35%, and it should be completed in 4-5 passes as far as possible to achieve sufficient refinement of austenite grains; The thickness to be warmed in the middle of the piece is controlled by 2 to 3 times the thickness of the finished product; The second stage is the finish rolling stage: the starting rolling temperature in the austenite non-recrystallized area is 880°C-900°C, the total rolling reduction rate is 50%-65%, and the final rolling temperature is controlled at 800-850°C; (3) Adopt UFC (controlled cooling method, the final cooling temperature is controlled at 500-600°C, and the cooling speed is controlled at 30-40°C/s; (4) After ultra-rapid cooling, the steel plate is air-cooled to room temperature. the