CN110669985A

CN110669985A - 750 MPa-level medium-temperature high-pressure boiler steel plate and production method thereof

Info

Publication number: CN110669985A
Application number: CN201910942504.8A
Authority: CN
Inventors: 王储; 艾芳芳; 欧阳鑫; 王勇; 胡昕明; 邢梦楠
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2019-09-30
Filing date: 2019-09-30
Publication date: 2020-01-10
Anticipated expiration: 2039-09-30
Also published as: CN110669985B

Abstract

The invention discloses a 750 MPa-level medium-temperature high-pressure boiler steel plate and a production method thereof. The steel contains 0.15-0.23% of C, 0.15-0.40% of Si, 1.20-1.60% of Mn, less than or equal to 0.005% of S, less than or equal to 0.015% of P, 0.20-0.40% of Mo, less than or equal to 0.30% of Cr, 0.20-0.50% of Cu, 0.05-0.15% of V, 0.04-0.10% of Nb, 0.03-0.10% of N, 0.02-0.05% of Als, and the balance of Fe and inevitable impurities. The heating temperature of a casting blank is 1150-1250 ℃, the initial rolling temperature of a recrystallization zone is 1050-1150 ℃, the final rolling temperature is 950-1100 ℃, the initial rolling temperature of a non-recrystallization zone is 830-900 ℃, the final rolling temperature is 810-850 ℃, the initial cooling temperature is 800-840 ℃, and the cooling speed is 15-30 ℃/s; the tempering temperature is 560-590 ℃, and the heat preservation time is 1.0-3.0 min/mm. The thickness of the finished steel plate is 80-100 mm, and the medium-temperature and high-pressure resistance performance is excellent.

Description

750 MPa-level medium-temperature high-pressure boiler steel plate and production method thereof

Technical Field

The invention relates to the field of metal materials, in particular to a 750 MPa-level medium-temperature high-pressure boiler steel plate and a production method thereof.

Background

The boiler is an energy conversion device, the energy input to the boiler is in the forms of chemical energy in fuel, electric energy, heat energy of high-temperature flue gas and the like, and steam, high-temperature water or an organic heat carrier with certain heat energy is output outwards after the conversion of the boiler, so that the boiler plays a very important role in national economic development for a long time, and is mainly used in the fields of thermal power stations, ships, locomotives, industrial and mining and the like. The medium-temperature high-pressure boiler container mainly refers to a boiler container facility with the use environment of 6.0MPa < pressure < 13.7MPa and 450 ℃ < temperature < 540 ℃, and because the raw material for manufacturing the boiler container is mainly steel, steel enterprises need to continuously develop new products to meet new requirements of boiler manufacturing industry.

Before the invention, the invention patent of 650 MPa-grade medium-temperature medium-pressure boiler steel plate and a production method thereof (CN107287506A) disclosed before the invention has the performance that the steel plate meets the use environment with medium pressure and low medium temperature, and the pressure-bearing capacity of the steel plate can not meet the requirement in a high-pressure working environment.

The invention discloses a steel for a vanadium-titanium composite treatment boiler and a pressure container and a production method thereof (CN102041448A), which improves the strength of the steel through vanadium-titanium composite treatment, but belongs to a grade with lower strength in the boiler container.

The invention discloses a low-alloy high-strength steel plate for a large-thickness boiler drum and a manufacturing method thereof (CN 103834873A). The disclosed steel plate is 100-150 mm thick, alloy elements such as Cr, Ni, Mo, Nb and the like are added to the components, and various properties of the steel plate are ensured through the traditional die-cast steel ingot, rolling and offline heat treatment processes, so that the process is complex and tedious.

The disclosed 16Mo3 steel plate for boilers or pressure vessels and a production method thereof (CN107746922A) have the performance meeting the requirements of medium and low pressure use environments, but have the pressure bearing capacity which can not meet the requirements in high pressure working environments.

The steel sheet according to the above-mentioned patent is limited to a low strength grade, a relatively low use temperature, a large thickness specification (100mm or more) of the steel sheet, and the like. There is a gap in the development of medium-temperature high-strength or ultra-high-strength medium-thin boiler steel plates.

Disclosure of Invention

In consideration of reduction and low-cost operation of the thermal power station boiler, the invention aims to obtain the medium-thin thickness specification boiler steel plate resistant to medium temperature and high pressure by a brand-new chemical composition design and adopting a production process route different from that of a large-thickness low-grade boiler steel plate.

The chemical component design adopted by the invention is to properly improve the carbon content, add Mo, Cr, Cu alloy elements, V, Nb micro alloy elements and N elements, and match with a proper production process, the developed medium-thin steel plate can bear medium temperature and high pressure, and the requirements of the upgrading process of equipment in a thermal power station on light weight and safety are met.

The specific technical scheme is as follows:

the 750 MPa-level medium-temperature high-pressure boiler steel plate comprises the following chemical components in percentage by mass: 0.15 to 0.23 percent of C, 0.15 to 0.40 percent of Si, 1.20 to 1.60 percent of Mn, less than or equal to 0.005 percent of S, less than or equal to 0.015 percent of P, 0.20 to 0.40 percent of Mo, 0 to 0.30 percent of Cr, 0.20 to 0.50 percent of Cu, 0.05 to 0.15 percent of V, 0.04 to 0.10 percent of Nb, 0.03 to 0.10 percent of N, 0.02 to 0.05 percent of Als, and the balance of Fe and inevitable impurities.

The mechanism of action of the alloy elements in the steel sheet is detailed as follows:

c is the most main element for improving the normal-temperature and high-temperature strength of the steel, the strength of the steel is improved through solid solution strengthening and precipitation strengthening, but the welding performance of the steel is influenced by too high carbon content, and the strength of the steel plate in a medium-temperature and high-pressure environment cannot be ensured by too low carbon content. Therefore, the content of C is set to be 0.15-0.23 percent.

Si is used as a reducing agent and a deoxidizing agent in the steelmaking process, and the combined action of Si and Mo in the invention is mainly reflected in the formation of molybdenum silicide, particularly the formation of molybdenum disilicide, an intermetallic compound, and the atomic combination in the crystal structure of the intermetallic compound has the characteristic of coexistence of metal bonds and covalent bonds, and has excellent high-temperature intrinsic characteristics. Meanwhile, Mo has the functions of solid solution strengthening and precipitation strengthening of MoC, grain refinement can be realized by means of inhibiting austenite grain growth by the MoC, and the improvement of the normal-temperature and high-temperature strength of the steel has a good effect on the high-temperature performance of the steel grade within the temperature range of 450-550 ℃, so that the Si content is limited to 0.15-0.40%, and the Mo content is limited to 0.20-0.40%.

Mn can increase the toughness, strength and hardness of steel, is an element for strongly stabilizing austenite, can effectively reduce the decomposition speed of austenite and improve the hardenability of steel, and the high Mn content can enhance the temper brittleness, so that the Mn content range is 1.20-1.60%.

S and P are harmful elements in steel, increase the brittleness of the steel, particularly grain boundary segregation, influence the heat strength of the steel, so the content of phosphorus and sulfur in the steel is reduced as much as possible, and S, P is respectively controlled to be less than or equal to 0.005 percent of S and less than or equal to 0.010 percent of P in the invention.

Cr is a strong carbide forming element, forms stable carbide with C in steel, and plays a role in improving the strength at normal temperature and high temperature, and the Cr content is set to be 0-0.3% in order to have sufficient strength for pipelines.

Cu improves the strength of steel through solid solution strengthening, and meanwhile, the element has the function of improving hardenability, and in addition, Cu can improve the oxidation corrosion resistance of the steel at high temperature, so the Cu content is limited to 0.20-0.50%.

V is combined with C or N in steel, and has strong precipitation strengthening effect. Compared with Nb, the V-containing carbonitride has low dissolution-precipitation equilibrium temperature, is favorable for remelting and precipitation under the condition of lower heating temperature, and plays a role in pinning grain boundaries. The carbon nitride of Nb can not be melted back and has a tendency to grow up when the two-phase zone is heated, and V can be melted back without the problem, so that the method is more suitable for heating the two-phase zone to realize the effects of grain refinement, precipitation strengthening and the like, and therefore, the content of V is limited to 0.05-0.15 percent.

The effect of Nb on refined grains is very obvious, NbC strain is induced to precipitate in the hot rolling process to hinder the recovery and recrystallization of deformed austenite, and the deformed austenite structure rolled in a non-recrystallization area is converted into a fine phase-change product during phase change through controlled rolling and controlled cooling so as to ensure that the steel has high strength and high toughness.

N is one of inevitable impurity elements in steel, and the content and precipitation strengthening are not easy to be too high, but N and nitrides such as Nb, V, Al and the like form element combination and can play the roles of pinning grain boundary refined grains and precipitation strengthening, so that the lower the content of the N element in the steel is, the better the content of the N element in the steel is, and the content of the N element in the steel is limited to 0.03-0.10 percent.

Als is a commonly used deoxidizer in steel. A small amount of aluminum is added into the steel, so that crystal grains can be refined, the impact toughness is improved, and the hot workability, the welding performance and the cutting processability of the steel are influenced if the aluminum content is too high, so that the content of Als is limited to 0.02-0.05 percent.

A production process of a 750 MPa-level medium-temperature high-pressure boiler steel plate comprises the following steps: the method comprises the following steps of molten iron pretreatment, converter smelting, LF-RH, continuous casting, casting blank heating, rolling, heat treatment and inspection. The method specifically comprises the following steps:

(1) the smelting process comprises the following steps: carrying out molten iron desulphurization pretreatment, selecting clean scrap steel to ensure that the alloy is clean and dry, stopping slag and tapping, requiring the thickness of a slag layer to be 60-80 mm, carrying out LF refining deep desulphurization treatment to produce reducing slag and ensure that impurities float sufficiently; feeding silicon and calcium into the LF furnace at a linear speed of 2-4 m/s and a linear feeding amount of 3-5 m/t; the RH deep treatment cycle time is 10-15 min.

(2) The casting process comprises the following steps: and (3) casting by adopting a continuous casting machine after vacuum breaking, wherein the superheat degree of molten steel is 20-30 ℃, and the casting process needs to be stable and constant. And (4) inserting the casting blank into a stack for slow cooling, wherein the stacking time for slow cooling is 30-40 h.

(3) A casting blank heating process: by controlling the heating process of the casting blank, the alloy elements are ensured to be fully dissolved in the solid solution, the growth of original austenite grains is effectively inhibited, the heating temperature of the casting blank is controlled to 1150-1250 ℃, the heating time is 4-6 hours, and the soaking time is 0.5-1.0 hour.

(4) The rolling process comprises the following steps: the rolling starting temperature of a recrystallization zone is 1050-1150 ℃, the rolling finishing temperature is 950-1100 ℃, the minimum deformation rate of a single pass is 15-25%, the total deformation rate is more than or equal to 60%, and the thickness of an intermediate blank is 3-4 times that of a finished steel plate; the rolling starting temperature of the non-recrystallization zone is 830-900 ℃, the rolling finishing temperature is 810-850 ℃, and the accumulated deformation rate is more than or equal to 65%. And cooling by adopting ACC after rolling, wherein the start cooling temperature is 800-840 ℃, the red returning temperature is 400-500 ℃, and the cooling speed is 15-30 ℃/s. The design of the parameters is determined based on the components and performance requirements of steel grades, the high-temperature strength is an important performance index because the steel is used for manufacturing a medium-temperature high-pressure boiler, the steel has certain C, Mn basic elements for improving the high-temperature strength, and V, Nb and N alloy elements are added, the grain refining effect can be furthest exerted through an optimal TMCP process, relatively refined pearlite and bainite structures are obtained, the composite structure is an important factor for ensuring the high-temperature strength performance of the steel plate, and the thickness of a finished product is 80-100 mm.

(5) A heat treatment process: although crystal grains of the steel plate rolled by TMCP are relatively refined, the steel plate still has a metastable stable structure, has certain structural stress and is unfavorable for high-temperature performance, and a more stable sorbite and tempered bainite structure is obtained by medium-high temperature tempering treatment, wherein the sorbite structure accounts for 60-70 percent, and the tempered bainite structure accounts for 30-40 percent. The tempering temperature is 560-590 ℃, the heat preservation time is 1.0-3.0 min/mm, the tempering temperature is not too high, the time is too long, the spheroidization of flaky carbides in sorbite is prevented, the precipitation at crystal boundary is prevented, and the heat strength of the steel is reduced.

Compared with the prior art, the invention has the following beneficial effects:

the invention has the beneficial effects that through a brand-new chemical component design and by adopting a TMCP (thermal mechanical control processing) and tempering production process route, the reduced boiler steel plate with the thin specification (the thickness of 80-100 mm) of medium temperature resistance and high pressure resistance is developed, and the reduced boiler steel plate has higher normal temperature strength, plasticity and toughness and medium temperature strength, R_eL≥450MPa、730MPa≤R_m≤830MPa，A≥21％、(0℃)KV₂Middle temperature interval steel plate R of more than or equal to 230J and 450-550 DEG C_p0.2Not less than 350MPa, wherein R at 450 DEG C_p0.2R is more than or equal to 370MPa at 500 DEG C_p0.2R is more than or equal to 360MPa at 550 DEG C_p0.2The microstructure is a sorbite and tempered bainite structure and the like, and the pressure is more than or equal to 350 MPa.

Detailed Description

The following examples are intended to illustrate the invention in detail, and are intended to be a general description of the invention, and not to limit the invention.

Table 1 shows the chemical composition of the steels of the examples of the present invention; table 2 shows steelmaking process parameters of the steels of the examples of the present invention; table 3 shows the rolling process parameters of the steels of the examples of the present invention; table 4 shows the heat treatment process parameters of the steels of the examples of the present invention; table 5 shows the mechanical properties of the steels of the examples.

TABLE 1 chemical composition wt% of steel sheet of inventive example

TABLE 2 Steel plate production steelmaking Process parameters of the examples

TABLE 3 examples rolling process parameters for steel plate production

TABLE 4 examples steel plate production heat treatment process parameters and metallographic structure

TABLE 5 mechanical properties of the examples

According to the results, the steel plate for the medium-temperature high-pressure boiler, provided by the invention, has the normal temperature R_eL≥535MPa，R_m≥740MPa，(0℃)KV₂More than or equal to 230J; r at 450 DEG C_p0.2R is more than or equal to 375MPa at 500 DEG C_p0.2R is more than or equal to 365MPa at 550 DEG C_p0.2More than or equal to 355MPa, and excellent medium temperature and high pressure resistance.

Claims

1. The 750 MPa-level medium-temperature high-pressure boiler steel plate is characterized in that the steel comprises the following chemical components in percentage by mass: 0.15 to 0.23 percent of C, 0.15 to 0.40 percent of Si, 1.20 to 1.60 percent of Mn, less than or equal to 0.005 percent of S, less than or equal to 0.015 percent of P, 0.20 to 0.40 percent of Mo0.30 percent of Cr, 0.20 to 0.50 percent of Cu, 0.05 to 0.15 percent of V, 0.04 to 0.10 percent of Nb, 0.03 to 0.10 percent of N, 0.02 to 0.05 percent of Als, and the balance of Fe and inevitable impurities.

2. The steel plate for a 750MPa grade medium temperature and high pressure boiler as claimed in claim 1, wherein the thickness of the steel plate is 80-100 mm, the normal temperature tensile strength is 730-830 MPa, and the steel plate R is in the medium temperature range of 450-550 DEG C_p0.2≥350MPa。

3. A method for producing a 750MPa grade medium temperature and high pressure boiler steel plate according to claim 1 or 2, the steel plate is produced by the following steps: the method comprises the steps of molten iron pretreatment, converter smelting, LF-RH, continuous casting, casting blank heating, rolling, heat treatment and inspection, and is characterized in that,

(1) the smelting process comprises the following steps: carrying out molten iron desulphurization pretreatment, selecting clean scrap steel to ensure that the alloy is clean and dry, stopping slag and tapping, requiring the thickness of a slag layer to be 60-80 mm, carrying out LF refining deep desulphurization treatment to produce reducing slag and ensure that impurities float sufficiently; feeding silicon and calcium into the LF furnace at a linear speed of 2-4 m/s and a linear feeding amount of 3-5 m/t; the RH deep treatment cycle time is 10-15 min;

(2) the casting process comprises the following steps: casting by using a continuous casting machine after vacuum breaking, wherein the superheat degree of molten steel is 20-30 ℃, the casting process needs to be stable and constant, the casting blank is inserted into a stack for slow cooling after being taken off the line, and the stack slow cooling time is 30-40 h;

(3) a casting blank heating process: controlling the heating temperature of the casting blank to 1150-1250 ℃, heating for 4-6 h, and soaking for 0.5-1.0 h;

(4) the rolling process comprises the following steps: the rolling starting temperature of a recrystallization zone is 1050-1150 ℃, the rolling finishing temperature is 950-1100 ℃, the minimum deformation rate of a single pass is 15-25%, the total deformation rate is more than or equal to 60%, and the thickness of an intermediate blank is 3-4 times that of a finished steel plate; the rolling starting temperature of the non-recrystallization zone is 830-900 ℃, the rolling finishing temperature is 810-850 ℃, and the accumulated deformation rate is more than or equal to 65%; cooling by adopting ACC after rolling, wherein the start cooling temperature is 800-840 ℃, the red returning temperature is 400-500 ℃, and the cooling speed is 15-30 ℃/s;

(5) a heat treatment process: the tempering temperature is 560-590 ℃, and the heat preservation time is 1.0-3.0 min/mm.