CN109266971B

CN109266971B - Reheating crack resistant W-containing high-strength low-alloy heat-resistant steel

Info

Publication number: CN109266971B
Application number: CN201811462079.4A
Authority: CN
Inventors: 王学; 李勇; 李健; 杨华春; 敬仕煜; 夏彩荥
Original assignee: Wuhan University WHU
Current assignee: Wuhan University WHU
Priority date: 2018-11-30
Filing date: 2018-11-30
Publication date: 2020-10-13
Anticipated expiration: 2038-11-30
Also published as: US20200173001A1; JP2020084314A; CN109266971A; JP6703630B1

Abstract

The invention provides reheating crack resistant W-containing high-strength low-alloy heat-resistant steel which comprises the following chemical components in percentage by mass: c: 0.04-0.11%, Si: 0.50% or less, Mn: 0.10-0.60%, P: 0.03% or less, S: 0.01% or less, Ni: 0.40% or less, Cr: 1.90-2.60%, V: 0.20 to 0.30%, Nb: 0.02 to 0.08%, Mo: 0.05-0.30%, W: 1.45-1.75%, Ti: 0.01-0.06%, B: 0.001 to 0.012%, Al: 0.03% or less, N: less than 0.01 percent, and the balance of Fe and inevitable impurities. The heat-resistant steel has excellent reheat crack resistance, and can be applied to high-temperature components of supercritical (super) critical thermal power generating units.

Description

Reheating crack resistant W-containing high-strength low-alloy heat-resistant steel

Technical Field

The invention belongs to the technical field of heat-resistant steel, and particularly relates to high-strength low-alloy heat-resistant steel which has excellent reheat crack resistance and is insensitive to intercrystalline cracks formed in a coarse grain heat affected zone in a postweld heat treatment or high-temperature service process.

Background

In order to reduce the environmental pollution caused by coal-fired power generation, efficient and clean supercritical (supercritical) thermal power generating units need to be developed. At present, the main way of improving the efficiency of a thermal power plant is to improve the temperature and the pressure of steam, which puts higher requirements on the high temperature resistance and the high pressure resistance of materials, and high-strength-grade heat-resistant steel needs to be developed to meet the requirement of improving boiler parameters.

Currently, there are two directions for the development of steel for boilers: one is ferritic heat-resistant steel, and the other is austenitic heat-resistant steel. Compared with austenitic heat-resistant steel, ferritic heat-resistant steel has the advantages of low cost, good heat conductivity, small thermal expansion rate, good thermal fatigue resistance and the like, so that the ferritic heat-resistant steel is widely applied to the ultra (supercritical) thermal power generating units. The development of the ferrite heat-resistant steel is divided into two main lines, one is that the content of a main heat-resistant alloy element Cr is gradually increased, and a stable tempered martensite structure is formed after proper heat treatment, such as 9-12% Cr medium alloy heat-resistant steel like T/P91, T/P92, T/P122, E911 and the like; and secondly, keeping the lower Cr content, adding alloy elements such as V, Nb, Mo, W and the like, and forming a stable bainite structure after proper heat treatment, such as novel low-alloy heat-resistant steel containing 2.25% of Cr, such as T/P23, T/P24 and the like. As one of the developments of ferritic heat-resistant steels, the high-temperature creep strength of the novel low-alloy bainitic heat-resistant steel is greatly improved compared with that of the conventional low-alloy heat-resistant steel, such as 2.25Cr-1.6WVNb steel (T23) containing W developed by Sumitomo and Mitsubishi heavy industry and 2.25Cr-1MoVTi steel (T24) containing Mo developed by Waulrex Mannesmann tubes, Germany, and the creep rupture strength at 600 ℃ of the novel low-alloy bainitic heat-resistant steel is 1.8 times that of the conventional 2.25Cr-1Mo steel (T22) and is close to that of T/P91 steel. The novel low-alloy heat-resistant steel also reduces the carbon content, so that the weldability is obviously improved, and the novel low-alloy heat-resistant steel is very suitable for manufacturing parts such as a water-cooled wall, a superheater, a reheater and the like of a supercritical (supercritical) boiler, and can also be used for manufacturing large-caliber thick-wall pipelines such as a header, a main steam pipe and the like.

However, in recent years, it has been found that the novel low-alloy heat-resistant steel such as T23 has a significant reheat cracking tendency, many leakage accidents caused by reheat cracking occur, the safe and stable operation of the unit is seriously affected, and a great economic loss is caused. Reheat cracking refers to an intergranular crack in the Coarse Grain Heat Affected Zone (CGHAZ) of a welded joint of metallic material that develops as a result of stress relaxation during post-weld heat treatment or high temperature service, and is also known as post-weld heat treatment crack or stress relaxation crack. The hardness of the heat affected zone of T23 and T24 steel in a welding state can be controlled below 350HV, and when a small-diameter pipe (the wall thickness is less than or equal to 10mm) is welded, cold cracks can not be generated even if postweld heat treatment is cancelled. However, reheat cracking may still occur under high temperature service conditions. The obvious reheat cracking tendency becomes a bottleneck problem for restricting the use of novel low-alloy heat-resistant steel such as T23, T24 and the like.

Low alloy steel such as Cr-Mo, Cr-Mo-V and the like has reheat crack tendency, and related research is reported in the last 60 years. The mechanism of reheat cracking of low alloy heat resistant steel has been extensively studied, and various low alloy heat resistant steels having susceptibility to reheat cracking have been developed. The prior art comprises the following technologies: such as a steel for high-temperature and high-pressure vessels, which has excellent creep brittleness resistance and reheat cracking resistance, Ca, Mg and rare earth elements are added to suppress the harmful effects of Cr, V, B, etc. on reheat cracking. A Cr-Mo steel having excellent reheat cracking resistance by controlling S, Al and Ca contents. The ferritic steel electric resistance welding boiler steel has excellent reheating crack resistance of welding seams. A Cr-Mo steel sheet having excellent strength, toughness, low-temperature crack resistance and reheat crack resistance. The steels are all traditional low-alloy heat-resistant steels and can be used for pressure vessels or subcritical and critical thermal power generating units with lower parameters. However, the novel low-alloy heat-resistant steel T23 for the supercritical (super) critical unit has high Cr content, and is added with precipitation strengthening elements such as W, V, Nb and Ti, and the reheat cracking tendency is more serious. As to Cr-Mo type low alloy heat resistant steel containing W, a heat resistant steel, it is pointed out that reheat cracking can be prevented in the case where the relation% N.ltoreq.% Ti +5 (% B) +0.004 is satisfied and the average grain size is less than 110 μm. However, recent studies have found that T23 steel satisfying the above relationship still has significant reheat crack susceptibility. Researches show that the reheating crack generation mechanism of the multi-element composite reinforced novel low-alloy heat-resistant steel is different from that of the traditional low-alloy heat-resistant steel, and is not caused by the segregation of N in the grain boundary described in the 'one kind of heat-resistant steel'.

Furthermore, there are also methods or measures proposed to avoid reheat cracking, as in the prior art: a method for eliminating reheating crack of welding seam features that the adjacent area of welding seam is locally heated to a temp higher than 400 deg.C (preferably 600 deg.C) before welding or after welding for cooling. And one-time fixed welding is carried out after all welding passes are finished by reserving the reheating crack prevention welding passes on the surface so as to prevent reheating cracks from generating and avoid generating notches at the toe parts to cause notch sensitivity. The heat-affected zone is cooled at a high cooling rate to have a martensite content of more than 80% so that the heat-affected zone is easily softened during annealing to prevent the occurrence of reheat cracks. A method for preventing the reheating cracks of the fillet weld of a pipe seat of a header can effectively prevent the reheating cracks of the fillet weld of the pipe seat of the header by applying a weld toe pressure channel weld bead of the fillet weld of the pipe seat. A method for eliminating reheating cracks of spot welding plate joints is mainly characterized in that a mold is used for carrying out spherical pressurization on welding spots of the joints and surrounding areas before postweld heat treatment to eliminate residual stress, and the spherical pressurization means that the surfaces of the mold, which are in contact with the spot welding joints, are spherical surfaces or curved surfaces similar to the spherical surfaces. In addition, it has been proposed to suppress reheat cracking by means of a severe welding process such as raising the preheating temperature, taking a multi-layer bead or a tempering bead, or adding an intermediate heat treatment, which increases the number of processes, increases the labor intensity, reduces the welding efficiency, greatly increases the production cost, and has an unsatisfactory effect of preventing reheat cracking, and thus the problem has not been solved fundamentally.

Aiming at the problem, the invention provides novel W-containing low-alloy heat-resistant steel with reheat cracking resistance, which maintains excellent high-temperature creep strength of T23 steel, has high reheat cracking resistance in a coarse grain heat affected zone, and is insensitive to reheat cracking during postweld heat treatment or service at the temperature of 500-750 ℃.

Disclosure of Invention

Aiming at the problems in the prior art, the technical scheme adopted by the invention for solving the problems in the prior art is as follows:

the W-containing high-strength low-alloy heat-resistant steel capable of resisting reheat cracks is characterized by comprising the following elements in percentage by mass: c: 0.04-0.11%, Si: 0.50% or less, Mn: 0.10-0.60%, P: 0.03% or less, S: 0.01% or less, Ni: 0.40% or less, Cr: 1.90-2.60%, V: 0.20 to 0.30%, Nb: 0.02 to 0.08%, Mo: 0.05-0.30%, W: 1.45-1.75%, Ti: 0.01-0.06%, B: 0.001 to 0.012%, Al: 0.03% or less, N: 0.01% or less, wherein the contents of C and B satisfy the following formula:

[％B]>-1.2×[％C]²+0.30×[％C]-0.01 (1)

the balance of Fe and inevitable impurities.

Further, the content of C in the heat-resistant steel is as follows: 0.04-0.08%, wherein the content of B in the heat-resistant steel is as follows: 0.004-0.01%.

Further, the content of C in the heat-resistant steel is as follows: 0.04-0.08%, wherein the content of B in the heat-resistant steel is as follows: 0.004-0.008%.

Further, the content of B in the heat-resistant steel is as follows: 0.004-0.012%.

Further, the content of B in the heat-resistant steel is as follows: 0.006-0.010%.

Further, the contents of C and B in the heat-resistant steel satisfy the following formula:

[％B]>-1.4×[％C]²+0.35×[％C]-0.0115 (2)。

the W-containing high-strength low-alloy heat-resistant steel capable of resisting reheat cracks is characterized by comprising the following elements in percentage by mass: c: 0.04-0.08%, Si: 0.50% or less, Mn: 0.10-0.60%, P: 0.03% or less, S: 0.01% or less, Ni: 0.40% or less, Cr: 1.90-2.60%, V: 0.20 to 0.30%, Nb: 0.02 to 0.08%, Mo: 0.05-0.30%, W: 1.45-1.75%, Ti: 0.01-0.06%, B: 0.001 to 0.012%, Al: 0.03% or less, N: 0.01% or less, wherein the contents of C and B satisfy the following formula:

[％B]>-1.4×[％C]²+0.35×[％C]-0.0115 (2)

the balance of Fe and inevitable impurities.

Further, the content of B in the heat-resistant steel is as follows: 0.004-0.01%.

Further, the content of B in the heat-resistant steel is as follows: 0.006-0.01%.

The reasons for the action of each element and the range thereof of the steel according to the present invention will be explained below. Unless otherwise specified,% of chemical composition means mass%.

C：0.04～0.11％

C forms carbides in steel, contributing to high-temperature strength, and in addition, it contributes to improvement of hardenability, avoiding formation of ferrite. Therefore, the content of C must be at least 0.04%. However, too much C increases the hardness of the weld heat affected zone, increasing the susceptibility to cold cracking, particularly reheat cracking. In addition, high C content steels become brittle when used at high temperatures for extended periods of time. Therefore, the upper limit of the C content is 0.11%, preferably 0.04 to 0.08%.

Si: less than 0.50%

Si is used as a deoxidizing element in the production of steel. It is also effective for improving oxidation resistance and high temperature corrosion resistance of steel, however, excessive Si content results in a decrease in creep plasticity and toughness during long-term use at high temperatures. Therefore, the upper limit of the Si content is 0.50% and the lower limit is the inevitable impurity content level. In order to ensure the deoxidation effect, the preferable range is 0.10 to 0.30%.

Mn：0.10～0.60％

As with Si, it is added as a deoxidizer, but too much addition results in creep embrittlement and a decrease in toughness. Therefore, the Mn content is at most 0.60%. In order to ensure the deoxidation effect, the preferable range is 0.20 to 0.50%.

P: less than 0.03%

P is present as an inevitable impurity in steel. If the content is high, reheat cracking is easily caused. Therefore, the P content is at most 0.03%. The content of P is preferably as low as possible, so there is no particular lower limit thereto. However, the production cost is increased by excessively lowering the P content, and it is preferably 0.001 to 0.01%.

S: less than 0.01%

S is present as an inevitable impurity as well as P. S is likely to segregate in the CGHAZ, resulting in the occurrence of reheat cracking. Therefore, the S content is limited to 0.01% or less. The content of S is preferably as low as possible, so there is no particular lower limit thereto. However, as in the case of P, an excessive reduction in S content leads to an increase in production cost, and is preferably 0.002 to 0.006%.

Ni: less than 0.4%

Ni is an austenite forming element. It inhibits the formation of the ferrite phase and ensures the stability of the tissue. The excessive addition of Ni lowers the plasticity during use at high temperatures, so that the Ni content is limited to 0.4% or less.

Cr：1.90～2.60％

Cr is indispensable for ensuring high-temperature oxidation resistance, high-temperature corrosion resistance and high-temperature strength. However, excessive addition thereof causes coarsening of carbides, eventually causing a decrease in high-temperature strength and a decrease in toughness. Therefore, the Cr content is limited to 1.90 to 2.60%.

V：0.20～0.30％

V forms fine carbides or carbonitrides in the steel, contributing to the improvement of creep strength. However, excessive addition thereof results in an increase in the growth rate of carbides, premature aggregation coarsening, premature disappearance of dispersion strength thereof and a decrease in toughness. Further, the excessive addition of V increases the precipitation density of carbonitride in the grains at the time of the post-weld heat treatment, increasing the reheat crack sensitivity. Therefore, the V content is limited to 0.20 to 0.30%.

Nb：0.02～0.08％

Nb forms fine, stable carbides or carbonitrides in the steel, contributing to the improvement of creep strength. Therefore, it is necessary to add at least 0.02% of Nb. However, excessive addition of Nb causes an increase in the carbide growth rate, premature aggregation coarsening, premature disappearance of dispersion strength, and a decrease in toughness. Therefore, the Nb content is limited to 0.02 to 0.08%.

Mo：0.05～0.35％

Mo increases the solid solution strength of the steel matrix, and precipitates as carbides to increase creep strength. In addition, it has a strong affinity for P, reducing the amount of P segregated at grain boundaries, so it contributes to reducing reheat cracking sensitivity. Therefore, it is necessary to make the content thereof not less than 0.05%. However, the excessive addition thereof decreases the toughness after long-term use, so the upper limit thereof is 0.35%.

W:1.45～1.75％

Like Mo, W increases the solid solution strengthening effect of the matrix and forms carbides to improve creep strength. In order to obtain these effects, the content thereof must be at least 1.45%. However, excessive addition results in coarse intermetallic compounds formed during service, resulting in a decrease in toughness. Therefore, the content thereof is limited to less than 1.75%.

Ti：0.01～0.06％

The addition of Ti fixes N and prevents the combination of N and B, which improves hardenability, prevents ferrite from occurring to reduce the room-temperature tensile strength, and improves the grain boundary strength and plasticity at high temperature. However, too high a Ti content lowers strength and toughness and increases reheat cracking sensitivity. Therefore, the Ti content is in the range of 0.01 to 0.06%.

B：0.001～0.012％

B can improve creep strength and creep rupture plasticity. In addition, in the postweld heat treatment process, B is segregated in CGHAZ crystal boundary, so that the crystal boundary plasticity can be improved, the precipitation growth and coarsening of carbide can be inhibited, the crystal boundary weakening is prevented, and the reheat crack sensitivity is reduced. To be effective, the B content and C content must satisfy [% B [ ]]>-1.2×[％C]²+0.30×[％C]-0.01. Preferably, when the B content and the C content satisfy [% B]>-1.4×[％C]²+0.35×[％C]And-0.0115 can achieve better reheat cracking resistance. Too high B content causes a significant deterioration in hot workability of the steel and also causes temper brittleness. In comprehensive consideration, the content of B element is preferably 0.001-0.012%, more preferably 0.004-0.010%, and most preferably 0.006-0.010%.

Al: less than 0.03%

Although Al is contained as a deoxidizer, excessive content of Al causes reduction in creep plasticity and toughness, and therefore the Al content is limited to 0.03% or less.

N: less than 0.01%

The solid solution of N in the matrix is detrimental to toughness and creep strength. In addition, an excessive amount of N forms a compound with B, which is not favorable for the B action. Therefore, the N content is limited to 0.01% or less.

The invention has the following advantages:

the purpose of inhibiting the reheating cracks of the low-alloy heat-resistant steel can be realized only by adjusting the contents of the C element and the B element, and the W-containing high-strength low-alloy heat-resistant steel resisting the reheating cracks is provided. Compared with the method of preventing reheating cracks by adopting harsh welding process and the like, the method provided by the invention fundamentally solves the problem of sensitivity of reheating cracks, does not cause additional cost increase on production, and has a more reliable result.

Drawings

FIG. 1 shows M in 2.25Cr-1.6WVNbNB steel₂₃C₆And MX type carbides as a function of carbon content;

FIG. 2 is a microstructure of steel;

FIG. 3 is a plug test specimen shape;

FIG. 4 is a simulated thermal cycle curve and sample shape and size;

fig. 5 is a graph showing the relationship between the B content and the C content in examples of the heat-resistant steel according to the present invention.

Wherein: 1-bottom plate, 2-bolt, 3-loading direction.

Detailed Description

The technical scheme of the invention is further described in detail by the following embodiments and the accompanying drawings:

the invention aims to provide novel low-alloy heat-resistant steel with reheat crack resistance, which greatly improves the reheat crack resistance on the basis of keeping the excellent high-temperature creep strength and low welding cold crack sensitivity of the novel low-alloy heat-resistant steel. The steel can be safely and reliably used in a supercritical (super) critical thermal power plant.

The influence of chemical components on the reheat crack sensitivity of the low-alloy heat-resistant steel is the largest. C and alloy elements such as W, Mo, V, Nb, Ti and the like all increase the reheat cracking tendency, but in the case of heat-resistant steel, W, Mo is solid-solution-strengthened in a matrix, and V, Nb, Ti and the like form fine dispersion carbonitride to provide precipitation strengthening, so that the elements are key elements for improving the high-temperature creep strength of the material. In order to obtain sufficient creep strength at high temperatures, it is necessary to add certain amounts of these alloying elements. The inventor researches a reheating crack generation mechanism of the novel low-alloy heat-resistant steel T23, and finds that with the development of smelting technology, the content of impurity elements of the novel heat-resistant steel can be controlled in a lower range and is no longer a main factor for reheating crack generation, and the precipitation of alloy carbide in the post-weld heat treatment process has a remarkable influence on the reheating crack generation. The method is different from the prior point that impurity elements are emphasized to be precipitated at the grain boundary of the traditional low-alloy heat-resistant steel, and the reheating crack of the traditional low-alloy heat-resistant steel is generated due to the weakening of the grain boundary, so that a thought is provided for developing novel low-alloy heat-resistant steel resisting the reheating crack.

The inventor researches on the mechanism of the CGHAZ reheat cracking of T23 steel to find that:

(1) most of the CGHAZ carbide is dissolved in the matrix during welding, and M containing Fe, Cr, W, Mo and other elements is precipitated in the grain boundary during postweld heat treatment₂₃C₆Form carbide, and grow up and coarsen rapidly, so that the bonding force between grain boundaries is reduced.

(2) Coarse grain boundary M₂₃C₆The massive precipitation of type carbides in a short time leads to a depletion of the nearby matrix alloying elements, thereby forming a softened region where strain preferentially accumulates under stress.

(3) Noncoherent M₂₃C₆Carbide also promotes nucleation of creep voids at grain boundaries, further accelerating grain boundary weakening.

(4) During short-term postweld heat treatment, the carbides precipitated in the crystal are mainly M₂₃C₆And M₇C₃The size of the alloy is small, and the alloy has certain strengthening effect, so that the strength in the crystal is improved.

(5) The strength in the crystal is high, the crystal boundary is obviously weakened, the crystal boundary in a coarse crystal area is preferentially deformed under the action of welding stress and thermal stress, holes are formed, microcracks are formed by aggregation, and finally, the crystal fracture is caused by expansion.

Carbide formed by elements such as Cr, W, Mo, and the like has a large influence on the reheat cracking of such heat-resistant steel, while elements such as V, Nb, Ti, and the like have a small influence on the reheat cracking. Therefore, from the viewpoint of suppressing reheat cracking, it is essential to control the CGHAZ grain boundaries and intragranular M₂₃C₆Precipitation of carbide. Carbon is forming M₂₃C₆The higher the carbon content of the steel, the more the CGHAZ is heat-treated after weldingThe more carbon there will be in the process to form carbides. Therefore, limiting the carbon content limits the amount of CGHAZ carbide precipitated. FIG. 1 shows the calculation of M in a typical T23 steel by Thermo-calc software₂₃C₆And MX carbide content as a function of carbon content (wherein the total amount of the alloy system is 1mol, M is suppressed in calculation)₆Phase C is formed, giving metastable results). As can be seen from this, MX and M₂₃C₆The phase contents all increase linearly with increasing C content, but M₂₃C₆The increase in phase content is significantly greater than that of the MX phase. When the C content is increased by 0.02%, 1mol of M in the steel₂₃C₆The type carbide increased by about 0.003mol, while the MX phase only increased by about 0.0002mol, less than 1/10 for the former. Namely, the reduction of C content can greatly reduce M₂₃C₆Content of type carbide, but less influence on MX type carbide. The MX phase is a main precipitation strengthening phase in the crystal and is very important for maintaining the high-temperature creep strength, the influence on the MX phase is small by properly controlling the carbon content, and the strengthening effect of the MX phase in the crystal can be still maintained, so that the high-temperature creep strength is ensured. Therefore, it is appropriate to limit the carbon content appropriately from the viewpoint of reducing the grain boundary carbide content and alleviating the weakening of the grain boundary during the CGHAZ post-weld heat treatment.

In addition, grain boundary carbides cause grain boundary weakening, which is related to the size and distribution thereof, and coarse carbides cause reduced coherence, and cause severe depletion of matrix alloying elements near the grain boundaries, thereby increasing grain boundary weakening. The fine carbides cause a much more slight decrease in the compatibility and element depletion. T23 steel has high susceptibility to reheat cracking due to M precipitated by grain boundaries during post-weld heat treatment of its CGHAZ₂₃C₆The type carbide is easy to aggregate and coarsen. The element B is easy to segregate in the steel in crystal boundary vacancies, inhibits the segregation of impurity elements, reduces the activation energy of the crystal boundary, plays a role in purifying the crystal boundary and strengthening the crystal boundary, and improves the plasticity of the crystal boundary; b can also enter M₂₃C₆In phase, a more stable M is formed₂₃(C，B)₆And inhibiting coarsening thereof. The addition of B is related to the C content in the steel, and the higher the C content is, the more B needs to be added to inhibit M₂₃C₆The aggregation of the phases coarsens.

The main idea of the invention is therefore to control the C content and B content of the steel. The design can reduce the grain boundary precipitation M in the process of postweld heat treatment₂₃C₆FIG. 5, which is obtained from the data of the following examples, shows the relationship between the B content present in the steel and the occurrence of reheat cracking with respect to the C content, in FIG. 5, the abscissa is the C content of the steel according to the present invention or comparative example, and the ordinate is the B content of the steel, open circles (○) indicate the steel in which reheat cracking does not occur, and closed circles (●) indicate the steel in which reheat cracking occurs, from which it is determined if [% C is present in the C content [% C%]And B content [% B]The relationship therebetween satisfies the following formula:

[％B]>-1.2×[％C]²+0.30×[％C]-0.01 (1)

reheat cracking can be prevented.

Steels having chemical compositions shown in Table 1 (containing impurities of Sn, As, Sb, Bi, Pb, etc. in a total amount of at most 0.04%) were prepared. The billet is obtained by refining outside an electric arc furnace and vacuum degassing treatment or smelting by an electroslag remelting method. Manufacturing bars by hot forging, wherein the specification is 55mm multiplied by 800 mm; the pipes with various specifications are manufactured by perforation and hot rolling, the outer diameter of the pipe covers various sizes from 38.1mm to 63.5mm, and the wall thickness covers various sizes from 4.5mm to 10 mm. And (4) carrying out normalizing and tempering heat treatment on the plate and the pipe. Normalizing at 1060 ℃, keeping the temperature for 2h, and then cooling in air; the tempering temperature is 760 ℃, and the furnace is cooled after heat preservation for 1 h. The microstructure of the steel of the present invention is all bainite as shown in FIG. 2.

Samples were taken for testing to evaluate reheat cracking susceptibility. The invention adopts 3 methods to evaluate the reheat crack sensitivity of steel, which are respectively as follows: the method comprises the following steps of performing post-weld heat treatment test, bolt test and short-time high-temperature creep rupture test on T23 pipe joints on water walls and header pipe seats.

The process of the actual post-weld heat treatment test of the joint comprises the following steps: welding the steel pipe according to actual production conditions, performing heat treatment at 730 +/-10 ℃ for 0.5 hour after welding to eliminate residual stress, performing magnetic powder and X-ray flaw detection on the joint after heat treatment, and detecting whether cracks are generated on the surface and inside of the joint.

The process of plugging the sample is as follows: the bolt 2 and base plate 1 samples shown in fig. 3 were machined, the bolt 2 was fitted into the central hole of the base plate 1, and a weld was applied to the base plate. The welded pins and base plate were placed for 24 hours to eliminate the effect of cold cracking. And then, mounting the test piece on a plug testing machine, heating to a testing temperature, preserving heat for 15min, and then loading a certain initial stress. By the time of fracture (t)_f) To determine reheat cracking susceptibility, t_f<24h, sensitivity; t is t_f>24h, insensitivity. The shorter the break time, the more sensitive it is to reheat cracking.

The short-time high-temperature creep rupture test comprises the following processes: the sample shown in fig. 4 was processed with the middle 10mm range as the gauge length portion, and the welding simulation test of the coarse crystal region was completed on a thermal simulator. The welding simulated thermal cycle curve is shown in fig. 3, and the simulated welding process comprises the following steps: TIG welding, preheating at 100 ℃, and the heat input is 25 kJ/cm. And (3) heating the sample to a test temperature of 500-750 ℃ after cooling, applying a constant strain rate of 0.5mm/min for stretching after heat preservation for 5s until the sample is broken, and measuring the reduction of area of the sample after the sample is broken. The magnitude of the material reheat cracking sensitivity is generally judged by the magnitude of the reduction of area (Z) of the broken sample according to the following criteria: 1) very sensitive, Z < 5%; 2) sensitive, 5% < Z < 10%; 3) slightly sensitive, 10% < Z < 20%; 4) insensitive, Z > 20%. In the present invention, the cracking was observed at all test temperatures Z of more than 20% without cracking, and the remainder with cracking.

Table 1 chemical composition (wt.%) of the example steels

In the prepared steels, 1-13 # are comparative examples, the components are mostly in the range specified by T23, and the contents of B and C are not satisfied [% B [ ]]>[％B]>-1.2×[％C]²+0.30×[％C]-0.01 relationship. As can be seen from Table 2, the No. 1-3 pipes cracked in the pipe joint postweld heat treatment test; 4-6 # is evaluated by a bolt test and is sensitive to reheat cracks within a wider heat treatment temperature range; 7-9 # is evaluated by a bolt test and a short-time creep rupture test of a simulated coarse crystal region, and is sensitive to reheat cracks within a wider heat treatment temperature range; cracking the No. 10 pipe joint in a pipe joint postweld heat treatment test, and evaluating the sensitivity of the pipe joint to reheat cracks at 550-750 ℃ by a simulated coarse-grain region short-time creep rupture test; the 11-13 # is evaluated by a short-time creep rupture test of a simulated coarse crystal region and is sensitive to reheat cracks within a certain heat treatment temperature range. The 3 test methods are used for evaluating that the steels have high reheat cracking tendency, reheat cracking is easily generated in a coarse crystal area in the postweld heat treatment or service process, and the reheat cracking resistance is poor.

14 to 32# are steels designed according to the present invention, and the composition satisfies [% B [ ]]>-1.2×[％C]²+0.30×[％C]-0.01. No. 14-16 cracks are not generated in the actual post-welding heat treatment test process of the joint, and no cracks are found on the surface layer and the inner part of the joint in nondestructive inspection. No. 17-18 actual joints are not cracked in the post-welding heat treatment test process, and nondestructive inspection shows that no cracks are generated on the surface layer and inside of the pipe joint; the heat treatment temperature range is wide (500-750 ℃), and the heat treatment temperature range is not sensitive to reheat cracking. The No. 19-20 is evaluated by a plug test and is insensitive to reheat cracks within the temperature range of 500-750 ℃. 21-32 # is evaluated by a short-time creep rupture test simulating a coarse crystal region, and is insensitive to reheat cracks within the temperature range of 500-750 ℃. The composition of the steel is insensitive to reheat cracks, no reheat cracks are generated in a coarse crystal region in the postweld heat treatment or service process, and the reheat crack resistance is excellent.

The test materials included rods and pipes of various specifications as long as the steel composition satisfied [% B [ ]]>-1.2×[％C]²+0.30×[％C]The relationship of-0.01, the resistance to reheat cracking is very good. Otherwise, it has better reheat cracking resistanceAnd (4) poor. The invention is not influenced by the material forming mode.

TABLE 2 reheat cracking evaluation test results of the steels of the examples

The protective scope of the present invention is not limited to the above-described embodiments, and it is apparent that various modifications and variations can be made to the present invention by those skilled in the art without departing from the scope and spirit of the present invention. It is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. The W-containing high-strength low-alloy heat-resistant steel capable of resisting reheat cracks is characterized by comprising the following elements in percentage by mass: c: 0.04-0.11%, Si: 0.50% or less, Mn: 0.10-0.60%, P: 0.03% or less, S: 0.01% or less, Ni: 0.40% or less, Cr: 1.90-2.60%, V: 0.20 to 0.30%, Nb: 0.02 to 0.08%, Mo: 0.05-0.30%, W: 1.45-1.75%, Ti: 0.01-0.06%, B: 0.001% -0.0045%, or 0.0055% -0.012%, Al: 0.03% or less, N: 0.01% or less, wherein the contents of C and B satisfy the following formula:

[％B]>-1.2×[％C]²+0.30×[％C]-0.01 (1)

the balance of Fe and inevitable impurities.

2. The high-strength low-alloy heat-resistant steel containing W and resistant to reheat cracking as claimed in claim 1, wherein: further, the content of C in the heat-resistant steel is as follows: 0.04-0.08%.

3. The high-strength low-alloy heat-resistant steel containing W and resistant to reheat cracking as claimed in claim 1, wherein: further, the contents of C and B in the heat-resistant steel satisfy the following formula:

[％B]>-1.4×[％C]²+0.35×[％C]-0.0115 (2)。

4. the high-strength low-alloy heat-resistant steel containing W and resistant to reheat cracking as claimed in claim 1, wherein: further, the content of C in the heat-resistant steel is as follows: 0.04-0.08%, wherein the content of C and B satisfies the following formula:

[％B]>-1.4×[％C]²+0.35×[％C]-0.0115 (2)

the balance of Fe and inevitable impurities.

5. The reheating crack resistant W-containing high-strength low-alloy heat-resistant steel as recited in claim 4, wherein: further, the content of B in the heat-resistant steel is as follows: 0.006-0.01%.