WO2023165611A1 - Steel for high-toughness corrosion-resistant christmas tree valve bodies smelted at high scrap ratio, and heat treatment method and production method therefor - Google Patents

Abstract

Disclosed in the present invention are a steel for high-toughness corrosion-resistant Christmas tree valve bodies smelted at a high scrap ratio, and a heat treatment method and a production method therefor. The steel for Christmas tree valve bodies mainly comprises the following components: C, Si, Mn, Cr, Mo, Ni, Cu, Al, Nb, Ti, B, La, Y, N, and the residual Sb+As+Pb+Bi+Sn in scrap steel which is greater than or equal to 0.035%. According to the present invention, the composition of the chemical components in the steel, the relationship among the components, and amounts are controlled, so that the steel valve body made of the steel for Christmas tree valve bodies has a tensile strength at the 1/4 thickness greater than or equal to 860 MPa, a yield strength greater than or equal to 680 MPa, KV₂ greater than or equal to 220 J at -46°C, A greater than or equal to 20%, Z greater than or equal to 70%, and a corrosion rate in the seawater environment less than or equal to 0.09 mm/a, and the performance thereof satisfies the requirements of a Christmas tree in a harsh environment.

Description

High-strength, toughness, corrosion-resistant subsea tree valve body steel smelted under high scrap steel ratio, heat treatment method and production method thereof technical field

The invention belongs to the technical field of alloy steel, and relates to a high-strength, toughness, corrosion-resistant underwater Christmas tree valve body steel smelted under a high ratio of scrap steel, a heat treatment method and a production method thereof.

Background technique

Under the premise of carbon neutrality and carbon peaking, the use of short-process electric furnace smelting to produce steel is considered to be the trend of steel production in the future. Because electric furnace smelting can use scrap steel on a large scale, thereby reducing carbon emissions, but scrap steel usually contains high residual elements, such as P, S, As, Sb, Sn, Bi, Pb, etc. In general, P and S can be removed during electric furnace smelting and subsequent refining, but As, Sb, Sn, Bi, and Pb are generally considered not to be removed during smelting. At present, electric furnaces are used to smelt high-quality steel, and a large proportion of about 50% of molten iron is usually added to dilute the residual elements in scrap steel. Although this method can reduce the proportion of residual elements, the use of too high proportion of molten iron is not conducive to carbon neutralization and carbon peaking.

The valve body of the subsea tree has high pressure and corrosion due to its decades of underwater service, so it has high requirements on the strength, toughness and corrosion resistance of the steel. Since the remaining elements will reduce the strength and toughness of steel and also harm the corrosion resistance, the steel used for Christmas trees usually requires Sb+As+Pb+Bi+Sn≤0.025%, so the proportion of scrap steel needs to be strictly controlled in smelting. How to eliminate the impact of residual elements on steel properties under the premise of high scrap steel ratio has attracted much attention.

Contents of the invention

The object of the present invention is to provide a kind of high strength and toughness anti-corrosion submerged tree valve body steel and its heat treatment method and production method smelted under the high scrap steel ratio, in Sb+As+Pb+Bi+Sn≥0.035% Under certain circumstances, the present invention can realize the yield strength of the Christmas tree valve body ≥ 680MPa, -46°C KV ₂ ≥ 220J, corrosion in seawater environment The speed is ≤0.09mm/a, which can meet the use requirements of the Christmas tree in the harsher seawater environment, and is suitable for the manufacture of underwater Christmas tree valve bodies.

In order to achieve the above object, the technical scheme that the present invention takes is as follows:

A high-strength, toughness, and corrosion-resistant subsea tree valve body steel smelted at a high steel scrap ratio, comprising the following chemical components in weight percentages: C 0.22%-0.28%, Si 0.15%-0.35%, Mn 1.7%-2.0% , Cr 0.5%～0.7%, Mo 0.3%～0.5%, Ni 0.80%～1.00%, Cu 0.30%～0.50%, Al 0.015%～0.035%, Nb 0.025～0.045%, Ti 0.0035～0.0055%, B 0.0005 ～0.0030%, La 0.0010～0.0030%, Y(yttrium) 0.0020～0.0050%, P≤0.015%, S≤0.015%, N 0.0070～0.0120%, O≤0.004%, Sb+As+Pb+Bi+Sn≥ 0.035%, the rest is Fe and other unavoidable impurities;

in,
A={2.5+30×[B-1.27×(N-0.002-0.29×Ti-0.15×Nb)]}×(1+4×Mn)×(1+2×Cr)×(1+3.5×
Mo), A≥90, preferably A is 90～130;
D=30×Ni+20×Mo+16×Cu+22×Mn-12×Si×Mn+28×C-10×C×Mn, D≥74.5, preferably D is 74.5-80;
X=26×Cu+4×Ni+1.2×Cr-1.5×Si-7×Cu×Ni-5×Mn, X≥1.8, preferably X is 1.8～5.0;
E=10×La+8×Y-(Sb+As+Pb+Bi+Sn), E≥0.002, preferably E is 0.002～0.015;

In the calculation formulas of A, D, X, and E values, the index value of each element = the content of the element in the steel × 100.

In order to produce a high-strength, toughness, corrosion-resistant subsea tree valve body with excellent strength, toughness, corrosion resistance and fatigue performance, which can meet the requirements of more severe underwater environments, the present invention performs the following controls:

C: C is the cheapest strengthening element in steel. For every increase of 0.1% solid solution C, the strength can be increased by about 450MPa. C and the alloying elements in the steel form precipitated phases to play a role in precipitation strengthening. C can significantly improve High hardenability, so that the core of the large-scale Christmas tree valve body obtains martensitic structure. However, as its content increases, the plasticity and toughness decrease, and high C content is harmful to corrosion performance, so the C content is controlled at 0.22% to 0.28%.

Si: Si is an effective solid-solution strengthening element in steel, which improves the strength and hardness of steel. Si can play a role in deoxidation during steelmaking and is a commonly used deoxidizer. However, Si is easy to segregate to austenite grain boundaries, which reduces the bonding force of grain boundaries and causes brittleness. In addition, Si is easy to cause element segregation in steel. Therefore, the Si content is controlled at 0.15% to 0.35%.

Mn: Mn can play a role in solid solution strengthening, and its solid solution strengthening ability is weaker than that of Si. Mn is an austenite stabilizing element that can significantly improve the hardenability of steel and reduce decarburization of steel. The combination of Mn and S can prevent Hot brittleness caused by S. But excessive Mn will reduce the plasticity of steel. Therefore, the Mn content is controlled at 1.7% to 2.0%.

Cr: Cr is a carbide forming element. Cr can improve the hardenability and strength of steel, but it is easy to cause temper brittleness. Cr can improve the oxidation resistance and corrosion resistance of steel, but when the Cr content is too high, it will increase the crack sensitivity. Cr content should be controlled at 0.50% to 0.70%.

Mo: Mo is mainly to improve the hardenability and heat resistance of steel. Mo solidly dissolved in the matrix can maintain a high stability of the steel structure during tempering, and can effectively reduce impurities such as P, S and As. The elements segregate at the grain boundaries, thereby improving the toughness of the steel and reducing temper brittleness. Mo reduces the stability of M ₇ C ₃ , and when the Mo content is high, acicular Mo ₂ C will be formed, which will lead to a decrease in the Mo content of the matrix. Mo can improve the strength of steel through the joint action of solid solution strengthening and precipitation strengthening, and can also change the toughness of steel by changing the precipitation of carbides. Therefore, Mo is controlled at 0.30% to 0.50%.

Ni: Ni can form an infinitely soluble solid solution with Fe. It is an austenite stabilizing element. It has the effect of expanding the phase area, increasing the stability of supercooled austenite, shifting the C curve to the right, and improving the hardenability of steel. Ni can refine the width of the martensite lath and improve the strength. Ni can significantly reduce the ductile-brittle transition temperature of steel and increase the low temperature toughness. The Ni content is controlled at 0.80% to 1.00%.

Cu: Cu expands the austenite phase region. Cu simple substance can be used as the second phase to significantly improve the strength, and can improve the tempering stability and strength of the structure. But if Cu is too high, it will cause Cu to be brittle. Therefore, the Cu content is controlled at 0.30% to 0.50%.

Al: Al is the main deoxidizer in steelmaking. Al combines with N to form fine and dispersed AlN, and maintains a coherent relationship with the matrix, which can strengthen and refine the structure, and can increase the fatigue crack initiation and propagation resistance. , thereby increasing the durability of the steel. Al content is controlled at 0.015% to 0.035%.

Nb: Nb is a strong C and N compound forming element. Nb (C, N) is finely dispersed and maintains a coherent relationship with the matrix, which can strengthen and refine the structure. The strengthening of the matrix can initiate and propagate fatigue cracks Increased resistance, thereby increasing fatigue strength. The Nb content is controlled at 0.025% to 0.045%.

Ti: Ti has a wide range of functions in steel. Ti can be used as a deoxidizer for deoxidation. Ti, C and N can form carbonitrides, precipitate in steel, play a role in precipitation strengthening, and can also pin grain boundaries to hinder grain growth. . The Ti content is controlled at 0.0035% to 0.0055%.

B: B is generally considered to be a trace element in steel and has a strong hardening effect. It can improve the toughness of steel while improving hardenability. However, due to the strong hardenability of B, the B content in the steel is not easy to be too high, so it is controlled at 0.0005-0.0030%.

La: La is a light rare earth element in steel, which can effectively reduce the grain boundary segregation of inclusions and steel residual elements in steel, and improve the bonding of grain boundaries. Thereby reducing the corrosion at the grain boundary of the steel and improving the corrosion performance. Therefore, it is controlled at 0.0010-0.0030%.

Y: Y is a heavy rare earth element in steel, which is particularly effective in reducing the segregation of Si and Mn in steel, and Y can increase the composite effect of Cr and Cu, and improve the corrosion resistance of steel. It can also sweep the grain boundaries of the remaining elements, and is known as a rare earth grain boundary sweeper, but excessive addition of Y will form individual large-sized hard Quality inclusions are not conducive to the toughness of steel, so the content is controlled at 0.0020-0.0050%

O and N: T.O forms oxide inclusions in steel, and T.O≤0.0040% is controlled; N can form fine precipitates with nitride-forming elements in steel to refine the structure, so N is controlled at 0.0070-0.0120%.

The invention makes full use of the beneficial effects of Mn, Cr, Mo and B elements on hardenability to ensure the hardenability of the thick-walled valve body. At the same time, use Nb and Ti to form nitrides with N to consume nitrogen, ensure that B elements exist in the steel in a solid solution state, and give full play to the hardenability effect, so as to ensure that 1/4 of the valve body wall thickness is still small when the wall thickness is ≥ 600mm. of tempered sorbite. Therefore, the above seven elements should satisfy:

A={2.5+30×[B-1.27×(N-0.002-0.29×Ti-0.15×Nb)]}×(1+4×Mn)×(1+2×Cr)×(1+3.5×Mo ), A≥90, the index value of each element in the formula = the content of the element in the steel × 100.

In order to ensure the low-temperature toughness of steel, toughening elements need to be limited. Ni is an element that can improve toughness at present, and Mo is beneficial to improve tempering stability, thereby improving the toughness of steel. Cu can precipitate fine nano-copper precipitates in steel, thereby improving the toughness of steel. Therefore, the contribution coefficients of the above three elements to toughness are 30, 20, and 16, respectively. Mn can promote the selection of steel in the phase transformation, so that the microstructure is finer and the toughness is improved, but the segregation of Si and Mn leads to the decrease of toughness, so the contribution of Mn to toughness has its own contribution, and there is an interaction with Si and Mn , so the coefficients are 22 and -12 respectively. The effect of C content on toughness also has two sides. On the one hand, it promotes phase transformation refinement and improves toughness. On the one hand, it interacts with Mn to promote the hardening of steel, resulting in lower toughness. Therefore, C contributes independently to toughness, and there is an interaction with C and Mn, so the coefficients are 28 and -10, respectively. Because P and S in the steel are also harmful to the toughness of the steel, but because the present invention has set the maximum content limit for the content of P and S, the harm of P and S to the toughness is not considered. Toughness Determination Factor of Steel

D=30×Ni+20×Mo+16×Cu+22×Mn-12×Si×Mn+28×C-10×C×Mn≥74.5, the index value of each element in the formula = the content of the element in the steel ×100.

In order to ensure better corrosion resistance at sea, the ratio of Si, Mn, Cu, Ni, and Cr needs to be limited, and the coefficient is 26 because Cu can improve the strength and significantly improve the corrosion resistance. Si and Mn will aggravate segregation, cause microstructure inhomogeneity and reduce erosion performance, so the coefficients are -1.5 and -5, respectively. Ni can improve stacking faults, significantly improve low-temperature toughness, and can passivate metals to improve erosion performance, so the coefficient of Ni is 4. Cr can strengthen the passivation film on the steel surface, so the coefficients are 1.2 respectively. Since the interaction between Cu and Ni will offset the corrosion resistance of the elements alone, the coefficients are -7 respectively; that is, X=26×Cu+4×Ni+1.2×Cr-1.5×Si-7×Cu×Ni- 5×Mn≥1.8, the index value of each element in the formula=the content of the element in the steel×100.

In order to reduce the influence of residual elements in high scrap steel ratio on steel properties, it is necessary to limit the content of La, Y and residual elements (Sb+As+Pb+Bi+Sn). La has a strong effect on the elimination of residual elements and segregation. The coefficient is 10. Although Y can clean the remaining elements, but it is a heavy rare earth element, its function will be affected, so the coefficient is 8. Therefore, it is required that E≥0.002; E=10×La+8×Y-(Sb+As+Pb+Bi+Sn), the index value of each element in the formula=the content of the element in the steel×100.

The metallographic structure of the steel for the valve body of the high-strength, toughness and corrosion-resistant subsea oil tree smelted at a high scrap steel ratio is tempered sorbite, and the grain size is 20-27 μm.

The tensile strength ≥ 860MPa, yield strength ≥ 680MPa, -46°C KV ₂ ≥ 220J, A ≥ 20 %, Z≥70%; corrosion rate in seawater environment ≤0.09mm/a; specifically, at 1/4 thickness of the valve body of the high-strength, toughness and corrosion-resistant subsea tree valve body smelted under high scrap steel ratio Tensile strength is 860-940MPa, yield strength is 680-750MPa, -46°C KV ₂ is 220-240J, A is 20-25%, Z is 70-75%; corrosion rate in seawater environment is ≤0.09mm/ a.

The high-strength, toughness, corrosion-resistant underwater Christmas tree valve body steel smelted at a high scrap steel ratio provided by the present invention A heat treatment method comprising the steps of:

(1) Quenching: Heat the valve body of the Christmas tree to 860-900°C, keep warm, and then water-cool;

(2) Step tempering: Heat the valve body of the Christmas tree to T1=450-550°C, keep it warm, then heat it to T2=650-700°C to keep it warm, and then water-cool it. The type and proportion of carbide precipitation are controlled by step tempering to improve toughness and corrosion fatigue life. In the first stage of step tempering, on the one hand, the internal and external temperature of the forging is guaranteed to be consistent, and on the other hand, the precipitated phase of the steel is mainly composed of fine M2C carbides, which improves the precipitation strength and offsets part of the strength and fine M2C that are reduced due to tempering. Carbide also helps to reduce the internal stress of the structure and improve the corrosion fatigue life; the temperature of the second stage of step tempering is higher than that of the first stage, which is conducive to the precipitation of precipitates such as M23C6 and M6C, and improves the toughness. If step tempering is not used, the types of steel precipitates will be reduced. Water cooling after tempering can improve work efficiency on the one hand and help the surface quality of forgings on the other hand.

In the step (1), the heating rate is 50-110° C./h, the holding time is t=0.8-1.2×S, S is the valve body wall thickness in mm, and t is in min.

In the step (2), the heating rate for the first heating is 50-110° C./h, the holding time is t1=0.8-1.2×S, S is the valve body wall thickness in mm, and the unit of t1 is min.

The above-mentioned heating rate can ensure that the temperature at different positions of the valve body is close; if the heating rate is too fast, the temperature gradient at different positions of the valve body will increase, which will increase the internal stress and increase the risk of cracks; Risk of tempering reactions, resulting in uncontrolled precipitated phase types and contents. The holding time is the key to controlling the content and size of the precipitated phase. If the holding time is too short, the precipitated phase will be less and the beneficial effect will be reduced. If the holding time is too long, the precipitated phase will increase, but the size of the precipitated phase will increase, which will reduce the dispersed distribution of the precipitated phase. effect. Excessively large precipitates also increase the risk of internal microcracks.

In the step (2), the heating rate for the second heating is 80-120° C./h, the holding time is t2=0.5-1.2×S, S is the valve body wall thickness in mm, and the unit of t2 is min. second step of ladder tempering The temperature rise rate in the second stage is higher than that in the first stage, because after the first stage tempering, the temperature gradient at different positions of the valve body has been reduced, so that the time cost can be reduced by increasing the temperature rise rate in the second stage, and the temperature rise rate Elevation also helps to control the precipitated phase size.

In the steps (1) and (2), the water cooling is all cooled to below 100°C.

The tempering process parameters should comply with Z=T2×(S/10+lgt2)/1000, 34.2≤Z≤35.8. The tempering parameters directly determine the mechanical properties and corrosion fatigue properties of the final product. If the tempering parameter is too large, the softening effect of the material will be large, resulting in a large decrease in the strength of the material and the strength cannot be guaranteed. It will also cause the size of the precipitate to be too large, weaken the precipitation strengthening effect, and increase the risk of microcracks in the steel and reduce the toughness. If the tempering parameter is small, the strength of the material will be insufficiently softened, the structural stress and internal stress will be large, and the toughness and corrosion fatigue performance will be reduced.

The production method of the high-strength, toughness, corrosion-resistant subsea tree valve body steel smelted at a high steel scrap ratio provided by the present invention comprises the following steps: electric arc furnace or converter smelting→LF furnace refining→RH or VD vacuum degassing→ Continuous casting of round billet→heating of round billet→forging into valve body→heat treatment→machining→flaw detection→packing and storage, wherein the heat treatment is carried out by the above heat treatment method.

In electric arc furnace or converter smelting, the feeding ratio of scrap steel is 70%, that is, 100 tons of molten steel, including 70 tons of scrap steel, 30 tons of molten iron and alloy; the diameter of the round billet is Φ380mm～Φ700mm.

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

1. The high-strength, toughness, corrosion-resistant subsea oil tree valve body steel smelted under high steel scrap ratio provided by the present invention, by controlling the composition and dosage of chemical components in the steel, makes its performance meet the needs of underwater x-mas trees in harsh environments ;

2. The relationship between B, N, Ti, Nb, Mn, Cr and Mo in the high-strength, toughness, corrosion-resistant subsea oil tree valve body steel smelted under the high scrap ratio provided by the present invention satisfies {2.5+30× [B-1.27×(N-0.002-0.29×Ti-0.15×Nb)]}×(1+4×Mn)×(1+2×Cr)×(1+3.5×Mo) ≥90 to ensure the hardenability of the subsea tree valve body;

3. The relationship between Ni, Mo, Cu, Mn, Si and C in the high-strength, toughness, corrosion-resistant subsea tree valve body steel smelted at a high scrap ratio provided by the present invention satisfies 30×Ni+20×Mo +16×Cu+22×Mn-12×Si×Mn+28×C-10×C×Mn≥74.5 to ensure the low temperature toughness of the subsea tree valve body;

4 The relationship between Cu, Ni, Cr, Si and Mn in the high-strength, toughness, corrosion-resistant subsea oil tree valve body steel smelted at a high scrap ratio provided by the present invention satisfies 26×Cu+4×Ni+1.2× Cr-1.5×Si-7×Cu×Ni-5×Mn≥1.8, which ensures that the subsea tree valve body has good anti-corrosion performance at sea;

5. between the remaining elements (Sb+As+Pb+Bi+Sn) present in the La, Y in the high-strength, toughness, corrosion-resistant underwater Christmas tree valve body steel smelted under the high scrap steel ratio provided by the present invention and the scrap steel The relation of satisfying 10×La+8×Y-(Sb+As+Pb+Bi+Sn) ≥ 0.002, in order to reduce the influence of residual elements in high scrap steel ratio on the performance of subsea tree valve body;

6. The heat treatment of the high-strength, toughness and corrosion-resistant subsea tree valve body steel smelted under the high scrap ratio provided by the present invention adopts the quenching + step tempering process for heat treatment, and the heating temperature T2 and holding time during the tempering treatment t2 is controlled to ensure that the overall performance of the steel used for the valve body of the subsea tree can meet the requirements of the subsea tree in harsh environments.

Description of drawings

Fig. 1 is the metallographic structure diagram of the steel for the submerged Christmas tree valve body in embodiment 3, and it can be seen that the crystal grains are fine;

Fig. 2 is a micrograph of the steel used for the valve body of the subsea tree in comparative example 2, and it can be seen that the crystal grains are coarse.

Detailed ways

The invention provides a high-strength, toughness, corrosion-resistant underwater Christmas tree valve body steel smelted at a high scrap steel ratio, Including the following chemical composition in weight percentage: C 0.22%~0.28%, Si 0.15%~0.35%, Mn 1.7%~2.0%, Cr 0.5%~0.7%, Mo 0.3%~0.5%, Ni 0.80%~1.00% , Cu 0.30%～0.50%, Al 0.015%～0.035%, Nb 0.025～0.045%, Ti 0.0035～0.0055%, B0.0005～0.0030%, La 0.0010～0.0030%, Y (yttrium) 0.0020～0.0050%, P ≤0.015%, S≤0.015%, N 0.0070～0.0120%, O≤0.004%, Sb+As+Pb+Bi+Sn≥0.035%, the rest is Fe and other unavoidable impurities;

in,
A={2.5+30×[B-1.27×(N-0.002-0.29×Ti-0.15×Nb)]}×(1+4×Mn)×(1+2×Cr)×(1+3.5×
Mo), A≥90;
D=30×Ni+20×Mo+16×Cu+22×Mn-12×Si×Mn+28×C-10×C×Mn, D≥74.5;
X=26×Cu+4×Ni+1.2×Cr-1.5×Si-7×Cu×Ni-5×Mn, X≥1.8;
E=10×La+8×Y-(Sb+As+Pb+Bi+Sn), E≥0.002;

In the calculation formulas of A, D, X, and E values, the index value of each element = the content of the element in the steel × 100.

The production method of the high-strength, toughness, corrosion-resistant and corrosion-resistant subsea tree valve body steel smelted at a high ratio of scrap steel comprises the following steps: electric arc furnace or converter smelting→LF furnace refining→RH or VD vacuum degassing→round billet continuous casting →Round billet heating→forging into valve body→heat treatment→machining→flaw detection→packing and storage.

Among them, electric furnace smelting: use high scrap steel ratio (scrap steel ratio ≥ 70%) to determine oxygen before tapping, measure the content of residual elements, and use steel retention during tapping to avoid slag;

LF furnace: C, Si, Mn, Cr, Ni, Mo, Cu, Nb, Ti, B, La, Y and other elements are adjusted to the target value;

Vacuum degassing: pure degassing time ≥ 15 minutes, to ensure [H] content ≤ 1.5ppm after vacuum treatment, to avoid white spots in the steel, causing hydrogen embrittlement;

Continuous casting: The target temperature of the molten steel in the tundish is controlled at 10-40°C above the liquidus temperature, and the round billet of φ380mm～φ700mm is continuously cast.

Forging route: round billet heating → forging → slow cooling.

Valve body heat treatment: trolley furnace heating → heat preservation → quenching → trolley furnace heating → heat preservation → quenching → tempering → heat preservation → water cooling.

Valve body processing route: valve body rough turning → flaw detection → valve body finish turning → grinding → flaw detection → packaging and storage.

Heat treatment is carried out according to the following steps:

(1) Quenching: Heat the christmas tree valve body to 860-900°C, keep it warm, and then water-cool it to below 100°C; the heating rate is 50-110°C/h, and the holding time is t=0.8-1.2×S, S is the wall thickness of the valve body, the unit is mm, and the unit of t is min;

(2) Ladder tempering: Heat the valve body of the Christmas tree to T1=450-550°C, keep it warm, then heat it to T2=650-700°C to keep it warm, and then water-cool it to below 100°C; the heating rate for the first heating is 50 ～110℃/h, the holding time is t1=0.8～1.2×S; the temperature rise rate of the second heating is 80～120℃/h, the holding time is t2=0.5～1.2×S, S is the valve body wall thickness, The unit is mm, and the unit of t2 is min; the tempering process parameters should meet Z=T2×(S/10+lgt2)/1000, 42≤Z≤48.

The performance detection method of the high strength, toughness, corrosion resistance, subsea tree valve body steel prepared by the above process and smelted at a high scrap steel ratio is as follows:

Organization: Sampling on the extension of the valve body, sampling within 1/4 of the thickness of the extension (500mm in thickness) for metallographic and grain size analysis.

Performance: Sampling on the extension of the valve body, taking tensile, impact and corrosion samples within 1/4 of the thickness of the extension (thickness is 500mm), and referring to GB/T228, GB/T229, GB/T 5776 for mechanical properties test.

The present invention will be described in detail below in conjunction with examples.

The chemical composition and weight percentage of the high-strength, toughness and corrosion-resistant subsea tree valve body steel smelted at high scrap steel ratio in each embodiment and comparative example are shown in Table 1, and the balance is iron and unavoidable impurities.

Claims (9)

1. A high-strength, toughness, corrosion-resistant subsea tree valve body steel smelted at a high ratio of scrap steel, characterized in that it includes the following chemical components in weight percentages: C 0.22%-0.28%, Si 0.15%-0.35%, Mn 1.7 %～2.0%, Cr 0.5%～0.7%, Mo 0.3%～0.5%, Ni 0.80%～1.00%, Cu 0.30%～0.50%, Al 0.015%～0.035%, Nb 0.025～0.045%, Ti 0.0035～0.0055% %, B 0.0005～0.0030%, La 0.0010～0.0030%, Y (yttrium) 0.0020～0.0050%, P≤0.015%, S≤0.015%, N 0.0070～0.0120%, O≤0.004%, Sb+As+Pb+ Bi+Sn≥0.035%, the rest is Fe and other unavoidable impurities;

   in,
   A={2.5+30×[B-1.27×(N-0.002-0.29×Ti-0.15×Nb)]}×(1+4×Mn)×(1+2×Cr)×(1+3.5×
   Mo), A≥90;
   D=30×Ni+20×Mo+16×Cu+22×Mn-12×Si×Mn+28×C-10×C×Mn, D≥74.5;
   X=26×Cu+4×Ni+1.2×Cr-1.5×Si-7×Cu×Ni-5×Mn, X≥1.8;
   E=10×La+8×Y-(Sb+As+Pb+Bi+Sn), E≥0.002;

   In the calculation formulas of A, D, X, and E values, the index value of each element = the content of the element in the steel × 100.

   The metallographic structure of the steel for the valve body of the high-strength, toughness and corrosion-resistant subsea oil tree smelted at a high scrap steel ratio is tempered sorbite, and the grain size is 20-27 μm.
2. The high-strength, toughness, and corrosion-resistant underwater Christmas tree valve body steel smelted at a high scrap steel ratio according to claim 1, wherein the high-strength, tough, corrosion-resistant underwater Christmas tree valve body smelted at a high scrap steel ratio Tensile strength ≥ 860MPa, yield strength ≥ 680MPa, -46°C KV ₂ ≥ 220J, A ≥ 20%, Z ≥ 70% at 1/4 thickness of steel valve body; corrosion rate in seawater environment ≤ 0.09mm/a .
3. The heat treatment method of the high-strength, toughness and corrosion-resistant subsea tree valve body steel smelted at a high steel scrap ratio as claimed in claim 1 or 2, wherein the heat treatment method comprises the following steps:

   (1) Quenching: Heat the valve body of the Christmas tree to 860-900°C, keep warm, and then water-cool;

   (2) Step tempering: Heat the valve body of the Christmas tree to T1=450-550°C, keep it warm, then heat it to T2=650-700°C to keep it warm, and then water-cool it.
4. The heat treatment method according to claim 3, characterized in that, in the step (1), the heating rate is 50-110°C/h, the holding time is t=0.8-1.2×S, and S is the valve body wall Thickness, in mm, t in min.
5. The heat treatment method according to claim 3, characterized in that, in the step (2), the heating rate for the first heating is 50-110° C./h, and the holding time is t1=0.8-1.2×S, and S is The wall thickness of the valve body is in mm, and the unit of t1 is min.
6. The heat treatment method according to claim 3, characterized in that, in the step (2), the heating rate for the second heating is 80-120° C./h, and the holding time is t2=0.5-1.2×S, and S is The wall thickness of the valve body is in mm, and the unit of t2 is min.
7. The heat treatment method according to claim 6, characterized in that the tempering process parameters should meet Z=T2×(S/10+lgt2)/1000, 34.2≤Z≤35.8.
8. The production method of high-strength, toughness, corrosion-resistant subsea tree valve body steel smelted under high steel scrap ratio as claimed in claim 1 or 2, characterized in that the production method comprises the following steps: electric arc furnace or converter smelting→ LF furnace refining→RH or VD vacuum degassing→round billet continuous casting→round billet heating→forging into a valve body→heat treatment→machining→flaw detection→packaging and storage, wherein the heat treatment adopts any one of claims 3-7 The heat treatment method described above is carried out.
9. The production method according to claim 8, characterized in that, in electric arc furnace or converter smelting, the feeding ratio of steel scrap is 70%; and the diameter of the round billet is Φ380mm-Φ700mm.