High-sulfur and high-chlorine resistant cast carbon steel and preparation method thereof

Abstract

The invention discloses high-sulfur and high-chlorine resistant cast carbon steel which comprises the following chemical components in percentage by mass: 0.12-0.18% of C, 0.70-1.0% of Mn, 0.40-0.60% of Si, less than or equal to 0.015% of P, less than or equal to 0.012% of S and the balance of Fe and residual elements: less than or equal to 0.01% of Cr, less than or equal to 0.10% of Ni, less than or equal to 0.05% of Mo, less than or equal to 0.05% of Cu and less than or equal to 0.04% of Al. The preparation method of the carbon steel mainly comprises the steps of decarburization, dephosphorization in an oxidation period, desulfurization in a reduction period, degassing and refining in an electric arc furnace. The carbon equivalent [CE] is less than or equal to 0.38%, the low-carbon C is less than or equal to 0.18%, the ultra-low-sulfur S is less than or equal to 0.012%, and the low-phosphorus P is less than or equal to 0.015%. The cast sulfur-resistant carbon steel can be suitable for gathering and transportation projects of natural gas fields with the highest content of hydrogen sulfide, is high-quality cast sulfur-resistant carbon steel with high sulfur and chloride ion corrosion resistance, saves cost by 1 billion yuan at a time, is substituted by the country, breaks through the oracle that 17 foreign valve manufacturers consider that carbon steel cannot be used, and solves the technical problem of material selection of national valve equipment with similar working conditions.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of cast steel materials, and in particular relates to a high-sulfur and high-chlorine resistant cast carbon steel and a preparation method thereof. Background Art

[0002] The natural gas in Puguang Gas Field in Sichuan, my country contains: H2S 12.31%~17.05%, CO2 7.89%~10.53%, chloride ions (Cl - ) content is 28200mg / l~64600mg / l, and the raw gas contains 340.6mg / m 3 ; Among them: sulfur, oxygen and carbon 316.2mg / m 3 , Methyl mercaptan 22.8mg / m 3 , Ethyl mercaptan 1.6mg / m 3 It also contains saturated water vapor, etc. According to the "Gas Reservoir Classification" (SY / T6168-2009), natural gas reservoirs with H2S concentrations of 2% to 10% are high-sulfur gas reservoirs. Sichuan Puguang Gas Field is the natural gas field with the highest content of hydrogen sulfide in my country, and is also one of the natural gas fields with the highest hydrogen sulfide content in the world. When using valve equipment made of ordinary carbon steel in such high-acid conditions, the valves are very prone to sulfide stress cracking (SSC) and hydrogen-induced stress corrosion cracking (HIC), which will cause hydrogen sulfide leakage and are not allowed to be used.

[0003] SSC means: when the medium contains H2S and water, the hydrogen atoms generated in the wet hydrogen sulfide environment penetrate into the interior of the steel and dissolve in the lattice, increasing the brittleness of the steel. The cracks formed under the action of external tensile stress or residual stress are called "sulfide stress corrosion cracking". HIC means: when the medium contains H2S and water, H2S reacts with the metal, hydrogen is precipitated during electrochemical corrosion, forming hydrogen-containing bubbles and hydrogen embrittlement, hydrogen-induced sulfide stress corrosion cracking, that is, hydrogen-induced stress corrosion cracking. The occurrence of HIC does not require external stress.

[0004] If sulfide stress cracking (SSC) and hydrogen-induced stress corrosion cracking (HIC) occur in valves for natural gas with high hydrogen sulfide and high chloride ions, it will cause extremely terrible consequences of hydrogen sulfide leakage. For example, when PetroChina was developing the Puguang gas field on December 23, 2003, its Luojia 16H gas well had a blowout, and the hydrogen sulfide gas that was ejected caused 243 people to be poisoned and killed, and a large number of cattle and pigs died. As a result, the development of the Puguang gas field was forced to stop.

[0005] After China National Petroleum Corporation was forced to stop developing the Puguang gas field, due to the shortage of natural gas in China, Sinopec later prepared to develop the Puguang gas field. At that time, 17 foreign valve manufacturers were eyeing this project. For valves for natural gas with high hydrogen sulfide and high chloride ions, foreign valve manufacturers mainly use Inconel625 and Incoloy825 nickel-based alloys or clad Inconel625 or Incoloy825 nickel-based alloys on the parts of carbon steel valves that contact the medium. The thickness of the nickel-based alloy cladding layer after processing must be ≥3mm, resulting in extremely high prices. According to the price quoted by foreign valves at that time, it was 1 billion yuan higher than the batch of 1,746 valves supplied by Lanzhou High Pressure Valve Co., Ltd.

[0006] Obviously, in high-acid working conditions, equipment made of ordinary carbon steel cannot meet normal use requirements, but if imported products are used, the economic pressure is self-evident. Summary of the invention

[0007] The invention provides a high-sulfur and high-chlorine resistant cast carbon steel and a preparation method thereof, which solves the technical problems of resistance to sulfide stress cracking (SSC), hydrogen-induced stress corrosion cracking (HIC) and chloride ion corrosion.

[0008] To achieve the above object, the present invention provides the following technical solutions: A high-sulfur and high-chlorine resistant cast carbon steel, whose chemical composition and mass percentage content are: C: 0.12%-0.18%, Mn: 0.70%-1.0%, Si: 0.40%-0.60%, P≤0.015%, S≤0.012%, and the rest are Fe and residual elements: Cr≤0.01%, Ni≤0.10%, Mo≤0.05%, Cu≤0.05%, Al≤0.04%.

[0009] A method for preparing high-sulfur and high-chlorine resistant cast carbon steel, wherein the smelting process mainly includes decarburization, dephosphorization during oxidation period, desulfurization during reduction period, degassing and electric arc furnace refining; the decarburization and dephosphorization during oxidation period adopt ore decarburization and dephosphorization method, adding 10kg / t to 20kg / t of molten steel of small pieces of iron ore, heating to remove moisture, and then adding in batches, after the last batch of ore is added, the clean boiling is maintained for more than 5 minutes, the temperature of the molten steel is greater than 1600°C during high-temperature boiling, and the slag is removed and removed cleanly; the desulfurization during reduction period, adding 15kg / t to 20kg / t of molten steel is added with lime (CaO), and the slag is repeatedly skimmed. The degassing and electric arc furnace refining are for deoxidation, removal of hydrogen and nitrogen, and the slag is cleaned. Aluminum ingots are inserted into the steel furnace according to a measurement of 1.5kg / t of molten steel, and silicon-calcium powder is added according to a measurement of 1.5kg / t of molten steel to form slag and skim it out. Finally, when the molten steel is tapped, 0.5kg / t of ferrotitanium is added to the bottom of the steel tapping trough or the ladle. Before adding, the ferrotitanium needs to be crushed to the size of soybeans and heated to 800°C.

[0010] The oxygen (O) content in the high-sulfur and high-chlorine resistant cast carbon steel product is ≤50 μg / g, the hydrogen (H) content is ≤3 μg / g, and the nitrogen (N) content is ≤80 μg / g.

[0011] The carbon equivalent [CE] of the high-sulfur and high-chlorine resistant cast carbon steel product is ≤0.38%.

[0012] The beneficial effects of the present invention are: (1) The carbon (C) content in the high-sulfur and high-chlorine resistant cast carbon steel of the present invention is strictly controlled to be lower than that in ordinary carbon steel, and the carbon equivalent [CE] is strictly controlled to be w(C)+w(Mn) / 6+[w(Cr)+w(Mo)+w(V)] / 5+[w(Ni)+w(Cu)] / 15, The carbon equivalent [CE] is ≤0.38%, which is lower than the carbon equivalent [CE] ≤0.50% of ordinary carbon steel (WCB), effectively reducing the steel's resistance to hydrogen sulfide stress corrosion cracking.

[0013] (2) The phosphorus content P in the high-sulfur and high-chlorine resistant cast carbon steel of the present invention is strictly controlled to be ≤0.015%, which is much lower than the P content in ordinary carbon steel, thereby reducing the harmful effect of P, a harmful element, on the cold brittleness of cast steel parts during casting.

[0014] (3) The sulfur content S in the high-sulfur and high-chlorine resistant cast carbon steel of the present invention is strictly controlled to be ≤0.012%, which is much lower than the S content in ordinary carbon steel. Reducing the S content in the steel will greatly reduce the harmfulness of hot cracking of cast steel parts caused by sulfides during casting and reduce the number of defects caused by sulfide inclusions in the steel.

[0015] (4) In the preparation method of the present invention, aluminum ingots and silicon-calcium powder are added to fully deoxidize and remove hydrogen and nitrogen. The formed Al2O3, CaO and SiO2 are easy to float in the molten steel, which has little effect on the quality of cast steel, ensuring that Al≤0.04% in the finished steel. Because the residual element chromium (Cr) is an element that increases the "carbon equivalent [CE]", increases the hardness of carbon steel and increases the cracks of carbon steel castings; therefore, the residual element Cr must be strictly controlled to be ≤0.01%. The non-metallic inclusions in the steel are minimized and there is no columnar segregation. The inclusions are 0.5-1 level, and the determination method of GB / T 10561 is carried out.

[0016] (5) In the preparation method of the present invention, titanium iron is added to refine the grains, improve the mechanical properties of the steel and its corrosion resistance to hydrogen sulfide and chloride ions, and ensure that the steel after heat treatment is an ultra-low sulfur high-purity steel with a grain size of 7 to 8, in accordance with the determination method of GB / T 6394.

[0017] (6) The high-sulfur and high-chlorine resistant cast carbon steel obtained by the preparation method of the present invention is a steel with a ferrite + uniform spheroidal carbide troostite structure, a low-carbon, ultra-low-sulfur, low-phosphorus fine-grained fully killed pure steel, and has the best resistance to hydrogen sulfide stress corrosion cracking. - ) solution, and has good strength and toughness; yield strength ≥250MPa, tensile strength ≥485MPa, elongation ≥22%, cross-sectional shrinkage ≥35%, hardness ≤200HB; the sample has passed the SSC (sulfide stress corrosion cracking) and HIC (hydrogen induced cracking) tests of the National Oil and Gas Field Material Corrosion Testing and Evaluation Center, filling the domestic technical gap in this field, saving 1 billion yuan in costs at one time, and generating good economic benefits. Moreover, it has achieved the goal of replacing imports with foreign ones, breaking the prediction of 17 foreign valve manufacturers that carbon steel is impossible to use, and solving the technical problem of material selection for valve equipment in similar working conditions in China. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of the present invention.

[0019] Example 1, a high-sulfur and high-chlorine resistant cast carbon steel, whose chemical composition and mass percentage content are: C: 0.12%, Mn: 0.7%, Si: 0.4%, P≤0.015%, S≤0.012%, and the rest are Fe and residual elements: Cr≤0.05%, Ni≤0.10%, Mo≤0.05%, Cu≤0.05%, Al≤0.04%.

[0020] Example 2, a high-sulfur and high-chlorine resistant cast carbon steel, whose chemical composition and mass percentage content are: C: 0.18%, Mn: 1.0%, Si: 0.60%, P≤0.015%, S≤0.012%, and the rest are Fe and residual elements: Cr≤0.05%, Ni≤0.10%, Mo≤0.05%, Cu≤0.05%, Al≤0.04%.

[0021] Example 3, a high-sulfur and high-chlorine resistant cast carbon steel, whose chemical composition and mass percentage content are: C: 0.15%, Mn: 0.82%, Si: 0.53%, P≤0.014%, S≤0.005%, and the rest are Fe and residual elements: Cr≤0.01%, Ni≤0.09%, Mo≤0.03%, Cu≤0.05%, Al≤0.03%.

[0022] The preparation methods of the above-mentioned embodiments 1-3 are as follows: The smelting process mainly includes decarburization, dephosphorization during oxidation period, desulfurization during reduction period, degasification and electric arc furnace refining; the decarburization and dephosphorization during oxidation period adopt ore decarburization and dephosphorization method, add 10kg / t to 20kg / t of molten steel of small pieces of iron ore, burn red to remove moisture, and then add in batches. After the last batch of ore is added, keep clean boiling for more than 5 minutes. When the temperature of the molten steel is greater than 1600℃ during high-temperature boiling, slag is removed and removed cleanly; the principle is: add iron ore (FeO) into the furnace, make the FeO concentration in the slag higher than the FeO concentration in the molten steel, and then diffuse into the molten steel to oxidize C in the molten steel, and the reaction is: Fe2O3(hematite)+Fe=3(FeO) (1) Fe3O4(magnetite)+Fe=4(FeO) (2) (FeO)+C=Fe+CO↑ (3) The generated CO molecules are insoluble in the molten steel and precipitate into CO bubbles. Since the partial pressure of hydrogen and nitrogen in the CO bubbles is low, the hydrogen and nitrogen in the molten steel can penetrate into the CO bubbles. The CO bubbles float to the surface of the molten steel and enter the atmosphere, achieving the effect of dehydrogenation and denitrification. The decarburization amount should be lower than the lower limit of the specification by 0.02% to 0.03%. The overall process of ore decarburization is: FeO+C= Fe+CO↑ (4) Ensure that the carbon (C) content of the finished product is 0.12% to 0.18% when the steel is tapped, and ensure that the carbon equivalent [CE] of the finished product is ≤ 0.38%.

[0023] Dephosphorization by ore method (P): 2P+5(FeO)=P2O5+Fe (5) When using the ore method to remove carbon (C) and dephosphorization (P), high temperature boiling should be maintained, and ore should be added in batches. After the last batch of ore is added, clean boiling should be maintained for more than 5 minutes. When the carbon content (C) is 0.02% to 0.03% lower than the specified value, the phosphorus (P) is more than 0.020% lower than the specified value, and the molten steel temperature is 10℃ to 15℃ higher than the tapping temperature, the slag can be removed, and it must be removed completely, and then the reduction period can begin.

[0024] Ensure that the phosphorus (P) content of the finished product is ≤0.015% when the steel is tapped.

[0025] In the reduction period desulfurization, 15kg / t to 20kg / t of molten steel is added with lime (CaO), and slag is repeatedly removed.

[0026] Sulfur (S) dissolved in steel usually exists in the form of FeS, and FeS can also exist in slag. The principle of desulfurization (S) is to use elements and element compounds with greater affinity for sulfur (S) than iron (Fe) to capture S in FeS, generating stable compounds that are only soluble in slag but not in molten steel, thereby causing S in molten steel to transfer to slag and be removed. The main elements that can generate stable compounds that are only soluble in slag but not in molten steel are: Ca, Mg, Na, and their sulfides are: CaS, MgS, NaS.

[0027] Mn and S can generate MnS, which has a low solubility in molten steel. Mn is the main alloying element in steel and is not used as a desulfurization material, but it actually plays a certain role in desulfurization. Lime (CaO) is usually the main desulfurization material during steelmaking, provided that the slag is alkaline, and its desulfurization (S) reaction formula is: FeS+CaO=FeO+CaS (6) During the oxidation period of the alkaline electric furnace, due to the high FeO, the desulfurization (S) effect is poor. The desulfurization is effective only when the slag has high alkalinity and low oxidation, high temperature and low slag viscosity, that is, desulfurization during the "reduction period" of steelmaking. In the slag of the "reduction period" of the alkaline electric furnace, the CaO content is usually 60%. The desulfurization reaction in the reduction period is carried out simultaneously with the deoxidation reaction. The deoxidation reaction reduces the FeO in the molten steel and slag to a minimum. The FeO content is usually 0.5%, thereby promoting the effective desulfurization reaction. Frequent slag removal can achieve the effective desulfurization reaction.

[0028] Ensure that the sulfur (S) content of the finished product is ≤0.012% when the steel is tapped.

[0029] The degassing and electric arc furnace refining are deoxidation, removal of hydrogen and nitrogen, and cleaning of slag. Aluminum ingots are inserted into the steel furnace at a rate of 1.5 kg / t of molten steel, and silicon-calcium powder is added at a rate of 1.5 kg / t of molten steel to form slag for slagging. Finally, when the molten steel is tapped, ferrotitanium at a rate of 0.5 kg / t of molten steel is added to the tapping trough or the bottom of the ladle. Before adding, the ferrotitanium needs to be crushed to the size of soybeans and heated to about 800°C.

[0030] The present invention is further described below by taking three heats of steel produced mainly according to the formulation of Example 3 as an example.

[0031] The chemical composition and mass percentage of the three furnaces of high-sulfur and high-chlorine resistant cast carbon steel actually smelted are shown in Table 1.

[0032] The gas content in the three furnaces of high-sulfur and high-chlorine resistant cast carbon steel actually smelted is shown in Table 2.

[0033] The inclusion grades of the three furnaces of high-sulfur and high-chlorine resistant cast carbon steel actually smelted are shown in Table 3.

[0034] The mechanical properties of the three furnaces of high-sulfur and high-chlorine resistant cast carbon steel actually smelted are shown in Table 4.

[0035] The SSC test results of three furnaces of high-sulfur and high-chlorine resistant cast carbon steel actually smelted are shown in Table 5.

[0036] Table 1 Chemical composition and mass percentage content (%) of high sulfur and high chlorine resistant carbon steel As can be seen from Table 1, compared with the chemical composition of ordinary carbon steel, W0219, WO221, W0222, the high-sulfur and high-chlorine resistant carbon steel prepared by the present invention has C≤0.18%, P≤0.015%, and S≤0.011%, which is a low-carbon, ultra-low-sulfur, and low-phosphorus steel.

[0037] Table 2 Gas content of high sulfur and high chlorine carbon steel (μg / g) It can be seen from Table 2 that the gas contents of W0219, WO221 and W0222 are oxygen (O) ≤ 50 μg / g, hydrogen (H) ≤ 3 μg / g, and nitrogen (N) ≤ 80 μg / g.

[0038] Table 3 Inclusion grade of high sulfur and high chlorine carbon steel Table 4 Mechanical properties of high sulfur and high chlorine resistant carbon steel It can be seen from Table 4 that the mechanical properties of W0219, WO221, W0222 are compared with ordinary carbon steel. The yield strength is increased by 4% to 16%, the tensile strength is increased by 2% to 5%, the elongation is increased by 31% to 59%, the cross-sectional shrinkage is increased by 50% to 84%, and the hardness is ≤200HB.

[0039] Table 5 SSC test results of high sulfur and high chlorine carbon steel It can be seen from Table 5 that the high-sulfur and high-chlorine resistant carbon steel sample prepared by the present invention passed the standard state and simulated state (SSC) test.

[0040] The high-sulfur and high-chlorine resistant carbon steel prepared by the present invention was initially used in the gathering and transportation project of the natural gas field with the highest hydrogen sulfide content in my country - the Sichuan Puguang Gas Field Phase I Gathering and Transportation Project. This is the first and largest system of high-sulfur resistant natural gas gathering and transportation project in China. At that time, the system needed a batch of valves made of high-sulfur and high-chlorine resistant materials. Such materials and valve products were not available in China at that time. The design institute originally planned to import foreign valve products, but the price was too high. Later, the system adopted high-pressure flat gate valves and stop valves made of high-sulfur and high-chlorine resistant carbon steel, which saved the country 1 billion yuan at one time. The 1,746 valve products supporting the device were imported from China and were the first valve products made of special materials developed in China, which accelerated the commissioning of the project. Under the special working conditions of the Puguang gas field, this series of low-carbon C≤0.18%, ultra-low sulfur S≤0.012%, low phosphorus P≤0.015% fine-grained fully killed pure steel, high sulfur and high chloride ion resistant carbon steel valve products were developed. After 15 years of field use tests since 2009, the valve quality meets the working conditions, has good operating effect, high stability, tight sealing, zero leakage, greatly reducing the maintenance cost of the system. As the key equipment of the device, this batch of valves is a strong guarantee for the safe operation of the device.

Claims

1. A high-sulfur and high-chlorine resistant cast carbon steel, characterized in that: Its chemical composition and mass percentage content are: C: 0.12%~0.18%, Mn: 0.70%~1.0%, Si: 0.40%~0.60%, P≤0.015%, S≤0.012%, and the rest are Fe and residual elements: Cr≤0.01%, Ni≤0.10%, Mo≤0.05%, Cu≤0.05%, Al≤0.04%.

2. The method for preparing a high-sulfur and high-chlorine resistant cast carbon steel according to claim 1, characterized in that: The smelting process mainly includes decarburization, dephosphorization during oxidation, desulfurization during reduction, degassing and electric arc furnace refining; the decarburization and dephosphorization during oxidation adopt ore decarburization and dephosphorization method, adding 10kg / t molten steel to 20kg / t molten steel of small pieces of iron ore, after red-hot dehydration, adding in batches, after the last batch of ore is added, keep boiling cleanly for more than 5 minutes, when the temperature of molten steel is greater than 1600℃, remove slag and remove cleanly; the desulfurization during reduction, adding 15kg / t molten steel The degassing and electric arc furnace refining are to remove oxidation, hydrogen and nitrogen, and the slag is cleaned. Aluminum ingots are inserted into the steel furnace according to the measurement of 1.5kg / t of molten steel, and silicon-calcium powder is added according to the measurement of 1.5kg / t of molten steel to form slag and scrape it out. When the molten steel is finally tapped, ferrotitanium with a measurement of 0.5kg / t of molten steel is added to the bottom of the steel tapping trough or the ladle. Before adding, the ferrotitanium needs to be crushed to the size of soybeans and heated to 800℃.

3. The method for preparing high-sulfur and high-chlorine resistant cast carbon steel according to claim 2, characterized in that: The oxygen O in the high-sulfur and high-chlorine resistant cast carbon steel product is ≤50 μg / g, the hydrogen H is ≤3 μg / g, and the nitrogen N is ≤80 μg / g.

4. The method for preparing high-sulfur and high-chlorine resistant cast carbon steel according to claim 2, characterized in that: The carbon equivalent [CE] of the high-sulfur and high-chlorine resistant cast carbon steel product is ≤0.38%.

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