A method for improving the SSC resistance of pressure vessel steel

Abstract

A method for improving the SSC resistance of pressure vessel steel comprises: after the steel plate is finished rolled and cooled to 440-460°C according to the original cooling method, forced rapid cooling is performed until it cools to room temperature. The added forced rapid cooling is directly carried out when the temperature drops to approximately 440-460°C during the cooling stage of the existing thermomechanically controlled rolling process. Aside from the addition of this forced rapid cooling process, other process conditions remain unchanged. The yield strength, tensile strength, and elongation of the pressure vessel steel produced using the inventive method do not change much, while having good and stable impact toughness and SSC resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of controlled rolling and controlled cooling microalloyed steel, and relates to a production method for improving the SSC resistance of pressure vessel steel. Background Art

[0002] Pressure vessel steel refers to a specialized type of steel with specialized requirements used in the manufacture of pressure vessels for the petroleum, chemical, gas separation, and gas storage and transportation industries. This special steel is crucial to the national economy and people's livelihood, and is in critical demand across various sectors of the national economy, including the petrochemical, energy, scientific research, and military industries. Pressure vessel steel faces demanding service conditions, with stringent requirements for chemical composition, strength, impact toughness, ductile-brittle transition temperature, weldability, and resistance to sulfide stress corrosion cracking (SSC). Among these, resistance to sulfide stress corrosion cracking (SSC) is even more stringent. Because pressure vessel steel used in the petroleum and chemical industries is susceptible to SSC when used in the separation and storage of petroleum and natural gas containing hydrogen sulfide and acidic media, SSC can have catastrophic consequences. Therefore, SSC is a crucial factor affecting the service life of pressure vessel steel.

[0003] Existing technologies have exposed numerous problems in actual large-scale industrial production. A common problem is the significant fluctuation in the SSC resistance of pressure vessel steel. The SSC resistance of different batches, and even samples from different locations within the same batch, can vary significantly, resulting in significant fluctuations (some samples may pass the standard test, while others may prematurely crack and fail). This SSC resistance fluctuates and cannot be stably controlled. In actual service applications, pressure vessel steel is used in a complex and diverse range of scenarios, significantly increasing the likelihood of being affected by tensile loads. Fluctuations in the SSC resistance of pressure vessel steel can lead to potentially significant bursting risks during field service. Under certain demanding service conditions, pressure vessel steel with significant fluctuations in SSC resistance is not permitted. Consequently, this widespread fluctuation in SSC resistance in large-scale production reduces the yield of pressure vessel steel products, limits the application and development of this type of steel, and has long plagued production practices, requiring urgent resolution. Therefore, developing pressure vessel steel with excellent and stable SSC resistance is a critical issue in pressure vessel steel production. Summary of the Invention

[0004] The purpose of the present invention is to eliminate slow cooling brittleness, significantly reduce the fluctuation of SSC resistance, and improve SSC resistance by simply regulating the cooling rate at a certain stage of the cooling process without changing the chemical composition of existing pressure vessel steel and the TMCP process.

[0005] The technical solution of the present invention:

[0006] A method for improving the SSC resistance of pressure vessel steel, wherein the chemical composition of the steel is as follows: C=0.15%-0.35%, Si=0.3%-0.4%, Mn=1.5%-1.7%, P≤0.02%, S≤0.01%, Cu=0.06%-0.08%, Ni=0.18%-0.22%, Cr=0.06%-0.09%, Mo=0.02%-0.05%, V=0.05%-0.09%, Nb=0.03%-0.04%, Ti=0.01%-0.04%, Al=0.02%-0.04%, and the remainder is Fe and unavoidable impurities; the method comprises the following process steps:

[0007] 1) After the steel plate is finished rolled, it is cooled to 650°C at a cooling rate of 0.5-1°C / s; then cooled to 440-460°C by air cooling;

[0008] 2) Use forced rapid cooling until it cools to room temperature and perform thermomechanical controlled rolling;

[0009] 3) After thermomechanical controlled rolling is completed, it is reheated to 450~650℃ for heat preservation and tempering treatment;

[0010] 4) After the insulation is completed, forced rapid cooling is adopted until the temperature reaches room temperature, and the rapid cooling method is at least one of water cooling, oil cooling and air cooling.

[0011] Principle of the invention:

[0012] In the method of reducing the fluctuation of the SSC resistance of existing pressure vessel steel and improving the SSC resistance of the present invention, the forced rapid cooling until cooling to room temperature can be performed by oil cooling, water cooling, or rapid fan cooling.

[0013] The present invention requires only the addition of a simple cooling device at a certain stage during the cooling process of existing TMCP production lines for pressure vessel steel (some steel mills already have similar cooling devices in the later stages of their production lines). By simply adjusting the cooling rate, fluctuations in SSC resistance can be significantly reduced, while maintaining essentially unchanged strength and excellent, stable impact toughness. This eliminates the need to deliberately pursue chemical composition design, TMCP processes, or modifications to existing rolling equipment to achieve the production of pressure vessel steel with excellent, stable SSC resistance.

[0014] The rationale for adopting this process is that, given the significant variations and fluctuations in SSC resistance between different batches of pressure vessel steel, and even between different locations within the same batch, it is reasonable to conclude that the content of individual elements in the chemical composition and variations or fluctuations in the rolling process during production are not the primary factors contributing to these variations in SSC resistance. After final rolling, pressure vessel steel cools into the ferrite + pearlite dual phase region, where new recrystallized ferrite grains are formed, and finally cools to room temperature. The cooling rate during this cooling process is a key factor influencing the microstructure of pressure vessel steel. If forced rapid cooling is not employed during the cooling process after final rolling, the cooling rate can vary depending on external factors such as the cooling method, ambient production temperature, and stacking load. Even within the same batch, the cooling rate can vary from location to location, resulting in cooling rate fluctuations. This cooling rate fluctuation inevitably leads to varying precipitation behavior of microalloying carbonitrides in the ferrite after the austenite-to-ferrite transformation.

[0015] An important characteristic of alloying elements in pressure vessel steel is their ability to form carbides, nitrides, and carbonitrides with the carbon and nitrogen elements in the steel. These precipitate as a second phase in the ferrite during hot rolling or the cooling process after rolling. Particularly in the temperature range of around 600°C, fine precipitates can precipitate from the supersaturated solid solution. For example, V, the main carbide in steel, V4C3, has the highest nucleation rate around 600°C, and the precipitation effect is most significant. The precipitation and growth of the alloying element second phase are closely related to the cooling rate during the cooling process. If the cooling rate is not fast enough, the precipitated second phase will undergo a certain degree of coarsening and growth; if the cooling rate is fast, the precipitated second phase will not coarsen and grow. Furthermore, the effect of cooling rate on the precipitation of secondary phases during the cooling process requires careful attention. For alloy steels containing elements such as Cr, Mn, Ni, and P, if held in the 450-650°C temperature range for an extended period, carbides of Cr and Ni, along with impurities such as P, As, Sn, and Sb, will precipitate at grain boundaries. Ni and Cr not only promote the segregation of these impurity elements but also segregate themselves. If cooling is followed by a slow cooling below the 450-650°C temperature range, these precipitated impurity phases will segregate and coarsen, dramatically increasing the steel's brittleness, a phenomenon known as slow cooling brittleness. The 450-650°C temperature range is the so-called slow cooling brittleness temperature range. Therefore, during the post-rolling cooling process, if the cooling rate below the slow cooling brittleness temperature range is not sufficiently rapid, slow cooling brittleness will occur. Conversely, if the cooling rate is relatively rapid, embrittlement will disappear or be suppressed. As mentioned above, during the cooling process after final rolling of pressure vessel steel, if forced rapid cooling is not adopted, the cooling rate or cooling speed will vary depending on external factors such as the cooling method, production environment temperature, and stacking volume, that is, the cooling rate will fluctuate. When the cooling rate is slower during the cooling rate fluctuation, slow cooling brittleness is easily generated, which promotes SSC, resulting in low SSC resistance. Conversely, when the cooling rate is faster during the cooling rate fluctuation, slow cooling brittleness does not occur, and therefore the steel has good and stable impact toughness and SSC resistance.

[0016] Given that fluctuations in cooling rate during the post-finish cooling process can lead to fluctuations in SSC resistance, an effective solution is to continue cooling to approximately 450°C (the lower limit of the slow-cold brittleness temperature range) after final rolling according to the conventional cooling method, followed by forced rapid cooling until the steel reaches room temperature. This avoids cooling rate fluctuations and eliminates slow-cold brittleness. The lower limit of the slow-cold brittleness temperature range is chosen to minimize the formation of a hard, brittle bainite phase during the post-rolling cooling process. From an environmental and cost-effective perspective, water cooling or rapid fan cooling is more attractive than oil cooling. Rapid cooling to room temperature, rather than simply cooling below the slow-cold brittleness temperature range, is based on the consideration that after hot rolling, heat transfer from the core causes the surface temperature to rise, resulting in a red-hot temperature. This red-hot temperature is typically higher than the slow-cold brittleness temperature range. Therefore, the subsequent cooling process from the red-hot temperature range to room temperature is more likely to result in slow-cold brittleness.

[0017] The pressure vessel steel produced by the method of the present invention has the following advantages compared with the same material produced under existing conditions: the pressure vessel steel produced by the method of the present invention has good and stable impact toughness and SSC resistance, and has almost no loss in strength. DETAILED DESCRIPTION Example 1

[0018] A method for producing pressure vessel steel, the chemical composition of which is shown in Table 1. The method of the present invention adds a forced controlled cooling step to the cooling process after the thermomechanically controlled rolling process. Specifically, after the thermomechanically controlled rolling process, the steel is cooled to 650°C at a cooling rate of 1°C / s, air-cooled between 650°C and 450°C, and water-cooled at 450°C until cooled to room temperature. The mechanical properties and SSC resistance of the materials obtained by these two methods are shown in Table 2. As can be seen from Table 2, the yield strength, tensile strength, and elongation of the steel plates according to the present invention are not significantly different from those of the steel plates not according to the present invention, while exhibiting excellent and stable impact toughness and SSC resistance.

[0019] Comparative Example 1: Steel plate was rolled on a rolling mill, following the thermomechanically controlled rolling process, cooled to 650°C at a rate of 1°C / s, and then air-cooled to room temperature. The resulting steel exhibited mechanical properties and SSC resistance, as shown in Table 2. Example 2

[0020] A pressure vessel steel sample (10 mm wall thickness) with the measured chemical composition shown in Table 1 was reheated to 650°C in a laboratory box furnace, then held at this temperature for 10 minutes. After the holding period, the sample was removed from the furnace and air-cooled to 450°C, followed by water cooling until it reached room temperature. The mechanical properties of the sample before and after treatment are shown in Table 2. As can be seen from Table 2, the yield strength, tensile strength, and elongation of the heat-treated sample are not significantly different from those in the as-rolled state, while the sample exhibits excellent and stable impact toughness and SSC resistance.

[0021] Comparative Example 2: Steel plate was rolled on a rolling mill, following the thermomechanically controlled rolling process, using a cooling rate of 1°C / s to 650°C, followed by air cooling to room temperature. The resulting steel exhibited mechanical properties and SSC resistance, as shown in Table 2.

[0022] Table 1 Chemical composition of example steel (wt%)

[0023] .

[0024] Table 2 Mechanical properties and SSC resistance of steel in Example 2.

[0025] .

[0026] Table 2 shows the SSC test results based on NACE TM 0177-2016 Test Method: Method A, with a loading of 80% x yield strength. Each sample group consists of three specimens. If one or more specimens in a sample group fail to fracture within 720 hours of testing, the group is considered to have failed the SSC test, denoted by an N. If all three specimens in the same group fail to fracture within 720 hours of testing, the group is considered to have passed the SSC test, denoted by a Y.

Claims

1. A method for improving the SSC resistance of pressure vessel steel, characterized by: The chemical composition of steel by weight percentage is C=0.15%~0.35%, Si=0.3%~0.4%, Mn=1.5%~1.7%, P≤0.02%, S≤0.01%, Cu=0.06%~0.08%, Ni=0.18%~0.22%, Cr=0.06%~0.09%, Mo=0.02%~0.05%, V=0.05%~0.09%, Nb=0.03%~0.04%, Ti=0.01%~0.04%, Al=0.02%~0.04%, and the rest is Fe and unavoidable impurities; The process steps include: 1) After the steel plate is finished rolled, it is cooled to 650°C at a cooling rate of 0.5-1°C / s; then cooled to 440-460°C by air cooling; 2) Use forced rapid cooling until it cools to room temperature and perform thermomechanical controlled rolling; 3) After thermomechanical controlled rolling is completed, it is reheated to 450~650℃ for heat preservation and tempering treatment; 4) After the insulation is completed, forced rapid cooling is adopted until the room temperature is reached; The forced rapid cooling method in steps 2) and 4) is at least one of water cooling, oil cooling and air cooling.