KR101359109B1 - Pressure vessel steel with excellent sulfide stress cracking resistance and low temperature toughness and manufacturing method thereof - Google Patents

Abstract

In one aspect of the present invention, the pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness is% by weight, C: 0.03 to 0.18%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, and Al: 0.005 to 0.1. %, Cu: 0.15 to 0.5%, Ni: 0.15 to 1.0%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.05%, Ca: 0.001 to 0.003%, N: 0.001 to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04-0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4-1.6, residual Fe and other unavoidable impurities.
In another aspect of the present invention, a method for manufacturing a pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness is% by weight, C: 0.03 to 0.18%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, and Al. : 0.005 to 0.1%, Cu: 0.15 to 0.5%, Ni: 0.15 to 1.0%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.05%, Ca: 0.001 to 0.003%, N: 0.001 to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04-0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4-1.6, balance Fe and other unavoidable impurities Reheating the slab, roughly rolling the reheated slab, hot-rolling the roughly rolled steel to a finishing temperature of 850 ° C to 950 ° C, followed by air cooling the hot-rolled steel material, and then normalizing heat treatment. And cooling the normalized heat treated steel.

Description

PRESSURE VESSEL STEEL WITH EXCELLENT SULFIDE STRESS CRACKING RESISTANCE AND LOW TEMPERATURE TOUGHNESS AND MANUFACTURING METHOD THEREOF}

The present invention relates to steels used in pressure vessels under pressure such as refining equipment and storage tanks for refining crude oil containing a large amount of H ₂ S, and more particularly, to sulfide stresses in an H ₂ S environment. The present invention relates to a steel having excellent crack resistance and excellent low temperature toughness when used at a low temperature, and a method of manufacturing the same.

In the case of pressure vessels, the required properties are determined by the temperature at which the container is used and the characteristics of the substance to be stored in the container. If the use temperature is low, low temperature toughness is required, and the deterioration of the steel due to corrosion depends on the kind of the storage material. Therefore, specific physical properties are required depending on the kind of the storage material. In recent years, the demand for steel materials which are resistant to the deterioration of materials by H ₂ S has been increasing in the steel materials required for the crude oil refining facility due to the increase of mining of crude oil having high H ₂ S content.

In addition, the demand for low-temperature toughness is also increasing, as the mining and refining environment is moving to a poorer environment where the operating temperature is lowered to below zero. There is an increasing demand for multi-functional steels that simultaneously require deterioration of materials and low-temperature toughness due to H ₂ S in pressure vessels.

In the environment containing H ₂ S, when hydrogen atoms generated by corrosion penetrate into the material from the outside and hydrogen atoms reach a critical concentration or more, cracks are generated and growth failure occurs. Hydrogen atoms that enter the material diffuse in the material and are captured by weak impurities, especially MnS and segregation bands, inclusions and the like. When the hydrogen atoms are concentrated in such portions, the hydrogen embrittlement deteriorates the mechanical properties of the material, and the stress applied locally increases, so that the maximum stress that the material can withstand is lowered. If the stress applied locally is greater than the stress that the material can withstand, the crack will grow and fracture will develop.

Particularly, when stress is applied like a pressure vessel, the cracks generated by hydrogen are connected in the vertical direction of the stress and the material is broken. Therefore, in order not only to minimize the number of places that can act as crack initiation points, such as MnS and non-metallic inclusions, but also to prevent cracks from propagating due to stress, the structure should be controlled to prevent crack propagation.

Generally, the lower the temperature, the lower the toughness at low temperature below the ductile-brittle transition temperature, so that the steel easily cracks and propagates even in the case of weak impact, which greatly affects the stability of the container. Therefore, a steel having a low use temperature should control the microstructure so that the soft-brittle transition temperature is lower than the use temperature so that brittleness does not occur at the use temperature. The impact toughness can be measured by the Charpy impact energy value, and the impact toughness is improved as the Charpy impact energy value is increased. In order to increase Charpy energy impact value, the addition of impurities such as sulfur or phosphorus should be minimized and the amount of alloying elements such as Ni should be appropriately added. The ductile-brittle transition temperature is closely related to the microstructure, and in terms of microstructure, it is preferable that pearlite acts as a starting point of cracking, and ferrite and pearlite interface facilitate crack propagation. Further, in order to make propagation of the cracks difficult, it is necessary to make many grain boundaries which are an obstacle to crack propagation. In other words, the finer the crystal grain, the more the crack propagation is interfered, which is helpful for lowering the transition temperature.

Accordingly, there is a growing need for the material degradation due to hydrogen and the improvement of the low temperature toughness in the H ₂ S environment. Therefore, many researches have been made in order to realize material deterioration due to hydrogen and improvement in low temperature toughness.

Patent document 1 proposes the 600MPa class pressure vessel steel excellent in the toughness used for materials, such as a boiler and a pressure vessel of a power plant. In the steel disclosed in Patent Document 1, Mo, B, and the like were added to improve the strength. However, these elements have a problem of impairing hydrogen organic crack resistance, and Ni has a problem that it is not economical because its addition amount is too large. In addition, Cu and Ca, which are essential for improving hydrogen organic crack resistance, are not included.

In addition, Patent Document 2 proposes a thick steel plate for pressure vessels having excellent strength of hydrogen-organic cracks, which can be stably used even in a H ₂ S (sour gas) gas atmosphere while satisfying the strength of 500 MPa. The thick steel plate disclosed in Patent Literature 1 is limited to 0.020% or less, but the upper limit thereof is too high to ensure hydrogen organic crack resistance, and Ca / S> 1.0 can be satisfied in the Ca addition range of 0.0005 to 0.005. There is no problem. In the manufacturing method, the control rolling is proposed under the unrecrystallized zone temperature, but it takes a long time to cool the steel sheet to the unrecrystallized zone temperature during the control rolling, so it is not economical to decrease the production productivity. In addition, since the structure is air-cooled after normalizing, there is a problem in that it is difficult to secure excellent low-temperature toughness and hydrogen organic crack resistance because the tissue is composed of ferrite pearlite tissue.

Korean Patent Publication No. 2004-0021117 Korean Patent Publication 0833070

The present invention is to provide a pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness in the H2S atmosphere by optimizing the components and manufacturing conditions of the steel and its manufacturing method.

In one aspect of the present invention, the pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness is% by weight, C: 0.03 to 0.18%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, and Al: 0.005 to 0.1. %, Cu: 0.15 to 0.5%, Ni: 0.15 to 1.0%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.05%, Ca: 0.001 to 0.003%, N: 0.001 to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04-0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4-1.6, residual Fe and other unavoidable impurities.

In another aspect of the present invention, a method for manufacturing a pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness is% by weight, C: 0.03 to 0.18%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, and Al. : 0.005 to 0.1%, Cu: 0.15 to 0.5%, Ni: 0.15 to 1.0%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.05%, Ca: 0.001 to 0.003%, N: 0.001 to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04-0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4-1.6, balance Fe and other unavoidable impurities Reheating the slab, roughly rolling the reheated slab, hot-rolling the roughly rolled steel to a finishing temperature of 850 ° C to 950 ° C, followed by air cooling the hot-rolled steel material, and then normalizing heat treatment. And cooling the normalized heat treated steel.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, a steel material having a ferrite having an area fraction of 20% or more and a ferrite having a residual ferrite and having a ferrite having an average grain size of 20 µm or less can be provided. Through the manufacture of the structured steel, the steel for pressure vessel has high resistance to cracking in the state of external stress applied in H ₂ S environment and has very good impact toughness at low temperature with ductile-brittle transition temperature below -10 ℃. Can provide.

1 (a) and (b) is a photograph of observing the microstructure of the invention examples 1 and 9, Figure 1 (c) is a photograph of observing the microstructure of Comparative Example 4.

The deterioration of the material caused by hydrogen generated by the corrosion under wet H ₂ S environment is mainly due to hydrogen organic cracking and sulfide stress cracking. Hydrogen organic cracking is a phenomenon in which cracks occur in the material in the absence of stress, whereas sulfide stress cracking is a phenomenon in which material cracks or fractures under stress in the material.

Sulfide stress cracks are easier to propagate by cracking because much more hydrogen penetrates into the material than hydrogen organic cracks and easily initiates cracking. Therefore, the inventors of the present invention have found out that the sulfide stress crack resistance is not necessarily superior due to the excellent hydrogen organic crack resistance. The inventors of the present invention have found that it is essential to control the component that can reduce the amount of hydrogen generated by corrosion and to control the microstructure so that the crack does not easily propagate even under stress in order to obtain an excellent sulfide stress cracking resistance.

A method for improving sulfide stress cracking resistance is to reduce the amount of hydrogen penetrating into the material. Since hydrogen is generated by corrosion, it is necessary to form a stable corrosion product film which can reduce the corrosion of the material or interfere with hydrogen intrusion into the material. In order to lower the corrosion rate or to form stable corrosion products on the surface, alloying elements such as Cu, Ni, Cr, Mo, etc. should be appropriately combined.

In particular, Cu and Ni are important elements that form a stable corrosion product on the surface in acidic solution to prevent the internal penetration of hydrogen, it is important to control the addition amount. It is necessary to make the hydrogen permeated into the material uniformly distributed in the material without accumulating in the hydrogen accumulation sites such as inclusions and internal defects.

This requires a site that can catch hydrogen inside the material and can be realized through the control of precipitates. Nb, V, and Ti fine precipitates act as a trap site of hydrogen to trap hydrogen in the material, thereby preventing hydrogen accumulation and thus improving sulfide stress cracking resistance. Therefore, it is essential to control the content of these elements and to control the rolling and tempering temperature and time so that fine precipitates such as Nb, V and Ti can be formed well. Finally, delaying the propagation of cracks generated by hydrogen is necessary to increase the susceptibility to sulphide stress bacteria. Since propagation of cracks progresses by breaking phase boundaries or phases with high hardness when abnormalities exist, it is essential to construct a microstructure as a single phase, and it is necessary to refine the grains in order to prevent crack growth due to the large grain boundaries. It is advantageous. It was also found that acicular ferrites with sub-grain boundaries in the grains as well as grain boundaries are the most desirable tissues for inhibiting crack propagation.

In addition, when the ductile-brittle transition temperature of the steel is lower than the use temperature, it can be said that it can be safely used as a steel having excellent low temperature toughness. Ductile-brittle transition temperatures depend primarily on the microstructure. In general, the elements that contribute to known toughness are effective in increasing upper shelf energy, but are not very effective in lowering the transition temperature. In order to lower the transition temperature, controlling the C-Mn ratio and refining the microstructure are the most effective methods. Since C-Mn is the most influential element, the C / Mn ratio control is essential to lower the transition temperature while maintaining the strength. The microstructure can lower the transition temperature when controlled in the same way as the sulfide stress cracking resistance improvement. In addition, since the crack is initiated at a minute low temperature, such as phase martensite (MA) at the time of impact, it is important not to make MA in terms of crack initiation, so it is preferable to appropriately limit the amount of addition of Si, Mo, etc., which is the MA element.

Therefore, the present inventors have conducted research to secure excellent sulfide stress cracking resistance, and as a result, in order to obtain excellent sulfide stress cracking resistance, it is possible to reduce the propagation of cracks easily under stress and component control which can reduce the amount of hydrogen generated by corrosion. It has been found that controlling tissue is essential and led to the present invention.

Hereinafter, one side of the present invention will be described in detail with respect to the sulfide stress cracking resistance and low temperature toughness steel.

In one aspect of the present invention, the pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness is% by weight, C: 0.03 to 0.18%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, and Al: 0.005 to 0.1. %, Cu: 0.15 to 0.5%, Ni: 0.15 to 1.0%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.05%, Ca: 0.001 to 0.003%, N: 0.001 to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04-0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4-1.6, residual Fe and other unavoidable impurities.

Carbon (C): 0.03-0.18 wt%

C is an element added to improve the strength, and its content can be improved by increasing its content, but as the addition amount increases, the fraction of harder phases such as pearlite and bainite increases, so that sulfide stress cracking resistance and low temperature toughness are increased. Inhibits. In order to improve sulfide stress cracking resistance and low temperature toughness, it is necessary to reduce the C content, but when C is less than 0.03% by weight, it is difficult to secure the strength, and when it exceeds 0.18% by weight, sufficient toughness cannot be obtained and the weldability is deteriorated. Since it is not preferable as molten steel, the range is limited to 0.03 to 0.18 weight%. From the viewpoint of sulfide stress cracking resistance and low temperature toughness, it is more preferable that C be 0.14% by weight or less.

Silicon (Si): 0.05 to 0.5 wt%

Si is an element added to control inclusions because it acts as a deoxidizer. In order to exhibit such an effect in the present invention, it is preferable that 0.05 wt% or more is included. On the other hand, when the content of silicon exceeds 0.5% by weight, it promotes the formation of MA to increase the soft-brittle transition temperature and inhibit the weldability. In addition, there is a problem of reducing the sulfide stress cracking resistance and low temperature toughness by increasing the amount of oxidation inclusion in the steel. Therefore, the silicon is preferably contained in 0.05 to 0.5% by weight.

Manganese (Mn): 0.5 to 2.0 wt%

Mn is an effective element to solidify the steel. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.5 wt% or more. When the content of manganese increases, the hardenability increases to increase the strength, but when it exceeds 2.0% by weight, there is a problem in that weldability is lowered. Therefore, the content of manganese is preferably included in 0.05 to 2.0% by weight. Since Mn is an element capable of improving strength without significantly deteriorating low-temperature toughness, it is more preferable to add Mn at 0.8% by weight or more.

Aluminum (Al): 0.005 to 0.1 wt%

Al is an element that is essentially added for deoxidation during steelmaking, and improves impact absorption energy. Like Al, Al reacts with oxygen to form oxide inclusions. When the Al content is less than 0.005% by weight, deoxidation is not sufficiently achieved. When the Al content is more than 0.1% by weight, not only impairs impact toughness but also forms a large amount of inclusions, thereby inhibiting hydrogen organic cracking resistance. Since there is a problem, the aluminum content is preferably limited to 0.005 ~ 0.1% by weight.

Copper (Cu): 0.15-0.5 wt%

Cu is an element added to improve strength and toughness of a steel and to improve corrosion resistance. Cu must be added in an amount of 0.15% by weight or more because it is dissolved in steel to improve strength and to form a protective film on the surface in an atmosphere containing hydrogen sulfide, thereby lowering the corrosion rate of the steel and reducing the amount of hydrogen diffused into the steel. However, since Cu is an element that causes cracks on the surface during hot rolling and inhibits surface quality, it is preferable to limit the upper limit to 0.5% by weight or less. It is more preferable to add Cu by 0.2 wt% or more for hydrogen permeation effect through stable surface corrosion product formation.

Nickel (Ni): 0.15 to 1.0 wt%

Ni is an element that reduces the corrosion rate of steel and improves the toughness of steel in acidic solution and is added for sulfide stress cracking resistance and toughness improvement. In addition, it also serves to suppress the occurrence of surface cracks in the steel due to the addition of Cu. In order to reduce surface cracks due to Cu addition, it is preferable to add 1.0 times or more of the Cu-added steel. Therefore, the lower limit of Ni is made 0.15% by weight or more, similarly to the lower limit of Cu. Although it is expected that sulfide stress cracking resistance and toughness will be improved by adding Ni, Ni is an expensive element, which leads to an increase in the price of steel, so it is preferable to limit the upper limit to 1.0% by weight.

Niobium (Nb): 0.01 to 0.05 wt%

Nb is dissolved at a temperature of around 1200 ° C and precipitates in the form of Nb (C, N) during hot rolling to increase the strength and acts as a trap site of hydrogen which has entered the steel, Thereby improving sulfide cracking resistance. In addition, the precipitate upon recrystallization generated during normalizing suppresses the nucleation site and ferrite grain growth, thereby refining the crystal grains and suppressing crack propagation. In order to exhibit such an effect in the present invention, 0.01% by weight or more should be added. However, the upper limit should be limited to 0.05% by weight or less since it may act as a starting point for the generation of sulfide stress cracks in the production of coarse crystals containing Nb depending on the correlation with the C, N and Ti contents and the playing conditions. Do.

Titanium (Ti): 0.01 ~ 0.05 wt%

Ti is an element which forms carbide or nitride and inhibits crystal growth of austenite phase during reheating, and ultimately serves to form fine homogeneous ferrite, which is an element for improving low-temperature toughness. In order to exhibit such an effect of the present invention, it is preferable to include 0.01 wt% or more. The finely dispersed Ti (C, N) precipitate acts as a strong hydrogen trap site to reduce the diffusion coefficient of hydrogen, thereby increasing sulfide stress cracking resistance. However, if the added amount is increased, Ti reacts with N in the steel and prevents formation of Nb (C, N) precipitates that are effective at low temperature toughness. Therefore, the low temperature toughness is inhibited. Therefore, limiting the upper limit to 0.05% by weight or less desirable.

Calcium (Ca): 0.001-0.003 wt%

Ca serves to shape the MnS inclusions. MnS is an inclusion with a low melting point and is stretched when rolled to serve as a starting point of hydrogen cracking. The added Ca reacts with MnS and surrounds MnS, which interferes with the stretching of MnS. The MnS spheroidization effect of Ca is closely related to the amount of S, but in order to exhibit the spheroidization effect, it should be added at least 0.001% by weight. Ca is an element having a low yield due to its high volatility, and it is preferable to limit the upper limit to 0.003 wt% or less in consideration of the load generated in the providing process. Therefore, the content of Ca is preferably included in 0.001 ~ 0.003% by weight.

Nitrogen (N): 0.001 to 0.01 wt%

Since N is difficult to completely remove industrially from steel, N is made into the lower limit 0.001 weight% which is the range which can accept the load in a manufacturing process. N forms nitrides with Al, Ti, Nb, V, etc., which hinders austenite grain growth, thereby improving toughness and improving strength, but when the content is excessively contained exceeding 0.01% by weight, N in solid state And N in these solid solution states adversely affect the toughness, so the range is preferably included in the range of 0.001 to 0.01 wt%.

Since the steel of the present invention can be used as a steel for pressure vessels, in consideration of this, the contents of the following C, Mn, Nb, Ti, V, and N elements preferably satisfy the following relationship.

C / Mn: 0.04 ~ 0.16

C and Mn are the elements that have the greatest influence on the strength and toughness of the steel. In order to improve the strength, it is desirable to increase these elements, but the toughness depends greatly on the ratio of these two elements. If the C / Mn value is less than 0.04, the tensile strength of the steel is less than 400 MPa, which makes it difficult to use, so it is preferable to limit the lower limit of C / Mn to 0.04. On the other hand, when the C / Mn ratio exceeds 0.16, the ductile-brittle transition temperature of the steel becomes more than -10 ° C and thus cannot be used at low temperatures. Therefore, in order to manufacture a pressure vessel steel having a tensile strength of 400 MPa or more and a ductile-brittle transition temperature of -10 ° C. or less, it is preferable to limit the ratio of C / Mn to 0.04 to 0.16.

((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4 ~ 1.6

Nb, Ti, and V are elements that form fine precipitates during rolling or heat treatment, and are useful for improving the toughness due to the increase in strength and grain refinement by the fine precipitates when these elements are added. In addition, when fine precipitates are uniformly distributed in the steel, it traps hydrogen in the steel, thereby preventing the concentration of hydrogen into defects, thereby improving sulfide stress cracking resistance. In order to take advantage of such microprecipitates, the amount and ratio of C and N addition elements, which form precipitates, must be appropriate. In other words, when the value of ((Nb + Ti + V) * 0.1) / (C * 0.01 + N) is less than 0.4, the excess C and N conditions are reduced, so the toughness is lowered. Therefore, the lower limit is preferably 0.4. If the value exceeds 1.6, it is preferable to limit the upper limit to 1.6 because the amount of Nb, Ti, and V added is excessively formed, resulting in coarse precipitates, which do not help to improve strength or toughness, but rather lower the toughness. Therefore, it is preferable that ((Nb + Ti + V) * 0.1) / (C * 0.01 + N) is contained in 0.4-1.6.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

However, since phosphorus and sulfur are generally referred to as impurities, a brief description thereof is as follows.

Phosphorus (P): 0.012 wt% or less

P is an element that is inevitably included in steel during steelmaking, and not only inhibits weldability and toughness but also easily segregates at the center of slab and austenite grain boundary during solidification. Preferably, in consideration of the load generated in the steelmaking process, it is preferable to limit the content to 0.012% by weight or less.

Sulfur (S): 0.0015 wt% or less

S is an impurity element and generally reacts with Mn to form MnS, which is elongated during rolling to act as a starting point for generating hydrogen organic cracks and to inhibit low temperature toughness. Therefore, it is preferable to reduce S as much as possible, but it is preferable to limit the range to 0.0015 wt% or less due to process constraints for S removal.

According to one aspect of the present invention, by satisfying the component system, it is possible to provide a pressure vessel steel material excellent in sulfide stress cracking resistance and low temperature toughness. However, when one or more of Cr, Mo, W and V described below may be further included, the effect of the present invention may be further improved.

Chromium (Cr): 0.05-0.5 wt%

Cr plays a role not only to increase the strength of the steel by increasing the incombustibility of the steel, but also to reduce the corrosion rate of the steel, thereby reducing the amount of hydrogen generated. To increase the strength and improve the hydrogen organic cracking resistance, it should be added at least 0.05% by weight. As the content of chromium increases, the strength increases, but when the content exceeds 0.5% by weight, the toughness of the steel is inhibited, so the upper limit is preferably limited to 0.5% by weight or less. Therefore, the content of chromium is preferably contained in 0.05 to 0.5% by weight.

Molybdenum (Mo): 0.05-0.3 wt%

Mo, like Cr, is an element that improves strength and resistance to hydrogen organic cracking. Mo is harder than Cr, so the effect is higher than that of Cr. In order to obtain the effect of Mo addition, it should be added at least 0.05% by weight. If it exceeds 0.3% by weight, the economical efficiency is not good, and the hardness is very easy to form MA, and the upper limit of low temperature toughness and sulfide stress crack resistance is inhibited. Silver is preferably limited to 0.3% by weight or less.

Tungsten (W): 0.05-0.3 wt%

W is an element that lowers the corrosion rate of steel in acidic solution. When W is added, the amount of hydrogen generated decreases due to the decrease in corrosion rate, thereby reducing the amount of hydrogen that penetrates into the steel, thereby improving resistance to sulfide stress cracking. In order to reduce the corrosion rate of steel, it should be added more than 0.05% by weight. Corrosion resistance is expected to increase with the addition of W, but W is not only an increase in the specific gravity of steel as a heavy metal, but also an increase in price, so the upper limit is preferably limited to 0.3 wt% or less.

Vanadium (V): 0.01% to 0.05% by weight

When the amount of N is sufficiently present in the steel, VN is formed, but it generally precipitates in the form of VC in the ferrite region. The vacancy carbon concentration at the time of transformation to austenite-ferrite is lowered, and VC provides a nucleation site for cementite formation. Therefore, rather than continuously forming Fe ₃ C at the grain boundary has a form of a discontinuous structure to increase the resistance to sulfide stress cracking. In order to exhibit this effect in the present invention, it should be added at least 0.01% by weight. On the other hand, when the content exceeds 0.05% by weight, coarse V precipitates are formed to inhibit toughness and become hydrogen accumulation sites in steel, thereby lowering resistance to hydrogen organic cracks. Therefore, the content of vanadium is preferably included in 0.01 to 0.05% by weight.

According to an aspect of the present invention, by satisfying the component system, it is possible to provide a pressure vessel steel material excellent in sulfide stress cracking resistance and low temperature toughness. The microstructure of the steel may include more than 20% acicular ferrite and the balance ferrite.

Hereinafter, a method of manufacturing a pressure vessel steel material having excellent sulfide stress cracking resistance and low temperature toughness, which is another aspect of the present invention, will be described in detail.

In another aspect of the present invention, a method for manufacturing a pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness is% by weight, C: 0.03 to 0.18%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, and Al. : 0.005 to 0.1%, Cu: 0.15 to 0.5%, Ni: 0.15 to 1.0%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.05%, Ca: 0.001 to 0.003%, N: 0.001 to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04-0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4-1.6, balance Fe and other unavoidable impurities Reheating the slab, roughly rolling the reheated slab, hot-rolling the roughly rolled steel to a finishing temperature of 850 ° C to 950 ° C, followed by air cooling the hot-rolled steel material, and then normalizing heat treatment. And cooling the normalized heat treated steel.

Reheat step

It is preferable to reheat the slab which satisfies the above-mentioned component system at 1050-1250 degreeC. According to the present invention, Mn and P segregation on the slab are relaxed by diffusion during reheating. When the reheating temperature is low, diffusion does not sufficiently take place, so segregation of Mn and P remains, It will damage the resistance. In case of Nb added steel, Nb added to steel is sufficiently solidified during reheating and fine precipitation during rolling or heat treatment improves strength and low temperature toughness. Therefore, it is desirable to limit the lower limit of the reheating temperature to 1050 ° C in order to alleviate segregation in the slab and to employ Nb. When the heating temperature is high, the relaxation of the segregation part and the solidification of Nb are easy, but at the same time, the grain size of the austenite is increased, and the low temperature toughness is deteriorated. Therefore, in order to obtain excellent low temperature toughness, it is preferable to limit the upper limit to 1250 캜.

Rough rolling  step

Grain refinement requires a rolling temperature and a reduction ratio that can sufficiently cause austenite recrystallization during rough rolling. In particular, since the recrystallization rate of the final three passes of rough rolling is the most important factor determining the final grain size, it is necessary to control the rough rolling end temperature and the reduction rate of the final three passes. The upper limit of the rough rolling end temperature is limited to 1100 ° C. When the temperature exceeds 1100 ° C, recrystallization easily occurs, but since the air-cooling waiting time increases from finishing rough finishing to finishing rolling, austenite grains grow and cause grain coarsening. On the other hand, when the rough rolling end temperature is less than 1000 ℃, partial recrystallization occurs even at a high rolling rate, the toughness is inferior because a non-uniform size of austenite is formed. Therefore, it is preferable to limit rough rolling end temperature to 1000 degreeC-1100 degreeC.

The recrystallization phenomenon is related not only to the rolling temperature but also to the reduction ratio. The lower the rolling temperature, the higher the reduction ratio required to generate recrystallization. Therefore, when the rolling reduction rate of the last three passes during rough rolling at which the rolling temperature is lowered is less than 10% per pass, sufficient recrystallization for grain refinement does not occur. Therefore, it is preferable to limit the rolling reduction per pass of the last three passes among the rough rolling passes to 10% or more.

Hot Finish Rolling Step

Deformation zones are formed in the grains during hot finishing rolling, and these deformation zones act as grain nucleation sites during phase transformation, and thus it is advantageous to form more strain zones during hot finishing rolling. In order to form more deformation zones, it is advantageous that the rolling temperature is low, so the finishing rolling start temperature is limited to 950 ° C or less. If the temperature exceeds 950 ° C, the strain energy applied during finishing rolling disappears at a high temperature, so that energy accumulation does not occur and thus does not significantly affect grain refinement. Therefore, as the temperature decreases, the accumulation of strain energy is advantageous, but the effect of reducing the pharmaceutical reduction ratio of the equipment is not only canceled, but also adverse effects occur during rolling in the austenite and ferrite two-phase spheres. Therefore, it is preferable to limit hot finishing rolling temperature to 850 degreeC or more.

Air cooling step

It is preferable to air-cool the hot finished rolled steel to 100 ° C or less. If the air cooling temperature exceeds 100 ℃ there is a risk of shortening the life of the facility due to high temperatures and safety accidents in the step of cutting and transporting the steel in order to perform the normalizing step to be described later.

Normalizing

When normalizing, the time required for complete austenite transformation to the center of the steel is required. Since the time varies depending on the thickness of the steel (t, mm), the normalizing time is determined according to the following formula.

Time = 1.3 * t + (10 ~ 30 minutes) ------------------------------------ Equation 1 )

Cooling stage

Air cooling after normalization is a general normalized heat-treated steel manufacturing process, but in the above-described ferrite and perlite abnormalities, sufficient sulfide stress cracking resistance and low temperature toughness cannot be obtained, so in the present invention, the steel is accelerated and cooled through water cooling after normalizing. In order to obtain a uniform single-phase structure having fine grains, cooling should be started at an Ar3 temperature or higher. Therefore, it is preferable to limit the cooling start temperature to 810 ° C or higher in consideration of the Ar3 temperature of the inventive steel.

In order to obtain a single-phase structure, the cooling end temperature must be lower than the ferrite transformation end temperature. In other words, the transformation temperature is terminated in water cooling, otherwise the untransformed austenite is transformed into pearlite when air-cooled after cooling, thereby inhibiting low temperature toughness and hydrogen organic crack resistance. Therefore, it is generally desirable to limit the cooling end temperature to 550 ° C. or less, which is the transformation end temperature of the carbon steel. Although it is related to the cooling rate, if the transformation end temperature is less than 250 ° C., the lower end of the cooling end temperature is limited to 250 ° C. because MA is generated and the low temperature toughness is inhibited. Therefore, the cooling end temperature is preferably limited to 250 ~ 550 ℃.

The microstructure can be controlled by adjusting the cooling rate. In order to produce acicular ferrite having a grain boundary, the cooling rate must be controlled in an appropriate range. The optimum cooling rate of the microstructure is closely related to the Ceq value described in Equation 2 below. The higher the Ceq, the easier bainite is produced even at a lower cooling rate. The lower the Ceq, the higher the cooling rate is required for the formation of acicular ferrite. Therefore, in order to control the acicular ferrite fraction, the cooling rate should be controlled according to the Ceq of the steel. If the cooling rate / Ceq is less than 15 ℃ / s can not form a sufficient fraction of acicular ferrite in the tissue. On the other hand, when the cooling rate / Ceq exceeds 25 ° C / s, bainite is produced, rather inhibiting sulfide stress cracking resistance and toughness. Therefore, the cooling rate / Ceq is preferably limited to 15-25 ° C / s.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 ------------------------- Equation 2)

In addition, the steel material may further include the annealing heat treatment at a temperature of 550 ~ 650 ℃. If the annealing heat treatment temperature is less than 550 ℃, it is difficult to remove the residual stress generated after normalizing, and if the annealing heat treatment temperature is higher than 650 ℃, a sudden decrease in strength and toughness of the steel material occurs. In addition, the annealing heat treatment time is limited to 10 to 90 minutes because it is difficult to remove the residual stress when the reference time is shorter, and when the length is longer than the reference time, a sudden decrease in strength and toughness of the steel material occurs.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

 Inventive Examples 1 to 15 and Comparative Examples 1 to 6 satisfying the component systems shown in Table 1 are then manufactured by using a continuous casting to produce a steel slab for the pressure vessel by a manufacturing method that satisfies the conditions of Table 2 below. It was.

After reheating Inventive Examples 1 to 15 and Comparative Examples 1 to 6 satisfying the component systems described in Table 1 as shown in Table 2 below, hot-rolled rolling was performed and air-cooled to prepare steel materials having a thickness of 30 mm, and manufactured steel materials. After normalizing 60 minutes, it was water-cooled.

No. Heating temperature (℃) Rough rolling end temperature (℃) Hot Finish Rolling Start Temperature (℃) Normalizing Temperature (℃) Cooling start temperature (℃) Cooling end temperature (℃) Cooling Rate / Ceq (℃ / s) Inventory 1 1120 1050 950 900 850 485 18 Inventive Example 2 1154 1080 950 890 840 450 19.5 Inventory 3 1180 1080 950 900 850 500 17.2 Honorable 4 1198 1090 940 920 860 420 20.8 Inventory 5 1100 1030 940 890 850 550 17.1 Inventory 6 1139 1050 930 900 850 380 22.4 Honorable 7 1190 1070 930 910 830 510 18.4 Inventive Example 8 1150 1040 950 890 860 480 17.6 Proposition 9 1120 1010 950 910 870 430 20.5 Honor
10 1135 1040 920 890 830 390 22.5 Honor
11 1170 1050 910 900 840 470 19.8 Honor
12 1130 1000 870 920 840 500 18.4 Honor
13 1100 1010 880 910 830 520 18.8 Honor
14 1120 1030 880 900 850 470 20.4 Honor
15 1135 1030 890 890 820 380 23.7 Comparative Example 1 1150 1050 930 890 850 550 16.4 Comparative Example 2 1145 1030 940 900 830 500 15.1 Comparative Example 3 1150 1020 980 920 860 460 19.5 Comparative Example 4 1208 1100 1000 890 850 450 17.6 Comparative Example 5 1198 1080 980 900 830 400 18.5 Comparative Example 6 1100 1050 950 910 840 510 17.3 Comparative Example 7 1150 1030 980 890 750 450 16.2 Comparative Example 8 1120 950 900 900 850 490 16.5 Comparative Example 9 1150 1030 950 890 800 630 13.4 Comparative Example
10 1270 1120 1030 890 830 400 21.4 Comparative Example
11 1150 1040 1010 900 750 550 12.4 Comparative Example
12 1220 1130 1030 960 880 540 17.6 Comparative Example
13 1060 960 850 950 890 450 21.7 Comparative Example
14 1190 1050 930 780 700 600 11.7

For the inventive examples and comparative examples prepared as described above, test specimens were prepared from thick steel sheets to evaluate sulfide crack resistance and toughness to measure ductility-brittle transition temperature and Charpy energy. Sulfide stress crack resistance was evaluated according to the constant load test according to NACE standard TM-0177, where the applied stress applied 80% of the actual yield stress and no breakage occurred in 720 hours before and after Failure occurred when the failure was evaluated. Low temperature toughness was calculated by the impact energy value obtained by Charpy impact test on a specimen having a V notch from 0 ° C. to −80 ° C., and the ductile-brittle transition temperature was calculated. In addition, the microstructure was observed using an optical microscope to measure the acicular ferrite fraction and the average grain size of the ferrite.

Table 3 below shows the yield strength, tensile strength, ductile-brittle transition temperature, Charpy impact energy at -20 ° C, sulfide stress cracking resistance, acicular ferrite fraction, and ferrite average grain size.

No. Yield strength (MPa) Tensile Strength (MPa) Ductile-brittle
Transition temperature (℃) -20 ℃ Charpy Energy (J) SSC resistance Needle Ferrite Fraction (%) Ferrite Grain Size (mm) Inventory 1 305.8 480.5 -35 337 Pass 24 18.5 Inventory 2 301.2 478.2 -35 312 Pass 28 17.4 Inventory 3 314.9 505.4 -30 264 Pass 35 15.8 Honorable 4 333.5 514.3 -30 247 Pass 40 19.4 Inventory 5 331.7 520.1 -30 267 Pass 48 18.6 Inventory 6 319.4 476.2 -35 325 Pass 28 15.4 Honorable 7 304.5 486.9 -40 354 Pass 25 16.2 Honors 8 316.4 495.2 -40 307 Pass 36 17.8 Proposition 9 322.9 524.5 -30 267 Pass 64 15.4 Inventory 10 324.1 515.8 -35 314 Pass 49 16.7 Exhibit 11 325.0 536.8 -25 257 Pass 61 15.0 Inventory 12 315.4 524.8 -30 281 Pass 45 19.4 Inventory 13 314.2 517.2 -30 275 Pass 47 15.8 Inventory 14 300.8 493.2 -40 344 Pass 40 13.6 Honorable Mention 15 348.6 511.7 -35 302 Pass 38 16.8 Comparative Example 1 314.8 524.1 -15 98 fail 16 20.5 Comparative Example 2 304.1 487.6 -20 124 fail 15 20.5 Comparative Example 3 311.4 479.6 -15 134 fail 18 21.4 Comparative Example 4 298.1 487.9 -10 67 fail 14 32.3 Comparative Example 5 289.4 498.7 -20 154 fail 15 35.7 Comparative Example 6 303.4 502.4 -15 84 fail 17 26.1 Comparative Example 7 302.9 469.8 -15 100 fail 14 27.5 Comparative Example 8 278.4 491.5 -20 134 fail 15 24.8 Comparative Example 9 289.6 448.5 -10 41 fail 18 28.3 Comparative Example 10 305.1 503.4 -15 71 fail 14 23.1 Comparative Example 11 289.6 451.0 -15 65 fail 17 25.4 Comparative Example 12 267.8 447.3 -5 25 fail 15 32.3 Comparative Example 13 431.1 598.4 -20 114 fail 19 24.1 Comparative Example 14 461.6 452.1 -15 74 fail 12 26.4

As shown in Table 1 to Table 3, Inventive Examples 1 to 15 are examples of applying the component and the manufacturing method satisfying the present invention, it can be confirmed that the fraction of acicular ferrite is 20% or more. In addition, the ductile-brittle transition temperature is all below -20 ℃, it can be seen that the Charpy impact energy is 250J at -20 ℃.

In addition, Inventive Examples 1 to 15 were also confirmed that the sulfide stress cracking resistance does not break for 720 hours under the conditions that the stress of 80% of the actual yield stress. In addition, as shown in FIG. 1, in the case of FIG. 1A, which is a result of observing the microstructure of Inventive Example 1, a ferrite grain having a needle fraction of 24% and having a size of about 18 μm is secured. You can see that you can. In (b) of FIG. 1 showing the microstructure of Inventive Example 9, it can be seen that ferrite grains having a needle size of 64% and having a size of about 15 μm can be obtained.

On the contrary, Comparative Examples 1 to 6 are outside the component range proposed in the present invention, and because the fraction of acicular ferrite is low, the transition temperatures are all -20 ° C or higher, and the Charpy impact energy is 67-154J at -20 ° C. The range was confirmed to be lower than the invention example.

In addition, Comparative Examples 7 to 13 are outside the manufacturing conditions proposed in the present invention, and because the fraction of acicular ferrite is low, the transition temperatures are all at least -20 ° C, and the Charpy impact energy is at 41 to 134J at -20 ° C. It was confirmed that the lower than the invention example.

In addition, as in Comparative Example 14, when the component and the manufacturing method deviate from the conditions proposed in the present invention, all of the transition temperature is -20 ° C or higher, and the Charpy impact energy at -20 ° C is lower than the invention example in the range of 74J. I could confirm it.

In addition, Comparative Examples 1 to 14 sulfide stress cracking resistance was also confirmed that the fracture occurs before 720 hours under the condition that the stress of 80% of the actual yield stress.

As shown in (c) of FIG. 1, when the microstructure of Comparative Example 4 was observed, ferrite grains having a very low acicular ferrite fraction of about 14% and having a size of 20 μm or more were confirmed.

Therefore, it is possible to confirm the advantageous effect of the present invention having high resistance to cracking in the state of applying external stress in the H ₂ S environment of the present invention and having excellent impact toughness at low temperatures of -10 ° C. or less at a soft-brittle transition temperature. there was.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (10)

By weight%, C: 0.03-0.18%, Si: 0.05-0.5%, Mn: 0.5-2.0%, Al: 0.005-0.1%, Cu: 0.15-0.5%, Ni: 0.15-1.0%, Nb: 0.01- 0.05%, Ti: 0.01% to 0.05%, Ca: 0.001% to 0.003%, N: 0.001% to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04 to 0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): 0.4 ~ 1.6, containing residual Fe and other unavoidable impurities, and having excellent stress cracking resistance and low temperature toughness, sulfide resistance, which is acicular ferrite and residual ferrite, over 20% by area fraction. Steel for containers.
The method of claim 1,
The steel is Mo: 0.05 ~ 0.3%, Cr: 0.05 ~ 0.5%, W; 0.05-0.3% and V: 0.01-0.05% Steel for pressure vessels excellent in sulfide stress cracking resistance and low temperature toughness, characterized in that it further comprises one or more of them.
delete The method of claim 1,
The average grain size of the ferrite is excellent in sulfide stress cracking resistance and low temperature toughness steel material, characterized in that 20㎛ or less.
By weight%, C: 0.03-0.18%, Si: 0.05-0.5%, Mn: 0.5-2.0%, Al: 0.005-0.1%, Cu: 0.15-0.5%, Ni: 0.15-1.0%, Nb: 0.01- 0.05%, Ti: 0.01% to 0.05%, Ca: 0.001% to 0.003%, N: 0.001% to 0.01%, P: 0.012% or less, S: 0.0015% or less, C / Mn: 0.04 to 0.16 and ((Nb + Ti + V) * 0.1) / (C * 0.01 + N): reheating the slab comprising 0.4-1.6, residual Fe and other unavoidable impurities;
Rough rolling the reheated slab;
Hot-rolling the roughly rolled steel at a temperature of 850 ° C to 950 ° C;
Air-cooling the hot finished rolled steel, and then performing a normalizing heat treatment for a time in the following relational formula 1; And
Method for producing a pressure vessel steel excellent in sulfide stress cracking resistance and low temperature toughness comprising the step of cooling the normalized heat treatment steel.
Relationship 1: Time = 1.3 * t + (10-30 minutes) (where t is the thickness of steel)
6. The method of claim 5,
The heating temperature of the reheating step is a method for producing a pressure vessel steel excellent in sulfide stress cracking resistance and low temperature toughness, characterized in that 1050 ~ 1250 ℃.
6. The method of claim 5,
The rough rolling step is a method for producing a pressure vessel steel, excellent in sulfide stress cracking resistance and low temperature toughness, characterized in that the finishing temperature is performed at a reduction ratio of 10% or more per final pass of the final rolling, and the end temperature is 1000 to 1100 ° C.
6. The method of claim 5,
The normalizing heat treatment step is a method of manufacturing a pressure vessel steel excellent in sulfide stress cracking resistance and low temperature toughness, characterized in that carried out at a temperature of 890 ~ 950 ℃.
6. The method of claim 5,
The cooling step is a method for producing a pressure vessel steel material excellent in sulfide stress cracking resistance and low temperature toughness performed at a cooling rate / Ceq of 15 ~ 25 ℃ / s from 810 ℃ to 250 ~ 550 ℃.
(The Ceq (carbon equivalent) satisfies Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, C, Mn, Cr, Mo, V, Cu, Ni Is weight percent)
The method according to any one of claims 5 to 9,
A method for producing a pressure vessel steel having excellent sulfide stress cracking resistance and low temperature toughness, further comprising a PWHT (Post Weld Heat Treatment) step of heat-treating the steel at a temperature of 550 to 650 ° C.

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