JP6152375B2 - Steel for pressure vessels excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance, manufacturing method thereof, and deep drawing product manufacturing method - Google Patents

Description

  The present invention relates to a steel material for a low temperature pressure vessel, and more specifically, a steel material for a pressure vessel excellent in low temperature toughness and resistance to hydrogen sulfide stress corrosion cracking (SSCC), a method for producing the same, and the above steel material The present invention relates to a method of manufacturing a deep drawing product used.

  A pressure vessel such as a refining facility or a storage tank for refining crude oil determines the physical properties required depending on the temperature at which the vessel is used and the characteristics of the substance stored in the vessel. In particular, when the use temperature is low, low temperature toughness is required, and the deterioration of the steel material due to corrosion differs depending on the type of storage material, so that special physical properties corresponding to the type of storage material are required.

Recently, as the mining of crude oil having a high hydrogen sulfide (H ₂ S) content increases, steel materials necessary for crude oil refining facilities are required to have high resistance to deterioration of the material due to hydrogen sulfide. In addition, since the mining and refining environment tends to change to a low temperature environment, improvement in low temperature toughness is required, and as a result, the steel materials used in pressure vessels applied to these also have corrosion resistance due to hydrogen sulfide and low temperature. The demand for materials that satisfy both toughness is increasing.

  On the other hand, conventionally, there is a method of manufacturing a cylinder for a pressure vessel by a spinning type processing method using a seamless pipe to manufacture a steel plate for a low-temperature pressure vessel having a high tensile strength. Used. However, since the cylinder manufactured by the method has a seam, the appearance is not beautiful and the physical properties of the seam portion may be deteriorated.

  In addition, conventionally used steel plates for low-temperature pressure vessels are made of seamless pipes and often contain vanadium (V), which is a strong carbide precipitation element. Therefore, when spheroidizing heat treatment is performed to impart workability, the strength of the steel material becomes excessively high due to the precipitation strengthening phenomenon of vanadium, and it is difficult to form directly in the subsequent deep drawing process. .

  Furthermore, since the conventional spheroidizing heat treatment for steel materials for low-temperature pressure vessels takes a long time of 90 minutes or more, it is disadvantageous in terms of the productivity and production cost of the steel materials, and decarburization can occur and the strength of the steel materials However, there is a risk of decline.

  One aspect of the present invention is a pressure superior in resistance to hydrogen sulfide stress corrosion cracking (SSCC) than conventional steel materials for low-temperature pressure vessels, effectively increasing the life and stability of the vessel, and excellent in low-temperature toughness. A steel material for containers, a method for producing the same, and a method for producing a deep-drawn product using the steel material are provided.

  One aspect of the present invention is mass%, carbon (C): 0.3-0.4%, silicon (Si): 0.15-0.40%, manganese (Mn): 0.4-1. 0%, aluminum (Al): 0.001 to 0.05%, chromium (Cr): 0.8 to 1.2%, molybdenum (Mo): 0.15 to 0.80%, nickel (Ni): 0.1% or less, calcium (Ca): 0.0005 to 0.0020%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, balance Fe and inevitable impurities The microstructure provides a steel material for pressure vessels excellent in low-temperature toughness and hydrogen sulfide stress corrosion cracking resistance, which consists of a three-phase composite structure of ferrite, bainite and martensite.

  According to another aspect of the present invention, a steel ingot satisfying the above-described composition is reheated at 1000 to 1250 ° C., and the reheated steel ingot is finish-rolled at 750 to 1000 ° C. to produce a hot rolled steel sheet. A step of normalizing the hot-rolled steel sheet, and a step of subjecting the hot-rolled steel sheet subjected to the normalization treatment to a spheroidizing heat treatment at a temperature of Ac1 to Ac3 for 30 minutes or more. Provided is a method for producing a steel material for pressure vessels excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance so that the rolling shape ratio per rolling pass represented by the following relational expression 2 satisfies 0.9 to 1.5. To do.

[Relational expression 2]
Rolling shape ratio = (rolling roll radius (mm) x reduction amount (mm)) ^0.5 / (average thickness of steel sheet (mm))
(Here, the average thickness of the steel sheet means the average value of the thicknesses on the entry side and the exit side of finish rolling.)

  Still another aspect of the present invention includes a step of deep drawing the pressure vessel steel manufactured by the above-described manufacturing method, and 1.6 t + (10 to 30 minutes) at 850 to 950 ° C. after the deep drawing (t A quenching step for holding for a period of t minutes when the thickness of the steel material expressed in mm is tmm, and a step of tempering at 550 to 625 ° C. after the quenching, A method of manufacturing a deep drawing product including

  According to the present invention, the time for the spheroidizing heat treatment can be dramatically shortened, so that cost and time can be saved, and even if the amount of the expensive alloy element added is reduced, the existing steel material for pressure vessels can be reduced. It is possible to provide a steel material that can remarkably improve resistance to hydrogen sulfide stress corrosion cracking.

  In order to develop a steel material excellent in low temperature toughness and resistance to hydrogen sulfide stress corrosion cracking, which is used as a material for pressure vessels such as refining equipment and storage tanks for refining crude oil. As a result of repeated research, it has been found that the target steel material can be produced by optimizing the production conditions even if the addition amount of the expensive alloy element is reduced, and the present invention has been completed.

  Hereinafter, the present invention will be described in detail.

  The steel material for pressure vessels excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance according to one aspect of the present invention is mass%, carbon (C): 0.3 to 0.4%, silicon (Si): 0.15-0.40%, Manganese (Mn): 0.4-1.0%, Aluminum (Al): 0.001-0.05%, Chromium (Cr): 0.8- 1.2%, molybdenum (Mo): 0.25 to 0.80%, nickel (Ni): 0.1% or less, calcium (Ca): 0.0005 to 0.0020%, phosphorus (P): 0 .015% or less, sulfur (S): 0.015% or less, preferably including the remainder Fe and inevitable impurities.

  Below, the reason for restrict | limiting an alloy component composition as mentioned above by this invention is demonstrated in detail. At this time, the content of each component means mass% unless otherwise specified.

C: 0.3 to 0.4%
Carbon (C) is an element added to ensure the strength of the steel material, and if its content is less than 0.3%, the strength decreases and the target strength cannot be ensured. If it exceeds 4%, the toughness decreases, which is not preferable. Therefore, the C content in the present invention is preferably limited to 0.3 to 0.4%.

Si: 0.15-0.40%
Silicon (Si) not only acts as a deoxidizer in the steelmaking process, but is a solid solution strengthening element and is an element advantageous for improving the strength. If the Si content is less than 0.15%, the above-described effects cannot be obtained. If the Si content exceeds 0.40%, the toughness is reduced, and an oxide film is formed on the surface of the steel sheet to improve the surface quality. There is a problem of lowering. Therefore, the Si content in the present invention is preferably limited to 0.15 to 0.40%.

Mn: 0.4 to 1.0%
Manganese (Mn) is an element that has an important influence on the strength and toughness of steel. If its content is less than 0.4%, it is difficult to expect the above-mentioned effects. There is a problem that the production cost is increased due to a decrease in the property, the cost of the alloy raw material is increased. Therefore, the Mn content in the present invention is preferably limited to 0.4 to 1.0%.

Al: 0.001 to 0.05%
Similar to Si, aluminum (Al) acts as a strong deoxidizer in the steelmaking process, and if its content is less than 0.001%, it is difficult to expect the above-described effects. On the other hand, if the Al content exceeds 0.05%, not only the deoxidation effect is saturated but also the production cost is increased, which is not preferable. Therefore, the Al content in the present invention is preferably limited to 0.001 to 0.05%.

Cr: 0.8-1.2%
Chromium (Cr) is an element advantageous for improving the hardenability of steel, and in order to obtain the effect, it is preferable to add 0.8% or more of Cr. However, Cr is an expensive element, and if its content exceeds 1.2%, the production cost is rapidly increased. Therefore, the Cr content in the present invention is preferably limited to 0.8 to 1.2%.

Mo: 0.15-0.80%
Molybdenum (Mo) is an element effective for improving the hardenability of steel, and is known as an element that prevents the occurrence of cracks due to sulfides. Further, it is an element effective for securing strength by precipitation of fine carbides after quenching and tempering. In order to acquire the effect mentioned above, it is preferable to add Mo 0.15% or more. However, Mo is also an expensive element, and if its content exceeds 0.80%, it causes an increase in manufacturing cost, which is not preferable. Therefore, the Mo content in the present invention is preferably limited to 0.15 to 0.80%, more preferably 0.3 to 0.7%.

Ni: 0.1% or less (excluding 0%)
Nickel (Ni) is a very effective element for improving the low-temperature toughness of steel. However, although it is a very expensive element, if its content is too large, it prevents spheroidization of carbides, so its content is reduced. It is preferable to limit to 0.1% or less.

Ca: 0.0005 to 0.0020%
Calcium (Ca) is an element that is advantageous for reducing the material anisotropy due to the rolling direction after rolling by spheroidizing inclusions that are elongated in the rolling direction like MnS. In order to acquire the said effect, it is preferable to add 0.0005% or more of Ca. However, if the content is too large, it reacts with O contained in the steel to produce non-metallic inclusions CaO and deteriorates the physical properties of the steel, so the upper limit may be limited to 0.0020%. preferable.

P: 0.015% or less Since phosphorus (P) is an element that deteriorates the low temperature toughness of steel, it is preferable to suppress the content to the maximum, and considering the steel making process, the content is 0.015%. It is preferable to limit to the following.

S: 0.015% or less Sulfur (S) is an element that impairs the low temperature toughness of steel, as in the case of P described above. However, in consideration of the steel making process, the content is preferably controlled to 0.015% or less.

  The remaining component of the present invention is iron (Fe). However, in an ordinary steel manufacturing process, unintentional impurities may be inevitably mixed from the raw material or the surrounding environment, so it is practically impossible to eliminate them. Such impurities are well known to those skilled in the ordinary steel manufacturing process, and the entire contents thereof are not specifically mentioned in this specification.

  It is preferable that the steel material of the present invention that satisfies the above-described component composition has a three-phase composite structure of ferrite, bainite, and martensite.

  Although explained more specifically below, in the manufacturing stage of the steel material provided by the present invention, after normalizing, the microstructure of the steel sheet may be formed into a three-phase composite structure of ferrite, bainite and martensite. it can.

  Among the three-phase composite structures, the martensite and bainite structures can secure the strength of the steel material and reduce the time for the spheroidizing heat treatment. More specifically, a low temperature transformation structure such as martensite, bainite, pearlite, etc. is characterized in that the finer the carbide, the faster the spheroidization rate, and usually martensite> bainite> pearlite. The spheroidizing time can be shortened in the order of.

  Ferrite is advantageous for securing the ductility of steel, and in particular, in the present invention, it is an advantageous structure for securing deep drawability.

  More specifically, it is preferable that martensite + bainite is included in the fine structure in an area fraction of 70 to 98%, and ferrite is included in an area fraction of 2 to 30%. If the martensite + bainite phase fraction exceeds 98%, the ductility of the steel will be insufficient, the formability, especially deep drawability, will decrease, and if the ferrite fraction exceeds 30%, sufficient ductility will be achieved. Since it can be secured, the moldability is excellent, but there is a problem that the time for the spheroidizing heat treatment is excessively increased.

  Moreover, the steel material for pressure vessels of this invention contains a carbide | carbonized_material, At this time, it is preferable that 50 area% or more of carbide | carbonized_materials whose sphericity represented by the following relational formula 1 is 2 or less is contained among total carbides. The upper limit is not particularly limited, but is actually about 95 area%, typically about 85 area%. The sphericity of the carbide in the present invention means the closeness of the shape of the carbide to the spherical shape, which affects the resistance to cracking due to sulfide in the steel material. If the carbide having a sphericity of 2 or less is less than 50% by area of the total carbide, sufficient resistance to hydrogen sulfide stress corrosion cracking cannot be ensured. More preferably, the carbide having a sphericity of 2 or less is included in 60% by area or more of the total carbides. In the following formula, “width” is the longest diameter in a two-dimensional image of carbide observed with a microscope or the like, and “length” means a diameter perpendicular to the width.

[Relational expression 1]
Sphericality of carbide = (Carbide width (nm)) / (Carbide length (nm))

  The steel sheet of the present invention having the above-described advantageous composition and microstructure is a normal one in the technical field to which the present invention belongs without undue repeated experimentation, if it is a person having ordinary knowledge in the technical field to which the present invention belongs. Can be easily manufactured using knowledge. However, the present invention provides a method for producing the steel sheet by a more advantageous production method found by the inventors of the present invention.

Steel for pressure vessels according to the present invention, reheating of the steel ingot satisfying the chemical composition proposed by the present invention - hot rolling - normalizing - can be prepared by performing a spheroidizing heat treatment, in the following, the respective The process conditions will be described in detail.

Reheating of a steel ingot Reheating of a steel ingot is preferably performed at 1000 to 1250 ° C. in a process of heating at a high temperature in order to perform subsequent hot rolling.

  If the reheating temperature is less than 1000 ° C, it is difficult to dissolve the solute atoms. If the reheating temperature exceeds 1250 ° C, the size of the austenite crystal grains becomes excessively large and the physical properties of the steel are deteriorated. .

Hot rolling The hot-rolled steel sheet can be produced by hot-rolling the reheated steel ingot, and the finish rolling is preferably performed at 750 to 1000 ° C.

  At the time of finish rolling, if the temperature is less than 750 ° C., the amount of non-recrystallized zone rolling becomes too large and material anisotropy occurs and the deep drawability deteriorates. It becomes coarse and the physical properties of the steel deteriorate.

  In the present invention, it is preferable to control the rolling shape ratio per rolling pass when performing finish rolling in the temperature range described above. More specifically, it is preferable that the rolling shape ratio per rolling pass represented by the following relational expression 2 satisfies 0.9 to 1.5. The reason for controlling the rolling shape ratio in the present invention is to control the shape ratio of the carbide formed after the subsequent spheroidizing heat treatment. When the rolling shape ratio is less than 0.9, the size of the crystal grains becomes coarse. When the rolling shape ratio exceeds 1.5, a load may be applied to the rolling mill. It is preferable to control to 5. More preferably, the rolling shape ratio is controlled to 0.95 to 1.2.

[Relational expression 2]
Rolling shape ratio = (rolling roll radius (mm) x reduction amount (mm)) ^0.5 / (average thickness of steel sheet (mm))
(Here, the average thickness of the steel sheet means the average value of the thicknesses on the entry side and the exit side of finish rolling.)

As normalizing heat treatment and spheroidization heat treatment above, by a normalizing heat treatment on the hot rolled hot-rolled steel sheet was produced, ferrite its microstructure, 3-phase composite structure of bainite and martensite Can be formed. At this time, the conditions sounded burn is not particularly limited.

When the baked break heat treatment is completed, it is preferable to perform spheroidizing heat treatment. This is for imparting appropriate workability necessary for deep drawing. At this time, the spheroidizing heat treatment is performed at Ac1 to Ac3 temperature for 30 minutes or more, preferably 30 to 90 minutes. However, if the heat treatment temperature is lower than Ac1, it takes a long time to spheroidize. There is a problem that it is difficult to form spheroidized carbide due to phase transformation.

The present invention, by no three-phase composite structure by the baked break heat treatment, the time of subsequent spheroidizing heat treatment can be reduced to below 90 min. This is very important in terms of energy and cost savings and productivity, considering that the spheroidizing heat treatment time required for conventional steel for deep drawing has exceeded 90 minutes.

  When the spheroidizing heat treatment is completed, a steel material for a pressure vessel intended in the present invention, that is, a steel material having a tensile strength of 700 MPa or less can be produced, and this can be used very suitably as a steel material for deep drawing. it can.

  Hereinafter, a method for producing a deep drawn product using the steel material for a pressure vessel produced by the production method according to the present invention will be described in detail.

  The steel material for a pressure vessel must have a tensile strength after deep drawing of 1200 MPa class. For that purpose, since it is preferable to transform the internal structure of the steel material into an austenite structure, it is preferable to hold the steel material at a high temperature for a certain period of time. More specifically, a deep drawn product having a target strength can be manufactured through a process of tempering after quenching at a high temperature.

Quenching
As described above, in order to transform the internal structure of the steel material after deep drawing into an austenite structure, a constant time at 850 to 950 ° C., preferably 1.6 t + (10 to 30 minutes) (t: mm It is preferable to hold for t minutes when the thickness is tmm.

  If the holding temperature is less than 850 ° C. or the holding time is less than 1.6 t + 10 minutes, it is difficult to re-dissolve the solid solution solute element, and it is difficult to ensure the strength. Also, whether the holding temperature exceeds 950 ° C. If the holding time exceeds 1.6 t + 30 minutes, crystal grains grow, and there is a problem that the low temperature toughness decreases as coarse crystal grains are formed.

  It is preferable to cool with water after being held in the temperature range described above.

Tempering
Since the quenched steel material may become brittle, it is preferable to perform tempering at 550 to 625 ° C. in order to impart appropriate low temperature toughness. If the tempering temperature is less than 550 ° C., the effect of tempering is not sufficient, so that it is difficult to ensure toughness, and if it exceeds 625 ° C., it is difficult to ensure strength.

  The steel material for a deep drawing pressure vessel manufactured according to the present invention shows a low strength excellent in formability in the process of deep drawing, but can have a tensile strength of 1100 to 1200 MPa after forming, and is −50 Joules or more − Since it has a low temperature impact toughness of 50 ° C., its utilization is wide and exhibits very good physical properties.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are merely examples for explaining the present invention in more detail and are not intended to limit the scope of rights of the present invention. The scope of the right of the present invention is determined by matters described in the claims and matters that can be reasonably inferred therefrom.

(Example)
Steel ingots having the composition shown in Table 1 below were reheated at 1150 ° C., and then hot finish rolled to produce each hot rolled steel sheet. Thereafter, the hot-rolled steel sheet was subjected to a normalizing treatment and then subjected to a spheroidizing heat treatment at 750 ° C. for 90 minutes. Next, after deep drawing, each deep drawn product was manufactured by quenching and tempering.

  At this time, the rolling temperature, rolling shape ratio, quenching, and tempering temperature during hot finish rolling were set to the conditions shown in Table 2 below.

（上表３における炭化物の分率（％）は、総炭化物のうち、本発明の関係式１による球形度が２以下である炭化物の分率を示したものである。また、Ｂはベイナイト、Ｍはマルテンサイト、Ｆはフェライトを意味する。）

(Carbide fraction (%) in Table 3 above indicates the fraction of carbides having a sphericity of 2 or less according to the relational expression 1 of the present invention among the total carbides. B is bainite, M stands for martensite and F stands for ferrite.)

  As shown in Table 3 above, the inventive materials 1 to 6 that satisfy all the composition and production conditions of the present invention include 50% or more of carbides having a sphericity of 2 or less, a yield strength of 1000 MPa or more, and a tensile strength of It can be seen that the strength is excellent at 1200 MPa or more, and the impact toughness at −50 ° C. is excellent. In particular, it can be seen that the inventive material according to the present invention is extremely excellent in resistance to hydrogen sulfide stress corrosion cracking, with an SSCC fracture time of 200 hours or longer.

  On the other hand, the comparative materials 1 to 3 whose rolling shape ratio does not satisfy the range proposed in the present invention have a fraction of carbide having a sphericity of 2 or less and less than 40%, and the resistance to SSCC is very inferior. I understand that. In particular, these comparative materials have a disadvantage of high production costs because a large amount of expensive Ni is added.

Claims (9)

In mass%, carbon (C): 0.3 to 0.4%, silicon (Si): 0.15 to 0.40%, manganese (Mn): 0.4 to 1.0%, aluminum (Al) : 0.001-0.05%, chromium (Cr): 0.8-1.2%, molybdenum (Mo): 0.15-0.80%, nickel (Ni): 0.1% or less (0 %), Calcium (Ca): 0.0005 to 0.0020%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, the balance Fe and inevitable impurities ,
Sphericity represented by the following equation 1 after the sphere Joka heat treatment including carbides is 2 or less than 50 area% of the total carbide, low-temperature toughness and hydrogen sulfide stress corrosion cracking resistance in high pressure vessel steel material.
[Relational expression 1]
Sphericality of carbide = (Carbide width (nm)) / (Carbide length (nm))
(Here, the width is the longest diameter in the two-dimensional image of the carbide, and the length means the diameter perpendicular to the width.)   The steel material for pressure vessels excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance according to claim 1, wherein the steel material further contains 0.3 to 0.7% of molybdenum (Mo). The steel material is
Before the spheroidizing heat treatment, it comprises a three-phase composite structure of ferrite, bainite and martensite, the bainite and martensite are included in an area fraction of 70 to 98%, and ferrite is included in an area fraction of 2 to 30%. The steel material for pressure vessels excellent in low-temperature toughness and hydrogen sulfide stress corrosion cracking resistance according to 1 or 2. The low temperature toughness and hydrogen sulfide stress corrosion according to any one of claims 1 to 3, wherein the steel material includes a carbide having a sphericity of 2 or less represented by the following relational expression 1 in an amount of 60 area% or more of the total carbide. Steel for pressure vessels with excellent crack resistance.
[Relational expression 1]
Carbide sphericity = (Carbide width (nm)) / (Carbide length (nm)) (where the width is the longest diameter in the two-dimensional image of the carbide and the length is the diameter perpendicular to the width) Means.) In mass%, carbon (C): 0.3 to 0.4%, silicon (Si): 0.15 to 0.40%, manganese (Mn): 0.4 to 1.0%, aluminum (Al) : 0.001-0.05%, chromium (Cr): 0.8-1.2%, molybdenum (Mo): 0.15-0.80%, nickel (Ni): 0.1% or less (0 %), Calcium (Ca): 0.0005 to 0.0020%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, the balance Fe and steel made of inevitable impurities Reheating the mass at 1000-1250 ° C .;
Finishing and rolling the reheated steel ingot at 750 to 1000 ° C. to produce a hot-rolled steel sheet;
Normalizing the hot-rolled steel sheet;
Subjecting the normalized hot-rolled steel sheet to spheroidizing heat treatment at Ac1 to Ac3 temperature for 30 minutes or more,
The finish rolling is performed so that the rolling shape ratio per rolling pass represented by the following relational expression 2 satisfies 0.9 to 1.5,
Manufacture of steel for pressure vessels excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance, containing 50% by area or more of the total carbide of carbides having a sphericity of 2 or less represented by the following relational expression 1 after spheroidizing heat treatment Method.
[Relational expression 1]
Carbide sphericity = (Carbide width (nm)) / (Carbide length (nm)) (where the width is the longest diameter in the two-dimensional image of the carbide and the length is the diameter perpendicular to the width) Means.)
[Relational expression 2]
Rolling shape ratio = (rolling roll radius (mm) x reduction amount (mm)) ^0.5 / (average thickness of steel sheet (mm))
(Here, the average thickness of the steel sheet means the average value of the thicknesses on the entry side and the exit side of finish rolling.)   The said steel ingot is a manufacturing method of the steel material for pressure vessels excellent in the low temperature toughness and hydrogen sulfide stress corrosion cracking resistance of Claim 5 which contains molybdenum (Mo) 0.3-0.7% further limiting.   The method for producing a steel material for a pressure vessel excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance according to claim 5 or 6, wherein the finish rolling is performed so that a rolling shape ratio satisfies 0.95 to 1.2.   The steel material for pressure vessels excellent in low temperature toughness and hydrogen sulfide stress corrosion cracking resistance according to any one of claims 5 to 7, which has a three-phase composite structure of ferrite, bainite and martensite after the normalization. Manufacturing method. Deep drawing the pressure vessel steel according to any one of claims 1 to 4,
After the deep drawing, hold for {1.6t + (10-30)} minutes (t: the thickness of the steel material expressed in mm) at 850 to 950 ° C., and then quenching.
Tempering at 550 to 625 ° C. after the quenching;
A method for manufacturing deep-drawn products.