KR20230090416A - Steel plate having excellent hydrogen induced craking resistance and low-temperature impact toughness, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material suitable for pressure vessels that can be used in petrochemical manufacturing facilities, storage tanks, etc., and more particularly, to a steel material excellent in hydrogen induced cracking (HIC) resistance and low-temperature impact toughness and manufacturing the same It's about how to do it.

Description

Steel with excellent resistance to hydrogen induced cracking and low-temperature impact toughness and its manufacturing method

The present invention relates to a steel material suitable for pressure vessels that can be used in petrochemical manufacturing facilities, storage tanks, etc., and more particularly, to a steel material excellent in hydrogen induced cracking (HIC) resistance and low-temperature impact toughness and manufacturing the same It's about how to do it.

Recently, pressure vessels used in industries such as mining, production, transportation, storage, refining, and power generation of energy resources have increased the demand for ultra-thick steel due to the enlargement of facilities due to the increase in use time. These steels are required to have a low carbon equivalent (Ceq) in order to secure the structural stability of the welded part. In addition, as the production of crude oil containing a large amount of H ₂ S increases, the steel for pressure vessels as described above requires hydrogen induced cracking (HIC) resistance, and as the use environment of these structures expands to extreme cold regions, excellent low temperature Impact toughness is required at the same time.

The cause of the hydrogen-induced cracking (HIC) is that corrosion occurs as the steel material comes into contact with wet hydrogen sulfide contained in crude oil, and hydrogen atoms generated by the corrosion invade and diffuse into the steel to form a molecular state in inclusions in the steel. come into existence As such, when hydrogen atoms are molecularized inside the steel, hydrogen gas is formed and gas pressure is generated, and brittle cracks are generated along the fragile structure inside the steel due to the pressure, and fracture occurs as they grow.

Therefore, as measures to improve the hydrogen-induced cracking resistance of steel materials used in a hydrogen sulfide atmosphere, a method of adding elements such as copper (Cu), a method of minimizing or controlling the shape of a hardened structure in which cracks easily occur and propagate , there are methods for controlling internal defects such as inclusions and voids in the steel material, which can act as the starting point of hydrogen accumulation and cracking.

Patent Document 1 proposes a method of increasing hydrogen-induced cracking resistance by appropriately controlling the shape of the voids inside the steel. Specifically, the shape of the void formed in the center of the steel material was to be obtained as spherical as possible, and the length ratio of the long side and the short side of the void was controlled to be 0.7 or more. However, since the shape of the voids formed during continuous casting is not constant, and there is a limit to uniformly controlling their shape through the rolling process, it is necessary to prepare an improvement measure because the hydrogen induced crack resistance of the steel material may show deviation. there is.

On the other hand, the steel for pressure vessels has a problem in stability as the impact toughness decreases as the operating temperature decreases. In particular, as the thickness of a steel material of the same strength increases, the toughness of the internal structure decreases more significantly. Therefore, it is necessary to properly manage the components or microstructure of the steel for pressure vessels applied to areas with low temperature environments so that impact toughness does not deteriorate even at low temperatures.

The rolling process is one of the representative methods of crystal grain refinement. When rolling is performed at a temperature at which recrystallization is possible, new austenite fine grains are created using internal stress generated by a rolling reduction as a driving force.

However, as the thickness of the steel material increases, the reduction force that can be applied by rolling is limited, so it is difficult to form fine crystal grains through rolling as the internal structure is closer to the center of the steel material, in particular. The grains of austenite tend to grow as the heating time increases and the higher the temperature at Ae3 or higher, some alloying elements have an effect of inhibiting the growth of austenite grains. These alloying elements are dissolved in steel and act as an obstacle to grain growth. Therefore, in the case of ultra-thick steel, in which grain refinement is difficult by rolling, the addition of such an alloying element should be considered together for crystal grain refinement.

In addition, in the case of quenching & tempering (QT) materials, it is common to perform water cooling and tempering heat treatment by reheating to the austenite single phase region after rolling and air cooling. Since this growth greatly reduces low-temperature impact toughness, a technology capable of solving this problem and securing excellent low-temperature impact toughness is required.

Korean Patent Registration No. 10-2164116

One aspect of the present invention relates to a steel material used in a hydrogen sulfide atmosphere, to provide a steel material excellent in hydrogen induced cracking resistance and low temperature impact toughness and a method for manufacturing the same.

The object of the present invention is not limited to the above. Additional tasks of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional tasks of the present invention from the contents described in the specification of the present invention.

In one aspect of the present invention, in weight%, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, the remainder including Fe and unavoidable impurities,

Inside the steel, the number of inclusions of one or more of Al-O-based, Ca-O-based and Al-Ca-O-based oxidizing inclusions having a size of 10 μm or more is 50 or less per 1 mm ² ,

It relates to a steel having excellent hydrogen induced cracking resistance and low-temperature impact toughness that satisfies the following [Relational Expression 1] and [Relational Expression 2].

[Relationship 1]

Ceq ≤ 0.45

(Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and the C, Mn, Cr, Mo, V, Cu, and Ni are the contents of each component (% by weight) ) is the value)

[Relationship 2]

1.2 ≤ Ca/S ≤ 4.0

(The above Ca and S are the content (% by weight) of each component)

Another aspect of the present invention, by weight%, C: 0.12 ~ 0.18%, Si: 0.2 ~ 0.5%, Mn: 0.8 ~ 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 ~ 0.045% , Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 ~0.004%, the remainder including Fe and unavoidable impurities, heating a steel slab satisfying the following [Relational Expression 1] and [Relational Expression 2] in a temperature range of 1100 to 1200 ° C;

Manufacturing a hot-rolled steel sheet by rough-rolling the heated steel slab at a temperature of 1050° C. or higher and finishing hot-rolling at a temperature of Ar3 or higher;

air-cooling the hot-rolled steel sheet;

Reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher, and reheating for (2.3t+30) minutes (where t means the thickness (mm) of steel) or more;

Quenching the reheated hot-rolled steel sheet to room temperature at a cooling rate of 0.4° C./s or more; and

Hydrogen-induced cracking resistance and low-temperature impact toughness including the step of tempering the cooled hot-rolled steel sheet for more than (3.4t + 30) minutes (where t means the thickness (mm) of steel) in the temperature range of 600 ~ 700 ℃ It relates to a manufacturing method of this excellent steel material.

[Relationship 1]

Ceq ≤ 0.45

(Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and the C, Mn, Cr, Mo, V, Cu, and Ni are the contents of each component (% by weight) ) is the value)

[Relationship 2]

1.2 ≤ Ca/S ≤ 4.0

(The above Ca and S are the content (% by weight) of each component)

According to one aspect of the present invention, after quenching & tempering (QT) and heat treatment after welding (PWHT), it is possible to provide a steel for pressure vessels having excellent resistance to hydrogen induced cracking and excellent low-temperature impact toughness.

Various advantageous advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

Figure 1 shows the results of ultrasonic testing performed on hydrogen induced cracking evaluation of Inventive Example 1 (a) in Examples of the present invention.
Figure 2 shows the results of ultrasonic testing performed on hydrogen induced cracking evaluation of Comparative Example 1 (b) in Examples of the present invention.

The terms used herein are intended to describe the present invention and are not intended to limit the present invention. Also, the singular forms used herein include the plural forms unless the related definition clearly dictates the contrary.

The meaning of "comprising" as used in the specification specifies a component, and does not exclude the presence or addition of other components.

Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. The terms defined in the dictionary are interpreted to have a meaning consistent with the related technical literature and the currently disclosed content.

The inventors of the present invention are of a method to secure the physical properties required for the material as the pressure vessel that can be used as petrochemical industry facilities, storage tanks, etc. is enlarged, used in a hydrogen sulfide atmosphere, and the use environment is expanded to extreme cold. Recognized the need for development. In particular, in the steel for pressure vessels having a certain thickness or more, a method for securing low-temperature impact toughness as well as resistance to hydrogen induced cracking was studied in depth. As a result, it was confirmed that it is possible to provide a steel for pressure vessels having target physical properties by controlling the relationship between the component composition and some components in the alloy design and at the same time optimizing the manufacturing conditions, and came to complete the present invention.

One embodiment of the steel of the present invention will be described in detail. First, the alloy composition of the steel of the present invention will be described in detail. In the steel of the present invention, in weight%, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004% , the remainder includes Fe and unavoidable impurities.

Additionally, one or more of Cu: 0.5% or less and Cr: 0.35% or less may be further included.

Carbon (C): 0.12 to 0.18% by weight (hereinafter, referred to as %, unless otherwise specified in the present invention, the content of each element is based on weight%.)

The C is an element effective in improving the strength of steel. In order to sufficiently obtain these effects, it is preferable to include 0.12% or more of the C. However, when the content exceeds 0.18%, the degree of segregation in the center of the steel material increases, and an island martensitic (MA) structure is formed, which greatly impairs hydrogen induced cracking resistance and low-temperature impact toughness, so 0.18% It is desirable not to exceed More advantageously, it may contain 0.15% or less.

Silicon (Si): 0.2 to 0.5%

The Si is not only used as a deoxidizing agent, but also is an element that is advantageous for improving strength and toughness of steel. In order to sufficiently obtain these effects, it is preferable that the Si content is 0.2% or more. However, if the content exceeds 0.5%, MA may be excessively formed, resulting in poor hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, the Si content is preferably 0.2 to 0.5%.

Manganese (Mn): 0.8 to 1.5%

The Mn is an element that is advantageous for improving the strength of steel through a solid solution strengthening effect. In order to sufficiently obtain the effect, it is preferable to include 0.8% or more of the Mn. However, when the content exceeds 1.5%, there is a problem in that hydrogen-induced cracking resistance and low-temperature impact toughness are greatly inhibited by combining with sulfur (S) in steel to form MnS. Therefore, the content of Mn is preferably 0.8 to 1.5%, more advantageously 1.0 to 1.5%.

Phosphorus (P): 0.15% or less

P is an element that is advantageous for improving strength and securing corrosion resistance of steel, but since it can greatly impair the impact toughness of steel, it is preferable to limit the content to as low as possible. In the present invention, even if the P is contained in a maximum of 0.015%, it is not unreasonable to secure the target physical properties, so it is preferable to set the content to 0.015% or less. However, considering the level that is unavoidably added, 0% can be excluded.

Sulfur (S): 0.003% or less

The S is an element that greatly inhibits hydrogen-induced cracking resistance and impact toughness of steel by combining with Mn in steel to form MnS or the like. Therefore, it is preferable to manage the S content as low as possible. In the present invention, even if the S is contained in a maximum of 0.003%, there is no difficulty in securing the target physical properties, so the content can be limited to 0.003% or less. However, considering the level that is unavoidably added, 0% can be excluded.

Aluminum (Al): 0.015 to 0.045%

Al is an element that can deoxidize molten steel at low cost. In order to sufficiently obtain the above-described effect, it is preferable to include 0.015% or more of Al, but if the content exceeds 0.045%, nozzle clogging during continuous casting may occur. This is not preferable because impact toughness may be greatly reduced due to the formation of Al-based oxidizing inclusions. Therefore, the Al is preferably included in 0.015 to 0.045%.

Niobium (Nb): 0.005 to 0.025%

The Nb precipitates in the form of NbC or Nb(C,N) to greatly improve the strength of the base material, and when reheated to a high temperature, the dissolved Nb suppresses recrystallization of austenite and ferrite or bainite transformation, thereby increasing the effect of refining the structure. You can get it. For this purpose, it is preferable to include 0.005% or more. However, if the content is excessive, undissolved Nb is formed in the form of TiNb (C, N), which can cause UT defects, hydrogen induced cracking resistance, and low-temperature impact toughness. Therefore, it is preferable not to exceed 0.025% do. More advantageously, it may contain 0.007 to 0.02%.

Nickel (Ni): 0.01 to 0.5%

The Ni is an element that can simultaneously improve the strength and low-temperature impact toughness of the base material, and it is preferable to include 0.01% or more of Ni in order to sufficiently obtain these effects. However, the Ni is an expensive element, and when the content exceeds 0.5%, there is a problem in that economical efficiency is greatly reduced. Therefore, the Ni content is preferably 0.01 to 0.5%.

Molybdenum (Mo): 0.01 to 0.12%

The Mo is an element that is advantageous for greatly improving the strength by greatly improving the hardenability of the steel. In order to sufficiently obtain these effects, it is preferable to include 0.01% or more of the Mo. However, Mo is an expensive element, and when excessively added, there is a concern that low-temperature impact toughness may be impaired by suppressing ferrite formation and forming bainite.

Vanadium (V): 0.005 to 0.03%

The V has a low melting temperature compared to other alloy elements, and has an effect of preventing a decrease in strength by precipitating in a heat-affected zone during welding. When strength is not sufficiently secured after post-weld heat treatment (PWHT) for the steel material as in the present invention, the strength improvement effect can be obtained by including 0.005% or more of V. However, when the content exceeds 0.03%, the fraction of the hard phase such as MA increases, and hydrogen-induced cracking resistance and low-temperature impact toughness may significantly decrease.

Titanium (Ti): 0.003% or less (excluding 0%)

When Ti is added together with N, TiN is formed, thereby reducing the occurrence of surface cracks due to the formation of AlN precipitates. However, if the content exceeds 0.003%, coarse TiN is formed during the reheating, QT heat treatment, or PWHT process of the steel slab, which may act as a factor impairing low-temperature impact toughness. Therefore, it is preferable to include Ti at 0.003% or less.

Nitrogen (N): 0.002 to 0.01%

When added together with Ti, N is an element that is advantageous for suppressing grain growth due to thermal effects during welding by forming TiN. When the Ti is added, it is preferable to include 0.002% or more of the N in order to sufficiently obtain the above-mentioned effects. However, if the content exceeds 0.01%, coarse TiN is formed and low-temperature impact toughness is impaired, which is not preferable. Therefore, the content of N is preferably 0.002 to 0.01%.

Calcium (Ca): 0.0005 to 0.004%

When the Ca is added to molten steel, it is possible to suppress the generation of MnS by combining with S forming MnS inclusions, and at the same time to form spherical CaS to suppress cracks caused by hydrogen induced cracking. In order to obtain the above-mentioned effect, it is preferable to include 0.0005% or more Ca, but when the content exceeds 0.004%, CaS is formed and the remaining Ca combines with oxygen (O) to form coarse oxidative inclusions, which It is elongated and destroyed during rolling and plays a role in promoting hydrogen induced cracking. Therefore, it is preferable to include the Ca at 0.0005 to 0.004%.

Additionally, in addition to the above composition, one or more of copper (Cu): 0.5% or less and chromium (Cr: 0.35% or less) may be further included.

Copper (Cu): 0.5% or less

The Cu is an element capable of greatly improving strength by solid solution strengthening, and is an element that effectively suppresses corrosion of a base material in a wet hydrogen sulfide atmosphere. However, in a strong acid atmosphere, the effect is not great, and if the content of Cu is excessive, the carbon equivalent is increased to impair weldability, and the surface quality of the product is greatly deteriorated. Therefore, when the Cu is added, it may be included in a maximum of 0.5%. However, in the present invention, since there is no problem in securing the target physical properties even if the Cu is not added, it should be noted that the Cu is not essential.

Chromium (Cr): 0.35% or less

The Cr is an element capable of preventing a decrease in strength by slowing down the decomposition rate of cementite during tempering or heat treatment after welding (PWHT). However, if the content exceeds 0.35%, coarse carbides increase and impact toughness can be greatly reduced. Therefore, it is preferable to include Cr at a maximum of 0.35%. However, in the present invention, since there is no problem in securing the target physical properties even if the Cr is not added, it should be noted that the Cr is not essential.

The remainder includes iron (Fe) and unavoidable impurities. Inevitable impurities can be unintentionally mixed in the normal steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. Further, the present invention does not entirely exclude the addition of other compositions than the aforementioned steel composition.

In order to secure the hydrogen-induced cracking resistance and low-temperature impact toughness in addition to the target level of strength of the steel of the present invention, in adding a certain amount of elements advantageous to the improvement of these physical properties, it is preferable to properly control their content. Accordingly, it is preferable that the carbon equivalent (Ceq) in the following [Relational Expression 1] is 0.45 or less. If the carbon equivalent (Ceq) exceeds 0.45, it may be advantageous to secure strength, but there is a concern that the physical properties after welding may be significantly impaired. In addition, if a large amount of alloying elements are included, economic feasibility is impaired due to cost increase, so the carbon equivalent (Ceq) is preferably 0.45 or less.

[Relationship 1]

Ceq ≤ 0.45

(Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and the C, Mn, Cr, Mo, V, Cu, and Ni are the contents of each component (% by weight) ) is the value)

In addition, the steel material of the present invention preferably satisfies the following [Relational Expression 2].

[Relationship 2]

1.2 ≤ Ca/S ≤ 4.0

(The above Ca and S are the content (% by weight) of each component)

If the Ca / S is less than 1.2, MnS is formed instead of CaS, and the center impact toughness and hydrogen induced cracking can be greatly increased, and if Ca / S is greater than 4, CaO-Al ₂ O ₃ and CaS are mixed Since inclusions in the state are formed, which may also cause poor impact toughness and hydrogen induced cracking, the Ca / S is preferably 1.2 to 4.0.

The microstructure of the steel material preferably has an area fraction of polygonal ferrite of 70% or more, an area fraction of pearlite of 20 to 30%, and the balance of bainite (including 0%). If the area fraction of the polygonal ferrite is less than 70%, impact toughness may be greatly reduced, and if the area fraction of pearlite is out of 20 to 30%, strength may be lowered or strength may be exceeded.

The average grain size of the polygonal ferrite is preferably 25 μm or less. When the average grain size of the polygonal ferrite exceeds 25 μm, impact toughness may be greatly reduced.

On the other hand, it is preferable that the number of oxidizing inclusions, such as Al-O, Ca-O, and Al-Ca-O, having a size of 10 μm or more, is 50 or less per 1 mm ² inside the steel. Oxidative inclusions having a size of less than 10 μm do not have a significant effect on the physical properties and do not have a large technical meaning, and when the number of oxidative inclusions exceeds 50/mm ² , the occurrence of hydrogen-induced cracking increases with a high probability. can

The characteristics of the microstructure of the above-described steel material are not significantly different before and after post-weld heat treatment (PWHT) described later.

The steel material has an average Crack Length Ratio (CLR) value of 10% or less in an experiment conducted under NACE TM0284 Solution A (strong acid) conditions, which is a related international standard, from the reference surface of the center of the width to the center.

The steel material has a yield strength of 260 MPa or more, a tensile strength of 485 MPa or more, and Charpy at -46 ° C. It has excellent strength and low-temperature impact toughness with an average shock absorption energy (CVN, -46℃) value of 150J or more.

The physical properties of the above-described steel materials may be physical properties of steel materials subjected to post-weld heat treatment (PWHT) on the steel materials.

Next, an embodiment of the manufacturing method of the steel of the present invention will be described in detail. The manufacturing method is manufactured by heating a steel slab satisfying the above-described alloy composition, hot rolling, cooling, reheating, quenching and tempering.

steel slab heating

It is preferable to perform homogenization treatment by heating a steel slab satisfying the above-mentioned alloy composition. At this time, it is preferable to heat to a temperature range of 1100 ~ 1200 ℃. If the heating temperature of the steel slab is less than 1100 ° C., precipitates (carbon, nitride) formed in the steel slab are not sufficiently re-dissolved, and thus the formation of precipitates is reduced in the process after hot rolling. On the other hand, when the temperature exceeds 1200 ° C., there is a concern that the austenite crystal grains become coarse and deteriorate the physical properties of the steel.

hot rolled

A hot-rolled steel sheet is manufactured by hot-rolling the heated steel slab as described above. After rough rolling the heated steel slab at a temperature of 1050 ° C. or higher, it is preferable to perform finish hot rolling at a temperature of Ar3 or higher.

If the temperature during the rough rolling is less than 1050 ℃, there is a problem that the temperature is lowered during the subsequent finish hot rolling. At this time, since it is important to prevent grains from being coarsened by sufficiently applying a reduction force during the rough rolling, it is preferable to give a reduction ratio of 10% or more in the final pass of the rough rolling. If the rolling force is not sufficient during rough rolling, there is a high possibility that grains will be coarsened after rough rolling.

In addition, if the finish hot rolling temperature is less than Ar3, the rolling load increases, and there is a possibility that quality defects such as surface cracks may occur.

The Ar3 can be expressed as follows.

Ar3 = 910-310C-80Mn-20Cu-55Ni-80Mo+119V+124Ti-18Nb+179Al

(Here, each element means the content (% by weight))

cooling and reheating

After air-cooling the hot-rolled steel sheet manufactured as described above to room temperature, it is preferable to re-heat it to a temperature of Ac3 or higher and maintain it for a certain period of time. Through the reheating process, it is possible to induce generation of a fine austenite structure and contribute to refinement of ferrite after water cooling. An austenite structure may be formed by reheating the hot-rolled steel sheet, but if the reheating temperature is lower than Ac3, the hot-rolled steel sheet structure may become a two-phase structure of ferrite and austenite. Therefore, the reheating is carried out in a temperature range of Ac3 or higher, preferably 830 to 930 ° C, and (2.3t + 30) minutes at the temperature so that a 100% austenite phase is sufficiently formed to the center of the hot-rolled steel sheet (where t is means the thickness (mm) of steel) or more. If the holding time is less than (2.3 t + 30) minutes, 100% austenizing is not achieved due to lack of resilience, resulting in abnormal reverse heat treatment, which may significantly reduce tensile and impact toughness. On the other hand, since the upper limit of the holding time has no physical meaning, it is not particularly limited, and a person skilled in the art can easily determine it in consideration of facility limitations.

The Ac3 can be represented as follows.

Ac3=93.2-436.5C+56Si-19.7Mn-26.6Ni+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al

(Here, each element means the content (% by weight))

Quenching and Tempering

The reheated hot-rolled steel sheet is preferably quenched to room temperature at a cooling rate of 0.4° C./s or more. If the cooling rate is less than 0.4 ° C / s during cooling, the microstructure may include coarsened ferrite and pearlite phases, thereby impairing strength and low-temperature impact toughness.

It is preferable to perform tempering heat treatment on the cooled hot-rolled steel sheet at a temperature range of 600 to 700 ° C for (3.4t + 30) minutes (here, t means the thickness (mm) of steel) or more. When the cooled hot-rolled steel sheet is heat treated at a temperature of less than 600 ° C, it is difficult to form fine precipitates and it is difficult to secure strength. It can be greatly inferior. If the tempering heat treatment time is less than (3.4 t + 30) minutes, heat treatment may be performed at a temperature lower than the target temperature due to insufficient reheating, so that strength may be secured, but impact toughness may increase. On the other hand, since the upper limit of the tempering heat treatment time has no technical meaning, it is not particularly limited and can be easily determined by a person skilled in the art considering facility limitations.

Cooling after the tempering heat treatment is not particularly limited, but may be performed by air cooling.

Welding is performed on the steel material manufactured as described above, and post-welding heat treatment (PWHT) may be performed. In general, since steel for pressure vessels is used after being welded, it is common to perform PWHT heat treatment in order to overcome toughness deterioration of the welded part. In the present invention, the welding and PWHT processes are not particularly limited. For example, it is necessary to stabilize the toughness after welding by performing PWHT (post-weld heat treatment) heat treatment for 1 hour or more per inch of steel thickness in the temperature range of 550 to 650 ° C.

If the temperature during the PWHT heat treatment is less than 550 ° C., long heat treatment is required, which may reduce economic feasibility. On the other hand, when the temperature exceeds 650 ° C., not only the effect of reducing strength is excessively increased, but also the impact toughness may be reduced due to coarsening of carbides.

The steel material after the PWHT heat treatment is air-cooled to room temperature, and a steel material composed of ferrite, pearlite, and bainite phase can be obtained.

Next, examples of the present invention will be described.

Of course, the following examples can be modified in various ways without departing from the scope of the present invention to those skilled in the art. The following examples are for understanding of the present invention, and the scope of the present invention should not be limited to the following examples and should not be defined, but should be defined by the claims described later as well as those equivalent thereto.

(Example)

A slab was manufactured by continuously casting molten steel having an alloy composition (% by weight, the remainder being Fe and unavoidable impurities) shown in Table 1 below. At this time, the slab was manufactured to a thickness of 700 mm.

division C Si Mn P S Al Nb Cu Cr Ni Mo V Ti Ca N Relation 1 Relation 2 Invention Example 1 0.160 0.370 0.950 0.010 0.001 0.030 0.010 0.250 0.250 0.350 0.100 0.010 0.002 0.0015 0.0035 0.430 1.5 Invention example 2 0.140 0.370 1.450 0.010 0.001 0.030 0.017 - - 0.350 0.100 0.007 0.002 0.0016 0.0035 0.426 1.6 Inventive example 3 0.150 0.350 1.250 0.008 0.001 0.030 0.016 0.100 0.150 0.300 0.100 0.007 0.002 0.0017 0.0035 0.436 1.7 Inventive example 4 0.155 0.370 1.400 0.009 0.001 0.030 0.017 - - 0.250 0.110 0.006 0.002 0.0015 0.0034 0.428 1.5 Inventive Example 5 0.160 0.250 1.050 0.008 0.001 0.030 0.013 0.100 0.100 0.300 0.080 0.010 0.002 0.0015 0.0030 0.400 1.5 Comparative Example 1 0.160 0.350 1.100 0.009 0.001 0.030 0.003 0.005 0.150 0.250 0.090 0.007 0.002 0.0003 0.0035 0.410 0.3 Comparative Example 2 0.185 0.370 1.000 0.010 0.001 0.030 0.017 0.100 0.100 0.350 0.100 0.010 0.002 0.0018 0.0033 0.424 1.8 Comparative Example 3 0.145 0.370 1.450 0.010 0.001 0.050 0.016 - - 0.300 0.100 0.008 0.002 0.0020 0.0033 0.428 2.0

In Table 1, relational expressions 1 and 2 are calculated as follows.

[Relationship 1]

Ceq ≤ 0.45

(Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and the C, Mn, Cr, Mo, V, Cu, and Ni are the contents of each component (% by weight) ) is the value)

[Relationship 2]

1.2 ≤ Ca/S ≤ 4.0

(The above Ca and S are the content (% by weight) of each component)

The playing slab was reheated to about 1000° C. or higher, forged to a thickness of about 400 mm, and then cooled in air.

After heating the forged slab to about 1100 ° C, rough rolling was performed at about 1050 ° C or higher, and then finish hot rolling was performed at about 980 ° C to obtain a hot-rolled steel sheet having a thickness of about 200 mm.

The hot-rolled steel sheet is air-cooled to room temperature, reheated to about 890° C., maintained for about 480 minutes, then water-cooled (?? quenched) to room temperature again, reheated to about 650° C., maintained (tempered) for about 710 minutes, and air-cooled after QT heat treatment. performed. Thereafter, the air-cooled hot-rolled steel sheet was heated to about 635 ° C. and held for about 1200 minutes to perform PWHT (post-weld heat treatment) heat treatment, and then air-cooled to room temperature to prepare a final steel product. Detailed conditions are shown in Table 2.

division slab
Extraction temperature (℃) Slab heating time (min.) End rolling temperature (℃) Reheat temperature (℃) Reheat holding time (min.) Tempering temperature (℃) Tempering holding time (min.) Invention example 1 1111 361 980 890 505 655 731 Invention example 2 1110 362 981 891 499 651 730 Invention Example 3 1109 360 980 890 506 650 729 Invention example 4 1110 359 981 891 506 654 728 Invention example 5 1108 356 982 890 506 655 730 Comparative Example 1 1112 359 979 888 507 654 731 Comparative Example 2 1110 360 985 889 509 653 732 Comparative Example 3 1109 361 980 890 507 651 734

The microstructure and mechanical properties of the steel material prepared as described above were evaluated. The microstructure was observed with an optical microscope, and the microstructure fraction, ferrite diameter, and number of inclusions were measured using an analysis program. At this time, the microstructure was measured at the point of t/4 (t is the thickness of the steel, mm) in the thickness direction of each steel, and the results are shown in Table 3 below.

In addition, the mechanical properties were evaluated at the 1/4t point in the thickness direction of each steel, and at this time, the tensile specimen was taken at each thickness direction point in the direction perpendicular to the rolling direction, and the tensile strength (TS), yield strength (YS) and elongation ( El) was measured, and the impact specimen was taken from a JIS No. 4 standard test piece at the 1/4t point in the thickness direction in the rolling direction, and the average impact toughness (CVN) at -46 ° C was measured. The results are shown in Table 3 below. was

The inclusions refer to oxidative inclusions such as Al-O, Ca-O, and Al-Ca-O systems having a size of 10 μm or more.

On the other hand, the hydrogen-induced cracking crack length ratio (CLR, %) in the longitudinal direction of the plate, which is used as an indicator of the hydrogen-induced cracking resistance of the steel plate, is 5% saturated with H ₂ S gas at 1 atm in accordance with the related international standard NACE TM0284. After immersing the specimen in NaCl + 0.5% CH ₃ COOH solution for 96 hours, the length of cracks is measured by ultrasonic testing, and the total length of each crack in the longitudinal direction of the specimen is calculated by dividing the total length of the specimen. It was evaluated and the results are shown in Table 3.

division microstructure (% area) Number of oxidizing inclusions
(pcs/mm ² ) tensile properties -46℃ CVN impact toughness (J) CLR (%) polaronal ferrite perlite bainite YP (MPa) TS (MPa) El. (%) Invention Example 1 78 22 0 12 347 497 37 251 0 Invention example 2 70 30 0 21 358 508 36 275 0 Invention example 3 72 22 6 17 355 499 36 246 0 Invention Example 4 73 20 7 15 345 500 36 228 0.6 Invention example 5 74 22 4 22 338 502 37 255 0.3 Comparative Example 1 78 22 0 28 321 480 38 241 10.5 Comparative Example 2 65 20 15 24 368 532 32 84 21.5 Comparative Example 3 71 29 0 86 355 504 37 198 16.4

As shown in Table 3, the inventive steels 1 to 5 manufactured by the alloy composition, component relationship, and manufacturing conditions proposed in the present invention have the microstructure, tensile properties, low-temperature impact toughness and hydrogen induced cracking resistance proposed in the present invention. value is satisfied.

On the other hand, in the case of Comparative Example 1, the content of Nb and Ca is outside the range proposed in the present invention, and the tensile strength is low due to the lack of Nb content, and the Ca / S ratio is outside the value suggested in the present invention. It can be seen that the CLR value deviated from the value suggested in the present invention due to insufficient control. Comparative Example 2 is a component system in which the C content is outside the range suggested in the present invention, and it is possible to sufficiently secure tensile properties, but the low-temperature impact toughness is outside the value suggested in the present invention, and the CLR value is also large due to the increase in the hard phase. It can be seen that increased In the case of Comparative Example 3, most of the components satisfy the values suggested in the present invention, but the content of Al is too large, and the tensile and low-temperature impact toughness are satisfied, but hydrogen induced cracking occurs due to the presence of Al-based oxide. As a starting point, it can be confirmed that the CLR value greatly deviated from the value suggested in the present invention.

On the other hand, Figures 1 and 2 show the ultrasonic flaw detection results of the test piece after the hydrogen induced cracking test at the 1/2t point in the center of the width for the steel materials of Inventive Example 1 and Comparative Example 1, respectively. In Inventive Example 1 of FIG. 1, hydrogen-induced cracking did not occur at all, whereas in Comparative Example 1 of FIG. 2, where Nb and Ca were outside the values suggested in the present invention, hydrogen-induced cracking occurred.

Claims (8)

In % by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025% , Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, the rest is Fe and unavoidable impurities;
Inside the steel, the number of inclusions of one or more of Al-O-based, Ca-O-based and Al-Ca-O-based oxidizing inclusions having a size of 10 μm or more is 50 or less per 1 mm ² ,
A steel having excellent hydrogen induced crack resistance and low temperature impact toughness that satisfies the following [Relational Expression 1] and [Relational Expression 2].
[Relationship 1]
Ceq ≤ 0.45
(Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and the C, Mn, Cr, Mo, V, Cu, and Ni are the contents of each component (% by weight) ) is the value)
[Relationship 2]
1.2 ≤ Ca/S ≤ 4.0
(The above Ca and S are the content (% by weight) of each component)
The method of claim 1,
The steel is Cu: 0.5% or less and Cr: 0.35% or less of hydrogen-induced cracking resistance and low-temperature impact toughness excellent steel.
The method of claim 1,
The steel has excellent hydrogen-induced crack resistance and low-temperature impact toughness in which the fraction of ferrite is 70% or more, the fraction of pearlite is 20 to 30%, and the balance is bainite (including 0%).
The method of claim 3,
The average grain size of the ferrite is less than 25㎛ hydrogen induced cracking resistance and low-temperature impact toughness excellent steel.
The method of claim 1,
After heat treatment (PWHT) of the steel after welding, the yield strength evaluated perpendicular to the rolling direction at the point t / 4 in the thickness direction of the steel (where t means the thickness (mm) of the steel) is 260 MPa or more, tensile A steel with excellent resistance to hydrogen induced cracking and low-temperature impact toughness with a strength of 485 MPa or more and an average Charpy impact absorption energy (CVN, -46 ° C) value of 150 J or more at -46 ° C.
In % by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025% , Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, the rest is Fe and heating a steel slab containing unavoidable impurities and satisfying the following [Relational Expression 1] and [Relational Expression 2] in a temperature range of 1100 to 1200 ° C.;
Manufacturing a hot-rolled steel sheet by rough-rolling the heated steel slab at a temperature of 1050° C. or higher and finishing hot-rolling at a temperature of Ar3 or higher;
air-cooling the hot-rolled steel sheet;
Reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher, and reheating for (2.3t+30) minutes (where t means the thickness (mm) of steel) or more;
Quenching the reheated hot-rolled steel sheet to room temperature at a cooling rate of 0.4° C./s or more; and
Tempering heat treatment of the cooled hot-rolled steel sheet for more than (3.4t + 30) minutes (where t means the thickness (mm) of steel) in the temperature range of 600 ~ 700 ℃
Method for producing a steel having excellent hydrogen-induced cracking resistance and low-temperature impact toughness comprising a.
[Relationship 1]
Ceq ≤ 0.45
(Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and the C, Mn, Cr, Mo, V, Cu, and Ni are the contents of each component (% by weight) ) is the value)
[Relationship 2]
1.2 ≤ Ca/S ≤ 4.0
(The above Ca and S are the content (% by weight) of each component)
The method of claim 6,
The steel slab is Cu: 0.5% or less and Cr: 0.35% or less of hydrogen-induced cracking resistance and low-temperature impact toughness excellent steel manufacturing method.
According to claim 6 or 7,
After welding the steel, manufacturing a steel having excellent hydrogen induced crack resistance and low-temperature impact toughness, including the step of PWHT (post-weld heat treatment) heat treatment for 1 hour or more per inch of steel thickness in a temperature range of 550 to 650 ° C. method.

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