WO2017111526A1 - Low-yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness - Google Patents

Abstract

An aspect of the present invention relates to a low-yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, the steel comprising, by weight, 0.02-0.10% of carbon (C), 0.5-2.0% of manganese (Mn), 0.05-0.5% of silicon (Si), 0.05-1.0% of nickel (Ni), 0.005-0.1% of titanium (Ti), 0.005-0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), and the balanced amount of Fe and inevitable impurities, the microstructure of which comprises, by area, 60% or more of acicular ferrite and a balanced amount of one or more phases of bainite, polygonal ferrite and martensite-austenite constituent (MA).

Description

High-strength, high-strength steel with excellent stress corrosion cracking resistance and low temperature toughness

The present invention relates to a high-resistance steel composite with excellent resistance to stress corrosion cracking resistance and low temperature toughness.

The steel used for the liquefied gas storage tank varies depending on the type of liquefied gas, but the liquefaction temperature of the gas is generally low temperature (-52 ° C in the case of LPG) at normal pressure, so that the low temperature toughness of the base metal and the weld is excellent. Has been required.

In addition, liquid ammonia (LAG) is known to cause stress corrosion cracking (SCC) of steel, and in the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC CODE), oxygen partial pressure, temperature, etc. In addition to regulating the operating conditions at the time of manufacture, the Ni content of the steel is limited to 5% or less and the actual yield strength is limited to 440 MPa or less.

In addition, when manufacturing a gas tank by welding steel materials for the gas tank, the stress relief of the weld portion is an important part. Thus, as a method of removing the weld stress, there is a PWHT (Post Welding Heat Treatment) method by heat treatment, and there is a mechanical stress relief (MSR) method for removing stress by adding hydrostatic pressure to the weld. . Among these, when the weld stress is removed using the mechanical stress relief (MSR) method, since the deformation due to the hydraulic pressure is applied to the base metal part, the yield ratio of the base material is limited to 0.8 or less. This is because when the stress of the MSR is applied to the base material due to the high-pressure water spray to remove the stress, if the yield strength and the tensile strength ratio are high, the yield strength, that is, the tensile strength may be reached and fracture may occur. This limits the difference between yield strength and tensile strength.

In particular, in the case of a gas tank, it is difficult to remove stress by the PWHT method because the size of the gas tank should be basically increased. Accordingly, most shipbuilders prefer a mechanical stress relief (MSR) method to manufacture a gas tank. The steel to be used requires a resistive ratio property.

As described above, in the tank in which the LPG and the LAG are mixed, the simultaneous achievement of the resistance ratio accompanying the upper limit of the yield strength from low temperature toughness and liquid ammonia has been a major problem.

On the other hand, Patent Document 1 has been proposed a technique for adding 6.5 to 12.0% Ni in order to implement excellent low-temperature toughness. In addition, Patent Literature 2 has proposed a technique of mixing tempered martensite and bainite by quenching a steel having a specific composition.

In general, however, the addition of a large amount of Ni produces many austenite phases with FCC lattice structures that are easily deformed due to the narrow interatomic spacing.Easily corroded when repeated stress and corrosion conditions are applied to these easily deformed FCC lattice structures This occurs and cracks are generated. Therefore, the present invention has a problem of inferior economic efficiency due to high expensive Ni content, and has a problem that may cause a decrease in stress corrosion cracking (SCC) resistance.

In addition, Patent Document 3 has been proposed a technique for softening only the surface layer of the steel sheet in order to implement a resistance compounding. However, this technique can achieve low-temperature toughness and resistance ratio, respectively, it is difficult to obtain the low-temperature toughness and resistance ratio at the same time.

Meanwhile, methods for improving the strength of steel, which are required for steel, include precipitation strengthening, solid solution strengthening, and martensite, but these methods improve strength while deteriorating toughness and elongation. There is a problem.

In addition, in the case of reinforcing the strength by miniaturizing the crystal grains by applying various manufacturing conditions, not only high strength can be obtained, but also the toughness deterioration can be prevented due to the reduction of the impact toughness transition temperature. Yield strength to generate ammonia stress corrosion (SCC) exceeds the upper limit of 440MPa, there is a problem that it is difficult to secure a resistance ratio.

Therefore, there is a demand for development of a high-resistance-strength high strength steel having excellent stress corrosion cracking resistance and low temperature toughness and a manufacturing method thereof.

(Prior art document)

(Patent Document 1) Patent Document 1: Japanese Patent Application Laid-Open No. 63-290246

(Patent Document 2) Patent Document 2: Japanese Patent Application Laid-Open No. 58-153730

(Patent Document 3) Patent Document 3: Japanese Patent Application Laid-Open No. 4-17613

One aspect of the present invention is to provide a high-resistance-ratio high-strength steel and its manufacturing method excellent in stress corrosion cracking resistance and low temperature toughness.

In addition, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the present invention by weight, carbon (C): 0.02 ~ 0.10%, manganese (Mn): 0.5 ~ 2.0%, silicon (Si): 0.05 ~ 0.5%, nickel (Ni): 0.05 ~ 1.0%, Titanium (Ti): 0.005 to 0.1%, Aluminum (Al): 0.005 to 0.5%, Niobium (Nb): 0.005% or less, Phosphorus (P): 0.015% or less, Sulfur (S): 0.015% or less Contains Fe and other unavoidable impurities, microstructure is area%, acicular ferrite is more than 60%, the remainder is bainite, polygonal ferrite, martensite-austenite constituent (MA) The present invention relates to a high-resistance-resistant high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness including at least one phase.

In addition, another aspect of the present invention is by weight, carbon (C): 0.02 ~ 0.10%, manganese (Mn): 0.5 ~ 2.0%, silicon (Si): 0.05 ~ 0.5%, nickel (Ni): 0.05 ~ 1.0%, Titanium (Ti): 0.005 to 0.1%, Aluminum (Al): 0.005 to 0.5%, Niobium (Nb): 0.005% or less, Phosphorus (P): 0.015% or less, Sulfur (S): 0.015% Hereinafter, heating the slab containing the remaining Fe and other unavoidable impurities to 1000 ~ 1200 ℃;

Rough rolling the heated slab at a temperature of 1100-900 ° C .;

Finishing rolling at a temperature between Ar ₃ + 100 ° C. and Ar ₃ + 30 ° C. based on the central temperature after the rough rolling; And

It relates to a stress corrosion cracking resistance and low-temperature toughness high strength steels excellent manufacturing method comprising the step of cooling to a temperature below 300 ℃ after the finish rolling.

In addition, the solution of the said subject does not enumerate all the characteristics of this invention. Various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, by controlling the alloy composition and the microstructure, there is an effect that can provide a high-resistance-ratio high-strength steel and excellent method for stress corrosion cracking resistance and low temperature toughness.

1 is a phase transformation diagram of the inventive steel A according to the cooling rate.

Figure 2 is a microstructure of 1 / 4t part of the steel sheet of A-5 of Comparative Example It is the photograph (1- (1) of FIG. 1) observed with the optical microscope.

Figure 3 is a microstructure of 1 / 4t part of the steel sheet of the invention A-1 It is the photograph (1- (2) of FIG. 1) observed with the optical microscope.

Figure 4 is a microstructure of 1 / 4t part of the steel sheet of A-6 of Comparative Example It is the photograph (1- (3) of FIG. 1) observed with the optical microscope.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

The present inventors have recognized that it is difficult to improve both ammonia stress corrosion cracking resistance and low temperature toughness, and studied in depth to solve this problem.

As a result, it was confirmed that by controlling the alloy composition and microstructure, it is possible to provide a high-resistance-ratio high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, and a method of manufacturing the same.

Hereinafter, the stress-corrosion crack resistance and low-temperature toughness excellent strength ratio high strength steel according to an aspect of the present invention will be described in detail.

In accordance with an aspect of the present invention, the stress-ratio crack resistance and the low-temperature toughness high strength steel having excellent low temperature toughness are% by weight, carbon (C): 0.02 to 0.10%, manganese (Mn): 0.5 to 2.0%, and silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1%, aluminum (Al): 0.005 to 0.5%, niobium (Nb): 0.005% or less, phosphorus (P) : 0.015% or less, sulfur (S): 0.015% or less, containing the remaining Fe and other unavoidable impurities,

Microstructure is the area%, acicular ferrite (Acicular Ferrite) is more than 60%, the rest includes at least one phase of bainite (Bainite), Polygonal Ferrite (Martensite-Austenite constituent).

First, the alloy composition of the high-resistance-strength high strength steel excellent in corrosion resistance and stress corrosion cracking resistance according to an aspect of the present invention will be described in detail. Hereinafter, the content of each component means weight%.

C (carbon): 0.02 to 0.10%

Since C is the most important element for securing basic strength, it needs to be contained in steel within an appropriate range, and in order to obtain such an addition effect, it is preferable to add C 0.02% or more.

If the C content is less than 0.02%, it is not preferable because it can lead to a decrease in yield ratio with a drop in strength. On the other hand, if the C content exceeds 0.10%, there is a problem that the yield strength upper limit that can cause ammonia stress corrosion cracking (SCC) is generated a large amount of low-temperature transformation phase, such as bainite.

Therefore, the content of C is preferably limited to 0.02 to 0.10%. More preferably, it is 0.05 to 0.08%.

Si (silicon): 0.05-0.5%

Si has the effect of strengthening the strength by solid solution strengthening effect, and is an element that is also usefully used as a deoxidizer in the steelmaking process.

If the Si content is less than 0.05%, the deoxidation effect and the strength improving effect may be insufficient. On the other hand, when the Si content is more than 0.5%, there is a problem in lowering the low temperature toughness and at the same time deteriorating the weldability.

Therefore, the content of the silicon is preferably limited to 0.05 ~ 0.5%. More preferably, it is 0.05 to 0.3%.

Mn (manganese): 0.5-2.0%

Manganese contributes to the ferrite grain refinement and is a useful element for enhancing strength by solid solution strengthening.

It is necessary to add more than 0.5% in order to obtain such a manganese effect. However, if the content exceeds 2.0%, the hardenability is excessively increased to promote the formation of upper bainite and martensite, greatly reducing the impact toughness and ammonia stress corrosion cracking (SCC) resistance, and the welding heat effect. Negative toughness is also lowered.

Therefore, the Mn content is preferably limited to 0.5 to 2.0%. More preferably, it is 1.0 to 1.5%.

Ni (nickel): 0.05-1.0%

Ni is an important element for facilitating cross slip of dislocations at low temperatures, improving impact toughness, improving hardenability, and improving strength. To achieve this effect, Ni is preferably added at least 0.05%. However, when the Ni content is more than 1.0%, it may cause ammonia stress corrosion cracking (SCC), and the manufacturing cost may also increase due to the high cost of Ni relative to other hardenable elements.

Therefore, the Ni content is preferably limited to 0.05 to 1.0%. More preferably, it is 0.2 to 0.5%.

Nb (niobium): 0.005% or less

Nb is known to have an effect of refining austenite by inhibiting recrystallization of austenite because Nb precipitated very finely in the form of NbC when reheated to a high temperature.

According to the microstructure, the yield strength may be excessively increased, and thus the yield strength may be exceeded, which may cause ammonia stress corrosion cracking (SCC). Therefore, Nb is preferably controlled at 0.005% or less. More preferably, it is 0.003% or less.

Ti (titanium): 0.005 to 0.1%

Titanium can greatly improve low-temperature toughness by forming oxides and nitrides in steel to suppress grain growth upon reheating, and is effective for miniaturizing welded microstructures.

To obtain this effect, it is necessary to add titanium at 0.005% by weight or more. However, if the content exceeds 0.1% by weight, there is a problem that low-temperature toughness is reduced due to clogging of the playing nozzle or crystallization of the center portion.

Therefore, the titanium content is preferably 0.005 to 0.1%. More preferably, it is 0.01 to 0.03%.

Al (aluminum): 0.005-0.5%

Aluminum is a useful element for deoxidizing molten steel, which needs to be added at 0.005% by weight or more. However, if the content exceeds 0.5% by weight it is not preferable because it causes nozzle clogging during continuous casting. Therefore, the aluminum content is preferably 0.005 to 0.5%. More preferably, it is 0.005 to 0.05%.

P (phosphorus): 0.015% or less

Phosphorus is an element that causes grain boundary segregation in the base metal and the welded part, which causes the problem of embrittlement of the steel, and thus it is necessary to actively reduce it. However, in order to reduce such phosphorus to an extreme, the load of the steelmaking process is intensified, and the above-mentioned problem does not occur significantly when the phosphorus content is less than 0.015%, so the upper limit thereof is limited to 0.015%, more preferably 0.010% by weight. .

S (sulfur): 0.015% or less

Sulfur (S) is an element that causes MgS and the like to cause thermal embrittlement and thus greatly impairs impact toughness. Therefore, the sulfur (S) is preferably controlled as low as possible, so the content is limited to 0.015% by weight or less, and more preferably 0.005% by weight. do.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Next, the microstructure of the resistive crack ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness according to an aspect of the present invention will be described in detail.

The microstructure of the steel of the present invention is the area%, acicular ferrite (60% or more), the rest of the bainite (Bainite), polygonal ferrite (Polygonal Ferrite), MA (Martensite-Austenite constituent) at least one phase It includes.

If the needle ferrite is less than 60% as the bainite fraction increases, the impact toughness may occur due to the increase in the hard phase, and the needle ferrite fraction is less than 60% as the Polygonal Ferrite fraction increases. In this case, deterioration of strength may occur. Therefore, the area fraction of acicular ferrite is preferably 60% or more.

In addition, when the pearlite is included, the tensile strength and the low temperature impact toughness may be inferior, and thus the microstructure of the steel of the present invention may not include pearlite.

In this case, the acicular ferrite may have a size of 30 μm or less as measured by a circular equivalent diameter. If the size exceeds 30 μm, impact toughness may be inferior.

In addition, the bainite is preferably granular bainite and upper bainite.

On the other hand, the bainite area fraction is preferably 30% or less. If the bainite area fraction exceeds 30%, it is necessary to limit the bainite fraction as it may exceed the upper limit of yield strength (440 MPa) that can cause ammonia stress corrosion cracking (SCC).

In addition, it is preferable that the said MA phase is 10 area% or less, and the magnitude | size measured by the circular equivalent diameter is 5 micrometers or less. Martensite-Austenite constituent (MA) is also known as iconic martensite.

If the fraction of the MA phase exceeds 10%, or the original equivalent diameter exceeds 5㎛, the toughness of the base material and the welded part tends to be greatly reduced, so it is necessary to limit the fraction and size of the MA phase.

Meanwhile, the steel material of the present invention that satisfies the above condition may have a yield ratio (YS / TS) of 0.85 or less, preferably 0.8 or less. In addition, the steel material may have excellent tensile strength of about 490 MPa or more, for example, about 510 to 610 MPa.

In addition, the upper limit of the steel yield strength does not exceed the upper limit of the yield strength for generating ammonia stress corrosion cracking (SCC) to 440MPa or less, it may be excellent in ammonia stress corrosion cracking (SCC) resistance.

In addition, the impact transition temperature of 1 / 4t portion in the thickness direction of the steel material can be excellent in low temperature toughness of -60 ℃ or less. Where t means the thickness of the steel.

In this case, the steel has a thickness of 6mm or more, preferably 6 to 50mm.

Thus, the steel of the present invention can secure both high strength, resistance ratio, excellent low temperature toughness and ammonia stress corrosion cracking (SCC) resistance.

Hereinafter, a method for manufacturing a high-resistance high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness according to another aspect of the present invention will be described in detail.

According to another aspect of the present invention, there is provided a method for producing a high-resistance steel sheet having excellent stress corrosion cracking resistance and low temperature toughness, comprising: heating a slab having the aforementioned alloy composition to 1000 to 1200 ° C;

Rough rolling the heated slab at a temperature of 1100-900 ° C .;

Finishing rolling at a temperature between Ar ₃ + 100 ° C. and Ar ₃ + 30 ° C. based on the central temperature after the rough rolling; And

And cooling to a temperature of 300 ° C. or less after the finishing rolling.

Heating stage

The slab having the alloy composition described above is heated to 1000 to 1200 ° C.

The slab heating temperature is preferably at least 1000 ° C, in order to solidify the Ti carbonitride formed during casting. In addition, if the slab heating temperature is too low, it is preferable to limit the lower limit to 1000 ° C. because the deformation resistance during rolling is so high that the rolling reduction per pass cannot be largely applied in the subsequent rolling process. However, when heating to excessively high temperature, austenite may coarsen and deteriorate toughness, so the upper limit of the heating temperature is preferably 1200 ° C.

Rough rolling stage

The heated slab is rough rolled at a temperature of 1100 ~ 900 ℃.

It is preferable to make rough rolling temperature more than the temperature (Tnr) at which recrystallization of austenite stops. The casting structure such as dendrites formed during casting by rolling is destroyed, and the effect of reducing the size of austenite can also be obtained. In order to obtain such an effect, the rough rolling temperature is preferably limited to 1100 ~ 900 ℃.

At this time, the rough rolling may be performed so that the last three passes have a reduction ratio per pass of 10% or more.

In order to give sufficient deformation to the center part during rough rolling, it is preferable that the rolling reduction per pass is 10% or more and the total cumulative rolling reduction is 30% or more for the last three passes during rough rolling.

In the early rolling during the rough rolling, the recrystallized structure causes grain growth due to the high temperature, but during the last three passes, the grain growth rate is slowed down as the bar is air-cooled in the rolling atmosphere. The rate of reduction of the pass is greatest for the particle size of the final microstructure.

In addition, when the rolling reduction per pass of the rough rolling is lowered, sufficient deformation is not transmitted to the center, and thus toughness may be reduced due to the coarsening of the center. Therefore, it is desirable to limit the rolling reduction per pass of the last three passes to 10% or more.

On the other hand, the total cumulative reduction rate during rough rolling is preferably set to 30% or more in order to refine the central tissue.

Finishing rolling steps

After the rough rolling, finish rolling at a temperature between Ar ₃ + 100 ° C. and Ar ₃ + 30 ° C. based on the central temperature.

This is to obtain a finer microstructure, and when the finish rolling is carried out at Ar ₃ (ferrite transformation start temperature) + 100 ℃ ~ Ar ₃ + 30 ℃ temperature, a large amount of strain bands are formed inside austenite to form ferrite nucleation By securing a large amount of features, it is possible to obtain an effect of securing a fine structure to the center of the steel.

If the finish rolling temperature is lowered below Ar ₃ + 30 ° C, the ferrite grain size becomes too fine, exceeding the upper limit of yield strength (440MPa) that generates ammonia stress corrosion cracking (SCC), and finish at the temperature exceeding Ar ₃ + 100 ° C. When rolled, it is not effective for fine grain size. Therefore, it is preferable to perform the finish rolling temperature between Ar ₃ + 100 ° C. and Ar ₃ + 30 ° C., and the microstructure of the steel sheet manufactured by performing finish rolling under such conditions is a composite structure having the characteristics as described above. Can be.

In this case, Ar ₃ may be calculated as Ar ₃ = 910- (310 * C)-(80 * Mn)-(55 * Ni), and each element symbol represents the content of each element measured in weight% unit. , Ar ₃ is a unit.

In addition, in order to effectively produce a large amount of strain bands in the austenite, it is more preferable to maintain the cumulative reduction ratio at the time of finish rolling at 60% or more, and to maintain the reduction ratio per pass except the final shape even rolling at 10% or more.

Cooling stage

After the finish rolling, the temperature is cooled to 300 ° C or lower.

Cooling is preferably started to cool at a temperature of Ar ₃ + 30 ℃ ~ Ar ₃ after the finish rolling to cool to 300 ℃ or less, such as 100 ~ 300 ℃ Finish Cooling Temperature (FCT, Finish Cooling Temperature).

If the cooling finish temperature (FCT, Finish Cooling Temperature) is more than 300 ℃, due to the tempering (Tempering) effect may be difficult to implement the resistance ratio by decomposing the fine MA phase, the cooling finish temperature is preferably 300 ℃ or less.

At this time, in the cooling step, the central cooling rate is 15 ° C./s or more at Bs-10 ° C. to Bs + 10 ° C., and then the central cooling rate is 10-50 ° C./s until 300 ° C. or less. Two stage cooling can be performed as much as possible.

In addition, the cooling start temperature may be Ar ₃ + 30 ℃ ~ Ar ₃ .

The first stage cooling starts cooling at the temperature of Ar ₃ + 30 ℃ ~ Ar ₃ after the finish rolling to the Bs-10 ℃ ~ Bs + 10 ℃ central cooling rate of the steel sheet is 15 ℃ / s or more, for example 30 ℃ / It is preferable to cool at a cooling rate of s or more.

When the central part cooling rate of the steel sheet is lower than 15 ° C / s from Bs-10 ° C to Bs + 10 ° C in the first stage cooling, coarse polygonal ferrite may be formed to lower tensile strength and impact toughness. Because of this.

In this case, Bs may be calculated as Bs = 830- (270 * C)-(90 * Mn)-(37 * Ni), and each element symbol represents the content of each element measured in weight% unit, and Bs The unit of is ° C.

The two-stage cooling is preferably cooled to a cooling rate of 10 ° C / s ~ 50 ° C / s to the central cooling rate of the steel sheet to 300 ° C or less, for example, 100 ~ 300 ° C cooling finish temperature after the first stage cooling.

When the cooling rate of the steel sheet in the two-stage cooling exceeds 50 ℃ / s, as shown in the microstructure of 1- (1) of FIG. 1, the bainite fraction is formed to 30 area% or more to ammonia stress corrosion cracking (SCC Yield strength exceeding the upper limit (440 MPa) for generating a), and there is a possibility of lowering the elongation and impact toughness due to excessive increase in strength.

On the other hand, when the cooling rate of the steel sheet in the two-stage cooling is less than 10 ℃ / s, coarse polygonal ferrite and pearlite is formed rather than the fine needle-like ferrite like the microstructure of 1- (3) of Figure 1 tensile strength There is a possibility that the 490 MPa or less and the Charpy transition temperature become -60 ° C or more.

According to the above-described manufacturing method it is possible to manufacture a high-resistance-ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

After reheating a 300 mm thick steel slab having the composition shown in Table 1 at a temperature of 1100 ° C., rough rolling was performed at a temperature of 1050 ° C. to prepare a bar. The cumulative reduction rate was roughly 30% for rough rolling. In addition, it calculates the Ar ₃ and Bs temperature according to the composition of each steel is shown in Table 1 below.

After the rough rolling, finish rolling was performed to satisfy the difference between the finish rolling temperature and the Ar ₃ temperature shown in Table 2 below to obtain a steel plate having the thickness shown in Table 2, and then cooled at various cooling rates through multi-stage cooling. . At this time, the cooling end temperature of one-step cooling was made into Bs temperature of each steel.

Microstructure, yield strength, tensile strength, yield ratio, Charpy impact transition temperature, ammonia stress corrosion cracking (SCC) test for the steel sheet prepared as described above and the results are shown in Table 3.

The microstructure was mirror-polished after taking specimens from 1 / 4t of steel plate, and then corroded with Nital corrosive solution and observed with optical microscope.

The fraction of MA phase was mirror-polished after specimens were taken from the 1 / 4t site, corroded with LePera corrosion solution, and then observed with an optical microscope.

In the tensile test, yield strength, tensile strength, and yield ratio were measured by collecting a JIS No. 4 specimen in a direction perpendicular to the rolling direction from a 1 / 4t portion of the steel sheet and performing a tensile test at room temperature.

Low-temperature impact toughness after making the V- notch test specimens were taken for the specimen in a direction perpendicular to the rolling direction from the 1 / 4t part of the steel sheet, - from 20 to -100 ℃ to 20 ℃ interval the Charpy impact test for each three per temperature The test was conducted once to derive the regression equation of each temperature average value, and the temperature of 100 J was determined as the transition temperature.

In addition, the ammonia stress corrosion cracking (SCC) test was carried out using the test solution and test conditions described in Table 4 by making a proof ring specimen, wherein the stress applied was 80% of the actual yield stress, 720 If no fracture occurred during the time, it was evaluated as passing. If the fracture occurred before 720 hours passed, it was evaluated as failed.

However, in Table 3, AF, B, PF, and MA mean AF: Acicular Ferrite, B: Bainite, PF: Polygonal ferrite, and MA: Martensite / Austenite.

As shown in Tables 1 to 3, the invention examples satisfying the composition and manufacturing conditions proposed by the present invention not only have high strength and high toughness, but also have excellent resistance to ammonia stress corrosion cracking (SCC). The yield ratio is 0.8 or less, it can be seen that the steel having a resistance yield ratio characteristics. In addition, as a result of observing the microstructure of the invention A-1 with a microscope, as shown in 1- (2) of Figure 1, the area percent, acicular ferrite (60% or more), the remainder bainite (Bainite), Polygonal Ferrite (Polygonal Ferrite), MA (Martensite-Austenite constituent) It can be confirmed that the mixed tissue consisting of one or more phases.

On the other hand, component composition satisfies the present invention, but in the case of Comparative Examples A-2, A-4, A-6, B-2, B-4 and B-6 where the manufacturing conditions do not satisfy the present invention, Polygonal The ferrite fraction was too high or the ferrite grain size was too coarse to secure tensile strength and low temperature toughness.

On the other hand, in Comparative Examples A-3, A-5, A-7 to B-3, B-5, and B-7, the Acicular Ferrite grain size is too small, the Bainite fraction is too high, or the MA phase is not at all. As it could not be produced, ammonia stress corrosion cracking (SCC) could occur. The yield strength exceeded the upper limit (440 MPa), resulting in ammonia stress corrosion cracking, and it was impossible to secure a resistance ratio and low temperature toughness.

In addition, the production conditions satisfy the present invention, but in the case of Comparative Examples C-1 to F-4 in which the composition of the composition does not satisfy the present invention, the Bainite fraction is too high, the Acicular Ferrite grain size is too small, or MA As the fraction of phase becomes too high, ammonia stress corrosion cracking (SCC) can occur. The yield strength exceeded the upper limit (440 MPa), resulting in ammonia stress corrosion cracking, and it was not possible to secure a resistance ratio and low temperature toughness.

Although described with reference to the embodiments above, those skilled in the art will understand that the present invention can be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

Claims (12)

1. By weight%, carbon (C): 0.02-0.10%, manganese (Mn): 0.5-2.0%, silicon (Si): 0.05-0.5%, nickel (Ni): 0.05-1.0%, titanium (Ti): 0.005 to 0.1%, aluminum (Al): 0.005 to 0.5%, niobium (Nb): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, remaining Fe and other unavoidable impurities Including,

   Microstructure is the area%, stress corrosion including at least 60% acicular ferrite, the remainder is at least one of bainite, polygonal ferrite, martensite-austenite constituent (MA) High-strength, high-strength steel with excellent crack resistance and low temperature toughness.
2. The method of claim 1,

   The needle-like ferrite is a high-strength-resistant high-strength steel with excellent stress corrosion cracking resistance and low temperature toughness, characterized in that the size measured by the equivalent diameter of 30㎛ or less.
3. The method of claim 1,

   The bainite is excellent in stress corrosion cracking resistance and low temperature toughness high strength steel, characterized in that less than 30 area%.
4. The method of claim 1,

   The MA phase is 10 area% or less, the stress corrosion cracking resistance and low-temperature toughness excellent strength ratio high strength steel, characterized in that the size measured by the equivalent diameter of the circle.
5. The method of claim 1,

   Yield ratio of the steel is less than 0.85, the tensile strength is 490MPa or more characterized in that the stress corrosion cracking resistance and low-temperature toughness excellent strength ratio high strength steel.
6. The method of claim 1,

   The yield strength of the steel is excellent in corrosion resistance cracking resistance and low temperature toughness high strength steel, characterized in that less than 440MPa.
7. The method of claim 1,

   A high strength corrosion-resistant high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, characterized in that the impact transition temperature of the steel is -60 ℃ or less.
8. By weight%, carbon (C): 0.02-0.10%, manganese (Mn): 0.5-2.0%, silicon (Si): 0.05-0.5%, nickel (Ni): 0.05-1.0%, titanium (Ti): 0.005 to 0.1%, aluminum (Al): 0.005 to 0.5%, niobium (Nb): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, remaining Fe and other unavoidable impurities Heating the slab to 1000 to 1200 ° C;

   Rough rolling the heated slab at a temperature of 1100-900 ° C .;

   Finishing rolling at a temperature between Ar ₃ + 100 ° C. and Ar ₃ + 30 ° C. based on the central temperature after the rough rolling; And

   Method for producing a high strength corrosion resistance cracking resistance and low temperature toughness resistance corrosion resistance comprising the step of cooling to a temperature below 300 ℃ after the finish rolling.
9. The method of claim 8,

   The cooling step is performed after the first stage cooling to the central cooling rate of 15 ℃ / s or more to Bs-10 ℃ ~ Bs + 10 ℃,

   A method for producing a high strength corrosion resistant cracking resistance and low temperature toughness, characterized by performing two-stage cooling so that the cooling rate of the central part is 10 to 50 ° C / s until 300 ° C or less.
10. The method of claim 8,

    Cooling start temperature is Ar ₃ + 30 ℃ ~ Ar _{3 The} stress corrosion cracking resistance and low-temperature toughness excellent resistance ratio high-strength steel manufacturing method characterized in that.
11. The method of claim 8,

    The rough rolling is a method of producing a high strength corrosion resistance cracking resistance and low temperature toughness crack resistance, characterized in that the last three passes are performed so that the reduction rate per pass is 10% or more.
12. The method of claim 8,

    The finish rolling is a method for producing a high strength corrosion resistant cracking resistance and low temperature toughness excellent resistance to corrosion, characterized in that the rolling reduction per pass 10% or more, cumulative reduction rate 60% or more.

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