CN108431274B

CN108431274B - Low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness

Info

Publication number: CN108431274B
Application number: CN201680075892.9A
Authority: CN
Inventors: 张成豪; 李学哲
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2015-12-23
Filing date: 2016-12-23
Publication date: 2021-12-07
Anticipated expiration: 2036-12-23
Also published as: CN108431274A; EP3395987A1; WO2017111526A1; EP3395987B1; KR20170075933A; KR101767778B1; CA3009137A1; EP3395987A4; US20180371588A1; JP2019504200A; CA3009137C; JP6691217B2

Abstract

One aspect of the present invention relates to a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness, comprising the following components in weight%: carbon (C): 0.02% -0.10%, manganese (Mn): 0.5 to 2.0%, silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1% and aluminum (Al): 0.005-0.5%, niobium (Nb): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): less than or equal to 0.015%, the balance being Fe and other unavoidable impurities, and the microstructure including, in area%, 60% or more acicular ferrite, and the balance being at least one of bainite, polygonal ferrite, and martensite-austenite components.

Description

Low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness

Technical Field

The present invention relates to a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness.

Background

Steel materials used for liquefied gas storage tanks vary depending on the type of liquefied gas, but the liquefaction temperature of the gas is generally low at normal pressure (minus 52 ℃ for LPG), and therefore, not only the base material but also the welded portion are required to have excellent low-temperature toughness.

Further, it is known that Liquefied Ammonia (LAG) causes Stress Corrosion Cracking (SCC) of steel materials, and therefore IGC CODE (international liquefied gas carrier vessel structure and equipment regulation) defines that the Ni content of steel materials is limited to 5% or less and the actual yield strength is limited to 440MPa or less while limiting the operating conditions during production such as oxygen partial pressure and temperature.

In addition, when a Gas Tank (Gas Tank) is manufactured by welding a steel material for the Gas Tank (Gas Tank), it is important to eliminate stress at a welded portion. Therefore, as a method of relieving Stress at the welded portion, there are, for example, a PWHT (post-weld heat treatment) method by heat treatment and a Mechanical Stress Relief (MSR) method of relieving Stress by applying hydrostatic pressure or the like to the welded portion. When the stress of the welded portion is relieved by a Mechanical Stress Relief (MSR) method, deformation due to water pressure also acts on the base material portion, and therefore the yield ratio of the base material is limited to 0.8 or less. This is because, when deformation of yield strength or more is applied to the base material by high-pressure water jet during stress relief by MSR, yield occurs, that is, tensile strength is reached if the ratio of yield strength to tensile strength is high, and thus, there is a possibility that breakage occurs, and therefore, the yield strength is limited to a large difference from the tensile strength.

In particular, since it is necessary to substantially increase the size of Gas tanks (Gas Tank) and it is difficult to remove stress by the PWHT method, most of shipyards select the Mechanical Stress Relief (MSR) method, and thus steel materials used for manufacturing Gas tanks (Gas Tank) are required to have low yield ratio characteristics.

As described above, in the field of gas tanks in which LPG and LAG are mixed, it is a significant object to achieve both low-temperature toughness and a low yield ratio defined by an upper limit of yield strength from liquefied ammonia.

Patent document 1 proposes a technique of adding 6.5% to 12.0% of Ni to realize excellent low-temperature toughness. Further, patent document 2 proposes a technique of applying a quenching and tempering treatment to a steel of a specific composition to mix Tempered (Tempered) martensite and bainite.

However, in general, if a large amount of Ni is added, many austenite phases having an easily deformable FCC lattice structure are generated due to a narrow atomic distance, and when repeated stress and a corrosive environment are applied to such an easily deformable FCC lattice structure, corrosion easily occurs, resulting in the generation of cracks. Therefore, the above invention has the following problems: due to the high content of high-priced Ni, the economy is lowered, and Stress Corrosion Cracking (SCC) resistance is also lowered.

Patent document 3 proposes a technique for softening only the surface layer of a steel sheet to reduce the yield ratio. However, although this technique can achieve low-temperature toughness and a low yield ratio, respectively, it is difficult to achieve both low-temperature toughness and a low yield ratio.

Further, as a method for improving the strength of another property steel required for the steel material, there are precipitation strengthening, solid solution strengthening, Martensite (martentite) strengthening, and the like, but these methods improve the strength, but deteriorate the toughness and elongation.

Further, when the strength is increased by grain refinement using various manufacturing conditions, not only high strength can be obtained, but also deterioration in toughness can be prevented due to a decrease in impact toughness transition temperature, but the yield strength is increased by grain refinement, and thus the yield strength upper limit 440MPa of ammonia Stress Corrosion Cracking (SCC) is exceeded, and it is difficult to secure a low yield ratio.

Therefore, it is required to develop a low yield ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness and a method for producing the same.

Prior art documents

Patent document 1: japanese patent laid-open publication No. Sho 63-290246

Patent document 2: japanese patent laid-open publication No. Sho 58-153730

Patent document 3: japanese patent laid-open publication No. Hei 4-17613

Disclosure of Invention

Technical problem

An aspect of the present invention is to provide a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness, and a method for manufacturing the same.

The technical problem to be solved by the present invention is not limited to the above. The technical problems to be solved by the present invention can be understood from the entire content of the present specification, and other technical problems to be solved by the present invention will not cause any difficulty for those skilled in the art to understand.

Technical scheme

One aspect of the present invention relates to a low yield ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness, comprising the following components in weight%: carbon (C): 0.02% -0.10%, manganese (Mn): 0.5 to 2.0%, silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1% and aluminum (Al): 0.005-0.5%, niobium (Nb): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): less than or equal to 0.015%, the balance being Fe and other unavoidable impurities, and the microstructure including, in terms of area%, 60% or more of at least one of Acicular Ferrite (Acicular Ferrite), and the balance being Bainite (Bainite), Polygonal Ferrite (Polygon Ferrite), and MA (Martensite-Austenitic constituent).

Another aspect of the present invention relates to a method for producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness, comprising the steps of:

heating a steel slab to 1000-1200 ℃, said steel slab comprising in weight percent: carbon (C): 0.02% -0.10%, manganese (Mn): 0.5 to 2.0%, silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1% and aluminum (Al): 0.005-0.5%, niobium (Nb): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): less than or equal to 0.015%, the balance being Fe and other unavoidable impurities;

carrying out rough rolling on the heated steel billet at the temperature of 1100-900 ℃;

after rough rolling, the temperature of the central part is controlled to be Ar₃+100℃～Ar₃Finish rolling is carried out at the temperature of +30 ℃; and

and cooling to a temperature of less than or equal to 300 ℃ after the finish rolling.

Incidentally, the above-described technical problem solutions do not list all the features of the present invention. The features of the present invention and the advantages and effects based on the features are described in further detail with reference to the following detailed description.

Effects of the invention

According to the present invention, by controlling the alloy composition and microstructure, a low yield ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness and a method for producing the same can be provided.

Drawings

FIG. 1 is a phase change diagram of invention Steel A based on the cooling rate.

FIG. 2 is a photograph of a microstructure 1/4t of the steel sheet of comparative example A-5 (1- (1) in FIG. 1) observed by an optical microscope.

FIG. 3 is a photograph showing the microstructure of steel sheet 1/4t in invention example A-1 (1- (2) in FIG. 1) observed with an optical microscope.

FIG. 4 is a photograph of a microstructure 1/4t of the steel sheet of comparative example A-6 observed with an optical microscope (1- (3) of FIG. 1).

Detailed Description

Preferred embodiments of the present invention are set forth below. However, the present invention can be modified in various ways, and the scope of the present invention is not limited to the following embodiments. In addition, embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

The present inventors have known that it is somewhat difficult to make both the ammonia-induced stress corrosion cracking resistance and the low-temperature toughness excellent, and have conducted intensive studies to solve this problem.

As a result, they have found that a low yield ratio high strength steel material excellent in both stress corrosion cracking resistance and low temperature toughness and a method for producing the same can be provided by controlling the alloy composition and microstructure, and have completed the present invention.

A low yield ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness according to an aspect of the present invention is described in detail below.

According to one aspect of the present invention, a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness comprises the following components in weight%: carbon (C): 0.02% -0.10%, manganese (Mn): 0.5 to 2.0%, silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1% and aluminum (Al): 0.005-0.5%, niobium (Nb): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): less than or equal to 0.015%, the balance being Fe and other unavoidable impurities,

the microstructure includes 60% or more of Acicular Ferrite (Acicilar Ferrite), and the balance of at least one of Bainite (Bainite), Polygonal Ferrite (Polygon Ferrite), and MA (Martensite-Austenitic constituent).

First, the alloy composition of a low yield ratio high strength steel excellent in stress corrosion cracking resistance and low temperature toughness according to one aspect of the present invention will be described in detail. The content unit of each component below is weight%.

C (carbon): 0.02 to 0.10 percent

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

If the C content is less than 0.02%, a decrease in strength and a decrease in yield ratio may be caused, and therefore, it is not preferable. If the C content is more than 0.10%, a large amount of low-temperature transformation phases such as bainite will be formed, and the yield strength may exceed the upper limit of the yield strength which may cause ammonia Stress Corrosion Cracking (SCC).

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

Si (silicon): 0.05 to 0.5 percent

Si has an effect of improving strength by a solid solution strengthening effect, and is also an element useful as a deoxidizer in a steel making process.

If the Si content is less than 0.05%, the deoxidizing effect and the strength-improving effect may be insufficient. If the Si content is more than 0.5%, there is a problem that the low-temperature toughness is lowered and the weldability is also deteriorated.

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

Mn (manganese): 0.5 to 2.0 percent

Manganese is an element that contributes to ferrite grain refinement and is useful for improving strength by solid solution strengthening.

In order to obtain such an effect of manganese, it is necessary to add 0.5% or more. However, if the manganese content is more than 2.0%, hardenability is excessively increased, and the formation of Upper bainite and martensite is promoted, which results in a significant decrease in impact toughness and resistance to ammonia Stress Corrosion Cracking (SCC), and also a decrease in toughness at portions affected by welding heat.

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

Ni (nickel): 0.05 to 1.0 percent

Ni is an important element for improving impact toughness by facilitating dislocation Cross slip (Cross slip) at low temperatures and for improving strength by improving hardenability, and it is preferable to add Ni in an amount of 0.05% or more in order to obtain such an effect. However, if the Ni content is more than 1.0%, ammonia Stress Corrosion Cracking (SCC) may be caused, and Ni is more expensive than other hardenable elements, and thus the manufacturing cost is also increased.

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

Nb (niobium): less than or equal to 0.005 percent

Nb has an effect of precipitating a small amount of Nb dissolved in the form of NbC during high-temperature reheating, and suppressing recrystallization of austenite to refine the structure.

Since such a structure refinement may cause an excessively high yield strength exceeding the upper limit of the yield strength that may cause ammonia Stress Corrosion Cracking (SCC), Nb is preferably controlled to 0.005% or less, more preferably 0.003% or less.

Ti (titanium): 0.005 to 0.1 percent

Titanium forms oxides and nitrides in steel to inhibit grain growth during reheating, can greatly improve low-temperature toughness, and is effective in refining the microstructure of a welded portion.

To obtain such an effect, it is necessary to add 0.005% by weight or more of titanium. However, if the titanium content is more than 0.1 wt%, there are problems that the continuous casting nozzle (nozzle) is clogged or the low-temperature toughness is lowered due to crystallization at the center portion.

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

Al (aluminum): 0.005 to 0.5 percent

Aluminum is an element useful for deoxidizing molten steel, and therefore, it is necessary to add 0.005% by weight or more. However, if the aluminum content is more than 0.5 wt%, it is undesirable because it causes clogging of the nozzle during continuous casting. Therefore, the aluminum content is preferably 0.005% to 0.5%, more preferably 0.005% to 0.05%.

P (phosphorus): less than or equal to 0.015 percent

Phosphorus is an element that causes grain boundary segregation in the base material and the welded portion, and causes embrittlement of steel, and therefore, it is necessary to actively reduce the content of phosphorus. However, when the content of phosphorus is minimized, the load on the steel making process is increased, and if the content of phosphorus is 0.015% or less, the above problem is not caused in many cases, so that the upper limit of the content of phosphorus is preferably limited to 0.015%, more preferably 0.010% by weight.

S (sulfur): less than or equal to 0.015 percent

Sulfur (S) is an element that causes hot shortness, forms MnS and the like to cause a large decrease in impact toughness, and is preferably controlled to be as low as possible, so the sulfur content is preferably limited to 0.015 wt% or less, and more preferably to 0.005 wt%.

The balance of the present invention is iron (Fe). However, the conventional manufacturing process inevitably involves mixing of unexpected impurities derived from raw materials or the surrounding environment, and thus the mixing of impurities cannot be excluded. These impurities are known to anyone skilled in the art of conventional manufacturing processes and therefore all relevant details are not repeated in this specification.

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

The microstructure of the steel material of the present invention contains 60% or more by area of Acicular Ferrite (Acicilar Ferrite), and the balance of at least one of Bainite (Bainite), Polygonal Ferrite (Polygonal Ferrite), and MA (Martensite-Austenitic consistency).

When the bainite (bainite) fraction is increased to result in an acicular ferrite fraction of less than 60%, an increase in hard phase may occur to result in deterioration of impact toughness. When the Polygonal Ferrite (Polygonal Ferrite) fraction is increased to result in an acicular Ferrite fraction of less than 60%, deterioration in strength may occur. Therefore, the area fraction of Acicular Ferrite (Acicular Ferrite) is preferably 60% or more.

Further, if pearlite is included, the tensile strength and low-temperature impact toughness may deteriorate, and therefore the microstructure of the steel material 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 an equivalent circle diameter. If the size is more than 30 μm, impact toughness may be deteriorated.

The bainite is preferably granular bainite (granular bainite) or upper bainite (upper bainite).

The bainite area fraction is preferably 30% or less. If the bainite area fraction is more than 30%, the upper limit of yield strength (440MPa) that may induce ammonia Stress Corrosion Cracking (SCC) is exceeded, and therefore the bainite fraction needs to be limited.

The MA phase is preferably 10% by area or less and 5 μm or less in size as measured by equivalent circle diameter. MA (martensite-austenite component) is also called island martensite.

When the fraction of the MA phase is more than 10% or the equivalent circle diameter is more than 5 μm, the toughness of the base material and the welded portion tends to be greatly reduced, and therefore, the fraction and size of the MA phase need to be limited.

The steel material of the present invention satisfying the above conditions may have a yield ratio (YS/TS) of 0.85 or less, preferably 0.8 or less. The steel material has excellent tensile strength of 490MPa or more, for example, about 510MPa to 610 MPa.

Further, the steel material has an upper limit of yield strength of 440MPa or less and does not exceed an upper limit of yield strength at which ammonia-induced Stress Corrosion Cracking (SCC) is initiated, and therefore has excellent ammonia-induced Stress Corrosion Cracking (SCC) resistance.

Further, the impact transformation temperature at 1/4t or less in the thickness direction of the steel material, where t represents the thickness of the steel material, may be equal to or less than-60 ℃, and may have excellent low-temperature toughness.

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

As described above, the steel material of the present invention can simultaneously secure high strength, low yield ratio, excellent low-temperature toughness, and ammonia-induced Stress Corrosion Cracking (SCC) resistance.

The method for producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to another aspect of the present invention will be described in detail below.

According to another aspect of the present invention, a method for producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness, comprises the steps of:

heating the steel billet with the alloy components to 1000-1200 ℃;

Heating step

The billet with the alloy components is heated to 1000-1200 ℃.

The billet heating temperature is preferably 1000 ℃ or higher in order to form solid solution of Ti carbonitride formed during casting. Further, if the billet heating temperature is too low, the deformation resistance at the time of rolling becomes too high, and the reduction per pass cannot be increased greatly in the subsequent rolling process, so that the lower limit thereof is preferably limited to 1000 ℃. However, if the heating temperature is too high, austenite becomes coarse, which may cause a decrease in toughness, and therefore the upper limit of the heating temperature is preferably 1200 ℃.

Rough rolling step

And carrying out rough rolling on the heated steel billet at the temperature of 1100-900 ℃.

The rough rolling temperature is preferably equal to or higher than the temperature (Tnr) at which austenite recrystallization stops. By the rolling, a cast structure such as dendrites formed during casting is broken, and an effect of reducing the size of austenite can be obtained. In order to obtain this effect, the rough rolling temperature is preferably limited to 1100 ℃ to 900 ℃.

In this case, the rough rolling may be performed so that the reduction ratio per pass of the final three passes is 10% or more.

In rough rolling, in order to sufficiently deform the steel sheet to the center portion, the reduction ratio of each pass is preferably 10% or more and the total cumulative reduction ratio is preferably 30% or more in the last three passes.

In rough rolling, the grain growth of the recrystallized structure occurs due to high temperature in the initial rolling, but when the final three passes of rolling are performed, the grain growth rate becomes slow as the steel strip is air-cooled in the process of waiting for rolling, and therefore the rolling reduction of the final three passes of rough rolling has the greatest influence on the grain size of the final microstructure.

Further, when the reduction ratio per pass of rough rolling is decreased, sufficient deformation is not transmitted to the central portion, and there is a possibility that toughness is decreased due to coarsening of the central portion. Therefore, the reduction ratio per pass of the last three passes is preferably limited to 10% or more.

In order to refine the center structure, the total cumulative reduction ratio in rough rolling is preferably set to 30% or more.

Finish rolling step

After rough rolling, the temperature of the central part is controlled to be Ar₃+100℃～Ar₃Finish rolling is carried out at a temperature of +30 ℃.

This is to obtain a finer microstructure if at Ar₃(ferrite transformation initiation temperature) +100 ℃ to Ar₃When finish rolling is performed at a temperature of +30 ℃, a large amount of deformed zones are generated in austenite, and a large number of ferrite nucleation sites are secured, so that the effect of securing the microstructure up to the center of the steel material can be obtained.

If the finishing temperature is reduced to less than Ar₃At +30 ℃ the ferrite grain size becomes too fine, which leads to exceeding the upper yield strength limit (440MPa) for inducing ammonia induced Stress Corrosion Cracking (SCC), if more than Ar₃Finish rolling at +100 ℃ has no effect on grain size refinement. Therefore, it is preferable to limit the finish rolling temperature to Ar₃+100℃～Ar₃Only when finish rolling is performed under such conditions at +30 ℃, the microstructure of the produced steel sheet can be made to be a composite structure having the characteristics as described above.

At this time, the Ar₃Can pass through Ar₃910- (310 × C) - (80 × Mn) - (55 × Ni), each element symbol represents the content of each element measured in units of weight%, and Ar₃The unit of (A) is [ deg. ] C.

In order to efficiently produce a large amount of deformed strips in the austenite, the cumulative reduction ratio is preferably maintained at 60% or more at the time of finish rolling, and more preferably at 10% or more per pass in addition to rolling for making the final shape uniform.

Step of Cooling

Cooling is preferably from Ar after finish rolling₃+30℃～Ar₃Until the Temperature is cooled to 300 ℃ or lower, for example, a Cooling end Temperature (FCT) of about 100 to 300 ℃.

If the Cooling completion Temperature (FCT) is greater than 300 ℃, the fine MA phase may be decomposed due to the Tempering effect, and it may be difficult to achieve a low yield ratio, and therefore the Cooling completion Temperature is preferably 300 ℃ or lower.

In this case, the step of cooling may be performed by performing the first step of cooling to Bs-10 ℃ to Bs +10 ℃ at a center portion cooling rate of 15 ℃/s or more, and then performing the second step of cooling to 300 ℃ or less at a center portion cooling rate of 10 ℃/s to 50 ℃/s.

Further, the cooling start temperature may be Ar₃+30℃～Ar₃。

The first cooling step is preferably performed from Ar after finish rolling₃+30℃～Ar₃And cooling the steel sheet to Bs-10 to Bs +10 ℃ at a cooling rate of 15 ℃/s or more, for example, 30 ℃/s or more, for the central portion of the steel sheet.

In the first cooling, if the cooling rate of the central portion of the steel sheet is less than 15 ℃/s until Bs-10 ℃ to Bs +10 ℃, coarse Polygonal Ferrite (Polygonal Ferrite) is formed, which may cause a decrease in tensile strength and impact toughness.

In this case, Bs can be calculated by Bs 830- (270 × C) - (90 × Mn) - (37 × Ni), each element symbol represents the content of each element measured in units of wt%, and Bs is in units of ℃.

The second cooling is preferably performed after the first cooling at a cooling rate of 10 ℃/s to 50 ℃/s for the central portion of the steel sheet to a cooling completion temperature of 300 ℃ or lower, for example, 100 ℃ to 300 ℃.

In the second cooling, if the cooling rate of the steel sheet is greater than 50 ℃/s, the bainite fraction is greater than or equal to 30 area% in the microstructure 1- (1) of fig. 1, which results in exceeding the upper limit of yield strength (440MPa) that induces ammonia Stress Corrosion Cracking (SCC), and the elongation and impact toughness may be lowered due to an excessive increase in strength.

On the contrary, in the second-step cooling, if the cooling rate of the steel sheet is less than 10 ℃/s, coarse polygonal ferrite and pearlite are formed without forming fine acicular ferrite as in the microstructure of 1- (3) of fig. 1, which may result in a tensile strength of 490MPa or less and a charpy impact transformation temperature of-60 ℃ or more.

According to the above-mentioned production method, a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness can be produced.

The present invention is described more specifically by examples below. It should be noted, however, that the following examples are only for illustrating the present invention to describe it in more detail, and are not intended to limit the scope of the present invention. The scope of the claims of the present invention depends on the contents of the claims and reasonable derivation thereof.

After reheating a billet having a thickness of 300mm having the composition of table 1 below to a temperature of 1100 c, rough rolling was performed at a temperature of 1050 c to manufacture a steel strip. The cumulative reduction rate in rough rolling was the same at 30%. In addition, Ar based on each steel component was calculated₃And Bs temperature and are reported in table 1 below.

After the rough rolling, the final rolling temperature and Ar shown in the following table 2 were satisfied₃The finish rolling was performed by the difference in temperature to obtain steel sheets having the thicknesses of table 2 below, and then cooling was performed at various cooling rates by multi-step cooling. At this time, the cooling end temperature of the first cooling is the Bs temperature of each steel.

The steel sheets thus produced were subjected to microstructure, yield strength, tensile strength, yield ratio, charpy impact transition temperature, and ammonia induced Stress Corrosion Cracking (SCC) tests, and the results are shown in table 3.

For the microstructure, samples were collected from 1/4t portions of the steel sheet, mirror-polished, etched with Nital etching solution, observed with an optical microscope, and then the phase fraction was obtained by image analysis.

The fraction of MA phase was determined by collecting a sample from 1/4t, mirror polishing, etching with a LePera etching solution, observing with an optical microscope, and image analysis.

For the tensile test, a JIS4 sample was sampled from 1/4t of a steel sheet in a direction perpendicular to the rolling direction, and a tensile test was performed at normal temperature to measure the yield strength, tensile strength, and yield ratio.

For low-temperature impact toughness, samples were collected from 1/4t portions of a steel sheet in a direction perpendicular to the rolling direction, and after V-notch samples were produced, Charpy impact tests were conducted at intervals of 20 ℃ from-20 ℃ to-100 ℃ for 3 times per temperature test, and then a regression equation of the average value of the respective temperatures was derived, and the temperature up to 100J was found as a transition temperature.

In the ammonia Stress Corrosion Cracking (SCC) test, a stress ring (proof ring) sample was prepared, and the test was performed using the test solutions and test conditions shown in table 4, and the stress applied at this time was actually 80% yield stress, and was evaluated as pass if no fracture occurred within 720 hours, and was evaluated as fail if fracture occurred before 720 hours.

[ TABLE 1 ]

[ TABLE 2 ]

[ TABLE 3 ]

In Table 3 above, AF, B, PF and MA represent Acicular Ferrite (AF: Acicilar Ferrite), Bainite (B: Bainite), Polygonal Ferrite (PF: Polygonal Ferrite) and Martensite-Austenite (MA: Martensite/Austenite).

[ TABLE 4 ]

As shown in the above tables 1 to 3, the inventive examples satisfying the components and the production conditions proposed in the present invention have not only high strength and high toughness but also excellent resistance to ammonia Stress Corrosion Cracking (SCC), and a low yield ratio of 0.8 or less. Further, as a result of observing the microstructure of invention example a-1 with a microscope, the microstructure shown in fig. 1 (2) is a mixed microstructure including 60% by area or more of Acicular Ferrite (Acicular Ferrite), and the balance of at least one of Bainite (Bainite), Polygonal Ferrite (Polygonal Ferrite), and MA (martensite-austenite component).

In contrast, comparative examples A-2, A-4, A-6, B-2, B-4 and B-6, in which the manufacturing conditions did not satisfy the present invention although the components satisfied the present invention, could not ensure tensile strength and low temperature toughness due to too high fraction of polygonal ferrite or too coarse ferrite grain size.

In addition, the comparative examples A-3, A-5, A-7 or B-3, B-5, B-7 exceeded the upper limit of yield strength (440MPa) that could induce ammonia Stress Corrosion Cracking (SCC) due to too small grain size of the acicular ferrite, too high fraction of bainite, or no MA phase at all, and thus ammonia stress corrosion cracking occurred, and low yield ratio and low temperature toughness could not be ensured.

Further, although the manufacturing conditions satisfied the present invention, the comparative examples C-1 to F-1, in which the components did not satisfy the present invention, exceeded the upper limit of yield strength (440MPa) that may induce Ammonia Stress Corrosion Cracking (SCC) due to too high a fraction of bainite or too small a grain size of acicular ferrite or too high a fraction of MA phase, whereby ammonia stress corrosion cracking occurred, and low yield ratio and low temperature toughness could not be ensured.

The present invention has been described above with reference to the embodiments, but various modifications and changes can be made by those skilled in the art without departing from the technical idea and field of the present invention described in the claims.

Claims

1. A low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness, comprising the following components in% by weight:

carbon (C): 0.02% -0.10%, manganese (Mn): 0.5 to 2.0%, silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1% and aluminum (Al): 0.005-0.5%, niobium (Nb): 0.003% or less excluding 0%, phosphorus (P): 0.015% or less, sulfur (S): less than or equal to 0.015%, the balance being Fe and other unavoidable impurities,

the microstructure comprises 60% or more of acicular ferrite, the balance of at least one of bainite, polygonal ferrite and martensite-austenite components, and

wherein the martensite-austenite phase is 8% by area or less, and the martensite-austenite phase has a size, as measured by an equivalent circle diameter, of 5 μm or less, and

wherein the yield strength of the steel material is 440MPa or less.

2. The steel material with low yield ratio and high strength excellent in stress corrosion cracking resistance and low-temperature toughness according to claim 1, characterized in that:

the acicular ferrite has a size of 30 μm or less as measured by an equivalent circle diameter.

3. The steel material with low yield ratio and high strength excellent in stress corrosion cracking resistance and low-temperature toughness according to claim 1, characterized in that:

the bainite is less than or equal to 30 area percent.

4. The steel material with low yield ratio and high strength excellent in stress corrosion cracking resistance and low-temperature toughness according to claim 1, characterized in that:

the yield ratio of the steel is less than or equal to 0.85, and the tensile strength is greater than or equal to 490 MPa.

5. The steel material with low yield ratio and high strength excellent in stress corrosion cracking resistance and low-temperature toughness according to claim 1, characterized in that:

the steel has an impact transition temperature of-60 ℃ or lower.

6. A method for producing a low-yield-ratio high-strength steel material excellent in stress corrosion cracking resistance and low-temperature toughness, comprising the steps of:

heating a steel slab to 1000-1200 ℃, said steel slab comprising in weight percent: carbon (C): 0.02% -0.10%, manganese (Mn): 0.5 to 2.0%, silicon (Si): 0.05 to 0.5%, nickel (Ni): 0.05 to 1.0%, titanium (Ti): 0.005 to 0.1% and aluminum (Al): 0.005-0.5%, niobium (Nb): 0.003% or less excluding 0%, phosphorus (P): 0.015% or less, sulfur (S): less than or equal to 0.015%, the balance being Fe and other unavoidable impurities;

cooling to a temperature of 250 ℃ or less after the finish rolling, an

Wherein the step of cooling is performed by performing the first step of cooling to Bs-10 ℃ to Bs +10 ℃ at a center portion cooling rate of 15 ℃/s or higher, and then performing the second step of cooling to 250 ℃ or lower at a center portion cooling rate of 10 ℃/s to 50 ℃/s.

7. The method of producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 6, characterized in that:

cooling start temperature Ar₃+30℃～Ar₃。

8. The method of producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 6, characterized in that:

the rough rolling is carried out in a mode that the reduction rate of each pass of the last three passes is more than or equal to 10%.

9. The method of producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 6, characterized in that:

the final rolling is performed in such a manner that the reduction ratio per pass is 10% or more and the cumulative reduction ratio is 60% or more.