JP6691217B2 - Low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness and method of manufacturing the same - Google Patents

Description

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

  The steel material used for the liquefied gas storage tank varies depending on the type of liquefied gas, but since the liquefaction temperature of the gas is normal pressure and low temperature (-52 ° C in the case of LPG), it goes without saying that it is the base material. However, excellent low temperature toughness has also been required for welded parts.

  In addition, liquid ammonia (LAG) is known to cause stress corrosion cracking (SCC) of a steel material, and in IGC CODE (International Code for Construction of Ships of Cleaning Liquids, Liquid-Ammonia (LAC)). It regulates the operating conditions during manufacturing such as oxygen partial pressure and temperature, limits the Ni content of steel to 5% or less, and limits the actual yield strength to 440 MPa or less.

  Moreover, when manufacturing a gas tank (Gas Tank) by welding the steel material for gas tanks (Gas Tank), it is important to remove the stress of a welding part. As a method for removing stress in a welded portion, there is a PWHT (Post Welding Heat Treatment) method by heat treatment, and a mechanical stress relief (MSR: Mechanical Stress Relief) method for removing stress by applying hydrostatic pressure to the welded portion. . Of these, when the stress of the welded portion is removed by using the mechanical stress relief (MSR) method, the base material is also deformed by water pressure, so the yield ratio of the base material is limited to 0.8 or less. There is. This is because, when stress is removed by using the MSR method, when a deformation higher than the yield strength is applied to the base material portion by high-pressure water injection, if the ratio of the yield strength and the tensile strength is high, the yield occurs, That is, since the tensile strength may be reached and fracture may occur, it is limited so that the difference between the yield strength and the tensile strength becomes large.

  Particularly, in the case of a gas tank, since it is basically necessary to increase the size, it is difficult to remove stress by the PWHT method. Therefore, many shipyards have selected a mechanical stress relieving (MSR) method, and a steel material for manufacturing a gas tank (Gas Tank) is required to have a low yield ratio characteristic.

  As described above, in a tank in which LPG and LAG are mixed, achieving both low temperature toughness and lowering the yield ratio due to the upper limit regulation of the yield strength of liquid ammonia is a major issue.

  On the other hand, Patent Document 1 proposes a technique of adding 6.5% to 12.0% Ni in order to achieve excellent low temperature toughness. Further, Patent Document 2 proposes a technique in which a steel having a specific composition is subjected to quenching and tempering treatment to mix tempered martensite and bainite.

  However, in general, when a large amount of Ni is added, the austenite phase having a FCC lattice structure that is easily deformed is generated because the interatomic distance is narrow, and stress and a corrosive environment are repeatedly generated in the FCC lattice structure that is easily deformed. If it is added, it easily corrodes and cracks occur. Therefore, the above-mentioned invention has a problem that it is inferior in economic efficiency due to a high content of expensive Ni, and has a problem of inducing a decrease in stress corrosion cracking (SCC) resistance.

  Further, Patent Document 3 proposes a technique of softening only the surface layer of a steel sheet in order to realize a low yield ratio. However, although this technique can achieve low temperature toughness and low yield ratio, respectively, it has a problem that it is difficult to obtain both low temperature toughness and low yield ratio.

  On the other hand, as a method for improving the strength of steel, which is a further property required for steel, precipitation strengthening, solid solution strengthening, martensite strengthening, and the like can be mentioned. In these methods, strength is improved, but toughness is improved. There is a problem of deteriorating elongation.

  Further, when strengthening the strength by applying various manufacturing conditions to refine the crystal grains, not only high strength can be obtained, but also the toughness deterioration can be prevented by decreasing the impact toughness transition temperature. However, the increase in the yield strength due to the refinement of crystal grains exceeds the upper limit (440 MPa) of the yield strength at which ammonia stress corrosion (SCC) can occur, which makes it difficult to secure a low yield ratio.

  Therefore, there is a demand for the development of a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness and a manufacturing method thereof.

JP-A-63-290246 JP-A-58-153730 JP-A-4-17613

  The present invention has been made in view of the above conventional problems, and an object 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 manufacturing method thereof. It is in.

  A low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to one embodiment of the present invention made to achieve the above object is 0.02% to 0.02% by weight of carbon (C). 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%, aluminum (Al) 0.005% to 0.5%, niobium (Nb) 0.005% or less, phosphorus (P) 0.015% or less, The content of sulfur (S) is 0.015% or less, the rest is Fe and other unavoidable impurities, the fine structure is% by area, acicular ferrite (Accular Ferrite) is 60% or more, and the rest is bainite ( Bainite), polygonal ferrite (Polyg) nal Ferrite), and characterized by comprising the MA (Martensite-Austenite constituent) 1 or more of.

The method for producing a high-strength steel material having a low yield ratio and excellent in stress corrosion cracking resistance and low temperature toughness according to one embodiment of the present invention, which is made to achieve the above object, is 0.02% by weight of carbon (C). ~ 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%, aluminum (Al) 0.005% to 0.5%, niobium (Nb) 0.005% or less, and phosphorus (P) 0.015. % Or less and 0.015% or less of sulfur (S), and the rest of the slab consisting of Fe and other unavoidable impurities is heated to 1000 ° C. to 1200 ° C., and the heated slab is heated to 1100 ° C. to 900 ° C. After the rough rolling at a temperature and after the rough rolling, the temperature of the central portion is used as a reference. A step of finish rolling at _{Ar 3 + 100 ℃ ~Ar 3 +} 30 ℃ temperature after the finish rolling, and having the the steps of cooling to a temperature of 300 ° C. or less.

  It should be noted that the means for solving the above problem does not enumerate all the features of the present invention. The various features of the present invention and the advantages and effects thereof will 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, it is possible to provide a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness, and a manufacturing method thereof.

It is a phase transformation diagram of invention steel A by a cooling rate. It is the photograph (1- (1) of FIG. 1) which observed the microstructure of the 1 / 4t part of the steel plate of A-5 which is a comparative example. It is the photograph (1- (2) of FIG. 1) which observed the microstructure of the 1 / 4t part of the steel plate of A-1 which is an example of an invention with the optical microscope. It is the photograph (1- (3) of FIG. 1) which observed the microstructure of the 1 / 4t part of the steel plate of A-6 which is a comparative example.

  Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be variously modified and implemented, and the technical scope of the present invention is not limited to the embodiments described below. Also, the embodiments of the present invention are provided to more fully explain the present invention to those having ordinary skill in the art.

  The present inventors have recognized that it is difficult to simultaneously improve ammonia stress corrosion cracking resistance and low temperature toughness, and have conducted diligent research to solve them.

  As a result, by controlling the alloy composition and the microstructure, it was confirmed that it is possible to provide 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, and to complete the present invention. I arrived.

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

  The low yield ratio and high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to an embodiment of the present invention contains, by weight%, carbon (C) of 0.02% to 0.10% and manganese (Mn). 0.5% -2.0%, silicon (Si) 0.05% -0.5%, nickel (Ni) 0.05% -1.0%, titanium (Ti) 0.005%- 0.1%, aluminum (Al) 0.005% to 0.5%, niobium (Nb) 0.005% or less, phosphorus (P) 0.015% or less, and sulfur (S) 0.015. % Or less, the rest is Fe and other unavoidable impurities, the fine structure is% by area, acicular ferrite (Accular Ferrite) is 60% or more, and the rest is bainite (Bainite), polygonal ferrite (Polygonal ferrite). Ferrite), and And MA (Martensite-Austenite constituent).

  First, an alloy composition of a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to an embodiment of the present invention will be described in detail. Hereinafter, the content of each component is shown by weight%.

C (carbon): 0.02% to 0.10%
Since C is the most important element for ensuring the basic strength, it must be contained in the steel within an appropriate range. In order to obtain such an addition effect, C is added by 0.02% or more.

  When the content of C is less than 0.02%, the yield ratio is reduced together with the reduction in strength. On the other hand, if the C content exceeds 0.10%, a large amount of low-temperature transformation phase such as bainite is generated, which exceeds the upper limit of the yield strength that induces ammonia stress corrosion cracking (SCC). is there.

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

Si (silicon): 0.05% to 0.5%
Si has the effect of strengthening by solid solution strengthening, and is an element that is also useful as a deoxidizer in the steelmaking process.

  If the Si content is less than 0.05%, the deoxidizing effect and the strength improving effect are insufficient. On the other hand, if the Si content exceeds 0.5%, there is a problem that the low temperature toughness decreases and the weldability also deteriorates.

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

Mn (manganese): 0.5% to 2.0%
Manganese is an element that contributes to the grain refinement of ferrite and is useful for improving strength by solid solution strengthening.

  In order to obtain such an effect, it is necessary to add manganese at 0.5% or more. However, when the content exceeds 2.0%, the hardening ability excessively increases, the formation of upper bainite (Upper bainite) and martensite is promoted, and the impact toughness and ammonia stress corrosion cracking (SCC) resistance are increased. Remarkably, the toughness of the heat-affected zone of welding is also reduced.

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

Ni (nickel): 0.05% to 1.0%
Ni is an important element for facilitating cross slip of dislocations (Cross slip) at a low temperature, improving impact toughness, improving hardenability, and improving strength. To obtain such effects, 0.05% or more is added. However, when the content of Ni exceeds 1.0%, ammonia stress corrosion cracking (SCC) is caused, and the manufacturing cost is increased because the cost is higher than that of other curable elements.

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

Nb (niobium): 0.005% or less Nb is solid-soluted when reheated to a high temperature and is extremely finely precipitated in the form of NbC, which has the effect of suppressing recrystallization of austenite and refining the structure. Are known.

  Since the yield strength is excessively increased due to such refinement of the structure and there is a possibility that the yield strength exceeding the upper limit of the yield strength that induces ammonia stress corrosion cracking (SCC) may be exceeded, Nb is controlled to 0.005% or less. It is more preferably 0.003% or less.

Ti (titanium): 0.005% to 0.1%
Titanium forms oxides and nitrides in steel and suppresses the growth of crystal grains during reheating, which can significantly improve low temperature toughness, and is also effective in refining the microstructure of the weld. is there.

  To obtain such an effect, titanium needs to be added in an amount of 0.005% or more. However, if the content exceeds 0.1%, there is a problem that the low temperature toughness is reduced due to clogging of the continuous casting nozzle and crystallization of the central portion.

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

Al (aluminum): 0.005% to 0.5%
Aluminum is an element useful for deoxidizing molten steel, and for that purpose, 0.005% or more must be added. However, if the content exceeds 0.5%, nozzle clogging occurs during continuous casting. Therefore, the content of aluminum is set to 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 material and the welded portion and has a problem of embrittlement of steel, so it is necessary to actively reduce it. However, in order to reduce such phosphorus to the limit, the load on the steelmaking process becomes large, and when the phosphorus content is 0.015% or less, the above-mentioned problem does not become large, so the upper limit is set. It is limited to 0.015%, more preferably 0.010%.

S (sulfur): 0.015% or less Sulfur (S) is an element that causes red heat embrittlement and forms MnS and the like and significantly impairs impact toughness, so it is controlled as low as possible. Therefore, the content is limited to 0.015% or less, and more preferably 0.005%.

  The remaining component of the alloy composition of the low yield ratio high strength steel according to the present invention is iron (Fe). However, in the usual manufacturing process, unintended impurities are inevitably mixed from the raw materials or the surrounding environment, and thus cannot be excluded. Since these impurities are well known to those skilled in the art of ordinary manufacturing processes, the entire contents thereof are not particularly mentioned herein.

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

  The microstructure of the low-yield ratio high-strength steel material according to the present invention is, in area%, 60% or more of acicular ferrite (Acicular Ferrite), and the rest is bainite (Bainite), polygonal ferrite (Polygonal Ferrite), and MA. (Martensite-Austenite constituent).

  When the fraction of bainite increases and the acicular ferrite is less than 60%, the impact toughness deteriorates due to the increase of hard phase. Further, when the fraction of polygonal ferrite increases and the fraction of acicular ferrite becomes less than 60%, the strength deteriorates. Therefore, the area fraction of acicular ferrite is set to 60% or more.

  Further, when pearlite is contained, the tensile strength and the low temperature impact toughness deteriorate, so the microstructure of the low yield ratio high strength steel according to the present invention does not contain pearlite.

  At this time, the acicular ferrite has a size of 30 μm or less measured as an equivalent circle diameter. If the size exceeds 30 μm, the impact toughness deteriorates.

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

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

  Further, MA (also referred to as MA phase) is preferably 10 area% or less, and the size measured as a circle equivalent diameter is preferably 5 μm or less. MA (Martensite-Austenite constituent) is also called island martensite.

  If the MA fraction exceeds 10% by area or the equivalent circle diameter exceeds 5 μm, the toughness of the base material and the welded portion tends to significantly decrease, so the MA fraction and size are limited. There is a need.

  On the other hand, the low yield ratio high strength steel material according to the present invention satisfying the above conditions has a yield ratio (YS / TS) of 0.85 or less, preferably 0.8 or less. Further, the present steel material has an excellent tensile strength of 490 MPa or more, for example, about 510 MPa to 610 MPa.

  In addition, the upper limit of the yield strength of the steel material according to the present invention is 440 MPa or less, which does not exceed the upper limit of the yield strength that causes ammonia stress corrosion cracking (SCC), and therefore has excellent ammonia stress corrosion cracking (SCC) resistance. Have.

  Further, the impact transition temperature of the ¼t portion in the thickness direction of the steel material is −60 ° C. or lower, which is excellent low temperature toughness. Here, t means the thickness of the steel material.

  At this time, the steel material has a thickness of 6 mm or more, preferably 6 mm to 50 mm.

  As described above, the low yield ratio and high strength steel material according to the present invention can secure high strength, low yield ratio, excellent low temperature toughness, and ammonia stress corrosion cracking (SCC) resistance.

  Hereinafter, a method for producing a low yield ratio and high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to an embodiment of the present invention will be described in detail.

According to an embodiment of the present invention, a method for manufacturing a high yield steel material having a low yield ratio and excellent resistance to stress corrosion cracking and low temperature toughness includes a step of heating a slab having the above alloy composition to 1000 ° C to 1200 ° C, and Rough rolling the slab at a temperature of 1100 ° C. to 900 ° C., after rough rolling, finishing rolling at a temperature of Ar ₃ + 100 ° C. to Ar ₃ + 30 ° C. based on the temperature of the central portion, and after finishing rolling, Cooling to a temperature of 300 ° C. or less.

<Heating stage>
A slab having the above alloy composition is heated to 1000 ° C to 1200 ° C.

  The heating temperature of the slab is 1000 ° C or higher. This is because the Ti carbonitride formed during casting is solid-dissolved. On the other hand, if the heating temperature of the slab is too low, the deformation resistance during rolling is too high to increase the rolling reduction per pass in the subsequent rolling step, so the lower limit is limited to 1000 ° C. However, if heated to an excessively high temperature, the austenite becomes coarse and the toughness decreases, so the upper limit of the heating temperature is 1200 ° C.

<Rough rolling stage>
The slab heated as described above is roughly rolled at a temperature of 1100 ° C to 900 ° C.

  The rough rolling temperature is set to a temperature (Tnr) or higher at which recrystallization of austenite stops. By rolling, the casting structure such as dendrite formed during casting is destroyed, and the effect of reducing the size of austenite is also obtained. In order to obtain such an effect, the temperature of rough rolling is limited to 1100 ° C to 900 ° C.

  At this time, rough rolling is performed so that the rolling reduction per pass is 10% or more for the last three passes.

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

  In the rough rolling, the structure recrystallized by the initial rolling causes the growth of crystal grains due to the high temperature, but during the last three passes, the bar is air-cooled while waiting for rolling, so that the crystal grains grow. Slow down. As a result, the rolling reduction in the last three passes during rough rolling has the greatest effect on the grain size of the final microstructure.

  In addition, when the rolling reduction per pass of rough rolling becomes low, sufficient deformation is not transmitted to the central portion, so that the toughness decreases due to the coarsening of the central portion. Therefore, the rolling reduction per pass of the last three passes is limited to 10% or more.

  On the other hand, in order to refine the structure of the central portion, the total cumulative rolling reduction during rough rolling is set to 30% or more.

<Finishing rolling stage>
After the rough rolling, finish rolling is performed at a temperature of Ar ₃ + 100 ° C. to Ar ₃ + 30 ° C. based on the temperature of the central portion.

This is to obtain a finer microstructure, and when finish rolling is performed at a temperature of Ar ₃ (ferrite transformation start temperature) + 100 ° C. to Ar ₃ + 30 ° C., a large amount of deformation zone is generated inside austenite. By securing a large amount of ferrite nucleation sites, the effect of securing a fine structure up to the central portion of the steel material can be obtained.

When the finish rolling temperature is lowered to less than Ar ₃ + 30 ° C., the ferrite grain size becomes too fine and exceeds the upper limit (440 MPa) of the yield strength that causes ammonia stress corrosion cracking (SCC). Further, if the finish rolling is performed at a temperature higher than Ar ₃ + 100 ° C., it is not effective in reducing the grain size. Therefore, performing finish rolling temperature at _{Ar 3 + 100 ℃ ~Ar 3 +} 30 ℃, by performing the finish rolling under the above conditions, the microstructure of the steel sheet to be produced, the composite structure having the characteristics described above .

At this time, Ar ₃ is calculated by Ar ₃ = 910− (310 × C) − (80 × Mn) − (55 × Ni), and each element symbol represents the content of each element measured in weight%. Indicated, the unit of Ar ₃ is ° C.

  Further, in order to effectively generate a large amount of deformation zone inside the austenite, the cumulative reduction rate during finish rolling is maintained at 60% or more, and the reduction rate per pass is adjusted except for the final shape-adjusting rolling. Keep above 10%.

<Cooling stage>
After finish rolling, it is cooled to a temperature of 300 ° C. or lower.

Cooling begins to cool at a temperature of _{Ar 3} + 30 ℃ _~Ar 3 after finish rolling, 300 ° C. or less, for example, 100 ° C. to 300 ° C. of about cooling stop temperature (FCT: Finish Cooling Temperature) until cool.

  When the cooling stop temperature (FCT) exceeds 300 ° C., fine MA (MA phase) is decomposed by the tempering effect and it is difficult to realize a low yield ratio, so the cooling stop temperature is set to 300 ° C. or lower.

  At this time, in the cooling stage, the cooling rate of the central portion is 10 to 100 degrees Celsius or less after performing one-step cooling so that the cooling rate of the central portion is 15 ° C./s or more from Bs−10 ° C. to Bs + 10 ° C. Two-stage cooling is performed so that the temperature becomes 50 ° C./s.

The cooling start temperature is _{Ar 3} + 30 ℃ _~Ar 3.

1 at the stage cooling, begins to cool at a temperature of _{Ar 3} + 30 ℃ _~Ar 3 after finish rolling, the cooling rate of the center portion of the steel plate to Bs-10 ℃ ~Bs + 10 ℃ is 15 ° C. / s or higher, e.g., 30 ° C. / s Cool to the above temperature.

  When the cooling rate of the central portion of the steel sheet is lower than 15 ° C / s from Bs-10 ° C to Bs + 10 ° C in the one-step cooling, coarse polygonal ferrite (Polygonal Ferrite) is formed, and tensile strength and impact toughness decrease. This is because.

  At this time, Bs is calculated by Bs = 830- (270 * C)-(90 * Mn)-(37 * Ni), and each element symbol indicates the content of each element measured in weight% unit, and Bs The unit is ° C.

  In the two-step cooling, after the one-step cooling, the cooling rate is 10 ° C./s to 50 ° C./s until the cooling stop temperature is 300 ° C. or lower, for example, 100 ° C. to 300 ° C.

  In the two-stage cooling, when the cooling rate of the steel sheet exceeds 50 ° C./s, the fraction of bainite formed becomes 30 area% or more, as in the fine structure shown in 1- (1) of FIG. The upper limit of yield strength (440 MPa) that causes ammonia stress corrosion cracking (SCC) is exceeded, and elongation and impact toughness decrease due to excessive increase in strength.

  On the other hand, when the cooling rate of the steel sheet is less than 10 ° C./s in the two-stage cooling, coarse polygonal instead of fine acicular ferrite as in the fine structure shown in 1- (3) of FIG. Ferrite and pearlite are formed, the tensile strength becomes 490 MPa or less, and the Charpy transition temperature becomes −60 ° C. or more.

  By the above-mentioned manufacturing method, a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness is manufactured.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the following examples are merely for showing one example of the present invention and for more detailed description, and do not limit the technical scope of the present invention.

A steel slab having a thickness of 300 mm and having the composition shown in Table 1 below was reheated to a temperature of 1100 ° C. and then rough-rolled at a temperature of 1050 ° C. to manufacture a bar. The cumulative rolling reduction during rough rolling was applied in the same manner as 30%. In addition, the Ar ₃ and Bs temperatures according to the composition of each steel were calculated and listed in Table 1.

After the rough rolling, the finish rolling is performed so as to satisfy the difference between the finish rolling temperature and the Ar ₃ temperature shown in Table 2 below to obtain a steel sheet having the thickness shown in Table 2, and then various cooling is performed by multistage cooling. Cooling was performed at the cooling rate. At this time, the cooling stop temperature of the one-step cooling was the Bs temperature of each steel.

  The microstructure, yield strength, tensile strength, yield ratio, Charpy impact transition temperature, and ammonia stress corrosion cracking (SCC) test were conducted on the steel sheet manufactured as described above, and the results are shown in Table 3.

  The microstructure was obtained by taking a test piece from a 1/4 t portion of a steel plate, mirror-polishing it, corroding it with a Nital etchant, and then observing it using an optical microscope to determine the phase fraction by image interpretation. I asked.

  The fraction of the MA phase was obtained by collecting a test piece from a 1/4 t portion, mirror-polishing it, corroding it with a LePera corrosive liquid, and then observing it using an optical microscope. I asked.

  In the tensile test, a JIS No. 4 test piece was sampled in a direction perpendicular to the rolling direction from the 1/4 t portion of the steel sheet, and the tensile test was performed at room temperature to measure the yield strength, the tensile strength, and the yield ratio.

  The low temperature impact toughness was obtained by taking a test piece from a 1 / 4t part of a steel plate in a direction perpendicular to the rolling direction and producing a V-notch test piece, and then a Charpy at 20 ° C to -100 ° C at 20 ° C intervals. Impact tests were performed 3 times at each temperature. A regression equation of each temperature average value was derived, and the temperature at which it became 100 J was determined as the transition temperature.

  Further, the ammonia stress corrosion cracking (SCC) test was carried out by producing proof ring test pieces and using the test solutions and test conditions shown in Table 4. At this time, the applied stress was 80% of the actual yield stress, and those which did not break until 720 hours were evaluated as pass, and those which were broken before 720 hours passed were evaluated as fail.

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

  As shown in Tables 1 to 3, the invention examples satisfying the component composition and manufacturing conditions proposed in the present invention not only have characteristics of high strength and high toughness, but also have ammonia stress corrosion cracking (SCC) resistance. It was confirmed that the steel material has excellent yield property and a yield ratio of 0.8 or less and low yield ratio characteristics. Moreover, as a result of observing the microstructure of Inventive Example A-1 with a microscope, as shown in 1- (2) of FIG. 1, the acicular ferrite is 60% or more in area%, and the rest is , Bainite, Polygonal Ferrite, and MA (Martensite-Austenite constituent).

  On the other hand, in Comparative Examples A-2, A-4, A-6, B-2, B-4, and B-6 in which the component compositions satisfy the range of the present invention, but the production conditions do not satisfy the present invention. However, the fraction of polygonal ferrite was too high, or the crystal grain size of ferrite was excessively coarse, and it was impossible 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 crystal grain size of the acicular ferrite was too small, or the bainite fraction was too high. Since it is generated or MA (MA phase) is not produced at all, ammonia stress corrosion cracking occurs beyond the upper limit of yield strength (440 MPa) at which ammonia stress corrosion cracking (SCC) occurs. It was impossible to secure low temperature toughness.

  Further, in Comparative Examples C-1 to F-4 in which the production conditions satisfy the present invention, but the component composition does not satisfy the scope of the present invention, the bainite fraction is excessively high or the acicular ferrite crystals are formed. Either the grain size is too small or the MA fraction is too high, so ammonia stress corrosion cracking (SCC) occurs. Above the yield strength upper limit (440 MPa), ammonia stress corrosion cracking occurs, and the low yield ratio and It was impossible to secure low temperature toughness.

  Although the present invention has been described above with reference to the embodiments, those skilled in the art can make various modifications to the present invention without departing from the technical scope of the present invention.

Claims (9)

Low yield ratio high strength steel,
% By mass , carbon (C) 0.02% to 0.10%, manganese (Mn) 0.5% to 2.0%, silicon (Si) 0.05% to 0.5%, nickel (Ni) is 0.05% to 1.0%, titanium (Ti) is 0.005% to 0.1%, aluminum (Al) is 0.005% to 0.5%, and niobium (Nb) is 0. 0.005% or less, phosphorus (P) 0.015% or less, sulfur (S) 0.015% or less, and the balance Fe and other unavoidable impurities,
The fine structure has an area% of 60% or more of acicular ferrite (Accular Ferrite), and the rest is 1 of bainite (Bainite), polygonal ferrite (Polygonal Ferrite), and MA (Martensite-Austenite constituent). Ri Do not from seeds or more,
The acicular ferrite has a size measured as a circle equivalent diameter of 30 μm or less,
The bainite is 30 area% or less,
The MA is 10 area% or less, low yield ratio high-strength steel size measured as equivalent circle diameter and excellent stress corrosion cracking resistance and low-temperature toughness characterized der Rukoto below 5 [mu] m. The low yield ratio and high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 1, wherein the steel material has a yield ratio of 0.85 or less and a tensile strength of 490 MPa or more. The low-yield ratio high-strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 1, wherein the steel material has a yield strength of 440 MPa or less. The low yield ratio and high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 1, wherein the impact transition temperature of the steel material is -60 ° C or less. It is a manufacturing method of the steel material as described in any one of Claim 1 thru | or 4, Comprising:
In mass %, carbon (C) is 0.02% to 0.10%, and manganese (Mn) is 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%, aluminum (Al) 0.005%
.About.0.5%, niobium (Nb) 0.005% or less, phosphorus (P) 0.015% or less, sulfur (S) 0.015% or less, and the balance Fe and other unavoidable impurities. Heating the slab to 1000 ° C to 1200 ° C,
Roughly rolling the heated slab at a temperature of 1100 ° C to 900 ° C;
After the rough rolling, finish rolling at a temperature of Ar ₃ + 100 ° C. to Ar ₃ + 30 ° C. based on the temperature of the central portion,
And a step of cooling to a temperature of 300 ° C. or lower after the finish rolling, a method for producing a low yield ratio high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness. In the cooling step, the cooling rate of the central portion is 15 ° C / s from Bs-10 ° C to Bs + 10 ° C.
After performing one-step cooling as described above,
The stress corrosion cracking resistance and the low temperature toughness are excellent according to claim 5 , wherein the cooling is performed in two stages so that the cooling rate of the central portion is 10 ° C / s to 50 ° C / s up to 300 ° C or less. Low yield ratio High strength steel manufacturing method. Cooling start temperature of said step of cooling is characterized by a _{Ar 3} + 30 ℃ _~Ar 3
The method for producing a low yield ratio and high strength steel material excellent in stress corrosion cracking resistance and low temperature toughness according to claim 5 . The low-yield excellent in stress corrosion cracking resistance and low temperature toughness according to claim 5 , wherein the rough rolling is performed so that a rolling reduction per pass for the last three passes is 10% or more. Method for producing high strength steel. The said finish rolling is performed so that the rolling reduction per pass may be 10% or more, and the cumulative rolling reduction may be 60% or more, The stress corrosion cracking resistance and the low temperature toughness excellent in low of Claim 5 characterized by the above-mentioned. Yield ratio High strength steel manufacturing method.

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