JP5176847B2 - Low yield ratio low temperature steel and method for producing the same - Google Patents

Description

  The present invention relates to a low yield ratio low temperature steel suitable for a steel material for a multipurpose tank loaded with a plurality of types of liquefied gases such as liquid ammonia and LPG, and a method for producing the same.

  In general, steel materials used in tanks require low temperature toughness of a base material and a weld heat affected zone (hereinafter referred to as “welded zone”) as a basic characteristic because the stored liquefied gas is low temperature.

  In addition, recent tanks are sometimes used as multipurpose tanks in which multiple types of liquefied gas are separately loaded in the same tank. In this case, for example, LPG is loaded or liquid ammonia is loaded after LPG is taken out. Is done. For this reason, in addition to low-temperature toughness, steel materials used for multipurpose tanks are required to have characteristics that can prevent stress corrosion cracking caused by ammonia. For stress corrosion cracking, the IGC Code, which is an international regulation on the structure and equipment of ships that carry liquefied gas, is a steel material for tanks that limits the Ni content to 5% or less, and the actual yield strength is 440 MPa. It is stipulated to keep it below.

  In addition, multipurpose tanks are required to have high tensile strength in order to increase capacity and to be mounted on ships. Ultimately, not only low-temperature toughness but also the upper limit of yield strength and high tensile strength are required for the steel materials. Along with this, a lower yield ratio is required. In response to this demand, various technologies have been proposed so far.

  Patent Document 1 discloses a process of decarburizing the surface so that the C content within 0.3 mm from the surface of the steel sheet material is 50% or less of the amount of the base material C, and after heating the surface decarburized steel sheet to the quenching temperature. And the manufacturing method of the steel plate which has the process of cooling so that a cooling rate may be 150 degrees C / sec or less in the temperature range of 800-500 degreeC is proposed. In the method proposed in this document, it is said that the surface hardness of the base material and the welded portion can be Hv 190 or less, and stress corrosion cracking due to ammonia can be prevented, but the process until obtaining the steel material is complicated. Furthermore, it cannot be said that low-temperature toughness can be secured in the base material and the welded portion.

  In Patent Documents 2 and 3, a steel material having a defined chemical composition is heated, hot rolled, and then subjected to accelerated cooling to obtain a multiphase structure, thereby obtaining a low yield ratio and excellent low temperature toughness. Methods for producing steel have been proposed. In the methods proposed in References 2 and 3, the area ratio of hard structures such as bainite and martensite becomes too large, and the stress-strain curve when the tensile test is performed becomes a round type with an unclear yield point. For this reason, the variation in yield strength increases, and it is difficult to stably obtain a low yield ratio. Furthermore, the flatness of the obtained steel material deteriorates due to the influence of residual stress caused by accelerated cooling, and additional man-hours for correcting the flatness are required, and high productivity cannot be expected.

JP 58-67830 A JP-A-11-293380 JP-A-2004-300493

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a low yield ratio low temperature steel that is excellent in mechanical properties of a base material and a welded portion and is suitable as a steel material for a multipurpose tank. . Moreover, the objective of this invention is providing the manufacturing method which can manufacture the steel for the low yield ratio low temperature with high productivity.

  In order to achieve the above object, the present inventors have a tensile strength (TS) of 490 to 610 MPa, a yield strength (YS) of 360 to 440 MPa, and a yield ratio ([YS / TS] × 100%) of 80%. Hereinafter, a steel material having mechanical properties such that the fracture surface transition temperature (vTrs) is −60 ° C. or less with respect to the low temperature toughness of the base material, and the absorbed energy (vE-55) at −55 ° C. is 50 J or more with respect to the low temperature toughness of the weld. With the goal of realizing this, various tests were conducted and extensive studies were conducted. As a result, the inventors have found that a steel material satisfying any mechanical characteristics can be obtained if the chemical composition and structure of the steel material and the hot rolling conditions are properly defined, and the present invention has been completed.

  The gist of the present invention is a low yield ratio low temperature steel shown in the following (A) and a method of manufacturing a low yield ratio low temperature steel shown in the following (B).

(A) In mass%, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 1.0 to 2.0%, P: 0.020% or less, S: 0.010% or less, Nb: 0.003% or less, Ti: 0.005 to 0.025%, sol. Al: 0.090% or less, and N: 0.001-0.010%, further Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.20% or less, and Mo: Contains at least two of 0.15% or less, the balance is Fe and impurities, and the SP1 calculated from the following formula (1) is 0.06 to 0.12% for low yield ratio for low temperature Steel, wherein the thickness is t / 4 from the surface, and the structure is a multiphase structure of a ferrite structure and a hard structure, and the area ratio of the ferrite structure is 80% or more, Among them, 15 or more hard structures satisfying the relationship of the following expression (2) are included in a 100 μm square region, the circle-equivalent average particle diameter of the hard structure is 2 to 5 μm, and the circle-equivalent average particle diameter of the ferrite structure is 7.5-20 μm, the ferrite set Low yield ratio steel for low temperature service which aspect ratio is equal to or more than 2.0.
SP1 = Cr / 5 + Mo / 3 + (Cu + Ni) / 15 (1)
However, the element symbol in the formula (1) indicates the content.
Hvh / Hvm ≧ 3.0 (2)
However, Hvh in the formula (2) indicates the Vickers hardness of the hard structure, and Hvm indicates the Vickers hardness of the ferrite structure.

  Here, the “position of t / 4 from the surface” includes not only the position of t (steel plate thickness) / 4 from the surface of the steel material but also the position in the vicinity thereof, specifically, 0 from the surface. Means a position within the range of .22t to 0.28t.

  The “ferrite structure” means a soft granular ferrite structure, and bainitic ferrite and ferrite structure constituting pearlite are excluded. The “hard structure” means a hard bainite and martensite structure.

  The “equivalent circle average particle diameter” means a radius when an average cross-sectional area of a crystal grain is obtained from a crystal grain size measured in accordance with JIS G 0552 for an arbitrary field of view and is replaced with a circle.

  In the steel for low yield ratio low temperature of the above (A), when two or more of Cu, Ni, Cr and Mo are contained, the Cu content is 0.05% or more and the Ni content is 0. 0.05% or more, Cr content is preferably 0.05% or more, and Mo content is preferably 0.03% or more.

(B)% by mass, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 1.0 to 2.0%, P: 0.020% or less, S: 0.010% or less, Nb: 0.003% or less, Ti: 0.005 to 0.025%, sol. Al: 0.090% or less, and N: 0.001-0.010%, further Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.20% or less, and Mo: A steel material containing two or more of 0.15% or less, the balance being Fe and impurities, and SP1 obtained from the above formula (1) being 0.06 to 0.12%, 1000 After heating to ℃ to 1200 ℃, rolling at a temperature of Ac ₃ point or higher at a reduction rate of 30% or higher, and further, at a temperature of (Ar ₃ point −40 ° C.) or higher and lower than Ac ₃ point, the rolling reduction rate is 30% or higher. Of low yield ratio low temperature steel, characterized in that rolling is finished at a temperature of (Ar ₃ point −40 ° C.) or more and less than 800 ° C. and cooled to room temperature at a cooling rate of 5 ° C./s or less. Production method.

  The steel for low yield ratio low temperature of the present invention has excellent low temperature toughness of the base metal and the welded portion at a low yield ratio by appropriately specifying the chemical composition and the state of the structure, and is suitable as a steel material for a multipurpose tank. Can be used. Moreover, in the manufacturing method of the low yield ratio low temperature steel according to the present invention, the water cooling treatment after rolling, which is a cause of deterioration in flatness, is unnecessary, so the steel material of the low yield ratio low temperature steel excellent in low temperature toughness is high. Can be manufactured with productivity.

  The reason why the low yield ratio low temperature steel of the present invention and the manufacturing method thereof are specified as shown in the above (A) and (B) will be described below. In the following description, “%” below means “mass%” unless otherwise specified.

1. About chemical composition C: 0.02-0.08%
C is an element that is extremely effective in increasing the strength of the steel material. However, if the content is less than 0.02%, not only the target tensile strength (490 to 610 MPa) can be secured, but also bainite and martensite necessary for achieving a low yield ratio (80% or less). The formation of a hardened structure such as is insufficient. On the other hand, if the C content exceeds 0.08%, the low-temperature toughness of the base material and the welded portion is lowered, and the weldability is also deteriorated. Therefore, the C content is 0.02 to 0.08%.

Si: 0.1 to 0.5%
Si is an element that is effective as a deoxidizer together with Al, and is extremely effective in increasing the strength of steel. However, if the content is less than 0.1%, these effects cannot be obtained. On the other hand, when the Si content exceeds 0.5%, the low temperature toughness of the welded portion is lowered. Therefore, the Si content is set to 0.1% to 0.5%.

Mn: 1.0-2.0%
Mn is an important element for increasing the hardenability and ensuring the strength and toughness of the steel material. However, if the content is less than 1.0%, the low temperature toughness of the base material cannot be improved. On the other hand, the content exceeding 2.0% not only saturates these effects, but also becomes a main factor of center segregation during the production of a slab (steel material) by continuous casting. Therefore, the Mn content is set to 1.0 to 2.0%.

P: 0.020% or less P is an element present as an impurity in steel. If the P content is reduced, the mechanical properties of the base metal and the low temperature toughness of the weld can be improved, and the center segregation can also be reduced. Therefore, the content should be as low as possible. For this reason, the P content is limited to 0.020% or less.

S: 0.010% or less S is also an element present as an impurity in steel. S promotes center segregation or produces stretched MnS and degrades the mechanical properties of the base metal and the welded portion, so its content should be as low as possible. For this reason, the S content is limited to 0.010% or less.

Nb: 0.003% or less Nb is an element that makes crystal grains fine even when added in a small amount, and is effective in securing a low yield ratio, but makes it difficult to generate ferrite having a moderately large grain size. For this reason, Nb is not positively added, and even when it is contained as an impurity, the content is limited to 0.003% or less.

Ti: 0.005-0.025%
Ti is an element that combines with N and precipitates finely in the slab as TiN, and suppresses the coarsening of austenite grains during heating, and is effective in refining the rolling structure. Moreover, when TiN exists in steel, the coarsening of the structure of a welding part is suppressed at the time of welding. That is, Ti is an element necessary for improving the toughness of the base material and the welded portion. If the Ti content is less than 0.005%, the effect is insufficient. On the other hand, if the Ti content exceeds 0.025%, the low temperature toughness of the welded portion deteriorates. Therefore, the Ti content is set to 0.005 to 0.025%.

sol. Al: 0.090% or less sol. Al is an element that is extremely effective in reducing dissolved oxygen in steel, and is contained in steel as a deoxidizer. Also, sol. Al also has the effect of fixing free N in steel as AlN and making it harmless. However, sol. Even if the content of Al is excessively increased, these effects are saturated. Therefore, sol. The Al content is 0.090% or less. Note that sol. When Al is contained by 0.005% or more, the above effect becomes remarkable. From the viewpoint of the cleanliness of steel, sol. The Al content is preferably 0.060% or less.

N: 0.001 to 0.010%
When N combines with Nb to form carbonitride, it increases the strength of the steel, and when combined with Ti to form TiN, N has the effect of refining the rolled structure. In order to obtain these effects, the N content needs to be 0.001% or more. However, the excessive content of N deteriorates the low temperature toughness of the weld. Therefore, the N content is set to 0.001 to 0.010%.

  In the steel for low yield ratio low temperature of the present invention, in order to reduce the yield ratio while ensuring the strength and low temperature toughness of the steel material, Cu: 0.50% or less, Ni: 0.50% or less, Cr : Two or more of 0.20% or less and Mo: 0.15% or less are contained so as to satisfy SP1: 0.06 to 0.12% given by the formula (1) described later.

Cu: 0.50% or less Cu is an element effective for improving the strength and low-temperature toughness of steel materials. However, if Cu is contained in an amount exceeding 0.50%, cracks occur during hot rolling, making manufacture difficult. Therefore, when it contains Cu, the content shall be 0.50% or less. In particular, it is desirable to contain 0.05% or more of Cu in order to effectively exhibit the effects due to the inclusion of Cu.

Ni: 0.50% or less Ni is an element effective for improving the strength and toughness of the steel material without adversely affecting the weldability and the toughness of the welded portion. However, excessive content of Ni is not only uneconomical but also undesirable for weldability. Moreover, when Ni is contained excessively, there is a risk of inducing stress corrosion cracking due to liquid ammonia. Therefore, when Ni is contained, the content is made 0.50% or less. In particular, it is desirable to contain 0.05% or more of Ni in order to effectively exhibit the effect of containing Ni.

Cr: 0.20% or less Cr is also an effective element for improving the strength and low temperature toughness of steel materials. However, when Cr is contained exceeding 0.20%, the low temperature toughness of the base material and the welded portion is deteriorated. Therefore, when Cr is contained, the content is made 0.20% or less. In particular, in order to effectively exhibit the effect of the Cr content, it is desirable to contain 0.05% or more of Cr.

Mo: 0.15% or less Mo is also an effective element for improving the strength and low-temperature toughness of steel materials. However, if the Mo content exceeds 0.15%, the low temperature toughness of the base metal and the welded portion deteriorates. Therefore, when Mo is contained, the content is made 0.15% or less. In particular, it is desirable to contain 0.03% or more of Mo in order to effectively exhibit the effect of containing Mo.

SP1: 0.06-0.12%
SP1 is a value obtained from the following formula (1), and the element symbol in the formula represents the content.
SP1 = Cr / 5 + Mo / 3 + (Cu + Ni) / 15 (1)

  To obtain a low yield ratio, the steel has a softer ferrite structure (hereinafter also referred to as “ferrite phase”) and a hard structure such as harder bainite and martensite (hereinafter also referred to as “hard phase”). It is effective to have a multiphase structure composed of Here, as the C content is increased, the hardness of the hard phase is increased and the tensile strength is increased, so that a low yield ratio is easily obtained, but the low temperature toughness is remarkably lowered. On the other hand, if the C content is reduced, soft ferrite increases, yield strength decreases, and tensile strength decreases, resulting in an increase in yield ratio. Therefore, as a method for increasing the tensile strength and reducing the low temperature toughness while reducing the C content, it is conceivable to contain Nb, V, etc. in the steel. In order to promote finer grain, the yield strength increases, resulting in an increase in yield ratio.

  On the other hand, when Cr, Mo, Cu, or Ni is contained in the steel, the concentration of C is promoted to the austenite phase delayed in transformation after rolling, and the hard phase can be made harder. For this reason, the present inventors have repeated research on the premise that the C content is relatively reduced, and can maintain the hardness of the hard phase even if the C content is reduced, and the ferrite grains are appropriately sized. The balance of the content of Cr, Mo, Cu, and Ni that can maintain the low-temperature toughness and further ensure low temperature toughness was found. In the present invention, the balance relationship between these contents is defined by equation (1), and the range of SP1 obtained from equation (1) is defined.

  When SP1 is less than 0.06%, the hardness of the hard phase cannot be maintained and the tensile strength is lowered, so that a low yield ratio cannot be obtained. On the other hand, if SP1 exceeds 0.12%, the hardenability becomes excessive and the low temperature toughness deteriorates. Therefore, SP1 is set to 0.06 to 0.12%.

2. About Microstructure The low yield ratio low temperature steel according to the present invention is a multiphase structure of a ferrite phase and a hard phase at a position t / 4 from the surface where the thickness of the steel material is t, and the ferrite phase occupies it. It is necessary that the area ratio is 80% or more. Here, “position at t / 4 from the surface” means a position at t / 4 from the surface of the steel material and a position in the vicinity thereof. Specifically, the observation of the structure is 0.22 t to 0. 0 from the surface. It can be performed at a position within the range of 28t.

  In a steel material having a dual phase structure of a ferrite phase and a hard phase, the soft ferrite phase yields before the hard phase during a tensile test, so the state of the ferrite phase has a great influence on the yield strength. In particular, the influence of the ferrite phase area ratio on the yield strength is extremely large. That is, when the area ratio of the ferrite phase is less than 80%, the target that the yield strength is 440 MPa or less is not satisfied. Therefore, the area ratio of the ferrite phase is defined as 80% or more. However, if the area ratio of the ferrite phase is too large, the hard phase is remarkably reduced and does not perform its function. Therefore, the area ratio of the ferrite phase is preferably 95% or less.

In the low yield ratio low temperature steel of the present invention, in addition to the above-mentioned definition of the ferrite area ratio, a hard phase satisfying the relationship of the following formula (2) at a position of t / 4 from the same surface as described above is a 100 μm square. It is necessary to include 15 or more in the visual field region.
Hvh / Hvm ≧ 3.0 (2)
(2) In formula, Hvh shows the Vickers hardness measured with the hard structure made into object, and Hvm shows the average value of the Vickers hardness measured several places with the ferrite structure.

  A low yield ratio can be achieved with a multiphase structure having a harder hard phase and a soft ferrite phase. For this reason, the present inventors have further researched, and at a position of t / 4 from the surface, a harder hard phase among the hard phases, that is, a hard phase satisfying the relationship of the formula (2) is in a region of 100 μm square. When it was less than 15, it was found that the tensile strength did not reach the target of 490 MPa or more. Therefore, it is defined that 15 or more hard phases satisfying the relationship of the above expression (2) are included in a 100 μm square region.

  Further, in the low yield ratio low temperature steel of the present invention, the equivalent circle average particle size of the hard phase satisfying the relationship of the above formula (2) needs to be 2 to 5 μm. When the equivalent circle average particle diameter of the hard phase is less than 2 μm, there is no effect of substantially increasing the tensile strength, and a low yield ratio cannot be obtained. On the other hand, when the equivalent-circle average particle diameter of the hard phase exceeds 5 μm, the hard phase becomes a starting point of fracture and the low temperature toughness deteriorates. Therefore, the circle-equivalent average particle diameter of the hard phase is defined as 2 to 5 μm.

  Further, the low yield ratio low temperature steel of the present invention has a circle-equivalent mean grain size of the ferrite structure of 7.5 to 20 μm at the position t / 4 from the same surface as described above, and an aspect ratio of 2.0 or less. It is necessary to be.

  As described above, the influence of the state of the ferrite structure on the yield strength is large, and the larger the equivalent circle average grain size of the ferrite structure is, the more effective the reduction of the yield strength is. That is, when the equivalent circle average particle diameter of the ferrite structure is less than 7.5 μm, it is difficult to achieve the target of yield strength of 440 MPa or less. On the other hand, in order to improve low temperature toughness, it is effective that the equivalent circular average grain size of the ferrite structure is small. That is, when the circle-equivalent average particle diameter of the ferrite structure exceeds 20 μm, it is difficult to achieve the target that the fracture surface transition temperature (vTrs) is −60 ° C. or less with respect to the low temperature toughness of the base material. From these, the circle-equivalent average particle diameter of the ferrite structure is defined as 7.5 to 20 μm.

  Furthermore, when the ferrite structure is work hardened due to the reduction after the ferrite transformation, the yield strength is remarkably increased and it is difficult to achieve a low yield ratio. At this time, the ferrite has a flat structure with a large aspect ratio. When the aspect ratio exceeds 2.0, it can be determined that the ferrite structure is work-hardened. Therefore, in the low yield ratio low temperature steel of the present invention, the aspect ratio of the ferrite structure is defined as 2.0 or less.

  In the present invention, when the state of the structure is defined as described above, observation at a position of t / 4 from the surface of the steel material is employed, but this is because the structure of the steel material varies depending on the position in the plate thickness direction. This is because the position is appropriate as a position representing the structural state of the steel material. Therefore, the position of t / 4 may be a position from the front surface side or a position from the back surface side.

3. In the method for producing low yield ratio steel for low temperature service of the present invention a method for manufacturing a steel material having the above chemical composition, after heating to 1000 ° C. to 1200 ° C., the rolling reduction in Ac ₃ point or more temperature 30% Rolled as above, further rolled at a temperature of (Ar ₃ point −40 ° C.) or higher and lower than Ac ₃ point at a reduction ratio of 30% or higher, and rolled at a temperature of (Ar ₃ point −40 ° C.) or higher and lower than 800 ° C. And cooling to room temperature at a cooling rate of 5 ° C./s or less. The rolling reduction is calculated using the rolling start thickness in the target temperature range as the denominator. For example, when rolling from 250 mm to 100 mm in a temperature range of Ac ₃ points or higher and then rolling from 100 mm to 10 mm in a temperature range of (Ar ₃ points −40 ° C.) or higher and lower than Ac ₃ points, Ac ₃ The reduction rate in the temperature range above the point is (250-100) / 250 × 100 = 60%, (Ar ₃ point−40 ° C.) or more, and the reduction rate in the temperature range below the Ac ₃ point is (100-10). / 100 × 100 = 90%.

  The heating temperature of the steel needs to be 1200 ° C. or less in order to suppress coarsening of the austenite crystal grains during heating. However, when the heating temperature of the steel is less than 1000 ° C., since the austenite crystal grains before rolling become fine, the ferrite grains obtained after rolling become extremely fine. Refinement of ferrite grains improves low-temperature toughness, but yield strength increases, making it difficult to achieve a low yield ratio. Therefore, the heating temperature of steel shall be 1000-1200 degreeC.

In the hot rolling, the rolling is performed at a temperature of Ac ₃ point or higher and the reduction ratio is 30% or higher because the austenite is sufficiently recrystallized to be finely divided and the finally obtained low temperature toughness is improved. . Preferably, rolling is performed at a temperature of Ac ₃ point or higher and a reduction rate of 60% or higher.

In rolling with less than Ac ₃ points, when sufficient strain is applied to unrecrystallized austenite, ferrite precipitation is promoted, and carbon in the ferrite is discharged to untransformed austenite. Thereby, a hard phase can be made harder and a low yield ratio is obtained. Therefore, rolling is performed at a temperature lower than Ac ₃ point and a reduction rate of 30% or more. However, when the final rolling temperature is 800 ° C. or higher, the carbon concentration to untransformed austenite is insufficient, and the target low yield ratio cannot be obtained. Therefore, the final rolling temperature is set to less than 800 ° C.

Furthermore, if the final rolling temperature is lowered, the ferrite grains are refined and the low temperature toughness is improved. However, when the final rolling temperature is less than (Ar ₃ point-40 ° C), the effect of work hardening of the ferrite grains The yield strength increases, and a low yield ratio cannot be obtained. Therefore, the final rolling temperature is set to (Ar ₃ point−40 ° C.) or higher.

  When accelerated cooling is used for cooling after rolling, the ferrite structure decreases, the area ratio of the hardened structure including bainite increases, the strength becomes too high, and the stress-strain curve during the tensile test becomes round. Since it becomes a mold, the desired mechanical properties cannot be obtained stably. Further, when accelerated cooling is used, the flatness tends to deteriorate due to the influence of residual stress, and a man-hour for correcting the flatness is required separately. For this reason, the cooling after rolling is performed at a cooling rate of 5 ° C./s or less. Air cooling is sufficient for cooling at a cooling rate of 5 ° C./s or less. Apart from this, heat treatments that change the structure should be avoided.

  As described above, the low yield ratio low temperature steel of the present invention has a low yield ratio and excellent low temperature toughness of the base metal and the welded portion by appropriately specifying the chemical composition and the state of the structure. It is suitable for these steel materials. Further, in the method for producing a low yield ratio low temperature steel according to the present invention, the water cooling treatment after rolling, which is a factor for deteriorating the flatness, is unnecessary, so the steel material of the low yield ratio low temperature steel excellent in low temperature toughness is high. Can be manufactured with productivity.

  In order to confirm the effects of the low yield ratio low temperature steel of the present invention and the method for producing the same, the following tests were conducted to evaluate each mechanical property. Steel having the chemical composition shown in Table 1 was melted in a converter, and a slab having a thickness of 250 mm was obtained using a continuous casting machine.

  The obtained slab was hot-rolled under the heating, rolling and cooling conditions shown in Table 2 to produce a steel plate having a plate thickness of 10 to 40 mm.

  For each of the obtained steel plates, various structural states and mechanical properties were investigated by the following methods. The results are shown in Table 3.

<Microstructure>
A cross section perpendicular to the rolling direction was cut out from each of the above steel plates to obtain a test piece. In the cross section in the plate thickness direction of each test piece, the structure is observed with a scanning electron microscope at a position of t / 4 part (t is the plate thickness) from the surface, the structure observation diagram is subjected to image analysis, and the ferrite circle equivalent average The particle size and the area ratio of the ferrite phase were determined.

  Furthermore, the Vickers hardness of the ferrite phase and the hard phase in the field of view of 100 μm square at the same position was measured according to JIS Z 2244. The Vickers hardness (Hvh) individually measured for the hard phase is divided by the average Vickers hardness (Hvm) of 10 measurements of the ferrite phase to calculate “Hvh / Hvm”, and the relationship of the above equation (2) The number of hard phases satisfying the conditions was investigated. Here, for the hard phase satisfying the relationship of the expression (2), the particle diameter was measured by image analysis of the structure observation chart by a scanning electron microscope, and the equivalent circle average particle diameter was obtained.

<Characteristics of base material>
The 14B test piece prescribed | regulated to JISZ2201 was extract | collected from the direction perpendicular | vertical to the rolling direction of each said steel plate, and the tension test was implemented. Yield strength (YS) is 360 to 440 MPa, tensile strength (TS) is 490 to 610 MPa, and yield ratio (YR: [YS / TS] × 100%) is 80% or less, and evaluation is performed. It was.

  In addition, in order to evaluate the low temperature toughness of the base metal part, a V-notch test piece specified in JIS Z 2202 was sampled from the position of t / 4 in a direction perpendicular to the rolling direction of each steel plate, and a Charpy test was performed. Carried out. The fracture surface transition temperature (vTrs) was calculated from the test results. The fracture surface transition temperature was evaluated in a favorable range of −60 ° C. or lower.

<Low temperature toughness of welds>
A test piece having a length of 600 mm and a width of 300 mm is cut out from each of the above steel plates, and a steel plate having a thickness of less than 25 mm is processed into an I-type groove, and a steel plate having a thickness of 25 mm or more is processed into an X-type groove. Each test piece was subjected to submerged arc welding with a heat input of about 35 kJ / mm to produce a welded joint. Samples were taken from each welded joint so that the end of the impact test piece was 1 mm from the surface of the steel sheet so that the notch position coincided with the fusion line, Charpy test was conducted, and the absorbed energy at -55 ° C (vE-55) Was measured. The absorption energy at −55 ° C. was evaluated in a favorable range of 50 J or more.

  As shown in Table 3, the steels of Test Nos. 1 to 10, which are examples of the present invention, satisfy all of the various conditions defined in the present invention, and thus have achieved the target mechanical characteristics. It had sufficient characteristics as a steel material for multipurpose tanks.

  On the other hand, in the test number 11 of the comparative example, since the C content is too much, the strength characteristics are satisfied, but the area ratio of the ferrite phase is lowered, and the particle size of the hard phase satisfying the relationship of the above formula (2) is further reduced. Since it was too large, the low temperature toughness deteriorated. The test number 14 of the comparative example also deteriorated the low-temperature toughness, which is because SP1 exceeded the range specified in the present invention due to excessive Mo content.

  In the test numbers 12 and 13 of the comparative examples, the strength characteristics did not satisfy the target. That is, in test number 12, since Nb was added, the ferrite grains were excessively refined, and the yield strength (YS) and the yield ratio (YR) exceeded the upper limit. In Test No. 13, SP1 was below the specified range of the present invention, so that a hard phase satisfying the relationship of the above formula (2) was not sufficiently obtained, and the yield strength and tensile strength did not reach the targets.

The steels having test numbers 15 to 18 of the comparative examples are produced under manufacturing conditions that are within the range of the chemical composition defined in the present invention but are outside the range defined in the present invention. In Test No. 15, the rolling end temperature was lower than Ar ₃ temperature −40 ° C., and the ferrite phase was work-hardened, so the aspect ratio of the ferrite phase exceeded the specified range of the present invention, the yield strength increased, and the low yield ratio was It was not obtained.

In Test Nos. 16 and 17, since the reduction rate at a temperature of Ar ₃ or lower was insufficient, the concentration of C into the hard phase was insufficient, and the tensile strength did not reach the target. In particular, in the test number 16, since the rolling end temperature was too high, the ferrite grains were also coarse. In test number 18, since the water cooling treatment was performed by cooling after rolling, the area ratio of the ferrite phase decreased and the yield strength increased.

  The low yield ratio low temperature steel of the present invention has a low yield ratio and is excellent in the low temperature toughness of the base metal and the welded portion, and is suitable as a steel material for a multipurpose tank. In addition, according to the method for producing low-yield ratio low-temperature steel of the present invention, low yield ratio low-temperature steel with excellent low-temperature toughness can be produced with high productivity without requiring additional steps such as correction of flatness. Can be manufactured.

Claims (3)

In mass%, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 1.0 to 2.0%, P: 0.020% or less, S: 0.010 % Or less, Nb: 0.003% or less, Ti: 0.005 to 0.025%, sol. Al: 0.090% or less, and N: 0.001-0.010%, further Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.20% or less, and Mo: Contains at least two of 0.15% or less, the balance is Fe and impurities, and the SP1 calculated from the following formula (1) is 0.06 to 0.12% for low yield ratio for low temperature Steel,
At a position t / 4 from the surface where the plate thickness is t, the structure is a multiphase structure of a ferrite structure and a hard structure, the area ratio of the ferrite structure is 80% or more, and the following (2 ) Containing 15 or more hard structures satisfying the relationship of the formula in a 100 μm square area, the hard structure has an equivalent circle average particle diameter of 2 to 5 μm, and the ferrite structure has an equivalent circle average particle diameter of 7.5 to 20 μm. A low yield ratio steel for low temperature, wherein the ferrite structure has an aspect ratio of 2.0 or less.
SP1 = Cr / 5 + Mo / 3 + (Cu + Ni) / 15 (1)
However, the element symbol in the formula (1) indicates the content.
Hvh / Hvm ≧ 3.0 (2)
However, Hvh in the formula (2) indicates the Vickers hardness of the hard structure, and Hvm indicates the Vickers hardness of the ferrite structure. Cu, Ni, comprise two or more of Cr and Mo, and the content of Cu is 0.05% or more and the content of Ni is 0.05% or more, the content of Cr 0. The low yield ratio low temperature steel according to claim 1, characterized in that it is at least 05% and the Mo content is at least 0.03%.
In mass%, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 1.0 to 2.0%, P: 0.020% or less, S: 0.010 % Or less, Nb: 0.003% or less, Ti: 0.005 to 0.025%, sol. Al: 0.090% or less, and N: 0.001-0.010%, further Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.20% or less, and Mo: a steel material containing two or more of 0.15% or less, the balance being Fe and impurities, and SP1 calculated from the following formula (1) being 0.06 to 0.12%,
After heating to 1000 ° C. to 1200 ° C., rolling is performed at a temperature of Ac ₃ point or higher at a reduction rate of 30% or higher, and (Ar ₃ point −40 ° C.) or higher and at a temperature lower than Ac ₃ point is 30%. Rolling as above, rolling is finished at a temperature of (Ar ₃ point−40 ° C.) or more and less than 800 ° C., and cooled to room temperature at a cooling rate of 5 ° C./s or less. Manufacturing method.
SP1 = Cr / 5 + Mo / 3 + (Cu + Ni) / 15 (1)
However, the element symbol in the formula (1) indicates the content.