JP4872917B2 - Low temperature steel and its manufacturing method - Google Patents

Description

  The present invention relates to a Ni-containing steel material for low temperature, particularly a Ni-containing steel material suitable as a structural material for a low temperature storage tank such as LNG, and a method for producing the same.

  The steel for low temperature for producing a storage tank of a low temperature substance such as LNG is required to have excellent fracture toughness from the viewpoint of ensuring safety. A typical example of steel that meets this requirement is 9% Ni steel.

  Conventionally, reduction of impurities such as P and S, reduction of C, and further three-stage heat treatment, that is, heat treatment of “quenching (Q), two-phase region quenching (L) and tempering (T)”, etc. Various improvements have been made on 9% Ni steel. On the other hand, addition of Mo has been studied as an alloy element effective for improving the strength and toughness of Ni-containing steel. This is because the addition of QLT or Mo increases the amount of retained austenite which is the basis for improving toughness. Such a state of the prior art is summarized as follows based on patent literature.

  Patent Document 1 discloses that a plate thickness of 40 mm manufactured by a three-stage heat treatment method (QLT) or a direct quenching-two-phase quenching method (DQ-LT) method in which Mo: 0.04 to 0.5% is added. The above 9Ni steel is disclosed.

  Patent Document 2 discloses a method for producing 9Ni steel having a thickness of 40 mm or more by a quenching-tempering method (QT) or a direct quenching-tempering method (DQ-T) method.

  In recent years, the price of steel materials has soared due to soaring alloy element prices. In the 9% Ni steel to which a large amount of expensive alloy elements such as Ni must be added, the increase in the price of the alloy elements causes a further increase in the price of the steel material. Therefore, in order to suppress the price of steel materials, it has become necessary to develop steel materials having performance equivalent to or better than 9% Ni steel, for example, excellent toughness, with a Ni content with low cost reduction. The following is a conventional technique related to such a low Ni type steel for low temperature.

  Patent Document 3 discloses a low-temperature steel containing 4.0 to 7.5% Ni and having an Ms point of 370 ° C. or lower. Moreover, the said patent document 2 shows the manufacturing method by the steel containing 7.5 to 10% Ni, and DQ-LT. Further, Patent Document 4 discloses a steel containing 5.5 to 10% Ni and a continuous casting method thereof.

Patent Documents 5 and 6 disclose steels containing 1.5 to 9.5% Ni and 0.02 to 0.08% Mo.
Japanese Unexamined Patent Publication No. 4-371520 JP-A-6-184630 JP-A-6-136483 JP-A-7-90504 JP-A-9-302445 JP 2002-129280 A

  However, Patent Document 1 does not describe the detailed conditions of rolling, and the Ni content exceeds 6% and is less than 8%, and the performance equivalent to the steel of the present invention described later is not obtained. .

  In Patent Document 2, a steel containing 7.52% Ni is exemplified as a comparative example. However, since the chemical composition and the manufacturing method of the steel are not appropriate, the amount of retained austenite is as low as 1.5%. It is sufficient and performance equivalent to 9% Ni steel cannot be obtained. Therefore, the steel is treated as a comparative example.

  Patent Document 3 discloses a method for improving the toughness of the weld heat-affected zone (HAZ), but discloses a design and manufacturing method of chemical components for obtaining a base material characteristic similar to 9% Ni steel. Furthermore, the characteristics of the base material itself are not disclosed.

  In the above Patent Document 2, it is described that the rolling reduction at 700 to 900 ° C. is 20 to 90%, but there is no description about the rolling reduction per pass, and the steel manufactured by the manufacturing method is −196. The toughness at ° C does not reach 250J.

  Patent Document 4 describes a component system that can be continuously cast, but does not disclose a manufacturing method or base material characteristics of the base material. Moreover, the minimum value of the Ni amount specifically shown is 9.08%, and means for obtaining a base material performance equivalent to 9% Ni steel with low Ni is not disclosed.

  Patent Documents 5 and 6 disclose a method for producing DQ-LT in which water cooling is stopped at 400 ° C. or lower. However, the heating temperature and rolling conditions are not disclosed. Further, the characteristics of about 7% Ni are not disclosed, and as shown in the examples, the base material toughness is 9% Ni steel and vTs is less than −196 ° C., whereas 5.0% The% Ni steel has a vTs of −160 ° C., and the 1.5Ni steel has a vTs of −125 ° C., which is directly related to toughness deterioration.

  As described above, each of the above-mentioned documents does not specifically disclose a steel having a performance equivalent to that of 9% Ni steel and a method for producing the same while being lower Ni than 9% Ni steel.

  An object of the present invention is to provide a steel material having a Ni content lower than that of 9% Ni steel and having performance equivalent to that of 9% Ni steel, and a method for producing the same.

  In order to achieve the above-mentioned object, the present inventor has examined the above-described prior art in detail. As a result, it was found that the conventional technique was insufficient in refining the structure and securing the amount of retained austenite. That is, there is a need for means for refining the base metal structure itself and stabilizing austenite even when the Ni content is less than 9% Ni steel.

  As a means for stabilizing austenite with a Ni content smaller than that of 9% Ni steel, it has been found that there are the following means in addition to the conventionally known trace addition of Mo.

  The first means is a method of lowering the Mf point at which martensitic transformation ends by introducing lattice defects into untransformed austenite. The transformation from austenite to martensite is a shear type transformation due to the movement of dislocations, and lattice defects present in the austenite hinder the movement of dislocations, delaying the end of the shear type transformation from austenite to martensite, and martensite. The Mf point that is the end point is shifted to the low temperature side. By shifting the Mf point to the low temperature side, the amount of austenite remaining at room temperature can be increased.

  The second means is refinement of the untransformed austenite phase. When untransformed austenite is transformed into martensite, the size of the minimum unit (lass) of martensite instantaneously generated by shearing transformation is about 0.5 to 1 μm in the thickness direction. This transformation is actually accompanied by volume expansion. Therefore, if the size of the untransformed austenite phase is equal to or smaller than the minimum unit of martensite, martensitic transformation due to expansion is remarkably suppressed, and austenite is more than obtained from the actual chemical component amount. It has been found that it stabilizes significantly.

  The amount of residual austenite of the low Ni steel thus obtained is not only comparable to the amount of residual austenite obtained with the conventional 9% Ni steel quenching and tempering material, but also has the following characteristics. That is, the retained austenite obtained by conventional 9% Ni steel quenching and tempering material has a needle-like shape in two-dimensional observation and a thin plate shape in three dimensions, and has a large aspect ratio. However, the austenite obtained from the low Ni steel is characterized by a very fine granular form in two-dimensional observation even though the total amount is approximately equivalent to 9% Ni steel. Therefore, retained austenite stably exists for the above-described reason even at low Ni.

  In order to introduce lattice defects (dislocations) into untransformed austenite and to refine the untransformed austenite phase, the conditions of heating, rolling and cooling are important. It is known that when a strong pressure is applied at a low temperature, a large amount of lattice defects (dislocations) are introduced and the subsequent structure becomes fine. In addition, for the above-mentioned miniaturization, it is particularly effective to add Nb as a trace element. This is because the fine precipitation of Nb (C, N) hinders the movement of dislocations and increases the density of lattice defects (dislocations) in austenite.

  This invention made | formed based on the above knowledge makes the following steel materials and its manufacturing method a summary.

( 1 ) By mass%, C: 0.01 to 0.1%, Si: 0.005 to 0.6%, Mn: 0.3 to 2%, Ni: more than 6% and less than 8%, sol .Al: 0.005 to 0.05%, N: 0.0005 to 0.005%, with the balance being Fe and impurities, satisfying the following formula (1), and having an area ratio A low-temperature steel material comprising 1.7% or more of austenite, an average aspect ratio of cementite contained is 5.0 or less, and an average equivalent circle diameter is 0.6 μm or less.
20C + 2.4Mn + Ni ≧ 10 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of the element.

( 2 ) By mass%, C: 0.01 to 0.1%, Si: 0.005 to 0.6%, Mn: 0.3 to 2%, Ni: more than 6% and less than 8%, sol .Al: 0.005 to 0.05%, N: 0.0005 to 0.005%, with the balance being Fe and impurities, satisfying the following formula (1), and having an area ratio 1.7% or more of austenite, the austenite has an average aspect ratio of 3.5 or less, an average equivalent-circle particle size of 1.0 μm or less, and an average aspect ratio of cementite contained The steel material for low temperature characterized by being 5.0 or less and having an average equivalent circle diameter of 0.6 μm or less.
20C + 2.4Mn + Ni ≧ 10 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of the element.

( 3 ) Further mass% instead of part of Fe, Mo: 0.1% or less, Cu: 2.0% or less, Cr: 0.8% or less, V: 0.08% or less, Nb: 0 0.08% or less, Ti: 0.03% or less, B: 0.0030% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less. (2) The steel material for low temperature as described in said (1) or (2) characterized by satisfy | filling Formula.
20C + 2.4Mn + Ni + 0.5Cu + 0.5Mo ≧ 10 (2)
However, the element symbol in the formula (2) indicates the content (% by mass) of the element.

( 4 ) A steel slab having the chemical composition described in any one of (1) to ( 3 ) above is heated to 850 to 1050 ° C (excluding 1050 ° C) , and 1 in a temperature range of 700 to 830 ° C. It is a manufacturing method that performs rolling with a cumulative reduction rate of 25% or more at 5% or more per pass, finishes rolling in a temperature range of 700 to 800 ° C., and immediately performs accelerated cooling to a temperature range of 200 ° C. or less. The cooling rate from the cooling start temperature to at least 600 ° C. in the accelerated cooling is 10 ° C./s or higher, the cooling rate from the cooling start temperature to 200 ° C. is 5 ° C./s or higher, and after the accelerated cooling, 650 ° C. A method for producing a steel material for low temperature characterized by tempering at the following temperature.

( 5 ) Between accelerated cooling after rolling and tempering at 650 ° C. or lower, heating to 600 to 800 ° C., and including two-phase region heat treatment for cooling up to 200 ° C. or lower at 5 ° C./s or higher. The method for producing a low-temperature steel material as described in ( 4 ) above.

  The reason why the chemical composition, metal structure and production conditions of the steel material are defined as described above in the present invention will be described in detail below. In addition, "%" about the component content of steel materials is "mass%".

C: 0.01 to 0.1%
C is an element that lowers the Mf point and is effective in stabilizing retained austenite. However, the martensite substrate itself is hardened, causing toughness deterioration more than improving toughness due to an increase in the amount of austenite. Therefore, it is important that the content of C is not less than the amount necessary to ensure strength, and an excessive amount that deteriorates toughness is avoided. If it is less than 0.01%, the strength is insufficient, and if it exceeds 0.1%, the toughness deteriorates. Therefore, the C content is set to 0.01 to 0.1%. A more desirable range is 0.03 to 0.07%.

Si: 0.005 to 0.6%
Si is effective as a deoxidizing element. It is also effective as an element that suppresses the precipitation of cementite and improves the stabilization of austenite during tempering. However, too much Si content causes toughness deterioration. Therefore, the content is made 0.005 to 0.6%. Desirable is 0.03 to 0.5%, and more desirable is 0.1 to 0.3%.

Mn: 0.3-2%
Mn is effective in stabilizing the austenite by lowering the Mf point, and a larger amount of austenite can be obtained as the content increases. However, when the Mn content is excessive, the toughness of the martensite substrate is deteriorated. Therefore, the content is set to 0.3 to 2.0%. A more desirable lower limit is 0.5%, and a more desirable lower limit is 0.7%. A desirable upper limit is 1.5%, and a more desirable upper limit is 1.0%.

Ni: more than 6% and less than 8% Ni is the most important element of the steel of the present invention because it increases the strength of the steel and contributes to the stabilization of austenite. The higher the content, the higher the strength, and the Mf point is lowered and the amount of retained austenite is increased. However, containing a large amount of Ni causes an increase in cost, so the content is made less than 8%. A more desirable upper limit is 7.5%. On the other hand, one of the objects of the present invention is to obtain a steel material having performance equivalent to that of 9% Ni steel. To achieve the object, a Ni content exceeding 6% is required. A more desirable lower limit is 6.5%.

sol.Al: 0.005 to 0.05%
Al is effective as a deoxidizing element like Si, and as an element for suppressing the precipitation of cementite and improving the stabilization of austenite during tempering. Furthermore, Al combines with N to become AlN, which also has the effect of contributing to the refinement of austenite grains during heating. Accordingly, it is necessary to contain 0.005% or more as sol.Al. However, too much Al content causes toughness deterioration. Therefore, the content is 0.005 to 0.05% as sol.Al. A more desirable content range is 0.02% to 0.04%.

N: 0.0005 to 0.005%
Since N is an element that contributes to the stabilization of austenite, it is desirable to add N. Moreover, it combines with Al to become AlN, which is effective for refining austenite grains during heating. In order to obtain these effects, a content of 0.0005% or more is necessary. However, excessive N deteriorates the martensite substrate, so its content needs to be 0.005% or less. A more desirable content range is 0.002% to 0.004%.

  One of the steel materials of the present invention is composed of Fe and impurities in addition to the above components. Another steel material of the present invention is a steel material containing one or more selected from Mo, Cu, Cr, V, Nb, Ti, B, Ca and Mg in addition to the above components. Hereinafter, these components will be described.

Mo: 0.1% or less Mo is effective in increasing the amount of austenite as an austenite stabilizing element in a low temperature range. In order to obtain this effect, a content of 0.01% or more is desirable. However, if the Mo content exceeds 0.1%, the toughness decreases due to deterioration of the martensite substrate, so it is necessary to make it 0.1% or less. The lower limit of the more desirable content is 0.02%, the desirable upper limit is 0.06%, and the more desirable upper limit is 0.05%.

Cu: 2.0% or less Cu is an element that stabilizes austenite in a solid solution state, and is desirably added. In order to obtain this effect, 0.05% or more is desirable. However, since solute Cu precipitates as ε-Cu by the tempering treatment, it is effective for increasing the strength but deteriorates toughness. Therefore, the upper limit of the content is set to 2.0%.

Cr: 0.8% or less Cr is an element effective in increasing the strength. To obtain this effect, 0.05% or more is desirable. However, if its content exceeds 0.8%, the toughness deteriorates, so 0.8% is made the upper limit.

V: 0.08% or less V is an element effective for increasing the strength of steel, and becomes a precipitate by tempering to strengthen the steel. In order to obtain the effect, the content is desirably 0.005% or more. However, if the content exceeds 0.08%, toughness deteriorates due to excessive precipitates, so the content is made 0.08% or less.

Nb: 0.08% or less Nb is effective in expanding the non-recrystallization temperature range in rolling and making the structure finer and tougher after rolling. In order to obtain this effect, it is desirable to contain 0.005% or more. However, if the content exceeds 0.08%, the toughness deteriorates, so the upper limit is made 0.08%.

Ti: 0.03% or less Ti is an element effective for preventing cracks in the slab. To obtain this effect, 0.005% or more is desirable. However, if the content exceeds 0.03%, the toughness deteriorates, so the upper limit is made 0.03%.

B: 0.0030% or less B is an element effective for increasing the strength. To obtain this effect, 0.0002% or more is desirable. However, if the B content exceeds 0.0030%, the toughness deteriorates, so the upper limit is made 0.0030%.

Ca: 0.0050% or less Ca is an element effective for improving toughness, and 0.0002% or more is desirable for obtaining this effect. However, if the content exceeds 0.0050%, the toughness deteriorates, so the upper limit is made 0.0050%.

Mg: 0.0050% or less Mg is an element effective for improving toughness. To obtain this effect, it is desirable to contain 0.0050% or more. However, if the content exceeds 0.0050%, the toughness deteriorates, so the upper limit is made 0.0050%.

20C + 2.4Mn + Ni ≧ 10, or 20C + 2.4Mn + Ni + 0.5Cu + 0.5Mo ≧ 10
In order to reduce the Ni content and obtain a toughness equivalent to that of 9% Ni steel, it is important to secure the amount of retained austenite. Although the amount of retained austenite obtained varies depending on the conditions of heating, rolling and heat treatment, it is also important to add chemical components that stabilize austenite. For this purpose, the value of “20C + 2.4Mn + Ni” or “20C + 2.4Mn + Ni + 0.5Cu + 0.5Mo” needs to be 10 or more. More preferably, it is 10.5 or more and 12 or less.

Austenite amount:
The amount of austenite in the steel is important as a means for improving toughness even with low Ni. In order to obtain the same toughness as 9% Ni steel even with low Ni steel, an austenite amount of 1.7% or more in area ratio is required. A desirable lower limit is 2.0%, and a more desirable lower limit is 3.0%. The higher the amount of austenite, the more effective is to improve toughness, so the upper limit is not specified, but if it exceeds 40%, the strength is insufficient. Therefore, the upper limit of the amount of austenite is preferably 40%.

Austenite form:
In order to stabilize austenite with a low Ni content, it is necessary to make the untransformed austenite phase fine. For this purpose, it is necessary to make austenite fine particles, and the aspect ratio must be 3.5 or less on average and the average equivalent circle diameter must be 1.0 μm or less. A desirable aspect ratio is 2.5 or less. Here, the equivalent circle diameter means the diameter of a circle when the projected area of austenite is regarded as a circle of the same area. The equivalent circle diameter means that obtained by measuring the structure observed when the steel material is cut in a plane parallel to the rolling direction (in the direction perpendicular to the plate thickness). The projected area of austenite is calculated using an image analyzer. Can be measured.

Form of cementite:
Cementite precipitates from the martensite substrate, and untransformed austenite decomposes and precipitates. The former precipitation reduces the strength of martensite and deteriorates toughness. Therefore, the size of cementite needs to be 0.6 μm or less in terms of the average equivalent circle diameter. The equivalent-circle diameter of cementite is the same as the equivalent-circle diameter of austenite described above, and is measured for cementite instead of austenite.

Next, the manufacturing method of said steel material is described.
(1) Heating of billet In order to improve the toughness of the steel material, it is important to refine the initial austenite grains, i.e., the austenite grains in the billet before rolling. This also contributes to an increase in Therefore, the heating temperature of the steel slab before rolling is set to 850 to 1050 ° C. Heating at a temperature lower than 850 ° C. results in insufficient strength, and heating at a temperature exceeding 1050 ° C. deteriorates toughness. A more desirable heating temperature is 900 to 1000 ° C.

(2) Rolling In order to refine the structure and increase the amount of austenite, sufficient rolling must be performed in the non-recrystallized region of austenite. Rolling of 5% or more per pass in the temperature range of 700 to 830 ° C. and a cumulative reduction ratio of 25% or more introduces lattice defects (dislocations) into austenite in the non-recrystallized austenite temperature range, and martensite of untransformed austenite It is necessary to suppress the transformation to. At this time, it is necessary to finish rolling at a temperature of 700 to 800 ° C. When the finishing temperature is lower than 700 ° C., the anisotropy of the steel material becomes remarkable. Further, when the finishing temperature exceeds 800 ° C., the toughness deteriorates.

(3) Cooling After completion of rolling, it is necessary to perform accelerated cooling to a temperature range of 200 ° C. or lower. Here, it is necessary to cool at least 10 ° C./s from the start of cooling to at least 600 ° C. This is to contain as many lattice defects (dislocations) introduced in finish rolling as possible. Further, in order to obtain a martensite structure, it is necessary to cool at a rate of 5 ° C. or higher from the cooling start temperature to 200 ° C. or lower. When accelerated cooling is stopped at a temperature higher than 200 ° C., sufficient martensite cannot be obtained and the strength deteriorates. The time from the rolling finish to the start of water cooling should be short, and it is desirable that the time from the end of rolling to the start of water cooling be within 30 seconds.

(4) Tempering After accelerated cooling, it is necessary to temper at a temperature of 650 ° C or lower. Thereby, martensite generated by cooling treatment, that is, quenching can be tempered, and the strength can be adjusted and the toughness can be improved. When tempering is performed at a temperature exceeding 650 ° C., the strength decreases.

(5) Two-phase region heating In order to further increase the amount of retained austenite, it is desirable to heat the two-phase region of ferrite and austenite before tempering. The two-phase region heat treatment needs to be a process of heating at 600 to 800 ° C. and then cooling to 200 ° C. or less at a cooling rate of 5 ° C./s. A more desirable range of the heating temperature for the two-phase region heat treatment is 680 to 750 ° C.

  A test material having the chemical composition shown in Table 1 was melted, and a steel plate having a thickness of 20 mm was made as a prototype. The manufacturing conditions are shown in Table 2. Tensile test pieces and Charpy test pieces were collected from 1/4 part (1/4 t part) of the thickness of the obtained steel sheet. The amount of austenite was measured by X-ray. In addition, the size and form of austenite and cementite were observed with a transmission electron microscope at a magnification of 40,000 for each of 20 visual fields to obtain an average aspect ratio, and an average equivalent-equivalent particle size was obtained with an image analyzer. Table 3 shows the measurement results.

  “Examples of the present invention” shown in Table 3 have the chemical composition defined in the present invention and are produced by the production method of the present invention. In addition to satisfying the above formula (1) or (2), austenite The area ratio, the form and the cementite form all satisfy the conditions defined in the present invention. And in the present invention example, YS is 585 MPa or more, TS is 690 to 825 MPa, and Charpy impact energy at −196 ° C. is 250 J or more.

In particular, it includes two requirements regarding the metal structure, namely, an austenite having an area ratio of 1.7% or more. (1) The austenite has an average aspect ratio of 3.5 or less and an average equivalent-equivalent grain size. It satisfies both (1) and (2) that it is 1.0 μm or less, and (2) that the aspect ratio of cementite is 5.0 or less on average and the average equivalent circle diameter is 0.6 μm or less. In some cases (test numbers T2, T4, T7 , T8, T10, T13 and T15), the absorbed energy is high toughness of 290 J or more.

  On the other hand, any of the comparative examples in which any of the chemical composition and other conditions are not within the range defined by the present invention has low impact energy and insufficient low temperature toughness.

  The mechanical properties of 9% Ni steel of the same thickness produced by the conventional method (quenching-tempering) are YS = 610 MPa, TS = 720 MPa, and Charpy absorbed energy at −196 ° C. is 280 J. From this, it can be seen that the steel of the present invention has the same performance as the 9% Ni steel although the amount of Ni is small.

  According to the present invention, a steel material having mechanical properties equivalent to or better than steel containing 9% Ni can be obtained even with a low Ni content of more than 6% and less than 8%. Since this steel material is inexpensive but has excellent low-temperature toughness, it is suitable as a structural material for storage tanks for low-temperature substances such as LNG.

Claims (5)

In mass%, C: 0.01 to 0.1%, Si: 0.005 to 0.6%, Mn: 0.3 to 2%, Ni: more than 6% and less than 8%, sol.Al: A steel material containing 0.005 to 0.05%, N: 0.0005 to 0.005%, the balance being Fe and impurities, and satisfying the following formula (1). A steel material for low temperature comprising 7% or more of austenite, an average aspect ratio of cementite contained is 5.0 or less, and an average equivalent circle diameter is 0.6 μm or less.
20C + 2.4Mn + Ni ≧ 10 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of the element. In mass%, C: 0.01 to 0.1%, Si: 0.005 to 0.6%, Mn: 0.3 to 2%, Ni: more than 6% and less than 8%, sol.Al: A steel material containing 0.005 to 0.05%, N: 0.0005 to 0.005%, the balance being Fe and impurities, and satisfying the following formula (1). The austenite contains 7% or more of austenite, and the austenite has an average aspect ratio of 3.5 or less and an average equivalent-equivalent particle size of 1.0 μm or less. A low-temperature steel material having an average equivalent circle diameter of 0.6 μm or less and having an average circle equivalent diameter of 0 or less.
20C + 2.4Mn + Ni ≧ 10 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of the element. Instead of part of Fe, it is further mass%, Mo: 0.1% or less, Cu: 2.0% or less, Cr: 0.8% or less, V: 0.08% or less, Nb: 0.08% Hereinafter, Ti: 0.03% or less, B: 0.0030% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, containing at least one selected from the following (2) The steel material for low temperature according to claim 1 or 2 , wherein the formula is satisfied.
20C + 2.4Mn + Ni + 0.5Cu + 0.5Mo ≧ 10 (2)
However, the element symbol in the formula (2) indicates the content (% by mass) of the element. A steel slab having the chemical composition according to any one of claims 1 to 3 is heated to 850 to 1050 ° C (excluding 1050 ° C), and 5% per pass in a temperature range of 700 to 830 ° C. In the above manufacturing method, after rolling at a cumulative reduction rate of 25% or more and finishing rolling in a temperature range of 700 to 800 ° C., the cooling is immediately accelerated to a temperature range of 200 ° C. or less. The cooling rate from the start temperature to at least 600 ° C. is 10 ° C./s or more, the cooling rate from the cooling start temperature to 200 ° C. is 5 ° C./s or more, and after the accelerated cooling, the temperature is 650 ° C. or less. A method for producing a low-temperature steel material, characterized by tempering. Between the accelerated cooling after rolling and the tempering at 650 ° C. or lower, it is heated to 600 to 800 ° C. and includes a two-phase region heat treatment for cooling up to 200 ° C. or lower at 5 ° C./s or more. The manufacturing method of the steel material for low temperature of Claim 4 .