JP5076423B2 - Method for producing Ni-containing steel sheet - Google Patents

Description

  The present invention relates to a method for producing a Ni-containing steel sheet that is suitable as a steel material for an LNG storage tank and has excellent low-temperature toughness, and particularly excellent in a balance between strength and toughness.

  In recent years, an increase in global energy demand and accompanying deterioration of the global environment have become a problem. Therefore, the demand for natural gas (LNG) as a clean energy source is increasing rapidly. With the increasing demand for natural gas (LNG), the construction of LNG storage tanks has recently been actively promoted both at home and abroad. Under such circumstances, there is an increasing demand for Ni-containing steel sheets with excellent low-temperature toughness such as 9% Ni steel used for the main body of LNG storage tanks. Moreover, in order to make effective use of the site for tank construction, LNG storage tanks tend to be large, and the steel sheets used have excellent low temperature toughness from the viewpoint of ensuring safety against brittle fracture. In addition, it is desired to have higher yield strength at the same time.

  As a manufacturing method of 9% Ni steel plate with excellent low temperature toughness, JIS G 3127 (nickel steel plate for low temperature pressure vessels (yield point or proof stress: 590 MPa or more)) and ASTM A553 are reheated and tempered (hereinafter referred to as QT). In addition, ASTM A844 specifies direct quenching and tempering (hereinafter referred to as DQT process). Both the QT process and the DQT process are heat treatments in which quenching is performed in one step.

In recent years, a heat treatment method has been proposed in which the low temperature toughness of a Ni-containing steel plate is stably further improved while maintaining the strength. For example, Non-Patent Document 1 discloses a first stage quenching (Q) in which martensite is obtained by quenching from the austenite zone, and a second stage quenching from the (γ + α) two-phase zone below the Ac ₃ transformation point. There is a commentary on the heat treatment (hereinafter referred to as QQ'T process) consisting of quenching (Q ') and tempering (T) tempering near the Ac ₁ transformation point. In the Q 'treatment in the QQ'T process, the martensite formed in the Q treatment is tempered to become tempered martensite, partly transforms into austenite, the structure is refined, and alloy elements are redistributed. Therefore, the subsequent rapid cooling forms martensite enriched with alloying elements and a small amount of retained austenite. When such a mixed structure is further tempered (T), a stable austenite phase enriched with alloying elements is precipitated, and impurities harmful to toughness such as C and N in the tempered martensite are austenite. Transition into phase. As a result, the microstructure is composed of a fine tempered martensite phase having extremely high toughness and a retained austenite phase that is stable even at extremely low temperatures. For this reason, it is said that the low temperature toughness of Ni-containing steel sheets is remarkably improved by applying the QQ'T process.

Based on this QQ'T process, various measures for improving strength and low temperature toughness have been proposed for Ni-containing steel sheets. For example, Patent Document 1 discloses that a Ni-containing thick steel sheet containing Ni: 7.5 to 12.0% is heated to a temperature not lower than the Ac ₃ point and cooled, followed by (Ac ₁ + 10 ° C.) to (Ac ₃ − A secondary quenching process in which the amount of austenite generated in the temperature range is 20 to 65 area% or 85 to 95 area% and cooled by heating, and further 500 ° C or more (Ac _1-10 ° C.) A method for producing a Ni-containing thick steel sheet using the QQ'T process is described. According to the technique described in Patent Document 1, excellent yield strength and low temperature toughness can be stably obtained even in a Ni-containing thick steel plate having a thickness of 50 mm or more.

Patent Document 2 discloses that steel containing C: 0.10% or less, Ni: 8.0 to 10.0%, impurities P being 0.001% or less, and S being 0.001% or less, after hot rolling, Ac ₃ point Describes a method for producing cryogenic steel materials using the QQ'T process, quenching from the above temperatures, then reheating to 680-710 ° C, quenching, and then tempering to 570-600 ° C . According to the technique described in Patent Document 2, even a very thick material exceeding a plate thickness of 40 mm can be a steel plate excellent in low-temperature toughness. In the technique described in Patent Document 2, if the hot rolling finish temperature is 800 to 900 ° C., the first stage quenching (Q) may be directly quenched (DQ).

Patent Document 3 includes a quenching process in which the mixture is heated between the Ac ₃ transformation point and 850 ° C. and water-cooled, and an intermediate quenching process in which the two-phase region between the Ac ₁ transformation point and the Ac ₃ transformation point is heated and cooled with water. Describes a heat treatment method for 9% Ni steel using the QQ'T process, which is performed at a heating rate of 50 ° C./min or higher in a temperature range of 500 ° C. or higher and further tempered at an Ac ₁ transformation point or lower. . According to the technique described in Patent Document 3, since the crystal grains are refined due to an increase in the heating rate at the time of quenching, the decrease in toughness due to the decrease in the cooling rate is compensated, and as a result, the toughness is not impaired. It is said that a steel plate having high yield strength can be manufactured.

In Patent Document 4, a slab containing Ni: 6.5 to 12.0% is subjected to diffusion heat treatment that is maintained at 1200 to 1350 ° C. for 24 hours or more, and then hot-rolled to obtain an Ac ₃ transformation point to (Ac ₃ transformation point). + 200 ° C) is heated and cooled to a temperature range, and then is heated and cooled to a two-phase region from (Ac ₁ transformation point + 50 ° C) to Ac ₃ transformation point, and the temperature is 450 ° C to (Ac ₁ transformation point + 70 ° C). A method for producing a low-temperature steel sheet using the QQ'T process is described. According to the technique described in Patent Document 4, the low temperature toughness of the Ni-containing low temperature steel sheet is improved, and the temperature range of the heat treatment is allowed to be wide and the productivity is improved.

Further, Patent Document 5 defines a Ni-containing thick steel sheet containing Ni: 7.5 to 12.0% and Nb: 0.003 to 0.03% in a temperature range of 980 to Ac ₃ points and a formula related to Nb and C. A primary quenching process in which the temperature is maintained at or above the temperature T _Q or less than T _Q and is maintained for a period of time equal to or less than t _Q defined by the equation related to the heating temperature, followed by cooling (Ac ₁ + 10 ° C) QQ'T process that performs secondary quenching treatment that holds and cools in the two-phase temperature range of ~ (Ac ₃ -10 ° C) and further temper treatment at 500 ° C or higher (Ac ₁ -10 ° C) or lower Describes a method for producing a Ni-containing thick steel plate using According to the technique described in Patent Document 5, excellent yield strength and low-temperature toughness can be stably obtained even in a Ni-containing thick steel plate having a thickness of 50 mm or more.
JP-A-9-256039 JP-A-6-179909 JP-A-8-27517 JP 9-41036 A JP-A-9-256040 The Japan Institute of Metals: 4th edition metal manual, p.801, Maruzen, published in December 1982

  All of the techniques described in Patent Documents 1 to 5 use the QQ'T process or the DQQ'T process. By using the QQ'T process or the DQQ'T process, while ensuring the strength, A Ni-containing thick steel sheet having a more stable and excellent low temperature toughness can be produced. However, since the QQ'T process or the DQQ'T process includes a plurality of heat treatment steps, in addition to the increase in manufacturing cost, the manufacturing step requires a long period of time, leaving a problem in productivity.

The present invention solves the above-mentioned problems of the prior art, and Ni-containing steel sheets that can produce Ni-containing steel sheets that have superior low-temperature toughness, excellent strength, and excellent strength-toughness balance with excellent productivity. It aims at providing the manufacturing method of. Here, “excellent in low temperature toughness” means that the absorbed energy when the Charpy impact test is performed at a test temperature of −196 ° C. is 150 J or more. The term “excellent in strength” here refers to a case where the yield strength is 600 MPa or more. “Excellent strength-toughness balance” means that (yield strength YS (MPa)) × (absorbed energy value at −196 ° C. vE ₋₁₉₆ (J)) is 100000 MPaJ or more. .

  In order to achieve the above-mentioned object, the present inventors do not use the DQQ'T process or the QQ'T process, select a process such as a QT process or a DQT process that performs a quenching process, The effects of heat treatment conditions on low temperature toughness and strength have been extensively studied. As a result, the present inventors further increased the temperature to a Ni-containing steel sheet in a quenched state after hot rolling, re-quenched or directly quenched after hot rolling, and more rapidly than conventional tempering. Furthermore, it has been found that a Ni-containing steel sheet having an excellent strength-toughness balance can be manufactured by shortening the process by performing a cooling heat treatment.

The present inventors first thought that, in order to stably obtain excellent low temperature toughness, it is necessary to sufficiently secure a retained austenite phase having high stability at low temperatures. However, when there is too much content of a retained austenite phase, yield strength will fall. For this reason, it is necessary to adjust the content of the retained austenite phase to an appropriate range in which both strength and low temperature toughness are excellent.
In the conventional QT or DQT process, it is difficult to secure a sufficient amount of retained austenite phase with little C and Ni concentration and high stability. Therefore, the present inventors have come up with the idea of setting the heating temperature of the heat treatment after quenching to a high temperature range of the two-phase temperature range. When the heating temperature is set to a high temperature range in the two-phase temperature range, a sufficient amount of austenite precipitates. However, the concentration of alloy elements such as C and Ni in the austenite is higher than when the heating temperature is low. Since it drops considerably, austenite becomes unstable and transforms to martensite after cooling. In order to suppress the precipitation amount of austenite, the heating and holding time may be shortened. However, in this case, Ni cannot be sufficiently concentrated in the austenite, and the austenite becomes unstable.

  Therefore, the present inventors have come up with the idea that the heat treatment after the quenching process is a process of rapidly heating and cooling to a high temperature range of the two-phase temperature range. Thereby, the transformation nucleus of α → γ transformation can be increased, and a large number of fine austenite can be dispersed and precipitated. By making the precipitated austenite finer, stability at low temperatures can be increased, thereby ensuring excellent low-temperature toughness. Furthermore, because of rapid heating in a short time, the tempering of the martensite phase that does not transform into austenite progresses. Therefore, it was found that even with the same component, it is possible to increase the strength compared to the prior art and to obtain a steel plate having an excellent strength-toughness balance.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.02 to 0.13%, Si: 0.04 to 0.5%, Mn: 0.2 to 1.0%, Ni: 7.0 to 10.0%, Al: 0.005 to 0.10%, the balance Fe and inevitable A steel material having a composition composed of impurities is hot-rolled into a steel sheet and then immediately quenched, followed by a heating rate of 5 ° C / s or more (Ac ₁ transformation point + 15 ° C) or more. , (Ac ₁ transformation point + 0.8 × (Ac ₃ transformation point-Ac ₁ transformation point)) Heat treatment to a temperature in the range below, followed by heat treatment to cool, low temperature toughness and strength-toughness A method for producing a Ni-containing steel sheet with excellent balance.

(2) By mass%, C: 0.02 to 0.13%, Si: 0.04 to 0.5%, Mn: 0.2 to 1.0%, Ni: 7.0 to 10.0%, Al: 0.005 to 0.10%, the balance Fe and inevitable a steel material having a composition consisting of impurities, after a steel sheet subjected to hot rolling, reheating quenching treatment by air cooling and then reheating and quenching to a temperature in the range of Ac ₃ transformation point to 850 ° C., then 5 After heating to a temperature in the range of (Ac ₁ transformation point + 15 ° C) or more and (Ac ₁ transformation point + 0.8 x (Ac ₃ transformation point-Ac ₁ transformation point)) at a rate of temperature rise of ℃ / s or more The manufacturing method of the Ni containing steel plate excellent in the low temperature toughness and strength-toughness balance characterized by performing sequentially the heat processing to cool.

(3) The method for producing a Ni-containing steel sheet according to (1) or (2), wherein the steel sheet is held for 10 seconds or less (including 0 seconds) after heating in the heat treatment.
(4) In any one of (1) to (3), in addition to the above composition, 1% selected from Mo: 0.5% or less, V: 0.1% or less, Cr: 0.5% or less in mass% The manufacturing method of the Ni containing steel plate characterized by containing seed | species or 2 or more types.

  (5) The method for producing a Ni-containing steel sheet according to any one of (1) to (4), further comprising Ti: 0.03% or less by mass% in addition to the above composition.

  According to the present invention, it is a simple process consisting of a single-stage quenching process and a tempering process, shortening the process, and having stable and excellent low-temperature toughness and desired high strength, and excellent strength-toughness balance. Ni-containing steel sheet can be produced efficiently and has a remarkable industrial effect. In addition, according to the present invention, an Ni-containing steel sheet having an excellent balance between strength and toughness can be manufactured at low cost, and there is an effect that it can greatly contribute to promotion of enlargement of the LNG tank and improvement of its safety.

First, the reasons for limiting the composition of the steel material (Ni-containing steel plate material) used in the present invention will be described. Hereinafter, mass% is simply expressed as%.
C: 0.02 to 0.13%
C is an element that increases the strength of steel, and in order to ensure a desired high strength, it needs to contain 0.02% or more. On the other hand, a content exceeding 0.13% causes a decrease in low temperature toughness. For this reason, C was specified in the range of 0.02 to 0.13%. In addition, from a viewpoint of ensuring hardenability and toughness ensuring, it is preferably 0.04 to 0.10%.

Si: 0.04-0.5%
Si acts as a deoxidizer and is an element that improves the strength of steel. To obtain such an effect, it needs to be contained in an amount of 0.04% or more. On the other hand, if the content exceeds 0.5%, the temper embrittlement susceptibility increases. For this reason, Si was specified in the range of 0.04 to 0.5%.
Mn: 0.2-1.0%
Mn is an element that improves hot ductility, improves hot workability, and contributes to strength improvement. In order to obtain these effects, it needs to be contained in an amount of 0.2% or more. On the other hand, even if the content exceeds 1.0%, the strength improving effect is saturated, the low temperature toughness is lowered, and the temper embrittlement sensitivity is further increased. For this reason, Mn was specified in the range of 0.2 to 1.0%. In addition, Preferably it is 0.4 to 0.8%.

Ni: 7.0 to 10.0%
Ni is an effective element for improving low temperature toughness, and further contributes to stabilization of the retained austenite phase. In order to obtain such an effect, the content of 7.0% or more is required. On the other hand, if the content exceeds 10.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, Ni is stipulated in the range of 7.0 to 10.0%.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer and has the effect of refining crystal grains. In order to acquire such an effect, it is necessary to contain 0.005% or more, but when it contains exceeding 0.10%, cleanliness will fall. For this reason, Al is prescribed | regulated to 0.005-0.10%.
The above-mentioned components are basic components, and in addition to this basic composition, one or more selected from Mo: 0.5% or less, V: 0.1% or less, Cr: 0.5% or less, and / or Alternatively, Ti: 0.03% or less can be selected and contained as necessary.

One or more selected from Mo: 0.5% or less, V: 0.1% or less, Cr: 0.5% or less
Mo, V, and Cr are all elements that contribute to strength improvement, and can be selected as necessary and contained in one or more. In particular, the inclusion of these elements is effective in the case of a thick material whose strength is significantly reduced. In order to obtain such effects, it is desirable to contain Mo: 0.03% or more, V: 0.02% or more, and Cr: 0.05% or more. On the other hand, if the content exceeds Mo: 0.5%, V: 0.1%, and Cr: 0.5%, the strength improvement allowance becomes small and the toughness of the base metal and the welded part decreases. For this reason, it is preferable to prescribe | regulate respectively Mo: 0.5% or less, V: 0.1% or less, and Cr: 0.5% or less.

Ti: 0.03% or less
Ti does not particularly affect the mechanical properties of the base material, but is an element that enhances the toughness of the weld heat affected zone, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.01% or more, but even if it contains exceeding 0.03%, an effect will be saturated and it will become economically disadvantageous. For this reason, Ti is preferably specified to be 0.03% or less.

The balance other than the above components is Fe and inevitable impurities.
In order to obtain a sound base material and a welded joint, it is preferable to reduce the inevitable impurities as much as possible to P: 0.01% or less and S: 0.01% or less.
In the present invention, a steel material having the above composition is used as a starting material. The manufacturing method of the steel material is not particularly specified, but any ordinary melting method and casting method can be applied. Especially, it is preferable to melt the molten steel which has an above-described composition with a normal melting furnace, such as a converter, and to make a slab (steel material) by a continuous casting method.

After heating the obtained steel material to a temperature at which hot rolling is possible, it is preferable to perform hot rolling to obtain a steel plate having a desired thickness. The obtained steel sheet is subjected to a direct quenching process in which quenching is performed immediately after the end of rolling, or air cooling is performed after the end of rolling, and then reheating and quenching is performed by reheating to a temperature in the range of Ac ₃ transformation point to 850 ° C. To make the steel sheet structure a uniform martensite structure.
Both the direct quenching process and the reheat quenching process are performed in order to obtain a uniform martensite structure. Therefore, in the case of direct quenching, it is necessary that the steel plate temperature (center) after the end of rolling is maintained at 600 ° C. or higher. When the steel plate temperature (center) after rolling is less than 600 ° C., it is difficult to ensure a uniform martensite structure even if it is rapidly cooled (water cooled) thereafter.

Further, in the case of reheating and quenching, it is necessary to reheat to a temperature at least above the Ac ₃ transformation point. If the reheating temperature is less than the Ac ₃ transformation point, a uniform martensite structure cannot be secured even if it is then rapidly cooled (water cooled). On the other hand, when the heating temperature exceeds 850 ° C., austenite grains become coarse during heating. For this reason, the reheating temperature of the reheating and quenching treatment is regulated to a temperature in the range of Ac ₃ transformation point to 850 ° C. After heating and holding at the reheating temperature described above, water cooling (quenching) is performed to obtain a uniform martensite structure. In addition, it has been confirmed that the direct quenching process and the reheating quenching process described above can obtain substantially the same effect.

The steel sheet that has been subjected to direct quenching treatment or reheating quenching treatment is then heated at a rate of 5 ° C./s or more (Ac ₁ transformation point + 15 ° C.) or more, (Ac ₁ transformation point + 0.8 × (Ac ₃ Transformation point—Ac ₁ transformation point)) After being heated to a temperature in the following range, a heat treatment for cooling is applied. In addition, the temperature increase rate here is an average temperature increase rate in the plate thickness direction, and it means an average temperature increase rate between 500-600 ° C.

This heat treatment is the most important thing in the present invention.
The temperature increase rate in this heat treatment is specified to be 5 ° C./s or more. When the heating rate is less than 5 ° C./s, the heating rate is slow and the nucleation frequency of austenite is low, so that the grain growth of precipitated austenite becomes remarkable, the retained austenite becomes coarse, and the tempering of martensite proceeds. Since the strength decreases, the desired low-temperature toughness and strength-toughness balance cannot be ensured. In addition, Preferably it is 10 degrees C / s or more, More preferably, it is 20 degrees C / s or more. In addition, the upper limit of the heating rate depends on the capacity of the heating device, so it is not necessary to specify it. However, considering the reality of what can be manufactured in the world as a current heating device, for example, induction heating method, / s is the upper limit.

The heating temperature in this heat treatment is defined as a temperature in the range of (Ac ₁ transformation point + 15 ° C.) or more and (Ac ₁ transformation point + 0.8 × (Ac ₃ transformation point−Ac ₁ transformation point)) or less. In addition, the temperature here means an average temperature in the thickness direction, and a temperature obtained by simulation calculation or the like from the thickness, surface temperature, cooling conditions, or the like can be used. For example, the temperature obtained by averaging the temperature distribution in the plate thickness direction using the difference method may be used as the average temperature.

If the heating temperature is less than (Ac ₁ transformation point + 15 ° C), a sufficient amount of precipitated austenite cannot be secured, and the tempering of the matrix (martensitic structure) is not sufficient, resulting in low temperature toughness. To do. On the other hand, if the heating temperature is higher than (Ac ₁ transformation point + 0.8 x (Ac ₃ transformation point-Ac ₁ transformation point)), the amount of transformation to austenite increases and the stability of the austenite phase decreases. The amount of residual austenite phase obtained decreases, and the strength decreases due to high temperature tempering. For this reason, the heating temperature in this heat treatment was defined as (Ac ₁ transformation point + 15 ° C.) or more and (Ac ₁ transformation point + 0.8 × (Ac ₃ transformation point−Ac ₁ transformation point)) or less.

Here, the transformation point is a transformation temperature obtained by an experiment (transformation measurement) with a temperature increase rate of 1 ° C./s. In Ni-containing steels, there is a discrepancy between the transformation point in the equilibrium state and the temperature at which the transformation actually occurs, mainly due to the microsegregation of Ni.
After the steel sheet temperature reaches the above heating temperature, it is preferable to cool immediately without holding (holding 0 s) or to cool after holding for 10 s or less. When the heating and holding time is longer than 10 s, the amount of transformation to austenite increases, the amount of residual austenite phase obtained decreases, and the tempering of the matrix (martensitic structure) proceeds and the strength decreases. For this reason, it is preferable to prescribe | regulate the heating holding | maintenance in above-mentioned heat processing as 10 s or less (0 s is included). The cooling after the heating and holding is not particularly required to be regulated such as air cooling or water cooling, but water cooling is preferable from the viewpoint of preventing temper embrittlement.

Such a rapid and short-time heat treatment can be performed by using, for example, a gas heating device or an induction heating device.
When performing the above-described heat treatment using an induction heating device, the induction heating device is preferably a so-called on-line system that is installed on a thick plate production line, for example. In the process of performing the heat treatment after the direct quenching treatment as in the present invention, it is preferable to use the on-line method particularly in terms of production efficiency. Of course, it goes without saying that the offline method may be used.

  Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace to form a small steel ingot (100 kg) and a hot-rolled slab (steel material) having a thickness of 100 mm. These steel materials were heated to 1100 ° C. and then hot-rolled to obtain steel plates having thicknesses shown in Table 2. Some steel plates are subjected to direct quenching (DQ), which is quenched (water-cooled) immediately after rolling, and some steel plates are air-cooled after rolling, and then re-heated and quenched (water-cooled). Quenching treatment (Q) was performed. Table 2 shows the quenching start temperature (rolling end temperature) of the direct quenching process and the heating temperature of the reheating quenching process.

The steel sheet that has been subjected to direct quenching treatment or reheating quenching treatment is then heated to the heating temperature shown in Table 2 at the rate of temperature rise shown in Table 2, and the heat treatment (T) for cooling after holding for the time shown in Table 2 is performed. gave. The cooling was air cooling.
For comparison, the heat treatment using the QQ′T process (Q temperature: 810 ° C., Q ′ temperature: 680 ° C., T temperature: 570 ° C.) was also performed (steel plate No. 1). The temperature increase rate of the heat treatment (T) was 0.5 ° C./s.

From the obtained steel plate, a test piece was collected and subjected to a tensile test and an impact test to evaluate strength and low temperature toughness. The test method was as follows.
(1) Tensile test Round bar test piece (parallel part) so that the direction perpendicular to the rolling direction (sheet width direction) is the tensile direction from the center of the thickness of the obtained steel sheet in accordance with the provisions of JIS Z 2201 (Diameter: 6 mmφ) was sampled and subjected to a tensile test at room temperature in accordance with the provisions of JIS Z 2241 to determine the yield strength YS and the tensile strength TS.

(2) Impact test V-notch test specimens were taken in the direction perpendicular to the rolling direction (sheet width direction) from the center of the thickness of the obtained steel sheet, and the test temperature was -196 in accordance with the provisions of JIS Z 2242. A Charpy impact test was performed at 0 ° C., and the absorbed energy vE ₋₁₉₆ (J) at −196 ° C. was obtained. Three tests were performed, and the obtained absorbed energy values were arithmetically averaged, and the average value was taken as the absorbed energy value of the steel sheet.

In addition, the test piece for residual austenite measurement was extract | collected from the plate | board thickness center part of the obtained steel plate, and the amount of retained austenite (volume%) in the 1 / 2t position of each steel plate was measured by X-ray diffraction.
The obtained results are shown in Table 3.

  Steel plates No. 1 to No. 3 are comparative examples using the QQ′T process and the QT process, which are conventional manufacturing processes. Steel plate No. 1 (comparative example) using the QQ'T process has increased retained austenite and improved low temperature toughness. Steel plate No. 3 using the QT process (comparative example) has a high heat treatment (T) heating temperature and a retained austenite content equivalent to that of the QQ'T process, but its stability is low and low temperature toughness is low. It is falling. Steel plate No. 2 using the QT process (comparative example) has a low amount of retained austenite and a lower low temperature toughness than the QQ'T process material (steel plate No. 1).

Each of the inventive examples has a low temperature toughness equivalent to or higher than that of the comparative example (steel plate No. 2) using the conventional QT process, and also has the same absorption as the comparative example (steel plate No. 2). When it has energy, it is a steel plate having higher strength and excellent strength-toughness balance.
On the other hand, in the comparative examples (steel plates No. 11 and No. 12) in which the heating temperature of the heat treatment (T) is out of the range of the present invention, stable retained austenite cannot be obtained sufficiently and the toughness is lowered. Moreover, both the comparative example (steel plate No. 28) in which the C content is outside the scope of the present invention and the comparative examples (steel plates No. 29, No. 30) in which the Ni content is outside the scope of the present invention have low temperature toughness. It is falling.

Claims (5)

% By mass
C: 0.02 to 0.13%, Si: 0.04 to 0.5%,
Mn: 0.2 to 1.0%, Ni: 7.0 to 10.0%,
Al: 0.005-0.10%
A steel material having a composition comprising the balance Fe and inevitable impurities, hot-rolled into a steel sheet, and then immediately quenched, followed by a quenching process at a rate of 5 ° C./s or more, (Ac ₁ transformation point + 15 ° C) above, (Ac ₁ transformation point +0.8 x (Ac ₃ transformation point-Ac ₁ transformation point)) A method for producing a Ni-containing steel sheet having excellent low temperature toughness and excellent strength-toughness balance. % By mass
C: 0.02 to 0.13%, Si: 0.04 to 0.5%,
Mn: 0.2 to 1.0%, Ni: 7.0 to 10.0%,
Al: 0.005-0.10%
The steel material containing the balance Fe and inevitable impurities is hot-rolled into a steel sheet, then air-cooled, and then reheated to a temperature in the range of Ac ₃ transformation point to 850 ° C. reheating quenching for quenching, then at 5 ° C. / s or more heating rate, (Ac ₁ transformation point + 15 ° C.) or higher, (Ac ₁ transformation point + 0.8 × (Ac ₃ transformation point -Ac ₁ transformation point )) A method for producing a Ni-containing steel sheet excellent in low temperature toughness and strength-toughness balance, characterized by sequentially performing heat treatment after heating to a temperature in the following range and cooling.   3. The method for producing a Ni-containing steel sheet according to claim 1, wherein the steel sheet is held for 10 s or less (including 0 s) after heating in the heat treatment.   In addition to the above composition, the composition further contains one or more selected from Mo: 0.5% or less, V: 0.1% or less, and Cr: 0.5% or less in mass%. The manufacturing method of the Ni containing steel plate in any one of 1-3.   The method for producing a Ni-containing steel sheet according to any one of claims 1 to 4, further comprising Ti: 0.03% or less by mass% in addition to the composition.