JP5928405B2 - Tempered steel sheet excellent in resistance to hydrogen-induced cracking and method for producing the same - Google Patents

Description

  The present invention relates to a tempered steel plate excellent in hydrogen-induced crack resistance and a method for producing the same. In particular, the present invention relates to a steel plate for a pressure vessel of oil or gas used in a wet hydrogen sulfide environment, a steel plate having a thickness of 10 to 60 mm suitable for a line pipe material for transporting petroleum or natural gas containing hydrogen sulfide, and a method for producing the same. .

  Steel pipes for oil and gas pressure vessels used in wet hydrogen sulfide environments, and line pipes that transport oil and natural gas containing hydrogen sulfide, in addition to strength, toughness and weldability, are also resistant to hydrogen-induced cracking (below) So-called sour resistance such as resistance to HIC and stress corrosion cracking resistance (hereinafter referred to as SCC resistance) is required. In hydrogen-induced cracking of steel, hydrogen ions due to corrosion reactions are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, and diffuse around non-metallic inclusions such as MnS in the steel and hard second-phase structures.・ Accumulated and cracked due to internal pressure.

  Several methods have been proposed to prevent such hydrogen-induced cracking. For example, in Patent Document 1, while lowering the S content in steel and adding an appropriate amount of Ca, REM, or the like, the formation of long extended MnS is suppressed, and a finely dispersed spherical CaS inclusion is formed. A technique for changing the above is disclosed. As a result, the stress concentration due to the sulfide inclusions is reduced, and the generation and propagation of cracks is suppressed, thereby improving the HIC resistance.

  Moreover, in patent document 2 and patent document 3, the reduction | decrease of the segregation by the reduction | decrease of the elements (C, Mn, P, etc.) with a high segregation tendency, the soaking process in a slab heating stage, and the transformation at the time of cooling after rolling are in progress. A technique for accelerating cooling is disclosed. This suppresses the generation of island martensite that becomes the starting point of cracks in the center segregation part and the generation of hardened structures such as martensite that becomes the propagation path of cracks.

  Patent Document 4 discloses a steel sheet in which Cu is added to form a protective film that suppresses hydrogen intrusion into steel on the steel surface.

  Recently, in Patent Document 5, Patent Document 6 and Patent Document 7, with respect to X80 grade high-strength steel sheet, while controlling the form of sulfide inclusions by addition of Ca at low S, low C-low Mn A method is disclosed in which center segregation is suppressed, and the accompanying strength reduction is compensated by addition of Cr, Mo, Ni, etc. and accelerated cooling.

Japanese Patent Laid-Open No. 54-110119 JP 61-60866 A JP-A-61-165207 JP-A-52-111815 JP-A-5-9575 JP-A-5-271766 JP 7-173536 A JP 2003-13138 A

  However, the above prior art has the following problems.

  As in the technique disclosed in Patent Document 1, by only controlling the form of sulfide inclusions, the occurrence of cracks due to oxide inclusions cannot be ignored with increasing strength.

  Even if a protective film that suppresses hydrogen intrusion to the steel surface is formed as in the technique disclosed in Patent Document 4, the effect cannot be expected in an environment having a low pH. For example, a NACE solution having a low pH does not provide a coating effect.

  The techniques disclosed in Patent Literature 2, Patent Literature 3, Patent Literature 5, Patent Literature 6, and Patent Literature 7 are all directed to the center segregation portion, but the portions other than the center segregation portion are considered. Absent. In high-strength steel sheets having strength of API standard X65 grade or higher manufactured by accelerated cooling or direct quenching, the steel plate surface portion with a high cooling rate is hardened compared to the inside, so that hydrogen-induced cracking occurs from the vicinity of the surface. There is a problem, and in order to prevent cracking from the surface, it is necessary to reduce the surface hardness.

  Conventionally, in order to reduce the surface hardness, the entire steel sheet is heated in a combustion gas atmosphere by a gas combustion furnace. Therefore, since it takes a long time to reach the target temperature, there is a problem that the strength is lowered not only to the steel plate surface layer portion but also to the steel plate center portion, and the specification of the steel material cannot be satisfied.

On the other hand, in the technique disclosed in Patent Document 8, only the surface layer portion is effectively heated by increasing the temperature rising rate of the steel sheet surface during tempering of the steel sheet to 10 ° C./sec or more, and the thickness average is obtained. However, there is a problem that it is necessary to heat using an induction heating device and cannot be reproduced by a normal heat treatment furnace.
The present invention solves the above problems and softens the surface layer of the steel sheet by tempering using a normal heat treatment furnace, but has reduced HIC resistance with reduced strength reduction at the center of the plate thickness and 1/4 part of the plate thickness. An object is to provide a tempered steel sheet having excellent yield strength: 415 MPa or more and tensile strength: 550 MPa or more and a method for producing the same.

  In order to solve the above-mentioned problems, the present inventors have earnestly studied the component composition and manufacturing method of a steel sheet, particularly accelerated cooling after rolling and a subsequent reheating process. As a result, the following knowledge was obtained.

  (A) The metal structure in the range from the steel sheet surface to 1 mm in the thickness direction is composed of one or two kinds selected from tempered martensite and tempered bainite.

  (B) The metal structure in the range of ± 1 mm from the thickness center to the thickness direction has a main phase composed of one or two kinds selected from tempered martensite and tempered bainite in an area fraction of 80% or more, The balance other than the main phase is at least one selected from ferrite, pearlite, cementite, and retained austenite.

  (C) Thickness where the hardness at the position of 1 mm from the steel sheet surface is 250 HV or less in terms of Vickers hardness, and the difference in hardness between the position of 1 mm from the steel sheet surface and the central part of the thickness is 60 HV or less in terms of Vickers hardness. Show directional hardness distribution.

The present invention has been made by further studying the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.5 to 2.0%, P: 0.050% or less, S: 0.050% or less, Al: 0.100% or less, carbon equivalent Ceq of 0.16 to 0.60, balance Fe and unavoidable impurities, range from steel plate surface to 1mm in thickness direction The metal structure of tempered martensite and tempered bainite consists of one or two kinds selected from tempered martensite and tempered bainite. The main phase consisting of one or two selected from the above is an area fraction of 80% or more, and the balance other than the main phase consists of one or more selected from ferrite, pearlite, cementite, retained austenite, From steel plate surface to plate The resistance to hydrogen-induced cracking is characterized in that the hardness at a position of 1 mm in the direction is Vickers hardness of 250 HV or less and the difference in hardness between the position of 1 mm from the steel sheet surface and the central part of the plate thickness is 60 HV or less in Vickers hardness. Excellent tempered steel sheet.

[2] Further, by mass%, Nb: 0.05% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V : The tempered steel sheet having excellent resistance to hydrogen-induced cracking as described in [1] above, containing at least one selected from 0.50% or less.

[3] Further, by mass%, it contains at least one selected from Ca: 0.0005 to 0.0050%, REM: 0.0050% or less, and Mg: 0.0050% or less. The tempered steel sheet having excellent resistance to hydrogen-induced cracking according to [1] or [2].

[4] After heating the steel material having the composition according to any one of [1] to [3] to a temperature range of 1050 to 1300 ° C. and hot rolling to a predetermined plate thickness to obtain a steel plate Subsequently, the steel plate was directly quenched from the Ar ₃ transformation point or higher, and after cooling was stopped in a temperature range of 400 ° C. or lower, the steel plate was heated in a heating furnace having an ambient temperature of 500 to 800 ° C., and then the heating furnace The steel plate is extracted from the heating furnace with the steel plate surface temperature immediately before extraction from the state where the steel plate surface temperature is 500 ° C. or higher and less than the Ac ₁ transformation point and the steel plate surface heating rate Vs satisfies the following formula (1). And a method for producing a tempered steel sheet excellent in resistance to hydrogen-induced cracking, characterized by air cooling.
Vs ≧ −0.0036 × t + 0.54 (1)
However, Vs: Temperature rising rate of steel plate surface (° C./min), t: Steel plate thickness (mm)

[5] After heating the steel material having the composition according to any one of [1] to [3] to a temperature range of 1050 to 1300 ° C. and hot rolling to a predetermined plate thickness to obtain a steel plate The steel plate is heated to a temperature not lower than the Ac ₃ transformation point, subsequently cooled from the Ar ₃ transformation point to a temperature of 400 ° C. or lower by water cooling, and further charged into a heating furnace having an atmospheric temperature of 500 to 800 ° C. And then the state of the steel plate immediately before extraction from the heating furnace is a state in which the steel plate surface temperature is 500 ° C. or higher and less than the Ac ₁ transformation point, and the steel plate surface heating rate Vs satisfies the following formula (1): A method for producing a tempered steel sheet having excellent resistance to hydrogen-induced cracking, wherein the steel sheet is extracted from a heating furnace and air-cooled.
Vs ≧ −0.0036 × t + 0.54 (1)
However, Vs: Temperature rising rate of steel plate surface (° C./min), t: Steel plate thickness (mm)

  According to the present invention, the reheat treatment (tempering treatment) after quenching can be performed by using a normal heat treatment furnace without using an induction heating apparatus, so that a tempered steel sheet having excellent resistance to hydrogen-induced cracking can be subjected to normal heat treatment. It can be manufactured using a furnace.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.05-0.30%
C is necessary to ensure the strength of the steel sheet, but if it is less than 0.05%, sufficient strength cannot be secured, and if it exceeds 0.30%, the strength becomes too high and coarse carbides are formed. Since it becomes easy, toughness and HIC resistance are deteriorated. Therefore, the C content is in the range of 0.05 to 0.30%. Preferably it is 0.06 to 0.16% of range.

Si: 0.05-0.60%
Si is added for deoxidation, but if it is less than 0.05%, the deoxidation effect is not sufficient, and if it exceeds 0.60%, island-shaped martensite is likely to be formed, so that toughness and weldability are deteriorated. . Therefore, the Si amount is in the range of 0.05 to 0.60%. Preferably it is 0.10 to 0.35% of range.

Mn: 0.5 to 2.0%
Mn is added to improve the strength and toughness of the steel, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.0%, the hardenability becomes too high, so that weldability and HIC resistance are increased. Deteriorates. Therefore, the Mn content is in the range of 0.5 to 2.0%. Preferably it is 1.0 to 1.7% of range.

P: 0.050% or less P is an inevitable impurity element, and deteriorates weldability and HIC resistance. This tendency becomes remarkable when it exceeds 0.050%. Therefore, the P content is 0.050% or less. Preferably it is 0.020% or less.

S: 0.0050% or less S is generally an MnS-based inclusion in steel, but its form is controlled from an MnS-based to CaS-based inclusion by addition of Ca. However, if the S content is large, the amount of CaS inclusions also increases, and a high-strength material can be a starting point for cracking. This tendency becomes remarkable when the S amount exceeds 0.0050%. Therefore, the S amount is 0.0050% or less. Preferably it is 0.0020% or less.

Al: 0.100% or less Al is added as a deoxidizer. In order to obtain this effect, addition of 0.010% or more is preferable. However, if it exceeds 0.100%, the HIC resistance is deteriorated due to a decrease in cleanliness. Therefore, the Al content is 0.100% or less. Preferably it is 0.050% or less.

Carbon equivalent Ceq: 0.16-0.60
If the carbon equivalent Ceq is less than 0.16, the hardenability is too low, the metal structure of the steel sheet is not desired, and the intended steel sheet strength cannot be ensured. On the other hand, if the carbon equivalent Ceq exceeds 0.60, the hardenability becomes too high, and the hardness at the 1 mm position from the surface after tempering does not become less than 250 HV in terms of Vickers hardness, and the HIC resistance decreases. For this reason, carbon equivalent Ceq shall be the range of 0.16-0.60. Preferably it is 0.30-0.50, More preferably, it is the range of 0.35-0.45.
The carbon equivalent Ceq is defined by the following formula.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
Here, the element symbol represents the content (% by mass) of each element.

  The above is the basic component of the present invention, but for the purpose of further improving the strength and toughness of the steel sheet, it contains one or more of Nb, Cu, Ni, Cr, Mo, V, Ca, REM and Mg shown below. May be.

Nb: 0.05% or less Nb suppresses grain growth during rolling or heating for quenching, and improves toughness by fine graining. In order to obtain this effect, 0.005% or more is preferably added. However, if the Nb amount exceeds 0.05%, the toughness of the weld heat affected zone deteriorates. Therefore, the Nb amount is 0.05% or less. Preferably it is 0.03% or less.

Cu: 0.50% or less Cu is an element effective for improving toughness and increasing strength. In order to obtain this effect, addition of 0.05% or more is preferable. However, if it exceeds 0.50%, cracks are likely to occur during slab heating, and weldability deteriorates. Therefore, when it contains Cu, the amount of Cu shall be 0.50% or less. Preferably it is 0.25% or less.

Ni: 0.50% or less Ni is an element effective for improving toughness and increasing strength. In order to obtain this effect, addition of 0.05% or more is preferable. However, if it exceeds 0.50%, stress corrosion cracking tends to occur. Therefore, when Ni is contained, the Ni content is 0.50% or less. Preferably it is 0.25% or less.

Cr: 0.50% or less Cr is an effective element for improving the hardenability and obtaining sufficient strength. In order to acquire this effect, 0.05% or more is preferable. However, if it exceeds 0.50%, weldability is deteriorated. Therefore, when Cr is contained, the Cr content is 0.50% or less. Preferably it is 0.40% or less.

Mo: 0.50% or less Mo is an element effective for improving toughness and increasing strength. In order to acquire this effect, 0.05% or more is preferable. However, if the content exceeds 0.50%, weldability and HIC resistance deteriorate. Therefore, when Mo is contained, the Mo amount is 0.50% or less. Preferably it is 0.35% or less.

V: 0.50% or less V is an element that increases strength without deteriorating toughness, weldability, and sour resistance. In order to acquire this effect, 0.01% or more is preferable. However, when it exceeds 0.50%, weldability is remarkably impaired. Therefore, when V is contained, the V amount is 0.50% or less. Preferably it is 0.10% or less.

Ca: 0.0005 to 0.0050%
Ca is an element indispensable for controlling the form of sulfide inclusions, but if it is less than 0.0005%, there is no effect, and if it exceeds 0.0050%, the effect is saturated, and rather the cleanliness decreases. To deteriorate the HIC resistance. Therefore, when it contains Ca, it is preferable to make Ca amount into the range of 0.0005 to 0.0050%. More preferably, it is the range of 0.0010-0.0030.

REM: 0.0050% or less, Mg: 0.0050% or less Both REM and Mg contribute to improvement of the toughness and ductility of the base material through the control of the form of sulfide. In order to acquire this effect, 0.0005% or more is preferable. However, if it exceeds 0.0050%, excessive inclusions may be generated, and the toughness may be reduced. Therefore, when REM and Mg are contained, the REM amount and the Mg amount are both 0.0050%. The following. Preferably it is 0.0030 or less.

  The balance other than the above components of the steel of the present invention is Fe and inevitable impurities. However, the content of elements other than those described above is not rejected as long as the effects of the present invention are not impaired.

2. About metallographic structure Metallographic structure in the range from the steel sheet surface to the thickness direction of 1 mm: One or two types selected from tempered martensite and tempered bainite. The desired strength cannot be obtained unless the structure is tempered martensite, tempered bainite, or a mixed structure of tempered martensite and tempered bainite. In addition, tempered martensite and tempered bainite are structures in which fine cementite is dispersed in ferrite with a high dislocation density, relatively high strength, and lath structure. Fewer two phases and excellent HIC resistance. Also, the toughness is excellent. For this reason, the metal structure in the range from the steel sheet surface to 1 mm in the sheet thickness direction is a structure composed of one or two kinds selected from tempered martensite and tempered bainite.

Metal structure in the range of ± 1mm in the thickness direction from the center of the plate thickness: One or two main phases selected from tempered martensite and tempered bainite are 80% or more in area fraction. During slab casting, impurities such as P and S are concentrated due to central segregation, and inclusions are likely to occur, and since C and Mn are also concentrated, a hard second phase is likely to occur and hydrogen-induced cracking is likely to occur. Position. However, since the cooling rate at the center of the plate thickness during quenching is slower than the surface of the steel plate, the hardness at the center of the plate thickness is lower than that near the surface. For this reason, it is not necessary for the central portion of the plate thickness to be composed only of tempered martensite, tempered bainite, or a mixed structure of tempered martensite and tempered bainite, which has excellent HIC resistance. It is necessary to. Here, the main phase means that the area ratio is 80% or more. The balance other than the main phase is at least one selected from ferrite, pearlite, cementite, and retained austenite. If the main phase is less than 80% in terms of the area ratio in the central portion of the plate thickness, soft ferrite increases, and the desired steel plate strength cannot be obtained. For this reason, the structure of the central portion of the plate thickness is such that the main phase has an area ratio of 80% or more. From the viewpoint of HIC resistance, the ratio of tempered martensite and / or tempered bainite having excellent HIC resistance is preferably high, and the area ratio of the main phase is preferably 90% or more.

3. About hardness The average hardness at a position of 1 mm from the steel sheet surface in the thickness direction: Vickers hardness of 250 HV or less The hardness in the vicinity of the steel sheet surface is an important factor for determining the HIC resistance. The higher the hardness, the lower the HIC resistance. If the average hardness at a position of 1 mm in the thickness direction from the steel sheet surface exceeds 250 HV, cracks occur in the HIC test. Preferably it is 240HV or less. In addition, if the hardness in the vicinity of the surface is increased, the bending workability of the steel material is degraded.

Hardness difference between the position 1 mm from the steel plate surface and the center of the plate thickness: 60 HV or less If the hardness difference between the position 1 mm from the surface of the steel plate and the center of the plate thickness exceeds 60 HV in terms of Vickers hardness, the hardness of the plate thickness center Is too low to obtain the desired strength. For this reason, the hardness difference between the position 1 mm from the surface of the steel sheet and the central part of the thickness is set to 60 HV or less in terms of Vickers hardness. Preferably it is 50HV or less.

4). Manufacturing conditions It is preferable that the steel having the above-described composition is melted by a melting means such as a converter or an electric furnace to form a steel material such as a slab by a continuous casting method or an ingot-bundling method. The melting method and the casting method are not limited to the methods described above. Then, it rolls to a desired shape and performs cooling and heating after rolling. The temperature of the steel material (steel material (slab) or steel plate) means the surface temperature of the steel material unless otherwise specified.

Heating temperature of steel material: 1050-1300 ° C
When the heating temperature is less than 1050 ° C., coarse carbides present in the steel material (slab) are not completely dissolved, so that the strength of the obtained steel sheet is likely to decrease. Moreover, since the slab temperature is low, the deformation resistance of the steel material is high, and the rolling efficiency also decreases. On the other hand, when the heating temperature of the steel material exceeds 1300 ° C., the toughness of the steel sheet obtained by coarsening the structure decreases, and the hardenability increases too much, and the surface hardness of the obtained steel sheet tends to increase. Become. For this reason, the heating temperature of the steel material was limited to the range of 1050 ° C to 1300 ° C.

Direct quenching (quenching starting temperature: Ar ₃ transformation point or more, cooling stop temperature: 400 ° C. or less)
After hot rolling is completed, in order to secure the base material strength and base material toughness, and to obtain a structure mainly composed of martensite, bainite, or a mixed structure of martensite and bainite, the Ar ₃ transformation From the temperature above the point, it is necessary to quench the steel sheet by forced cooling. When the quenching start temperature is lower than the Ar ₃ transformation point, a part of austenite is transformed into ferrite and the desired strength and structure cannot be obtained. Therefore, the quenching start temperature and Ar ₃ transformation point or more. The reason for direct quenching until reaching 400 ° C. or less is to complete the transformation from austenite to martensite or bainite to maintain the strength of the base material, and to obtain a desired structure excellent in HIC resistance.

Steel plate surface temperature during tempering: 500 ° C. or higher and less than Ac ₁ transformation point When the maximum temperature reached on the steel plate surface during tempering is less than 500 ° C., the heating temperature is too low, so that tempering of martensite or bainite caused by direct quenching is sufficient. Therefore, the recovery of the reduced toughness is insufficient. In addition, the hardness in the vicinity of the surface cannot be reduced below the target, and the HIC resistance is not sufficient. On the other hand, when the maximum temperature of the steel sheet surface becomes more Ac ₁ transformation temperature, ferrite by reverse transformation occurs, such as toughness and strength of the surface layer portion significantly decreases. For this reason, the maximum temperature reached on the steel sheet surface during tempering is set to a range of 500 ° C. or higher and lower than the Ac ₁ transformation point.

Temperature rising rate Vs of steel plate surface temperature during tempering: Vs ≧ −0.0036 × t + 0.54
Generally, in tempering, in order to make the whole steel plate a uniform structure, the steel plate is extracted from the heating furnace after the temperature of the whole steel plate is as uniform as possible. However, in the present invention, tempering is terminated in a state where there is a temperature difference between the surface and the inside of the steel sheet, and the steel sheet is extracted from the heating furnace. This is because the steel sheet excellent in HIC resistance intended by the present invention needs to suppress the hardness in the vicinity of the surface in order to ensure the HIC resistance, and tempering in the vicinity of the surface needs to be advanced. This is because it is necessary to suppress tempering inside the steel sheet in order to ensure the strength of the steel sheet.

  As a representative position inside the steel plate, a thickness of 1/4 is considered. When the temperature of the steel plate surface and the thickness ¼ was calculated by heat conduction analysis, it was found that the temperature difference between the steel plate surface and the thickness ¼ became smaller as the heating rate Vs on the steel plate surface became smaller. Further, in the state where the temperature increase rate Vs on the steel sheet surface satisfies [Vs ≧ −0.0036 × t + 0.54], it is satisfied that “plate thickness surface layer temperature−plate thickness 1/4 temperature ≧ 5 ° C.”. all right. Where Vs is the rate of temperature rise (° C./min) on the surface of the steel sheet, and t is the thickness (mm) of the steel sheet. In such a state, the steel sheet surface layer and the plate thickness ¼ can be tempered with a temperature difference of 5 ° C. or more. Naturally, a larger temperature difference is produced inside the steel plate (for example, plate thickness 1/2) than the plate thickness 1/4.

When performing tempering with a temperature difference, compared to normal tempering, the steel plate surface layer can be tempered while suppressing the decrease in strength inside the steel plate, so the steel plate surface layer is softened while maintaining the steel plate strength. it can. If the temperature difference between the plate thickness surface layer and the plate thickness ¼ is 5 ° C. or more, the target surface hardness and the target steel plate strength can be ensured. For this reason, the state of the steel plate immediately before extracting the steel plate from the heating furnace is such that the steel plate surface temperature is 500 ° C. or higher and less than the Ac ₁ transformation point, and the temperature rising rate Vs of the steel plate surface is Vs ≧ −0.0036 × t + 0.54. As a satisfactory condition, the steel plate is extracted from the heating furnace.

  The steel sheet extracted from the heating furnace is air-cooled. The reason why the steel sheet extracted from the heating furnace is air-cooled is that when the tempered steel sheet is water-cooled, a temperature difference is generated between the surface and the inside of the steel sheet and the steel sheet may be distorted. For example, in addition to normal air cooling, mist cooling by spraying water or cooling forcibly blowing air with a fan can be applied. In addition, although the upper limit of the temperature increase rate Vs on the steel plate surface immediately before extracting the steel plate is not particularly defined, when the temperature increase rate Vs on the steel plate surface is too large, the temperature difference between the steel sheet surface layer and the plate thickness ¼ is large. The inside of the steel sheet is not sufficiently tempered, and as-quenched martensite or bainite may remain and the toughness and HIC resistance may not be excellent. Therefore, the steel sheet is kept until Vs ≦ −0.0036 × t + 2.18 is satisfied. It is preferable to hold in a heating furnace.

Reheating and quenching (reheating temperature: Ac ₃ transformation point or higher, quenching start temperature: Ar ₃ transformation point or higher, cooling stop temperature: 400 ° C. or lower)
Instead of direct quenching after hot rolling, reheating quenching can also be performed. After the hot rolling, the steel sheet is reheated to the Ac ₃ transformation point or higher, water cooling is started from the temperature of the Ar ₃ transformation point or higher, and the water cooling is performed to a temperature of 400 ° C. or lower. Reheating temperature is untransformed of ferrite remaining to be Ac less than ₃ transformation point, a desired strength, the tissue can not be obtained. Therefore, the reheating temperature is set to Ac ₃ transformation point or more. Further, when the water cooling start temperature is lower than the Ar ₃ transformation point, a part of austenite is transformed to ferrite before the start of water cooling, and a desired strength and structure cannot be obtained. Therefore, the quenching start temperature and Ar ₃ transformation point or more. The reason for water cooling until reaching 400 ° C. or lower is to complete the transformation from austenite to martensite or bainite to maintain the strength of the base material, and to obtain a desired structure having excellent HIC resistance. After the reheating and quenching is completed, the steel sheet is tempered. The tempering conditions are the same as the tempering performed after the direct quenching already described.

As the Ar ₃ transformation point, Ac ₁ transformation point, and Ac ₃ transformation point, values calculated from the following equations are used.

Ar ₃ transformation point (° C) = 900-332C + 6Si-77Mn-20Cu-50Ni-18Cr-68Mo
Ac ₁ transformation point (℃) = 750.8-26.6C + 17.6Si-11.6Mn-22.9Cu-23Ni + 24.1Cr + 22.5Mo
−39.7V−5.7Ti + 232.4Nb−169.4Al−894.7B
Ac ₃ transformation point (℃) = 937-476.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr + 38.1Mo
+ 124.8V + 136.3Ti-19.1Nb + 198.4Al
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al: content of each element (mass%))

A steel material (slab) having a steel composition shown in Table 1 was cast by a converter-ladder refining-continuous casting method to obtain a slab having a thickness of 250 mm. Here, steel types A to D are invention steels, and steel type E is a comparative steel. These slabs were hot-rolled and subjected to direct quenching (DQ) -tempering treatment or reheating quenching (RQ) -tempering treatment. Table 2 shows the manufacturing conditions.
The obtained steel sheet was subjected to structure observation, hardness measurement, tensile property evaluation, base material toughness measurement, and HIC resistance test in the following manner. The obtained results are shown in Table 3.

[Tissue observation]
As for the structure of the steel sheet, a sample with a cross section perpendicular to the rolling direction was taken, the cross section was polished to a mirror surface, then corroded with a methanolic nitric acid solution, and from the steel sheet surface to 1.0 mm in the thickness direction, and from the center of the thickness A range of ± 1 mm in the thickness direction was photographed at a magnification of 400 with an optical microscope, and the range was photographed with a plurality of continuous screens. Phases in the range were identified from the photograph, and the area fraction of each phase was determined. The structures of tempered martensite and tempered bainite are both structures in which fine carbides are dispersed between the laths, and these cannot be distinguished. Therefore, tempered martensite and tempered bainite have the same phase. In the steel sheet before tempering, it is possible to distinguish martensite and bainite by the presence or absence of cementite in the crystal grains.

[Hardness measurement]
As for the structure of the steel sheet, a sample having a cross section perpendicular to the rolling direction was collected, and after the cross section was polished to a mirror surface, the Vickers hardness was measured in accordance with JIS Z2244. The load of the Vickers indenter was 10 kg. Five points were measured at a position 1 mm from the surface and a position where the plate thickness was 1/2, and the Vickers hardness at 5 points was averaged to obtain a hardness at a position 1 mm from the surface and a position where the plate thickness was 1/2.

[Tensile properties]
In accordance with JIS Z2241, a No. 5 tensile test piece having a full thickness was taken from the steel sheet with the direction perpendicular to the rolling direction as the test piece longitudinal direction, and a tensile test was performed.

[Base material toughness]
V-notch test specimens were collected from the direction perpendicular to the rolling direction at the plate thickness 1/2 position of each steel plate in accordance with JIS Z 2202 (1998), and conformed to JIS Z 2242 (1998). Then, each steel plate was subjected to a Charpy impact test at three temperatures, the absorbed energy at a test temperature of 0 ° C. was determined, and the base material toughness was evaluated. The average value of the three absorbed energy at the test temperature of 0 ° C. (sometimes referred to as vE ₀ ) was 50 J or more, and the base material toughness was excellent.

[HIC resistance test]
The HIC resistance is obtained by placing a steel sample (width 30 mm x length 60 mm x plate thickness) in a 5% NaCl + 0.5% CH ₃ COOH aqueous solution (normal NACE solution) saturated with hydrogen sulfide having a pH of about 3 for 96 hours. Immerse, then cut the sample parallel to the length direction at the center of the width, polish the plate thickness cross section (cross section parallel to the rolling direction and perpendicular to the width direction) to the mirror surface, observe the cross section at 20 times, crack It was evaluated by the HIC test of investigating the presence or absence. Those in which no cracks were observed were accepted, and those in which even one crack was observed were rejected.

  The results are shown in Table 3. Here, no. 1-5, no. No. 10 is an invention example. 6-9, no. 11 is a comparative example. Each of the inventive examples has a yield strength of 415 MPa or more, a tensile strength of 550 MPa or more, an average hardness of a surface layer portion of 1 mm to 5 mm from the steel plate surface is 250 HV or less, and a hardness of 1 mm from the surface and the center of the plate thickness. The hardness distribution in the plate thickness direction with a difference of 60 HV or less was shown, and the HIC resistance was also good.

Claims (5)

In mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.5 to 2.0%, P: 0.050% or less, S: 0.050 % Or less, Al: 0.100% or less, Ti: 0.007 to 0.012% , the carbon equivalent Ceq is 0.30 to 0.50 , and the balance is Fe and unavoidable impurities. The metal structure in the range up to 1 mm in the sheet thickness direction is composed of one or two kinds selected from tempered martensite and tempered bainite, and the metal structure in the range of ± 1 mm in the sheet thickness direction from the center of the sheet thickness, One or two main phases selected from tempered martensite and tempered bainite are 80% or more in area fraction, and the remainder other than the main phase is selected from ferrite, pearlite, cementite, and retained austenite. From one or more Furthermore, the hardness at the position of 1 mm from the surface of the steel sheet in the thickness direction is 250 HV or less in terms of Vickers hardness, and the hardness difference between the position of 1 mm from the surface of the steel sheet and the central portion of the thickness is 60 HV or less in terms of Vickers hardness. Tempered steel sheet with excellent resistance to hydrogen-induced cracking.   Further, in terms of mass%, Nb: 0.05% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.00. The tempered steel sheet having excellent resistance to hydrogen-induced cracking according to claim 1, comprising at least one selected from 50% or less.   Furthermore, by mass%, it contains at least one selected from Ca: 0.0005 to 0.0050%, REM: 0.0050% or less, and Mg: 0.0050% or less. 1. A tempered steel sheet having excellent resistance to hydrogen-induced cracking according to 1 or 2. A method for producing a tempered steel sheet according to any one of claims 1 to 3,
A steel material having the component composition according to any one of claims 1 to 3 is heated to a temperature range of 1050 to 1300 ° C and hot-rolled to a predetermined plate thickness to obtain a steel plate, and subsequently the steel plate is subjected to Ar ₃ After quenching directly from the transformation point and above, after stopping cooling in a temperature range of 400 ° C. or less, the steel plate is charged in a heating furnace having an ambient temperature of 500 to 800 ° C., and then immediately before extraction from the heating furnace. The steel sheet surface temperature is 500 ° C. or more and less than the Ac ₁ transformation point, and the steel sheet surface temperature rise rate Vs satisfies the following formula (1), and the steel sheet is extracted from the heating furnace and air-cooled. A method for producing a tempered steel sheet having excellent hydrogen-induced cracking resistance.
Vs ≧ −0.0036 × t + 0.54 (1)
However, Vs: Temperature rising rate of steel plate surface (° C./min), t: Steel plate thickness (mm) A method for producing a tempered steel sheet according to any one of claims 1 to 3,
A steel material having the composition according to any one of claims 1 to 3 is heated to a temperature range of 1050 to 1300 ° C and hot-rolled to a predetermined plate thickness to obtain a steel plate, and then the steel plate is subjected to Ac ₃ transformation. Is heated to a temperature equal to or higher than the point, subsequently cooled to a temperature of 400 ° C. or lower by water cooling from the Ar ₃ transformation point or higher, further charged in a heating furnace having an atmospheric temperature of 500 to 800 ° C., and then heated. The state of the steel plate immediately before extraction from the heating furnace is defined as a state in which the steel plate surface temperature is 500 ° C. or higher and less than the Ac ₁ transformation point, and the steel plate surface heating rate Vs satisfies the following formula (1). A method for producing a tempered steel sheet excellent in hydrogen-induced cracking resistance, characterized by extracting air and air cooling.
Vs ≧ −0.0036 × t + 0.54 (1)
However, Vs: Temperature rising rate of steel plate surface (° C./min), t: Steel plate thickness (mm)