KR102192969B1 - High-strength ultra-thick steel sheet with excellent brittle crack propagation stopping properties and its manufacturing method - Google Patents

Abstract

It is an object of the present invention to provide a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties and a method of manufacturing the same when the sheet thickness is 70 mm or more. By mass %, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.01% or less, S: 0.005% or less, Ti: 0.005 to 0.03%, Al: 0.005 to 0.080% And N: 0.0050% or less, Ceq defined in the following formula (1) is 0.36 or more and 0.40 or less, and the remainder is a component composition consisting of Fe and inevitable impurities, and ( 211) A surface integration degree of 1.2 or more, a (200) surface integration degree of 1.7 or more on the rolled surface on the steel plate surface, and a Charpy fracture surface transition temperature (vTrs) at a position of 1/4 of the plate thickness is -40 A high-strength ultra-thick steel sheet having an excellent brittle crack propagation stopping property of 70 mm or more with a plate thickness of 70 mm or less, and having a Charpy fracture surface transition temperature (vTrs) on the surface of the steel sheet of -80°C or less.

Description

High-strength ultra-thick steel sheet with excellent brittle crack propagation stopping properties and its manufacturing method

The present invention relates to a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties having a thickness of 70 mm or more, which is used for large structures such as ships, offshore structures, low-temperature storage tanks, and construction/civil structures, and a manufacturing method thereof.

In large structures such as ships, offshore structures, low-temperature storage tanks, and construction/civil structures, when an accident accompanying brittle destruction occurs, the impact on the social economy and the environment is large. For this reason, the improvement in safety is always required for the large-sized structure, and the steel sheet used as a material for the large-sized structure is required to have a high level of brittle crack propagation stopping properties at use temperature.

In ships such as container ships and bulk carriers, a high-strength thick steel plate is used for the hull shell plate due to its structure. In recent years, with an increase in the size of the hull, a further increase in strength is required, and thickening of the thick steel plate used as a material is in progress.

In general, the brittle crack propagation stopping property of a steel sheet tends to deteriorate as the strength or thickness increases. For this reason, demands for brittle crack propagation stopping properties for thick steel plates used for large structures are also becoming more sophisticated.

Here, as a means for improving the brittle crack propagation stopping property of a steel sheet, a method of increasing the Ni content in steel has been known. For example, in a reservoir tank of liquefied natural gas (LNG), 9% Ni steel is used on a commercial scale.

However, an increase in the Ni content in the steel inevitably causes a significant increase in manufacturing cost. For this reason, it is difficult to apply 9% Ni steel to applications other than the LNG reservoir tank.

On the other hand, for a relatively thin steel plate having a plate thickness of less than 50 mm, which does not reach cryogenic temperatures such as LNG, for example, used for ships or line pipes, fine graining is achieved by TMCP method and low temperature toughness is improved, Excellent brittle crack propagation stopping properties can be realized.

In addition, in order to improve the brittle crack propagation stopping property without increasing the alloy cost, a steel sheet in which the structure of the surface layer portion is ultrafine is proposed in Patent Document 1.

The steel sheet having excellent brittle crack propagation stopping properties described in Patent Document 1 was completed by focusing on the fact that when the brittle crack propagates, the shear lip (plastic deformation region) generated in the surface layer of the steel sheet is effective in improving the brittle crack propagation stopping property. It is characterized by absorbing the propagation energy of the brittle crack that propagates by miniaturizing the crystal grains of the shear lip portion. In addition, in Patent Document 1, after cooling the surface layer to the Ar ₃ transformation point or less by controlled cooling after hot rolling, the process of stopping the controlled cooling and reheating the surface layer to the Ar ₃ transformation point or more is repeated once or more. During the implementation, it is described that by applying a reduction to the steel sheet to cause repeated transformation, or to generate an ultra-fine ferrite structure or bainite structure in the surface layer portion by processing recrystallization.

In Patent Document 2, in a steel sheet having a microstructure mainly composed of ferrite-pearlite, in order to improve the brittle crack propagation stopping property, both surface portions of the steel sheet have a circle equivalent particle diameter: 5 µm or less, and an aspect ratio. : It is important to suppress variation in ferrite grain size while constituting a layer having a ferrite structure having two or more ferrite grains in an area ratio of 50% or more, and as a method of suppressing this variation, the maximum reduction rate per pass during finish rolling It is described that the local recrystallization phenomenon is suppressed by making it 12% or less.

Patent Literature 3 discloses a technique on the extension of TMCP that improves the brittle crack propagation stop property by focusing not only on the refinement of ferrite grains but also on the subgrains formed in the ferrite grains. Specifically, in a steel plate having a plate thickness: 30 to 40 mm, complicated temperature control such as cooling and reheating of the surface layer of the steel plate is not required,

(a) rolling conditions to ensure fine ferrite grains,

(b) rolling conditions for generating a fine ferrite structure in a portion of 5% or more of the steel plate thickness,

(c) a rolling condition in which a subgrain is formed by rearranging the electric potential introduced by processing (rolling) in the fine ferrite while developing an aggregated structure by thermal energy, and

(d) A technique for improving the brittle crack propagation stop characteristic by cooling conditions for suppressing coarsening of the formed fine ferrite grains and fine sub grain grains is described.

Further, in controlled rolling, a method of improving the brittle crack propagation stopping characteristic by applying a reduction in the transformed ferrite to develop an aggregate structure is also known. This is a method of increasing the resistance against brittle fracture by generating a separation on the fracture surface of the steel sheet in a direction parallel to the sheet surface to relieve the stress at the tip of the brittle crack.

For example, in Patent Document 4, the (110) plane X-ray intensity ratio is set to 2 or more by controlled rolling, and the area ratio of coarse grains having a circle equivalent diameter of 20 µm or more is set to 10% or less. It is described to improve.

Patent Document 5 discloses a welded structural steel having excellent brittle crack propagation stopping properties at a joint, wherein the X-ray surface strength ratio of the (100) surface in the rolled surface within the thickness of the joint is 1.5 or more. , It is described that the brittle crack propagation stop characteristic is excellent due to the deviation of the angle of the stress load direction and the crack propagation direction due to the development of the texture.

In addition, in Patent Document 6, the brittle crack propagation stop characteristic that develops an aggregate structure in each portion in the plate thickness direction (the position of 1/4 plate thickness, the center portion of the plate thickness, etc.) by defining the average reduction rate in controlled rolling A method for producing an excellent welded structural steel sheet is described.

In addition, in a large container ship exceeding 6,000 TEU in recent years, a thick steel plate having a plate thickness of 70 mm or more is used. In Non-Patent Literature 1, the brittle crack propagation stop characteristic of a steel plate having a plate thickness of 65 mm was evaluated, and the result that the brittle crack did not stop in the large brittle crack propagation stop test of the base metal is reported.

Japanese Patent Publication No. Hei 7-100814 Japanese Unexamined Patent Publication No. 2002-256375 Japanese Patent Publication No. 3467767 Japanese Patent Publication No. 3548349 Japanese Patent Publication No. 2659661 Japanese Patent Publication No. 5733425

Long brittle crack propagation behavior in thick shipbuilding steels, Japanese Society of Marine Engineers Lecture No. 3, 2006, pp359-362

In the techniques described in Patent Documents 1 and 2, a specific structure is obtained by reheating after cooling only the surface layer of the steel sheet once, and further processing during reheating. For this reason, it is not easy to control on an actual production scale, and in particular, in the production of a thick material having a plate thickness of 70 mm or more, it is a process with a large load on the rolling facility and the cooling facility.

In addition, all of the steel sheets described in Patent Documents 1 to 6 are based on manufacturing conditions and disclosed experimental data, and the thickness of the sheet: about 50 mm to 70 mm is the main target, and for application to a thick material of 70 mm or more, predetermined It is not clear whether or not the characteristics of are obtained, and the crack propagation characteristics in the plate thickness direction required for large structures have not been verified at all.

In addition, in Non-Patent Literature 1, the ESSO test of a specimen has shown the result that the value of Kca at a use temperature of -10°C is less than 3000 N/mm ^1.5 , and that of Non-Patent Literature 1 In the technology, it is suggested that in the case of a large structure to which a steel plate having a plate thickness exceeding 50 mm is applied, it cannot be said that safety is sufficiently secured.

In view of such circumstances, an object of the present invention is to provide a high-strength ultra-thick steel sheet excellent in brittle crack propagation stopping properties when the sheet thickness is 70 mm or more, and a manufacturing method thereof.

In order to solve the above problems, the inventors have repeatedly studied a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties even with a sheet thickness of 70 mm or more, and a manufacturing method for stably obtaining the steel sheet. As a result, the (211) plane integration degree on the rolled surface in the center of the plate thickness is set to 1.2 or more, and the (200) plane on the rolled surface in the steel plate surface (simply referred to as "surface") It has an aggregated structure with a degree of integration of 1.7 or more, and the Charpy wavefront transition temperature (vTrs) at the position of 1/4 of the plate thickness, which is an index of toughness, is -40°C or less, and the Charpy wavefront transition temperature (vTrs) at the steel plate surface It was found that an ultra-thick steel sheet of -80°C or less has very excellent brittle crack propagation stopping properties.

The present invention has been completed by further examining the above findings. The summary structure of the present invention is as follows.

[1] In mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.01% or less, S: 0.005% or less, Ti: 0.005 to 0.03%, Al: 0.005 -0.080% and N: 0.0050% or less, Ceq defined in the following formula (1) is 0.36 or more and 0.40 or less, and the balance consists of Fe and unavoidable impurities, and the rolled surface in the center of the plate thickness The (211) plane integration degree at is 1.2 or more, and the (200) plane integration degree at the rolled surface at the steel sheet surface has an aggregate structure of 1.7 or more, and the Charpy fracture surface transition temperature (vTrs) at the position of 1/4 thickness of the plate A high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties of 70 mm or more in a plate thickness of -40°C or less and a Charpy fracture surface transition temperature (vTrs) on the steel sheet surface (vTrs) of -80°C or less.

Ceq = C + Mn/6 + Cu/15 + Ni/15 + Cr/5 + Mo/5 + V/5...(1)

Here, C, Mn, Cu, Ni, Cr, Mo, and V in Formula (1) mean the content (mass%) of each element, and when it does not contain it, it is set as 0.

[2] The above component composition further contains one or two or more of Nb: 0.005 to 0.05%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.5%, and Cr: 0.01 to 0.5% in mass% High-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties according to [1] below.

[3] The above component composition is further by mass%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.0100% or less of 1 type or 2 High-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties according to [1] or [2] containing more than a species.

[4] After heating the steel material having the component composition according to any one of [1] to [3] to a temperature of 1000 to 1200°C, the temperature at the center of the plate thickness is the cumulative reduction ratio in the austenite recrystallization temperature range When the temperature at the center of the plate thickness is at least 50% in the austenite non-recrystallization temperature range, the surface temperature of the plate thickness is below the Ar ₃ transformation point, and the temperature at the center of the sheet thickness is above the Ar ₃ transformation point. A high-strength ultra-thick steel sheet with excellent brittle crack propagation stopping properties of 70 mm or more thick, which is cooled down to a cooling stop temperature of 500 °C or less at a cooling rate of 0.5 °C/s or higher after hot rolling under conditions of a cumulative reduction ratio of more than 20%. Manufacturing method.

[5] A method for producing a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping characteristics according to [4], in which the temperature at the center of the plate thickness is tempered to a temperature less than the Ac ₁ transformation point after cooling to a cooling stop temperature of 500°C or less.

According to the present invention, since the texture in the plate thickness direction is appropriately controlled, even an extremely thick steel plate having a plate thickness of 70 mm or more has excellent brittle crack propagation stopping properties, excellent toughness, and high strength. Further, according to the present invention, by optimizing the rolling conditions, it is possible to stably manufacture a high-strength ultra-thick steel sheet in an industrially very simple process. For example, by applying the high-strength ultra-thick steel plate of the present invention to a deck member that is joined to the hatch side coaming in the strong deck structure of container ships and bulk carriers in the shipbuilding field, it contributes to the improvement of the safety of the ship and is very industrial. useful.

Hereinafter, an embodiment of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Component composition>

Hereinafter, each component is demonstrated. In addition, "%" which shows the content of a component means "mass%".

C: 0.03 to 0.20%

C is an element that improves the strength of steel. In the present invention, in order to ensure the desired strength, the C content is set to 0.03% or more. Moreover, when the C content exceeds 0.20%, not only the weldability is deteriorated, but also toughness is adversely affected. For this reason, the C content is set in the range of 0.03 to 0.20%. In addition, the lower limit is preferably 0.05% or more. The upper limit is preferably 0.15% or less.

Si: 0.03 to 0.5%

Si is effective as a deoxidation element and as a strengthening element of steel. When the Si content is less than 0.03%, these effects cannot be obtained. On the other hand, when the Si content exceeds 0.5%, not only the surface properties of the steel are impaired, but the toughness is extremely deteriorated. Therefore, the Si content is in the range of 0.03 to 0.5%.

Mn: 0.5 to 2.2%

Mn is contained as a strengthening element. If the Mn content is less than 0.5%, the effect is not sufficient. On the other hand, if it exceeds 2.2%, weldability deteriorates, and the cost of a steel sheet also rises. Therefore, the Mn content is in the range of 0.5 to 2.2%.

P: 0.01% or less, S: 0.005% or less

P and S are unavoidable impurities in steel. When the content of these increases, the toughness deteriorates. In a steel sheet having a plate thickness of 70 mm or more, in order to maintain good toughness, the P content is suppressed to 0.01% or less and the S content is suppressed to 0.005% or less. Moreover, 0.006% or less of P content, and 0.003% or less of S content are more preferable ranges.

Ti: 0.005 to 0.03%

Ti has an effect of forming nitride, carbide, or carbonitride by containing a trace amount, miniaturizing crystal grains, and improving the toughness of the base metal. The effect is obtained by making the Ti content 0.005% or more. On the other hand, when the Ti content exceeds 0.03%, the toughness of the base metal and the welding heat affected zone decreases. Therefore, the Ti content is in the range of 0.005 to 0.03%.

Al: 0.005 to 0.080%

Al acts as a deoxidizing agent. In order to use Al as a deoxidizing agent, it is necessary to make the Al content 0.005% or more. Moreover, when the Al content exceeds 0.080%, the toughness decreases, and the toughness of the weld metal portion decreases when welding is performed. For this reason, the Al content is set in the range of 0.005 to 0.080%. In addition, the lower limit is preferably 0.020% or more. The upper limit is preferably 0.060% or less.

N: 0.0050% or less

N binds with Al in the steel, adjusts the crystal grain size during rolling, and strengthens the steel. In order to obtain this effect, it is preferable to make the N content 0.0010% or more. On the other hand, when the N content exceeds 0.0050%, toughness deteriorates. In the present invention, the N content is set to 0.0050% or less.

The above is the basic composition of the present invention, and the balance is Fe and unavoidable impurities.

In the present invention, in order to further improve the properties, it is possible to contain one or two or more of Nb, Cu, Ni, and Cr in addition to the above component composition.

Nb: 0.005 to 0.05%

Nb, as NbC, precipitates during ferrite transformation or reheating, and contributes to increase in strength. Further, Nb has an effect of expanding the non-recrystallized region in rolling in the austenite region, and contributes to fine graining of ferrite. For this reason, Nb content is also effective in improving toughness. The effect is exhibited by making the Nb content 0.005% or more. When the Nb content exceeds 0.05%, coarse NbC may precipitate, resulting in a decrease in toughness. Therefore, when it contains Nb, it is preferable to make Nb content into 0.005 to 0.05%.

Cu: 0.05 to 1.0%

Cu is an element that enhances the hardness of steel. This element can be contained in order to directly contribute to the improvement of the strength after rolling and to improve functions such as toughness, high temperature strength, or weather resistance. These effects are exhibited by making the Cu content 0.05% or more. On the other hand, excessive Cu content deteriorates toughness and weldability. In a steel sheet having a thickness of 70 mm or more, the Cu content is preferably 0.05 to 1.0% as a range in which toughness and weldability are not deteriorated while maintaining sufficient strength.

Ni: 0.05 to 1.5%

Ni is an element that improves the hardness of steel. Ni directly contributes to the improvement of the strength after rolling, and may be contained in order to improve functions such as toughness, high temperature strength, or weather resistance. These effects are exhibited by making the Ni content 0.05% or more. On the other hand, excessive Ni content deteriorates toughness and weldability. In a steel sheet having a thickness of 70 mm or more, the Ni content is preferably 0.05 to 1.5% as a range in which toughness and weldability are not deteriorated while maintaining sufficient strength.

Cr: 0.01 to 0.5%

Cr is an element that enhances the stiffness of steel. This element may directly contribute to the improvement of the strength after rolling, and may be contained in order to improve functions such as toughness, high temperature strength, or weather resistance. These effects are exhibited by making the Cr content 0.01% or more. On the one hand, excessive inclusion deteriorates toughness and weldability. It is preferable that the Cr content is 0.01 to 0.5% as a range in which toughness and weldability are not deteriorated while maintaining sufficient strength even with a plate thickness of 70 mm or more.

In the present invention, in order to further improve the properties, it is possible to contain one or two or more of Mo, V, B, Ca, and REM in addition to the above component composition.

Mo: 0.01 to 0.5%

All of Mo are elements that increase the qualities of steel. This element may directly contribute to the improvement of the strength after rolling, and may be contained in order to improve functions such as toughness, high temperature strength, or weather resistance. These effects are exhibited by making the Mo content 0.01% or more. On the one hand, excessive inclusion deteriorates toughness and weldability. The Mo content is preferably 0.01 to 0.5% as a range in which toughness and weldability are not deteriorated while maintaining sufficient strength even with a plate thickness of 70 mm or more.

V: 0.001 to 0.10%

V is an element that improves the strength of steel by precipitation strengthening precipitated as V(CN). This effect is exhibited by making the V content 0.001% or more. However, when the V content exceeds 0.10%, the toughness may decrease. For this reason, when V is contained, it is preferable to make the V content into the range of 0.001 to 0.10%.

B: 0.0030% or less

B is an element that enhances the etchability of steel, and the above effect is obtained even in a trace amount such as 0.0030% or less of B content. Moreover, when the B content exceeds 0.0030%, the toughness of the welded portion decreases. Therefore, in the case of containing B, the B content is preferably 0.0030% or less. In addition, from the viewpoint of obtaining the above effect, the lower limit of the B content is preferably 0.0006%.

Ca: 0.0050% or less, REM: 0.0100% or less

Ca and REM improve toughness by miniaturizing the structure of the welding heat affected zone. Even if it contains these components, since the effect of this invention is not impaired, you may contain it as needed. However, when excessively contained, coarse inclusions may be formed and the toughness of the base material may be deteriorated. Therefore, in the case of containing these components, it is preferable to set the upper limit of the content to Ca: 0.0050% and REM: 0.0100%.

Ceq: 0.36 or more and 0.40 or less

In the high-strength ultra-thick steel sheet of the present invention, Ceq represented by the following formula (1) is adjusted to 0.36 or more and 0.40 or less, in addition to each component composition being in the range of the content. When Ceq is less than 0.36, it becomes difficult to increase the degree of integration of the (211) plane on the rolled surface in the center of the sheet thickness. Moreover, in order to ensure weldability, Ceq is made into 0.40 or less.

Ceq = C + Mn/6 + Cu/15 + Ni/15 + Cr/5 + Mo/5 + V/5...(1)

C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) mean the content (mass %) of each element, and when it does not contain, it is set as 0.

<collective organization>

The high-strength ultra-thick steel sheet of the present invention has a (211) plane integration degree of 1.2 or more on the rolled surface in the center of the plate thickness, and a (200) plane on the rolled surface in the surface (range of 1 mm below the surface from the electrode surface). It has an aggregate structure that satisfies the density of 1.7 or more. By adopting the above-described component composition and controlling the texture to satisfy the above range under the manufacturing conditions described later, a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties can be obtained.

From the above, in the present invention, the high-strength ultra-thick steel sheet of the present invention has an effect of excellent strength, toughness, and brittle crack propagation stopping properties even if the plate thickness is 70 mm or more by controlling the component composition and texture.

<Manufacturing method>

The molten steel of the said component composition is melt|dissolved in a converter etc., and it is made into a steel raw material (slab) by continuous casting etc., After heating at 1000-1200 degreeC, hot rolling is performed.

When the heating temperature is less than 1000°C, a sufficient time for rolling in the austenite recrystallization temperature range cannot be ensured. On the other hand, when the heating temperature exceeds 1200°C, the austenite grains become coarse, resulting in a decrease in toughness, as well as significant oxidation loss, resulting in a decrease in yield. Therefore, the heating temperature of the steel material is in the range of 1000 to 1200°C. From the viewpoint of improving the toughness of the steel sheet, the preferable range of the heating temperature is 1000° C. or higher and the upper limit is 1150° C. or lower. In addition, the temperature of the steel material means the temperature at the center of the thickness of the steel plate.

In hot rolling, first, rolling is performed in which the temperature at the center of the sheet thickness makes the cumulative reduction ratio in the austenite recrystallization temperature range 10% or more. By setting the cumulative reduction ratio in this temperature range to 10% or more, the Charpy fracture surface transition temperature (vTrs) at the position of 1/4 of the plate thickness can achieve -40°C or less. If the cumulative reduction ratio is less than 10%, fine graining of austenite is insufficient, the toughness is not improved, and the Charpy fracture surface transition temperature at the position of 1/4 of the plate thickness cannot achieve -40°C or less. The upper limit of the cumulative reduction ratio is not particularly limited, but the cumulative reduction ratio is preferably 45% or less since the effect of improving the fine grain size is reduced. In addition, in the case of the component composition of the present invention, the condition is, preferably, the cumulative reduction ratio in the temperature range included in 1100 to 950°C in the hot rolling is 10% or more.

Further, rolling is performed in which the temperature at the center of the sheet thickness is 50% or more in a cumulative reduction ratio in the austenite non-recrystallization temperature range. By making the cumulative reduction ratio in this temperature range 50% or more, an aggregate structure in which the (211) plane integration degree on the rolled surface in the center of the sheet thickness is 1.2 or more is obtained. Conversely, if the cumulative reduction ratio in this temperature range is less than 50%, an aggregate structure in which the (211) plane integration degree on the rolled surface in the center of the sheet thickness becomes 1.2 or more cannot be obtained. The upper limit of the cumulative reduction ratio is not particularly limited, but it is preferably 75% or less so as not to impair the rolling efficiency. In addition, in the case of the component composition of the present invention, the above conditions preferably have a cumulative reduction ratio of 50% or more in a temperature range included in 950 to 700°C in hot rolling.

Further, in the present invention, in the hot rolling, the plate thickness and surface temperature to the Ar ₃ transformation point or less also in excess of 20% when the cumulative rolling reduction in the temperature of the plate thickness center in the temperature range more than Ar ₃ transformation point. It is an important requirement in the present invention, and by performing hot rolling under this condition, the degree of integration of the (200) plane on the rolled surface on the surface of the steel sheet can be developed. And by performing hot rolling under this condition, it is possible to obtain a (200) plane integration degree on the rolled surface on the steel plate surface of 1.7 or more and a Charpy fracture surface transition temperature (vTrs) on the steel plate surface to -80°C or less. If the cumulative reduction ratio is 20% or less when the sheet thickness surface is in this temperature range, the desired texture and vTrs cannot be obtained. Here, the Ar ₃ transformation point is represented by the following formula.

Ar ₃ = 910-310C-80 Mn-20Cu-15Cr-55Ni-80Mo

The element symbol in the above formula means the content (mass%) of each element, and the one not containing is 0.

In addition, the temperature of the surface A preferred temperature range in the rolling from below Ar ₃ transformation point is Ar ₃ transformation point ~ - is (Ar ₃ transformation point - 80) ℃. In addition, the temperature at the center of the sheet thickness is at least the Ar ₃ transformation point, and the temperature range preferable for rolling is (Ar ₃ transformation point + 80)° C. to Ar ₃ transformation point.

In addition, in the hot rolling in the present invention, rolling outside of the above-specified temperature range is not limited, and at least, a reduction in the prescribed cumulative reduction ratio may be performed in the prescribed temperature range.

The steel sheet after rolling is cooled to a cooling stop temperature of 500°C or less at a cooling rate of 0.5°C/s or more. When the cooling rate is less than 0.5°C/s, the (211) plane integration degree on the rolled surface at the center of the sheet thickness cannot be ensured at least 1.2. Moreover, when the cooling stop temperature does not satisfy 500 degreeC or less, desired strength and texture cannot be obtained.

Further, in the case of performing a tempering treatment after cooling to a cooling stop temperature of 500°C or less, it is necessary to perform the temperature at the center of the plate thickness below the Ac ₁ transformation point. This is because when the tempering treatment is more than the Ac ₁ transformation point, the texture developed during rolling is lost. Here, the Ac ₁ transformation point is represented by the following formula.

Ac ₁ = 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B

The element symbol in the formula means the content (mass%) of each element, and the one not containing is 0.

In addition, in the above description, the temperature at the center of the sheet thickness is obtained by calculation of heat transfer from the surface temperature of the steel sheet measured with a radiation thermometer. In addition, the temperature condition in the cooling condition after rolling is the temperature at the center of the plate thickness, and the cooling rate also means the average cooling rate calculated based on the temperature at the center of the plate thickness.

Example

Next, an embodiment of the present invention will be described.

The molten steel of each component composition shown in Table 1 was melt|dissolved in a converter, and it was made into the steel material by a continuous casting method. After hot rolling to 70-100 mm of sheet thickness, it cooled and obtained the test steel shown in Table 2. In Table 2, heating conditions, hot rolling conditions, and cooling conditions are shown. Moreover, about the thing which tempered after cooling, the tempering temperature was also shown.

[Table 1]

[Table 2]

With respect to the obtained steel sheet, a JIS 14A test piece of Φ 14 mm was taken from the position of 1/4 of the sheet thickness, and a tensile test was conducted to measure yield strength (YS) and tensile strength (TS). It evaluated that YS was 390 MPa or more and TS was 510 MPa or more as good.

A JIS No. 4 impact test piece was taken from the position of 1/4 of the plate thickness and the surface of the steel plate so that the direction of the longitudinal axis of the test piece was parallel to the rolling direction, and a Charpy impact test was performed to obtain the Charpy fracture surface transition temperature (vTrs). The vTrs at the position of 1/4 of the sheet thickness was -40°C or less and the vTrs on the surface of the steel sheet was -80°C or less, evaluated as having good toughness.

In addition, in order to evaluate the texture of the steel sheet, the degree of integration of the (211) plane on the rolled surface in the center of the sheet thickness and rolling on the steel sheet surface (the steel sheet surface refers to a range of 1 mm below the surface from the pole surface) The (200) plane density in the plane was measured, respectively.

The degree of surface integration was measured using an X-ray diffraction apparatus (manufactured by Rigaku Electric Co., Ltd.) and a Mo source.

Next, in order to evaluate the brittle crack propagation stop property, a temperature gradient ESSO test was performed to obtain a Kca value at -10°C (hereinafter, also referred to as Kca (-10°C) N/mm ^1.5 ). . The thing with Kca (-10 degreeC) of 6000 N/mm ^1.5 or more was made favorable.

Table 3 shows the results of these tests.

[Table 3]

From the results shown in Table 3, in the example of the present invention, the (211) plane integration degree on the rolled surface in the center of the plate thickness was 1.2 or more, and the (200) plane integration degree on the rolled surface in the steel plate surface was 1.7. It has the above texture and has excellent toughness when the Charpy fracture surface transition temperature (vTrs) at the position of 1/4 thickness of the plate is -40°C or less and the Charpy fracture surface transition temperature (vTrs) at the surface of the steel sheet is -80°C or less. In addition, Kca (-10°C) was 6000 N/mm of ^1.5 or more, and excellent brittle crack propagation stopping properties were obtained.

On the other hand, the comparative example deviating from the present invention does not satisfy any of YS, TS, collective structure, and vTrs. Moreover, all the values of Kca (-10°C) were not satisfied.

Claims (6)

By mass%,
C: 0.03 to 0.20%,
Si: 0.03 to 0.5%,
Mn: 0.5 to 2.2%,
P: 0.01% or less,
S: 0.005% or less,
Ti: 0.005 to 0.03%,
Al: 0.005 to 0.080%
And N: 0.0050% or less
Containing, Ceq defined in the following formula (1) is 0.36 or more and 0.40 or less, and the balance is a component composition consisting of Fe and inevitable impurities
It has an aggregate structure in which the (211) plane integration degree on the rolled surface in the center of the plate thickness is 1.2 or more, and the (200) plane integration degree on the rolled surface in the steel plate surface is 1.7 or more,
The Charpy wavefront transition temperature (vTrs) at the position of 1/4 of the plate thickness is -40°C or less,
Charpy fracture surface transition temperature (vTrs) on the surface of the steel sheet is -80°C or less,
Yield strength (YS) of 390 MPa or more, tensile strength (TS) of 510 MPa or more,
High-strength ultra-thick steel plate with excellent brittle crack propagation stopping properties with a thickness of 70 mm or more
Ceq = C + Mn/6 + Cu/15 + Ni/15 + Cr/5 + Mo/5 + V/5...(1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V in Formula (1) mean the content (mass%) of each element, and when it does not contain it, it is set as 0. The method of claim 1,
The component composition is further mass%,
Nb: 0.005 to 0.05%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.5%
And Cr: 0.01 to 0.5%
High-strength ultra-thick steel sheet with excellent brittle crack propagation stopping properties containing one or two or more of. The method of claim 1,
The component composition is further mass%,
Mo: 0.01 to 0.5%,
V: 0.001 to 0.10%,
B: 0.0030% or less,
Ca: 0.0050% or less,
REM: 0.0100% or less
High-strength ultra-thick steel sheet with excellent brittle crack propagation stopping properties containing one or two or more of. The method of claim 2,
The component composition is further mass%,
Mo: 0.01 to 0.5%,
V: 0.001 to 0.10%,
B: 0.0030% or less,
Ca: 0.0050% or less,
REM: 0.0100% or less
High-strength ultra-thick steel sheet with excellent brittle crack propagation stopping properties containing one or two or more of. After heating the steel material having the component composition according to any one of claims 1 to 4 at a temperature of 1000 to 1200°C,
The temperature at the center of the plate thickness is 10% or more in the austenite recrystallization temperature range, the temperature at the center of the plate thickness is 50% or more in the austenite non-recrystallization temperature range, and the plate thickness surface temperature is Ar ₃ After performing hot rolling under the condition that the cumulative reduction ratio exceeds 20% when the temperature of the transformation point or lower and the center of the plate thickness is in the temperature range of the Ar ₃ transformation point or higher,
A method of manufacturing a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties of 70 mm or more in thickness, which is cooled to a cooling stop temperature of 500 °C or less at a cooling rate of 0.5 °C/s or more The method of claim 5,
A method of manufacturing a high-strength ultra-thick steel sheet having excellent brittle crack propagation stopping properties in which it is cooled down to a cooling stop temperature of 500° C. or less and then tempered to a temperature below the Ac ₁ transformation point.