JP2005281807A - High strength steel plate having low temperature toughness and weld heat affected zone toughness and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel plate having a high strength satisfying a tensile strength of ≥900 MPa, and also having excellent base metal low temperature toughness and weld heat affected zone toughness, and to provide a method for producing the same. <P>SOLUTION: A steel stock having a composition comprising, by mass, 0.005 to 0.030% C, 0.01 to 0.60% Si, 1.50 to 3.50% Mn, ≤0.020% P, ≤0.0050% S, 0.001 to 0.100% Al, 0.10 to <1.60% Ni, ≤0.60% Cr, ≤1.00% Cu, 0.01 to 0.60% Mo, 0.01 to 0.10% Nb, 0.005 to 003% Ti, 0.0002 to 0.0015% B, 0.0005 to 0.0100% O and 0.001 to 0.010% N, and the balance Fe with inevitable impurities, and in which carbon equivalent defined by Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 is 0.50 to 0.68% is reheated at 1,000 to 1,250°C, and is next subjected to hot rolling in which the cumulative draft at ≥750°C reaches ≥50%. The hot rolling is finished at ≥750°C, and then, it is cooled from the temperature range of ≥700°C to a cooling stop temperature in the range of 450 to 300°C at a cooling rate of ≥10°C/s, and is thereafter air-cooled to a room temperature. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-strength steel plate suitable for use in welded steel structures such as line pipes, ships, offshore structures, building machines, building structures, bridges, tanks, and penstocks, It relates to the improvement of weld heat affected zone (HAZ) toughness. Note that “excellent in low temperature toughness” means a case where the absorbed energy at −40 ° C. in the Charpy impact test has a base material toughness of 200 J or more. “Excellent welding heat-affected zone toughness” means toughness with an absorbed energy at −20 ° C. of 80 J or more in the Charpy impact test of the welding heat-affected zone of SAW welding with a heat input of 100 kJ / cm or less. It shall be a case.

  Generally, as the strength of the steel sheet increases, the low temperature toughness tends to decrease. For this reason, it is not easy to provide good low-temperature toughness particularly in a high-strength steel sheet having a tensile strength of 900 MPa or more. For this reason, various proposals have been made in order to improve the low temperature toughness of high strength steel sheets having a tensile strength of 900 MPa or more.

  For example, Patent Document 1 discloses that C: 0.03-0.10%, Mn: 1.7-3.0%, Mo: 0.1-0.8%, Nb 0.01-0.10%, Ti: 0.005-0.03%, low C-high Mn- The main component is Mo-Nb-Ti, Si content is as high as 0.8% or more, the structure is mainly composed of lower bainite, the tensile strength is 900MPa or more, and low temperature toughness and weld heat affected zone toughness are excellent. An ultra-high strength steel sheet is disclosed. However, in the technique described in Patent Document 1, C is as high as 0.03 to 0.10%, and there has been a limit in improving HAZ toughness.

  Patent Document 2 includes C: 0.01 to 0.2%, Ti: 0.003 to 0.05%, Mg: 0.0001 to 0.01%, and an Mg-containing oxide having a particle size of 0.2 to 5 μm as a nucleus. Composite particles in which one of the nitrides is deposited alone or in combination with both, and a composite particle in which one of the sulfides and nitrides is deposited alone with the Mg-containing oxide having a particle size of 0.005 to 0.2 μm as the core Is dispersed in steel, and the mixed structure of bainite and martensite in the heat affected zone is 80% or more of the whole structure. A line pipe steel pipe is disclosed. However, the technique described in Patent Document 2 requires a strict management of the steel making process, and has a problem that the manufacturing cost increases.

Patent Document 3 includes C: 0.03 to 0.12%, at least Ni: 1.0%, and further includes appropriate amounts of Mo, Nb, Cu, Ti, Al, and N, and has a fine-grained low bainite and fine-grained structure. An ultra-high-strength austenized steel with excellent cryogenic toughness that has a microstructure containing lath martensite, fine-grained bainite, and residual austenite up to 10% by volume has been proposed. However, the technique described in Patent Document 3 has a problem that the amount of C is high, and in order to obtain excellent weld heat-affected zone toughness, it is necessary to use a high Ni system of 2.0% or more, which increases the material cost. .
JP 2002-146471 A JP 2001-303191 A Special table 2002-534601 gazette

  The present invention solves the above-mentioned problems of the prior art, has a high strength of a tensile strength of 900 MPa or more, is excellent in the low temperature toughness of the base material, and is excellent in the weld heat affected zone toughness, and its An object is to provide a manufacturing method.

In order to achieve the above-described problems, the present inventors diligently studied various factors affecting the base material toughness and the weld heat affected zone (hereinafter also referred to as HAZ) toughness at a tensile strength of 900 MPa or more. as a result,
C: 0.030% or less, Mn: 1.5% or more, Ni: less than 1.6%, B: 2 to 15 ppm, extremely low C-high Mn-low Ni-trace B component system, and carbon equivalent Ceq is 0.54 The components are optimized to satisfy ~ 0.63, and water cooling after controlled rolling is stopped in the temperature range of 450 ° C to 300 ° C, and then air-cooled, so that the amount of carbide precipitated in the lath is extremely high. It was found that a large amount of a bainitic ferrite-based structure was produced in a large amount, the strength-toughness balance of the base material was improved, and good HAZ toughness was obtained. As a result, it has a tensile strength of 900MPa or more and excellent low temperature toughness of the base metal, and also has excellent HAZ toughness (CGHAZ and ICCGHAZ) of heat input equivalent to 50kJ / cm, high strength with excellent low temperature toughness and HAZ toughness. The knowledge that a steel plate can be manufactured was acquired.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.005 to 0.030%, Si: 0.01 to 0.60%, Mn: 1.50 to 3.50%, P: 0.020% or less, S: 0.0050% or less, Al: 0.001 to 0.100%, Ni: 0.10 %: Less than 1.60%, Cr: 0.60% or less, Cu: 1.00% or less, Mo: 0.01 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.03%, B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%, N: 0.001 to 0.010%, the following equation (1): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
(Where Ceq (%): carbon equivalent, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by the formula is contained so as to satisfy 0.50 to 0.68%, the balance is composed of Fe and inevitable impurities, and the structure contains a bainitic ferrite phase of 90% or more by volume. And high strength steel sheet with excellent low temperature toughness and weld heat affected zone toughness, characterized by having a tensile strength of 900 MPa or more.
(2) In (1), in addition to the said composition, V: 0.005-0.100% is further contained by the mass%, The high strength steel plate characterized by the above-mentioned.
(3) In (1) or (2), in addition to the above-mentioned composition, it further contains, by mass%, one or two selected from Ca: 0.010% or less and REM: 0.020% or less High-strength steel sheet characterized.
(4) By mass%, C: 0.005 to 0.030%, Si: 0.01 to 0.60%, Mn: 1.50 to 3.50%, P: 0.020% or less, S: 0.0050% or less, Al: 0.001 to 0.100%, Ni: 0.10 %: Less than 1.60%, Cr: 0.60% or less, Cu: 1.00% or less, Mo: 0.01 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.03%, B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%, N: 0.001 to 0.010%, the following formula (1) Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Where Ceq (%): carbon equivalent, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
A steel material having a carbon equivalent Ceq defined by the formula of 0.50 to 0.68%, preferably composed of the balance Fe and inevitable impurities, is heated to 1000 to 1250 ° C., and then at 750 ° C. or higher. The hot rolling is performed so that the cumulative rolling reduction of 50% or more is finished, the hot rolling is finished at 750 ° C or higher, and then the temperature range of 700 ° C or higher is 450 ° C to 300 ° C at a cooling rate of 10 ° C / s or higher. A method for producing a high-strength steel sheet excellent in low temperature toughness and weld heat affected zone toughness, characterized by cooling to a cooling stop temperature in a temperature range and then air cooling to room temperature.
(5) In (4), in addition to the said composition, V: 0.005-0.100% is further contained by the mass%, The manufacturing method of the high strength steel plate characterized by the above-mentioned.
(6) In (4) or (5), in addition to the above-mentioned composition, it further contains, by mass%, one or two selected from Ca: 0.010% or less and REM: 0.020% or less High-strength steel sheet characterized.

  According to the present invention, a high-strength steel plate having a high tensile strength of 900 MPa or more and excellent in low-temperature toughness and weld heat-affected zone toughness of a base material can be produced at a high efficiency and at a low cost. There is an effect.

  The reason for limiting the composition of the steel sheet of the present invention will be described first.

C: 0.005-0.030%
C is an element that increases the strength of the steel, and in order to obtain a desired high strength, the content of 0.005% or more is required. On the other hand, if the content exceeds 0.030%, a steel sheet having a tensile strength of 900 MPa or more deteriorates weldability, easily causes weld cracking, and lowers the base metal toughness and HAZ toughness. For this reason, C was limited to the range of 0.005 to 0.030%.

Si: 0.01-0.60%
Si is an element that acts as a deoxidizing agent and further increases the strength of the steel material by solid solution strengthening. In order to obtain such an effect, a content of 0.01% or more is required, but a content exceeding 0.60% significantly deteriorates the HAZ toughness. For this reason, Si was made into the range of 0.01 to 0.60%.

Mn: 1.50 to 3.50%
Mn is an element that has the effect of improving the hardenability of steel and improving toughness. In the present invention, it is necessary to contain 1.50% or more, but if it exceeds 3.50%, the weldability may be deteriorated. There is. For this reason, in this invention, Mn was limited to 1.50 to 3.50% of range.

P: 0.020% or less P is an element that increases the strength by solid solution strengthening. However, in order to deteriorate toughness and weldability, P is preferably reduced as much as possible in the present invention, but up to 0.020% is acceptable. For this reason, P was limited to 0.020% or less.

S: 0.0050% or less S is present as a sulfide in steel and exhibits an effect of reducing ductility. For this reason, it is desirable to reduce S as much as possible, but it is acceptable up to 0.0050%.

Al: 0.001 to 0.100%
Al acts as a deoxidizer in the steelmaking process, and in the present invention, it needs to be contained in an amount of 0.001% or more. However, if it exceeds 0.100%, the toughness is lowered. For this reason, Al was limited to the range of 0.001 to 0.100%.

Ni: 0.10% to less than 1.60%
Ni has the effect of improving the hardenability of the steel and improving the toughness. Such an effect is recognized at a content of 0.10% or more, but a content exceeding 1.60% tends to cause an increase in production cost. For this reason, Ni was limited to the range of 0.10 to 1.60%.

Cr: 0.60% or less
Cr is an inexpensive element that improves the hardenability of steel. Such an effect is recognized at a content of 0.1% or more, but a content exceeding 0.60% deteriorates weldability and toughness. For this reason, Cr was limited to 0.60% or less.

Cu: 1.00% or less
Cu has an effect of improving hardenability, and is preferably contained in an amount of 0.1% or more in the present invention. On the other hand, the content exceeding 1.00% increases the risk of causing hot brittleness. For this reason, Cu was limited to 1.00% or less.

Mo: 0.01 ~ 0.60%
Mo is an element having an effect of improving the hardenability of steel, and such an effect is recognized with a content of 0.01% or more. However, a content exceeding 0.60% deteriorates weldability and toughness. For this reason, Mo was limited to the range of 0.01 to 0.60%.

Nb: 0.01-0.10%
Nb has the effect of suppressing the recrystallization of austenite grains in the hot rolling process, facilitating the expansion of austenite grains by hot rolling, and refining the transformation structure after water cooling to improve strength and toughness. . Such an effect becomes remarkable when the content is 0.01% or more, but when the content exceeds 0.10%, the weldability and the HAZ toughness deteriorate. For this reason, Nb was limited to a range of 0.01% to 0.10%.

Ti: 0.005-0.03%
Ti combines with N in the steel to form TiN, suppresses the coarsening of crystal grains and contributes to the improvement of HAZ toughness, and also reduces the solid solution N and ensures the effect of improving the hardenability of B. Have Such an effect is recognized when the content is 0.005% or more. However, when the content exceeds 0.03%, TiN becomes coarse, the effect of refining γ grains disappears, and toughness deteriorates. For this reason, Ti was limited to the range of 0.005 to 0.03%.

B: 0.0002-0.0015%
B is an element that improves the hardenability of the steel in a small amount, and such an effect is recognized with a content of 0.0002% or more, but when it exceeds 0.0015%, the weldability and toughness are deteriorated. For this reason, B was limited to the range of 0.0002 to 0.0015%.

O: 0.0005 to 0.0100%
O generates Ti-based oxides and SiO ₂ -MnO-based oxides and has an action of preventing the austenite grain size of HAZ from becoming coarse. Such an effect becomes remarkable when the content is 0.0005% or more. However, if the content exceeds 0.0100%, a large amount of coarse oxide inclusions are generated. Toughness and HAZ toughness deteriorate. For this reason, O was limited to 0.0005 to 0.0100% of range.

N: 0.001 to 0.010%
N is an element that produces TiN together with Ti and has the effect of improving HAZ toughness by suppressing the coarsening of the austenite grain size of HAZ and promoting ferrite formation. Such an effect is recognized when the content is 0.001% or more. On the other hand, if the content exceeds 0.010%, the amount of solute N harmful to toughness increases and toughness deteriorates. For this reason, N was limited to the range of 0.001 to 0.010%.

  In addition to the above basic components, V: 0.005 to 0.100%, and / or Ca: 0.010% or less, REM: 0.020% or less can be included.

V: 0.005-0.100%
V combines with C and N and precipitates as carbide or nitride, and has the effect of increasing the strength of the steel by precipitation hardening. Such an effect is recognized at a content of 0.005% or more, but if it exceeds 0.100%, the weldability deteriorates. For this reason, it is preferable to limit V to 0.005 to 0.100% of range.

One or two selected from Ca: 0.010% or less, REM: 0.020% or less
Both Ca and REM are sulfide forming elements, and are elements that spheroidize sulfides and improve the ductility of steel. However, the content exceeding Ca: 0.010% and REM: 0.020% deteriorates toughness. For this reason, it was limited to Ca: 0.010% or less and REM: 0.020% or less.

In the present invention, the above-described components are within the above-mentioned range, and the following formula (1): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Where Ceq (%): carbon equivalent, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is contained so as to satisfy 0.50 to 0.68%.

  When Ceq is less than 0.50%, as shown in FIG. 2, it is not possible to secure a base material strength with a tensile strength of 900 MPa or more. On the other hand, if Ceq exceeds 0.68%, the base material toughness cannot be secured as shown in FIG. For this reason, Ceq was limited to the range of 0.50 to 0.68%.

  The balance other than the above components is Fe and inevitable impurities.

  The steel sheet of the present invention has the above-described composition and has a structure mainly composed of bainitic ferrite. The structure mainly composed of bainitic ferrite refers to a structure containing bainitic ferrite having a volume ratio of 90% or more regardless of the position in the plate thickness direction. When the structural fraction of bainitic ferrite is less than 90%, martensite increases, so it has a high strength of 900 MPa or more, and it is difficult to have both excellent base material low temperature toughness and HAZ toughness. As phases other than bainitic ferrite, mixing of upper bainite, martensite and ferrite with a volume ratio of 10% or less is acceptable.

  Bainitic ferrite is a structure in which cementite does not substantially precipitate in the lath, and by making C: 0.03% or less, as shown in FIG. It can be an organization. This makes it possible to obtain excellent CGHAZ (Coarse-grain HAZ) toughness and ICCGHAZ (Intercritically-reheated coarse-grain HAZ) toughness. On the other hand, when C exceeds 0.03%, the base metal structure at the center of the plate thickness becomes a structure mainly composed of lower bainite in which cementite is precipitated in the lath, as shown in FIG. 4, and CGHAZ and ICCGHAZ toughness to degrade. Note that this difference in structure is not limited to the center portion of the plate thickness.

  Below, the manufacturing method of this invention steel plate is demonstrated.

  The molten steel having the above composition is melted by a normal melting means such as a converter or an electric furnace, and steel such as a steel slab or slab is obtained by a normal casting method such as a continuous casting method or an ingot-bundling method. It is preferable to use a raw material. The melting method and the casting method are not limited to the methods described above.

  A steel material having the above composition is heated to 1000 to 1250 ° C. and then hot-rolled.

  If the heating temperature is less than 1000 ° C., the hot deformation resistance is too high to obtain a high rolling reduction per time, and the hot rolling productivity decreases. When a precipitate-forming element such as V or Nb is contained, these elements are not sufficiently dissolved in austenite, and it is difficult to sufficiently exhibit the effects of these elements. On the other hand, when the heating temperature exceeds 1250 ° C., the crystal grains become coarse, and the amount of oxidation loss increases and the frequency of furnace repairs increases. For this reason, the heating temperature of the steel material was limited to a range of 1000 to 1250 ° C.

  Next, the heated steel material is subjected to hot rolling at a rolling end temperature of 750 ° C. or higher to obtain a thick steel plate.

  When the rolling end temperature is less than 750 ° C., ferrite precipitates during rolling, and a desired structure cannot be obtained even if a cooling process is performed thereafter, and a desired strength cannot be ensured. The cumulative rolling reduction in the temperature range of 750 ° C. or higher, preferably 950 ° C. or lower is set to 50% or higher. If the cumulative rolling reduction is less than 50%, sufficient austenite grain refinement cannot be achieved, and thus the base material toughness cannot be secured.

  After the hot rolling, the thick steel plate is cooled at a cooling rate of 10 ° C./s or more from a temperature range of 700 ° C. or more. When the cooling start temperature is less than 700 ° C., ferrite already increases too much at the start of cooling, and thus it becomes impossible to secure a desired strength after cooling. Further, when the cooling rate is less than 10 ° C./s, a structure other than bainitic ferrite is generated, and the structural fraction of bainitic ferrite is lowered, so that a desired strength cannot be ensured. For this reason, it is preferable that a cooling rate shall be 10 degrees C / s or more.

  The thick steel plate is cooled to a cooling stop temperature in a temperature range of 450 ° C. or lower and 300 ° C. or higher at the above cooling rate.

  If the cooling stop temperature exceeds 450 ° C, it will cause coarsening of bainitic ferrite and an extremely low dislocation density, and as shown in Fig. 1, it will not be possible to secure the tensile strength of the base material of 900 MPa or more. . On the other hand, if the cooling stop temperature is less than 300 ° C., diffusible hydrogen and residual stress in the steel sheet become excessive, and the base metal toughness and HAZ toughness deteriorate. For this reason, the cooling stop temperature is preferably limited to a temperature in the temperature range of 450 ° C. or lower and 300 ° C. or higher. More preferably, it is 450 ° C. or lower and 350 ° C. or higher.

  In actual operation, the temperature of the steel sheet is generally controlled by the surface temperature of the steel sheet, and the average temperature in the entire thickness direction is calculated in real time, and the temperature control and speed control are generally performed based on this average temperature. Is. Therefore, “temperature” in the present invention means the average temperature of the entire steel sheet, “cooling rate” means the average cooling rate of the entire steel sheet, and “temperature increase rate” means the average temperature increase rate of the entire steel sheet.

  Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace and made into a 100 mm thick slab (steel material) by a continuous casting method. Subsequently, these steel materials were subjected to hot rolling under the conditions shown in Table 2 and subsequently subjected to cooling treatment under the conditions shown in Table 2 to obtain thick steel plates (plate thickness: 16 to 20 mm).

  In addition, from the position of 1 / 2t in the thickness direction of the obtained thick steel plate, a specimen for tissue observation is collected, and the structure is observed with a scanning electron microscope and a transmission electron microscope to identify the tissue and determine the tissue fraction. Asked. The texture fraction is determined by measuring the average austenite (γ) particle size by line segmentation using a scanning electron microscope, randomly selecting 10 particles having the average γ particle size, and setting each group in the γ particle. Was obtained as a cross-sectional area ratio, and an average value of the ten cross-sectional area ratios was converted to a volume ratio as a structure fraction of each phase at each position of the steel sheet.

  In addition, a No. 4 tensile test specimen was taken from the position of the obtained steel plate in the thickness direction 1 / 2t in accordance with the provisions of JIS Z 2201, and a tensile test was conducted in accordance with the provisions of JIS Z 2241. 0.2% yield strength YS and tensile strength TS were determined.

In addition, a Charpy impact test specimen with a V-notch standard dimension was taken from the position of the obtained thick steel plate in the thickness direction 1 / 2t according to JIS Z 2202, and conformed to JIS Z 2242. A Charpy impact test was performed to determine the absorbed energy vE- ₄₀ (J) at -40 ° C.

In addition, a reproducible heat cycle test piece is taken from the obtained steel plate, and a reproducible welding heat cycle is applied that gives a reproducible heat cycle of 1 cycle of maximum heating temperature: 1400 ° C, cooling time of _800-500 ° C t _800-500 = 57s. After applying test (applying reproducible thermal cycle equivalent to bond part with welding heat input of 50kJ / cm) (TP1) and similar reproducible thermal cycle, maximum heating temperature: 800 ° C, cooling time of 800-500 ° C t ₈₀₀ A reproducible welding thermal cycle test (TP2) was performed, which gave a reproducible thermal cycle of ₋₅₀₀ = 57 s and gave a reproducible thermal cycle of 2 cycles. Charpy impact test specimens with V-notch standard dimensions are collected from these test pieces that have been given a reproducible thermal cycle in accordance with JIS Z 2202, and Charpy impact tests are conducted in accordance with JIS Z 2242 regulations. Then, the absorbed energy vE- ₂₀ (J) at -20 ° C was obtained, and the HAZ toughness was evaluated.

  The obtained results are shown in Table 3.

  In addition, the steel plate temperature and cooling rate in Table 2 were displayed using average temperature and average cooling rate.

Each of the examples of the present invention is a steel sheet having high tensile strength: 900 MPa or more, excellent base material toughness of vE- ₄₀ : 200 J or more, and excellent HAZ toughness of vE- ₂₀ : 80 J or more. On the other hand, in the comparative example that is out of the scope of the present invention, one of the characteristics is deteriorated.

  For example, in the comparative example (steel plate No. 8) in which the B content is outside the scope of the present invention, the reproduced HAZ toughness (Tp1) after applying one thermal cycle is deteriorated. Further, in the comparative example (steel plate No. 9) in which the B content deviates from the range of the present invention, the reproduced HAZ toughness after being given two cycles of heat cycle is deteriorated. Further, in the comparative example (steel plate No. 10) in which the B content and Ceq are out of the scope of the present invention, the base metal strength and the reproduced HAZ toughness after applying the 1-cycle and 2-cycle thermal cycles are all deteriorated. .

  Moreover, in the comparative example (steel plate No. 11) in which the C content is outside the scope of the present invention, the microstructure is outside the scope of the present invention, and the reproduced HAZ toughness after two cycles of thermal cycling is deteriorated. . Moreover, in the comparative example (steel plate No. 12) in which the C and B contents are out of the range of the present invention, the microstructure is out of the range of the present invention, and the base material strength and the heat cycle of 1 cycle or 2 cycles are reduced. All of the reproduced HAZ toughness after application has deteriorated.

  In addition, a comparative example (steel plate No. 13) in which the cumulative rolling reduction at 750 ° C. to 950 ° C. is low and the structure deviates from the scope of the present invention, the cooling rate deviates from the preferred range of the present invention, and the structure departs from the scope of the present invention. In the comparative example (steel plate No. 14) that deviates, and in the comparative example (steel plate No. 15) in which the cooling stop temperature deviates from the preferred range of the present invention and the structure deviates from the range of the present invention, the base metal strength or HAZ toughness deteriorates. ing. Further, in the comparative example (steel plate No. 16) in which the cooling stop temperature deviates from the preferred range of the present invention and the structure deviates from the range of the present invention, the base material toughness is deteriorated.

It is a graph which shows the influence of the cooling stop temperature which acts on a base material tensile characteristic. 4 is a graph showing the relationship between the carbon equivalent Ceq and the base metal tensile strength of a steel plate produced with a cooling stop temperature of 450 ° C. or lower. It is a graph which shows the relationship between the carbon equivalent Ceq and base material toughness of the steel plate manufactured by making cooling stop temperature into 450 degrees C or less. It is a scanning electron microscope structure photograph which shows an example of the base material structure | tissue of the plate | board thickness center part of a comparative example. It is a scanning electron micrograph structure | tissue photograph which shows the base material structure | tissue of the plate | board thickness center part of the example of this invention.

Claims (4)

% By mass
C: 0.005 to 0.030%, Si: 0.01 to 0.60%,
Mn: 1.50 to 3.50%, P: 0.020% or less,
S: 0.0050% or less, Al: 0.001 to 0.100%,
Ni: 0.10% or more and less than 1.60%, Cr: 0.60% or less,
Cu: 1.00% or less, Mo: 0.01-0.60%,
Nb: 0.01-0.10%, Ti: 0.005-0.03%,
B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%,
N: 0.001 to 0.010%
Is contained so that the carbon equivalent Ceq defined by the following formula (1) satisfies 0.50 to 0.68%, and the balance is composed of Fe and inevitable impurities, and bainitic ferrite having a volume ratio of 90% or more A high-strength steel sheet excellent in low-temperature toughness and weld heat-affected zone toughness, characterized by having a structure including a phase and a tensile strength of 900 MPa or more.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
Where Ceq (%): carbon equivalent,
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)   The high-strength steel sheet according to claim 1, further comprising, in addition to the composition, V: 0.005 to 0.100% by mass.   The high content according to claim 1 or 2, further comprising one or two selected from Ca: 0.010% or less and REM: 0.020% or less in mass% in addition to the composition. Strength steel plate. % By mass
C: 0.005 to 0.030%, Si: 0.01 to 0.60%,
Mn: 1.50 to 3.50%, P: 0.020% or less,
S: 0.0050% or less, Al: 0.001 to 0.100% or less,
Ni: 0.10% or more and less than 1.60%, Cr: 0.60% or less,
Cu: 1.00% or less, Mo: 0.01-0.60%,
Nb: 0.01-0.10%, Ti: 0.005-0.03%,
B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%,
N: 0.001 to 0.010%
Is heated to 1000 to 1250 ° C., and the cumulative rolling reduction at 750 ° C. or higher is 50. The steel material having a carbon equivalent Ceq defined by the following formula (1) satisfies 0.50 to 0.68%. % Hot rolling is completed at 750 ° C or higher, and then the cooling stop temperature in the temperature range of 450 ° C to 300 ° C at a cooling rate of 10 ° C / s from the temperature range of 700 ° C or higher. A method for producing a high strength steel sheet excellent in low temperature toughness and weld heat affected zone toughness, characterized by cooling to room temperature and then air cooling to room temperature.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
Where Ceq (%): carbon equivalent,
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)

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