JP2006118007A - High strength steel having excellent toughness in weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel and a high strength steel sheet capable of obtaining excellent HAZ toughness at the time of big heat input welding under the securance of base metal toughness and low temperature cracking resistance while maintaining a high strength in a 780 MPa class. <P>SOLUTION: The invention has a composition comprising, by mass, 0.010 to 0.080% C, 0.02 to 1.00% Si, 1.10 to 2.90% Mn, 0 to 0.030% P, 0 to 0.010% S, ≤0.20% Al, 0.40 to 2.40% Ni, 0.50 to 1.95% Cr, 0.16 to 1.10% Mo, 0.002 to 0.030% Ti and 0.0058 to 0.0120% N also so as to satisfy 1.0≤[Ti]/[N]<4.0, and the balance Fe with inevitable impurities, and in which AS value and DL value defined by the following formulae satisfy AS≥3.60 and DL≤2.80, and has a structure mainly composed of bainitic ferrite: AS=[Mn]+[Ni]+2×[Cu], and DL=2.5×[Mo]+30×[Nb]+10×[V]; wherein, [X] denotes the content (mass%) of the element X. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-strength steel material having a tensile strength of 780 MPa or more, which is used for, for example, a bridge, a building, a ship, a penstock, a tank, and other large structures. (Hereinafter sometimes referred to as “HAZ”).

  In high strength steel plates of 780 MPa or more, a large amount of alloy components are added from the viewpoint of securing the strength of the base metal. Therefore, when the cooling rate is fast under small heat input welding conditions, the HAZ hardens and weld cracks (cold cracks) occur. Cheap. In order to prevent this, preheating at 100 ° C. or higher is performed during welding. If this preheating can be omitted, the construction efficiency can be greatly increased, and the cost can be reduced. For this reason, a high-tensile steel plate of 780 MPa class or more that has excellent cold cracking resistance is desired.

A parameter called Pcm (%) defined by the following formula has been proposed as an index of cold cracking resistance. Conventionally, Japanese Patent Application Laid-Open No. 9-3591 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2001-200334 (Patent Document 2). As described in, Nb, V, and Mo, which can improve the cold cracking resistance by restricting Pcm, while being hard to increase Pcm and improving the hardenability by adding a small amount, are actively added. By doing so, the strength of the base material has been secured.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B]
However, [C]-[B] represent mass% of each element.

On the other hand, with the recent increase in size of structures, welding of large cross-section members (large heat input welding) has become inevitable. In this case, there has been a problem that the structure of the HAZ becomes coarse and the HAZ toughness decreases. It was. Up to now, as techniques for improving the HAZ toughness of steel materials, for example, JP-A-9-104949 (Patent Document 3) utilizes TiN, or JP-A-2002-121641 (Patent Document 4). A technique for improving the HAZ toughness by utilizing Ti-containing oxide inclusions has been proposed.
JP-A-9-3591 Japanese Patent Laid-Open No. 2001-200334 JP-A-9-104949 JP 2002-121641 A

As described above, Nb, V, and Mo are positively added to the high-strength steel sheet of 780 MPa class in order to ensure low-temperature cracking resistance. For this reason, bainite and steel that act as resistance to crack propagation during bainite transformation. Since the block is coarsened and coarse hard MA (Martensite-Austenite Constituent: mixture of martensite and austenite) is generated as the second phase, there is a problem that the base metal toughness and the HAZ toughness deteriorate.
In recent years, there has been an increasing demand for improved structural safety, such as improved seismic resistance, and high heat input welding with high base metal toughness and cold cracking resistance in high-strength steel sheets of 780 MPa or higher. Improvement of the HAZ toughness at the time is required. However, the conventional HAZ toughness improvement technology does not satisfy such a demand.

  The present invention has been made in view of such a problem, and obtains excellent HAZ toughness at the time of high heat input welding while ensuring the base material toughness and low temperature cracking resistance while having a high strength of 780 MPa class. An object of the present invention is to provide a high-strength steel material and a steel plate that can be used.

One of the points of the present invention is that when designing a steel component, it is not limited to Pcm, which has been regarded as an index of cold cracking resistance until now, but the component design is performed in consideration of the steel structure, that is, the amount of C is extremely low. Nb, V, and Mo, which adversely affect the base material toughness and HAZ toughness, are suppressed, and the hardenability improving elements Mn and Ni, or further Cu are positively added. This is in that a structure mainly composed of bainitic ferrite is generated regardless of whether the cooling rate after hot rolling is high or low.
Another point of the present invention is that, as a result of investigating the cause of the decrease in absorbed energy in the HAZ near the bond during high heat input welding, the HAZ is caused by the coarsening of the prior austenite grain (γ grain) diameter. Based on this knowledge, we obtained the knowledge that the HAZ toughness is deteriorated because the entire structure is coarsened. Based on this knowledge, the component system that can stabilize TiN that can be finely dispersed to a high temperature and refine the old γ grains. It is in the point.

That is, the high-strength steel material or high-strength steel sheet of the present invention is mass%,
C: 0.010 to 0.080%,
Si: 0.02 to 1.00%,
Mn: 1.10 to 2.90%,
P: 0 to 0.030%,
S: 0 to 0.010%,
Al: 0.20% or less,
Ni: 0.40 to 2.40%,
Cr: 0.50 to 1.95%,
Mo: 0.16 to 1.10%,
Ti: 0.002 to 0.030%,
N: 0.0058 to 0.0120%
1.0 ≦ [Ti] / [N] <4.0
The balance is composed of Fe and inevitable impurities, and AS and DL values defined by the following formulas are AS ≧ 3.60 and DL ≦ 2.80, and the structure is mainly composed of bainitic ferrite. Is.
AS = [Mn] + [Ni] + 2 × [Cu]
DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V]
However, [X] represents content (mass%) of element X.

  The steel material of the present invention or the high-strength steel plate further includes (1) Cu: 1.60% or less, (2) B: 0.0050% or less, Nb: 0.100% or less, V: 0.060 in addition to the chemical components. % Or less, (3) One or two of Ca and REM in total, 0.0050% or less, (4) Mg: 0.0050% or less, (5) Hf: 0.050% or less Zr: Any one or two of 0.100% or less, (6) W: 5.0% or less, Co: Any one or two of 5.0% or less Elements can be further contained alone or in combination.

  Further, when the high-strength steel sheet is manufactured by heating the steel having the above components to the austenite temperature, hot rolling, and cooling, it is preferable to set the finishing temperature of the hot rolling to 870 ° C. or less. By setting the finishing temperature to 870 ° C. or less, the old γ grains after welding are refined, and the HAZ toughness at the time of high heat input welding can be further improved.

  According to the steel material and steel sheet of the present invention, C is extremely low, and Mn and Ni, or even Cu is positively added so that the AS value is 3.60 or more, while addition of Mo, Nb, and V is performed. Since the DL value is suppressed to 2.80 or less, the structure is mainly composed of bainitic ferrite regardless of the cooling rate after hot rolling, even when the plate thickness is thick. It is excellent in base material strength and base material toughness. Furthermore, since a relatively large amount of N is added within a predetermined [Ti] / [N] ratio range, TiN can be stabilized at a high temperature, which can refine old γ grains near the bond. Therefore, even when high heat input welding is performed, excellent HAZ toughness can be obtained.

  The first important point on the composition of the steel sheet of the present invention is that AS ≧ 3.60 in order to ensure a predetermined base material strength of Mn, Ni and Cu, which are hardenability improving elements under an extremely low C content. On the other hand, Nb, V, and Mo are positively suppressed so as to satisfy DL ≦ 2.80 in order to ensure the base material toughness. First, the structure and characteristics produced after hot rolling by the steel components of the steel sheet of the present invention will be described with reference to the CCT diagram.

  FIG. 1 shows CCT diagrams of an extremely low C steel (A) to which Mn, Ni, and Cu according to the present invention are positively added, a conventional high C steel (B1), and a low C steel (B2). In the figure, BF represents bainitic ferrite, GBF represents granular bainitic ferrite, M represents martensite, B represents bainite, and F represents ferrite. From the same figure, in the steel sheet of the present invention, BF is 85% or more, more preferably 90% or more in terms of area ratio, regardless of whether the cooling after hot rolling is high cooling rate (CR1) or low cooling rate (CR2). Thus, a fine bainite (bainitic ferrite) structure in which the second phase MA is finely dispersed can be obtained. With such a structure mainly composed of BF, even a thick plate having a thickness of about 50 mm or more can obtain a strength of 780 MPa or more as a mechanical property of the base material and has excellent toughness. In addition, as described above, since the entire structure becomes BF having a low hardness cooling rate sensitivity at both high cooling rate (CR1) and low cooling rate (CR2), small heat input welding conditions (heat input number) kJ / cm) can reduce the hardness of HAZ (improve cold cracking resistance), and also during low-speed cooling under high heat input welding conditions (heat input several hundred kJ / cm) A relatively good HAZ toughness can be obtained. On the other hand, conventional high-C steel (B1) produces ferrite and coarse bainite at a high cooling rate (CR1), and as a result, coarse and massive MA forms, resulting in a decrease in the strength and toughness of the base material. It was also difficult to ensure the HAZ toughness during the medium heat input welding.

Next, the 2nd main point on the component of this invention steel plate is demonstrated.
Although the base material toughness and the HAZ toughness are improved by making the structure mainly composed of the above-mentioned BF, the structure is presupposed to ensure sufficient HAZ toughness even under high heat input welding of about 800 kJ / cm. As described above, it is important to add a relatively large amount of N in a range where the [Ti] / [N] ratio is 1.0 to 4.0 in order to suppress the coarsening of the prior γ grain size in HAZ.
The reason why the HAZ toughness is improved under large heat input welding by adding a large amount of N is not necessarily clear, but is presumed as follows. First, it is considered that by increasing the N content, the driving force at the time of TiN generation is increased, and TiN can be finely dispersed even if manufactured by a conventional method. At the same time, it is considered that the stability of TiN at a high temperature could be increased by controlling the balance of addition of N and Ti as described above. That is, by increasing N and adding N and Ti, refinement of old γ grains in the vicinity of the bond is stably achieved, and the variation in HAZ toughness is greatly improved (reduced), and the appropriateness of AS and DL is further improved. In combination with proper adjustment, the microstructure in the γ grains after transformation (bainitic ferrite) can be refined, and it is speculated that these have ensured excellent HAZ toughness even after high heat input welding. The
According to the research of the present inventor, when the parent phase is a ferrite pearlite structure as in the prior art, the toughness is deteriorated if solid solution N exists in the parent phase, and thus the N cannot be sufficiently increased. It was found that the toughness does not deteriorate even when the N content is increased because the solid phase can be concentrated in the second phase MA by making the parent phase a BF-based structure as in the invention.

Here, the reason for limiting the components of the high-strength steel material and the steel plate of the present invention will be described. All units are mass%.
C: 0.010-0.080%
C is an element necessary for ensuring the strength of the base material. If it is less than 0.010%, even if a hardenability improving element is positively added, a base material strength of 780 MPa or more cannot be secured. On the other hand, if it exceeds 0.080%, a large amount of MA is generated, and the base material toughness and the HAZ toughness are deteriorated. For this reason, the lower limit of the C content is 0.01%, preferably 0.020% or more, more preferably 0.030% or more, while the upper limit is 0.080%, preferably 0.8%. It should be 070%, more preferably 0.060%.

Si: 0.02 to 1.00%
Si has a solid solution strengthening action, but if added excessively, a large amount of MA is generated in the base material and HAZ, and the base material toughness and HAZ toughness deteriorate. Therefore, the lower limit of Si content is 0.02%, preferably 0.10%, and the upper limit is 1.00%, preferably 0.80%.

Mn: 1.10 to 2.90%
Mn is an element effective for improving the hardenability and ensuring the strength and toughness. However, when added excessively, the strength becomes excessive, and the base metal toughness and the HAZ toughness are lowered. For this reason, the lower limit of the amount of Mn is 1.10%, preferably 1.40%, more preferably 1.70%, and still more preferably 1.90%.

P: 0 to 0.030% or less, S: 0 to 0.010% or less These elements are easily segregated impurity elements and adversely affect the base material toughness and HAZ toughness. Is 0.030% or less, and S is 0.010% or less.

Al: 0.20% or less Al is added as a deoxidizing element. However, if excessively added, a large amount of MA is generated, and the base material toughness and the HAZ toughness deteriorate. For this reason, the upper limit of the amount of Al is 0.20%, preferably 0.15%, more preferably 0.10%.

Ni: 0.40 to 2.40%
Ni improves the low temperature toughness and hardenability of the steel to improve the strength, and is also effective in preventing hot cracking and weld hot cracking. However, when excessively added, scale wrinkles are likely to occur. For this reason, the lower limit of the Ni amount is 0.40%, preferably 0.60%, more preferably 0.80%, still more preferably 1.00% or more, and the upper limit is 2.40%.

Cr: 0.50 to 1.95%
Cr increases the strength of the base metal and the welded portion, but if added excessively, it degrades the base metal toughness and HAZ toughness. For this reason, the lower limit of the Cr amount is 0.50%, preferably 0.70%, more preferably 1.00%, and the upper limit is 1.95%, preferably 1.70%, more preferably 1.50. %.

Mo: 0.16 to 1.10%
Mo is effective for improving the hardenability and ensuring high strength, and is an effective element for preventing temper brittleness. However, when added excessively, the base material toughness and the HAZ toughness are lowered. For this reason, the lower limit of the Mo amount is 0.16%, preferably 0.22%, more preferably 0.25%, still more preferably 0.40%, and the upper limit is 1.10%, preferably 0.80. %, More preferably 0.60%.

Ti: 0.002 to 0.030%
Ti combines with N to form nitrides, refines the HAZ austenite grains during welding, and is an effective element for improving HAZ toughness. If the Ti content is less than 0.002%, the effect of refining is too small, so the lower limit is 0.002%, preferably 0.007%, more preferably 0.010%, and even more preferably 0.012%. To do. On the other hand, if added excessively, TiN becomes coarse and may deteriorate the base metal toughness and HAZ toughness, so the upper limit is 0.030%, preferably 0.025%, more preferably 0.020%. .

N: 0.0058 to 0.0120%
N is an important element for improving the HAZ toughness at the time of high heat input welding together with Ti, and combines with Ti to form TiN to refine the austenite grains at the time of high heat input welding, thereby reducing the HAZ inertia. Has the effect of improving. However, excessive addition of N adversely affects the base material toughness and the HAZ toughness. In order to effectively exhibit the effect of N, the lower limit of the N amount is 0.0058%, preferably 0.0060%, more preferably 0.0070%, and still more preferably 0.0080, The upper limit is 0.0120%, preferably 0.0100%, more preferably 0.0090%.

[Ti] / [N]: 1.0 to 4.0
When the ratio of [Ti] / [N] is less than 1.0, the solute N becomes excessive, and the base metal toughness and the HAZ toughness deteriorate. On the other hand, when it exceeds 4.0, TiN becomes difficult to finely disperse, and the base material toughness and the HAZ toughness are also lowered. For this reason, the lower limit of the ratio is 1.0, and the upper limit is 4.0, preferably 3.0, more preferably 2.0.

AS value: 3.60 or more The addition amount of Mn, Ni, and Cu is closely related to the base material strength and HAZ toughness, and Cu is about twice as strong as Mn and Ni, and has a high strength improvement effect. In order to increase the base metal strength to 780 MPa or more in the range from high cooling rate to low cooling rate after hot rolling, it is necessary to set the AS value to 3.60 or more, as will be apparent from Examples described later. As a result, a BF amount of the mother phase can be obtained by 85 area% or more. Since the base metal toughness and the HAZ toughness increase as the amount of BF increases, the amount of BF is preferably 90 area% or more, more preferably 95 area% or more. For this purpose, the AS value is increased. The addition amount of Mn, Ni and Cu described later is adjusted. As the AS value is higher, BF transformed at a low temperature is obtained at a low cooling rate (high heat input welding), and the amount of BF increases. For this reason, the AS value is preferably 4.00 or more, more preferably 4.50 or more, and further preferably 5.00 or more.

DL value: 2.80 or less Mo has the effect of improving hardenability as described above. Nb and V, which will be described later, have the same action. On the other hand, when these elements are added excessively, a coarse bainite structure is generated, and the base metal toughness and the HAZ toughness deteriorate. Such a deterioration effect of toughness is not uniform for each element, and when Mo is set to 1 by experiments by the inventors, Nb is about 12 times and V is about 4 times. As will be apparent from the examples described later, in order to ensure good Maemura toughness of vE _-20 = 200 J or more, the DL value is 2.80 or less, preferably 2.50 or less, more preferably 2.00. Hereinafter, the addition of Mo, Nb, and V should be limited so that it is more preferably 1.50%, and still more preferably 1.00 or less.

  The steel plate of the present invention is formed by the balance Fe and unavoidable impurities in addition to the above components, but does not hinder the addition of elements that further improve the characteristics within a range that does not impair the effects and effects of the above components. For example, (1) Cu: 1.60% or less, (2) B: 0.0050% or less, Nb: 0.100% or less, V: less than 0.060%, (3) Ca, One or two types of REM in total of 0.0050% or less, (4) Mg: 0.0050% or less, (5) Hf: 0.050% or less, Zr: 0.100% or less Or two or more (6) W: 5.0% or less, Co: Any one or two elements selected from the group of 5.0% or less alone or in combination Can do. The reasons for limiting these auxiliary elements will be described below.

Cu: 1.60% or less Cu has the effect of improving the base metal strength by solid solution strengthening and precipitation strengthening, and improving the hardenability although not as much as Mo, Mn, Ni, and Cr. In order to effectively exhibit such an action, it is desirable to add 0.30% or more, more preferably 0.50% or more. However, if it exceeds 1.60%, the toughness of the base metal and the HAZ toughness will be lowered, so the upper limit of the Cu content is 1.60%, preferably 1.40%, more preferably 1.20%, Preferably it is 1.00%.

B: 0.0050% or less B has an action of improving the hardenability and improving the HAZ toughness. In particular, the effect is great when welding with a large heat input. Addition of 0.0005% or more is preferable in order to effectively exhibit such action. When added in a large amount, the base material toughness and HAZ toughness are deteriorated. For this reason, the upper limit of the amount of B is made 0.0050%, preferably 0.030%, more preferably 0.0020.

Nb: 0.100% or less Nb, like B, improves the hardenability. That is, solid solution Nb has the effect of improving the hardenability of the base material and improving the strength of the base material and the welded joint. However, if added excessively, the strength becomes excessive and deteriorates the base material toughness and HAZ toughness. It becomes like this. For this reason, the upper limit of the amount of Nb is made 0.100%, preferably 0.040%, more preferably 0.020%.

V: Less than 0.060% V, like B and Nb, improves hardenability by adding a small amount. Moreover, there is an effect of increasing the temper softening resistance. However, if added excessively, the strength becomes excessive and the base metal toughness and the HAZ toughness are deteriorated. For this reason, the upper limit of the V amount is 0.060%, preferably 0.050%, more preferably 0.040%.

Ca, REM: 0.0050% or less in total These elements have an effect of reducing anisotropy by controlling the form of inclusions to spheroidize MnS, and have an effect of improving HAZ toughness. However, when added excessively, the base material toughness is deteriorated. For this reason, the total of these elements is 0.0050%, preferably 0.0030%.

Mg: 0.0050% or less Mg has the effect of improving HAZ toughness by forming MgO and suppressing the coarsening of HAZ austenite grains. However, when added excessively, the base material toughness is deteriorated. For this reason, the upper limit is made 0.0050%, preferably 0.0035%.

Zr: 0.100% or less Hf: 0.050% or less Zr and Hf are elements that are effective in improving HAZ toughness by forming nitrides with N to refine HAZ austenite grains during welding. . However, when added excessively, it returns and lowers the base metal toughness and the HAZ toughness. For this reason, the upper limit of the amount of Zr is set to 0.100%, preferably 0.050%, and the upper limit of the amount of Hf is set to 0.050%, preferably 0.030.

W: 5.0% or less Co: 5.0% or less W and Co are effective for improving hardenability and securing strength easily with a small amount. W further has the effect of improving the temper softening resistance. On the other hand, if added excessively, the strength becomes too high, and on the contrary, the base metal toughness and the HAZ toughness are lowered. Therefore, the upper limit of these elements is 5.0%, preferably 2.5%.

The high-strength, high-toughness steel sheet of the present invention can be produced by a conventional method. The steel slab is heated to an austenite temperature range, preferably about AC _{3 to} 1350 ° C., then hot-rolled, hot-rolled, and air-cooled. Alternatively, it may be cooled at an average cooling rate of about 60 ° C./sec or less by direct cooling. In order to suppress the production of MA and GBF as much as possible, accelerated cooling is preferably performed at an average cooling rate of about 5 ° C./sec or more. This accelerated cooling may be performed up to a temperature range below the BF transformation point (about 650 to 400 ° C.). In order to ensure the BF transformation point or lower, the temperature may be lowered to about 200 ° C. or lower. Note that accelerated cooling is performed at a temperature of at least 800 ° C. because the cooling rate is high at high temperatures.

  The hot rolling finishing temperature may be 1000 ° C. or lower as in a conventional method, but by setting it to 870 ° C. or lower, the HAZ toughness can be further improved. The reason for this is that if unrecrystallized zone rolling is performed a lot when rolling the base metal, there will be many transformation nuclei (strain due to rolling) during reverse transformation under the influence of welding heat. This is because the old γ grain size is finer than finishing at a high temperature (above 870 ° C.). For these reasons, it is desirable to set the finishing temperature to 870 ° C. or lower. However, the HAZ toughness can be more effectively improved by setting the finishing temperature to preferably 800 ° C. or lower, more preferably 750 ° C. or lower. Moreover, by performing such low temperature finish rolling, the lower limit of AS can be expanded downward to about 3.20 as compared with the case of hot rolling by a conventional method.

  By the above manufacturing method, after hot rolling, BF is 85% or more, preferably 90% or more in area% over a high cooling rate to a low cooling rate, and the balance is high strength and high formed by GBF and MA. A tough structure is obtained. Since MA is finely generated at the interface of BF or GBF, the toughness is not deteriorated as in the case of massive MA. However, since less toughness provides better toughness, it is preferably 5.0 area% or less, more preferably 3. It should be 0% or less.

As described above, the steel sheet of the present invention can obtain a structure mainly composed of BF from the high cooling rate to the low cooling rate after the hot rolling, so that a relatively thick steel plate, for example, a thickness of about 50 mm is obtained. Even if it has a strength of 780 MPa or more, it has good base material toughness, HAZ toughness, and low temperature cracking resistance.
Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.

  The slabs (thickness 250 mm) obtained by melting the steel shown in Tables 1 to 3 and casting the molten metal were heated at 1150 ° C., and then hot-rolled, and the finishing temperatures shown in Tables 4 and 5 were obtained. The hot rolling was finished at, and the temperature range of 800 to 200 ° C. was directly cooled (on-line water cooling) at an average cooling rate of 10 ° C./sec. In addition, the steel of the sample of Table 4 and 5 respond | corresponds to the steel of the steel number (Tables 1-3) similar to a sample number.

  From the obtained hot-rolled sheet (thickness: 50 mm), a specimen for observing the structure was sampled from a quarter of the thickness of the hot-rolled sheet and subjected to optical microscope observation (400 times magnification). And the remainder was formed by GBF and MA. In addition, in order to measure the area fraction of BF, the tissue observation test piece was subjected to nital corrosion, and then the tissue was photographed at a magnification of 1000 using an SEM (scanning electron microscope), and the photographed image was image analysis software (named Image- Pro, manufactured by Planetron), and the area ratio of BF was determined. The results are also shown in Tables 4 and 5. In addition, about the example of an invention (sample No. 1-59), when MA amount was measured, it was 3.0 area% or less.

In addition, a tensile test and an impact test were performed in the following manner to examine the mechanical properties of the base material. As for the pass level, the tensile strength is 780 MPa or more, and the toughness is 200 J or more in absorbed energy (vE ₋₂₀ ).
-Tensile test A JIS No. 4 test piece was obtained from a 1/4 thickness portion of each steel plate, a tensile test was performed, and 0.2% yield strength and tensile strength were measured.
-Impact test A JIS No. 4 test piece was sampled from a 1/4 thickness portion of each steel plate, subjected to a Charpy impact test, an absorption energy (vE- ₂₀ ) at -40 ° C was determined, and a base material toughness was evaluated.

Further, the HAZ toughness was examined in the following manner.
Perform one-pass large heat input welding (electroslag welding) with a heat input of 800 kJ / cm, extract a JIS No. 4 test piece from HAZ 0.5 mm away from the bond (melting line), conduct a V-notch Charpy impact test, Absorption energy (vE _-40 ) at 40 ° C was measured. At this time, the number of samples was set to 5, the average value was obtained, and the HAZ toughness was evaluated. The acceptable level is that the absorbed energy (vE _-40 ) is 150 J or more on average.
In addition, about the invention example, based on the y-type weld crack test method prescribed | regulated to JISZ3158, the state which cooled the steel plate used for the test to 0 degreeC and -20 degreeC (Root crack prevention preheating temperature = 0 degreeC, -20 (° C.), and coating arc welding was performed at a heat input of 1.7 kJ / mm, and the low temperature cracking resistance was examined. No cracking occurred at any temperature.

The survey results are shown in Tables 4 and 5. From the table, the inventive examples have a base material toughness of 780 MPa or more and vE _-20 of all 200 J or more, and have high strength and excellent base material toughness. Further, regarding the HAZ toughness at the time of high heat input welding of 800 J / cm, vE- ₄₀ has an absorbed energy of 150 J or more, and it was confirmed that the HAZ toughness is excellent also at the high heat input welding.
On the other hand, comparative examples (Table 5, Nos. 81 to 115) in which any of the alloy composition, [Ti] / [N], AS value, and DL value are out of the scope of the invention, Despite accelerated cooling at about 10 ° C / sec, most of the HAZ toughness does not reach about 60J, and the base material vE- ₂₀ is generally less than 200J, and the base metal toughness is inferior. Met.

It is a typical CCT figure for demonstrating the relationship between the cooling rate after hot rolling of conventional steel and this invention steel, and a structure | tissue.

Claims (9)

mass%
C: 0.010 to 0.080%,
Si: 0.02 to 1.00%,
Mn: 1.10 to 2.90%,
P: 0 to 0.030%,
S: 0 to 0.010%,
Al: 0.20% or less,
Ni: 0.40 to 2.40%,
Cr: 0.50 to 1.95%,
Mo: 0.16 to 1.10%,
Ti: 0.002 to 0.030%,
N: 0.0058 to 0.0120%
1.0 ≦ [Ti] / [N] <4.0
The balance is composed of Fe and inevitable impurities, and AS and DL values defined by the following formulas are AS ≧ 3.60 and DL ≦ 2.80, and the structure is mainly composed of bainitic ferrite. A high-strength steel sheet excellent in toughness of the weld heat-affected zone.
AS = [Mn] + [Ni] + 2 × [Cu]
DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V]
However, [X] represents content (mass%) of element X.   Furthermore, Cu: 1.60% or less of the high-strength steel material according to claim 1.   The high-strength steel material according to claim 1 or 2, further comprising any one or more of B: 0.0050% or less, Nb: 0.100% or less, and V: less than 0.060%.   Furthermore, the high strength steel materials of any one of Claim 1 to 3 which contain 0.0050% or less of 1 type or 2 types of Ca and REM in total.   Furthermore, Mg: 0.0050% or less, The high-strength steel material of any one of Claim 1 to 4.   The high-strength steel material according to any one of claims 1 to 5, further comprising any one or two of Hf: 0.050% or less and Zr: 0.100% or less.   The high strength steel material according to any one of claims 1 to 6, further comprising any one or two of W: 5.0% or less and Co: 5.0% or less.   A high-strength steel plate formed by the high-strength steel material according to any one of claims 1 to 7. A method for producing a high-strength steel sheet, wherein the steel having the component according to any one of claims 1 to 7 is heated to an austenite temperature, hot-rolled at a finishing temperature of 870 ° C or lower, and cooled.

Images