JP4252949B2 - Low yield ratio high-tensile steel sheet with small acoustic anisotropy and excellent weldability, and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile steel sheet having a tensile strength as high as 780 MPa or more, a yield ratio as low as 85% or less, a small acoustic anisotropy, and excellent base material toughness and weldability.

  In steel plates for bridges and buildings (thick steel plates), if there is a defect in the welded part, this part is likely to be the starting point of fracture occurrence, so the presence or absence of the defective part is investigated by ultrasonic testing. In such a case, work such as repairing the part is generally performed. However, in a steel plate that is remarkably changed in sound speed depending on the flaw detection direction, that is, a steel plate with high acoustic anisotropy, it is impossible to detect the exact position of the weld defect in the ultrasonic flaw detection test. The acoustic anisotropy is required to be small.

  As described above, in the field of offshore structures, building structures, etc., the acoustic anisotropy of the steel sheet is required to be small in order to simplify the detection of welding defects from the improvement of welding construction efficiency, and further −40 There is a demand for a 780 MPa class high-tensile steel sheet that secures weldability (HAZ toughness, weld crack resistance) and base metal toughness at extremely low temperatures such as ° C.

  In addition, in recent years, particularly in the field of high-rise building structures that require earthquake resistance, it is possible to absorb the fracture energy added to the structure during an earthquake and prevent the collapse of the structure. A steel material having a low YR is demanded, but generally, a high strength steel sheet tends to have a high YR. YR (%) is represented by YS (0.2% yield strength) / TS (tensile strength) × 100.

  Conventionally, it has been difficult to secure low-temperature toughness in high-tensile steel sheets of 780 MPa class or higher, but in recent years, various techniques for improving base material toughness have been proposed. For example, in JP-A-11-172365 (Patent Document 1), the cumulative reduction ratio of non-recrystallization zone rolling in hot rolling is 50% or more so that the aspect ratio of prior austenite (γ) grains is 3 or more. In Japanese Patent Application Laid-Open No. 2001-220644 (Patent Document 2), hot rolling is performed at a rolling finishing temperature (FRT) of 850 ° C. or lower so that the average flatness of old γ grains is 50% or lower. However, Japanese Patent Laid-Open No. 2001-200334 (Patent Document 3) discloses that the bainite lath width can be reduced by setting the cumulative rolling reduction at Ar3 point or higher and lower than 900 ° C. in hot rolling to 10 to 50%. JP-A-9-3591 (Patent Document 4) describes that the lath length is shortened by hot rolling at a cumulative reduction of 30% or more in the recrystallization temperature range.

  On the other hand, in a high strength steel of 780 MPa class or higher, there is a problem that HAZ toughness deteriorates during high heat input welding. The reason is that when the heat input is increased, the cooling rate of the HAZ is decreased and the toughness is reduced by generating coarse island martensite. This problem occurs in both thick and thin objects when performing high heat input welding. For this reason, it is usual that welding heat input is restricted to 5 kJ / mm or less at the time of welding construction, and the welding efficiency has been inevitably lowered.

Various techniques for improving the HAZ toughness have been proposed for this problem. For example, Japanese Patent Laid-Open No. 2000-160281 (Patent Document 5) describes the former γ by making C low and actively adding hardenability improving elements Mn, Cr, Mo, or further finely dispersing TiN. In order to refine the grains, JP-A-6-65680 (Patent Document 6) has a low C, and further refinement of the old γ grains by fine dispersion of Ta ₂ O ₃ is disclosed in JP-A-5-171341. Japanese Patent Laid-Open No. 7-233437 discloses that Ti and Mg are added as essential components and the former γ grains are refined by dispersing oxides to promote the formation of intragranular ferrite. Japanese Patent Laid-Open No. 2-254120 (Patent Document 9) discloses that the hardenability can be improved by Pcm ≦ 0.24 and Ceq ≧ 0.45 under B-free. , B free It has been described in utilize precipitation strengthening by Cu.

  Moreover, as a high-tensile steel plate having excellent weldability and a low yield ratio, JP-A-6-248336 (Patent Document 10) and JP-A-6-248337 (Patent Document 11) describe welding of B-added steel. In order to improve the properties, a method for producing a high-strength steel sheet using no precipitation of B and utilizing precipitation hardening of V by quenching and tempering is described in order to ensure the strength associated with a decrease in hardenability due to the absence of B. Yes.

JP 11-172365 A JP 2001-220644 A Japanese Patent Laid-Open No. 2001-200334 JP-A-9-3591 JP 2000-160281 A JP-A-6-65680 JP-A-5-171341 JP-A-7-233437 JP-A-2-254120 JP-A-6-248336 JP-A-6-248337

The technology related to the improvement of the base metal toughness is that the addition amount of Mn, Cu, and Ni having the effect of lowering the transformation point is generally small and the Ar ₃ point is high, so that the rolling temperature in the non-recrystallized region of austenite is lowered. Since there is a limit to the improvement in base metal toughness due to low temperature rolling, it has been possible to obtain excellent base metal toughness such that the absorbed energy (vE _-50 ) in the Charpy impact test at -50 ° C is 100 J or more. I could not.

  In addition, the technologies related to the improvement of the HAZ toughness are to prevent the hardening of the HAZ at a high cooling rate by lowering the C, and any decrease in strength due to the lowering of C can be any of Nb, Mo, V, Or it is going to supplement by adding complex. However, when these elements are positively added, there is a problem that a bainite block that acts as resistance to crack propagation during bainite transformation becomes coarse, and the base metal toughness and HAZ toughness deteriorate.

  Moreover, in the technique of obtaining a hot rolled steel sheet having a low yield ratio while ensuring the weldability, a large amount of alloying elements such as C and V must be added in order to compensate for the strength reduction caused by the absence of B. For this reason, there is a problem that the weldability is deteriorated and the base material toughness is deteriorated.

  On the other hand, any technique does not consider reducing acoustic anisotropy, and has a problem in terms of acoustic anisotropy.

  The present invention has been made in view of such problems, and has a low yield ratio and a high base metal toughness and weldability (weld crack resistance, HAZ toughness) while having a high tensile strength of 780 MPa or more. In addition, an object is to provide a high-strength steel sheet having a small acoustic anisotropy and capable of easily detecting defects during welding and a method for manufacturing the same.

  As a result of conducting various experimental studies on the above-mentioned problems, the present inventor has determined the component design in consideration of the steel structure mainly composed of bainitic ferrite, that is, after limiting C to an extremely low amount, Without particularly controlling the cooling rate after hot rolling by suppressing the addition of Nb, V, Mo, which adversely affects HAZ toughness, and positively adding the hardenability improving elements Mn, Ni, Cu In addition, a structure mainly composed of bainitic ferrite (sometimes referred to as “BF”) can be generated at any of a high cooling rate and a low cooling rate, and thereby excellent base material toughness and welding can be achieved. It has been found that the sexuality can be realized. In addition, the amount of MA (Martensite-Austenite Constituent) contained in the organization together with BF as the main component is 10 area% or less and its hardness is increased to increase the bainitic nature of MA. It has been found that the YR is lowered by setting the hardness ratio to ferrite (MA hardness / BF hardness) to 1.1 or more. Further, it has been found that the acoustic anisotropy can be reduced by controlling the long axis / short axis ratio (flatness) of the prior austenite grains within a predetermined range. Further, it has been found that the flatness of the prior austenite can be realized by performing a predetermined amount of hot rolling in a specific temperature range according to the chemical composition of the steel. The present invention has been completed based on these findings.

That is, the high-tensile steel plate of the present invention is mass%, C: 0.010 to 0.080%, Si: 0.02 to 0.50%, Mn: 1.10 to 3.00%, Cu: 1 .60% or less, Ni: 0.40 to 2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0.200% or less, N: 0.0100% or less, Cr: Including 0.30 to 2.00%, Mo: 0.10 to 1.10%, Ti: 0.002 to 0.030%, the balance is Fe and inevitable impurities, and is defined by the following formula AS value and DL values AS ≧ 4.00, a DL ≦ 2.80, (excluding 0%.) 10 area% or less tissue of MA in the sheet thickness 1/4 sites unrealized, balance bainitic It consists of ferrite and granular bainitic ferrite, the bainitic ferrite in the structure Wherein an area ratio of 85% or more, and (sometimes referred to as "average oblateness".) The average value of the long axis / short axis of prior austenite grains is 1.0 to 3.0, further Beini of the MA The hardness ratio to tick ferrite is 1.10 or more. The plate thickness ¼ portion means a portion having a depth of ¼ of the plate thickness from the plate surface, and the structure observation surface at the plate thickness ¼ portion is, as usual, the plate thickness direction (with respect to the plate surface). And a plane (cross section perpendicular to the rolling direction, TD plane) including the vertical direction) and the rolling direction (length direction).
AS = [Mn] + [Ni] + 2 × [Cu]
DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V]
However, [X] represents content (mass%) of element X.

  In the structure of the high-tensile steel sheet, in order to further improve the base metal toughness, the average value of the equivalent circle diameter of the prior austenite grains (sometimes referred to as “average equivalent circle diameter”) is 70 μm or less, preferably 65 μm or less. More preferably, the thickness is 60 μm or less. The circle equivalent diameter means a diameter of a circle having an area equivalent to the area of the prior austenite grains.

Further, as the chemical component, (1) B: 0.0050% or less, (2) Nb: 0.10 % or less, V: 0.30% or less, one or two, (3) Ca : 0.0050% or less, Rare earth element (REM): Any one or two of 0.0100% 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) Co: 2.50% or less, W: Any one or two of 2.50 or less alone Or in combination.

Moreover, the suitable manufacturing method of the said high strength steel plate heats the steel which has the said component to Ar3 point-1300 degreeC, cools after hot-rolling 50% or more of the total reduction in a partial recrystallization temperature range. Further, the amount of MA and the hardness ratio to bainitic ferrite (the hardness of MA / the hardness of bainitic ferrite) are adjusted by reheating in the two-phase region of austenite / ferrite. Further, after reheating in the two-phase region, tempering can be performed so that MA does not decompose and disappear at a temperature below the Ar1 point. By performing such tempering, the base material toughness can be further improved. The partial recrystallization temperature range means that a steel plate test piece having an austenite grain size of 100 ± 10 μm was squeezed under conditions of a strain rate of 10 / second and an equivalent strain of 0.2, and the structure was frozen after 10 seconds by, for example, water cooling. Sometimes 20 to 80 vol% refers to a temperature range where recrystallized grains are formed.

According to the high-tensile steel sheet of the present invention, C is extremely low and Mn, Ni, and Cu are positively added so that the AS value is 4.00 or more, while addition of Mo, Nb, and V is performed in DL. Since the components were adjusted so that the value was 2.80 or less, it is difficult to cause crack propagation even when the plate thickness is as thick as 50 mm or more regardless of the cooling rate after hot rolling. ferrite can be a microstructure which chromatic an area ratio of 85% or more, while a high strength, provided excellent base material toughness, and excellent weldability (resistance to weld cracking resistance, HAZ toughness) a. In addition, since the hardness of MA is increased by reheating in the two-phase region, and the hardness ratio of MA to bainitic ferrite is 1.10 or more, the strength is as low as 85% or less while being high strength of 780 MPa or more. Yield ratio can be realized and the average flatness of the prior austenite grains can be reduced to 1.0-3.0, so that the acoustic anisotropy can be reduced and the defect detection work during welding is simplified. Can be

  The main point on the composition of the steel sheet of the present invention is that the amount of Nb, V, and Mo, which adversely affects the HAZ toughness and base metal toughness, is limited (DL ≦ 2.80) after limiting the C amount to an extremely low amount. The hardenability improving elements Mn, Ni, and Cu are positively added (AS ≧ 4.00). First, the structure and characteristics produced by hot rolling with the steel components of the steel sheet of the present invention will be described first 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 is Beiniti' Cheops ferrite, GBF is granulite label Initi' Cheops ferrite, M represents martensite, B is bainite, F is shows a ferrite.
From the figure, in the steel sheet (A) of the present invention, the BF is 85% or more in area ratio (such as this ) in both the high cooling rate (CR1) and the low cooling rate (CR2) after the hot rolling. A structure containing 85% by area or more of BF may be referred to as “organization mainly composed of BF.”) , More preferably 90% or more. With a structure mainly composed of BF (a trace amount of GBF may be generated in the remaining part), even if it is a thick plate having a thickness of 50 mm or more without any quenching or tempering heat treatment, As mechanical properties, a strength of 780 MPa or more is obtained, and excellent toughness is provided. Moreover, as described above, since the matrix structure is mainly BF having a low cooling rate sensitivity at both the high cooling rate (CR1) and the low cooling rate (CR2), the hardness of the HAZ is reduced under small heat input welding conditions. It is possible to reduce (improve cold cracking resistance), and to ensure HAZ toughness even under high heat input welding conditions.

  On the other hand, the conventional high-C steel (B1) generates a considerable amount of M at a high cooling rate (CR1). Therefore, the hardness is sensitive to the cooling rate, and the HAZ at the time of small heat input welding is high. It was difficult to achieve both reduction in hardness and strength and toughness of the base material. In addition, conventional high C steel (B1) and low C steel (B2) produce F or GBF at a medium cooling rate or a low cooling rate (CR2), and as a result, coarse and massive MA is produced. Moreover, the base metal strength and toughness were lowered, and the HAZ toughness at the time of high heat input welding could not be secured.

Next, the relationship between the acoustic anisotropy of the steel sheet and the flatness of prior austenite grains (γ grains) will be described. For acoustic anisotropy, the transverse wave sound velocity ratio C _SL / C _SC defined in JIS Z 3060 (the transverse wave sound velocity value C _SL obtained with the vibration direction as the L direction (rolling direction) and the C direction (the direction perpendicular to the rolling direction)). (M / sec) and C _SC (m / sec)).
The present inventor has determined that the shear wave sound speed ratio C _SL / C _SC is a low value such as 1.020 or less, that is, low acoustic anisotropy, that is, the shear wave sound speed ratio (C _SL / C _SC ) and the flatness of the old γ grains. The relationship with the rate was investigated. The result is shown in FIG. From FIG. 3, it was found that when the flatness of the old γ grains is 3.0 or less (minimum value is 1.0), the low acoustic anisotropy such that the shear wave sound velocity ratio is 1.020 or less is achieved. From the viewpoint of acoustic anisotropy, the flatness of the old γ grains is preferably 1.8 or less, more preferably 1.6 or less. FIG. 3 is obtained from the examples described later.

Further, according to the investigation by the present inventor, it has been found that there is a close relationship between the average equivalent circular diameter of the old γ grains and the base material toughness (vE _-50 ). FIG. 4 shows the relationship between the average equivalent circular diameter of the old γ grains and the base metal toughness (vE _-50 ). From FIG. 4, the smaller the average equivalent circular diameter of the old γ grains, the more the base toughness (vE _{− 50} ) is improved. Accordingly, it is desirable that the average equivalent circle diameter of the prior γ particle diameter is 70 μm, preferably 60 μm or less, more preferably 40 μm or less. FIG. 4 is obtained from the examples described later.

In general, since MA is harder than the parent phase, it is known that when MA is left, the yield ratio decreases, but the base material toughness also decreases. For example, in a normal steel material, when the yield ratio is to be 85% or less, it is necessary to leave the MA amount at about 10% or more. On the other hand, when such an amount of MA remains, the toughness of the base metal is remarkably lowered and the desired characteristics cannot be exhibited. For this reason, normally, steel of the component system which is hard to produce MA is used so that MA does not remain, and forced cooling is performed after rolling. If the MA still remains, a tempering process is applied to further decompose the MA.
However, as a result of a detailed investigation by the present inventor, when the steel of the present invention having AS ≧ 4.00 and DL ≦ 2.80 is used, the hardness of MA with respect to the hardness of BF (matrix) and the MA While maintaining the base material toughness by adjusting the balance with the amount, that is, the MA amount is 10% or less in area ratio and the hardness ratio of MA to BF is 1.10 times or more, It was found that the yield ratio could be 85% or less.
FIG. 5 shows the BF of MA for a steel material whose MA content and base material toughness are approximately the same level (MA amount is about 2 to 4% and base material toughness [vE _-50 ] is about 120 to 140 J). The relationship between the hardness ratio and the yield ratio is summarized, but the figure shows that the yield ratio is 85% or less when the hardness ratio is 1.10 times or more. However, when the amount of MA exceeds 10 area%, the base material toughness deteriorates even when the hardness ratio is 1.10 times or more (sample No. 66 in Table 9 in Examples below and 75, sample No. 80 in Table 10). FIG. 5 is obtained from the examples described later.

In the case of the steel according to the present invention, the reason why the yield ratio is lowered by increasing the hardness of MA even if the amount of MA is small is not necessarily clear, but it is presumed to be as follows. That is, in the present invention, the amount and hardness of MA are adjusted by performing a two-phase region heat treatment after hot rolling, but the concentration of C to MA occurs during this heat treatment, and the hardness of MA is reduced. It seems to have risen. At that time, it is speculated that movable dislocations are introduced around martensite in MA as C is concentrated. It is thought that by introducing this movable dislocation, a situation that yields easily is obtained.
Since the feature of the present invention is to effectively utilize the presence of such MA, the MA must be present. In order to effectively exhibit the effect of MA, it is desirable that it be 0.5 area% or more, more preferably 1.0 area% or more, and still more preferably 2.0 area% or more. However, if the amount of MA exceeds 10 area%, the influence on the base material may occur. Therefore, the upper limit of the MA amount is 10 area%, preferably 9.0 area% or less, more preferably It is good to set it as 8.0 area% or less, More preferably, it is 7.0 area% or less.

Next, the reasons for limiting the components of the high-tensile steel sheet of the present invention will be described in detail. 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%, it becomes impossible to ensure a base material strength of 780 MPa or more even if Mn, Ni and Cu, which are hardenability improving elements, are positively added. On the other hand, if it exceeds 0.080%, martensite is generated instead of bainitic ferrite on the high cooling rate side, and the low temperature cracking resistance deteriorates. By adding 0.010% or more of C and limiting it to 0.080% or less, and simultaneously adding appropriate amounts of Mn, Ni, Cu and Cr, the low temperature cracking resistance of HAZ during small heat input welding and the mother It is possible to achieve both material strength and to improve the toughness of HAZ at the time of large heat input. For this reason, the lower limit of the C amount is 0.010%, preferably 0.030%, while the upper limit is 0.080%, preferably 0.060%.

Si: 0.02 to 0.50%
Si is an element having a deoxidizing action. If the Si content is less than 0.02%, its effect is too small. On the other hand, if it exceeds 0.50%, the weldability and the base metal toughness are deteriorated. For this reason, the lower limit of the Si amount is 0.02%, and the upper limit is 0.50%, preferably 0.20%.

Mn: 1.10 to 3.00%, Ni: 0.40 to 2.50%, Cu: 1.60% or less These elements have an effect of improving the hardenability, and have a low cooling rate from a high cooling rate. Facilitates the formation of bainitic ferrite over speed, and by aggressive addition and ultra-low C, both HAZ toughness and cold cracking resistance at the time of small heat input welding are achieved, and the strength and toughness of the base metal In addition, the HAZ toughness during high heat input welding can be improved.

That, Mn is effective strength improves hardenability, to ensure the toughness is too small action according is less than 1.10%, whereas rather low temperature toughness is degraded 3.00 percent. Therefore, the lower limit of the amount of Mn is 1.10%, preferably 1.30%, more preferably 1.40%, and the upper limit is 3.00%, preferably 2.20%, more preferably 2.10. %.
Ni also improves the low temperature toughness and hardenability of the steel to improve the strength, and is effective in preventing hot cracking and weld hot cracking. If the amount of Ni is less than 0.40%, these effects are too small. On the other hand, if it exceeds 2.50%, scale wrinkles are likely to occur. For this reason, the lower limit of the Ni amount is 0.40%, preferably 0.50%, and the upper limit is 2.50%, preferably 2.00%.
Cu is not as hard as Mo, Mn, Ni, and Cr, but improves hardenability, and improves the strength of the base metal by solid solution strengthening and precipitation strengthening. Addition of 0.10% or more, more preferably 0.50% or more, and still more preferably 0.80% or more is desirable for effectively exhibiting such an action. However, if it exceeds 1.60%, the toughness of the base metal and the HAZ toughness at the time of high heat input welding are lowered, so the upper limit of the Cu amount is 1.60%, preferably 1.20%.

AS value: 4.00 or more The addition amount of Mn, Ni, and Cu is closely related to the strength of the base material, 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 the high cooling rate to the low cooling rate, the AS value is 4.00 or more, preferably 4.20 or more, more preferably 4.20 or more, as will be apparent from the examples described later. It is necessary to add Mn, Ni and Cu so as to be 40 or more.

P: 0.030% or less P, which is an impurity element, adversely affects the toughness of the base metal and the welded portion, so it is limited to 0.030% or less. Preferably it is 0.010% or less.

S: 0.010% or less S is an element that forms MnS and lowers the ductility. Particularly in high-strength steel, the effect is large, so 0.010% or less, preferably 0.005% or less. Good.

Al: 0.200% or less Al has an effect of improving the base material toughness by deoxidation and refinement of the microstructure. Addition of 0.010% or more, more preferably 0.020% or more is desirable in order to effectively exhibit such action. However, if added in excess, the toughness of the base material is rather lowered, so the upper limit is made 0.200%. Preferably it is 0.060% or less.

N: 0.0100% or less N has an effect of combining with Ti described later to form TiN to refine the austenite grains during high heat input welding and improve the HAZ toughness. Addition of 0.0020% or more, more preferably 0.0040% or more is desirable in order to effectively exhibit such action. However, excessive addition of N adversely affects the base metal toughness and HAZ toughness, so the upper limit is made 0.0100%, preferably 0.0080%, more preferably 0.0060% or less.

Cr: 0.30 to 2.00%
Cr increases the strength of the base metal and the welded portion. However, if the Cr content is less than 0.30%, such an effect is too small. On the other hand, if it exceeds 2.00%, the weldability and the HAZ toughness are deteriorated. For this reason, the lower limit of the Cr content is 0.30%, preferably 0.50%, more preferably 0.70%, and the upper limit is 2.00%, preferably 1.50%, more preferably 1.00. %.

Mo: 0.10 to 1.10%
Mo is effective for improving hardenability and ensuring strength, and has an effect of preventing temper brittleness. If the amount of Mo is less than 0.10%, this effect is too small, so the lower limit of the amount of Mo is 0.10%, preferably 0.15%. On the other hand, Mo has an effect of suppressing recrystallization, and if added excessively, coarse austenite grains are formed after rolling, the transformed bainite blocks (bundles of bainitic ferrite) are coarsened, and the toughness of the base material is deteriorated. Moreover, Mo is easily segregated at the austenite grain boundary, and if added excessively, the frequency of nucleation during transformation is lowered, and the bainite block after transformation is coarsened to deteriorate the base material toughness and HAZ toughness. For this reason, the upper limit of the Mo amount is 1.10%, preferably 0.60%.

DL value: 2.80 or less Mo and Nb and V described later have the effect of improving the hardenability, but on the other hand, the bainite block is coarsened and the base material toughness and the HAZ toughness are deteriorated. Such a deterioration effect of the base material toughness is not uniform for each element. When Mo is set to 1 through experiments by the inventors, Nb is about 12 times and V is about 4 times. As will be apparent from the examples described later, by suppressing the addition of Mo, Nb, and V so that the DL value is 2.80 or less, preferably 2.50 or less, more preferably 2.00 or less, a bainite block is obtained. The above-mentioned AS ≧ 4.00 and the amount of reduction in the partial recrystallization temperature range to be 50% or more of the total hot rolling reduction, the average equivalent circle diameter of the old γ grains is about 70 μm. It is refined to the following, and it is possible to ensure the Maemura toughness of vE _-50 ≧ 100 J or more and also to have a good HAZ toughness.

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 amount of Ti is less than 0.002%, the effect of refining is too small. On the other hand, if it exceeds 0.030%, the HAZ toughness is deteriorated. For this reason, the lower limit of the Ti amount is 0.002%, preferably 0.005%, and the upper limit is 0.030%, preferably 0.020%.

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) B in the following range, (2) One or two of Nb and V in the following range, (3) One or more of Ca and REM in the following range, (4) Mg, (5) any one or two of Zr and Hf in the following range, and (6) any one or two elements of Co and W in the following ranges, Alternatively, it can be added in combination.

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 0.045%. More preferably, the content is 0.0010 to 0.0040%.

Nb: 0.10% or less Solid solution Nb has the effect of improving the hardenability of the base material and improving the strength of the base metal and the welded joint, and can be added as necessary. On the other hand, solute Nb suppresses the recovery of processed austenite and suppresses recrystallization, so that coarse austenite grains are formed after rolling, the bainite block after transformation becomes coarse, and the base metal toughness is significantly reduced. Nb tends to segregate at the austenite grain boundaries, and if added excessively, the frequency of nucleation during transformation is reduced, and the bainite block after transformation is coarsened to deteriorate the base metal toughness and HAZ toughness. For this reason, the upper limit of the Nb amount is 0.10%, preferably 0.020%, more preferably 0.015%.

V: 0.30% or less V has the effect of increasing hardenability and temper softening resistance when added in a small amount, and can be added as necessary. On the other hand, V suppresses the recovery of processed austenite and suppresses recrystallization, so that coarse austenite grains are formed after rolling, the bainite block after transformation becomes coarse, and the base material toughness is significantly reduced. V is easily segregated at the austenite grain boundary, and if added excessively, the nucleation frequency at the time of transformation is lowered, the bainite block after transformation is coarsened, and the base metal toughness and the HAZ toughness are deteriorated. For this reason, the upper limit of V amount is set to 0.30%. The content is preferably 0.20% or less, more preferably 0.10% or less.

Ca: 0.0050% or less, REM: 0.0100% or less Ca and REM have the effect of reducing anisotropy by controlling the form of inclusions to spheroidize MnS. If the Ca content exceeds 0.0050% and the REM content exceeds 0.0100%, the added amount is excessive, so that the toughness of the base material is changed and deteriorated. Therefore, the upper limit of the Ca content 0.0050%, preferably 0.0030% to the upper limit of the REM 0.0100%, preferably to 0.0070%. In order to effectively use each of the above elements, it is preferable to contain Ca: 0.0005% or more and REM: 0.0010% or more.

Mg: 0.0050% or less Mg has the effect of improving the HAZ toughness by forming MgO and suppressing the coarsening of HAZ austenite grains. In order to effectively utilize this action, it is preferable to contain Mg: 0.0001% or more. Mg: If it exceeds 0.0050%, the added amount is excessive, so that the toughness of the base material is changed and deteriorated. For this reason, the upper limit of the amount of Mg is made 0.0050%, preferably 0.0035%.

Zr: 0.100% or less, Hf: 0.050% or less Zr and Hf are elements effective for improving HAZ toughness by forming nitrides with N and refining HAZ austenite grains during welding. is there. However, if added excessively, the base metal toughness and HAZ toughness are reduced. For this reason, the upper limit of the Zr amount is 0.100%, and the upper limit of the Hf amount is 0.050%.

Co: 2.50% or less, W: 2.50% or less Co and W are effective elements for improving hardenability and ensuring strength easily. In the case of W, temper softening resistance is further improved. Has the effect of On the other hand, when added excessively, the base metal toughness and the HAZ toughness are deteriorated. For this reason, the upper limits of the Co amount and the W amount are each 2.50%, preferably 1.00%.

Next, the manufacturing method of the low yield ratio high tension steel plate of this invention is demonstrated.
In the production method of the present invention, it is necessary to strictly control the hot rolling conditions in order to control the form of the old γ grains on the premise that the steel having the above chemical composition (the balance unavoidable impurities) is used. . Furthermore, after obtaining the hot-rolled steel sheet having a structure composed mainly of BF, for the adjustment of the yield ratio, i.e., subjected to reheating at two-phase region in order to adjust the amount and hardness of the MA, mother necessary Tempering is performed to adjust toughness. The other steps and conditions for producing the steel plate of the present invention are not particularly limited, and the usually used high-strength steel plate production steps and conditions (temperature, time, etc.) can be appropriately employed.

  As the rolling conditions in the production method of the present invention, the steel slab is heated to an Ar3 point to 1300 ° C. to completely austenite, and then hot rolled. In hot rolling, it is particularly important to roll 50% or more of the total reduction amount, preferably 70% or more of the total reduction amount in the partial recrystallization temperature range. By rolling in such a temperature range, as will be apparent from the examples described later, the average flatness and average equivalent circle diameter so as to equiax the morphology of the old γ grains in the steel sheet using the phenomenon of partial recrystallization. Can be controlled to a predetermined value.

  Since the partial recrystallization temperature range varies depending on the chemical composition of the steel sheet, the temperature range may be examined by an appropriate experiment before hot rolling. That is, a steel plate test piece having the same chemical composition as the steel plate to be manufactured was prepared, and the test piece was heated to a temperature at which the austenite grain size was 100 ± 10 μm, and then the test piece was subjected to a strain rate of 10 / sec. A temperature range in which the recrystallized grains become 20 to 80 vol% when the structure is frozen under a condition of strain of 0.2 and frozen after 10 seconds, for example, by water cooling, that is, a partial recrystallization temperature range is obtained in advance.

  The cooling conditions after the hot rolling are air cooling or water cooling to a temperature below the Bs point (about 200 ° C.). Since the amount of MA is adjusted by the subsequent reheating treatment in the two-phase region, from this viewpoint, rapid cooling (in this case, MA is difficult to produce) or air cooling may be used, but the amount of BF in the steel structure may be In order to make more, it is preferable to cool with water. The water cooling here means a cooling rate of about 3 ° C./sec or more, preferably 5 ° C./sec or more.

  After hot rolling and cooling to obtain a predetermined low temperature transformation structure, reheating is typically performed in a two-phase region of about 700 to 900 ° C. after cooling in order to reduce the yield ratio. Thereby, while producing MA from BF (matrix), carbon is concentrated in MA, and the amount of MA is adjusted to 10 area% or less, and the hardness ratio of MA to BF is adjusted to 1.10 or more.

  Further, after the reheating, tempering can be performed at a temperature of Ar 1 point or less as necessary. In this case, the holding temperature and holding time are adjusted so that MA is not completely decomposed and disappears. Such tempering can improve the base material toughness while realizing a low yield ratio. Cooling from the tempering temperature is not particularly limited, and may be air-cooled.

  Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.

  Steels shown in Tables 1 to 5 are melted by a normal melting method to form slabs, and as shown in Tables 6 to 10, after heating under the conditions shown in the same table, hot rolling is performed, and the temperature is 200 ° C. or lower. After air cooling (average cooling rate: about 5 to 10 ° C./sec), reheating was performed for about 15 to 30 minutes in the two-phase region, and further tempering treatment was performed as necessary, followed by cooling with air cooling. In addition, each sample of Tables 6-10 was manufactured using the steel of the same number of Tables 1-5. The amount of reduction in the partial recrystallization temperature range indicates the ratio (%) of the reduction rate rolled in the partial recrystallization temperature range to the total reduction rate in hot rolling.

The obtained hot-rolled sheet was sampled from a 1/4 part of the thickness of the hot-rolled sheet and subjected to optical microscope observation (magnification 400 times). As a result, BF was the main component, and the balance was MA. Or it became the structure | tissue which consists of MA and a trace amount GBF. In order to measure these area fractions, etc., the structure observation specimen was corroded with 2% nitric acid-ethanol solution (generic name: 2% nital solution), and then the structure was photographed at a magnification of 1000 using a scanning electron microscope (SEM). The photographed images were analyzed using image analysis software (named Image-Pro, manufactured by Planetron), and the area ratios of BF, GBF, and MA were determined.
Also, the hardness of BF (matrix) and MA was measured at a load of 1 g using a micropicker tester (manufactured by Akashi Seisakusho), and the average value of three points excluding the highest and lowest values was obtained. Based on this average hardness, the hardness ratio of MA to BF was determined.

  Further, the flatness and equivalent circle diameter of the old γ grains were determined by the following procedure using the structure observation specimen. The mirror-polished test piece is subjected to a corrosion treatment using an AGS solution manufactured by Yamamoto Kagaku Tool Research Co., Ltd. or a 2% nital solution. Corrosion conditions are 5 to 10 minutes at room temperature for the AGS solution and 5 to 30 seconds at room temperature for the 2% nital solution. The test piece after corrosion is observed at a magnification of 400 times using an optical microscope and photographed. The obtained micrographs are analyzed using image analysis software (named Image-Pro Plus, Media Cybernetics) to determine the equivalent circle diameter, and the length of the old γ grain long axis and short axis. The flatness (major axis / minor axis) value is calculated.

Moreover, the acoustic anisotropy was investigated using the sample steel plate. The acoustic anisotropy is determined in accordance with JIS Z 3060 according to the transverse wave sound velocity value C _SL (m / second) in the vibration direction L direction (rolling direction) and the transverse wave sound velocity value C _SC (m / second) in the C direction (rolling perpendicular direction). Seconds), and the transverse sound velocity ratio C _SL / C _SC was determined and evaluated.

In addition, a tensile test and an impact test were performed in the following manner to examine the mechanical properties of the base material.
The tensile test is performed using a JIS No. 4 test piece taken from a 1/4 thickness portion of each steel plate, 0.2% proof stress (YS) and tensile strength (TS) are measured, and the yield ratio (YS / TS). X100%). Moreover, the impact test performed the Charpy impact test at -50 degreeC using the JIS4 test piece extract | collected from the plate | board thickness 1/4 site | part of each steel plate, and calculated | required the absorbed energy (vE- ₅₀ ). In the present invention, TS ≧ 780 MPa, YR ≦ 85%, and base metal toughness vE ₋₅₀ ≧ 100 (J) were set as acceptable levels.

Furthermore, HAZ toughness and low temperature cracking resistance are as follows for all of those with a tensile strength of 780 MPa or more and a base material toughness of vE _-50 ≧ 100 (J) and some of which did not meet the acceptance criteria. I examined the sex.
For HAZ toughness, welding (submerged arc welding) is performed at a heat input of 5 kJ / mm, 10 kJ / mm, and 15 kJ / mm, and a JIS No. 4 specimen is taken from the specimen collection site 3 shown in FIG. A Charpy impact test was conducted to determine the absorption energy (vE _-40 ) of the bond portion, and vE _-40 ≧ 80 J was set as an acceptable level. In the figure, 1 is a steel plate, 2 is a weld metal part, 3 is a specimen collection part, and is located on the groove opening side from the center of the plate thickness. Super large heat input welding with a heat input of 15 kJ / mm was carried out in order to see the influence of alloy elements when the cooling rate was very slow.
The low temperature cracking resistance was measured based on the y-type weld cracking test method defined in JISZ3158 by covering arc welding with a heat input of 1.7 kJ / mm and measuring the root cracking prevention preheating temperature. The preheating temperature of 0 ° C. indicates that the steel plate subjected to the test was welded in a state cooled to 0 ° C., and no crack was generated after welding.

The survey results are shown in Tables 6 to 10. 3 shows the relationship between the average flatness of the old γ grains and the acoustic anisotropy (plotted sample Nos. 1 to 12, 89 to 93), and the relationship between the average equivalent circular diameter of the old γ grains and the base material toughness. FIG. 4 (Plotted Sample Nos. 1-4), FIG. 5 (Plotted Sample Nos. 43, 49-52, 96-99), AS Value and Tensile Strength, Relationship between the Hardness Ratio of MA to BF and YR Is shown in FIG. 6 (Plotted Sample Nos. 11, 18 to 20, 63 , 73, 83 ).

FIG. 3 shows that low acoustic anisotropy is obtained such that the flatness of the old γ grains is 3.0 or less and the transverse wave sound velocity ratio is 1.020 or less. Further, as shown in FIG. 4, as the average equivalent circle diameter of the old γ grains is refined, the base material toughness (vE _-50 ) is improved, and the average equivalent circle diameter of the old γ grains is reduced to 70 μm or less. It can be seen that the energy is about 100 J or more. FIG. 5 also shows that a high strength steel sheet with a low yield ratio of 85% or less can be obtained by setting the hardness ratio to 1.1 or more. FIG. 6 also shows that a high-strength steel sheet having a tensile strength of 780 MPa or more can be obtained by setting the AS value to 4.00 or more.

Further, from Tables 6 to 8, the invention examples show that the base material toughness is that vE- ₅₀ is 100 J or more, and the low temperature cracking resistance does not cause root cracking even when the steel plate temperature is 0 ° C. Excellent cold cracking resistance. Moreover, also about the HAZ toughness, it was confirmed that the toughness of the bond part is excellent in both the small heat input welding and the large heat input welding .

On the other hand, as shown in Tables 9 and 10, the comparative example in which the alloy composition (including AS value and DL value) is out of the scope of the invention has a tensile strength of less than 780 MPa even if the manufacturing conditions are appropriate. The base material toughness vE- ₅₀ was less than 100 J and did not reach the acceptable level. Moreover, even if the alloy composition is within the scope of the invention as in sample Nos. 84, 87, 89 to 93, if the manufacturing conditions are inappropriate and the amount of reduction in the partial recrystallization temperature range is less than 50%, The anisotropy exceeded 1.020, and the acoustic anisotropy deteriorated. In addition, as shown in sample Nos. 96 to 99, even when the components, heating hot rolling conditions, and tempering temperature are appropriate, YR exceeds 85% in the case where reheating in the two-phase region was not performed. The target level was not reached.

The typical CCT figure for demonstrating the relationship between the cooling rate at the time of manufacture of this invention steel and a structure | tissue is shown. Sectional explanatory drawing of the steel plate weld part which shows the extraction | collection site | part of the test piece for investigating the HAZ toughness in an Example is shown. It is a figure which shows the relationship between the average oblateness of the old gamma grain in Example, and acoustic anisotropy. It is a figure which shows the relationship between the average equivalent circular diameter of an old γ grain, and base material toughness in an Example. It is a figure which shows the relationship between the hardness ratio of MA with respect to BF, and YR in an Example. It is a figure which shows the relationship between AS value and tensile strength in an Example.

Claims (10)

mass%
C: 0.010 to 0.080%,
Si: 0.02 to 0.50%,
Mn: 1.10 to 3.00%
Cu: 1.60% or less,
Ni: 0.40 to 2.50%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.200% or less,
N: 0.0100% or less Cr: 0.30-2.00%,
Mo: 0.10 to 1.10%,
Ti: 0.002 to 0.030%,
And the balance is Fe and inevitable impurities, and AS and DL values defined by the following formulas are AS ≧ 4.00 and DL ≦ 2.80, and the structure at the 1/4 thickness region is MA. (excluding 0%.) 10 area% or less viewed contains, and the balance of the bainitic ferrite and granular labeling initiative ticks ferrite comprises the bainitic ferrite 85% or more at an area ratio in a tissue, and the former Acoustic anisotropy characterized in that the average value of major axis / minor axis of austenite grains is 1.0 to 3.0 and the hardness ratio of MA to bainitic ferrite is 1.10 or more A low-yield ratio high-tensile steel plate with small weldability and excellent weldability.
AS = [Mn] + [Ni] + 2 × [Cu]
DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V]
However, [X] represents content (mass%) of element X.   The low-yield-ratio high-tensile steel sheet according to claim 1, wherein in the structure, an average value of equivalent circle diameters of prior austenite grains is 70 µm or less.   The low yield ratio high tensile strength steel sheet according to claim 1 or 2, further comprising B: 0.0050% or less.   The low yield ratio high tensile strength steel sheet according to any one of claims 1 to 3, further comprising any one or two of Nb: 0.10% or less and V: 0.30% or less.   Furthermore, the low yield ratio high-tensile steel sheet according to any one of claims 1 to 4, further comprising any one or two of Ca: 0.0050% or less and rare earth elements: 0.0100% or less.   Furthermore, Mg: The low yield ratio high-tensile steel plate of any one of Claim 1 to 5 containing 0.0050% or less. Furthermore, the low yield ratio high-tensile steel sheet according to any one of claims 1 to 6, containing any one or two of Hf: 0.050% or less and Zr: 0.100% or less. The low yield ratio high-tensile steel sheet according to any one of claims 1 to 7, further comprising any one or two of Co: 2.50% or less and W: 2.50% or less.   A steel plate having a composition described in any one of claims 1 to 8 is heated to an Ar3 point to 1300 ° C and an austenite grain size is set to 100 ± 10 µm. 2) When the structure is frozen after 10 seconds and the structure is frozen after 10 seconds, 50% or more of the total reduction amount is hot-rolled in a partial recrystallization temperature range in which 20 to 80 vol% becomes recrystallized grains, and then cooled. The acoustic anisotropy according to any one of claims 1 to 8, further comprising adjusting the amount of MA and the hardness ratio with respect to bainitic ferrite by reheating in a two-phase region of austenite and ferrite. Of low yield ratio high tensile strength steel sheet with small weldability and excellent weldability.   The manufacturing method according to claim 9, wherein after reheating in the two-phase region, tempering is performed so that MA does not decompose and disappear at a temperature of Ar 1 point or less.

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