JP2007177326A - High tensile strength thin steel sheet having low yield ratio and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength thin steel sheet having a YR (yield ratio) of ≤80% and a TS (tensile strength) of ≥590 MPa. <P>SOLUTION: A steel stock having a composition comprising 0.045 to 0.18% C, 0.05 to 0.50% Si and 0.6 to 2.0% Mn, comprising proper amounts of P, S, Al and N, and further comprising Mo and W in such a manner that (Mo+W/2) satisfies 0.08 to 0.70%, and the balance Fe with inevitable impurities, and having a weld crack sensitivity parameter Pcm of ≤0.22% is subjected to: hot rolling in which rolling finishing temperature reaches the one in the range of 800 to 950°C at the surface; and cooling treatment composed of primary cooling in which cooling is performed till the central part reaches a temperature in the range of 750 to 650°C at a cooling rate of ≥10°C/s immediately after the rolling, air cooling for 2 to 10s, and the subsequent secondary cooling in which cooling is performed at a cooling rate of ≥10°C/s and is stopped at a temperature in the range of 500 to 650°C at the central part. By the above process, the high tensile strength thin steel sheet simultaneously satisfying a TS of ≥590 and a YR of ≤80%, and also having excellent weldability and toughness can be obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength thin steel plate suitable for use in bridges, shipbuilding, construction, line pipes, construction machinery, and the like, and a method for producing the same, and particularly to a building structure that is subjected to large plastic deformation due to an earthquake and requires earthquake resistance. The present invention relates to a high-tension thin-walled steel sheet having a low yield ratio, which is suitable for goods. In the present invention, the “thin steel plate” refers to a thick plate having a thickness of 19 to 40 mm.

In recent years, steel sheets having excellent earthquake resistance have been required for building structures and the like from the viewpoint of ensuring safety during an earthquake. Moreover, it has been clarified from the conventional research results that the steel plate having a lower yield ratio is superior in earthquake resistance, and it is obliged to use steel materials having a yield ratio of 80% or less for the building structure.
In addition, with an increase in the height of a building structure and an increase in span, the application of, for example, a 590 MPa class high-tensile steel material having a higher strength than before, to the building structure is increasing.

Conventionally, 590 MPa class high-strength steel materials having a low yield ratio used for building structures are generally manufactured by performing multiple heat treatments such as two-phase region heat treatment and subsequent tempering heat treatment. It was. However, multiple heat treatments such as two-phase region heat treatment and subsequent tempering heat treatment have caused problems such as a rise in manufacturing costs and an increase in the manufacturing period.
In order to solve such a problem, it is conceivable to omit the two-phase region heat treatment or the tempering heat treatment, that is, non-tempering. For example, in Patent Document 1, accelerated cooling is performed after rough rolling to supercool austenite (γ), then finish rolling to promote ferrite (α) transformation is performed, and accelerated cooling is further performed after finish rolling. Has disclosed a method for producing a high-tensile steel material with a low yield ratio that achieves both high toughness and a low yield ratio by appropriately controlling the refinement of the soft phase α and the ratio of the soft phase to the hard phase. Yes. According to the technique described in Patent Document 1, a low-yield-ratio high-strength steel material can be manufactured without adding expensive alloy elements or requiring complicated heat treatment with low productivity.

In Patent Document 2, Patent Document 3, and Patent Document 4, when heating and rolling a steel slab in which the content of C, Si, Mn, Nb, Ti, V, Al, etc. is adjusted to an appropriate range, 800 ° C. A method for producing a 590 to 690 MPa class high-tensile steel sheet having a low yield ratio, in which the rolling amount at the following temperature is 5 to 15 mm and the rolling is completed at a temperature equal to or lower than the Ar ₃ transformation point, has been proposed. However, in the techniques described in Patent Document 2, Patent Document 3, and Patent Document 4, the thickness is limited to 25 mm or less, and the yield ratio level is set to 85% or less, from the viewpoint of ensuring earthquake resistance. Is not sufficient, and requires addition of a large amount of an alloying element such as Ti, leaving a problem from the viewpoint of ensuring the toughness of the heat affected zone.

  Patent Document 5 contains C: 0.02 to 0.04%, solid solution B: 0.0002 to 0.002%, a composition having a formula CEN related to the alloy element content within a predetermined range, bainite as a main component, and islands. 590 MPa class non-tempered type low yield ratio high tensile strength steel sheets having a structure in which 0.8 to 2.5 volume% of martensite is dispersed have been proposed. Patent Document 6 includes C: 0.025 to 0.045%, Si, Mn, Al, Nb, B, and N are adjusted to an appropriate range, and Ti is limited to an appropriate range in relation to N. Rolling was started after heating the steel material with a composition within the prescribed range CEN related to alloy element content to the Nb solid solution temperature or higher and 1250 ° C or lower, and the cumulative reduction ratio was 60% in the austenite non-recrystallization temperature range. 590MPa class non-refining type low in which the rolling is finished at {(non-recrystallization temperature of austenite) -80 ° C} or more, air cooled, and the island-like martensite phase is finely dispersed in bainite. A method for producing a high yield strength steel sheet has been proposed. However, in the techniques described in Patent Document 5 and Patent Document 6, the carbon content of the steel material is reduced to 0.02 to 0.04% or 0.025 to 0.045% to further reduce carbon, and in particular, to ensure the strength of thick materials. However, it is necessary to add a large amount of alloying elements, which increases the manufacturing cost. Furthermore, in the technique described in Patent Document 6, it is essential to lower the rolling finishing temperature, and there is a problem that stable production becomes difficult from the viewpoint of rolling mill load.

  Further, Patent Document 7 contains C: 0.05 to 0.15%, Mo: 0.7 to 1.2%, further contains Si, Mn, P, S, Nb, Al, N, and Pcm is 0.25% or less. Rolling is completed at a temperature of 700 to 800 ° C. with a cumulative reduction amount of 950 ° C. or less at 50% or more and allowed to cool, and the microstructure at the 1/4 thickness position in the thickness direction is polygonal or pseudo. There has been proposed a method for producing a non-tempered low yield ratio high strength steel excellent in high temperature strength and having a structure mainly composed of polygonal ferrite. However, in the technique described in Patent Document 7, the rolling finishing temperature is lowered to increase the strength and the yield ratio. However, the amount of alloying elements added is large, and in particular, Mo should be added in an amount of 0.7% or more. There is a problem that the manufacturing cost is inevitable.

On the other hand, in Patent Documents 8 to 13, a low yield ratio non-tempered high tension that uses accelerated cooling to increase strength and achieve both high strength and low yield ratio in order to reduce the amount of alloying elements added. Steel manufacturing methods have been proposed.
The techniques described in Patent Document 8, Patent Document 9, and Patent Document 10 are such that the steel slab is subjected to hot rolling that completes rolling at the Ar ₃ transformation point or higher, and then is cooled to a predetermined temperature before starting cooling. This is a method in which ferrite is generated by air cooling until it becomes, and cooling is performed from a two-phase region to achieve a low yield ratio. However, in such a method, even if the cooling start temperature is slightly different, the ferrite generation rate is different, so that the material variation becomes large. For this reason, when a steel sheet is actually produced by this method, it is necessary to strictly control the cooling start temperature, and there is a problem that stable production becomes difficult.

  In Patent Document 11, hot rolling is completed at a temperature of 720 ° C or higher with a cumulative reduction amount in the austenite non-recrystallization temperature range of 30% or higher, and accelerated cooling is started from a temperature of 680 ° C or higher. Start, stop accelerated cooling at a temperature of 150 to 350 ° C., and make the structure of 1/4 thickness in the thickness direction into a structure containing 1 to 10% of martensite or martensite-austenite mixed phase, low yield ratio A method for producing high-strength steel has been proposed.

Patent Document 12 discloses that a steel piece containing C: 0.005 to 0.03% has a cumulative reduction ratio of 30% or more in a temperature range of 1000 to 1250 ° C. after heating and a cumulative reduction in an austenite non-recrystallization temperature range. Non-refined, high strength, low yield ratio, high rate of 30% or more, hot rolling with rolling end temperature of Ar ₃ point or higher, and after hot rolling, cooling is accelerated at 0.1-20 ° C / s A method for producing a tough steel plate has been proposed. As a result, a high strength of 700 MPa or more and a low yield ratio of 80% or less can be achieved.

Patent Document 13 states that a steel slab containing C: 0.02% or more and less than 0.05% is heated to a cumulative reduction amount of 30% or more in a temperature range of 1100 ° C. or less after rolling and rolled at a temperature of Ae ₃ temperature or more. Of low yield ratio 570MPa class high strength steel with accelerated cooling at a cooling rate of 3K / s or higher from Ae ₃ temperature to 300 ° C after hot rolling is completed. A method has been proposed.
Japanese Patent Laid-Open No. 10-306316 JP-A-4-104419 Japanese Unexamined Patent Publication No. H4-221017 Japanese Patent Laid-Open No. 5-171271 JP 2000-219934 JP 2002-53912 A JP 2004-84068 A Japanese Unexamined Patent Publication No. 63-219523 JP 63-223123 A Japanese Unexamined Patent Publication No. 1-301819 JP 2002-3983 A Japanese Patent Laid-Open No. 2003-147477 JP 2004-339550 A

  However, in the technique described in Patent Document 11, distortion may occur in the steel sheet or residual stress may occur in the steel sheet because the temperature at the time of cooling stop varies greatly in the steel sheet. For this reason, in the steel plate manufactured by the technique described in Patent Document 11, when the steel plate is gas-cut, distortion called a so-called cut-off camber occurs, so that there is a problem that strict management of manufacturing conditions is required. is there.

Further, in the technique described in Patent Document 12, as in the techniques described in Patent Document 5, Patent Document 6, and Patent Document 7, since the C is reduced, a large amount of expensive alloy elements are required to increase the strength. Requires addition. For this reason, there is a problem that the manufacturing cost increases.
Further, the technique described in Patent Document 13 has a problem in that since the C content is lowered, it is necessary to add a large amount of an expensive alloy element in order to increase the strength, resulting in an increase in manufacturing cost. Furthermore, in the technique described in Patent Document 13, since the cooling stop temperature is lowered to 300 ° C. or lower, the temperature at the time of cooling stop varies greatly in the steel plate, and the steel plate is distorted or Residual stress may occur. For this reason, in the steel sheet manufactured by the technique described in Patent Document 13, when the steel sheet is gas-cut, distortion called a so-called striation camber occurs, so that there is a problem that strict management of manufacturing conditions is required. is there.

  The present invention solves the problems of the prior art as described above, online without the need for off-line heat treatment, and further adds a large amount of expensive alloy elements as described above, and strictly controls manufacturing conditions. A high-strength thin steel plate that has excellent weldability and has a low yield ratio, and preferably has a small material variation in the thickness direction, and can produce the high-tensile thin steel plate at a low cost. It aims at providing the manufacturing method of a steel plate. The “high-tensile thin steel plate” in the present invention refers to a steel plate (thick plate) having a thickness of 19 mm to 40 mm and a high strength of tensile strength TS: 590 MPa or more. The “low yield ratio” refers to a yield ratio of 80% or less. In addition, “the material fluctuation in the thickness direction is small” means that the yield strength and tensile strength at the full thickness of the steel plate (hereinafter referred to as the full thickness position) and the yield strength at the position of the thickness 1/2 t The difference between the tensile strength and each is 40 MPa or less.

  In order to achieve the above-mentioned object, the present inventors diligently studied the factors affecting the increase in strength and the decrease in yield ratio. As a result, a steel material having a composition containing Mo and / or W at 0.08 to 0.70 mass% in Mo + W / 2 is hot rolled to form a thin steel plate, and then the thin steel plate is subjected to accelerated cooling under predetermined conditions. Thus, it was found that a high-strength thin steel sheet that is non-tempered and that simultaneously satisfies a high strength of tensile strength TS: 590 MPa or more and a low yield ratio of 80% or less can be produced. In addition to satisfying the above conditions, and further limiting the C content to 0.045 to 0.08%, non-refining, tensile strength TS: high strength of 590MPa or more and low yield ratio of 80% or less In addition, the difference in yield strength and tensile strength between the full thickness position and the 1 / 2t position is 40MPa or less at the same time, with low yield ratio and small material fluctuation in the thickness direction. It has been found that thin steel plates can be produced.

First, the experimental results on which the present invention is based will be described.
One of Cu, Ni, Cr, Mo, W, V, Nb, Ti, B with mass%, 0.08% C-0.25% Si-1.25% Mn-0.018% P-0.002% S Or, adding two or more kinds, hot rolling the slab with a constant carbon equivalent Ceq (= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14) to 0.39% to make a 20mm thick steel plate, Immediately after hot rolling, accelerated cooling is performed until the temperature at the center of the plate reaches 700 ° C (primary cooling stop temperature), then air cooling for 4 s, and then the temperature at the center of the plate again reaches 580 ° C (stops secondary cooling) (Accelerated cooling) until the temperature reached, and then air cooling. The obtained steel sheet was subjected to a tensile test by collecting a JIS No. 5 test piece in accordance with the provisions of JIS Z 2201, and the tensile properties (tensile strength TS, yield strength YS) were obtained. The obtained results are shown in FIG. FIG. 1 shows that Mo + W / 2 needs to be 0.08% or more in order to ensure the tensile strength TS: 590 MPa or more and the yield ratio YR: 80% or less.

Next, the influence of the primary cooling stop temperature in the accelerated cooling after hot rolling was investigated.
A steel material having a mass of 0.06% C-0.25% Si-1.35% Mn-0.012% P-0.003% S-0.24% Mo-0.025% Al-0.0025% N did. At this time, the rolling end temperature of the hot rolling was 900 ° C. on the surface. Immediately after the end of hot rolling, the average cooling rate is 30 ° C./s, and the cooling at the center of the plate thickness is accelerated to a temperature within the range of 800 to 550 ° C. (primary cooling stop temperature), and then for 3 s. After air cooling, accelerated cooling was performed until the temperature at the center of the plate thickness reached 520 ° C. (secondary cooling stop temperature) at an average cooling rate of 30 ° C./s again, and then air cooling was performed.

  From the obtained steel plate, a JIS No. 5 full-thickness tensile test piece was collected in accordance with the provisions of JIS Z 2201, and a tensile test was performed. FIG. 2 shows the relationship between the obtained tensile properties (yield strength YS, tensile strength TS, and yield ratio YR) and the primary cooling stop temperature. From Fig. 2, when the primary cooling stop temperature is higher than 750 ° C or lower than 650 ° C, the yield ratio YR is higher than 80%, and primary cooling is necessary to ensure a low yield ratio. It can be seen that the stop temperature needs to be in the range of 750-650 ° C. The reason why the yield ratio YR is high when the primary cooling stop temperature is higher than 750 ° C. or lower than 650 ° C. is that the vicinity of the surface layer becomes bainite and the hardness increases. From this result, it is possible to suppress bainite formation near the surface layer by temporarily stopping cooling at a temperature in the range of 750 to 650 ° C., and lower the yield ratio even when using a full thickness tensile specimen. I got the knowledge that I can.

In addition, the effect of the air cooling time after stopping the primary cooling in the accelerated cooling after hot rolling was investigated.
A steel material containing 0.06% C-0.25% Si-1.35% Mn-0.012% P-0.003% S-0.24% Mo-0.025% Al-0.0025% N is hot rolled to a thickness of 25 mm. A steel plate was used. At this time, the hot rolling end temperature was 900 ° C. on the surface. Immediately after the end of hot rolling, the cooling rate is 30 ° C / s, and the cooling is accelerated until the temperature at the center of the plate reaches 650 ° C (primary cooling stop temperature), and the air cooling time is changed to 0 to 20s. After air cooling, accelerated cooling was performed again at an average cooling rate of 30 ° C./s until the temperature at the center of the plate thickness reached 550 ° C. (secondary cooling stop temperature), and then air cooling was performed.

  From the obtained steel plate, a JIS No. 5 full-thickness tensile test piece was collected in accordance with the provisions of JIS Z 2201, and a tensile test was performed. FIG. 3 shows the relationship between the obtained tensile properties (yield strength YS, tensile strength TS, and yield ratio YR) and air cooling time. From Fig. 3, when the air cooling time is 0 s (no air cooling) or 1 s, the yield ratio exceeds 80%, and when the air cooling time exceeds 10 s, the tensile strength TS decreases and 590 MPa. It is far below. The reason why the yield ratio exceeds 80% when the air cooling time is less than 2 s is that the structure near the surface layer becomes bainite and the hardness increases. On the other hand, if the air cooling time is longer than 10 s, the ferrite transformation proceeds excessively during air cooling, resulting in a decrease in strength. For this reason, in order to ensure both a low yield ratio and high strength, it was found that the air cooling time must be in the range of 2 to 10 s.

Moreover, the influence of the secondary cooling stop temperature in the accelerated cooling after hot rolling was investigated.
A steel material containing 0.06% C-0.25% Si-1.35% Mn-0.012% P-0.003% S-0.24% Mo-0.025% Al-0.0025% N at mass% is subjected to hot rolling and has a thickness of 25 mm. Steel plate. At this time, the hot rolling end temperature was 900 ° C. on the surface. Immediately after the hot rolling is completed, the average cooling rate is 30 ° C./s, the cooling is accelerated until the temperature at the center of the plate thickness reaches 720 ° C. (primary cooling stop temperature), and after air cooling for 5 s, again, The cooling was performed at an average cooling rate of 30 ° C./s until the temperature at the center of the plate thickness reached 700 to 300 ° C. (secondary cooling stop temperature), and then air-cooled.

  From the obtained steel sheet, a JIS No. 5 full-thickness tensile test piece was collected in accordance with JIS Z 2201 and subjected to a tensile test. FIG. 4 shows the relationship between the obtained tensile properties (yield strength YS, tensile strength TS, and yield ratio YR) and the secondary cooling stop temperature. From FIG. 4, when the secondary cooling stop temperature is less than 500 ° C., the yield ratio exceeds 80%, and when it exceeds 650 ° C., the tensile strength TS decreases and falls below 590 MPa. From such a thing, in order to ensure both a low yield ratio and high intensity | strength, the knowledge that it was necessary to make secondary cooling stop temperature into the range of 650-500 degreeC was acquired.

  In this way, by subjecting a thin steel plate (thick plate) having a thickness of 40 mm or less to the above-described hot rolling and the cooling treatment having the primary cooling and the secondary cooling immediately after the hot rolling, other than the surface layer The central structure is mainly composed of a ferrite phase and contains a predetermined amount of hard phase.Furthermore, the fast-cooled surface layer is tempered by recuperation, resulting in a structure with a low yield ratio. It was newly found that it becomes a high-tensile thin steel plate having a low yield ratio of 80% or less.

  Moreover, in mass%, 0.06% C-0.25% Si-1.32% Mn-0.015% P-0.001% S is used as a basic component, among Cu, Ni, Cr, Mo, W, V, Nb, Ti, B. A slab with one or two or more added carbon constant Ceq of 0.40% is hot-rolled into a 20mm thick steel plate, and the temperature at the center of the plate thickness is 700 ° C immediately after hot rolling. After accelerating cooling until reaching the (primary cooling stop temperature), and then air cooling for 4 s, the cooling was accelerated until the temperature at the center of the plate thickness reached 580 ° C. (secondary cooling stop temperature), and then air cooling was performed. In accordance with JIS Z 2201, the JIS No. 5 full thickness test piece and the JIS No. 4 test piece are taken from the full thickness position, and a tensile test is conducted on the obtained steel plate. Properties (tensile strength TS, yield strength YS) were determined. The obtained results are shown in FIG.

From Fig. 5, the tensile strength TS: 590MPa or more and the yield ratio YR: 80% or less are satisfied, and the yield strength, the difference in tensile strength, ΔYS, ΔTS between the full thickness position and the plate thickness 1 / 2t position are In both cases, it was found that (Mo + W / 2) must be adjusted to 0.08 to 0.20% in order to make steel sheets satisfying 40 MPa or less.
In addition, in mass%, 0.01 to 0.11% C-0.25% Si-1.35% Mn-0.014% P-0.002% S-0.13% Mo, Cu, Ni, Cr, V, Nb, Ti, B One or two or more of them are added, and a slab with a constant carbon equivalent Ceq of 0.39% is hot-rolled into a 20mm-thick steel plate. Accelerated cooling to 680 ° C (primary cooling stop temperature), then air-cooled for 5s, then accelerated cooling until the center of the plate thickness reaches 620 ° C (secondary cooling stop temperature), and then air-cooled . In accordance with JIS Z 2201, the JIS No. 5 full thickness test piece and the JIS No. 4 test piece are taken from the full thickness position, and a tensile test is conducted on the obtained steel plate. Properties (tensile strength TS, yield strength YS) were determined. The obtained result is shown in FIG.

From Fig. 6, the tensile strength TS: 590MPa or more and the yield ratio YR: 80% or less are satisfied, and the yield strength, the difference in tensile strength, ΔYS, ΔTS between the full thickness position and the plate thickness 1 / 2t position are In both cases, it was found that the C content must be adjusted to 0.045 to 0.08% in order to make steel sheets satisfying 40 MPa or less.
The present invention has been completed based on the above-described findings and further studies. That is, the gist of the present invention is as follows.

(1) In mass%, C: 0.045 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.6 to 2.0%, P: 0.020% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.0060% Including Mo and / or W in the following formula (1)
0.08 ≦ Mo + W / 2 ≦ 0.70 (1)
(Where, Mo, W: content of each element (mass%))
Is contained, the balance is Fe and inevitable impurities, and the following formula (2) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
The weld cracking susceptibility index Pcm shown in Fig. 2 is 0.22% or less, and the structure in the central part in the thickness direction is a composite structure containing ferrite as the main phase and 20% by volume or less of the hard phase. A high-strength thin steel plate having a low yield ratio.

(2) In mass%, C: 0.045 to 0.08%, Si: 0.05 to 0.50%, Mn: 0.6 to 2.0%, P: 0.020% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.0060% Including Mo and / or W in the following formula (1a)
0.08 ≦ Mo + W / 2 ≦ 0.20 (1a)
(Where, Mo, W: content of each element (mass%))
Is contained, the balance is Fe and inevitable impurities, and the following formula (2) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
The weld cracking susceptibility index Pcm shown in Fig. 2 is 0.22% or less, and the structure in the central part in the thickness direction is a composite structure containing ferrite as the main phase and 20% by volume or less of the hard phase. A high-strength thin steel plate with a low yield ratio and small material fluctuation in the thickness direction.

(3) In (1) or (2), in addition to the above composition, it is also in mass%, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.05 to 1%, V: 0.01 to 0.1% Nb: 0.005-0.1%, Ti: 0.005-0.05%, B: 0.0002-0.0050%, It is set as the composition containing 1 type, or 2 or more types, The high tension thin-walled steel plate characterized by the above-mentioned.
(4) In any one of (1) to (3), in addition to the above-mentioned composition, it further contains one or two of Ca: 0.0002 to 0.0050% and REM: 0.0002 to 0.0050% in mass%. A high-tensile thin steel plate characterized by having a composition.

(5) The steel material is subjected to hot rolling and a cooling treatment that is accelerated and cooled immediately after the hot rolling to obtain a steel plate. The steel material is mass%, C: 0.045 to 0.18%, Si. : 0.05-0.50%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.0060% or less, and further Mo and / or W (1) formula
0.08 ≦ Mo + W / 2 ≦ 0.70 (1)
(Where, Mo, W: content of each element (mass%))
Is contained, the balance is Fe and inevitable impurities, and the following formula (2) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
The steel material having a composition having a weld crack sensitivity index Pcm of 0.22% or less, and the hot rolling is hot rolling with a rolling end temperature in the range of 800 to 950 ° C., and the cooling The primary cooling is accelerated cooling to the primary cooling stop temperature at a cooling rate of 10 ° C / s or more at an average cooling rate to a temperature in the center of the plate thickness ranging from 750 to 650 ° C, and then air cooling for 2 to 10s. Then, the secondary cooling that accelerates the cooling at an average cooling rate of 10 ° C./s or more and stops the accelerated cooling at the secondary cooling stop temperature at which the center of the plate thickness is in the range of 500 to 650 ° C. A method for producing a high-tensile thin steel plate having a low yield ratio, characterized in that the treatment comprises:

(6) The steel material is subjected to hot rolling and a cooling treatment that is accelerated and cooled immediately after the hot rolling to obtain a steel plate. The steel material is mass%, C: 0.045 to 0.08%, Si : 0.05-0.50%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.0060% or less, and further Mo and / or W (1a) formula
0.08 ≦ Mo + W / 2 ≦ 0.20 (1a)
(Where, Mo, W: content of each element (mass%))
Is contained, the balance is Fe and inevitable impurities, and the following formula (2) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
A steel material having a composition with a weld crack sensitivity index Pcm of 0.22% or less,
The hot rolling is hot rolling in which the rolling end temperature is a temperature in the range of 800 to 950 ° C on the surface,
The above cooling process is accelerated at an average cooling rate of 10 ° C./s or more to a primary cooling stop temperature at which the central portion of the plate thickness reaches a temperature in the range of 750 to 650 ° C., and then for 2 to 10 seconds. Air cooling, followed by accelerated cooling at an average cooling rate of 10 ° C / s or higher and secondary cooling to stop the accelerated cooling at the secondary cooling stop temperature at which the center of the plate thickness is in the range of 500 to 650 ° C A method for producing a high-strength thin steel plate having a low yield ratio and having a small material variation in the thickness direction, characterized in that the treatment comprises cooling.

(7) In (5) or (6), in addition to the composition, the steel material is further in mass%, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.05 to 1%, V : 0.01 to 0.1%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.05%, and B: 0.0002 to 0.0050% Manufacturing method.
(8) In any one of (5) to (7), in addition to the composition, the steel material may further be mass%, Ca: 0.0002 to 0.0050%, REM: 0.0002 to 0.0050%, or A method for producing a high-tension thin-walled steel sheet, comprising a composition containing two types.

  According to the present invention, a large amount of expensive alloying elements, strict control of manufacturing conditions, and offline two-phase region heat treatment are not required, and weldability and toughness are excellent, and a low yield ratio is achieved. In addition, it is possible to manufacture a high-strength thin steel plate having a small variation in material in the thickness direction without any tempering. Moreover, according to this invention, there exists an effect that the high-tensile thin steel plate which has the outstanding characteristic can be provided easily and cheaply.

The high-strength thin steel plate of the present invention is a steel plate (thick plate) having a thickness of 40 mm or less, preferably 19 mm or more, having a predetermined composition and structure, a tensile strength TS: high strength of 590 MPa or more, and a yield ratio. YR: A high-strength steel sheet having a low yield ratio of 80% or less.
First, the reason for limiting the composition of the high-strength thin steel plate of the present invention will be described. In the following, unless otherwise stated,% means mass%.

C: 0.045-0.18%
C is an element that improves the strength of steel. In the present invention, it is necessary to contain 0.045% or more in order to ensure a tensile strength TS: 590 MPa or more. However, when C is contained excessively exceeding 0.18%, the cold cracking sensitivity is increased. For this reason, in this invention, C was limited to 0.045 to 0.18% of range. If the content exceeds 0.08%, the difference in strength, ΔYS, ΔTS at the full thickness position and the 1 / 2-thickness position, becomes 40 MPa or more. For this reason, C is preferably limited to a range of 0.045% to 0.08% for uses that require a small material variation in the thickness direction.

Si: 0.05-0.50%
Si acts as a deoxidizer and needs to contain 0.05% or more in steelmaking. On the other hand, if it exceeds 0.50%, the toughness of the base material is lowered. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.10 to 0.40%.
Mn: 0.6-2.0%
Mn is an element that improves the strength through improving the hardenability of steel. In order to ensure such an effect, the content of 0.6% or more is required. On the other hand, if the content exceeds 2.0%, the weldability is significantly reduced. For this reason, in this invention, Mn was limited to 0.6 to 2.0% of range. In addition, Preferably it is 0.80 to 1.60%.

P: 0.020% or less P is an element inevitably contained in steel as an impurity, and it is desirable to reduce it as much as possible in order to reduce the toughness of steel. In particular, a content exceeding 0.020% significantly reduces toughness. For this reason, in the present invention, P is limited to 0.020% or less.
S: 0.005% or less S is an element inevitably contained in steel as an impurity, and it is desirable to reduce as much as possible in order to reduce the toughness of steel and the drawing in the thickness direction tensile test. In particular, when the content exceeds 0.005%, the above-described tendency of deterioration of the characteristics becomes remarkable. Therefore, S is limited to 0.005% or less.

Al: 0.1% or less
Al is an element that acts as a deoxidizer and is the most widely used element as a deoxidizer in the deoxidation process of molten steel. In order to acquire such an effect, it is desirable to contain 0.001% or more. However, if it exceeds 0.1%, a coarse oxide is formed and the ductility of the steel sheet base material is remarkably lowered. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.020 to 0.080%.

N: 0.0060% or less When N is present as solute N, it lowers the base metal toughness and weld heat affected zone toughness after strain aging. For this reason, N was limited to 0.0060% or less.
In addition to the components described above, the present invention further contains Mo and / or W.
Mo and W are important elements for reducing the yield ratio while securing the strength. In the present invention, Mo or W, or Mo and W is expressed by the following formula (1).
0.08 ≦ (Mo + W / 2) ≦ 0.70 (1)
(Where, Mo, W: content of each element (mass%))
Is contained so as to satisfy. If (Mo + W / 2) is less than 0.08%, it is impossible to ensure that the tensile strength TS is 590 MPa or more and the yield ratio YR is 80% or less. On the other hand, when Mo and / or W is contained so that (Mo + W / 2) exceeds 0.70%, the weldability is lowered and the manufacturing cost (material cost) is increased. Therefore, Mo and / or W are specified to be contained so that (Mo + W / 2) is in the range of 0.08 to 0.70%, that is, to satisfy the expression (1). In addition, the following (1a) formula is used for the usage in which it is required to reduce the material variation in the thickness direction.
0.08 ≦ Mo + W / 2 ≦ 0.20 (1a)
(Where, Mo, W: content of each element (mass%))
Is preferably defined so as to satisfy the above. This is because when (Mo + W / 2) increases beyond 0.20, the strength differences ΔYS and ΔTS between the full thickness position and the 1/2 t thickness position increase beyond 40 MPa, and the material variation in the thickness direction is significant. Become.

Pcm: 0.22% or less Pcm is a weld crack susceptibility index indicating crack susceptibility during welding. The following formula (2) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
Defined by In addition, when calculating Pcm value using (2) Formula, the element which is not contained shall be calculated as zero.

  When the Pcm value becomes high, it is necessary to perform pre-heat treatment and / or post-heat treatment at the time of welding. Although it depends on the welding conditions and restraint conditions of the welded part, welding can be performed without preheating by setting the Pcm value to 0.22% or less. For this reason, in the present invention, Pcm is limited to 0.22% or less. In addition, Preferably, it is 0.20% or less.

The high-strength thin steel plate of the present invention has the above-described components as basic components, and, if necessary, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.05 to 1%, V: 0.01 -0.1%, Nb: 0.005-0.1%, Ti: 0.005-0.05%, B: One or more of 0.0002-0.0050%, and / or Ca: 0.0002-0.0050%, REM: 0.0002-0.0050 1 type or 2 types of% may be contained.
Cu, Ni, Cr, V, Nb, Ti, and B are all elements that improve the strength of the steel sheet, and can be selected to contain one or more as required.
Cu: 0.03-1%
Cu is an effective element that improves the strength without reducing toughness. In order to obtain such an effect, it is necessary to contain 0.03% or more. On the other hand, the content exceeding 1% frequently causes surface defects during hot rolling. For this reason, it is preferable to limit Cu to 0.03 to 1%.

Ni: 0.03-2%
Ni is an effective element that improves strength without reducing toughness, and in order to obtain such an effect, it needs to be contained in an amount of 0.03% or more. On the other hand, if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit Ni to 0.03 to 2% of range.

Cr: 0.05 to 1%
Cr is an effective element that improves the strength without significantly increasing the alloy cost. In order to obtain such an effect, it is necessary to contain 0.05% or more. On the other hand, the content exceeding 1% reduces weldability. For this reason, Cr is preferably limited to a range of 0.05 to 1%.

V: 0.01 to 0.1%
V is an effective element for improving the strength of the steel sheet by precipitation strengthening, and in order to obtain such an effect, the content of 0.01% or more is required. On the other hand, if the content exceeds 0.1%, weldability decreases. For this reason, it is preferable to limit V to the range of 0.01 to 0.1%.
Nb: 0.005-0.1%
Nb is an effective element that improves the strength of the steel sheet by precipitation strengthening, and in order to obtain such an effect, the Nb content needs to be 0.005% or more. On the other hand, if the content exceeds 0.1%, weldability decreases. For this reason, Nb is preferably limited to a range of 0.005 to 0.1%.

Ti: 0.005-0.05%
Ti is an element effective for improving the strength of the steel sheet by precipitation strengthening, fixing solute N, and improving the toughness of the heat affected zone. For this purpose, it needs to contain 0.005% or more. To do. On the other hand, if it exceeds 0.05% and contains excessively, the weld heat-affected zone toughness decreases. For this reason, Ti is preferably limited to a range of 0.005 to 0.05%.

B: 0.0002-0.0050%
B is an effective element that improves the hardenability and thereby improves the strength of the steel sheet by containing a trace amount, and in order to obtain such an effect, the content of 0.0002% or more is required. On the other hand, when it contains exceeding 0.0050%, weldability will fall. For this reason, it is preferable to limit B to the range of 0.0002 to 0.0050%.

Ca and REM are elements that improve ductility in the thickness direction and weld heat-affected zone toughness, and can be selected as necessary to contain one or two kinds.
Ca: 0.0002 to 0.0050%
By fixing S, Ca has the effect of suppressing the formation of MnS, improving the drawing characteristics in the thickness direction, and improving the weld heat affected zone toughness. In order to acquire such an effect, 0.0002% or more needs to be contained. On the other hand, an excessive content exceeding 0.0050% lowers the base metal toughness. Therefore, Ca is preferably limited to a range of 0.0002 to 0.0050%.

REM: 0.0002 to 0.0050%
REM fixes S to suppress the generation of MnS, thereby improving the drawing characteristics in the thickness direction, and also has the effect of improving the weld heat affected zone toughness. In order to acquire such an effect, 0.0002% or more needs to be contained. On the other hand, an excessive content exceeding 0.0050% lowers the base metal toughness. Therefore, REM is preferably limited to a range of 0.0002 to 0.0050%.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include O: 0.01% or less, Zr: 0.01% or less, Co: 0.01% or less, Sn: 0.01% or less, Pb: 0.01% or less, and Sb: 0.01% or less.
The high-tensile thin steel plate of the present invention has the above-described composition, and further, the structure in the central part in the thickness direction has a composite structure including ferrite as a main phase and a hard phase of 20% by volume or less. The central portion in the thickness direction means a range excluding the surface layer portion, that is, a region of 1/4 to 1/2 of the thickness.

  A steel sheet having a low yield ratio is obtained by using a ferrite phase, which is a soft phase, as a main phase in the thickness direction central structure of the steel sheet. The main phase is a phase containing 50% or more by volume. And desired high intensity | strength is securable by including a hard phase below 20 volume%. In addition, it is preferable that the hard phase is mainly a MA phase (MA phase: an abbreviation for island martensite) of 10% by volume or less, and the rest is a phase containing pearlite and / or bainite. Mo and W contribute significantly to the hardness increase of the hard phase.

  Incidentally, in the surface layer other than the central part of the steel plate, it is rapidly cooled at the time of cooling, a bainite phase is formed and the hardness is increased, but in the present invention, the surface layer is tempered by recuperation by air cooling after primary cooling. Become. For this reason, the hardness of a surface layer falls and a yield ratio also falls. In the high-tensile thin steel plate of the present invention, the hardness of the surface layer is preferably 300 HV10 or less from the viewpoint of lowering the yield ratio. In addition, the surface layer as used in the field of this invention is an area | region except a center part, and shall mean the range within 2 mm from the surface in the plate | board thickness direction.

Below, the manufacturing method of the high-tensile thick steel plate of this invention is demonstrated.
The molten steel having the above composition is melted in a conventional melting furnace such as a converter, and is made into a slab (steel material) by a conventional method such as a continuous casting method or an ingot-bundling method. Next, the steel material is subjected to hot rolling and a cooling process for accelerated cooling immediately after the hot rolling to obtain a thin steel plate.
Hot rolling is rolling in which the rolling end temperature is a temperature in the range of 800 to 950 ° C on the surface.

The heating temperature for hot rolling is not particularly limited, but is preferably heated to a temperature in the range of 1050 to 1250 ° C. When the heating temperature is less than 1050 ° C., the deformation resistance increases and the rolling mill load increases. On the other hand, if it exceeds 1250 ° C., surface flaws are likely to occur during hot rolling. Therefore, the heating temperature is desirably a temperature in the range of 1050 to 1250 ° C.
The rolling end temperature of the hot rolling is a temperature in the range of 800 to 950 ° C on the surface. If the rolling end temperature is less than 800 ° C, the desired strength cannot be ensured. On the other hand, if the temperature exceeds 950 ° C, the base material toughness of the steel sheet decreases. For this reason, the rolling end temperature of hot rolling was limited to a temperature in the range of 800 to 950 ° C. on the surface. In addition, Preferably it is 800-900 degreeC.

  The cooling process includes primary cooling that accelerates cooling to a primary cooling stop temperature at an average cooling rate of 10 ° C./s or higher to a temperature in the center of the plate thickness ranging from 750 to 650 ° C., and then between 2 and 10 seconds. Air cooling, followed by accelerated cooling at an average cooling rate of 10 ° C / s or higher, and secondary cooling that stops accelerated cooling at the secondary cooling stop temperature at which the center of the plate thickness is in the range of 500 to 650 ° C The process consisting of The average cooling rate is a value defined by the cooling rate at the position where the thickness is 1/4 t.

The accelerated cooling rate is preferably higher from the viewpoint of uniformly cooling the steel sheet, and the average cooling rate is 10 ° C./s or more. When the average cooling rate is less than 10 ° C / s, the tendency for non-uniform cooling becomes strong, the generation of cooling strain and residual stress increases, and the shape defect and the cutting camber occur when gas cutting the steel sheet. Increased risk.
Further, the primary cooling stop temperature is a temperature at which the central portion of the plate thickness is in the range of 750 to 650 ° C. When the primary cooling stop temperature is higher than 750 ° C., the desired tensile strength cannot be secured. On the other hand, if the primary cooling stop temperature is less than 650 ° C., the yield ratio exceeds 80%, and a desired low yield ratio cannot be ensured. For this reason, the primary cooling stop temperature was limited to a temperature within the range of 750 to 650 ° C.

  The air cooling time after stopping the primary cooling was in the range of 2 to 10 s. If the air cooling time is less than 2 s, the surface layer becomes bainite and the hardness increases, and the yield ratio exceeds 80%. On the other hand, if the air cooling time exceeds 10 s, the ferrite transformation proceeds excessively and the strength decreases. It becomes difficult to ensure the desired tensile strength. For this reason, the air cooling time after the primary cooling stop was limited to the range of 2 to 10 s.

  Further, the secondary cooling stop temperature is set to a temperature at which the central portion of the plate thickness is in the range of 500 to 650 ° C. When the secondary cooling stop temperature is higher than 650 ° C., the strength is lowered and the desired tensile strength cannot be secured. On the other hand, if the secondary cooling stop temperature is less than 500 ° C., the yield ratio exceeds 80%, and a desired low yield ratio cannot be ensured. For this reason, the secondary cooling stop temperature was limited to a temperature at which the thickness center portion is in the range of 500 to 650 ° C. It should be noted that the secondary cooling stop temperature is preferably set to a high temperature of 500 ° C. or higher in order to reduce residual stress that causes a cutting camber during gas cutting.

In the accelerated cooling in the present invention, it is necessary to control the cooling rate of the steel sheet within a desired range. Examples of the cooling rate control method include a method of adjusting water density, a method of repeating strong cooling and air cooling a plurality of times, a method of repeating strong cooling and weak cooling, and the like. There is no loss of effect. In order to reduce the cooling strain and residual stress of the steel sheet after accelerated cooling, the average cooling rate is controlled by repeating strong cooling and air cooling with a water density of 1.5 ton / m ² / min or more multiple times. The method is most desirable. In addition, it is preferable to set it as air cooling after stopping accelerated cooling.

Example 1
The steel material having the composition shown in Table 1 was subjected to hot rolling and cooling treatment (air cooling after stopping secondary cooling) under the conditions shown in Table 2 to obtain a thin steel plate having the thickness shown in Table 2.
The obtained thin steel plate was subjected to a structure observation, a tensile test, a Charpy impact test, a hardness test, and a y-type weld cracking test to investigate the structure, tensile properties, toughness, hardness, and weld cracking property. The test method was as follows.

(1) Microstructure observation A specimen for microstructural observation is collected from the obtained thin steel plate, the cross section in the L direction is polished, corroded with nital, and the cross sectional structure is observed with a scanning electron microscope at a thickness of 1/4 t or more in 5 views or more. Observation, imaging, and tissue fraction were determined by image analysis.
(2) Tensile test JIS No. 5 test piece was collected from the thin steel plate obtained in accordance with the provisions of JIS Z 2201, and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Strength YS, tensile strength TS, yield ratio YR).

(3) Charpy impact test V-notch test specimens were collected from the position of 1 / 4t thickness of the obtained thin steel plate in accordance with the provisions of JIS Z 2242, Charpy impact test was conducted, and the absorbed energy at 0 ° C vE ₀ (J) and fracture surface transition temperature vTrs (° C.) were determined.
(4) Hardness test From the thin steel plate obtained, a hardness test piece was collected, and the cross-sectional hardness distribution was determined using a Vickers hardness tester (load: 98N) in the thickness direction for the thickness cross section. The hardness at the lower 1 mm position was determined.

(5) y-type weld crack test Using the y-type weld crack test piece collected in accordance with JIS Z 3158 from the obtained thin steel plate, conduct a weld crack test at 25 ° C to investigate the presence of cracks. did.
The obtained results are shown in Table 3.

Each of the inventive examples is a high-strength thin steel plate having a low yield ratio that satisfies a tensile strength TS of 590 MPa or more and a yield ratio YR of 80% or less, and has a good toughness of vE ₀ of 100 J or more. Furthermore, no cracks were generated in the y-type weld cracking test at 25 ° C., and excellent weldability (weld cracking property) was obtained. On the other hand, in comparative examples that are out of the scope of the present invention, the strength is insufficient, the yield ratio exceeds 80%, the toughness is reduced, or weld cracks occur at 25 ° C.

(Example 2)
The steel material having the composition shown in Table 4 was subjected to hot rolling and cooling treatment (air cooling after stopping the secondary cooling) under the conditions shown in Table 5 to obtain a thin steel plate having the thickness shown in Table 5.
The obtained thin steel plate was subjected to a structure observation, a tensile test, a Charpy impact test, and a y-type weld cracking test to investigate the structure, tensile properties, toughness, and weld cracking property. The test method was as follows as in Example 1.

(1) Microstructure observation A specimen for microstructural observation is collected from the obtained thin steel plate, the cross section in the L direction is polished, corroded with nital, and the cross sectional structure is observed with a scanning electron microscope at a thickness of 1/4 t or more by 5 fields Observation, imaging, and tissue fraction were determined by image analysis.
(2) Tensile test JIS No. 5 test piece (full thickness) from the full thickness position of the thin steel plate obtained, and JIS No. 4 test piece from the half thickness position, respectively, in accordance with the provisions of JIS Z 2201. Tensile tests were conducted in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength YS, tensile strength TS, yield ratio YR).

(3) Charpy impact test V-notch specimens were sampled from the obtained thin steel plate thickness 1 / 4t position and plate thickness 1 / 2t position in accordance with JIS Z 2242, and Charpy impact test was conducted. The absorption energy vE ₀ (J) at 0 ° C. and the fracture surface transition temperature vTrs (° C.) were determined.
(4) y-type weld crack test Using the y-type weld crack test specimen collected from the thin steel plate obtained in accordance with JIS Z 3158, a weld crack test was conducted at 25 ° C to check for cracks. did.

  The results obtained are shown in Table 5.

Each of the inventive examples is a high-strength thin steel plate having a low yield ratio that satisfies a tensile strength TS of 590 MPa or more and a yield ratio YR of 80% or less, and has a good toughness of vE ₀ of 100 J or more. Furthermore, no cracks are generated in the y-type weld cracking test at 25 ° C., and it has excellent weldability (weld cracking property). Further, in the present invention example in which the C content is within the preferred range of the present invention, the strength difference ΔYS, ΔTS between the full thickness position and the thickness 1/2 t position is both 40 MPa or less, and the material variation in the thickness direction is small. It is a tension thin wall steel plate. In the present invention example (steel plate No. 2-52 to 2-60) in which the amount of C or (Mo + W / 2) is outside the preferred range of the present invention, ΔYS and ΔTS exceed 40 MPa, and the material variation in the thickness direction is It is getting bigger. On the other hand, in comparative examples that are out of the scope of the present invention, the strength is insufficient, the yield ratio exceeds 80%, the toughness is reduced, or weld cracks occur at 25 ° C.

It is a graph which shows the relationship between tensile strength TS, yield strength YS, yield ratio YR, and (Mo + W / 2) amount. 6 is a graph showing the relationship between tensile strength TS, yield strength YS, yield ratio YR, and primary cooling stop temperature. 5 is a graph showing the relationship between tensile strength TS, yield strength YS, yield ratio YR and air cooling time. 6 is a graph showing the relationship between tensile strength TS, yield strength YS, yield ratio YR, and secondary cooling stop temperature. It is a graph which shows the relationship between tensile strength TS, yield strength YS, yield ratio YR, and ΔYS, ΔTS and the amount of (Mo + W / 2). It is a graph which shows the relationship between tensile strength TS, yield strength YS, yield ratio YR, and ΔYS, ΔTS and the amount of C.

Claims (8)

mass%
C: 0.045 to 0.18%, Si: 0.05 to 0.50%,
Mn: 0.6 to 2.0%, P: 0.020% or less,
S: 0.005% or less, Al: 0.1% or less,
N: 0.0060% or less, Mo and / or W are contained so as to satisfy the following formula (1), the balance is Fe and inevitable impurities, and the weld cracking sensitivity index represented by the following formula (2) It has a composition with Pcm of 0.22% or less, and has a low yield ratio characterized in that the structure in the central part in the thickness direction is a composite structure containing ferrite as the main phase and a hard phase of 20% by volume or less. High tensile thin steel plate.
Record
0.08 ≦ Mo + W / 2 ≦ 0.70 (1)
Where Mo, W: content of each element (mass%)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%) mass%
C: 0.045 to 0.08%, Si: 0.05 to 0.50%,
Mn: 0.6 to 2.0%, P: 0.020% or less,
S: 0.005% or less, Al: 0.1% or less,
N: containing 0.0060% or less, further containing Mo and / or W so as to satisfy the following formula (1a), the balance consisting of Fe and inevitable impurities, and a weld cracking sensitivity index represented by the following formula (2) It has a composition with Pcm of 0.22% or less, and has a low yield ratio characterized in that the structure in the center in the thickness direction is a composite structure containing ferrite as the main phase and 20% by volume or less of the hard phase. A high-strength thin steel plate with little material fluctuation in the thickness direction.
Record
0.08 ≦ Mo + W / 2 ≦ 0.20 (1a)
Where Mo, W: content of each element (mass%)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)   In addition to the above composition, it is also mass%, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.05 to 1%, V: 0.01 to 0.1%, Nb: 0.005 to 0.1%, Ti: 0.005 to The high-tension thin-walled steel sheet according to claim 1 or 2, characterized in that the composition contains one or more of 0.05% and B: 0.0002 to 0.0050%.   The composition according to any one of claims 1 to 3, wherein in addition to the composition, the composition further includes one or two of Ca: 0.0002 to 0.0050% and REM: 0.0002 to 0.0050% in mass%. The high-tensile thin-walled steel sheet according to crab. The steel material is subjected to hot rolling and a cooling treatment that is accelerated and cooled immediately after the hot rolling, and when making a steel plate, the steel material is mass%,
C: 0.045 to 0.18%, Si: 0.05 to 0.50%,
Mn: 0.6 to 2.0%, P: 0.020% or less,
S: 0.005% or less, Al: 0.1% or less,
N: 0.0060% or less, Mo and / or W are contained so as to satisfy the following formula (1), the balance is Fe and inevitable impurities, and the weld cracking sensitivity index represented by the following formula (2) A steel material having a composition with a Pcm of 0.22% or less,
The hot rolling is hot rolling in which the rolling end temperature is a temperature in the range of 800 to 950 ° C on the surface,
The above cooling process is accelerated at an average cooling rate of 10 ° C./s or more to a primary cooling stop temperature at which the central portion of the plate thickness reaches a temperature in the range of 750 to 650 ° C., and then for 2 to 10 seconds. Air cooling, followed by accelerated cooling at an average cooling rate of 10 ° C / s or higher and secondary cooling to stop the accelerated cooling at the secondary cooling stop temperature at which the center of the plate thickness is in the range of 500 to 650 ° C A method for producing a high-strength thin steel plate having a low yield ratio, characterized in that the treatment comprises cooling.
Record
0.08 ≦ Mo + W / 2 ≦ 0.70 (1)
Where Mo, W: content of each element (mass%)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%) The steel material is subjected to hot rolling and a cooling treatment that is accelerated and cooled immediately after the hot rolling, and when making a steel plate, the steel material is mass%,
C: 0.045 to 0.08%, Si: 0.05 to 0.50%,
Mn: 0.6 to 2.0%, P: 0.020% or less,
S: 0.005% or less, Al: 0.1% or less,
N: containing 0.0060% or less, further containing Mo and / or W so as to satisfy the following formula (1a), the balance consisting of Fe and inevitable impurities, and a weld cracking sensitivity index represented by the following formula (2) A steel material having a composition with a Pcm of 0.22% or less,
The hot rolling is hot rolling in which the rolling end temperature is a temperature in the range of 800 to 950 ° C on the surface,
The above cooling process is accelerated at an average cooling rate of 10 ° C./s or more to a primary cooling stop temperature at which the central portion of the plate thickness reaches a temperature in the range of 750 to 650 ° C., and then for 2 to 10 seconds. Air cooling, followed by accelerated cooling at an average cooling rate of 10 ° C / s or higher and secondary cooling to stop the accelerated cooling at the secondary cooling stop temperature at which the center of the plate thickness is in the range of 500 to 650 ° C A method for producing a high-strength thin steel plate having a low yield ratio and having a small material variation in the thickness direction, characterized in that the treatment comprises cooling.
Record
0.08 ≦ Mo + W / 2 ≦ 0.20 (1a)
Where Mo, W: content of each element (mass%)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)   In addition to the above composition, the steel material is further in mass%, Cu: 0.03-1%, Ni: 0.03-2%, Cr: 0.05-1%, V: 0.01-0.1%, Nb: 0.005-0.1% Ti: 0.005-0.05%, B: It is set as the composition containing 1 type, or 2 or more types in 0.0002-0.0050%, The manufacturing method of the high strength thin-walled steel plate of Claim 5 or 6 characterized by the above-mentioned.   The steel material further comprises a composition containing at least one of Ca: 0.0002 to 0.0050% and REM: 0.0002 to 0.0050% in mass% in addition to the composition. A method for producing a high-tensile thin steel plate according to any one of 5 to 7.

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