JP7272471B2 - steel plate - Google Patents

Description

The present invention relates to steel materials used for various steel structures in the fields of ships, construction, civil engineering, etc., and in particular, excellent welding even when high heat input welding with a welding heat input exceeding 20.0 kJ / mm is performed. The present invention relates to a steel sheet having heat affected zone toughness.

As steel materials become stronger and thicker, there is an increasing demand for high heat input welding, such as submerged arc welding, electrogas welding, and electroslag welding, which are excellent in production efficiency. Various steels for high heat input welding have been proposed because the toughness of the weld heat affected zone (hereinafter sometimes referred to as HAZ) that is welded with high heat input is lowered. For example, a technique of finely dispersing TiN in steel to suppress coarsening of austenite grains in the weld heat-affected zone and a technique of using TiN as ferrite transformation nuclei in the weld heat-affected zone have been put to practical use.

Suppression of microstructure coarsening using TiN precipitates is economically useful and widely used. On the other hand, in the high temperature range where TiN melts in the weld heat-affected zone, these effects cannot be obtained, and the dissolved TiN causes an excess of solid solution Ti and solid solution N, which embrittles the ground structure. However, there is a problem that the toughness is remarkably lowered.

Therefore, in Patent Document 1, TiOx with a particle size of 5 μm or less (where x: 0.65 to 1.3) is finely dispersed in steel among Ti oxides that are difficult to dissolve even in the high temperature region of the weld heat affected zone. , a technique has been proposed to improve the toughness of the weld heat affected zone by using it as a nucleus for generating acicular ferrite in the weld heat affected zone. In Patent Document 2, B, N and sol. Techniques have been proposed to improve the toughness of the weld heat-affected zone by adjusting the amount of Al and positively precipitating BN that refines the weld heat-affected zone.

In addition, in Patent Document 3, the amount of Ti—B—N in the composition is adjusted so that the HAZ toughness is in a high toughness region, and further Ca or Ce is added to give the effect of improving toughness by controlling the morphology of inclusions. techniques have been proposed. Patent Document 4 also proposes a technology to improve the toughness of large heat input welds by adding REM, which forms stable sulfur oxides even in the bond part of welding, with a low N-low Ti composition. It is Furthermore, in Patent Document 5, Ca-based nonmetallic inclusions that act as transformation nuclei and promote ferrite transformation in the weld heat-affected zone are finely dispersed in the steel by appropriately controlling the Ca, O, and S contents. A technique for improving the weld heat affected zone toughness of high heat input welding exceeding 20.0 kJ/mm has been disclosed.

JP-A-57-51243 JP-A-62-170459 JP-A-60-204863 Japanese Patent Publication No. 4-14180 Japanese Patent No. 3546308

However, with the technique using Ti oxide described in Patent Document 1, it is difficult to uniformly finely disperse the oxide, especially in mass production, and the toughness of the weld heat-affected zone cannot be stably ensured. There is a problem. In the technique described in Patent Document 2, cracks originating from inclusions mainly composed of nitride may occur during casting. In addition, with the techniques in Patent Documents 3 and 4, it is difficult to sufficiently suppress the grain growth of austenite in the weld heat affected zone with high heat input welding exceeding 20.0 kJ / mm, so the toughness of the joint becomes unstable. There is a problem of becoming Moreover, the technique described in Patent Document 5 has a problem that it is difficult to ensure stable toughness.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet having excellent toughness in the heat affected zone of high heat input welding where the welding heat input is 20.0 kJ/mm or more.

In order to solve the above problems, the inventors made various studies and obtained the following findings.

In order to suppress the coarsening of the structure in the weld heat-affected zone by utilizing the TiN precipitates, which are excellent in industrial productivity, it is essential to appropriately control the size of the TiN precipitates in the base steel plate. be. That is, by securing a certain amount or more of TiN precipitates of a size that remains undissolved even when exposed to a high temperature above the thermodynamically calculated melting temperature, coarsening of the structure in the weld heat-affected zone is stabilized. It is possible to obtain a steel sheet with excellent toughness in the weld heat affected zone. In addition, in the present invention, in addition to controlling TiN precipitates, by controlling the amounts of S, Ca and O, coarsening of the structure in the weld heat affected zone is suppressed, and excellent toughness is achieved in the weld heat affected zone. can be obtained.

The present invention has been completed through further studies based on the findings obtained above, and the gist thereof is as follows.
[1] The component composition is mass%,
C: 0.030 to 0.120%,
Si: 0.01 to 0.15%,
Mn: 0.80-2.00%,
P: 0.020% or less,
S: 0.0005 to 0.0050%,
Al: 0.005 to 0.100%,
Ti: 0.005 to 0.030%,
N: 0.0030 to 0.0080%,
Ca: 0.0005 to 0.0030%,
O: contains 0.0040% or less,
the balance being Fe and unavoidable impurities,
Further, a steel sheet containing S, Ca, and O so as to satisfy the following formula (1), and having a mass ratio of 40% or more of TiN precipitates having an equivalent circle diameter exceeding 0.1 μm.
0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 (1)
However, each element symbol indicates the content (% by mass) of each element.
[2] The component composition is further, in mass%,
Cu: 1.00% or less,
Ni: 1.50% or less,
Cr: 1.00% or less,
Mo: 0.50% or less,
V: 0.50% or less, and
Nb: The steel sheet according to [1], containing one or more selected from 0.05% or less.
[3] The component composition is further, in mass%,
B: 0.0025% or less,
Mg: 0.0050% or less,
Zr: 0.0200% or less,
REM: The steel sheet according to [1] or [2], containing at least one selected from 0.0200% or less.

INDUSTRIAL APPLICABILITY According to the present invention, a steel sheet having excellent toughness in the heat-affected zone of high heat input welding with a welding heat input of 20.0 kJ/mm or more is obtained, which is industrially very useful.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail. First, the chemical composition that the steel sheet of the present invention should have will be described. In the following description, % indications relating to chemical components all mean % by mass.

C: 0.030-0.120%
C is an element that increases the strength of steel materials, and in order to secure the strength required for structural steel, it is necessary to contain 0.030% or more. Therefore, the lower limit of the C content is 0.030%. The C content is preferably 0.040% or more, more preferably 0.050% or more, still more preferably 0.060% or more. On the other hand, if C exceeds 0.120%, island martensite (hereinafter sometimes referred to as MA) is likely to form in the weld heat affected zone, leading to a decrease in toughness, so the upper limit is 0.120%. and The C content is preferably 0.100% or less, more preferably 0.090% or less, still more preferably 0.085% or less.

Si: 0.01-0.15%
Si is an element added as a deoxidizing agent when steel is melted, and its content must be 0.01% or more. Therefore, the Si content should be 0.01% or more. The Si content is preferably 0.02% or more, more preferably 0.03% or more, still more preferably 0.04% or more, and most preferably 0.06% or more. However, if it exceeds 0.15%, the toughness of the base metal is lowered, and the tendency to form island-shaped martensite is increased in the heat-affected zone of high heat input welding, which may lead to a drop in toughness. Therefore, the Si content should be 0.15% or less. The Si content is preferably 0.13% or less, more preferably 0.10% or less, and still more preferably 0.09% or less.

Mn: 0.80-2.00%
The Mn content is set to 0.80% or more in order to ensure the strength of the base material. The Mn content is preferably 1.00% or more, more preferably 1.20% or more, still more preferably 1.40% or more, and most preferably 1.50% or more. On the other hand, if the Mn content exceeds 2.00%, the toughness of the HAZ is remarkably deteriorated, so the Mn content is made 2.00% or less. The Mn content is preferably 1.90% or less, more preferably 1.85% or less, still more preferably 1.80% or less, and most preferably 1.70% or less.

P: 0.020% or less P promotes the formation of MA in the HAZ near the bond portion and greatly reduces the toughness, so the P content is set to 0.020% or less. The P content is preferably 0.015% or less, more preferably 0.012% or less. In addition, the lower limit of the P content is not particularly limited. However, excessive dephosphorization causes an increase in cost, so the P content is preferably 0.002% or more.

S: 0.0005 to 0.0050%
S is an element necessary for forming MnS or CaS that acts as a ferrite nucleation site. Therefore, the S content should be 0.0005% or more. The S content is preferably 0.0010% or more, more preferably 0.0015% or more. However, since an excessive S content causes a decrease in the toughness of the base material, the S content is made 0.0050% or less. The S content is preferably 0.0040% or less, more preferably 0.0035% or less, still more preferably 0.0030% or less.

Al: 0.005-0.100%
Al is an element contained for deoxidizing steel, and the Al content is made 0.005% or more. The Al content is preferably 0.010% or more, more preferably 0.020% or more, still more preferably 0.030% or more. However, when the content exceeds 0.100%, not only the toughness of the base metal but also the toughness of the weld metal is lowered. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.085% or less, more preferably 0.070% or less, still more preferably 0.065% or less.

Ti: 0.005-0.030%
Ti precipitates in the base material as TiN when the molten steel solidifies, and contributes to improvement of base material toughness by suppressing coarsening of austenite grains. Also, during welding, TiN suppresses the coarsening of the structure in the weld heat-affected zone and serves as a transformation nucleus of ferrite, contributing to an increase in toughness. In order to obtain such effects, the content must be 0.005% or more. Therefore, the Ti content should be 0.005% or more. The Ti content is preferably 0.008% or more, more preferably 0.011% or more, still more preferably 0.015% or more. On the other hand, when the Ti content exceeds 0.030%, the precipitated TiN becomes excessively coarse, and the above effect cannot be obtained. Therefore, the Ti content should be 0.030% or less. The Ti content is preferably 0.027% or less, more preferably 0.024% or less, and still more preferably 0.020% or less.

N: 0.0030 to 0.0080%
N generates TiN and contributes to the improvement of toughness, so the N content is made 0.0030% or more. The N content is preferably 0.0035% or more, more preferably 0.0040% or more. On the other hand, if it exceeds 0.0080%, when the TiN is held at a high temperature due to the welding thermal cycle and TiN melts, there is a concern that N dissolved in the base structure becomes excessive and the toughness deteriorates. From the above, the N content is set to 0.0080% or less. The N content is preferably 0.0070% or less, more preferably 0.0065% or less, still more preferably 0.0070% or less.

Ca: 0.0005-0.0030%
Ca has the effect of fixing S and improving the toughness. To obtain this effect, the Ca content is set to 0.0005% or more. The Ca content is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if the Ca content exceeds 0.0030%, the effect saturates, so the Ca content is made 0.0030% or less. The Ca content is preferably 0.0025% or less, more preferably 0.0020% or less.

O: 0.0040% or less Since O indirectly affects the formation of composite sulfides in which MnS precipitates on CaS, the O content is made 0.0040% or less. The O content is preferably 0.0030% or less, more preferably 0.0025% or less. In addition, the lower limit of the O content is not particularly limited. However, excessive reduction of the oxygen content causes an increase in cost, so the O content is preferably 0.0003% or more.

Also, in the present invention, S, Ca, and O must satisfy the following formula (1).
0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 (1)
However, each element symbol indicates the content (% by mass) of each element.
(1) If the value of "(Ca-(0.18 + 130 x Ca) x O) / 1.25 / S" in the formula (hereinafter referred to as the A value) is 0 or less, CaS does not crystallize and S Precipitates as MnS alone, elongates in the rolling direction during steel sheet production, and lowers the toughness of the base material. In addition, MnS is melted in the weld heat affected zone, so excellent toughness cannot be obtained. Therefore, the A value is greater than 0. The A value is preferably 0.1 or more, more preferably 0.2 or more, and still more preferably 0.3 or more. On the other hand, when the A value is 1 or more, most of the S is fixed by Ca, and MnS, which serves as ferrite formation nuclei, does not precipitate on CaS, so ferrite does not form in the weld heat affected zone, and the effect of improving toughness cannot be obtained. . Therefore, the A value should be less than 1. The A value is preferably 0.8 or less, more preferably 0.7 or less.

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

In the present invention, in addition to the above components, one or more selected from Cu, Ni, Cr, Mo, V and Nb are contained as selective elements within the following ranges for the purpose of improving strength. can be done.

Cu: 1.00% or less Cu is an element that is effective in increasing the strength of steel sheets. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less, more preferably 0.30% or less. On the other hand, in order to obtain such an effect, when Cu is contained, the Cu content is preferably 0.03% or more. The Cu content is more preferably 0.04% or more.

Ni: 1.50% or less Ni improves the toughness and strength of the steel sheet, but excessive addition reduces the toughness of the base metal and HAZ and presses the manufacturing cost. Therefore, when Ni is contained, the Ni content is set to 1.50% or less. The Ni content is preferably 1.0% or less, more preferably 0.50% or less, still more preferably 0.30% or less. On the other hand, in order to obtain such an effect, when Ni is contained, the Ni content is preferably 0.03% or more. More preferably, the Ni content is 0.04% or more.

Cr: 1.00% or less Cr is an element that is advantageous in increasing the strength of the steel sheet, but excessive content reduces the toughness of the base metal and HAZ. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less, more preferably 0.50% or less, still more preferably 0.30% or less. On the other hand, in order to obtain such an effect, when Cr is contained, the Cr content is preferably 0.02% or more. More preferably, the Cr content is 0.03% or more.

Mo: 0.50% or less Mo is an element advantageous for increasing the strength of the steel sheet, but excessive content reduces the toughness of the base metal and HAZ. Therefore, when Mo is contained, the Mo content shall be 0.50% or less. The Mo content is preferably 0.40% or less, more preferably 0.30% or less, still more preferably 0.20% or less. On the other hand, in order to obtain such effects, when Mo is contained, the Mo content is preferably 0.003% or more. More preferably, the Mo content is 0.004% or more.

V: 0.50% or less V is an element advantageous for increasing the strength of the steel sheet, but excessive content reduces the toughness of the base metal and HAZ. Therefore, when V is contained, the V content should be 0.50% or less. The V content is preferably 0.40% or less, more preferably 0.30% or less, still more preferably 0.20% or less. On the other hand, in order to obtain such an effect, when V is contained, the V content is preferably 0.003% or more. More preferably, the V content is 0.004% or more.

Nb: 0.05% or less Nb greatly contributes to the strength improvement of steel sheets, but excessive content may cause an increase in upper bainite and island-shaped martensite in the weld heat-affected zone structure, resulting in a decrease in toughness. Connect. Therefore, when Nb is contained, the Nb content is made 0.05% or less. The Nb content is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.02% or less. On the other hand, in order to obtain such effects, when Nb is contained, the Nb content is preferably 0.002% or more. More preferably, the Nb content is 0.003% or more.

In addition to the above components, the steel material of the present invention can further contain one or more selected from B, Mg, Zr and REM as selective elements within the following ranges.

B: 0.0025% or less B forms BN in the weld heat-affected zone to reduce solid solution N, and also serves as ferrite transformation nuclei to form ferrite to improve toughness. In order to obtain such an effect, when B is contained, the B content should be 0.0003% or more. The B content is preferably 0.0005% or more, more preferably 0.0008% or more. However, when the B content exceeds 0.0025%, the toughness of the base metal and HAZ is lowered. Therefore, when B is contained, the B content should be 0.0025% or less. The B content is preferably 0.0020% or less, more preferably 0.0018% or less.

Mg: 0.0050% or less, Zr: 0.0200% or less, REM: 0.0200% or less Mg, Zr, and REM are all elements that have an effect of improving toughness by dispersing oxides. In order to exhibit such an effect, when Mg, Zr and REM are contained, the Mg content should be 0.0005% or more, and the Zr content and REM content should be 0.0010% or more, respectively. preferable. More preferably, the Mg content is 0.0010% or more, and the Zr content and REM content are each 0.0015% or more. On the other hand, even if Mg exceeds 0.0050% and Zr and REM each exceed 0.0200%, the effect is only saturated. Therefore, when these elements are contained, the Mg content should be 0.0050% or less, and the Zr content and REM content should each be 0.0200% or less. Preferably, the Mg content is 0.0030% or less, and the Zr and REM contents are each 0.01% or less.

Next, the structure of the steel sheet of the present invention will be explained.

40% or more by mass of TiN precipitates having an equivalent circle diameter of more than 0.1 μm;
Regarding the TiN precipitates in the steel sheet, the mass ratio (hereinafter also referred to as the P value) of the precipitates with an equivalent circle diameter exceeding 0.1 μm in the total amount of precipitates is 40% or more. TiN remains unmelted even when high heat input welding exceeding . As a result, it suppresses subsequent austenite grain growth and contributes to improving the toughness of the heat affected zone and the steel plate. Therefore, the mass ratio of precipitates having an equivalent circle diameter exceeding 0.1 μm in the total amount of TiN precipitates is set to 40% or more. The mass ratio of precipitates having an equivalent circle diameter of more than 0.1 μm in the total amount of TiN precipitates is preferably 45% or more, more preferably 50% or more. On the other hand, if the mass ratio of precipitates with a large size (equivalent circle diameter) increases excessively, the precipitates may become coarse and may become the starting point of fracture. It is preferable that the mass ratio of precipitates having a diameter exceeding 0.1 μm is 98% or less. In the total amount of TiN precipitates, the mass ratio of precipitates having an equivalent circle diameter of more than 0.1 μm is more preferably 98% or less, further preferably 95% or less. In addition, precipitates having an equivalent circle diameter of more than 2.0 μm may become starting points of brittle fracture, so it is desirable to reduce them as much as possible.

In order to control the proportion of precipitates exceeding 0.1 μm in equivalent circle diameter, for example, the average cooling rate from 1450° C. to 1300° C. during casting is adjusted to 0.5° C./sec or less. Due to Ostwald growth after precipitation, the mass ratio of precipitates having an equivalent circle diameter exceeding 0.1 μm can be made 40% or more. When the cooling rate is higher than 0.5 ° C./sec, the proportion of precipitates with an equivalent circle diameter of 0.1 μm or less increases, and TiN is formed during high heat input welding exceeding 20.0 kJ / mm. Most of them are dissolved, and subsequent grain growth cannot be sufficiently suppressed.

Next, the method for manufacturing the steel sheet of the present invention will be described.

The steel sheet of the present invention can be produced by conventionally known methods other than the above average cooling rate during casting. For example, steel melted in a converter, an electric furnace, etc. is secondary refined by RH degassing, etc., and after adjusting the steel components to the above-mentioned appropriate range, steel such as slabs is produced through continuous casting or ingot making-blooming processes. be the material. The average cooling rate may be controlled during continuous casting or ingot casting. Next, the steel material is reheated, hot-rolled to form a steel sheet of desired dimensions, and then left to cool, or after the hot rolling, accelerated cooling, direct quenching-tempering, and reheating and quenching. It can be manufactured through processes such as - tempering, reheating and normalizing - tempering. The plate thickness range obtained in the present invention is 9 mm to 50 mm.

The steel sheet of the present invention has excellent toughness in the large heat input affected zone where the welding heat input is 20.0 kJ/mm or more. Specifically, when a Charpy impact test at -40 ° C is performed in a large heat input affected zone where the welding heat input is 20.0 kJ / mm or more, the impact absorption value (vE _{-40 ° C} ) exceeding 100 J is obtained.

Examples of the present invention are described below. It should be noted that the steel sheet and the method for producing the same according to the present invention are not limited to the examples.

No. 1 having the component composition shown in Table 1 was melted using a 150 kg high-frequency melting furnace. Steel Nos. 1 to 18 were melted and cast at the average cooling rate shown in Table 2 to form steel ingots, which were then hot rolled to form steel plates with a thickness of 50 mm. TiN precipitates were quantified for the obtained steel sheet using the QUANPASS method (see JP-A-2010-12778). Specifically, a 10 mm square plate-shaped metal sample is cut out from the 1/4 position of the plate thickness of the steel plate, the metal sample is electrolyzed in an electrolytic solution, precipitates etc. are extracted, filtered by size and quantitatively analyzed. A repeated test was conducted. From the quantification results, the P value was defined as the mass ratio of TiN precipitates having an equivalent circle diameter exceeding 0.1 μm among all TiN precipitates.

In order to evaluate the toughness of the heat affected zone of the weld, a simulated heat cycle test simulating high heat input welding was performed. A test piece of width 80 mm x length 80 mm x thickness 15 mm is taken from the 1/4 position of the plate thickness of the steel plate, heated to 1450 ° C., and then cooled between 800 and 500 ° C. in 300 seconds. After applying a simulated heat cycle. , 2 mm V-notch Charpy test pieces were taken from these test pieces. The obtained Charpy test piece was subjected to a Charpy impact test at a test temperature of -40°C to evaluate the toughness. A good result was obtained when the average impact absorption value (vE _-40°C ) of the three test results exceeded 100J. The simulated thermal cycle conditions correspond to the thermal history of the bond portion in the case of electrogas welding with a heat input of 30.0 kJ/mm simulating one-pass welding with a plate thickness of 50 mm.

Table 2 also shows the test results of the average cooling rate from 1450°C to 1300°C during casting, the P value, and the toughness of the weld heat affected zone.

発明例である鋼板Ｎｏ．１～１０、１８は、大入熱溶接熱影響部において優れた靭性を示した。一方で、鋼の成分組成もしくはＰ値が本発明範囲外である鋼板Ｎｏ.１１～１７においては、大入熱溶接熱影響部の靭性が発明例に比べて低かった。

Steel plate No. which is an invention example. 1 to 10 and 18 showed excellent toughness in the heat affected zone of high heat input welding. On the other hand, in steel sheets Nos. 11 to 17, in which the chemical composition or P value of the steel is outside the range of the present invention, the toughness of the heat affected zone of high heat input welding was lower than that of the invention examples.

Claims (3)

The component composition is mass%,
C: 0.030 to 0.120%,
Si: 0.01 to 0.15%,
Mn: 0.80-2.00%,
P: 0.020% or less,
S: 0.0005 to 0.0050%,
Al: 0.005 to 0.100%,
Ti: 0.005 to 0.030%,
N: 0.0030 to 0.0080%,
Ca: 0.0005 to 0.0030%,
O: contains 0.0040% or less,
the balance being Fe and unavoidable impurities,
Furthermore, S, Ca, and O are contained so as to satisfy the following formula (1), and among the TiN precipitates, the precipitates having an equivalent circle diameter of more than 0.1 μm and 2.0 μm or less account for 40% or more by mass. steel plate.
0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 (1)
However, each element symbol indicates the content (% by mass) of each element. The component composition is further, in mass%,
Cu: 1.00% or less,
Ni: 1.50% or less,
Cr: 1.00% or less,
Mo: 0.50% or less,
V: 0.50% or less, and
The steel sheet according to claim 1, containing one or more selected from Nb: 0.05% or less. The component composition is further, in mass%,
B: 0.0025% or less,
Mg: 0.0050% or less,
Zr: 0.0200% or less,
The steel sheet according to claim 1 or 2, containing one or more selected from REM: 0.0200% or less.