KR20230051276A - steel plate - Google Patents

Abstract

An object of the present invention is to provide a steel sheet having excellent toughness in a high heat input welding heat affected zone where the welding heat input is 20.0 kJ/mm or more. Component composition, in mass%, C: 0.030 to 0.120%, Si: 0.01 to 0.15%, Mn: 0.80 to 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: 0.0040% or less, the balance being Fe and unavoidable impurities, and S, Ca, and O are the following (1) A steel sheet containing so as to satisfy the formula and having a mass ratio of 40% or more of TiN precipitates exceeding 0.1 µm in equivalent circle diameter. 0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 . (1) However, each element symbol represents the content (mass %) of each element.

Description

steel plate

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

Large heat input with excellent production efficiency, such as submerged arc welding, electrogas welding, and electroslag welding, in welding construction accompanying the increase in strength and thickness of steel materials The demand for application of welding is increasing. Since the toughness of the welding heat-affected zone (hereinafter sometimes referred to as HAZ) welded with high heat input decreases, various steels for high heat input welding have been proposed. For example, a technique of finely dispersing TiN in steel to suppress coarsening of austenite grains in a weld heat-affected zone and a technique of using it as a ferrite transformation nucleus in a weld heat-affected zone have been put into practical use.

Suppression of tissue coarsening using TiN precipitates is economically useful and widely used. On the other hand, these effects are not obtained in a high-temperature region at which TiN melts in the heat-affected zone of the weld. There was a problem that the microstructure) was embrittled and the toughness was remarkably lowered.

Therefore, in Patent Document 1, TiOx having a particle size of 5 μm or less (provided that 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 heat-affected zone in the weld heat-affected zone. A technique for improving the toughness of a weld heat-affected zone by using it as a generation nucleus of acicular ferrite has been proposed. Patent Literature 2 proposes a technique for improving the toughness of the heat-affected zone by adjusting the amounts of B, N, and sol.Al in the component composition to actively precipitate BN that refines the heat-affected zone.

Further, in Patent Literature 3, a technique of adjusting the amount of Ti-B-N in the component composition so that the HAZ toughness is in the high toughness region, and adding Ca or Ce to give a toughness improving effect by controlling the shape of inclusions is proposed. there is. Patent Literature 4 also proposes a technique for improving the toughness of a welded joint with high heat input by setting the component composition to a low-N-low Ti system and adding REM that forms a stable sulfur oxide even in a welded bond portion. Further, in Patent Literature 5, Ca-based nonmetallic inclusions, which become transformation nuclei and promote ferrite transformation in the heat-affected zone of welding, are finely dispersed in steel by appropriately controlling the Ca, O, and S contents, and the concentration exceeds 20.0 kJ/mm. A technique for improving the toughness of a heat-affected zone in welding with high heat input is disclosed.

Japanese Unexamined Patent Publication No. 57-51243 Japanese Unexamined Patent Publication No. 62-170459 Japanese Unexamined Patent Publication No. 60-204863 Japanese Patent Publication Hei 4-14180 Japanese Patent No. 3546308

However, the technology using Ti oxide described in Patent Literature 1 has a problem in that it is difficult to uniformly finely disperse the oxide, particularly in mass production, and thus the toughness of the heat-affected zone cannot be secured stably. In the technique described in Patent Literature 2, cracks originating from nitride-mainly inclusions may be generated during casting. Further, in the techniques in Patent Literatures 3 to 4, it is difficult to sufficiently suppress the grain growth of austenite in the heat-affected zone by welding with a large heat input exceeding 20.0 kJ/mm. There is a problem that the toughness becomes unstable. In addition, the technique described in Patent Literature 5 has a problem that it is difficult to ensure stable toughness.

The present invention has been made in view of the above situation, and an object of the present invention is to provide a steel sheet having excellent toughness in a high heat input welding heat affected zone where the welding heat input is 20.0 kJ/mm or more.

In order to solve the said subject, inventors repeated various examinations and obtained the following recognition.

In order to suppress coarsening of the structure in the heat-affected zone using TiN precipitates having excellent industrial productivity, it is important to appropriately control the size of TiN precipitates in the base steel sheet. That is, by securing a certain amount or more of TiN precipitates having a size that remains without melting even when exposed to a high temperature at a temperature equal to or higher than the thermodynamically calculated melting temperature, the coarsening of the structure in the weld heat affected zone is stably suppressed, and welding A steel sheet having excellent toughness in the heat-affected zone can be obtained. Further, in the present invention, by controlling the amount of S, Ca, and O in addition to controlling the precipitates of TiN, coarsening of the structure in the heat-affected zone is suppressed, and excellent toughness is obtained in the heat-affected zone. You can get a steel plate.

The present invention was completed by further examination based on the recognition obtained above, and the gist thereof is as follows.

[1] Component composition, in mass%,

C: 0.030% to 0.120%;

Si: 0.01 to 0.15%;

Mn: 0.80 to 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 is Fe and unavoidable impurities,

Further, a steel sheet containing S, Ca, and O 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 account for 40% or more in terms of mass ratio.

0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 . (One)

However, each element symbol represents the content (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

The steel sheet according to [1], containing at least one selected from Nb: 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;

The steel sheet according to [1] or [2], containing at least one selected from REM: 0.0200% or less.

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

(Mode for implementing the invention)

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described in detail. First, the component composition that the steel sheet of the present invention should have will be described. In the following description, all % indications regarding chemical components mean % by mass.

C: 0.030 to 0.120%

C is an element that increases the strength of steel materials, and needs to be contained in an amount of 0.030% or more in order to secure strength required as structural steel. Therefore, the lower limit of the C content is made 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, when C exceeds 0.120%, island-like martensite (hereinafter sometimes referred to as MA) tends to be formed in the weld heat-affected zone, leading to a decrease in toughness, so the upper limit is set at 0.120%. . 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 to 0.15%

Si is an element added as a deoxidizer when melting steel, and the content of 0.01% or more is required. Therefore, the Si content is made 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, when it exceeds 0.15%, the toughness of the base material is lowered, and the generation tendency of island martensite is increased in the heat-affected zone of high heat input welding, resulting in a decrease in toughness in some cases. Therefore, the Si content is made 0.15% or less. The Si content is preferably 0.13% or less, more preferably 0.10% or less, still more preferably 0.09% or less.

Mn: 0.80 to 2.00%

As for Mn, in order to ensure the strength of the base material, the Mn content is set to 0.80% or more. The Mn content is preferably 1.00% or more, more preferably 1.20% or more, even more preferably 1.40% or more, and most preferably 1.50% or more. On the other hand, when the Mn content exceeds 2.00%, the toughness of the HAZ deteriorates remarkably, so it is set to 2.00% or less. Further, 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

Since P promotes MA production in the HAZ near the bond portion and greatly reduces toughness, 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 P content is not specifically limited. However, since excessive removal of P causes an increase in cost, 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 serving as a ferrite nucleation site. For this reason, S content is made into 0.0005% or more. The S content is preferably 0.0010% or more, and more preferably 0.0015% or more. However, excessive inclusion of S causes a decrease in base metal toughness, so the S content is set to 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 to 0.100%

Al is an element contained for deoxidation of steel, and the Al content is set to 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 it contains more than 0.100%, not only the toughness of the base metal but also the toughness of the weld metal is reduced. Therefore, the Al content is made 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 to 0.030%

Ti contributes to the improvement of base metal toughness by precipitating in the base material as TiN at the time of solidification of molten steel, and suppressing the coarsening of austenite grains. Further, during welding, TiN suppresses the coarsening of the structure in the weld heat-affected zone and serves as a ferrite transformation nucleus, contributing to high toughness. In order to obtain such an effect, 0.005% or more of containing is required. Therefore, the Ti content is made 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 Ti is contained exceeding 0.030%, the precipitated TiN excessively coarsens, and the said effect is not obtained. Therefore, the Ti content is made 0.030% or less. The Ti content is preferably 0.027% or less, more preferably 0.024% or less, still more preferably 0.020% or less.

N: 0.0030 to 0.0080%

Since N generates TiN and contributes to improving toughness, the N content is set to 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%, there is a concern that when TiN is dissolved at a high temperature by a welding heat cycle, dissolved N in the dough structure becomes excessive and deteriorates toughness. From the above, the N content is made 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 to 0.0030%

Ca has an effect of improving toughness by fixing S. In order to obtain the effect, Ca content is made into 0.0005% or more. The Ca content is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, since the effect is saturated when the Ca content exceeds 0.0030%, the Ca content is made 0.0030% or less. The Ca content is preferably 0.0025% or less, and more preferably 0.0020% or less.

O: 0.0040% or less

Since O indirectly affects the formation of complex sulfide in which MnS is precipitated on CaS, the O content is set to 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 O content is not specifically limited. However, the O content is preferably 0.0003% or more, since an excessive reduction in the amount of oxygen causes an increase in cost.

In addition, in the present invention, S, Ca, and O need to satisfy the following formula (1).

0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 . (One)

However, each element symbol represents the content (mass %) of each element.

(1) When the value of "(Ca - (0.18 + 130 × Ca) × O) / 1.25 / S" (hereinafter referred to as the A value) in the formula is 0 or less, CaS does not crystallize and S precipitates as a single MnS , It elongates in the rolling direction during steel sheet manufacturing, reducing the toughness of the base material. In addition, excellent toughness cannot be obtained because MnS melts in the weld heat affected zone. Therefore, the value of A should be greater than 0. A value is preferably 0.1 or more, more preferably 0.2 or more, still more preferably 0.3 or more. On the other hand, when the A value is 1 or more, since most of S is fixed by Ca and MnS, which serves as a ferrite formation nucleus, is not precipitated on CaS, ferrite is not generated in the weld heat-affected zone, and the effect of improving toughness is not obtained. don't Therefore, the value of A is less than 1. 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 among Cu, Ni, Cr, Mo, V and Nb may be contained as an optional element in the following range for the purpose of strength improvement and the like. .

Cu: 1.00% or less

Cu is an element effective for increasing the strength of a steel sheet, but when added excessively, there is a concern that it promotes cracking of cast steel ingots and lowers the toughness of the steel sheet. Therefore, when containing Cu, Cu content is made into 1.00 % or less. Cu content is preferably 0.50% or less, more preferably 0.30% or less. On the other hand, in order to acquire such an effect, when containing Cu, it is preferable to make Cu content into 0.03 % or more. As for Cu content, it is more preferable to make it 0.04 % or more.

Ni: 1.50% or less

While Ni improves the toughness of a steel plate and also raises strength, excessive addition reduces the toughness of a base material and HAZ, and presses manufacturing cost. Therefore, when containing Ni, Ni content is made into 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 containing Ni, it is preferable to make Ni content into 0.03 % or more. As for Ni content, it is more preferable to make it 0.04 % or more.

Cr: 1.00% or less

Cr is an element that is advantageous for increasing the strength of the steel sheet, but excessive inclusion lowers the toughness of the base material and HAZ. Therefore, when it contains Cr, Cr content is made into 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 containing Cr, it is preferable to make Cr content into 0.02 % or more. As for Cr content, it is more preferable to make it 0.03 % or more.

Mo: 0.50% or less

Mo is an element that is advantageous for increasing the strength of the steel sheet, but an excessive amount of Mo deteriorates the toughness of the base material and HAZ. Therefore, when containing Mo, Mo content is made into 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 an effect, when containing Mo, it is preferable to make Mo content into 0.003 % or more. As for Mo content, it is more preferable to make it 0.004 % or more.

V: 0.50% or less

V is an element that is advantageous for increasing the strength of the steel sheet, but excessive inclusion lowers the toughness of the base material and HAZ. Therefore, when V is contained, the V content is made 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, in the case of containing V, the V content is preferably 0.003% or more. As for V content, it is more preferable to make it 0.004 % or more.

Nb: 0.05% or less

Although Nb contributes greatly to improving the strength of a steel sheet, an excessive amount of Nb may cause an increase in upper bainite or island martensite in the structure of the weld heat affected zone, leading to a decrease in toughness. Therefore, when Nb is contained, Nb content is made into 0.05 % or less. The Nb content is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.02% or less. On the other hand, in order to obtain such an effect, when containing Nb, it is preferable to make Nb content into 0.002% or more. As for Nb content, it is more preferable to make it 0.003 % or more.

In addition to the above components, the steel materials of the present invention may further contain at least one selected from B, Mg, Zr and REM as an optional element in the following range.

B: 0.0025% or less

B generates BN in the weld heat-affected zone, reduces solute N, and forms ferrite transformation nuclei to generate ferrite to improve toughness. In order to obtain such an effect, when B is contained, B content is made into 0.0003% or more. The B content is preferably 0.0005% or more, more preferably 0.0008% or more. However, if B is contained in excess of 0.0025%, the toughness of the base material and HAZ will be lowered. For this reason, when B is contained, B content is made into 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

All of Mg, Zr, and REM are elements having an effect of improving toughness by dispersing oxides. In order to express these effects, when Mg, Zr, and REM are contained, it is preferable that the Mg content is 0.0005% or more, and the Zr content and REM content are each 0.0010% or more. It is more preferable that 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 it contains more than 0.0050% of Mg and more than 0.0200% of each of Zr and REM, the effect is only saturated. Therefore, when these elements are contained, the Mg content is 0.0050% or less, and the Zr content and REM content are each 0.0200% or less. Preferably, the Mg content is 0.0030% or less, and the Zr content and REM content are each 0.01% or less.

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

Among the TiN precipitates, the precipitates exceeding 0.1 μm in equivalent circle diameter account for 40% or more in terms of mass ratio,

With regard to the TiN precipitates in the steel sheet, the mass ratio of the precipitates exceeding 0.1 μm in equivalent circle diameter among the total amount of precipitates is 40% or more (hereinafter also referred to as P value), so that the large heat input exceeding 20.0 kJ/mm welding Even in the case of carrying out, TiN remains undissolved. As a result, subsequent grain growth of austenite is suppressed, contributing to improvement of the heat-affected zone and toughness of the steel sheet. Therefore, among the total precipitates of TiN precipitates, the mass ratio of precipitates exceeding 0.1 µm in equivalent circle diameter is set to 40% or more. Of the total amount of TiN precipitates, the mass ratio of precipitates exceeding 0.1 µm in equivalent circle diameter is preferably 45% or more, more preferably 50% or more. On the other hand, if the mass ratio of precipitates having a large size (equivalent circle diameter) increases excessively, the precipitates may become coarse and become the starting point of fracture. It is preferable that the excess precipitate is 98% or less in terms of mass ratio. Among the total amount of TiN precipitates, the mass ratio of precipitates exceeding 0.1 µm in equivalent circle diameter is more preferably 98% or less, and still more preferably 95% or less. In addition, since precipitates exceeding 2.0 μm in equivalent circle diameter may become a starting point of brittle fracture, it is desirable to reduce them as much as possible.

In order to control the ratio of precipitates exceeding 0.1 μm in equivalent circle diameter, for example, Ostwald after precipitation by adjusting the average cooling rate from 1450 ° C. to 1300 ° C. to be 0.5 ° C./sec or less during casting. ) The mass ratio of precipitates exceeding 0.1 µm in equivalent circle diameter can be set to 40% or more by growth. When the cooling rate is greater than 0.5° C./sec, the ratio of precipitates of 0.1 μm or less in equivalent circle diameter increases, and most of TiN is dissolved during welding with a large heat input exceeding 20.0 kJ/mm, and subsequent Lip growth cannot be suppressed sufficiently.

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

The steel sheet of the present invention can be manufactured by a conventionally known method for manufacturing methods other than the average cooling rate at the time of casting described above. For example, after secondary refining of steel smelted in a converter or electric furnace with RH degassing, etc. to adjust the steel component to the above appropriate range, continuous casting or ingot casting -Slabbing process) to make steel materials such as slabs. In addition, the average cooling rate may be controlled during continuous casting or ingot casting. Then, the steel material is reheated, hot rolled to obtain a steel sheet having a desired size, and then subjected to a process of cooling, or, after the hot rolling, accelerated cooling, direct quenching-tempering, and reheating quenching - It can be manufactured through a process such as tempering, reheating, normalizing, and tempering. The range of the board thickness obtained by this invention is 9 mm - 50 mm.

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

Example

Examples of the present invention are described below. In addition, the steel sheet and its manufacturing method of the present invention are not limited to the examples.

Using a 150-kg high-frequency melting furnace, steels No. 1 to 18 having the component compositions shown in Table 1 were melted, cast at the average cooling rate shown in Table 2 into steel ingots, and then hot-rolled to obtain a thickness of 50 It was made into a steel plate of mm. For the obtained steel sheet, TiN precipitates were quantified using the QUANPASS method (refer to Japanese Laid-Open Patent Publication No. 2010-12778). Specifically, a 10 mm square plate-shaped metal sample is cut out from a position of 1/4 of the plate thickness of the steel plate, the metal sample is electrolyzed in an electrolyte solution, precipitates, etc. are extracted, and filtering and quantification according to size are performed. A test repeating the analysis was conducted. From the quantitative results, the mass ratio of TiN precipitates having a size exceeding 0.1 μm in equivalent circle diameter among all TiN precipitates was taken as P value.

Further, in order to evaluate the toughness of the heat-affected zone of the weld, a reproducible heat cycle test simulating high heat input welding was conducted. A test piece having a width of 80 mm × a length of 80 mm × a thickness of 15 mm is taken from a position of 1/4 of the plate thickness of the steel plate, heated to 1450 ° C, and then subjected to a reproducible heat cycle of cooling between 800 and 500 ° C in 300 seconds. A 2 mm V-notch Charpy test piece was taken from the test piece. The obtained Charpy test piece was subjected to a Charpy impact test at a test temperature of -40°C to evaluate toughness. A case where the average shock absorption value (vE _-40°C ) of the three test results exceeded 100 J was regarded as a good result. The above simulated heat 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 at a plate thickness of 50 mm.

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

Steel sheet Nos. 1 to 10 and 18, which are invention examples, exhibited excellent toughness in the heat-affected zone of high heat input welding. On the other hand, in the steel sheets No. 11 to 17 whose component compositions or P values were outside the scope of the present invention, the toughness of the high heat input welding heat affected zone was lower than that of the invention examples.

Claims (3)

Component composition, in mass%,
C: 0.030% to 0.120%;
Si: 0.01 to 0.15%;
Mn: 0.80 to 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 is Fe and unavoidable impurities,
Further, a steel sheet containing S, Ca, and O 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 account for 40% or more in terms of mass ratio.
0<(Ca−(0.18+130×Ca)×O)/1.25/S<1 . (One)
However, each element symbol represents the content (mass %) of each element. According to claim 1,
Ingredient composition, 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
A steel sheet containing at least one selected from Nb: 0.05% or less. According to claim 1 or 2,
Ingredient composition, further, in mass%,
B: 0.0025% or less;
Mg: 0.0050% or less;
Zr: 0.0200% or less;
REM: 0.0200% or less, a steel sheet containing at least one selected from among.