Steel sheet having excellent heat affected zone toughness and method for manufacturing same

Abstract

The present invention relates to: a steel sheet which is used for ships and the like and in which a heat affected zone (HAZ) formed by welding has excellent toughness; and a method for manufacturing the same.

Description

Technical Field

[0001] The present disclosure relates to steel for ships and the like, and more particularly, to steel having excellent toughness in a heat-affected zone (HAZ) formed by welding, and a method for manufacturing the same. Background Art

[0002] Recently, as the area of Arctic sea ice has rapidly decreased due to the temperature increase caused by global warming, the interest in opening Arctic shipping routes has increased. It is necessary to build ships for opening Arctic routes or ships that operate as icebreakers capable of breaking ice in emergency situations in the Arctic routes.

[0003] An icebreaker is a ship that opens a route by breaking ice on the water surface. To date, most icebreakers have been military ships or exploration ships, but recently, as the interest in Arctic shipping routes has increased, the use of icebreakers has expanded to general merchant ships and cruise ships. For example, Russia is the most active country in building icebreakers due to its geographical characteristics. As of 2020, there are approximately 40 icebreakers operating globally, including Yermak, Arktika, and Sibir, and an increase in the construction of icebreakers is expected in the future.

[0004] The steel for the hull of an icebreaker must have excellent impact toughness even at extremely low temperatures to withstand the low temperature of the Northern Sea Route, and at the same time, high strength is required to protect the hull. In other words, it is necessary to ensure both high strength and high toughness, and for this purpose, a large amount of alloying elements are added.

[0005] When building an icebreaker at a shipyard, in order to improve productivity, it is advantageous to increase the heat input during welding of the steel, and a steel that does not deteriorate the toughness of the welding heat-affected zone (HAZ) even when the heat input during welding is increased as described above. However, as described above, in order to ensure strength, a large amount of alloying elements are added to the steel for an icebreaker, and as a result, the toughness of the welding HAZ is significantly reduced.

[0006] Generally, in order to ensure the toughness of a welding HAZ manufactured with a high heat input, the nitrogen content is increased to generate fine TiN precipitates and refine the grain size of the welding HAZ (Patent Document 1). However, in this case, due to the high nitrogen content, the impact toughness of the base material may deteriorate due to free nitrogen (free N). To prevent this, a large amount of boron (B) is added to form BN, thereby preventing the deterioration of toughness. However, if the added amount is not carefully controlled, the toughness may deteriorate additionally due to the appearance of free boron (free B). In addition, during the casting process for manufacturing a slab, adding a large amount of nitrogen may cause microcracks on the surface of the slab, and therefore, the method using high nitrogen can be considered an ineffective method.

[0007] Meanwhile, attempts have been made to refine the grain size of the welded HAZ by utilizing fine oxides to ensure toughness, but it is very difficult to uniformly disperse the oxides that have been formed as fine grains at high temperatures into the steel, and it is uncertain whether the necessary oxides can actually be selectively refined only throughout the steel and whether it has a toughness improvement effect.

[0008] Therefore, there is a need for steel manufacturing techniques that can ensure the strength and toughness of the base material while ensuring excellent toughness in the welded HAZ.

[0009] (Patent Document 1) Japanese Patent Application Laid-Open No. 2005-200716 Summary of the Invention

[0010] Technical Problem

[0011] One aspect of the present disclosure is to provide such a steel plate and a method for manufacturing the same: excellent toughness of the heat-affected zone (HAZ) can be ensured even when welding a steel plate having high strength and high toughness with a certain amount of heat input.

[0012] The problems of the present disclosure are not limited to the above matters. Other tasks of the present disclosure are described throughout the specification, and those skilled in the art to which the present disclosure pertains will have no difficulty in understanding other tasks of the present disclosure based on the content described in the specification of the present disclosure.

[0013] Technical Solution

[0014] According to one aspect of the present disclosure, a steel plate having excellent toughness in the heat-affected zone (HAZ) contains, by weight %: C: 0.03% to 0.06%, Mn: 1.5% to 1.7%, Si: 0.05% to 0.2%, Al: 0.01% to 0.04%, Ni: 0.6% to 0.9%, Mo: 0.1% to 0.2%, Cr: 0.1% to 0.3%, Ti: 0.01% to 0.02%, Nb: 0.005% to 0.02%, N: 0.0035% to 0.0070%, P: 0.008% or less, S: 0.002% or less, and the balance of Fe and unavoidable impurities,

[0015] wherein, in the welded HAZ welded with a heat input of 150 KJ / cm to 200 KJ / cm, the MA fraction in the region from the fusion line (FL) to FL + 1 mm is 4% or less in terms of area fraction.

[0016] According to another aspect of the present disclosure, a method for manufacturing a steel plate having excellent heat-affected zone (HAZ) toughness includes: heating a steel slab to 1050°C to 1150°C, the steel slab containing by weight%: C: 0.03% to 0.06%, Mn: 1.5% to 1.7%, Si: 0.05% to 0.2%, Al: 0.01% to 0.04%, Ni: 0.6% to 0.9%, Mo: 0.1% to 0.2%, Cr: 0.1% to 0.3%, Ti: 0.01% to 0.02%, Nb: 0.005% to 0.02%, N: 0.0035% to 0.0070%, P: 0.008% or less, S: 0.002% or less, and the balance of Fe and inevitable impurities;

[0017] Rough rolling the heated steel slab at a temperature of 900°C or higher;

[0018] After rough rolling, finish hot rolling the rough-rolled steel slab at a temperature of 800°C or higher to manufacture a hot-rolled steel plate; and

[0019] Cooling the t / 4 point of the thickness (t) of the hot-rolled steel plate to 700°C or lower at a cooling rate of 15°C / second.

[0020] Advantageous Effects

[0021] According to the present disclosure, a steel plate and a method for manufacturing the same can be provided: the steel plate not only has excellent strength and toughness of the base material, but also ensures excellent toughness of the welded HAZ. Such a steel plate can be applied to various fields, such as icebreakers and structural members in extremely low temperature environments.

[0022] Various beneficial advantages and effects of the present disclosure are not limited to the above, and will be more easily understood during the process of describing specific embodiments of the present disclosure. Detailed Embodiments

[0023] The terms used in this specification are intended to describe the present disclosure and are not intended to limit the present disclosure. In addition, unless the relevant definition clearly indicates the opposite meaning, the singular forms used in this specification include the plural forms.

[0024] As used in the specification, the meaning of "comprising" is to specify the components and does not exclude the presence or addition of other components.

[0025] Unless otherwise defined, all terms, including technical terms and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art to which the present invention pertains. Terms defined in the dictionary are interpreted to have a meaning consistent with the relevant technical literature and the currently disclosed content.

[0026] The inventors of the present disclosure have conducted in-depth research on techniques for ensuring the toughness, particularly low-temperature toughness, of the formed welded HAZ by welding a steel plate having high strength and toughness with a medium heat input, for example, a heat input of about 150 KJ / cm to 200 KJ / cm. As a result, the present inventors have completed the present disclosure upon recognizing that by minimizing the MA phase in the microstructure of the base material and optimizing the nitrogen (N) content to finely precipitate TiN precipitates, it is possible to achieve the technical objective of preventing cracks and toughness deterioration by controlling the MA (martensite-austenite constituent) phase near the fusion line (FL) in the welded HAZ even when applying medium heat input welding.

[0027] First, one aspect of the steel plate of the present disclosure will be described in detail.

[0028] The steel plate contains, by weight %, C: 0.03% to 0.06%, Mn: 1.5% to 1.7%, Si: 0.05% to 0.2%, Al: 0.01% to 0.04%, Ni: 0.6% to 0.9%, Mo: 0.1% to 0.2%, Cr: 0.1% to 0.3%, Ti: 0.01% to 0.02%, Nb: 0.005% to 0.02%, N: 0.0035% to 0.0070%, P: 0.008% or less, S: 0.002% or less, and the balance of Fe and inevitable impurities. Hereinafter, each alloy composition will be described.

[0029] Carbon (C): 0.03% to 0.06% (hereinafter, the content of each alloy composition is provided in weight %)

[0030] C is the most important element for ensuring basic strength, and thus it needs to be contained in the steel plate within an appropriate range. If the C content exceeds 0.06%, the hardenability may increase, a large amount of MA may be generated, and the toughness of the welded HAZ may decrease. If the C content is less than 0.03%, the strength may decrease, and thus, the C content is preferably 0.03% to 0.06%.

[0031] Silicon (Si): 0.05% to 0.2% and aluminum (Al): 0.01% to 0.04%

[0032] Si and Al are alloying elements necessary for precipitating dissolved oxygen in molten steel in the form of slag for deoxidation during the steelmaking and casting processes. When manufacturing the steel plate using a converter, it is preferable to contain 0.05% of Si and 0.01% or more of Al. However, Si and Al are alloying elements that may coarsely form Si and Al composite oxides or coarsely form martensite islands in the microstructure of the welded HAZ, and thus, it is preferable to contain 0.2% or less of Si and 0.04% or less of Al.

[0033] Manganese (Mn): 1.5% to 1.7%

[0034] Since Mn is a useful element that increases strength through solid solution strengthening and improves hardenability to form low-temperature transformation phases, in order to ensure a yield strength of 500 MPa or higher, it is preferably contained in an amount of 1.5% or more. However, if the Mn exceeds 1.7%, due to the excessive increase in hardenability, it may promote the formation of upper bainite and martensite in the welded HAZ and the matrix material structure, thereby significantly reducing toughness. Therefore, the Mn content is preferably 1.5% to 1.7%.

[0035] Nickel (Ni): 0.6% to 0.9%

[0036] Ni is an important element that improves impact toughness by promoting cross-slip of dislocations at low temperatures and improves hardenability to increase strength. In order to improve the impact toughness of high-strength steel with a yield strength of 500 MPa or greater and the impact toughness of the bainite structure in the welded HAZ, Ni is preferably contained in an amount of 0.6% or more. However, if the Ni content exceeds 0.9%, the hardenability may increase excessively and low-temperature transformation phases may be formed, which instead reduces toughness and also increases the manufacturing cost. Therefore, the Ni is preferably not more than 0.9%.

[0037] Niobium (Nb): 0.005% to 0.02%

[0038] Nb precipitates in the form of NbC or NbCN to increase the strength of the matrix material. In addition, Nb dissolved during reheating at high temperatures precipitates very finely in the form of NbC during rolling, which has the effect of suppressing the recrystallization of austenite and refining the structure. Therefore, it is preferred that Nb is contained in an amount of 0.005% or more. However, excessive addition of Nb may cause brittle cracks at the edges of the steel plate, and due to the formation of a large number of martensite islands (MA) in the welded HAZ, there may be a problem of reduced toughness. Therefore, it is preferred that Nb does not exceed 0.02%.

[0039] Titanium (Ti): 0.01% to 0.02%

[0040] Ti precipitates as TiN during reheating, which inhibits the growth of grains in the matrix material and the welded HAZ, thereby significantly improving low-temperature toughness. And preferably, in order for the effective precipitation of TiN, Ti is contained in an amount of 0.01% or more. However, if the Ti content exceeds 0.02%, there may be problems such as reduced low-temperature toughness due to clogging of the casting nozzle or central crystallization and reduced toughness of the welded HAZ due to coarse TiN precipitation (due to a reduced Ti / N ratio). Therefore, preferably, Ti does not exceed 0.02%.

[0041] Nitrogen (N): 0.0035% to 0.0070% (35 ppm to 70 ppm)

[0042] N combines with Ti to precipitate TiN, thereby preventing the growth of prior austenite grains and exhibiting the effect of refining the grain size. To form fine TiN precipitates, N preferably contains at least 35 ppm. However, excessive addition of N may lead to a decrease in toughness due to the appearance of free nitrogen (free N) and slab cracks caused by AlN precipitation, and thus, N is preferably 70 ppm or less. More preferably, N is 45 ppm to 60 ppm.

[0043] Molybdenum (Mo): 0.1% to 0.2%

[0044] Mo is an element that increases hardenability and improves strength. In the present disclosure, to ensure the required strength, it is preferred to contain 0.1% or more of Mo. However, excessive inclusion of Mo may lead to a decrease in toughness due to excessive increase in strength, and thus, Mo is preferably not more than 0.2%.

[0045] Chromium (Cr): 0.1% to 0.3%

[0046] Cr is an element that improves strength through solid solution strengthening, and to ensure the strength required in the present disclosure, it is preferred to contain 0.1% or more of Cr. However, excessive addition of Cr may lead to excessive increase in strength or reduction in toughness due to carbide precipitation, and thus, the Cr content is preferably 0.3% or less.

[0047] Phosphorus (P): 0.008% (80 ppm) or less and Sulfur (S): 0.002% (20 ppm) or less

[0048] P and S are elements that cause intergranular brittleness or embrittlement by forming coarse inclusions, and to improve the resistance to brittle crack propagation, preferably, P is controlled to 80 ppm or less and S is controlled to 20 ppm or less.

[0049] The balance contains iron (Fe), and since it may be inevitable to mix in unexpected impurities from raw materials or the surrounding environment during the normal manufacturing process, it is impossible to exclude such impurities. Since these impurities may be known to any person skilled in the art during the manufacturing process, all of their details are not specifically mentioned in this specification.

[0050] Preferably, the steel plate of the present disclosure has a matrix material yield strength of 500 MPa or higher and an impact transition temperature of -40 °C or lower.

[0051] Meanwhile, preferably, the fraction of martensite-austenite (MA) in the fusion line (FL) to FL+1 mm region in the weld heat affected zone (HAZ) of the steel of the present disclosure welded with a medium heat input (about 150 KJ / cm to 200 KJ / cm) is 4% or less in terms of area fraction, and the impact toughness measured at -20 °C in the FL to FL+1 mm region is 33 J or more. By minimizing the MA fraction in the FL to FL+1 mm region of the weld HAZ, the low-temperature toughness of the medium heat input weld zone can be ensured. During medium heat welding, the microstructure of the weld HAZ is not particularly limited, but since a large amount of alloying components are added to the steel plate of the present disclosure to ensure strength, a microstructure that is not conducive to toughness is formed. Therefore, it is important to ensure toughness by minimizing the MA phase. The microstructure of the weld HAZ may include, for example, a mixed phase of granular bainite and upper bainite.

[0052] Meanwhile, the microstructure of the base material of the steel plate of the present disclosure is not particularly limited, but as an example, the microstructure of the base material may include a mixed phase of acicular ferrite, granular bainite, and upper bainite.

[0053] Hereinafter, an aspect of the method for manufacturing the steel plate of the present disclosure will be described in detail.

[0054] The steel plate of the present disclosure can be manufactured by reheating a steel slab that satisfies the above composition, performing rough rolling and finish rolling, and then cooling. Each process will be described in detail below.

[0055] Slab reheating: 1050 °C to 1150 °C

[0056] Preferably, the steel slab that satisfies the above composition is reheated in the temperature range of 1050 °C to 1150 °C. The reheating temperature can be set to 1050 °C or higher so that the carbonitrides of Ti and / or Nb formed during casting can dissolve. In addition, in order to fully dissolve the carbonitrides of Ti and / or Nb, it is more preferable to heat to 1080 °C or higher. However, since there is a concern that austenite may coarsen when reheated at too high a temperature, the reheating temperature is preferably 1150 °C or lower.

[0057] Rough rolling: 900 °C or higher

[0058] The reheated steel slab is subjected to rough rolling to adjust its shape. Preferably, the rough rolling temperature is higher than or equal to the temperature Tnr at which austenite recrystallization stops. Therefore, it is preferable to perform rough rolling at a temperature of 900 °C or higher. The effect of reducing the grain size can also be obtained through the recrystallization of coarse austenite and the destruction of the casting structure such as dendrites formed during casting by rolling. In order to fully cause recrystallization and refine the structure, it is preferable that the total cumulative reduction ratio of rough rolling is 40% or more.

[0059] Finish rolling: 800 °C or higher

[0060] To introduce the austenite structure of the above rough-rolled steel plate into an inhomogeneous microstructure, finish rolling is performed. To apply the maximum deformation to the structure, finish rolling is preferably performed at a temperature of 800 °C or higher. To generate the finest structure, the cumulative reduction ratio of finish rolling is preferably 50% or greater. If the finish rolling temperature is lower than 800 °C, ferrite may precipitate during air cooling after rolling and before water cooling, which may reduce the strength. Therefore, finish rolling is preferably performed at 800 °C or higher.

[0061] Cooling after rolling: Cool to a temperature of 700 °C or lower at a cooling rate of 15 °C / second or greater at the t / 4 point (t: thickness of the steel plate)

[0062] If the cooling rate is less than 15 °C / second or the cooling termination temperature exceeds 700 °C, the microstructure may not be properly formed, making it difficult to ensure a yield strength of 500 MPa or higher. There is no specific limit on the upper limit of the cooling rate in the present disclosure. However, since in the field to which the present disclosure pertains, the cooling rate can be 100 °C / second or greater, as a preferred example, the cooling rate is 200 °C / second or less.

[0063] Embodiments of the invention

[0064] Hereinafter, embodiments of the present disclosure will be described. It should be understood that those skilled in the art can modify the following embodiments in various ways without departing from the scope of the present disclosure. The following embodiments are intended to assist in understanding the present disclosure, and the scope of the present disclosure should not be limited to the following embodiments, but should be determined by the appended claims and their equivalents.

[0065] (Example)

[0066] A steel billet with a thickness of 300 mm and having the composition shown in Table 1 below (and the remaining Fe and inevitable impurities) is reheated to a temperature of 1110 °C, then continuously rough-rolled at 980 °C, and then finish-rolled at 860 °C. Thereafter, the resulting structure is cooled to 620 °C to 560 °C at a cooling rate of 25 °C / second to 37 °C / second to manufacture a steel plate. However, in Table 2 below, in Comparative Example 5, a steel billet having the composition of Invention Steel 2 was used, reheated and rough-rolled under the same conditions as above, but finish-rolled at 730 °C, and then the resulting structure was cooled to 610 °C at a cooling rate of 7 °C / second to manufacture a steel plate.

[0067] For the steel plates manufactured as above, the yield strength and the impact transition temperature were measured, and the results are shown in Table 2. In addition, the steel plates manufactured were welded with a heat input of 150 KJ / cm to 200 KJ / cm, and the impact toughness and the microstructure of the fusion line (FL) to FL + 1 mm of the welded HAZ were analyzed, and the results are shown in Table 2. The MA fraction was measured optically by the LePera etching method.

[0068] [Table 1]

[0069]

[0070] [Table 2]

[0071]

[0072] It can be seen that all the inventive examples satisfying the conditions of the present disclosure have a matrix material yield strength of 500 MPa or greater, an impact transition temperature of -40°C or lower, an MA fraction in the welded HAZ from the fusion line (FL) to FL + 1 mm welded with a heat input of 150 KJ / cm to 200 KJ / cm of 4% or less, and an impact toughness at -20°C measured in the region from the fusion line to FL + 1 mm of 33 J or greater.

[0073] In contrast, it can be seen that Comparative Example 1 contains more C than that proposed in the present disclosure, and due to the formation of a large amount of martensite (MA) phase in the welded HAZ, the impact toughness measured in the region from FL to FL + 1 mm at -20°C is less than 33 J.

[0074] Comparative Example 2 contains less Ni than that proposed in the present disclosure, and it can be seen that due to insufficient Ni addition, even if the MA fraction is 4% or less, the impact toughness measured at FL at -20°C is less than 33 J, resulting in a decrease in toughness.

[0075] Comparative Example 3 contains a large amount of Si and Nb proposed in the present disclosure, and it can be seen that due to the formation of a large amount of MA phase in the welded HAZ, the impact toughness measured in the region from FL to FL + 1 mm at -20°C is less than 33 J.

[0076] It can be seen that Comparative Example 4 contains a larger amount of Ti and a smaller amount of N than that proposed in the present disclosure, and therefore, TiN precipitates coarsely in the welding zone, resulting in an increase in the particle size. Thus, even if the MA fraction is 4% or less, the impact toughness measured in the region from FL to FL + 1 mm at -20°C is less than 33 J.

[0077] Meanwhile, Comparative Example 5 is a case where the components satisfy the suggestions of the present disclosure but do not satisfy the manufacturing process. It can be seen that the MA fraction of the welded HAZ after welding is 4% or less and the impact toughness measured in the FL to FL + 1 mm region at -20 °C is 33 J or more, but some ferrite is formed during rolling and air cooling, and due to the slow cooling rate, the low-temperature transformation phase is not properly formed, and thus, the yield strength of the base material is manufactured to be 500 MPa or less.

Claims

1. A steel plate having excellent heat affected zone (HAZ) toughness, the steel plate comprising, by weight %, C: 0.03% to 0.06%, Mn: 1.5% to 1.7%, Si: 0.05% to 0.2%, Al: 0.01% to 0.04%, Ni: 0.6% to 0.9%, Mo: 0.1% to 0.2%, Cr: 0.1% to 0.3%, Ti: 0.01% to 0.02%, Nb: 0.005% to 0.02%, N: 0.0035% to 0.0070%, P: 0.008% or less, S: 0.002% or less, and the remainder of Fe and unavoidable impurities, in, In the weld HAZ welded at a heat input of 150 KJ / cm to 200 KJ / cm, the MA fraction in the region from the fusion line (FL) to FL+1 mm is 4% or less in terms of area fraction. 2 . The steel sheet according to claim 1 , wherein the impact toughness at −20° C. in the FL to FL+1 mm region is 33 J or more. 3 . The steel plate according to claim 1 , wherein a microstructure of the steel plate comprises a mixed phase of acicular ferrite, granular bainite, and upper bainite. 4 . The steel plate according to claim 1 , wherein the microstructure of the weld HAZ comprises a mixed phase of granular bainite and upper bainite. 5 . The steel sheet according to claim 1 , wherein a yield strength of a base material of the steel sheet is 500 MPa or more and an impact transition temperature of the base material of the steel sheet is −40° C. or less.

6. A method for manufacturing a steel plate having excellent heat affected zone (HAZ) toughness, the method include: heating a steel slab to 1050° C. to 1150° C., the steel slab comprising, in weight %, C: 0.03% to 0.06%, Mn: 1.5% to 1.7%, Si: 0.05% to 0.2%, Al: 0.01% to 0.04%, Ni: 0.6% to 0.9%, Mo: 0.1% to 0.2%, Cr: 0.1% to 0.3%, Ti: 0.01% to 0.02%, Nb: 0.005% to 0.02%, N: 0.0035% to 0.0070%, P: 0.008% or less, S: 0.002% or less, and the remainder of Fe and inevitable impurities; Rough rolling the heated steel billet at a temperature of 900° C. or higher; After the rough rolling, the rough-rolled steel billet is finish hot-rolled at a temperature of 800° C. or higher to produce a hot-rolled steel plate; as well as The hot-rolled steel sheet was cooled to 700° C. or less at a cooling rate of 15° C. / sec at a point t / 4 of the thickness (t).

7. The method according to claim 6, wherein the total reduction is 40% or more.

8. The method according to claim 6, wherein the cumulative reduction ratio of the finish rolling is 50% or more.

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