Steel material with excellent toughness in the heat-affected zone during welding and method for manufacturing the same

Abstract

The present invention aims to provide a steel material that can ensure excellent toughness in the heat-affected zone of the weld even when a steel material with high strength and high toughness is welded with a constant heat input, and a method for manufacturing the same. [Solution] The present invention In weight percent, it contains C: 0.03~0.06%, Mn: 1.5~1.7%, Si: 0.05~0.2%, Al: 0.01~0.04%, Ni: 0.6~0.9%, Mo: 0.1~0.2%, Cr: 0.1~0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less. The method is characterized by including the steps of: heating a steel slab, the remainder of which consists of Fe and unavoidable impurities, to 1050-1150°C; roughly rolling the heated steel slab at a temperature of 900°C or higher; finishing rolling the heated steel slab at a temperature of 800°C or higher after the rough rolling to produce a hot-rolled steel sheet; and cooling the temperature at point t / 4 of the thickness t of the hot-rolled steel sheet to 700°C or lower at a cooling rate of 15°C / s.

Description

Technical Field

[0001] The present invention relates to a steel material having excellent toughness in the heat-affected zone of welding and a method for manufacturing the same. More specifically, it relates to a steel material used for ships and the like, and relates to a steel material having excellent toughness in the heat-affected zone (HAZ) even when a steel material having high strength and high toughness is welded with a certain heat input and a method for manufacturing the same.

Background Art

[0002] Recently, as the sea ice area in the Arctic Ocean rapidly decreases due to the temperature rise caused by global warming, the interest in opening the Arctic Sea Route has been increasing. Ships for developing such an Arctic Sea Route or ships for operating on the Arctic Sea Route need to be built as icebreakers that can crush icebergs in case of emergency.

[0003] The above-mentioned icebreaker refers to a ship that breaks the ice on the water surface to open a route for navigation. Most of the icebreakers until now have been military or exploration ships, but recently, as the interest in the Arctic Sea Route has increased, the scope of use has been expanding to general merchant ships and cruise ships. As an example, Russia is the most active country in building icebreakers due to its regional characteristics. As of 2020, more than 40 icebreakers such as the Ermak, the Arctica, and the Sibir are operating globally. It is expected that the construction of icebreakers will further increase in the future.

[0004] The steel material used for the hull of the above-mentioned icebreaker needs to have excellent impact toughness even at extremely low temperatures in order to withstand the low temperature of the Arctic Sea Route. At the same time, high strength is required to protect the hull. That is, it is necessary to ensure high strength and high toughness at the same time, and for this purpose, a large amount of alloy elements are added.

[0005] Shipbuilders, when constructing icebreakers, find it advantageous to increase the heat input during welding to improve productivity. Therefore, they require steel that does not experience a decrease in toughness in the heat-affected zone even when the heat input is increased during welding. However, as mentioned above, the steel used in icebreakers has a large amount of alloying elements added to ensure strength, which leads to a problem where the toughness of the heat-affected zone after welding is significantly reduced.

[0006] Generally, to ensure the toughness of the heat-affected zone (HIR) produced with high heat input, a method is used (Patent Document 1) that increases the nitrogen content and generates fine TiN precipitates to refine the particle size of the HIR. However, in this case, the high nitrogen content is prone to causing a decrease in the impact toughness of the base material due to free nitrogen (Free N). To prevent this, a large amount of boron (B) can be added to form BN, which can prevent a decrease in toughness. However, if the amount of addition is not carefully controlled, the generation of free boron (Free B) can lead to an additional decrease in toughness. Furthermore, when a large amount of nitrogen is added, there is a problem of inducing microcracks on the slab surface during the continuous casting process for slab production, so the method of utilizing high nitrogen content is not considered an effective method.

[0007] On the other hand, there are attempts to ensure toughness by utilizing fine oxides to refine the particle size of the heat-affected zone during welding. However, it is extremely difficult to actually disperse the oxides that are pre-formed at high temperatures into a fine, uniform state within the steel material. Therefore, there is uncertainty as to whether it is actually possible to selectively refine only the necessary oxides throughout the steel material, and whether this has any effect on improving toughness.

[0008] Therefore, there is a need for steel manufacturing technology that can ensure the strength and toughness of the base material while simultaneously ensuring excellent toughness in the heat-affected zone during welding. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2005-200716 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The present invention aims to provide a steel material that can ensure excellent toughness in the heat-affected zone of the weld even when a steel material with high strength and high toughness is welded with a constant heat input, and a method for manufacturing the same.

[0011] The problems that the present invention will address are not limited to those described above. Further problems that the present invention will address are described throughout the specification, and any person with ordinary skill in the art to which the present invention belongs will have no difficulty understanding these further problems from the contents of the specification. [Means for solving the problem]

[0012] The steel material exhibiting excellent toughness in the heat-affected zone of the weld according to the present invention contains, by weight percent, C: 0.03~0.06%, Mn: 1.5~1.7%, Si: 0.05~0.2%, Al: 0.01~0.04%, Ni: 0.6~0.9%, Mo: 0.1~0.2%, Cr: 0.1~0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less, with the remainder being Fe and unavoidable impurities. It is characterized in that, in the heat-affected zone (HAZ) of a weld welded at a heat input of 150~200 KJ / cm, the MA fraction in the region from the fusion line (FL) to FL+1 mm is 4% or less in area fraction.

[0013] The present invention provides a method for producing steel with excellent heat-affected zone toughness, comprising the steps of: heating a steel slab containing, by weight %, C: 0.03~0.06%, Mn: 1.5~1.7%, Si: 0.05~0.2%, Al: 0.01~0.04%, Ni: 0.6~0.9%, Mo: 0.1~0.2%, Cr: 0.1~0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less, with the remainder being Fe and unavoidable impurities, to 1050~1150°C; roughly rolling the heated steel slab at a temperature of 900°C or higher; and, after the rough rolling, finishing rolling at a temperature of 800°C or higher to produce a hot-rolled steel sheet; and This method is characterized by including a step of cooling the temperature at point t / 4 of the thickness t of the hot-rolled steel sheet to 700°C or less at a cooling rate of 15°C / s. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a steel material and a method for manufacturing the same that not only has excellent strength and toughness in the base material but also ensures excellent toughness in the heat-affected zone of the weld. Such a steel material can be applied in various fields, such as icebreakers and structures in cryogenic environments.

[0015] The diverse yet significant advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of describing specific embodiments of the present invention. [Modes for carrying out the invention]

[0016] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Furthermore, singular expressions used herein also include plural expressions unless the related definition clearly indicates otherwise.

[0017] In specifications, the term "includes" specifies a particular component and does not exclude the existence or addition of other components.

[0018] Unless otherwise specified, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries are to be interpreted as having the meaning corresponding to the relevant technical documents and the content currently disclosed.

[0019] The inventors of this invention have conducted extensive research on techniques to ensure the toughness, particularly low-temperature toughness, of the weld heat-affected zone formed by welding a steel material with high strength and toughness using intermediate heat input welding, for example, with a heat input of about 150-200 kJ / cm. As a result, they recognized that by minimizing the island-like martensite (MA) phase in the microstructure of the base material and optimizing the nitrogen (N) content to finely precipitate TiN precipitates, it is possible to control the MA phase near the fusion line (FL) in the weld heat-affected zone, even when applying intermediate heat input welding, thereby preventing cracks and toughness degradation, and thus arrived at the present invention.

[0020] First, the steel material of the present invention will be described in detail.

[0021] The above steel materials contain, by weight percent, C: 0.03-0.06%, Mn: 1.5-1.7%, Si: 0.05-0.2%, Al: 0.01-0.04%, Ni: 0.6-0.9%, Mo: 0.1-0.2%, Cr: 0.1-0.3%, Ti: 0.01-0.02%, Nb: 0.005-0.02%, N: 0.0035-0.0070%, P: 0.008% or less, S: 0.002% or less, with the remainder being Fe and unavoidable impurities. The composition of each alloy is described below.

[0022] Carbon (C): 0.03~0.06% (The content of carbon in each alloy composition below is in weight percent) Since C is the most important element for ensuring basic strength, it needs to be contained in steel within an appropriate range. When the content of C exceeds 0.06%, the hardening ability is improved, a large amount of island-shaped martensite (MA) is generated, and the toughness of the weld heat-affected zone decreases. When it is less than 0.03%, it causes a decrease in strength. Therefore, the content of C is preferably 0.03 - 0.06%.

[0023] Silicon (Si): 0.05 - 0.2% and Aluminum (Al): 0.01 - 0.04% Si and Al are essential alloy elements for precipitating dissolved oxygen in molten steel in the form of slag during the steelmaking and continuous casting processes to perform deoxidation work. When manufacturing steel using a converter, it is preferably contained with Si of 0.05% and Al of 0.01% or more. However, when contained in a large amount, it is an alloy element that can coarsely generate Si, Al composite oxides or coarsely generate a large amount of island-shaped martensite in the fine structure of the weld heat-affected zone. Therefore, it is preferably contained with Si of 0.2% or less and Al of 0.04% or less.

[0024] Manganese (Mn): 1.5 - 1.7% Mn is a useful element that improves strength by solid solution strengthening and improves the hardening ability so that a low-temperature transformation phase is generated. Therefore, in order to ensure a yield strength of 500 MPa or more, it is preferably contained with Mn of 1.5% or more. However, when it exceeds 1.7%, due to an excessive increase in hardening ability, it can promote the generation of upper bainite and martensite in the weld heat-affected zone and the base metal structure, greatly decreasing the toughness. Therefore, the content of Mn is preferably 1.5 - 1.7%.

[0025] Nickel (Ni): 0.6 - 0.9% The above-mentioned Ni is an important element for improving impact toughness by facilitating cross-slip of dislocations at low temperatures, thereby improving hardening ability and increasing strength. To improve impact toughness in high-strength steel with a yield strength of 500 MPa or more, and in the bainite structure of the weld heat-affected zone, it is preferable that Ni be included at a concentration of 0.6% or more. However, if it exceeds 0.9%, the hardening ability increases excessively, generating a low-temperature transformation phase, which can actually decrease toughness and increase manufacturing costs. Therefore, it is preferable that it not exceed 0.9%.

[0026] Niobium (Nb): 0.005~0.02% The above-mentioned Nb precipitates in the form of NbC or NbCN, improving the strength of the base material. Furthermore, Nb dissolved during reheating at high temperatures precipitates very finely in the form of NbC during rolling, suppressing austenite recrystallization and refining the microstructure. Therefore, it is preferable that the above-mentioned Nb is present in an amount of 0.005% or more, but if added excessively, it may cause brittle cracks at the corners of the steel material, and problems such as a decrease in toughness due to the formation of a large amount of island-like martensite (MA) in the heat-affected zone of the weld are possible, so it is preferable that it does not exceed 0.02%.

[0027] Titanium (Ti): 0.01-0.02% The above-mentioned Ti precipitates as TiN during reheating, suppressing grain growth in the base material and the heat-affected zone of the weld, and significantly improving low-temperature toughness. Therefore, it is preferable to include 0.01% or more for effective TiN precipitation. However, if it exceeds 0.02%, problems such as clogging of the continuous casting nozzle and a decrease in low-temperature toughness due to crystallization in the center may occur, as well as a decrease in the toughness of the heat-affected zone of the weld as the Ti / N ratio decreases and the TiN precipitates become coarser. Therefore, it is preferable not to exceed 0.02%.

[0028] Nitrogen (N): 0.0035~0.0070% (35~70 ppm) The above-mentioned N combines with Ti to precipitate TiN, preventing the growth of prior austenite grains and refining the grain size. It is preferable that the N content be 35 ppm or higher to form fine TiN precipitates. However, if present in excessive amounts, it may cause a decrease in toughness due to free nitrogen (Free N) generation and slab cracking due to AlN precipitation; therefore, it is preferable that the N content be 70 ppm or lower. A N content of 45-60 ppm is more preferable.

[0029] Molybdenum (Mo): 0.1-0.2% The above-mentioned Mo is an element that increases hardening ability and improves strength. In this invention, it is preferable that the above-mentioned Mo is contained in an amount of 0.1% or more in order to ensure the required strength. However, if it is included in an excessive amount, the toughness may decrease due to an excessive increase in strength, so it is preferable that it does not exceed 0.2%.

[0030] Chromium (Cr): 0.1-0.3% The above-mentioned Cr is an element that improves strength through solid solution strengthening, and it is preferable that it be included in an amount of 0.1% or more in order to ensure the strength required in the present invention. However, since excessive addition may lead to an over-increase in strength or a decrease in toughness due to carbide precipitation, it is preferable that its content be 0.3% or less.

[0031] Phosphorus (P): 0.008% (80 ppm) or less and sulfur (S): 0.002% (20 ppm) or less The above-mentioned P and S are elements that induce grain boundary brittleness or form coarse inclusions, thereby inducing brittleness. To improve resistance to brittle crack propagation, it is preferable to control the levels of P to 80 ppm or less and S to 20 ppm or less.

[0032] The remainder consists of iron (Fe), and since unintended impurities from the raw materials or surrounding environment are inevitably introduced during the normal manufacturing process, it is impossible to eliminate them. These impurities are easily recognizable to any competent technician during the manufacturing process, and therefore, their full nature is not specifically mentioned in this specification.

[0033] The steel material of the present invention preferably has a base material yield strength of 500 MPa or higher and an impact transition temperature of -40°C or lower.

[0034] On the other hand, it is preferable that in the heat-affected zone of the weld obtained by welding the steel material of the present invention with a moderate heat input (approximately 150-200 kJ / cm), the MA fraction in the region from the fusion line (FL) to FL+1 mm is 4% or less in area fraction, and the impact toughness at -20°C measured 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 heat-affected zone, low-temperature toughness in the moderate heat input weld can be ensured. The microstructure of the heat-affected zone during moderate heat input welding is not particularly limited, but in the steel material of the present invention, a large amount of alloying components are added to ensure strength, resulting in a microstructure that is not good for toughness. Therefore, it is important to minimize the MA phase to ensure toughness. As an example, the microstructure of the heat-affected zone may include a mixed phase of granular bainite and upper bainite.

[0035] On the other hand, the microstructure of the base material of the steel material of the present invention 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.

[0036] Next, the steel manufacturing method of the present invention will be described in detail.

[0037] The steel material of the present invention can be manufactured by reheating a steel slab satisfying the above-described composition, followed by rough rolling and finish rolling, and then cooling. Each process will be described in detail below.

[0038] Slab reheating: 1050~1150℃ It is preferable to reheat the steel slab satisfying the above-mentioned composition to a temperature range of 1050 to 1150°C. By reheating to a temperature of 1050°C or higher, the carbonitrides of Ti and / or Nb formed during casting can be dissolved. Furthermore, to sufficiently dissolve the carbonitrides of Ti and / or Nb, it is more preferable to heat to 1080°C or higher. However, if the reheating temperature is excessively high, the austenite may coarseen, so it is preferable that the reheating temperature be 1150°C or lower.

[0039] Rough rolling: 900℃ or higher The reheated steel slab is subjected to rough rolling to adjust its shape. The rough rolling temperature is preferably above the temperature at which austenite recrystallization stops (Tnr), and therefore it is preferable to perform the rough rolling at a temperature of 900°C or higher. Rolling destroys the cast structure, such as dendrites formed during casting, and also reduces the grain size through the recrystallization of coarse austenite. To ensure sufficient recrystallization and refine the structure, the total cumulative reduction ratio of the rough rolling is preferably 40% or more.

[0040] Finishing rolling: 800℃ or higher Finish rolling is performed to introduce the austenitic structure of the roughly rolled steel sheet into a non-uniform fine structure. To impart maximum deformation to the structure, it is preferable to perform the finish rolling at a temperature of 800°C or higher. To generate the finest structure, it is preferable that the cumulative reduction ratio of the finish rolling be 50% or higher. If the finish rolling temperature is below 800°C, ferrite will precipitate during air cooling before water cooling after the end of rolling, reducing the strength; therefore, it is preferable to perform the finishing rolling at 800°C or higher.

[0041] Cooling after rolling: The temperature at point t / 4 is cooled to a temperature of 700°C or lower at a cooling rate of 15°C / s or higher (t: thickness of the steel plate). If the cooling rate is less than 15°C / s or the cooling completion temperature exceeds 700°C, the microstructure will not be properly formed, making it difficult to secure a yield strength of 500 MPa or more. The upper limit of the cooling rate is not particularly limited in this invention, but since a cooling rate of 100°C / s or more is possible in the art to which this invention belongs, it is preferable, as a preferred example, that the cooling rate is 200°C / s or less. [Examples]

[0042] The following describes embodiments of the present invention. It goes without saying that the following embodiments can be modified in various ways without departing from the scope of the invention, by anyone with ordinary skill in the art to which the present invention pertains. The following embodiments are for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be defined not only by the claims described below, but also by equivalent claims.

[0043] (Examples) A 300 mm thick steel slab having the composition shown in Table 1 below (the remainder being Fe and unavoidable impurities) was reheated to 1110°C, then rough-rolled continuously at 980°C, and finished-rolled at 860°C. After this, the steel material was manufactured by cooling at a cooling rate of 25-37°C / s to 620-560°C. However, in Table 2 below, Comparative Example 5 used a steel slab having the composition of Inventive Steel 2, and under the same conditions as above, reheating and rough-rolling were performed, but the finish-rolling was carried out at 730°C, and then the steel material was manufactured by cooling at a cooling rate of 7°C / s to 610°C.

[0044] The yield strength and impact transition temperature of the steel materials manufactured as described above were measured, and the results are shown in Table 2. Furthermore, welding was performed on the manufactured steel materials with a heat input between 150 and 200 kJ / cm, and the impact toughness and microstructure of the heat-affected zone (HAZ) from the fusion line (FL) to FL+1 mm were analyzed, and the results are shown in Table 2. The above MA fractions were measured from fractions revealed by the LePera etching method via optics.

[0045] [Table 1]

[0046] [Table 2]

[0047] In all of the examples of inventions that satisfy the conditions of the present invention, the yield strength of the base material is 500 MPa or more, the impact transition temperature is -40°C or less, the MA fraction in the Fusion Line (FL) to FL+1 mm region of the weld heat-affected zone welded with a heat input of 150 to 200 kJ / cm is 4% or less, and the impact toughness at -20°C measured in the Fusion Line to FL+1 mm region is 33 J or more.

[0048] In contrast, Comparative Example 1 contains more C than the present invention, and as a large amount of island-like martensite (MA) phase is generated in the heat-affected zone of the weld, the impact toughness measured in the FL to FL+1 mm region becomes less than 33 J at -20°C.

[0049] Comparative Example 2 contains less Ni than presented in the present invention, and it can be seen that the insufficient amount of Ni causes a decrease in toughness, resulting in an impact toughness of less than 33 J at -20°C, even though the MA fraction is 4% or less.

[0050] Comparative Example 3 contains a high amount of Si and Nb as presented in the present invention, and it can be seen that as a large amount of MA phase is generated in the heat-affected zone during welding, the impact toughness measured in the FL to FL+1 mm region becomes less than 33 J at -20°C.

[0051] Comparative Example 4 contains more Ti than presented in the present invention, has a low N content, and as TiN precipitates coarsely in the weld, the particle size increases. As a result, even though the MA fraction is 4% or less, the impact toughness measured in the FL~FL+1mm region is less than 33J at -20°C.

[0052] On the other hand, Comparative Example 5 satisfies the components presented in the present invention but does not satisfy the manufacturing process. In this case, the MA fraction of the heat-affected zone after welding is 4% or less, and the impact toughness measured in the FL to FL+1 mm region is 33 J or more at -20°C. However, it was confirmed that some ferrite was formed during finish rolling and air cooling, and the low-temperature transformation phase was not adequately generated due to the slow cooling rate, resulting in the base material's yield strength being 500 MPa or less.

Claims

1. In weight percent, it contains C: 0.03-0.06%, Mn: 1.5-1.7%, Si: 0.05-0.2%, Al: 0.01-0.04%, Ni: 0.6-0.9%, Mo: 0.1-0.2%, Cr: 0.1-0.3%, Ti: 0.01-0.02%, Nb: 0.005-0.02%, N: 0.0035-0.0070%, P: 0.008% or less, S: 0.002% or less, with the remainder being Fe and unavoidable impurities. A steel material exhibiting excellent toughness in the heat-affected zone (HAZ) of a weld, characterized in that the MA fraction in the region from the fusion line (FL) to FL+1 mm is 4% or less in terms of area fraction, when welded with a heat input of 150 to 200 kJ / cm.

2. The steel material with excellent weld heat-affected zone toughness according to claim 1, characterized in that the impact toughness at -20°C in the FL to FL+1 mm region is 33 J or more.

3. The steel material having excellent toughness in the weld heat-affected zone, as described in claim 1, is characterized in that the microstructure of the steel material includes a mixed phase of acicular ferrite, granular bainite, and upper bainite.

4. The steel material with excellent toughness in the weld heat-affected zone according to claim 1, characterized in that the microstructure of the weld heat-affected zone includes a mixed phase of granular bainite and upper bainite.

5. The steel material having excellent toughness in the weld heat-affected zone, as described in claim 1, characterized in that the base material yield strength of the steel material is 500 MPa or more and the impact transition temperature is -40°C or lower.

6. A steel slab containing, by weight percent, C: 0.03-0.06%, Mn: 1.5-1.7%, Si: 0.05-0.2%, Al: 0.01-0.04%, Ni: 0.6-0.9%, Mo: 0.1-0.2%, Cr: 0.1-0.3%, Ti: 0.01-0.02%, Nb: 0.005-0.02%, N: 0.0035-0.0070%, P: 0.008% or less, S: 0.002% or less, with the remainder being Fe and unavoidable impurities, is heated to 1050-1150°C. The step of roughly rolling the heated steel slab at a temperature of 900°C or higher, The steps include: producing a hot-rolled steel sheet by performing finish rolling at a temperature of 800°C or higher after the rough rolling, and A method for manufacturing steel material with excellent heat-affected zone toughness of the weld, characterized by including a step of cooling the temperature at point t / 4 of the thickness t of the hot-rolled steel sheet to 700°C or less at a cooling rate of 15°C / s.

7. The method for producing steel with excellent heat-affected zone toughness in the weld, as described in claim 6, characterized in that the total reduction ratio of the rough rolling is 40% or more.

8. The method for producing steel with excellent toughness in the weld heat-affected zone, as described in claim 6, characterized in that the cumulative reduction ratio of the finish rolling is 50% or more.

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