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

Abstract

The present invention provides a steel material capable of ensuring excellent toughness in a weld heat-affected zone even when the steel material has high strength and high toughness and is welded with a heat input of a certain level or more, and a method for manufacturing the same. [Solution] The present invention relates to a steel material used for ships and the like, which contains, by weight, 0.04 to 0.07% carbon (C), 1.5 to 1.7% manganese (Mn), 0.1 to 0.3% silicon (Si), 0.01 to 0.04% aluminum (Al), 0.7 to 1.0% nickel (Ni), 0.05 to 0.30% molybdenum (Mo), 0.010 to 0.018% titanium (Ti), 0.01 to 0.03% niobium (Nb), 0.003 to 0.006% nitrogen (N), 0.007% or less phosphorus (P), and 0.002% or less sulfur (S), with the remainder being Fe and unavoidable impurities, The heat affected zone (HAZ) of welds welded with a heat input of 100 to 200 KJ / cm is characterized by containing bainite phase with an area fraction of 90% or more in the region from the fusion line (FL) to FL+3 mm.

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, the present invention 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) formed by welding and a method for manufacturing the same.

Background Art

[0002] Recently, as the sea ice area in the Arctic rapidly decreases due to the rising temperature caused by global warming, the interest in opening the Arctic sea route has been increasing. It has been observed that the temperature in the Arctic region has risen by 3 - 4°C over the past 50 years, and a further temperature rise of 6 - 7°C is expected in the next 100 years. Due to such a temperature rise, the Arctic sea ice in summer has decreased by about 40% since 1980, and the thickness of the sea ice has also become thinner, further increasing the potential for the utilization of the Arctic sea route.

[0003] A ship for developing the Arctic sea route or a ship for operating the Arctic sea route needs to be built as an icebreaker that can crush icebergs in an emergency. An icebreaker refers to a ship that crushes the ice on the water surface to open a route for navigation.

[0004] Most of the icebreakers so far have been military or exploration ships. Recently, with the increasing interest in the Arctic sea route, the scope of their use has expanded to general merchant ships and cruise ships (tourist 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.

[0005] On the other hand, the steel material used for the hull of an icebreaker needs to have excellent impact toughness even at extremely low temperatures in order to withstand the low temperature of the Arctic sea route, and at the same time, high strength is required to protect the hull.

[0006] In shipbuilding, increasing the heat input during steel welding is advantageous for improving productivity when constructing icebreakers. However, increasing the heat input during welding leads to a decrease in the tensile strength and toughness of the heat-affected zone. Therefore, there is a demand for steel materials that do not experience a decrease in toughness in the heat-affected zone even when the heat input during welding is increased.

[0007] Generally, to ensure the toughness of the heat-affected zone of a weld manufactured with a high heat input, a method is used to refine the particle size of the heat-affected zone by increasing the nitrogen content to generate fine TiN precipitates (Patent Document 1). However, in this case, the impact toughness of the base material tends to decrease due to the free nitrogen (Free N) associated with the high nitrogen content, and as the hardening ability decreases due to the smaller particle size, the tensile strength of the heat-affected zone decreases, and if a large amount of low-temperature transformation phase is generated, a problem arises in which the toughness decreases.

[0008] Therefore, there is a need for steel manufacturing technology that can ensure excellent toughness in the heat-affected zone of the weld, even when welding is performed with increased heat input, while maintaining the strength and toughness of the base material. [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 objective of the present invention is to provide a steel material that can ensure excellent toughness in the heat-affected zone even when welding a steel material having high strength and high toughness with a heat input above a certain amount, 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 pertains will have no difficulty understanding these further problems from the contents of the specification. [Means for solving the problem]

[0012] This invention contains, by weight percent, carbon (C): 0.04-0.07%, manganese (Mn): 1.5-1.7%, silicon (Si): 0.1-0.3%, aluminum (Al): 0.01-0.04%, nickel (Ni): 0.7-1.0%, molybdenum (Mo): 0.05-0.30%, titanium (Ti): 0.010-0.018%, niobium (Nb): 0.01-0.03%, nitrogen (N): 0.003-0.006%, phosphorus (P): 0.007% or less, sulfur (S): 0.002% or less, with the remainder consisting of Fe and unavoidable impurities. The heat-affected zone (HAZ) of a weld welded with a heat input of 100-200 kJ / cm contains 90% or more of the bainite phase in the region from the fusion line (FL) to FL+3 mm, providing a steel material with excellent toughness in the heat-affected zone.

[0013] The present invention provides a method for manufacturing steel with excellent toughness in the heat-affected zone of a weld, comprising the steps of: reheating a steel slab having the above-described alloy composition to 1100-1180°C; roughly rolling the heated steel slab at a temperature of 900°C or higher; finishing the rough rolling at a temperature of 800°C or higher to produce a hot-rolled steel material; and cooling the hot-rolled steel material at a cooling rate of 10°C / s or higher until the temperature at point t / 4 of the thickness (t, in mm) is 600°C or lower. [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 strength and 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, the singular forms used herein also include plural forms unless the relevant definitions indicate a clearly opposite meaning.

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

[0018] Unless otherwise defined, 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 in accordance with the relevant technical literature and the content of the present disclosure.

[0019] The inventors of this invention have conducted extensive research on techniques to improve the toughness, particularly the low-temperature toughness, of the weld heat-affected zone formed when performing intermediate heat input welding of approximately 100-200 kJ / cm on steel materials with high strength and toughness.

[0020] As a result, we confirmed that the technical objectives of the present invention can be achieved by optimizing the alloy composition and manufacturing conditions to ensure that the microstructure near the fusion line (FL) of the weld heat-affected zone is in the hard phase, while controlling the grain size, thus completing the present invention.

[0021] The present invention will be described in detail below.

[0022] The steel material excellent in toughness of the weld heat affected zone according to the present invention can be composed of an alloy composition in weight %, carbon (C): 0.04 to 0.07%, manganese (Mn): 1.5 to 1.7%, silicon (Si): 0.1 to 0.3%, aluminum (Al): 0.01 to 0.04%, nickel (Ni): 0.7 to 1.0%, molybdenum (Mo): 0.05 to 0.30%, titanium (Ti): 0.010 to 0.018%, niobium (Nb): 0.01 to 0.03%, nitrogen (N): 0.003 to 0.006%, phosphorus (P): 0.007% or less, and sulfur (S): 0.002% or less.

[0023] Hereinafter, the reason for restricting the alloy composition of the steel material provided by the present invention as described above will be described in detail.

[0024] On the other hand, unless otherwise specified in the present invention, the content of each element is based on weight, and the ratio of the structure is based on area.

[0025] Carbon (C): 0.04 to 0.07% Carbon (C) is the most important element for ensuring the strength of not only the base metal but also the weld heat affected zone, and thus it needs to be contained in the steel within an appropriate range.

[0026] When the content of C exceeds 0.07%, the hardening ability is improved and the strength becomes excessively high, and there is a problem that the toughness of the weld heat affected zone decreases due to the precipitation of fine hard phases. On the other hand, when the content is less than 0.04%, it causes a decrease in strength, which is not preferable.

[0027] Therefore, the above C can be contained in an amount of 0.04 to 0.07%. More preferably, it can be contained in an amount of 0.045% or more and 0.065% or less. <​​​​​​​​In this invention, since the objective is to ensure a strength of 610 MPa or higher in the heat-affected zone during welding, it is preferable that the material contains 1.5% or more of Mn. However, if the content exceeds 1.7%, the hardening ability becomes excessively high, which may conversely lead to a significant decrease in the toughness of the heat-affected zone during welding.

[0030] Therefore, in the present invention, the above Mn can be contained in an amount of 1.5 to 1.7%.

[0031] Silicon (Si): 0.1-0.3% and Aluminum (Al): 0.01-0.04% Silicon (Si) and aluminum (Al) are essential elements for deoxidation during steelmaking and continuous casting processes, as they precipitate dissolved oxygen in molten steel in the form of slag. When manufacturing steel using an electric furnace, it is preferable that Si be at least 0.1% and Al be at least 0.01%. However, if the content of these elements is excessive, coarse Si-Al composite oxides may be formed, or a large amount of fine hard phase may be formed in the microstructure of the heat-affected zone of the weld. Therefore, it is preferable that Si be at least 0.3% and Al be at least 0.04%.

[0032] Nickel (Ni): 0.7-1.0% Nickel (Ni) is an important element for improving impact toughness by facilitating cross-slip of potential at low temperatures, thereby enhancing hardening ability and increasing strength.

[0033] In this invention, to form a hard phase (preferably a bainite phase) near the melting line of the heat-affected zone of the weld, and to improve the impact toughness of such a hard phase structure, it is preferable to include 0.7% or more Ni. However, if the content exceeds 1.0%, the hardening ability becomes excessive, which leads to a decrease in toughness and a significant increase in manufacturing costs.

[0034] Therefore, in the present invention, the above Ni can be contained in an amount of 0.7 to 1.0%.

[0035] Molybdenum (Mo): 0.05~0.30% Molybdenum (Mo) is a useful element for improving the hardening ability of steel and increasing its strength. In order to ensure the strength targeted in this invention, it is preferable to include Mo at a concentration of 0.05% or more. However, if the content is excessive, the strength may become excessively high and the toughness may decrease, so it is preferable that the content does not exceed 0.30%.

[0036] Titanium (Ti): 0.010~0.018% Titanium (Ti) has the effect of significantly improving toughness by precipitating into TiN during reheating and suppressing grain growth in the base material and the heat-affected zone of the weld. To effectively precipitate TiN, it is preferable to include 0.010% or more of Ti. However, if the content exceeds 0.018%, problems arise such as clogging of the continuous casting nozzle or crystallization in the center, which reduces low-temperature toughness. In addition, the Ti-to-N content ratio (N / Ti) becomes low, and as the TiN precipitates become coarser, the toughness of the heat-affected zone of the weld may decrease.

[0037] Therefore, in the present invention, the above Ti can be contained in an amount of 0.010 to 0.018%.

[0038] Niobium (Nb): 0.01-0.03% Niobium (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.

[0039] To fully obtain the effects described above, it is preferable to contain 0.01% or more of Nb. However, if the content exceeds 0.03%, there is a high possibility that brittle cracks will occur at the corners of the steel material, and a large amount of fine hard phase may be generated in the heat-affected zone of the weld, potentially reducing toughness.

[0040] Therefore, in the present invention, the above Nb can be contained in an amount of 0.01 to 0.03%.

[0041] Nitrogen (N): 0.003~0.006% Nitrogen (N) exhibits a grain size refinement effect by inhibiting the growth of prior austenite grains by bonding with Ti and precipitating on TiN. Thus, in order to form fine TiN precipitates, it is preferable to contain 0.003% or more of the above-mentioned N. However, if the content exceeds 0.006%, not only does the toughness decrease due to the generation of free nitrogen (Free N), but there is also the problem of AlN precipitation inducing slab cracks.

[0042] Therefore, the above N can contain 0.003 to 0.006%, and more favorably, it can contain 0.004% or more and 0.005% or less.

[0043] Phosphorus (P): 0.007% or less and sulfur (S): 0.002% or less Phosphorus (P) and sulfur (S) are elements that induce grain boundary brittleness or brittleness by forming coarse inclusions. For the purpose of improving the resistance of brittle crack propagation in steel materials, the amount of P can be limited to 0.007% or less, and the amount of S can be limited to 0.002% or less.

[0044] While it is preferable for these elements to be present at 0%, 0% can be excluded considering that they may inevitably be added.

[0045] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from the raw materials or the surrounding environment, and these cannot be eliminated. Since these impurities are easily recognizable to any technician in the normal manufacturing process, their details are not specifically mentioned herein.

[0046] The steel material of the present invention having the alloy composition described above can have a microstructure consisting of a single phase of bainite, or bainite with an area fraction of 90% or more and the remainder being acicular ferrite. As a result, the above steel material has a tensile strength of 610 to 770 MPa and an impact toughness of 33 J or more at -20°C.

[0047] On the other hand, the weld heat-affected zone formed by welding the steel material of the present invention with a moderate heat input (approximately 100-200 kJ / cm) preferably contains a bainite phase with an area fraction of 90% or more in the region from the fusion line (FL) to FL+3 mm.

[0048] If the bainite phase in the above-mentioned molten zone is less than 90%, it will not be possible to achieve the target level of intensity.

[0049] The molten line region described above may be a single bainite phase, but it can also contain an acicular ferrite phase in addition to bainite. Furthermore, trace amounts of the MA phase may be present within the bainite and acicular ferrite structure, and in this case, the amount of MA phase present does not impair the physical properties of the molten line.

[0050] Furthermore, it is preferable that the average grain size of the prior austenite in the region from the fusion line (FL) to FL+3mm is 100 μm or less. By refining the grain size in the fusion line region in this way, it is possible to advantageously secure the target level of strength and toughness.

[0051] Specifically, the region from the fusion line (FL) to FL+3mm exhibits excellent strength and low-temperature toughness, with a tensile strength of 610 MPa or higher and an impact toughness of 33 J or higher at -20°C.

[0052] The following describes in detail a method for producing steel materials with excellent toughness in the heat-affected zone of the weld, relating to another aspect of the present invention.

[0053] 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.

[0054] Slab reheating: 1100~1180℃ It is preferable to reheat the steel slab satisfying the alloy composition described above in a temperature range of 1100 to 1180°C. It is preferable to set the reheating temperature to 1100°C or higher to sufficiently solidify the Ti and / or Nb carbonitrides formed during casting. However, if the reheating temperature is excessively high, the austenite may coarseen, so it is preferable that the reheating temperature be 1180°C or lower.

[0055] Rough rolling: 900℃ or higher The reheated steel slab is then 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.

[0056] In order to induce sufficient recrystallization and refine the microstructure during rough rolling at the temperatures mentioned above, it is preferable that the total cumulative reduction ratio during rough rolling be 40% or more.

[0057] Finishing rolling: 800℃ or higher Hot-rolled steel is produced by performing finish rolling to introduce the austenitic structure of the roughly rolled steel sheet into a non-uniform fine structure. Specifically, it is preferable to perform the finish rolling at a temperature of 800°C or higher in order to impart maximum deformation to the structure. If the finish rolling temperature is below 800°C, ferrite will precipitate during cooling after the rolling is completed, reducing the strength, so it is preferable to perform it at 800°C or higher.

[0058] In order to generate the finest possible structure during finish rolling at the temperatures mentioned above, it is preferable that the cumulative reduction rate of the finish rolling is 50% or more.

[0059] Cooling after rolling: Cooling at point t / 4 until the temperature is 600°C or below at a cooling rate of 10°C / s or more (t: thickness of hot-rolled steel (unit: mm)). If the cooling rate of the hot-rolled steel material manufactured as described above is less than 10°C / s, or if the cooling completion temperature exceeds 600°C, the microstructure of the base material may become coarser, affecting the microstructure of the heat-affected zone formed by subsequent welding. The upper limit of the above 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 above cooling rate is 200°C / s or less. In this invention, the cooling completion temperature is not particularly limited, and it may be cooled down to room temperature.

[0060] The hot-rolled steel material, once cooled as described above, can be welded. This welding can be performed with a moderate heat input, for example, a heat input of 100-200 kJ / cm. Any welding method that can accommodate this heat input is acceptable. An unrestricted example would be EGW (electric welding).

[0061] When the hot-rolled steel material according to the present invention is welded with internal heat input, the formed weld heat-affected zone has a fine structure in which the molten line region mainly consists of the bainite phase, thus exhibiting excellent low-temperature toughness as well as strength.

[0062] The present invention will be described more specifically below with reference to examples. However, such examples are merely illustrative of how the present invention can be carried out, and the present invention is not limited by such examples. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0063] A 300 mm thick steel slab having the composition shown in Table 1 below (the remainder being Fe and unavoidable impurities) was reheated at 1140°C, then subjected to continuous rough rolling at 980°C, followed by finish rolling at 880°C. After this, the steel material was manufactured by cooling at a cooling rate of 16-35°C / s until the temperature at the 1 / 4t point reached 470-580°C. During this process, the rough rolling was performed with a reduction ratio of 40% or more, and the finish rolling was performed with a cumulative reduction ratio of 50% or more.

[0064] The microstructure of the steel materials manufactured as described above was measured, and the results are shown in Table 2.

[0065] Furthermore, the steel materials manufactured as described above were welded with a heat input of between 100 and 200 kJ / cm. The microstructure and mechanical properties (tensile strength, low-temperature impact toughness) of the heat-affected zone (HAZ) from the fusion line (FL) to FL+3 mm were then analyzed, and the results are shown in Table 2.

[0066] The microstructure of the above-mentioned steel material (base material) and molten wire was observed using an optical microscope, then separated using EBSD equipment, and the fraction was measured.

[0067] The tensile strength of the steel material and molten wire was measured using a universal tensile machine, and the low-temperature impact toughness was measured using a Charpy impact tester at -20°C, where the Charpy impact absorption energy (CVN) value was measured.

[0068] [Table 1]

[0069] [Table 2]

[0070] As shown in Tables 1-3 above, it can be confirmed that the inventive steels 1-4 that satisfy all the alloy compositions and manufacturing conditions restricted by the present invention exhibit excellent strength and low-temperature toughness in the heat-affected zone of the weld. In particular, it can be seen that the heat-affected zone of the inventive steels has a tensile strength of 610 MPa or more and satisfies an impact toughness of 33 J or more at -20°C.

[0071] On the other hand, in Comparative Example 1, where the content of C and Mo in the alloy composition restricted by the present invention was excessive, the impact toughness deteriorated because the strength of the heat-affected zone during welding became excessively high compared to the inventive steel.

[0072] Comparative Example 2 showed that the Mn content was insufficient, and as a bainite phase was not sufficiently formed in the heat-affected zone of the weld, the strength of the heat-affected zone deteriorated.

[0073] Comparative Example 3 had insufficient Ni content, resulting in a significant deterioration of the low-temperature toughness of the heat-affected zone during welding, specifically 13 J at -20°C.

[0074] Comparative Example 4 shows a case where the Ti content is excessive, while the N and Nb content is insufficient. As a result, coarse TiN precipitates in the weld, and the particle refinement effect by NbC is insufficient, leading to a coarse prior austenite grain size, and consequently, a deterioration in low-temperature toughness.

Claims

1. In weight percent, it contains carbon (C): 0.04-0.07%, manganese (Mn): 1.5-1.7%, silicon (Si): 0.1-0.3%, aluminum (Al): 0.01-0.04%, nickel (Ni): 0.7-1.0%, molybdenum (Mo): 0.05-0.30%, titanium (Ti): 0.010-0.018%, niobium (Nb): 0.01-0.03%, nitrogen (N): 0.003-0.006%, phosphorus (P): 0.007% or less, sulfur (S): 0.002% or less, with the remainder consisting of Fe and unavoidable impurities. A steel material characterized in that the heat-affected zone (HAZ) of a weld, when welded with a heat input of 100 to 200 kJ / cm, contains bainite phase in an area fraction of 90% or more within the molten line region, which is a range of 3 mm from the molten line toward the base metal.

2. The steel material according to claim 1, characterized in that the average particle size of prior austenite in the melting line region is 100 μm or less.

3. The steel material according to claim 1, characterized in that the melting line region has a tensile strength of 610 MPa or more and an impact toughness of 33 J or more at -20°C.

4. The steel material according to claim 1, characterized in that the steel material contains bainite in a microstructure with an area fraction of 90% or more (including 100%) and the remainder being acicular ferrite.

5. The steel slab, containing, by weight percent, carbon (C): 0.04-0.07%, manganese (Mn): 1.5-1.7%, silicon (Si): 0.1-0.3%, aluminum (Al): 0.01-0.04%, nickel (Ni): 0.7-1.0%, molybdenum (Mo): 0.05-0.30%, titanium (Ti): 0.010-0.018%, niobium (Nb): 0.01-0.03%, nitrogen (N): 0.003-0.006%, phosphorus (P): 0.007% or less, sulfur (S): 0.002% or less, with the remainder being Fe and unavoidable impurities, is reheated at 1100-1180°C. The step of roughly rolling the heated steel slab at a temperature of 900°C or higher, The step of manufacturing hot-rolled steel by performing finish rolling at a temperature of 800°C or higher after the rough rolling, A step of cooling the hot-rolled steel material at a cooling rate of 10°C / s or more until the temperature at point t / 4 of the thickness (t, in mm) of the material reaches 600°C or less, and The step includes welding the hot-rolled steel material after the aforementioned cooling, The method for manufacturing steel according to any one of claims 1 to 4, characterized in that the welding is performed with a heat input of 100 to 200 kJ / cm.

6. The method for manufacturing steel according to claim 5, characterized in that the rough rolling step is performed with a reduction ratio of 40% or more.

7. The method for manufacturing steel according to claim 5, characterized in that the finish rolling step is performed with a cumulative reduction ratio of 50% or more.

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