Heat-treatable steel sheet having excellent low-temperature toughness in welded zone and manufacturing method therefor

Abstract

The present invention provides a steel sheet comprising, by wt%, carbon (C): 0.11% - 0.14%, silicon (Si): 0.1% - 0.3%, manganese (Mn): 0.9% - 1.3%, soluble aluminum (Sol. Al): 0.015% - 0.055%, phosphorus (P): 0 (exclusive) - 0.015% (inclusive), sulfur (S): 0 (exclusive) - 0.003% (inclusive), nickel (Ni): 1.7% - 2.2%, chromium (Cr): 0.2% - 0.6%, and molybdenum (Mo): 0.2% - 0.6%, with the balance being iron and inevitable impurities, wherein the microstructure of the heat-affected zone in welding has a maximum size of effective grain of 50 μm or less with a grain boundary misorientation angle of 15° or more, and comprises, by area fraction, 90–98% of lower bainite and upper bainite in total, and less than 5% of martensite-austenite constituent.

Description

Heat-treated steel plate with excellent low-temperature toughness of welded joints and manufacturing method thereof

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly, to a steel plate having excellent weld toughness and a yield strength of 690 MPa and a method for manufacturing the same.

[0002] Due to global CO2 emission regulations, research into the capture, storage, and transportation of liquefied carbon dioxide (LCO2) emitted from industry is growing. In response, the shipbuilding industry is developing vessels to store and transport LCO2. They are designing LCO2 storage containers equipped with materials with superior low-temperature toughness and a yield strength of 690 MPa. While CO2 has a liquefaction point of approximately -55°C at low pressure (8 bar), the application of high-strength materials at 690 MPa allows for designs at medium pressure (19 bar), raising the liquefaction point to approximately -35°C.

[0003] However, in general carbon steel, it is not easy to secure strength and toughness at the same time in high-strength steel because it shows an inversely proportional behavior in which ductility and toughness relatively decrease as strength increases.

[0004] The problem to be solved by the present invention is to provide a steel plate having excellent weld toughness with a yield strength of 690 MPa and a method for manufacturing the same. Specifically, the present invention provides a steel plate having excellent base material and weld impact toughness at -40°C and weld fracture toughness at -35°C in a steel plate having a yield strength of 690 MPa or higher, and a method for manufacturing the same.

[0005] According to another aspect of the present invention, a steel sheet comprises, in wt%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): more than 0 and 0.015% or less, sulfur (S): more than 0 and 0.003% or less, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder of iron and unavoidable impurities, and a microstructure of a weld heat-affected zone has an effective grain maximum size of 50㎛ or less with an inter-grain orientation difference of 15° or more, and includes lower bainite and upper bainite in a total of 90 to 98% in area fraction, and martensite on the surface. Includes less than 5%.

[0006] Additionally, it may further contain copper (Cu): 0 to 0.3%.

[0007] In addition, when the above steel plate is applied as a welding base material, the impact toughness value measured for the weld heat affected zone (HAZ) under the condition of a heat input of 15 kJ / cm can have a value of 50 J or more at -40°C, and the fracture toughness value can have a value of 0.1 mm or more at -35°C.

[0008] In addition, when the above steel plate is applied as a welding base material, the impact toughness value measured for the weld heat affected zone (HAZ) under the condition of a heat input of 35 kJ / cm can have a value of 50 J or more at -40°C, and the fracture toughness value can have a value of 0.1 mm or more at -35°C.

[0009] Additionally, the yield strength may be 690 MPa or more, the elongation may be 14% or more, and the impact toughness value may be 100 J or more at -40°C.

[0010] A method for manufacturing a steel sheet according to one aspect of the present invention comprises the steps of: (a) heating a steel material containing, in wt%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): more than 0 and 0.015% or less, sulfur (S): more than 0 and 0.003% or less, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder iron and unavoidable impurities, to a temperature of 1100°C to 1250°C; (b) hot-rolling the steel material and air-cooling it to room temperature; (c) a step of reheating the steel material to a temperature of 850°C to 950°C for 30 to 120 minutes; (d) a step of rapidly cooling the reheated steel material to room temperature and quenching it; and (e) a step of tempering the rapidly cooled steel material at a temperature of 550°C to 650°C; wherein, in the steel plate prepared through the step (e), the microstructure of the weld heat-affected zone has an effective grain maximum size of 50 μm or less, with an orientation difference between grains of 15° or more.

[0011] Additionally, it may further contain copper (Cu): 0 to 0.3%.

[0012] Additionally, the hot rolling in step (b) above can be performed at a final temperature of 750°C to 950°C.

[0013] Additionally, the thickness of the hot-rolled steel material of step (b) may be 10 mm to 50 mm.

[0014] Additionally, the cooling rate during quenching in step (d) may exceed 10°C / s.

[0015] According to the present invention, a steel plate having excellent weld toughness and a yield strength of 690 MPa and a method for manufacturing the same can be realized. For example, a steel plate having excellent base material and weld impact toughness at -40°C and weld fracture toughness at -35°C can be realized in a steel having a yield strength of 690 MPa or higher, and a method for manufacturing the same.

[0016] Figure 1 is a flowchart illustrating a method for manufacturing a steel plate according to one embodiment of the present invention.

[0017] Figure 2 is a photograph of the microstructure of the weld heat affected zone (HAZ) observed under conditions of heat inputs of 15 kJ / cm and 35 kJ / cm for a steel plate according to Example 2 among experimental examples of the present invention, taken with a scanning electron microscope.

[0018] Figure 3 is a photograph of the microstructure of the weld heat affected zone (HAZ) observed under conditions of heat inputs of 15 kJ / cm and 35 kJ / cm for a steel plate according to Comparative Example 3 among experimental examples of the present invention, taken with a scanning electron microscope.

[0019] Figure 4 is a photograph of the weld heat affected zone (HAZ) observed under conditions of heat inputs of 15 kJ / cm and 35 kJ / cm for a steel plate according to Example 2 among experimental examples of the present invention, observed using an electron backscatter diffractometer.

[0020] Figure 5 is a photograph of the weld heat affected zone (HAZ) observed under conditions of heat inputs of 15 kJ / cm and 35 kJ / cm for a steel plate according to Comparative Example 3 among experimental examples of the present invention, observed using an electron backscatter diffractometer.

[0021] Hereinafter, the present invention will be described in detail. When describing the present invention, if it is determined that specific descriptions of related known technologies or configurations may unnecessarily obscure the gist of the present invention, such detailed descriptions will be omitted.

[0022] And the terms described below are terms defined in consideration of their functions in the present invention, and may vary depending on the intention or custom of the user or operator, so their definitions should be made based on the contents throughout this specification explaining the present invention.

[0023] As described above, steel plates applicable to liquefied carbon dioxide storage containers mounted on ships require excellent low-temperature toughness and high strength. The present invention can provide a steel plate having excellent weld toughness with a yield strength of 690 MPa and a method for manufacturing the same. That is, in order to simultaneously secure a strength of 690 MPa, impact toughness of the base material and weld at a temperature of -40°C, and fracture toughness of the weld at a temperature of -35°C, the influence of alloy components is examined by focusing on the structure composition of the weld, and the present invention provides a steel plate capable of securing a structure with a high toughness value in the weld heat-affected zone (HAZ) and a method for manufacturing the same.

[0024] steel plate

[0025] A steel sheet according to one embodiment of the present invention contains, in wt%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): more than 0 and 0.015% or less, sulfur (S): more than 0 and 0.003% or less, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder of iron and unavoidable impurities.

[0026] The above impurities mean those that are not intentionally added, and in one embodiment of the present invention, for example, one or more selected from the group consisting of Ti, Nb, B, and Ca may be included. Ti and Nb may be included as the impurities in weight percent, for example, 0.025% or less, and B and Ca may be included as the impurities in weight percent, for example, 0.003% or less.

[0027] Additionally, the steel plate may further include copper (Cu): 0 to 0.3%.

[0028] Below, the role and content of each component included in the steel plate according to one embodiment of the present invention are described. The content below refers to weight percent.

[0029] Carbon (C): 0.11%~0.14%

[0030] Carbon (C) is an element that increases strength, but it is the main element that forms martensite (Martensite Austenite constituent; MA), which is weak in toughness. If the carbon content is too excessive, it is detrimental to the toughness. However, since the steel grade according to the present embodiment, which requires quenching after heat treatment, requires martensite structure formation, it is essential to add an appropriate amount of carbon, which is a main element that increases hardenability. If the carbon content is less than 0.11%, the formation of martensite structure is detrimental, which causes a problem of lowering the toughness of the final product. Therefore, it is necessary to add more than 0.11%. On the other hand, if the carbon content is too excessive, the toughness of the weld joint is deteriorated due to the formation of a large amount of martensite. Therefore, the upper limit of the carbon content can be limited to 0.14%.

[0031] Silicon (Si): 0.1% to 0.30%

[0032] Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process together with aluminum, and can also have a solid solution strengthening effect. In addition, it can be used as an element to improve hardenability in steel for heat treatment. The silicon may be added in a content ratio of 0.1% or more of the total weight of the steel sheet according to one embodiment of the present invention. In addition, the silicon may be added in a content ratio of 0.30% or less of the total weight of the steel sheet according to one embodiment of the present invention. When the silicon content exceeds 0.30% of the total weight and is added in a large amount, there is a risk of brittle fracture occurring because the martensite formed in the heat affected zone (HAZ) of the weld is not decomposed.

[0033] Manganese (Mn): 0.9%~1.3%

[0034] Manganese (Mn) is an element that improves the toughness of the heat-affected zone (HAZ) by suppressing the formation of grain boundary ferrite in the HAZ during cooling after welding by increasing the hardenability of steel. To achieve this effect, manganese needs to be added in an amount of 0.9% or more. However, if manganese is added excessively, exceeding 1.3%, segregation of central manganese may cause unevenness in the structure, which may deteriorate the toughness of the HAZ. Therefore, it is desirable to limit the upper limit to 1.3%.

[0035] Aluminum (Al): 0.015% to 0.055%

[0036] Aluminum (Al) is the primary deoxidizer in steel. Therefore, to achieve its effective deoxidizing effect, its content must be at least 0.015%. If the content exceeds 0.055%, not only will the deoxidizing effect become saturated, but oxides will also increase, reducing toughness. Therefore, it is recommended to limit the upper limit to 0.055%.

[0037] Phosphorus (P): 0 to 0.015%

[0038] Phosphorus (P) is an element that causes grain boundary segregation in base metals and welds, and its content must be actively reduced to prevent steel embrittlement. However, reducing P to near-limit levels increases the load on the steelmaking process. However, since the above problem does not significantly occur at phosphorus contents below 0.015%, it is desirable to limit the upper limit to 0.015%.

[0039] Sulfur (S): 0 to 0.003%

[0040] Sulfur (S) is an element that causes red-hot embrittlement in steel, and its content must be actively reduced to prevent this embrittlement. However, reducing sulfur to near-limit levels increases the load on the steelmaking process. Since the above problem does not significantly occur at sulfur contents below 0.003%, it is desirable to limit the upper limit to 0.003%.

[0041] Nickel (Ni): 1.7%~2.2%

[0042] Nickel (Ni) is a useful element that can simultaneously increase strength and toughness, and is an effective element for improving low-temperature impact toughness. When the nickel content is 1.7% or more, the above effect can be exhibited. In addition, nickel can also improve hardenability when added in large amounts, and thus can be utilized as an element to improve the problem of reduced hardenability due to the addition of chromium and molybdenum. However, when the nickel content exceeds 2.2%, it is economically disadvantageous because it is an expensive alloy. Therefore, in a steel sheet according to one embodiment of the present invention, the upper limit of the nickel content is controlled to 2.2 wt%.

[0043] Chromium (Cr): 0.2%~0.6%

[0044] Chromium is an element that increases hardenability and thus yield strength and tensile strength by solid solution. According to an embodiment of the present invention, adding an appropriate amount of chromium is essential for forming martensite during quenching, and this effect can be achieved when the chromium content is 0.2% or more. However, when the chromium content exceeds 0.6%, an excessive low-temperature metamorphic structure is formed in the weld heat-affected zone, which reduces low-temperature toughness and may increase manufacturing costs. Therefore, in a steel sheet according to an embodiment of the present invention, the upper limit of the chromium content is controlled to 0.6%.

[0045] Molybdenum (Mo): 0.2%~0.6%

[0046] Molybdenum (Mo) has the effect of increasing the hardenability of steel, similar to chromium described above, thereby increasing the strength of the steel. Specifically, it has the effect of delaying transformation during the cooling process after heat treatment, thereby increasing the strength. To fully achieve this effect, it is desirable to add at least 0.2%. However, as an expensive element, excessive addition of molybdenum is economically disadvantageous and can excessively increase the hardness of the weld, thereby reducing the toughness. Therefore, in the steel sheet according to one embodiment of the present invention, the upper limit of the molybdenum content is controlled to 0.6%.

[0047] Meanwhile, optionally, the steel sheet according to one embodiment of the present invention may further contain copper.

[0048] Copper (Cu): 0% or more but less than 0.3%

[0049] Copper (Cu) is an element effective in increasing strength and improving toughness. However, when the copper content exceeds 0.3%, surface defects may occur, and there is a problem that the carbon equivalent increases with increasing content. Therefore, in a steel sheet according to one embodiment of the present invention, the upper limit of the copper content is controlled to 0.3%.

[0050]

[0051] The steel sheet according to embodiments of the present invention is a steel alloy designed to have a carbon content in the range of 0.11 to 0.14%, and has sufficient hardenability, so that addition of B is unnecessary. In addition, the steel sheet according to embodiments of the present invention does not intentionally add Nb and Ti, so that although the austenite grains are very coarse, the hardenability secured by the high carbon content can form a fine grain structure. In addition, the steel sheet according to embodiments of the present invention can secure advantageous toughness by adding a large amount of Ni in the range of 1.7 to 2.2%.

[0052] More specifically, the steel sheet according to the embodiments of the present invention having the alloy element composition as described above does not intentionally add Nb and Ti, and thus the austenite grains in the heat affected zone (HAZ) of the weld are very coarse, while the intergranular spacing is narrow due to the hardenability secured by the high carbon content, and accordingly, the high-angle effective grains are very dense. That is, the maximum size of the effective grains with an intergranular orientation difference of 15° or more is, for example, 50 ㎛ or less, so that a very dense intragranular structure can be formed.

[0053] In addition, a steel plate having the alloy element composition as described above may have a composite structure in which the weld heat-affected zone contains lower bainite and upper bainite in a total of 90 to 98% and contains martensite on the surface in a less than 5%.

[0054] In addition, the steel plate having the alloy element composition as described above can have a yield strength of 690 MPa or more and an elongation of 14% or more, and furthermore, since Ni, which is advantageous for toughness, is added in a large amount within the above-described range, the steel plate can have a material having an impact toughness value of 100 J or more at -40°C.

[0055] In addition, when a steel plate having the alloy element composition as described above is applied as a welding base material, it can have excellent toughness values ​​as follows. More specifically, the impact toughness value measured for the heat affected zone (HAZ) of the weld under the condition of a heat input of 15 kJ / cm can have a value of 50J or more at -40℃, and the fracture toughness value can have a value of 0.1mm or more at -35℃, and the impact toughness value measured for the heat affected zone (HAZ) of the weld under the condition of a heat input of 35 kJ / cm can have a value of 50J or more at -40℃, and the fracture toughness value can have a value of 0.1mm or more at -35℃.

[0056] Hereinafter, a method for manufacturing a steel plate according to one embodiment of the present invention having the above-described alloy element composition will be described.

[0057] Method of manufacturing steel plates

[0058] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a steel plate according to one embodiment of the present invention. Referring to FIG. 1, the method for manufacturing the steel plate includes: (a) a step of heating a steel material having the above-described composition to a temperature of 1100°C to 1250°C; (b) a step of hot-rolling the steel material and air-cooling it to room temperature (S110); (c) a step of reheating the steel material to a temperature of 850°C to 950°C for 30 minutes to 120 minutes (S120); (d) a step of quenching the reheated steel material by rapidly cooling it to room temperature (S130); and (e) a step of tempering the rapidly cooled steel material at a temperature of 550°C to 650°C (S140).

[0059] Specifically, the present invention heats a steel material comprising, in wt%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): more than 0 and 0.015% or less, sulfur (S): more than 0 and 0.003% or less, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder of iron and unavoidable impurities, to a temperature of 1100°C to 1250°C. The steel material may further include copper (Cu): more than 0 and 0.3% or less.

[0060] Through the heating process of such steel, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur. In this case, the steel may be a slab plate manufactured through a continuous casting process. If the reheating temperature of the steel is lower than 1100℃, there is a problem that the heating temperature is insufficient, resulting in an increase in the rolling load. Conversely, if the reheating temperature exceeds 1250℃, the austenite grains rapidly coarsen or decarburization occurs, making it difficult to secure the strength and low-temperature toughness of the manufactured steel.

[0061] Next, the steel is hot rolled to produce a hot-rolled steel sheet of the required thickness, and then cooled to room temperature (S110).

[0062] The thickness of the above hot-rolled steel sheet is preferably 100 mm or less, and more preferably 10 to 50 mm. Hot rolling may be performed using AR (As Rolled), Controlled Rolling, or TMCP. The rolling end temperature is preferably completed within the range of 750 to 950°C above the temperature Ar3 at which ferrite is formed.

[0063] Next, the rolled steel is reheated at a temperature of 850°C to 950°C for 30 to 120 minutes (S120).

[0064] The above reheating is intended to reversely transform the hot-rolled steel sheet composed of ferrite and pearlite into a single austenite phase. However, if the reheating temperature is lower than 850°C, austenitization is not sufficiently achieved, resulting in coarse soft ferrite being mixed in, which reduces the hardness of the final product. On the other hand, if the reheating temperature exceeds 950°C, the austenite grains become coarser, which has the effect of increasing the hardenability, but there is the problem of the low-temperature toughness of the steel being inferior.

[0065] Meanwhile, if the heating time is less than 30 minutes, austenitization does not occur sufficiently, preventing the subsequent rapid cooling phase transformation from occurring. On the other hand, if the heating time exceeds 120 minutes, the austenite grains become coarser, which has the effect of improving hardenability, but there is also the problem of low-temperature toughness being inferior.

[0066] Next, the reheated steel is quenched to room temperature (S130).

[0067] As an example, high strength can be achieved by quenching at a cooling rate exceeding 10°C / s, and then high toughness can be achieved through a subsequent tempering process. According to an embodiment of the present invention, a completely martensite matrix structure can be formed by quenching the steel to room temperature.

[0068] Next, the rapidly cooled steel is tempered at a temperature of 550°C to 650°C (S140).

[0069] Tempering is a process for changing the martensite structure formed by quenching into tempered martensite. Heat treatment is performed below the Ac1 temperature where phase transformation begins, and has the characteristic that the higher the temperature, the lower the strength and the higher the toughness. The tempering temperature applied in the present invention is 550℃ to 650℃. When heat treatment is performed below 550℃, the strength increases but the toughness decreases, and when tempering is performed above 650℃, there is a problem that the strength is insufficient due to the coarsening of carbides and material softening.

[0070] The steel plate manufactured using the above-described manufacturing method can obtain a tempered martensite structure.

[0071] In addition, in the welded heat affected zone of the steel plate manufactured by the above-described manufacturing method, a microstructure can be obtained in which the maximum effective grain size is 50㎛ or less and the orientation difference between grains is 15° or more, and the microstructure can have a composite structure in which most of the structure is composed of upper / lower bainite and some of the structure is composed of martensite.

[0072] The above steel plate may have a material having a yield strength of 690 MPa or more, an elongation of 14% or more, and an impact toughness value of 100 J or more at -40°C. Furthermore, when the above steel plate is applied as a welding base material, the impact toughness value measured for the heat affected zone (HAZ) of the weld under the condition of a heat input of 15 kJ / cm may have a value of 50 J or more at -40°C, and the fracture toughness value may have a value of 0.1 mm or more at -35°C, and the impact toughness value measured for the heat affected zone (HAZ) of the weld under the condition of a heat input of 35 kJ / cm may have a value of 50 J or more at -40°C, and the fracture toughness value may have a value of 0.1 mm or more at -35°C.

[0073] Experimental example

[0074] Hereinafter, the structure and operation of the present invention will be described in more detail through preferred embodiments and comparative examples. However, these are presented as only a few examples of the present invention and should not be construed as limiting the present invention in any way.

[0075] Anything not described here will be omitted as it is technically feasible for those skilled in this field to infer.

[0076] Steels of comparative examples and examples each having the alloy compositions shown in Table 1 below were prepared, and rolling and heat treatment steps were performed, respectively.

[0077] Ingredients CSiMnPSAlCuNiCrMoExample 10.1190.201.120.0100.0020.0330.151.90.480.52Example 20.1300.251.050.0090.0020.0250.102.030.440.41Comparative Example 10.0920.201.100.0080.0020.0330.161.230.490.48Comparative Example 20.0820.201.050.0070.0010.0240.151.5 40.500.48Comparative Example 30.0820.201.190.0050.0010.0240.291.700.400.42Comparative Example 40.1020.181.110.0080.0010.0320.121.920.420.46Comparative Example 50.1540.221.080.0070.0020.0360.141.980.480.46Comparative Example 60.1120.171.040.0080.0030.0420.181.580.430.42

[0078] Referring to Table 1, the steel of Example 1 contains, in wt%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): 0 to 0.015% or less, sulfur (S): 0 to 0.003% or less, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder iron (Fe), and Example 2 further contains copper (Cu): 0 to 0.3% or less.

[0079] In contrast, Comparative Examples 1 and 2 do not satisfy the range of carbon (C): 0.11% to 0.14%, nickel (Ni): 1.7% to 2.2%, Comparative Examples 3, 4 and 5 do not satisfy the range of carbon (C): 0.11% to 0.14%, and Comparative Example 6 does not satisfy the range of nickel (Ni): 1.7% to 2.2%.

[0080] The steel material of the above-mentioned composition was subjected to simulated rolling (As-roll) at a slab reheating temperature (SRT): 1150℃, a temperature of the holding section (FRT): 850℃ to 1000℃, and a product was manufactured by applying the same heat treatment conditions: a post-rolling reheating temperature: 905℃, a post-rolling reheating time: 60 minutes, a tempering temperature: 600℃, and a tempering time: 60 minutes.

[0081] Table 2 shows the properties of the base material and welded joints of the steels used in comparative and exemplary embodiments. Since the present invention was conducted on a lab scale, practical welding was not possible, so a heat-affected zone (HAZ) simulation test was performed. The thermal cycle was applied based on a heat input (15 to 35 kJ / cm) typically applicable to shipbuilding. More specifically, the maximum heating temperature is 1350℃, and after maintaining it for 5 seconds, at a heat input of 15kJ / cm, the cooling rate was 75.9℃ / s in the 1350~800℃ section, 24.2℃ / s in the 800~500℃ section, and 5.8℃ / s in the 500~300℃ section. At a heat input of 35kJ / cm, the cooling rate was 28.0℃ / s in the 1350~800℃ section, 9.0℃ / s in the 800~500℃ section, and 2.0℃ / s in the 500~300℃ section. The reason why the welding heat inputs were applied as 15kJ / cm and 35kJ / cm is because these are the minimum and maximum heat input conditions used when welding the corresponding steel in the production of tanks in shipbuilding companies. As described later, under the condition of a heat input of 35 kJ / cm, the cooling rate is slow compared to a heat input of 15 kJ / cm, so the overall grain size becomes coarser and the MA phase size and fraction increase, showing a tendency for the overall impact toughness to decrease. The fracture and impact tests were conducted as Charpy tests at -35°C and -40°C.

[0082] Table 3 shows the microstructure of the weld. More specifically, the effective grain size, which is a high-angle grain size with grain boundaries of 15° or more, measured by electron back scatter diffraction (EBSD) analysis on the examples and comparative examples, and the respective fractions constituting the microstructure are shown in Table 3. In the microstructure in Table 3, LB stands for Lower Bainite, UB stands for Upper Bainite, MA stands for Martensite-Austenite Constituent, GB stands for Granular Bainite, and TM stands for Tempered Martensite.

[0083]

[0084] Physical propertiesMaterial mechanical propertiesWelded joint propertiesYS (MPa)TS (MPa)El (%)Impact toughness(J@-40℃)Heat input(15kJ / cm)Heat input(35kJ / cm)Impact toughness(J@-40℃)Fracture toughness(mm@-35℃)Impact toughness(J@-40℃)Fracture toughness(mm@-35℃)Example 176884219.82521580.1881240.158Example 277485919.92471330.1711150.199Comparative example 172878620.42492010.168290.07 2Comparative Example 281585617.9227770.180360.307Comparative Example 372678021.6250500.026240.037Comparative Example 474583518.41881240.114460.068Comparative Example 584489617.5168850.086660.045Comparative Example 675484818.7194940.064540.054

[0085] ClassificationBase materialWelding heat affected zone (CGHAZ)MicrostructureHeat input (15kJ / cm)Heat input (35kJ / cm)Average AGS (μm)High angleEffective grain size (μm)Microstructure fraction (%)Average AGS (μm)High angleEffective grain size (μm)Microstructure fraction (%)LBUBGBMALBUBGBMAExample 1TM262.444.495--5Less than296.547.280Less than15-5Example 2TM287.542.495--5Less than284.345.88510-5Comparative Example 1TM146.640.585555Less than177.260.63040205-10Comparative Example 2TM125.039.56025105 Less than 140.259.5-60305-10Comparison Example 3TM219.4111.52040305-10250.6124.1-207010More than Comparison Example 4TM255.849.4801055 Less than 286.575.24030205-10Comparison Example 5TM228.438.290--5-10246.640.8855-5-10Comparison Example 6TM257.548.26515105-10263.464.83030305-10

[0086] Referring to Table 2, the specimens of Examples 1 and 2 and the specimens of Comparative Examples 1 to 6 exhibit strength, elongation and toughness values ​​in the range of a yield strength of 690 MPa or more, a tensile strength of 770 to 940 MPa, an elongation of 14% or more, and an impact toughness value of 100 J or more at -40°C. In addition, when the specimens are applied as a welding base material, the specimens of Examples 1 and 2 may have an impact toughness value of 50 J or more at -40°C measured for the weld heat affected zone (HAZ) under both the conditions of a heat input of 15 kJ / cm and a heat input of 35 kJ / cm, and a fracture toughness value of 0.1 mm or more at -35°C. On the other hand, it can be confirmed that the specimens of Comparative Examples 1 to 6 exhibit impact toughness values ​​and / or fracture toughness values ​​measured for the weld heat affected zone (HAZ) under conditions of a heat input of 15 kJ / cm or a heat input of 35 kJ / cm, which fall below the range of the present invention.

[0087] Referring to Table 3, the specimens of Examples 1 and 2 can contain lower bainite and upper bainite in the total of 90 to 98%, and can contain less than 5% of island martensite in the microstructure of the weld heat-affected zone (HAZ) under both the heat input conditions of 15 kJ / cm and 35 kJ / cm. It can be confirmed that Examples 1 and 2 secure hardenability and hardenability according to sufficient carbon content, thereby suppressing the formation of granular bainite phase (GB) vulnerable to toughness and the formation of MA phase at coarse grain boundaries in the weld zone. In contrast, Comparative Examples 1 to 4 are steel grades with low carbon content, so it can be confirmed that the granular bainite phase (GB) vulnerable to toughness is formed at a high fraction in the microstructure of the weld heat-affected zone (HAZ) because hardenability is not secured.

[0088] More specifically, Comparative Example 3 is a steel grade with a low carbon content, which exhibits inferior impact toughness and fracture toughness. This is because, when the carbon content is low, the hardenability is insufficient, which leads to the development of granular bainite, especially in the weld heat affected zone, and thus a high fraction of coarse MA phase, which is detrimental to toughness, is generated. Comparative Examples 4 and 6 are steel grades in which only the carbon content and the nickel content are outside the range of the present invention, and granular bainite is observed at a heat input of 35 kJ / cm, and the maximum size of the high-angle grain boundaries is outside the range of the present invention, thereby exhibiting inferior impact toughness and fracture toughness. Comparative Example 5 is a steel grade in which the carbon content is excessively added beyond the standard range, and because the hardenability is high, the high-angle grain size is very small in all heat input ranges, and the microstructure is also formed as dense lower bainite, but it can be confirmed that it exhibits inferior fracture toughness due to the high fraction of MA phase.

[0089] FIG. 2 is a photograph of the microstructure of the weld heat affected zone (HAZ) of a steel plate according to Example 2 among experimental examples of the present invention, taken with a scanning electron microscope, and FIG. 3 is a photograph of the microstructure of the weld heat affected zone (HAZ) of a steel plate according to Comparative Example 3 among experimental examples of the present invention, taken with a scanning electron microscope.

[0090] Referring to Fig. 2, Fig. 2(a) and Fig. 2(b) are representative microstructure photographs observed under the conditions of heat input of 15 kJ / cm and 35 kJ / cm, respectively, in Example 2, showing that the formation of granular bainite (GB), which is vulnerable to toughness, in the weld zone is effectively suppressed, and the structure is formed with fine-spaced lower bainite (LB) and upper bainite (UB) structures as the main components. More specifically, as shown in Table 3, it can be confirmed that the microstructure of the heat-affected zone (HAZ) of Example 2 mostly includes lower bainite (LB) under the condition of heat input of 15 kJ / cm, and also that the microstructure of the heat-affected zone (HAZ) of Example 2 mostly includes lower bainite (LB) and some upper bainite (UB) under the condition of heat input of 35 kJ / cm.

[0091] Referring to FIG. 3, FIG. 3(a) and FIG. 3(b) are representative microstructure photographs observed under the conditions of heat input of 15 kJ / cm and 35 kJ / cm, respectively, in Comparative Example 3, showing that granular bainite (GB), which is vulnerable to toughness, is formed in the heat affected zone (HAZ) of the weld. More specifically, as shown in Table 3, it can be confirmed that the specimen of Comparative Example 3 contains a certain portion of the microstructure of the heat affected zone (HAZ) of the weld under the condition of heat input of 15 kJ / cm, and also contains mostly granular bainite (GB) in the microstructure of the heat affected zone (HAZ) of the weld under the condition of heat input of 35 kJ / cm.

[0092] Meanwhile, referring to Table 3, it can be confirmed that the specimens of Examples 1 and 2 have a very coarse austenite grain size (AGS) in the weld heat affected zone (HAZ) under both the heat input conditions of 15 kJ / cm and 35 kJ / cm, but the intergranular lath spacing is narrow due to the high carbon content, and thus the maximum effective grain size for a high-angle intergranular orientation difference of 15° or more is 50㎛ or less, thereby having a very fine intragranular structure. On the other hand, it can be confirmed that the specimens of Comparative Examples 1 to 6 have a relatively coarse intragranular size because the maximum effective grain size for a high-angle intergranular orientation difference of 15° or more exceeds 100㎛ in the weld heat affected zone (HAZ) under the heat input conditions of 15 kJ / cm or 35 kJ / cm.

[0093] FIG. 4 is a photograph of the effective grains of the weld heat affected zone (HAZ) of the steel sheet according to Example 2 among the experimental examples of the present invention observed through an electron backscatter diffractometer, and FIG. 5 is a photograph of the effective grains of the weld heat affected zone (HAZ) of the steel sheet according to Comparative Example 3 among the experimental examples of the present invention observed through an electron backscatter diffractometer. More specifically, electron back scatter diffraction (EBSD) analysis was performed on two specimens of Example 2 and Comparative Example 3 to show grain color maps. The grain color map is represented in various colors according to the orientation of the grains, and the area indicated by a single color can be used as one crystallographic domain factor representing the effective grain size of the specimen. In FIG. 4 and FIG. 5, grains having a high inclination angle of 15° or more between grains were distinguished by being represented in different colors.

[0094] Referring to FIG. 4, FIG. 4(a) and FIG. 4(b) show that the internal structure of the weld heat-affected zone (HAZ) of the steel plate according to Example 2 is very dense under both heat input conditions of 15 kJ / cm and 35 kJ / cm, respectively. That is, it can be confirmed that the effective grain size having a high inclination angle is small and that it has a fine intra-granular structure, and this is because most of the microstructure of the weld heat-affected zone (HAZ) of the steel plate according to Example 2 includes lower bainite (LB) and upper bainite (UB) having relatively small effective grain sizes.

[0095] Referring to FIG. 5, FIG. 5(a) and FIG. 5(b) show that the internal structure of the weld heat affected zone (HAZ) of the steel plate according to Comparative Example 3 is relatively coarse under both heat input conditions of 15 kJ / cm and 35 kJ / cm, respectively. That is, it can be confirmed that the maximum size of effective grains having a high inclination angle has a coarse intragranular structure, and this is because most of the microstructure of the weld heat affected zone (HAZ) of the steel plate according to the Comparative Example includes granular bainite (GB) having a relatively large effective grain size.

[0096] A comprehensive review of the results in Tables 2 and 3 above confirms that impact toughness and fracture toughness are closely related not only to the structural characteristics but also to the intragranular grain size. In a bainite or martensite matrix, even if the austenite grains are large, if the internal structure is dense, that is, if the effective grain size is small and the intergranular orientation difference is high, such as 15° or more, the toughness is excellent. This is because when a brittle crack propagates, it encounters resistance at the interface formed by the high angle and its direction is changed, which confirms that a more improved toughness can be secured.

[0097] It will be understood that the present invention encompasses not only the disclosed embodiments, but also various modifications and equivalent embodiments that can be derived from the disclosed embodiments by those skilled in the art. Therefore, the technical protection scope of the present invention should be defined by the following claims.

Claims

1. Contains, by weight%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): 0 to 0.015% or less, sulfur (S): 0 to 0.003% or less, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder of iron and unavoidable impurities. The microstructure of the weld heat affected zone is The maximum effective grain size is 50㎛ or less, and the inter-grain orientation difference is 15° or more. In terms of area fraction, it contains lower bainite and upper bainite in total of 90 to 98%, and contains martensite on the surface in less than 5%. Steel plate.

2. In paragraph 1, Copper (Cu): Steel sheet containing more than 0 and not more than 0.3%.

3. In paragraph 1 or 2, When the above steel plate is applied as a welding base material, the impact toughness value measured for the weld heat affected zone (HAZ) under the condition of a heat input of 15 kJ / cm has a value of 50J or more at -40℃, and the fracture toughness value has a value of 0.1mm or more at -35℃. Steel plate.

4. In paragraph 1 or 2, When the above steel plate is applied as a welding base material, the impact toughness value measured for the weld heat affected zone (HAZ) under the condition of a heat input of 35 kJ / cm has a value of 50J or more at -40°C, and the fracture toughness value has a value of 0.1mm or more at -35°C. Steel plate.

5. In paragraph 1 or 2, Yield strength: 690 MPa or more, elongation: 14% or more, impact toughness value at -40℃: 100J or more, Steel plate. 6.(a) A step of heating a steel material containing, by weight%, carbon (C): 0.11% to 0.14%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 0.9% to 1.3%, dissolved aluminum (Sol. Al): 0.015% to 0.055%, phosphorus (P): more than 0 and not more than 0.015%, sulfur (S): more than 0 and not more than 0.003%, nickel (Ni): 1.7% to 2.2%, chromium (Cr): 0.2% to 0.6%, molybdenum (Mo): 0.2% to 0.6%, and the remainder of iron and unavoidable impurities, to a temperature of 1100°C to 1250°C; (b) a step of hot rolling the above steel and cooling it in the air to room temperature; (c) a step of reheating the above steel material at a temperature of 850°C to 950°C for 30 to 120 minutes; (d) a step of quenching the reheated steel by rapidly cooling it to room temperature; and (e) a step of tempering the rapidly cooled steel at a temperature of 550°C to 650°C; including, The microstructure of the weld heat-affected zone in the steel plate prepared through the above step (e) is as follows: The maximum effective grain size is 50㎛ or less and the inter-grain orientation difference is 15° or more. Method for manufacturing steel plates.

7. In paragraph 6, Copper (Cu): Containing more than 0 but not more than 0.3%, Method for manufacturing steel plates.

8. In paragraph 6, The hot rolling in step (b) above is performed at a final temperature of 750°C to 950°C. Method for manufacturing steel plates.

9. In paragraph 6, The thickness of the hot-rolled steel material of the above step (b) is 10 mm to 50 mm. Method for manufacturing steel plates.

10. In paragraph 6, The cooling rate during quenching in the above step (d) is greater than 10℃ / s. Method for manufacturing steel plates.

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