Low-yield-ratio steel material with excellent toughness of welding heat affected zone and manufacturing method of low-yield-ratio steel material

Abstract

The present invention relates to a steel material for use as a material for building structures, ship structures, marine structures, and the like. More specifically, the present invention relates to a low-yield-ratio steel material having excellent toughness in a welding heat-affected zone, and a method for manufacturing the same.

Description

[0001] This application is a divisional application of Chinese patent application filed on October 25, 2019, with application number "201980070261.1" and invention title "Low yield strength ratio steel with excellent toughness in the weld heat-affected zone and its manufacturing method". Technical Field

[0002] This invention relates to a type of steel used in building structures, ship structures, marine structures, and the like. More specifically, this invention relates to a low yield strength ratio steel with excellent toughness in the weld heat-affected zone and a method for manufacturing the same. Background Technology

[0003] With the trend of building structures becoming taller and larger, the steel used as materials for these structures is also becoming larger, and is being replaced by thicker steel that is thicker than existing steel.

[0004] For such thick steel, in addition to requiring the base material to have high strength and impact toughness, it is usually also required to have a low yield strength ratio. Moreover, the higher the strength and the thicker the material, the more welding is required, which greatly increases the time and cost of welding.

[0005] Therefore, when high heat input welding is used, the number of welding passes is reduced, thereby improving welding efficiency. This not only reduces costs but also improves the stability of the welded structure.

[0006] However, compared to ordinary welding, high heat input welding widens the heat-affected zone (HAZ) of the steel, raises the temperature of the HAZ to a relatively high range, and leads to the growth of austenite grains, eventually forming a coarse structure. This greatly increases the likelihood of a decrease in the impact toughness of the weld HAZ.

[0007] On the other hand, in order to solve the problems mentioned above, a technique has been proposed to appropriately distribute high-temperature stable Ti-based carbides and nitrides in steel to delay grain growth in the heat-affected zone during welding.

[0008] For example, Patent Document 1 discloses a structural steel that, as a technique utilizing TiN precipitates, exhibits an impact toughness of approximately 200 J at 0°C (approximately 300 J for the base material) when using a heat input of 100 J / cm (maximum heating temperature 1400°C). Specifically, the Ti / N content ratio (Ti / N) is controlled to be 4–12, resulting in the precipitation of 5.8 × 10⁻⁵ TiN precipitates with a grain size of less than 0.05 μm. 3 pcs / mm 2 ~8.1×10 4 pcs / mm 2TiN precipitates with a grain size of 0.03–0.2 μm were precipitated at a density of 3.9 × 10⁻⁶. 3 pcs / mm 2 ~6.2×10 4 pcs / mm 2 This is done to refine the ferrite, thereby ensuring the toughness of the welded parts.

[0009] However, the aforementioned patent document 1 has the problem that excessive formation of carbon and nitrides leads to too many cracks on the surface of the billet during continuous casting. When using billets with multiple surface cracks to produce thick plate products, cracks will also appear on the surface of the final product. Therefore, there are disadvantages such as surface repair problems or difficulty in repair itself, which may result in the manufacture of defective products.

[0010] Therefore, Patent Document 2 discloses a steel that suppresses surface cracks that may occur in Patent Document 1 by controlling the surface crack sensitivity index (Cs) to an appropriate level, and has excellent weld heat-affected zone toughness by controlling elements such as Si or Cr that have a negative impact on the toughness of the weld heat-affected zone.

[0011] However, although the steel in the aforementioned patent document 2 shows improved toughness in the weld heat-affected zone compared to the prior art, it cannot meet the requirement of a low yield strength ratio in building structures, ship structures, and marine structures.

[0012] Such problems may also occur in the steel using precipitates such as TiN in Patent Document 1. During the steel manufacturing process, precipitates are also generated in the base material. Therefore, the precipitation hardening effect leads to an increase in yield strength, making it difficult to achieve a low yield strength ratio.

[0013] In addition, elements that typically inhibit grain growth in the weld heat-affected zone will, in most cases, also increase the yield strength due to the grain growth inhibition effect. Therefore, it is necessary to study a scheme that can simultaneously achieve the toughness of the weld heat-affected zone and the low yield strength ratio of the base material.

[0014] Patent Document 1: Japanese Patent Publication No. 1999-140582

[0015] Patent Document 2: Korean Patent Publication No. 2016-0078772 Summary of the Invention

[0016] Technical issues

[0017] One aspect of this invention is to provide a type of steel and a method for manufacturing the same, wherein the steel has a low yield strength ratio and high toughness of the base material, and the weld heat-affected zone exhibits excellent toughness.

[0018] The problems to be solved by this invention are not limited to those described above. Other problems to be solved by this invention are described throughout this specification, and those skilled in the art will have no difficulty understanding these other problems.

[0019] Technical solution

[0020] This invention provides a low yield strength ratio steel with excellent weld heat-affected zone toughness. The steel, by weight percent, comprises: carbon (C): 0.05–0.09%, manganese (Mn): 1.5–1.6%, silicon (Si): 0.2–0.3%, aluminum (Al): 0.02–0.05%, nickel (Ni): 0.4–0.5%, phosphorus (P): less than 0.02%, sulfur (S): less than 0.01%; and at least one element selected from titanium (Ti): 0.005–0.02%, niobium (Nb): 0.01–0.05%, copper (Cu): 0.1–0.3%, chromium (Cr): 0.1–0.2%, and molybdenum (Mo): 0.05–0.10%; at least one element selected from boron (B): less than 5 ppm and nitrogen (N): less than 60 ppm, with the balance being Fe and other unavoidable impurities.

[0021] The microstructure has a thickness of 20–100 mm. When the thickness is 20–40 mm, the microstructure comprises, by area fraction, 40–50% low-temperature bainite, 3–6% island martensite (MA), and the balance acicular ferrite. When the thickness is greater than 40 mm–60 mm, the microstructure comprises, by area fraction, 35–40% low-temperature bainite, 3–5% island martensite (MA), and the balance acicular ferrite. When the thickness is greater than 60 mm–100 mm, the microstructure comprises, by area fraction, 30–35% low-temperature bainite, 3–5% island martensite (MA), and the balance acicular ferrite.

[0022] Another aspect of the present invention provides a method for manufacturing a low yield strength ratio steel with excellent toughness in the weld heat-affected zone, comprising: a step of reheating a steel billet satisfying the above alloy composition at 1100–1250°C; a step of rough rolling the reheated steel billet at 900–1000°C; a step of hot finishing rolling the rough rolled billet at 830–870°C to manufacture a hot-rolled steel plate; a step of cooling the hot-rolled steel plate to 250–500°C at a cooling rate of 3–200°C / s; and a step of air cooling the cooled steel plate to room temperature.

[0023] Invention Effects

[0024] According to the present invention, a steel can be provided that not only exhibits excellent strength and toughness of the base material, but also excellent toughness of the weld heat-affected zone during high heat input welding. Furthermore, since the steel of the present invention has a low yield strength ratio, it can be appropriately used as a structural steel. Attached Figure Description

[0025] Figure 1 Images are shown of microstructures of inventive examples and comparative examples according to one aspect of the present invention.

[0026] Figure 2 An image showing an island-shaped martensite phase as an example of an invention according to one aspect of the present invention is shown.

[0027] Figure 3 The results show the measured impact toughness (-10°C) of the welded portion and the heat-affected zone of the inventive steel according to one aspect of the invention, based on the location of the weld. Detailed Implementation

[0028] To fundamentally address the issues of high yield strength ratio in the base material and weakened toughness in the weld heat-affected zone (HAZ) during high heat input welding when manufacturing thick steel for use in existing building structures, the inventors conducted in-depth research. The results confirmed that by optimizing the steel's alloy composition and manufacturing conditions, not only the strength, yield strength ratio, and toughness of the base material can be controlled, but also the microstructure of the weld heat-affected zone formed during welding can be controlled, thereby ensuring a weld heat-affected zone with excellent toughness, thus completing this invention.

[0029] In particular, during high heat input welding such as submerged arc welding (SAW), the present invention can ensure excellent toughness of the weld heat-affected zone (HAZ), thereby providing a suitable steel for structural applications.

[0030] The present invention will now be described in detail.

[0031] According to one aspect of the invention, a low yield strength ratio steel with excellent weld heat-affected zone toughness is preferably composed, by weight%, of at least one element selected from the following: carbon (C): 0.05–0.09%, manganese (Mn): 1.5–1.6%, silicon (Si): 0.2–0.3%, aluminum (Al): 0.02–0.05%, nickel (Ni): 0.4–0.5%, phosphorus (P): less than 0.02%, sulfur (S): less than 0.01%; and titanium (Ti): 0.005–0.02%, niobium (Nb): 0.01–0.05%, copper (Cu): 0.1–0.3%, chromium (Cr): 0.1–0.2%, and molybdenum (Mo): 0.05–0.10%; and boron (B): less than 5 ppm and nitrogen (N): less than 60 ppm.

[0032] The reasons for controlling the alloy composition of the steel provided by the present invention will be described in detail below. In this context, unless otherwise stated, the content of each component refers to weight percent, and the proportion of the microstructure is based on area.

[0033] Carbon (C): 0.05–0.09%

[0034] Carbon (C) is the element that has the greatest impact on ensuring the strength of steel, so it needs to be included in steel in an appropriate amount.

[0035] In this invention, if the C content is less than 0.05%, the strength of the steel decreases too much, making it difficult to use as structural steel. On the other hand, if the C content is greater than 0.09%, the carbon equivalent (Ceq) becomes too large, and due to the greatly increased hardenability of the base material and the welded part, there is a problem of decreased toughness in the welded part.

[0036] Therefore, in this invention, the content of C is preferably 0.05 to 0.09%, more preferably 0.06 to 0.08%.

[0037] Manganese (Mn): 1.5–1.6%

[0038] Manganese (Mn) improves the hardenability of steel, which helps ensure strength. However, in this invention, the Mn content needs to be appropriately limited to ensure the toughness of the weld heat-affected zone (HAZ).

[0039] Generally, manganese (Mn) does not severely impair the toughness of the weld heat-affected zone (HAZ), but it tends to segregate in the center of the steel plate's thickness. At these segregated areas, the Mn content is much higher than the average, leading to the formation of brittle structures that severely compromise the toughness of the HAZ. Therefore, in this invention, it is preferable to add 1.6% or less of Mn. However, if the Mn content is too low, ensuring the strength of the steel plate becomes difficult; therefore, it is preferable to limit the lower limit to 1.5%.

[0040] Silicon (Si): 0.2–0.3%

[0041] Silicon (Si) is an element required to improve the strength of steel plates and for deoxidation of molten steel. Therefore, a Si content of 0.2% or more is preferred. However, while Si inhibits the formation of cementite during the decomposition of unstable austenite, it promotes island martensite (MA) structure, which can significantly reduce the toughness of the weld heat-affected zone. Therefore, it is preferable to limit the Si content to 0.3% or less. If the Si content exceeds 0.3%, coarse Si oxides will form, and brittle fracture may occur starting from these inclusions, which is undesirable.

[0042] Aluminum (Al): 0.02–0.05%

[0043] Aluminum (Al) is an element that can inexpensively deoxidize molten steel. Therefore, an Al content of 0.02% or more is preferred. However, if the Al content exceeds 0.05%, it can cause nozzle blockage during continuous casting. Furthermore, dissolved Al may promote the formation of island-like martensite in the welded portion, thus potentially impairing the toughness of the weld.

[0044] Nickel (Ni): 0.4~0.5%

[0045] Nickel (Ni) is an element that is beneficial for simultaneously improving the strength and toughness of the base material. Therefore, the Ni content can be 0.4% or higher. However, Ni is a high-valence element, and if its content exceeds 0.5%, it is economically disadvantageous, and weldability may deteriorate.

[0046] Phosphorus (P): less than 0.02%

[0047] Phosphorus (P) is beneficial in improving strength and ensuring corrosion resistance, but it significantly impairs impact toughness. Therefore, the P content is best kept as low as possible, preferably with an upper limit of 0.02%. However, considering the degree of unavoidable contamination, 0% can be ruled out.

[0048] Sulfur (S): less than 0.01%

[0049] Sulfur (S) is an element that forms MnS and other elements that greatly impair impact toughness. Its content should ideally be as low as possible, preferably with an upper limit of 0.01%. However, considering the degree of unavoidable contamination, 0% can be excluded.

[0050] In addition to the alloy components mentioned above, the low yield strength ratio steel of the present invention may also contain at least one of the following elements: titanium (Ti): 0.005-0.02%, niobium (Nb): 0.01-0.05%, copper (Cu): 0.1-0.3%, chromium (Cr): 0.1-0.2%, and molybdenum (Mo): 0.05-0.10%.

[0051] Titanium (Ti): 0.005–0.02%

[0052] Titanium (Ti) combines with nitrogen (N) to form tiny nitrides, which effectively mitigate grain coarsening and reduce toughness near the weld fusion line. If the Ti content is too low, the coarsening suppression effect will not be fully realized due to insufficient Ti nitride content; therefore, it is preferable to add 0.005% or more. However, if the Ti content is too high, the formation of large Ti nitrides can reduce the grain boundary fixation effect; therefore, it is preferable to limit the upper limit to 0.02%.

[0053] Niobium (Nb): 0.01–0.05%

[0054] Niobium (Nb) is an effective element for improving the strength of steel, but it significantly reduces the toughness of the weld heat-affected zone, therefore the Nb content needs to be appropriately limited. In particular, during the reverse transformation to the austenite region near the weld fusion line, Nb carbonitrides precipitate at the austenite grain boundaries, impairing toughness. Therefore, it is preferable to limit it to 0.05% or less. However, to ensure strength, it is preferable that the Nb content is 0.01% or more.

[0055] Copper (Cu): 0.1–0.3%

[0056] Copper (Cu) is an element that helps minimize the decrease in toughness of the base material and increase the strength of steel. To achieve this effect, a content of 0.1% or more copper can be included. However, if the Cu content is too high, it will significantly impair the surface quality of the product. Therefore, it is preferable to limit the Cu content to below 0.3%.

[0057] Chromium (Cr): 0.1–0.2%

[0058] Chromium (Cr), similar to Cu, is an element that helps minimize the decrease in toughness of the base material and increase the strength of the steel. To achieve this effect, a Cr content of 0.1% or higher can be obtained. However, if the Cr content exceeds 0.2%, it leads to a significant reduction in weldability and is therefore undesirable.

[0059] Molybdenum (Mo): 0.05~0.10%

[0060] Molybdenum (Mo) has the effect of significantly improving the hardenability of steel and inhibiting the formation of ferrite phase even when added in small amounts, and it is beneficial to greatly improve strength. Therefore, a content of 0.05% or more Mo can be included. However, if the Mo content exceeds 0.10%, it greatly increases the hardness of the welded parts and impairs toughness, which is therefore undesirable.

[0061] The low yield strength ratio steel of the present invention may also contain one or more elements selected from boron (B): less than 5 ppm and nitrogen (N): less than 60 ppm.

[0062] Boron (B): below 5 ppm

[0063] Boron (B) is an element that improves hardenability even when added in trace amounts. However, if the B content exceeds 5 ppm, it may precipitate or crystallize at grain boundaries, significantly impairing low-temperature impact toughness. In particular, in this invention, in addition to B, it also contains Mn, Ni, Mo, etc., which ensure hardenability; therefore, an excessively high B content may have negative effects.

[0064] Therefore, in this invention, the content of B is limited to less than 5 ppm.

[0065] Nitrogen (N): below 60 ppm

[0066] Nitrogen (N) can suppress grain growth caused by welding heat effects by forming TiN precipitates when added together with titanium (Ti). However, when added in excess, the formation of coarse TiN not only impairs low-temperature impact toughness but also leads to surface cracking due to the formation of AlN. Therefore, it is preferable to contain a maximum of 60 ppm.

[0067] The balance component of this invention is iron (Fe). However, unexpected impurities from raw materials or the surrounding environment are inevitably introduced during conventional manufacturing processes, making it impossible to eliminate these impurities. These impurities are known to anyone skilled in conventional manufacturing processes, and therefore all related details are not elaborated upon in this specification.

[0068] Preferably, the steel of the present invention having the above alloy composition has a thickness of 20 to 100 mm, and its microstructure includes low-temperature bainite, acicular ferrite and island martensite phases.

[0069] In particular, the present invention preferably controls the fraction of fine structures according to the thickness of the steel.

[0070] Specifically, when the steel has a thickness of 20–40 mm, the fine microstructure preferably comprises 40–50% low-temperature bainite, 3–6% island martensite (MA), and the balance acicular ferrite by area fraction. When the thickness is greater than 40 mm–60 mm, the microstructure preferably comprises 35–40% low-temperature bainite, 3–5% island martensite (MA), and the balance acicular ferrite by area fraction. Furthermore, when the thickness is greater than 60 mm–100 mm, the microstructure preferably comprises 30–35% low-temperature bainite, 3–5% island martensite (MA), and the balance acicular ferrite by area fraction.

[0071] In this invention, the thinner the steel, the faster the cooling rate during rolling. Therefore, by further incorporating a low-temperature bainite phase, there is a tendency for increased strength. The low-temperature bainite phase plays a major role in ensuring the strength of the base material and welded parts in this invention; however, an excessive amount of low-temperature bainite phase is detrimental to elongation and impact toughness. Therefore, in this invention, it is necessary to closely control the cooling rate and the fraction of fine-grained phases according to the steel thickness.

[0072] When the low-temperature bainite phase in the microstructure of the steel is insufficient, the target strength level cannot be ensured. On the other hand, when the fraction of acicular ferrite or island martensite phase is insufficient, the low-temperature toughness may decrease rapidly.

[0073] Furthermore, when the island-shaped martensite phase is included according to the above fraction, the maximum length of the long axis of the island-shaped martensite phase is 1 μm or less, and preferably there are 20 or less distributed in one austenite grain.

[0074] When island-like martensite phases are uniformly distributed in steel at a small size, they can hinder fracture propagation, thus playing a major role in comprehensively improving the steel's strength, impact toughness, and other mechanical properties. Therefore, by ensuring that the island-like martensite phases are distributed as uniformly as possible in the steel, the effects based on the island-like martensite phase can be improved. However, when the size of the island-like martensite phases becomes too large or their distribution is uneven, they may become fracture initiation points or fracture propagation paths, thereby impairing mechanical properties.

[0075] In this invention, when the steel contains the island-shaped martensite phase, its size and distribution need to be controlled. Specifically, by controlling the size and distribution as described above, the effect based on the island-shaped martensite phase can be obtained.

[0076] The steel provided by this invention has the fine microstructure described above, thereby having the following effects: a low yield strength ratio of less than 85% and a high tensile strength of more than 600 MPa, while also ensuring excellent toughness with a Charpy impact energy of more than 100 J at -10°C.

[0077] Furthermore, the steel of the present invention can be welded, and can have a post-weld island martensite (MA) fraction of 3-6% in the weld heat-affected zone (HAZ). Besides the island martensite phase, the remaining microstructure consists of low-temperature bainite and acicular ferrite. On the other hand, when the thickness of the steel is greater than 40 mm, the post-weld island martensite (MA) fraction in the weld heat-affected zone (HAZ) can be 3-5%.

[0078] If the fraction of island martensite in the weld heat-affected zone is less than 3%, the strength and toughness may be significantly reduced. On the other hand, if the fraction of island martensite is greater than 5% or 6%, or if the size or distribution of the island martensite exceeds the expected range, it may become a fracture initiation point or propagation path, greatly impairing toughness and ductility. In this paper, the distribution of island martensite in the weld heat-affected zone refers to the distribution level of the island martensite phase in the base material. It should be noted that the microstructure of the weld heat-affected zone is also the same as or similar to the microstructure phase distribution of the base material with controlled thickness.

[0079] As described above, the microstructure of the heat-affected zone after welding is controlled in the steel of the present invention, thus ensuring excellent toughness with a Charpy impact energy of over 100J at -10°C.

[0080] On the other hand, in this invention, high heat input welding can be used as a method for welding the steel. As an example, submerged arc welding (SAW) with a welding heat input of 200 KJ / cm or more can be used.

[0081] In other words, even with the high heat input welding described above, the weld heat-affected zone with minimal toughness deterioration can be obtained for the steel of the present invention.

[0082] The following will describe in detail another aspect of the invention a method for manufacturing low yield strength ratio steel with excellent weld heat-affected zone toughness.

[0083] First, after preparing a steel billet that meets the above alloy composition, the billet can be reheated at 1100-1250°C.

[0084] If the billet is reheated above 1250°C, the austenite grains become coarse, making it impossible to obtain steel with the desired properties. On the other hand, if the temperature is below 1100°C, it becomes difficult to resolution the carbonitrides such as Ti and / or Nb carbonitrides formed in the billet during casting.

[0085] Therefore, in this invention, the steel billet can be reheated at 1100-1250°C.

[0086] Hot rolling of the reheated steel billet produces hot-rolled steel sheets. This hot rolling process can involve both roughing and finishing rolling.

[0087] The rough rolling can be carried out at a temperature range of 900–1000°C, while the finish rolling can be carried out at a temperature range of 830–870°C. If the temperature during rough rolling is below 900°C, it will be difficult to ensure the target temperature during subsequent finish rolling, which may result in poor quality. Furthermore, if the temperature during finish rolling is above 870°C, the steel's toughness may decrease due to the formation of coarse microstructures, while if the temperature is below 830°C, it will be difficult to control the shape of the sheet metal.

[0088] Therefore, roughing and finishing rolling can be performed within the above temperature range.

[0089] Hot-rolled steel sheets manufactured in this way can be cooled to 250-500°C at a cooling rate of 3-200°C / s.

[0090] In this invention, by controlling the cooling process of the manufactured hot-rolled steel sheet, a fine microstructure that is conducive to ensuring a low yield strength ratio can be formed.

[0091] If the cooling rate is less than 3°C / s, the hardenability of the steel plate is insufficient, making it impossible to properly form a low-temperature bainitic structure and instead generating a pearlite-based microstructure, resulting in a significant decrease in strength and potentially reduced toughness. On the other hand, if the cooling rate is greater than 200°C / s, martensite is primarily formed instead of low-temperature ferrite and ferrite phases, leading to excessively high strength, poor toughness, and a high yield strength ratio, which fails to ensure physical properties suitable for the application of this invention.

[0092] On the other hand, depending on the thickness of the hot-rolled steel sheet manufactured as described above, the present invention can employ different cooling rates. Specifically, when the thickness of the hot-rolled steel sheet is 20 to 40 mm, a cooling rate of 80 to 200 °C / s is preferred; when the thickness of the hot-rolled steel sheet is greater than 40 mm to 60 mm, a cooling rate of 20 °C / s or more to less than 80 °C / s is preferred; and when the thickness of the hot-rolled steel sheet is greater than 60 mm to 100 mm, a cooling rate of 3 °C / s or more to less than 20 °C / s is preferred.

[0093] Furthermore, when cooling at the aforementioned rate, if the final temperature is below 250°C, excessive martensite formation occurs, resulting in a significant increase in strength but potentially a decrease in toughness. On the other hand, if the final temperature is above 500°C, the low-temperature bainite phase will not form properly, making it difficult to ensure a low yield strength ratio.

[0094] To ensure the aforementioned cooling rate, water cooling can be used, preferably starting at 780–860°C. If the cooling start temperature is too low, the temperature will enter the two-phase region of ferrite and austenite before cooling begins. Due to the formation of some ferrite, the strength of the steel may decrease significantly. On the other hand, if the cooling start temperature is too high, reheating will occur at the center of the steel's thickness during cooling, which may cause reverse microstructure transformation. This reduces the chance of static recrystallization after rolling, and increases the likelihood of forming an inhomogeneous microstructure.

[0095] On the other hand, after completing the above cooling process, it can be cooled to room temperature.

[0096] On the other hand, a process of high heat input welding can also be performed on the hot-rolled steel plate obtained after air cooling. As the high heat input welding method, submerged arc welding (SAW) with a welding heat input of 200 KJ / cm or more can be used.

[0097] The weld heat-affected zone (HAZ) formed after high heat input welding can contain 3 to 6% of island martensite (MA) phase in area, thereby ensuring Charpy impact energy of over 100 J at -10°C in the weld heat-affected zone.

[0098] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for a more detailed description and are not intended to limit the scope of the invention. The scope of the invention depends on the content of the claims and what is reasonably derived therefrom.

[0099] Methods of implementing the invention

[0100] (Example)

[0101] Steel billets with the alloy composition shown in Table 1 are subjected to a series of processes (reheating-hot rolling-cooling) according to the manufacturing conditions in Table 2 to produce hot-rolled steel sheets. After cooling according to Table 2, the sheets are then cooled to room temperature.

[0102] Table 1

[0103]

[0104] (In Table 1, B* is expressed in ppm.)

[0105] Table 2

[0106]

[0107] The mechanical properties (yield strength (YS), tensile strength (TS), elongation (E1), yield ratio (YR)) and impact toughness (CVN, -10℃) of each hot-rolled steel sheet were measured, and the results are shown in Table 3 below. For the tensile test, specimens were collected from JIS 5 specification specimens at a location 1 / 4t (where t represents the steel sheet thickness (mm)) in the thickness direction along a direction perpendicular to the rolling direction (transverse). Each collected specimen was subjected to a tensile test at room temperature. For the impact toughness, specimens were collected from ASTM E23 specification specimens at a location 1 / 4t in the thickness direction along a direction perpendicular to the rolling direction (transverse). Three Charpy V-Notch impact tests were performed at each measurement temperature, and the average value was obtained.

[0108] For the fraction of fine structure, after collecting specimens from the residue of the impact toughness specimens for observation of the structure, the fraction (area %) of bainite and acicular ferrite at low temperature was first measured by SEM according to the method specified in ASTM E560 specification, and the fraction of island martensite was measured by optical microscope after etching with Lepera reagent. The results are shown in Table 3 below.

[0109] Table 3

[0110]

[0111] (In Table 3, B represents the low-temperature bainite phase, AF represents the acicular ferrite phase, and MA represents the island martensite phase.)

[0112] As shown in Tables 1 to 3 above, Examples 1 to 7 of the invention, which meet the alloy composition and manufacturing conditions proposed in this invention, have the expected fine microstructure, thus exhibiting excellent strength and ductility, as well as a low yield strength ratio and excellent impact toughness. It can be seen that such effects can be ensured regardless of the steel plate thickness.

[0113] On the other hand, Comparative Examples 4 to 7, which do not meet the alloy composition proposed in this invention, share the common feature of having difficulty in ensuring a low yield strength ratio, with Comparative Examples 5 and 7 also having poor impact toughness.

[0114] On the other hand, for Comparative Examples 1 to 3, where the alloy composition meets the requirements of the present invention but the manufacturing conditions deviate from those of the present invention, the strength or toughness is poor.

[0115] Figure 1 Images of the fine structures of Invention Example 2 (thickness 40 mm), Invention Example 3 (thickness 60 mm), Invention Example 4 (thickness 80 mm), and Comparative Example 6 (thickness 20 mm) observed using a scanning electron microscope (SEM) are shown. Additionally, Figure 2 An image showing the island-shaped martensite phase observed in Invention Example 2 (thickness 40 mm) is shown.

[0116] like Figure 1 and Figure 2 As shown, the inventive example sufficiently forms low-temperature bainite and acicular ferrite phases, and island-shaped martensite phases are formed in multiple locations. On the other hand, the comparative example does not sufficiently form low-temperature bainite and acicular ferrite phases, and the bainite phase formed is estimated to have been formed at high temperatures based on its shape. Due to this difference in microstructure, it is believed that the comparative example will exhibit lower strength than the inventive example.

[0117] On the other hand, some of the hot-rolled steel sheets manufactured above are subjected to submerged arc welding (SAW) with a heat input of 226 KJ / cm.

[0118] Impact toughness (CVN (-10℃, -20℃, -40℃)) was measured after collecting samples from the fusion line (FL) within the weld heat-affected zone formed after the welding was completed. Further, impact toughness (CVN) was also measured after collecting samples at FL+2, FL+5, and FL+10, and the results are shown in Table 4 below (where +2, +5, and +10 refer to locations 2 mm, 5 mm, and 10 mm away from the fusion line in the base material direction, respectively). For impact toughness, three Charpy V-Notch impact tests were performed at each temperature (-10℃, -20℃, -40℃) and the average value was obtained.

[0119] Table 4

[0120]

[0121] As shown in Table 4 above, the weld heat-affected zone (FL) obtained by welding hot-rolled steel sheets (Example of the Invention) manufactured based on the alloy composition and manufacturing conditions proposed in this invention using the SAW method exhibits excellent impact toughness (see Table 4). Figure 3 ).

[0122] On the other hand, in Comparative Example 7, which has the same thickness as Invention Example 4, the impact toughness of the fusion line in the weld heat-affected zone is poor after welding. Furthermore, in Comparative Example 5, which has a relatively thinner thickness, the impact toughness is also very poor throughout the entire weld heat-affected zone.

Claims

1. A low yield strength ratio steel with excellent toughness in the weld heat-affected zone, wherein, The steel, by weight percent, contains carbon (C): 0.05-0.09%, manganese (Mn): 1.5-1.6%, silicon (Si): 0.2-0.3%, aluminum (Al): 0.02-0.05%, nickel (Ni): 0.4-0.5%, phosphorus (P): less than 0.02%, and sulfur (S): less than 0.01%. as well as Titanium (Ti): 0.005–0.02%, Niobium (Nb): 0.01–0.05%, Copper (Cu): 0.1–0.3%, Chromium (Cr): 0.1–0.2%, and Molybdenum (Mo): 0.05–0.10%; Boron (B): less than 5 ppm and Nitrogen (N): less than 60 ppm, with the balance containing Fe and other unavoidable impurities. It has a thickness of 20-100mm. When the thickness is 20–40 mm, the microstructure, by area fraction, comprises 40–50% low-temperature bainite, 3–6% island martensite (MA), and the balance acicular ferrite. When the thickness is greater than 40 mm to 60 mm, the microstructure, by area fraction, comprises 35 to 40% low-temperature bainite, 3 to 5% island martensite (MA), and the balance acicular ferrite. When the thickness is greater than 60 mm to 100 mm, the microstructure, by area fraction, comprises 30 to 35% low-temperature bainite, 3 to 5% island martensite (MA), and the balance acicular ferrite.

2. The low yield strength ratio steel with excellent toughness in the weld heat-affected zone according to claim 1, wherein, The maximum length of the long axis of the island-shaped martensite phase is less than 1 μm, and there are less than 20 island-shaped martensite phases in a single austenite grain.

3. The low yield strength ratio steel with excellent toughness in the weld heat-affected zone according to claim 1, wherein, The steel has a tensile strength of 600 MPa or higher, a yield strength ratio of 85% or lower, and a Charpy impact energy of 100 J or higher at -10℃.

4. The low yield strength ratio steel with excellent toughness in the weld heat-affected zone according to claim 1, wherein, The steel has a post-weld island martensite (MA) fraction of 3-6% in the weld heat-affected zone (HAZ).

5. The low yield strength ratio steel with excellent toughness in the weld heat-affected zone according to claim 4, wherein, The Charpy impact energy of the weld heat-affected zone (HAZ) at -10℃ is above 100J.

6. A method for manufacturing a low yield strength ratio steel with excellent toughness in the weld heat-affected zone, comprising: The step of reheating the steel billet at 1100–1250°C, wherein the steel billet, by weight%, comprises: carbon (C): 0.05–0.09%, manganese (Mn): 1.5–1.6%, silicon (Si): 0.2–0.3%, aluminum (Al): 0.02–0.05%, nickel (Ni): 0.4–0.5%, phosphorus (P): less than 0.02%, and sulfur (S): less than 0.01%; and Titanium (Ti): 0.005–0.02%, Niobium (Nb): 0.01–0.05%, Copper (Cu): 0.1–0.3%, Chromium (Cr): 0.1–0.2%, and Molybdenum (Mo): 0.05–0.10%; Boron (B): less than 5 ppm and Nitrogen (N): less than 60 ppm, with the balance containing Fe and other unavoidable impurities; The step of rough rolling the reheated steel billet at 900-1000°C; The step of hot finishing rolling at 830-870°C after rough rolling to manufacture hot-rolled steel plates; The step of cooling the hot-rolled steel sheet to 250-500°C at a cooling rate of 3-200°C / s; and The step of air cooling to room temperature after cooling.

7. The manufacturing method according to claim 6, wherein, Regarding the cooling When the thickness of the hot-rolled steel plate is 20-40 mm, it is cooled at a rate of 80-200 °C / s. When the thickness of the hot-rolled steel plate is greater than 40mm to 60mm, it is cooled at a rate of 20℃ / s to less than 80℃ / s. When the thickness of the hot-rolled steel plate is greater than 60 mm to 100 mm, it is cooled at a rate of more than 3 °C / s to less than 20 °C / s.

8. The manufacturing method according to claim 6, wherein, The cooling is achieved through water cooling, and cooling begins at 780–860°C.

9. The manufacturing method according to claim 6, further comprising: The step of performing high heat input welding on the air-cooled hot-rolled steel sheet.

10. The manufacturing method according to claim 9, wherein, The high heat input welding is performed by submerged arc welding (SAW) with a welding heat input of 200 KJ / cm or more.

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