Extra thick steel plate with excellent strength and low-temperature impact toughness and manufacturing method thereof

Abstract

The present invention provides an extra thick steel plate that can ensure both excellent strength and low temperature toughness, in an extra thick steel plate to which a normalizing heat treatment method is applied, in which the plate is heat treated after rolling and then air cooled. [Solution] The steel sheet is characterized in that it contains, by weight, 0.06-0.1% C, 0.3-0.5% Si, 1.35-1.65% Mn, 0.015-0.04% Al, 0.015-0.04% Nb, 0.15-0.4% Cr, 0.005-0.02% Ti, 0.3-0.5% Ni, 0.002-0.008% N, 0.01% or less P, 0.003% or less S, with the balance being Fe and other unavoidable impurities, and the microstructure is composed of, by area fraction, more than 80% polygonal ferrite having an average grain size of 40 μm or less, and the balance being pearlite having an average grain size of 20 μm or less.

Description

[Technical Field]

[0001] The present invention relates to an extra-thick steel plate having excellent strength and low-temperature impact toughness and a manufacturing method thereof, and more particularly to an extra-thick steel plate having excellent strength and low-temperature impact toughness and applicable to various industries such as various frames of ships and marine structures, infrastructure industry materials such as bridges and construction, and materials for wind power substructures, and a manufacturing method thereof. [Background technology]

[0002] In recent years, a characteristic of most infrastructure and energy industries is that structures are becoming larger due to the minimization of installation costs and the deterioration of installation environments, and they are gradually being moved to cold regions and polar regions.

[0003] In line with these trends toward larger structures and colder climates, it is expected that demand for extra-heavy steel materials over 100 mm thick will increase among the structural steel materials used in all industrial fields, and it will be necessary to ensure stable low-temperature impact toughness.

[0004] However, metallurgical disadvantages of extra-heavy steel are that it is difficult to achieve high strength due to reduced rolling and cooling limitations. Adding excessive alloying elements to achieve strength not only increases costs but also rapidly reduces toughness. Low-temperature rolling to compensate for the reduced toughness is limited by product specifications, and removing alloying elements that adversely affect toughness causes a decrease in strength.

[0005] Methods for producing extra-thick steel plates include a general rolling method, a thermomechanical controlled process (TMCP) rolling method, a method in which heat treatment is performed after rolling and then quenching, and a normalizing method in which heat treatment is performed after rolling and then air cooling.

[0006] The general rolling method is a method of rolling without controlling the rolling temperature, and is mainly applied to general steels that do not require impact toughness, so there are limitations to its application to steel materials that require low-temperature impact toughness.

[0007] The thermomechanical controlled rolling method performs temperature control to roll in the recrystallized region and the unrecrystallized region, and ensures strength and impact toughness by cooling as needed. However, the long waiting time required to adjust the rolling temperature to produce extra-thick steel plates can lead to a serious decrease in productivity.

[0008] The method of heat treating and quenching after rolling uses a high carbon component containing 0.12% or more C by weight, which results in a significant decrease in toughness and is problematic in terms of the cost of the heat treatment.

[0009] The normalizing method, which involves rolling, heat treatment, and air cooling, ensures grain refinement through reheat treatment after normalizing with a relatively high C content, but there is a risk of a decrease in impact toughness due to the formation of a large amount of pearlite.

[0010] The reality is that development of extra-thick steel plates with a thickness of 100 mm or more is necessary to solve these problems and ensure both excellent strength and low-temperature toughness. Summary of the Invention [Problem to be solved by the invention]

[0011] An object of the present invention is to provide an extra heavy steel plate to which a normalizing heat treatment method is applied, in which heat treatment is performed after rolling and air cooling is performed, which can ensure both excellent strength and low-temperature toughness. [Means for solving the problem]

[0012] As a means for achieving the above-mentioned object, an extra heavy steel plate according to the present invention may contain, by weight%, C: 0.06 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.35 to 1.65%, Al: 0.015 to 0.04%, Nb: 0.015 to 0.04%, Cr: 0.15 to 0.4%, Ti: 0.005 to 0.02%, Ni: 0.3 to 0.5%, N: 0.002 to 0.008%, P: 0.01% or less, S: 0.003% or less, with the balance being Fe and other unavoidable impurities, and the microstructure may be composed of, by area fraction, more than 80% polygonal ferrite having an average grain size of 40 μm or less and the balance being pearlite having an average grain size of 20 μm or less.

[0013] The extra thick steel plate of the present invention may have a value of the following formula (1) of 3.6 or more.

[0014] (1)[Mn]+5([Ni]+[Cr])

[0015] In the formula (1), [Mn], [Ni], and [Cr] represent the weight percentage of each element.

[0016] In the extra thick steel plate of the present invention, the thickness t of the entire steel plate may be 100 to 200 mm, the yield strength at 1 / 4t from the outermost surface may be 320 MPa or more, and the impact toughness energy value at -60 to -40°C may be 200 J or more.

[0017] As another means for achieving the above-mentioned object, the method for producing an extra heavy steel plate of the present invention comprises, by weight %, C: 0.06 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.35 to 1.65%, Al: 0.015 to 0.04%, Nb: 0.015 to 0.04%, Cr: 0.15 to 0.4%, Ti: 0.005 to 0.02%, Ni: 0.3 to 0.5%, N: 0.002 to 0.008%, P: 0.01 The method may include a step of reheating a slab containing 0.003% or less of S and the balance being Fe and other unavoidable impurities, a step of rough rolling the reheated slab with a reduction of 70 to 120 mm so that the residual reduction is 25 to 35%, followed by hot rolling including finish rolling, a step of normalizing heat treatment, and a step of air cooling, and the thickness t of the entire steel plate may be 100 to 200 mm.

[0018] In the method for producing an extra thick steel plate of the present invention, the finish rolling may be performed with a reduction of 70 to 110 mm.

[0019] In the method for producing an extra thick steel plate of the present invention, the rough rolling may be carried out at 1000°C or higher.

[0020] In the method for producing an extra thick steel plate of the present invention, the finish rolling may be carried out at a temperature of 850 to A3°C.

[0021] In the method for producing an extra thick steel plate of the present invention, the finishing temperature of the hot rolling may be 820 to 910°C. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide an extra-thick steel plate having a thickness of 100 to 200 mm and having excellent strength and impact toughness through control of an alloy composition based on a low C component system and control of manufacturing conditions such as rolling reduction, and a manufacturing method thereof. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a photograph of the microstructure of an extra-thick steel plate according to an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] In order to achieve the above-mentioned object, the extra heavy steel plate of the present invention may contain, by weight%, C: 0.06 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.35 to 1.65%, Al: 0.015 to 0.04%, Nb: 0.015 to 0.04%, Cr: 0.15 to 0.4%, Ti: 0.005 to 0.02%, Ni: 0.3 to 0.5%, N: 0.002 to 0.008%, P: 0.01% or less, S: 0.003% or less, and the balance being Fe and other unavoidable impurities, and the microstructure may be composed of, by area fraction, more than 80% polygonal ferrite having an average grain size of 40 μm or less and the balance being pearlite having an average grain size of 20 μm or less.

[0025] Preferred embodiments of the present invention will be described below. However, the embodiments of the present invention may be modified in various different forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0026] The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Furthermore, it should be noted that the terms "comprise" or "comprises" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0027] On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted in an overly ideal or formal sense. For example, in this specification, the singular expression includes the plural expression unless there is a clear exception in the context.

[0028] Furthermore, in this specification, when tolerances for manufacturing and materials inherent in the meanings mentioned are presented, terms such as "about," "substantially," and the like are used to mean a numerical value or a value close to that numerical value, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosure content in which precise and absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0029] In this specification, the term "average crystal grain size" refers to the equivalent circular diameter (ECD) of a crystal grain.

[0030] Steel that has undergone normalizing heat treatment has a higher carbon content than steel that has undergone controlled rolling, which involves controlled rolling and cooling to ensure strength, and therefore tends to have poor impact toughness even after heat treatment. In addition, if the heat treatment temperature is too high or the treatment time is too long, grain growth can cause a decrease in strength compared to the steel sheet in the rolled state before heat treatment.

[0031] The present invention aims to ensure excellent strength and impact toughness by controlling the alloy composition based on a low C component system and by controlling the rolling conditions.

[0032] The extra thick steel plate of the present invention contains, by weight, C: 0.06 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.35 to 1.65%, Al: 0.015 to 0.04%, Nb: 0.015 to 0.04%, Cr: 0.15 to 0.4%, Ti: 0.005 to 0.02%, Ni: 0.3 to 0.5%, N: 0.002 to 0.008%, P: 0.01% or less, S: 0.003% or less, and the balance being Fe and other unavoidable impurities.

[0033] The reasons for limiting the chemical composition of the extra thick steel plate will be specifically explained below.

[0034] The C content is 0.06 to 0.1 wt%.

[0035] C is a solid solution strengthening element that bonds with Nb and other elements in steel to form carbides, improving tensile strength. Therefore, in the present invention, C may be added in an amount of 0.06 wt% or more. However, excessive C content can lead to excessive pearlite formation, resulting in poor impact and fatigue properties at low temperatures. Furthermore, as the solute C content increases, impact properties also decrease. In consideration of this, the upper limit of the C content in the present invention may be limited to 0.1 wt%. More preferably, the C content may be 0.07 to 0.09 wt%.

[0036] The Si content is 0.3 to 0.5 wt %.

[0037] Si, together with Al, plays a role in deoxidizing molten steel and improving yield strength and tensile strength, so in the present invention, Si may be added in an amount of 0.3 wt% or more. However, excessive Si content can hinder the diffusion of C and promote the formation of island martensite (Martensite Austenite Constituent, MA), which may result in poor impact properties and fatigue properties at low temperatures. In consideration of this, in the present invention, the upper limit of the Si content may be limited to 0.5 wt%.

[0038] The Mn content is 1.35 to 1.65 wt%.

[0039] Mn is a solid solution strengthening element, and in the present invention, Mn may be added in an amount of 1.35 wt% or more. However, an excessive Mn content may cause a decrease in toughness due to the formation of MnS inclusions and central segregation, so in the present invention, the upper limit of the Mn content may be limited to 1.65 wt%.

[0040] The Al content is 0.015 to 0.04 wt%.

[0041] Al acts as a main deoxidizer for steel and may be added in an amount of 0.015 wt% or more to fix N. However, excessive Al content may decrease low-temperature toughness due to an increase in the fraction and size of Al2O3 inclusions, and may induce the formation of island martensite in the base metal and weld heat-affected zone, which may degrade impact properties and fatigue properties at low temperatures. In consideration of this, the upper limit of the Al content in the present invention may be limited to 0.04 wt%.

[0042] The Nb content is 0.015 to 0.04 wt%.

[0043] Nb strengthens the solid solution or precipitates carbides, thereby suppressing recrystallization during rolling or cooling and refining the structure, resulting in increased strength. To ensure strength, 0.015 wt% or more of Nb may be added in the present invention. However, excessive Nb content may promote the formation of island martensite due to its carbon affinity, which may result in poor impact and fatigue properties at low temperatures. In consideration of this, the upper limit of the Nb content in the present invention may be limited to 0.04 wt%.

[0044] The Cr content is 0.15 to 0.4 wt%.

[0045] Cr is an element that is advantageous for increasing the hardenability of steel and improving its strength, so in the present invention, Cr may be added in an amount of 0.15 wt% or more. However, excessive Cr content not only reduces weldability but also increases manufacturing costs as an expensive element. In consideration of this, the upper limit of the Cr content in the present invention may be limited to 0.4 wt%.

[0046] The Ti content is 0.005 to 0.02 wt%.

[0047] Ti combines with solute N to form TiN, which can degrade impact properties and surface quality. Furthermore, the formed TiN suppresses coarsening of the microstructure and contributes to refinement, thereby improving toughness. Taking this into consideration, Ti may be added in an amount of 0.005 wt% or more in the present invention. However, excessive Ti content can cause coarsening of precipitates, which can lead to fracture. Furthermore, Ti that does not combine with N may remain in the steel and form TiC, which reduces the toughness of the base metal and weld. Taking this into consideration, the upper limit of the Ti content in the present invention may be limited to 0.02 wt%.

[0048] The Ni content is 0.3 to 0.5 wt%.

[0049] Since Ni is an element that can improve both strength and toughness, Ni may be added in an amount of 0.3 wt% or more in the present invention. However, if the Ni content is excessive, the effect of improving strength and toughness becomes saturated and there is a problem that the manufacturing cost increases. Therefore, in the present invention, the upper limit of the Ni content may be limited to 0.5 wt%.

[0050] The N content is 0.002 to 0.008 wt%.

[0051] N forms precipitates together with Ti, Nb, Al, etc., and refines the austenite structure during reheating, resulting in improved strength and toughness. Taking this into consideration, N may be added in an amount of 0.002 wt% or more in the present invention. However, excessive N content may induce surface cracks at high temperatures and remain in the steel as solid solution N, potentially reducing toughness. Therefore, the upper limit of the N content in the present invention may be limited to 0.008 wt%.

[0052] The P content is 0.01% by weight or less.

[0053] P is an element that is inevitably contained in steel during the manufacturing process and induces grain boundary segregation, thereby embrittling the steel. In consideration of this, the upper limit of the P content in the present invention is preferably limited to 0.01 wt%.

[0054] The S content is 0.003% by weight or less.

[0055] S is an element that is inevitably contained in steel during the manufacturing process, and it combines with Mn to form MnS, which reduces low-temperature toughness. In consideration of this, the upper limit of the S content in the present invention is preferably limited to 0.003 wt%.

[0056] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, not all of the contents of these impurities are specifically mentioned in this specification.

[0057] The extra thick steel plate of the present invention can satisfy the above-mentioned alloy composition and the value of the following formula (1) of 3.6 or more.

[0058] (1)[Mn]+5([Ni]+[Cr])

[0059] In the formula (1), [Mn], [Ni], and [Cr] represent the weight percentage of each element.

[0060] In order for an extra thick steel plate having a thickness of 100 to 200 mm to satisfy the desired strength and impact toughness when the C content is 0.1 wt % or less, the value of formula (1) is preferably 3.6 or more.

[0061] The microstructure of the extra heavy steel plate according to one embodiment of the present invention may be composed of, in area fraction, more than 80% polygonal ferrite and the remainder pearlite. More preferably, the microstructure may be composed of, in area fraction, more than 80% to 90% polygonal ferrite and 10% to less than 20% pearlite.

[0062] In order to ensure strength, a fine microstructure is advantageous. For example, the average grain size of polygonal ferrite may be 40 μm or less, and the average grain size of pearlite may be 20 μm or less.

[0063] The attached Figure 1 is a photograph of the microstructure of an extra-thick steel plate according to one example of the present invention. Referring to Figure 1, it can be seen that polygonal ferrite with an average grain size of 40 μm or less is distributed at an area fraction of 80 to 90%, and pearlite with an average grain size of 20 μm is distributed at an area fraction of 10 to 20%. Referring to Figure 1, it can be seen that pearlite is spheroidized by carbon diffusion and formed at the grain boundaries and within the grains.

[0064] The extra heavy steel plate according to the present invention has excellent yield strength and low-temperature impact toughness. In one example, the extra heavy steel plate may have an overall thickness t of 100 to 200 mm, a yield strength of 320 MPa or more at ¼ t from the outermost surface, and an impact toughness energy value of 200 J or more at −60 to −40°C.

[0065] The method for producing an extra thick steel plate according to the present invention will be described in detail below.

[0066] The method for producing an extra thick steel plate of the present invention may include the steps of reheating a slab having the above-mentioned alloy composition, hot rolling it, normalizing heat treatment, and air cooling it.

[0067] First, a slab satisfying the above-mentioned alloy composition may be reheated at 1020 to 1150°C. If the reheating temperature is less than 1020°C, Ti, Nb, etc. may not be fully dissolved, which may result in a decrease in strength. On the other hand, if the reheating temperature exceeds 1150°C, austenite grains may become coarse, which may result in a decrease in toughness.

[0068] Hot rolling is carried out in the order of rough rolling and finish rolling. Rough rolling is performed at a recrystallization temperature of 1000°C or higher, and finish rolling is performed at a non-recrystallization temperature of 850 to A3°C. It is advantageous to perform rolling at a temperature close to the A3°C temperature to achieve a grain refinement effect, but it may also be performed at 850°C or higher in consideration of productivity. The A3°C temperature varies depending on the steel type, but is generally around 910°C. The finish temperature for hot rolling is preferably 820 to 910°C.

[0069] Furthermore, the present invention relates to extra heavy steel plates having an overall thickness of 100 to 200 mm, and since the total reduction amount of hot rolling is not large at about 200 mm, the allocation of passes between rough rolling and finish rolling is important. According to the present invention, rough rolling may be performed with a reduction amount of 70 to 120 mm so that the remaining reduction amount is 25 to 35%. Here, the remaining reduction amount is the percentage of the total reduction amount that can be used for finish rolling to reduce the product to the final thickness after rough rolling. Finish rolling may be performed with a reduction amount of 70 to 110 mm. In the above-mentioned rolling process, the extra heavy steel plate according to the present invention has a thickness of 100 to 200 mm.

[0070] The hot-rolled steel sheet may be subjected to a normalizing heat treatment. For example, the normalizing heat treatment may be performed by heating the steel sheet to 880 to 920°C and then maintaining the temperature range for 200 to 300 minutes. The normalized steel sheet is then air-cooled to be manufactured into a final product.

[0071] The present invention will be described in more detail through the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

[0072] {Example} Molten steel having the alloy composition shown in Table 1 below was continuously cast to produce a slab. The produced slab was reheated, rough rolled, finish rolled, normalizing heat treatment, and air-cooled under the production conditions shown in Table 2 below to produce an extra heavy steel plate with a thickness of 100 to 200 mm. The value of formula (1) in Table 2 is a value derived by substituting the alloy composition shown in Table 1. In Comparative Example 4 in Table 2, the normalizing heat treatment was omitted, and the extra heavy steel plate was produced by air-cooling immediately after rolling.

[0073] [Table 1]

[0074] [Table 2]

[0075] Table 3 shows the results of measuring the microstructure and physical properties of the manufactured extra thick steel plates.

[0076] In Table 3, "Impact (-40°C)" and "Impact (-60°C)" refer to the impact toughness energy values ​​at -40°C and -60°C, respectively. The yield strength, tensile strength, and impact toughness energy values ​​refer to the physical property values ​​at 1 / 4t from the outermost surface, where t is the total thickness of the manufactured steel plate.

[0077] [Table 3]

[0078] Referring to Tables 1 to 3, it can be seen that the examples of the invention that satisfy the alloy composition and manufacturing conditions defined in the present invention all have a yield strength of 320 MPa or more at 1 / 4t from the outermost surface, where t is the total thickness of the manufactured steel plate, and an impact toughness energy value of 200 J or more at -60°C to -40°C, thereby demonstrating excellent yield strength and low-temperature impact toughness.

[0079] In Comparative Example 1, the C content was excessive, which resulted in excessive pearlite formation, and although the strength increased, the low-temperature impact toughness rapidly deteriorated.

[0080] In Comparative Example 2, the value of formula (1) was less than 3.6, and sufficient strength could not be ensured.

[0081] In Comparative Example 3, the alloy composition defined in the present invention was satisfied, but the finish rolling temperature was too high compared to the temperature range defined in the present invention, resulting in coarse ferrite grains and a deterioration in strength and low-temperature impact toughness.

[0082] Comparative Example 4 satisfied the alloy composition and formula (1) defined by the present invention, but was not subjected to normalizing heat treatment, and although the strength was satisfactory, the low-temperature impact toughness deteriorated.

[0083] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0084] The extra thick steel plate according to an embodiment of the present invention can ensure excellent strength and impact toughness by controlling the alloy composition based on a low carbon component system and the manufacturing process such as the reduction rate.

Claims

1. The alloy contains, by weight, C: 0.06 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.35 to 1.65%, Al: 0.015 to 0.04%, Nb: 0.015 to 0.04%, Cr: 0.15 to 0.4%, Ti: 0.005 to 0.02%, Ni: 0.3 to 0.5%, N: 0.002 to 0.008%, P: 0.01% or less, S: 0.003% or less, and the balance being Fe and other unavoidable impurities; The ultra-thick steel plate is characterized in that the microstructure is composed of, in terms of area fraction, more than 80% polygonal ferrite having an average grain size of 40 μm or less, and the remainder being pearlite having an average grain size of 20 μm or less.

2. 2. The extra thick steel plate according to claim 1, wherein the value of the following formula (1) is 3.6 or more: (1) [Mn] + 5 ([Ni] + [Cr]) (In the formula (1), [Mn], [Ni], and [Cr] represent the weight percentage of each element.)

3. The extra-thick steel plate according to claim 1, characterized in that the thickness t of the entire steel plate is 100 to 200 mm, the yield strength at 1 / 4t from the outermost surface is 320 MPa or more, and the impact toughness energy value at -60 to -40 ° C. is 200 J or more.

4. reheating a slab containing, by weight, 0.06-0.1% C, 0.3-0.5% Si, 1.35-1.65% Mn, 0.015-0.04% Al, 0.015-0.04% Nb, 0.15-0.4% Cr, 0.005-0.02% Ti, 0.3-0.5% Ni, 0.002-0.008% N, 0.01% or less P, and 0.003% or less S, with the balance being Fe and other unavoidable impurities; a hot rolling step in which the reheated slab is rough rolled at a reduction of 70 to 120 mm so that the residual reduction is 25 to 35%, and then finish rolled; A normalizing heat treatment step; and air cooling, The thickness t of the entire steel plate is 100 to 200 mm, A method for manufacturing an extra-thick steel plate, characterized in that the microstructure is composed of more than 80% polygonal ferrite having an average grain size of 40 μm or less in area fraction, and the remainder pearlite having an average grain size of 20 μm or less.

5. The method for producing an extra thick steel plate according to claim 4, wherein the finish rolling is performed with a reduction of 70 to 110 mm.

6. The method for producing an extra thick steel plate according to claim 4, wherein the rough rolling is performed at 1000°C or higher.

7. The method for producing an extra thick steel plate according to claim 4, characterized in that the finish rolling is performed at a start temperature of 850 to A3°C.

8. The method for producing an extra thick steel plate according to claim 4, characterized in that the finish start temperature of the hot rolling is 820 to 910°C.

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