KR20210037112A - High strength structural steel - Google Patents

Abstract

According to one aspect of the present invention, high-strength structural steel with excellent thickness-direction tensile properties comprises 0.13-0.15 wt.% of carbon (C), 0.3-0.4 wt.% of silicon (Si), 1.35-1.55 wt.% of manganese (Mn), 0-0.02 wt.% (excluding 0 wt.%) of phosphorus (P), 0-0.003 wt.% (excluding 0 wt.%) of sulfur (S), 0.015-0.055 wt.% of aluminum (Al), 0-0.0005 wt.% (excluding 0 wt.%) of boron (B), 0.05-0.15 wt.% of copper (Cu), 0.03-0.04 wt.% of niobium (Nb), 0.1-0.2 wt.% of nickel (Ni), 0-0.1 wt.% (excluding 0 wt.%) of chromium (Cr), 0.003-0.013 wt.% of titanium (Ti), 0-0.01 wt.% (excluding 0 wt.%) of vanadium (V), 0-0.08 wt.% (excluding 0 wt.%) of molybdenum (Mo), and the remainder consisting of iron (Fe) and inevitable impurities. The thickness-direction reduction of area (RA) is 35% or higher.

Description

High strength structural steel {HIGH STRENGTH STRUCTURAL STEEL}

The present invention relates to a high-strength steel material, and more particularly, to a high-strength structural steel having excellent tensile properties in the thickness direction.

In general, structural steels, especially building steels, are considered to be important in the properties of the longitudinal and transverse directions. However, in welded structural materials, if the thickness of the steel sheet is large and a large tensile stress is applied in the thickness direction, an internal structure destruction phenomenon called lamellar tearing may occur on the surface parallel to the rolled surface. Therefore, it is recommended to guarantee the tensile characteristics in the thickness direction. It is being asked.

The section reduction rate for the tensile test in the thickness direction of a structural 50kg class steel material with lamella tearing resistance is generally required to be guaranteed at least 20%, and in particular, EN standards require securing a section reduction rate of at least 35%.

In steel materials used for welding structures, lamellar tearing, that is, internal fracture, is likely to occur in severely deformed welds, and is mainly caused by non-metallic inclusions. In addition, sulfur (S) may be severely segregated, resulting in local concentration of sulfides. In order to guarantee the reduction rate of the section in the tensile test in the thickness direction, there are various protections such as macro pores and hydrogen embrittlement control, but research is focused on a method of controlling segregated inclusions in steel materials such as manganese sulfide (MnS). Is in progress.

Conventionally, as a method of controlling the shape of the inclusions, a method of adding calcium (Ca) to the alloy component to spheroidize the shape of the inclusions has been mainly used, but this method does not prevent the formation of the main inclusions, MnS itself, as well as sulfur. There is a point that it cannot prevent the tensile embrittlement property in the thickness direction due to the segregation of (S).

As a related technology, there is Korean Patent Publication No. 10-1827750 (registered on February 5, 2018, high-strength hot-rolled steel sheet and a method of manufacturing the same).

An object to be solved by the present invention is to provide a high-strength structural steel having excellent tensile properties in the thickness direction.

The high-strength structural steel having excellent tensile properties in the thickness direction according to an aspect of the present invention is, by weight, carbon (C): 0.13 to 0.15%, silicon (Si): 0.3 to 0.4%, manganese (Mn): 1.35 to 1.55 %, phosphorus (P): greater than 0 and less than 0.02%, sulfur (S): greater than 0 and less than 0.003%, aluminum (Al): 0.015 to 0.055%, boron (B): greater than 0 and less than 0.0005%, copper (Cu): 0.05 to 0.15%, niobium (Nb): 0.03 to 0.04%, nickel (Ni): 0.1 to 0.2%, chromium (Cr): more than 0 and less than 0.1%, titanium (Ti): 0.003 to 0.013%, vanadium (V) : More than 0 0.01% or less, Molybdenum (Mo): more than 0 0.08%, and residual iron (Fe) and other unavoidable impurities, and a thickness direction cross-sectional reduction ratio (RA) of 35% or more.

In one embodiment, the thickness direction section reduction ratio RA may be 65 to 75%.

In one embodiment, the steel is a thick steel plate of thickness: 83 ~ 100T, tensile strength (TS) of 500 ~ 570 MPa, yield strength (YS) of 350 ~ 420 MPa, and elongation (EL) of 20 ~ 30% Can represent.

In one embodiment, the steel material may have a shock absorption energy of 100J or more at a low temperature of -20°C, and a transition temperature of -40°C or less.

According to the present invention, it is possible to manufacture a steel sheet having a high thickness direction cross-sectional reduction rate (RA; 35% or more) through the control of the alloy component, and the cleanliness of the extremely thick material is secured by suppressing the formation of inclusions without the addition of other alloy components. In addition, when used in welded structures, it is possible to develop a steel material having resistance to lamellar tearing, which is useful in the steel industry for building structures.

Figure 1 is a graph showing the measurement of the cross-sectional reduction rate according to the content of sulfur (S).
2 is a graph showing the results of measuring the physical properties of the specimens of Comparative Examples.
3 is a graph showing the results of an impact simulation test on the specimens of Comparative Examples and Examples.
4 is a graph showing the results of a thickness direction toughness test on specimens of Comparative Examples and Examples.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the present invention. The present invention may be implemented in various different forms, and is not limited to the embodiments described herein. The same reference numerals are assigned to the same or similar components throughout the present specification. In addition, detailed descriptions of known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted.

The present invention aims to provide a steel material that can guarantee a reduction in thickness direction required in a structural steel by reducing the content of sulfur (S), which causes inclusions and segregation, to suppress the formation of MnS, which is the main inclusion.

High-strength steel with excellent tensile properties in the thickness direction

One aspect of the present invention, the high-strength steel having excellent tensile properties in the thickness direction is weight %, carbon (C): 0.13 ~ 0.15%, silicon (Si): 0.3 ~ 0.4%, manganese (Mn): 1.35 ~ 1.55%, phosphorus (P): more than 0 and less than 0.02%, sulfur (S): more than 0 and less than 0.003%, aluminum (Al): 0.015 to 0.055%, boron (B): more than 0 and less than 0.0005%, copper (Cu): 0.05 to 0.15 %, Niobium (Nb): 0.03 to 0.04%, Nickel (Ni): 0.1 to 0.2%, Chromium (Cr): more than 0 and less than 0.1%, Titanium (Ti): 0.003 to 0.013%, Vanadium (V): More than 0 0.01% or less, molybdenum (Mo): contains more than 0 and 0.08% or less.

Other than the above components, the rest is made of iron (Fe) and impurities that are inevitably contained in the steelmaking process.

Hereinafter, the role and content of each component included in the high-strength steel having excellent tensile properties in the thickness direction according to the present invention will be described as follows.

Carbon (C): 0.13 to 0.15% by weight

Carbon (C) is an element that affects the strength, toughness, and toughness of welds of steel. In addition, as an element that increases the hardenability of steel, it increases the fraction of pearlite by delaying ferrite transformation during cooling after hot finish rolling, thereby increasing not only yield strength but also tensile strength. However, if the content is less than 0.13% by weight of the entire steel sheet, it is possible to secure sufficient tensile strength through the addition of alloying elements, but it is difficult to secure the desired yield strength and elongation. Conversely, when the amount of carbon (C) exceeds 0.15% by weight, the possibility of cracking the slab due to the formation of primary ferrite is high, resulting in a decrease in toughness and weldability, and a higher percentage of pearlite phase, resulting in a desired microstructure. It becomes difficult to control. Therefore, it is preferable to add the content of carbon (C) in an amount of 0.13 to 0.15% by weight of the entire steel sheet.

Silicon (Si): 0.3 to 0.4% by weight

Silicon (Si) acts as a deoxidizing agent and is an element that works effectively for solid solution strengthening. In addition, it is effective in improving toughness and ductility of steel by inducing ferrite formation as a ferrite stabilizing element. In order to exert an effect, silicon (Si) should be added in an amount of 0.3% by weight or more. However, by generating a red scale in the heating furnace, when a large amount is added, it may give a problem of deteriorating the surface of the steel, and also has a problem of deteriorating weldability due to the generation of oxides. Therefore, it is preferable to limit the addition amount of silicon (Si) to 0.4% by weight or less of the total steel sheet.

Manganese (Mn): 1.35 ~ 1.55% by weight

Manganese (Mn) is a substitutional element having an atomic diameter similar to that of iron (Fe). It is very effective in solid solution strengthening and is an element effective in securing strength after heat treatment by improving the hardenability of steel. In addition, as an austenite stabilizing element, it can contribute to refinement of crystal grains of ferrite by delaying the transformation of ferrite and pearlite. If the amount of manganese (Mn) added is less than 1.35% by weight, the effect is insignificant, and if it is added in excess of 1.55% by weight, the carbon equivalent is increased to greatly reduce weldability, and MnS publications and central segregation in the slab/coil occur. As a result, the ductility and impact characteristics of the steel are greatly reduced. Therefore, the content of manganese (Mn) is preferably limited to 1.35 to 1.55% by weight of the entire steel sheet.

Phosphorus (P): more than 0 and not more than 0.02% by weight

In the case of phosphorus (P), there is a high probability of segregation during the manufacturing process of steel, and the segregation of phosphorus decreases toughness and causes delayed fracture that is destroyed after a certain period of time after forming. Therefore, the content of phosphorus (P) in the steel is preferably limited to 0.018% by weight or less.

Sulfur (S): more than 0 and 0.003% by weight or less

Sulfur (S) impairs the toughness and weldability of the steel and combines with manganese (Mn) to form MnS inclusions, thereby lowering the corrosion resistance and impact properties of the steel. In addition, MnS, the main inclusion, induces lamella tearing, an internal fracture phenomenon in steel used for welding structures, lowers the tensile strength in the thickness direction of the steel, and is the cause of tensile embrittlement in the thickness direction due to segregation of sulfur (S). do. Accordingly, in the present invention, the content of the sulfur (S) is limited to 0.003% by weight or less of the total steel sheet.

Aluminum (Al): 0.015 ~ 0.055% by weight

Aluminum (Al) acts as a deoxidizing agent, and it is preferable to add 0.015 to 0.055% by weight of the entire steel sheet. When aluminum (Al) is added in an amount of less than 0.015% by weight, the deoxidation effect is insignificant, and when it is added in an amount exceeding 0.055% by weight, it combines with nitrogen (N) present in the steel to form a coarse AlN-based nitride.

Boron (B): more than 0 and 0.0005% by weight or less

Boron (B) is an element that segregates at the austenite grain boundary and improves hardenability by suppressing the formation of ferrite, which is a soft structure, when cooled, but when it exceeds 0.0005% by weight, it has a problem of grain boundary brittleness, so its content should be less than 0.0005% by weight. It is desirable to limit it to.

Copper (Cu): 0.05 to 0.15% by weight

Copper (Cu) is a metal that has a low melting point of an alloy, and is an element that makes descaling difficult by melting at the grain boundaries of the steel sheet surface during hot rolling to create solid scale. Therefore, it is preferable to limit the content of copper (Cu) to 0.05 to 0.15% by weight.

Niobium (Nb): 0.03 to 0.04% by weight

Since niobium (Nb) combines with carbon to form carbides that affect the increase in strength, it is preferable to add 0.03 to 0.04% by weight of the entire steel sheet. When niobium (Nb) is added in an amount of less than 0.03% by weight, sufficient reinforcing effect cannot be obtained, and when it exceeds 0.04% by weight, manufacturing cost increases and it is difficult to secure ductility.

Nickel (Ni): 0.1 to 0.2% by weight

Nickel (Ni) is an element that increases toughness and promotes graphitization. The nickel (Ni) may be added in a content ratio of 0.1 to 0.2% by weight. When the content of nickel (Ni) is less than 0.1% by weight, the effect of adding nickel (Ni) is insufficient and the toughness decrease is large, and when it exceeds 0.2% by weight, it is uneconomical and causes residual austenite to occur, causing embrittlement.

Chromium (Cr): greater than 0 and less than or equal to 0.013% by weight

Chromium (Cr) is an element that improves hardenability and forms a downward effect on yield strength. In addition, chromium (Cr) is an element that has a high effect of inhibiting phase transformation at high temperatures of ferrite and pearlite, and when added to C-Mn steel as a ferrite stabilizing element, the diffusion of carbon is delayed due to the solute interference effect, thereby affecting particle size refinement. The chromium (Cr) is preferably added in an amount of 0.013% by weight or less of the total weight. If the content of chromium (Cr) is added in excess of 0.013% by weight of the total weight, it may give a problem that properties are deteriorated in terms of toughness and hardenability of the steel.

Titanium (Ti): 0.003 ~ 0.013% by weight

Titanium (Ti) generates Ti (C, N) precipitates having high high temperature stability, thereby preventing coarsening of austenite grains in the slab heating step, thereby improving the toughness of the steel material. When titanium (Ti) is added in an amount of less than 0.003% by weight, sufficient reinforcing effect cannot be obtained, and when it exceeds 0.013% by weight, it can reduce the impact and DWTT characteristics of steel by generating coarse precipitates, and manufacturing cost increases and ductility There is a difficulty in securing. Therefore, the titanium (Ti) is preferably added in an amount of 0.003 to 0.013% by weight of the entire steel sheet.

Vanadium (V): greater than 0 and less than or equal to 0.01% by weight

Vanadium (V) improves strength by bonding with carbon, but since it is an element that increases strength compared to cost, the present invention limits the amount to be added to 0.01% by weight or less.

Molybdenum (Mo): more than 0 and not more than 0.08% by weight

Molybdenum (Mo) is an element that improves strength by effectively acting on solid solution strengthening. However, as elements that improve hardenability, since the elongation is greatly reduced when a large amount is added, the content is limited to 0.08% by weight or less in the present invention.

The remaining component of the present invention is iron (Fe). However, in a typical manufacturing process, since unintended impurities may inevitably be mixed from the raw material or the surrounding environment, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.

The steel material according to the present invention having the above alloy composition is applied to a thick steel plate of thickness: 83 ~ 100T, tensile strength (TS) of 500 ~ 570 MPa, yield strength (YS) of 350 ~ 420 MPa, and 20 ~ 30% of It shows the elongation rate (EL). In addition, the shock absorption energy at a low temperature of -20°C is 100J or more, the transition temperature is -40°C or less, and the section reduction ratio (RA) indicating the tensile properties in the thickness direction is about 65 to 75. Accordingly, the steel material of the present invention satisfies 20% or more, which is a thickness direction section reduction rate required for a structural 50kg-class steel material, and in particular, can guarantee a thickness direction section reduction rate of 35% or more required by the EN standard.

A 50 kg-class structural steel can be manufactured by controlling the above-described component system and the process conditions described later. Hereinafter, a method of manufacturing a high-strength hot-rolled steel sheet that satisfies another aspect of the present invention, a resistance recovery ratio, will be described in detail.

Method for manufacturing high-strength steel with excellent tensile properties in the thickness direction

Another aspect of the present invention relates to a method of manufacturing a high-strength steel material having excellent tensile properties in the thickness direction. The method of manufacturing a high-strength steel material having excellent tensile properties in the thickness direction according to an aspect of the present invention includes the step of reheating a steel slab satisfying the alloy composition of the above-described steel material at a reheating temperature (SRT) of 1,125 to 1,175°C (S110), the heating. Hot-rolling the steel slab at a finish rolling temperature (FRT) of 800 to 840° C. (S120), and cooling the hot-rolled plate (S130).

Slab reheating step (S110)

In the slab reheating step (S110), the steel slab having the above alloy composition is reheated to re-use of segregated components during casting and re-use of precipitates. Specifically, in% by weight, carbon (C): 0.13 to 0.15%, silicon (Si): 0.3 to 0.4%, manganese (Mn): 1.35 to 1.55%, phosphorus (P): greater than 0 and 0.02% or less, sulfur ( S): more than 0 and 0.003% or less, aluminum (Al): 0.015 to 0.055%, boron (B): more than 0 and 0.0005% or less, copper (Cu): 0.05 to 0.15%, niobium (Nb): 0.03 to 0.04%, Nickel (Ni): 0.1 to 0.2%, chromium (Cr): more than 0 0.1%, titanium (Ti): 0.003 to 0.013%, vanadium (V): more than 0 0.01% or less, molybdenum (Mo): more than 0 0.08 % Or less, and the steel slab containing the remaining iron (Fe) and other impurities is heated at a slab reheating temperature (SRT) of 1,125°C to 1,175°C for 2 hours or more. The slab may be a steel slab manufactured by a continuous casting process performed before the slab reheating step (S110).

If the slab reheating temperature is less than 1,125°C, the heating temperature is insufficient, and thus there is a problem that the rolling load increases during hot rolling. In addition, there is a problem in that the austenite crystal grains rapidly coarsen because the precipitates do not reach the solid solution temperature and cannot be re-precipitated as fine precipitates during hot rolling, and thus the growth of austenite grains cannot be suppressed. In addition, when the reheating temperature exceeds 1,175°C, the austenite grains rapidly coarsen or decarburization occurs, making it difficult to secure the strength and low-temperature toughness of the manufactured steel.

Hot rolling step (S120)

After heating the slab as described above, hot rolling is performed on the heated slab (S120). It is preferable to perform the rolling temperature above the austenite recrystallization region. More preferably, the heated slab is subjected to finish rolling at an average reduction rate of 10 to 40% per pass, and finish rolling temperature (FRT): 800 to 840°C to obtain a rolled sheet material with a thickness of about 83 to 100T. Acquire. By rolling, the cast structure such as dendrite formed during casting is destroyed, and the effect of reducing the size of austenite can be obtained. In order to obtain these effects, the rolling temperature is preferably controlled to 800 ~ 840 ℃. If the finish rolling temperature exceeds 840°C, there is a concern that the quality of the steel sheet may deteriorate due to the occurrence of surface scale of the steel sheet. In addition, at a rolling temperature of less than 800°C, grains are refined to increase strength, but may cause an increase in rolling load and a decrease in productivity.

Cooling step (S130)

After the hot rolling, the rolled sheet is cooled to a predetermined temperature. Specifically, the rolled hot-rolled sheet is cooled at an average cooling rate of 30 to 100°C/sec. The cooling may be air cooling or water cooling. In order to control the internal structure of the steel sheet, it is important to control the cooling rate. At a cooling rate of less than 30℃/sec, it is not sufficiently cooled, and there is a possibility of causing scale generated at high temperatures. There is a disadvantage that it is difficult to secure the flatness of the plate at the cooling rate.

The steel sheet manufactured under the above alloy composition and process conditions is a structural thick steel sheet of thickness: 83 to 100T, tensile strength (TS) of 500 to 570 MPa, yield strength (YS) of 350 to 420 MPa, and elongation of 20 to 30%. It represents (EL). In addition, it exhibits a high impact absorption energy of 100J or more at a low temperature of -20°C, the transition temperature is as low as -40°C or less, and a reduction ratio (RA) of about 65 to 75, which indicates tensile properties in the thickness (Z) direction. Accordingly, the steel material of the present invention satisfies 20% or more, which is a thickness direction section reduction rate required for a structural 50kg-class steel material, and in particular, can guarantee a thickness direction section reduction rate of 35% or more required by the EN standard.

1 is a graph showing the measurement of the cross-sectional reduction rate according to the content of sulfur (S) in order to determine the maximum sulfur (S) content for securing the cross-sectional reduction rate.

Referring to FIG. 1, when the content of sulfur (S) is 30 ppm or more, it is difficult to control the section reduction ratio (RA) in the thickness direction of 35% or more required for structural steel materials in EN standards to 35% or more, and the content of sulfur (S) is 30 ppm. Hereinafter, it can be seen that the generation of MnS inclusions can be suppressed and the internal cleanliness can be improved, thereby securing a reduction ratio in the cross section in the ErP direction.

Hereinafter, the present invention will be described in detail through examples, but these are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the scope of description of these examples. Contents not described herein can be sufficiently technically inferred by those skilled in this technical field, and thus description thereof will be omitted.

Example

Steel slab specimens of Examples and Comparative Examples having the component composition of Table 1 (wt%, the remainder being Fe and inevitable impurities) were prepared, and slab reheating, hot rolling, and cooling were performed for each specimen.

ingredient C Si Mn P S Al B Comparative example 0.138 0.338 1.415 0.0125 0.0035 0.04 0.0005 Example 0.132 0.352 1.399 0.0145 0.0025 0.033 0.0005 ingredient Cu Nb Cr Ti V Mo Comparative example 0.116 0.035 0.03 0.009 - - Example 0.091 0.033 0.03 0.011 - -

In particular, as shown in Table 1, the sample of Comparative Example had a content of sulfur (S) of 0.0035% by weight, and the sample of Example had a content of sulfur (S) of 0.0025% by weight.

Subsequently, yield strength (YP), tensile strength (TS), and elongation (EL) were measured for the specimens of Comparative Examples and Examples, respectively, and the results are shown in the graph of FIG. And shown in the graph of FIG. 4.

2 is a graph showing the results of measuring the physical properties of the specimens of Comparative Examples, Figure 3 is a graph showing the results of the impact simulation test on the specimens of Comparative Examples and Examples, and Figure 4 is a specimen of Comparative Examples and Examples It is a graph showing the result of performing the toughness test in the thickness direction for.

2, in the case of tensile strength (TS), compared to the comparative example, a slightly reduced value was shown in the specimen of the Example, and the yield strength (YS) and the elongation (EL) were similar in the Comparative Example and the Example. .

Referring to FIG. 3 showing the impact test result, at 0°C, the comparative example and the example showed similar values, but the higher the temperature (-40°C), the higher the absorbed energy value in the sample of the example.

And, referring to FIG. 4 showing the results of the tensile test in the thickness direction, the specimen of the example exhibited a higher value than that of the specimen of the comparative example. This result can be said that by limiting the content of sulfur (S) in the alloy component to 30 ppm or less, the generation of MnS inclusions is suppressed and the internal cleanliness of the steel material is improved, thereby increasing the reduction in section in the thickness direction.

According to the present invention described above, a steel sheet having a high thickness direction cross-sectional reduction ratio (RA; 35% or more) can be manufactured through the control of the alloy component. Therefore, it is possible to develop a steel material having resistance to lamellar tearing when used in a welded structure, as well as securing cleanliness for extremely thick materials by suppressing the formation of inclusions without the addition of other alloy components such as calcium (Ca). Useful for the steel industry.

In the above, the embodiments of the present invention have been described mainly, but various changes or modifications may be made at the level of those skilled in the art. These changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims (4)

In% by weight, carbon (C): 0.13 to 0.15%, silicon (Si): 0.3 to 0.4%, manganese (Mn): 1.35 to 1.55%, phosphorus (P): more than 0 0.02% or less, sulfur (S): More than 0 0.003% or less, Aluminum (Al): 0.015 to 0.055%, Boron (B): more than 0 0.0005% or less, Copper (Cu): 0.05 to 0.15%, Niobium (Nb): 0.03 to 0.04%, Nickel (Ni ): 0.1 to 0.2%, chromium (Cr): more than 0 and less than 0.1%, titanium (Ti): 0.003 to 0.013%, vanadium (V): more than 0 and less than 0.01%, molybdenum (Mo): more than 0 and less than 0.08%, And residual iron (Fe) and other inevitable impurities,
Characterized in that the thickness direction section reduction rate (RA) is 35% or more,
Structural steel. The method of claim 1,
It characterized in that the thickness direction section reduction rate (RA) is 65 to 75%,
Structural steel. The method of claim 1,
The steel is a thick steel plate of thickness: 83 ~ 100T,
Characterized in that it exhibits a tensile strength (TS) of 500 to 570 MPa, a yield strength (YS) of 350 to 420 MPa, and an elongation (EL) of 20 to 30%,
Structural steel. The method of claim 1,
The steel material is characterized in that the shock absorption energy at a low temperature of -20 ℃ is more than 100J, the transition temperature is -40 ℃ or less,
Structural steel.

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