KR20200062467A - Hot-rolled steel sheet for earthquake-resistant steel pipe and method for manufacturing the same - Google Patents

Abstract

An objective of the present invention is to provide a high-strength hot-rolled steel sheet with excellent economic efficiency and a manufacturing method thereof. According to an aspect of the present invention, the manufacturing method of a high-strength hot-rolled steel sheet comprises: a step of heating a steel slab comprising 0.04-0.07 wt% of carbon (C), 0-0.03 wt% (excluding 0 wt%) of silicon (Si), 1.3-1.5 wt% of manganese (Mn), 0-0.018 wt% (excluding 0 wt%) of phosphorus (P), 0-0.005 wt% (excluding 0 wt%) of sulfur (S), 0.01-0.05 wt% of aluminum (Al), 0.11-0.13 wt% of titanium (Ti), 0.02-0.03 wt% of niobium (Nb), 0-0.01 wt% (excluding 0 wt%) of vanadium (V), 0-0.06 wt% (excluding 0 wt%) of nitrogen (N), 0-0.1 wt% (excluding 0 wt%) of chromium (Cr)+molybdenum (Mo)+nickel (Ni), 0-0.1 wt% (excluding 0 wt%) of copper (Cu)+tin (Sn), and the balance iron (Fe) and inevitable impurities to 1,140-1,200°C; a step of hot-rolling the heated steel slab at 870-910°C to obtain a hot-rolled sheet; and a step of cooling the hot-rolled sheet to 570-610°C at an average cooling rate of 10-30°C and then winding the hot-rolled sheet.

Description

Hot rolled steel sheet and its manufacturing method{HOT-ROLLED STEEL SHEET FOR EARTHQUAKE-RESISTANT STEEL PIPE AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel sheet and a method of manufacturing the same, and more particularly, to a hot rolled steel sheet having a tensile strength of 700 MPa or more through hot rolling and a method of manufacturing the same.

Recently, at construction sites, the demand for reducing construction time and transportation costs is increasing through the application of high-strength materials to increase the economical efficiency of construction, and the reduction of construction time and transportation cost through weight reduction of components. In the case of the existing high-strength hot-rolled steel sheet, niobium (Nb) and vanadium (V) precipitates are used as a main strength securing mechanism. In order to manufacture a 70 kg-class hot-rolled steel sheet through this method, the manufacturing cost is increased due to the addition of a large amount of ferroalloy. Becomes. In order to solve this problem, it is necessary to develop a hot-rolled material having a tensile strength of 70 kg and an economical cost for alloy iron.

As a related technology, there is Korean Patent Registration No. 10-1827750 (registered on Feb. 2018, high-strength hot-rolled steel sheet and manufacturing method thereof).

The problem to be solved by the present invention is to provide a 70kg class hot rolled steel sheet having excellent economic efficiency and a manufacturing method thereof by replacing expensive alloy elements with other components.

High-strength hot-rolled steel sheet according to an aspect of the present invention, by weight, carbon (C): 0.04 ~ 0.07%, silicon (Si): more than 0 0.03% or less, manganese (Mn): 1.3 ~ 1.5%, phosphorus (P ): more than 0 0.018% or less, sulfur (S): more than 0 0.005% or less, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.11 to 0.13%, niobium (Nb): 0.02 to 0.03%, vanadium (V): more than 0 and less than 0.01%, nitrogen (N): more than 0 and less than 006%, including residual iron (Fe) and other unavoidable impurities, sum of chromium (Cr), molybdenum (Mo), nickel (Ni) It is characterized in that the content is more than 0 and 0.1% or less, and the total content of copper (Cu) and tin (Sn) is more than 0 and 0.1% or less.

In the present invention, the hot-rolled steel sheet may have a tensile strength (TS): 700 MPa or more, a yield strength (YP): 550 MPa or more, and an elongation (EL): 20% or more.

Method for manufacturing a high-strength hot-rolled steel sheet according to another aspect of the present invention, by weight%, carbon (C): 0.04 ~ 0.07%, silicon (Si): more than 0 0.03% or less, manganese (Mn): 1.3 ~ 1.5%, Phosphorus (P): more than 0 and 0.018% or less, sulfur (S): more than 0 and 0.005% or less, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.11 to 0.13%, niobium (Nb): 0.02 to 0.03 %, Vanadium (V): more than 0 and less than 0.01%, nitrogen (N): more than 0 and 006% or less, chromium (Cr) + molybdenum (Mo) + nickel (Ni): more than 0 and 0.1% or less, copper (Cu) + Tin (Sn): heating the steel slab containing residual iron (Fe) and other unavoidable impurities to a temperature of 1,140 to 1,200° C., more than 0 and 0.1% or less; Hot-rolling the heated steel slab at 870-910°C to obtain a hot-rolled sheet material; And cooling the hot rolled sheet to 570 to 610°C at an average cooling rate of 10 to 30°C, and then winding it up.

In the present invention, the hot-rolled steel sheet may have a tensile strength (TS): 700 MPa or more, a yield strength (YP): 550 MPa or more, and an elongation (EL): 20% or more.

According to the present invention, it is very economical because it can secure the tensile strength of 700MPa or more while limiting the addition amount of niobium (Nb) and vanadium (V) for increasing the strength compared to the cost and using the precipitation hardening element titanium (Ti). It is advantageous.

1 is a process flow chart schematically showing a method of manufacturing a high strength hot rolled steel sheet according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art to which the present invention pertains can easily practice. The present invention can be implemented in many different forms, and is not limited to the embodiments described herein. Throughout this specification, the same reference numerals are assigned to the same or similar components. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the subject matter of the present invention are omitted.

As a result of repeated research, the inventors of the present invention limit the addition amount of niobium (Nb) and vanadium (V) having a low strength increase compared to the cost among alloying elements, and add titanium (Ti) to reduce the manufacturing cost. A component system capable of securing tensile strength could be set.

Hereinafter, the high-strength hot-rolled steel sheet of the present invention will be described in detail.

High strength hot rolled steel sheet

High-strength hot-rolled steel sheet, which is one aspect of the present invention, by weight, carbon (C): 0.04 to 0.07%, silicon (Si): more than 0 to 0.03% or less, manganese (Mn): 1.3 to 1.5%, phosphorus (P) : More than 0 0.018% or less, sulfur (S): more than 0 0.005% or less, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.11 to 0.13%, niobium (Nb): 0.02 to 0.03%, vanadium ( V): more than 0 and less than 0.01%, nitrogen (N): more than 0 and less than 006%, chromium (Cr) + molybdenum (Mo) + nickel (Ni): more than 0 and less than 0.1%, copper (Cu) + tin (Sn) : It contains more than 0 and 0.1% or less.

The rest of the above components are made of iron (Fe) and impurities which are inevitably contained in the steelmaking process.

Hereinafter, the role and content of each component included in the high-strength hot-rolled steel sheet that satisfies the resistivity ratio according to the present invention will be described.

Carbon (C): 0.04 to 0.07% by weight

Carbon (C) is an element that affects the strength, toughness and toughness of welds. In addition, as an element to increase the hardenability of the steel material, by increasing the fraction of pearlite by delaying the ferrite transformation upon cooling after hot finish rolling, the tensile strength as well as the yield strength is increased. However, if the content is less than 0.04% by weight of the entire steel sheet, it is possible to secure sufficient tensile strength through the addition of alloy elements, etc., but it is difficult to secure the desired yield strength and elongation. Conversely, when the amount of carbon (C) added exceeds 0.07% by weight, the possibility of cracks in the slab due to the formation of super ferrite increases, leading to a decrease in toughness and a decrease in weldability, and a high percentage of pearlite phase to achieve the desired microstructure. It becomes difficult to control. Therefore, it is preferable to add the content of carbon (C) at 0.04 to 0.07% by weight of the entire steel sheet.

Silicon (Si): more than 0 and 0.03% by weight or less

Silicon (Si) acts as a deoxidizer and is an element that effectively acts to strengthen solid solution. It is also effective in improving the toughness and ductility of steel by inducing ferrite formation as a ferrite stabilizing element. However, by creating a red scale in the heating furnace, it may cause a problem of deteriorating the surface of the steel when added in large quantities, and also has a problem of deteriorating weldability due to oxide formation. Therefore, it is preferable to limit the addition amount of the silicon (Si) to 0.03% by weight or less of the entire steel sheet.

Manganese (Mn): 1.3 to 1.5% by weight

Manganese (Mn) is a substitutional element having an atomic diameter similar to iron (Fe), which is very effective for solid solution strengthening and improves the hardenability of steel and is effective in securing strength after heat treatment. In addition, as an austenite stabilizing element, it is possible to contribute to the refinement of ferrite grains by delaying ferrite and pearlite transformation. If the amount of manganese (Mn) added is less than 1.3% by weight, the effect is negligible, and when it exceeds 1.5% by weight, the carbon equivalent is increased to significantly reduce the weldability, generating MnS publications and generating central segregation in the slab/coil. By doing so, the ductility and impact characteristics of the steel are greatly reduced. Therefore, the content of manganese (Mn) is preferably limited to 1.3 to 1.5% by weight of the entire steel sheet.

Aluminum (Al): 0.01 ~ 0.05 wt%

Aluminum (Al) acts as a deoxidizer and it is preferable to add 0.01 to 0.05% by weight of the entire steel sheet. When aluminum (Al) is added in an amount of less than 0.01% by weight, the deoxidizing effect is negligible, and when it is added in an amount of more than 0.05% by weight, it is combined with nitrogen (N) present in the steel to produce coarse AlN-based nitride.

Titanium (Ti): 0.11 ~ 0.13% by weight

Titanium (Ti) can improve the toughness of the steel by preventing the coarsening of the austenite grains in the slab heating step by generating a high temperature stability Ti(C, N) precipitate. Titanium (Ti) is less than 0.11% by weight, a sufficient strengthening effect cannot be obtained, and if it exceeds 0.13% by weight, the coarse precipitates can be generated to deteriorate the impact of the steel and DWTT properties, and the manufacturing cost increases and the ductility increases. It is difficult to secure. Therefore, the titanium (Ti) is preferably added in an amount of 0.11 to 0.13% by weight of the entire steel sheet.

Niobium (Nb): 0.02 to 0.03% by weight

Niobium (Nb) is combined with carbon to form carbides that affect the increase in strength, so it is good to add 0.02 to 0.03% by weight of the entire steel sheet. When niobium (Nb) is added in an amount of less than 0.02% by weight, a sufficient reinforcing effect cannot be obtained, and when it exceeds 0.03% by weight, the manufacturing cost increases and there is difficulty in securing ductility.

Vanadium (V): more than 0 and less than 0.01% by weight

Vanadium (V) combines with carbon to improve strength, but the present invention limits its addition amount to 0.01% by weight or less because the strength increase compared to cost is an element that is inferior.

Total content of chromium (Cr), molybdenum (Mo), and nickel (Ni): more than 0 and 0.1% by weight or less

Chromium (Cr), molybdenum (Mo), and nickel (Ni) are elements that improve strength by effectively acting in solid solution strengthening. However, as elements to improve the hardenability, since the elongation is greatly reduced when added in large amounts, in the present invention, their total content is limited to 0.1% by weight or less.

Total content of copper (Cu) and tin (Sn): more than 0 and 0.1% by weight or less

Copper (Cu) and tin (Sn) are metals with a low melting point of alloys, and are elements that make descaling difficult by forming a solid scale by melting at the grain boundaries of the steel sheet during hot rolling. Therefore, it is preferable to limit the total content of copper (Cu) and tin (Sn) to 0.1% by weight or less.

Other inevitable additive elements: phosphorus (P), sulfur (S), nitrogen (N)

Other inevitable elements include phosphorus (P), sulfur (S), and nitrogen (N). In the case of phosphorus (P), there is a high probability of segregation during the steel manufacturing process, and segregation of phosphorus degrades toughness and causes delayed destruction, which occurs after a certain time after molding. Therefore, the content of phosphorus (P) in the steel is preferably limited to 0.018% by weight or less. Sulfur (S) inhibits the toughness and weldability of the steel and combines with manganese (Mn) to form MnS, thereby reducing the corrosion resistance and impact properties of the steel. Thus, in the present invention, the content of the sulfur (S) was limited to 0.005% by weight or less of the entire steel sheet. Further, nitrogen (N) combines with niobium (Nb) and the like to form carbonitrides to refine crystal grains, but solid nitrogen increases to compete with titanium (Ti) in steel to form coarse TiN-based precipitates. TiN is not sufficiently employed in the steel even during the slab reheating process, resulting in coarse precipitate. Therefore, the amount of nitrogen (N) is limited to 60 ppm or less.

The remaining component of the invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the surrounding environment, and thus cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.

The hot rolled steel sheet according to the present invention having the above-described alloy composition exhibits a tensile strength (TS) of 700 MPa or more, a yield strength (YP) of 550 MPa or more, and an elongation of 16% or more. According to the hot rolled steel sheet of the present invention, since the increase in strength compared to the cost limits the amount of niobium (Nb) and vanadium (V) for heat and uses titanium (Ti), which is a precipitation hardening element, it is possible to secure a tensile strength of 700 MPa or more, which is economical. Very advantageous in terms.

By controlling the above-described component system and the process conditions described below, a low-cost 70 kg grade hot rolled steel sheet can be manufactured. Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet that satisfies another aspect of the present invention, that is, resistive ratio, will be described in detail.

Manufacturing method of high strength hot rolled steel sheet

1 is a process flow chart schematically showing a method of manufacturing a high strength hot rolled steel sheet according to an embodiment of the present invention.

Another aspect of the present invention is a method of manufacturing a high-strength hot-rolled steel sheet that satisfies the resistivity ratio, re-heating a steel slab that satisfies the alloy composition ratio of the hot-rolled steel sheet at a temperature of 1,180 to 1,220°C, and 880 to 920 the heated steel slab. Hot rolling at a finish rolling temperature of ℃, and cooling the hot rolled sheet material and then winding at a temperature of 560 ~ 640 ℃.

Slab reheating step (S110)

The steel slab having the above alloy composition is reheated to restock the segregated components during casting and restock. Specifically, by weight, carbon (C): 0.04 to 0.07%, silicon (Si): more than 0 and 0.03% or less, manganese (Mn): 1.3 to 1.5%, phosphorus (P): more than 0 and 0.018% or less, sulfur (S): more than 0 0.005% or less, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.11 to 0.13%, niobium (Nb): 0.02 to 0.03%, vanadium (V): more than 0 and less than 0.01% , Nitrogen (N): more than 0 and 006% or less, chromium (Cr) + molybdenum (Mo) + nickel (Ni): more than 0 and 0.1% or less, copper (Cu) + tin (Sn): more than 0 and 0.1% or less, and The steel slab containing the remaining iron (Fe) and other impurities is heated for 2 hours or more at SRT (Slab Reheating Temperature) of 1,180°C to 1,220°C. The slab may be a steel slab manufactured by a continuous casting process performed before the slab reheating step (S110).

When the slab reheating temperature is less than 1,180°C, the heating temperature is not sufficient, and there is a problem in that the rolling load increases during hot rolling. In addition, there is a problem in that the austenite grains are rapidly coarsened because the precipitates do not reach the solid solution temperature, and thus do not reprecipitate as fine precipitates during hot rolling, so that the grain growth of austenite cannot be suppressed. In addition, when the reheating temperature exceeds 1,220°C, there is a problem in that it is difficult to secure the strength and low-temperature toughness of the steel produced by the austenite crystal grains being rapidly coarsened or decarburization.

Hot rolling step (S120)

After heating the slab as described above, hot rolling is performed on the heated slab. It is preferable to perform the rolling temperature in the austenite recrystallization region or more. More preferably, finish rolling temperature (FDT): finish rolling is performed at 880 to 920°C. By rolling, a casting structure such as dendrites formed during casting is destroyed, and an effect of reducing the size of austenite can be obtained. In order to obtain this effect, it is preferable to control the rolling temperature to 880 ~ 920 ℃. When the finish rolling temperature exceeds 920°C, there is a concern that the quality of the steel sheet is deteriorated due to surface scale generation of the steel sheet. In addition, at a rolling temperature of less than 880°C, crystal grains are refined to increase strength, but may increase rolling load and decrease productivity.

Cooling and coiling step (S130)

After the hot rolling, the hot rolled sheet material is cooled to a predetermined coiling temperature. Specifically, the rolled hot-rolled sheet material is cooled to a coiling temperature 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°C/sec, cooling is not sufficiently performed, which may cause scale generated at high temperature, and exceeds 100°C/sec. At the cooling rate, there is a disadvantage that it is difficult to secure the flatness of the plate material.

The cooling is preferably cooled to the coiling temperature. In the present invention, it is preferable that the coiling temperature satisfies 560 to 640°C. The coiling temperature is to ensure an appropriate amount of ferrite and pearlite, and when the coiling temperature is too high, coarse ferrite and pearlite are generated, making it difficult to secure strength. When the coiling temperature exceeds 640°C, the yield ratio decreases due to the formation of coarse grains, but toughness may decrease and problems may occur that fall below the target strength, whereas when the coiling temperature is lower than 560°C, the tissue As it becomes fine, strength and toughness may increase, but it is difficult to meet the elongation.

Hereinafter, the present invention will be described in detail through examples, but this is only a preferred embodiment of the present invention, and the scope of the present invention is not limited by the scope of the examples. The contents not described here will be sufficiently technically inferred by those skilled in the art, and thus the description thereof will be omitted.

Example

Steel slabs of Examples and Comparative Examples having the component composition (% by weight, the rest being Fe and inevitable impurities) of Tables 1 and 2 below were prepared, and the slabs were reheated, hot rolled, cooled, and coiled under the conditions shown in Table 3. Was done. Thereafter, yield strength (YP), tensile strength (TS), and elongation (EL) were measured, respectively, and the results are shown in Table 3.

division Ingredient (% by weight) C Si Mn P S Al Ti Example 0.054 0.012 1.34 0.007 0.004 0.023 0.12 Comparative Example 1 0.065 0.016 1.41 0.014 0.003 0.033 0.03 Comparative Example 2 0.0664 0.231 1.7 0.0174 0.0013 0.037 0.02 Comparative Example 3 0.0564 0.198 1.494 0.0151 0.001 0.028 0.011 Comparative Example 4 0.58 0.503 1.78 0.013 0.001 0.039 0.06 Comparative Example 5 0.12 0.933 2.15 0.013 0.003 0.037 0.003 Comparative Example 6 0.174 0.077 1.417 0.0134 0.0019 0.031 0.001 Comparative Example 7 0.0872 0.288 2.267 0.0119 0.0004 0.032 0.002

division Ingredient (% by weight) Nb V Cr+Mo+Ni N Cu+Sn Example 0.024 0.001 0.03 0.0044 0.022 Comparative Example 1 0.056 0.002 0.03 0.0053 0.019 Comparative Example 2 0.069 0.04 0.25 0.0050 0.014 Comparative Example 3 0.064 0.059 0.18 0.0037 0.014 Comparative Example 4 0.048 0.004 0.59 0.0036 0.022 Comparative Example 5 0.002 0.004 0.41 0.0039 0.020 Comparative Example 6 0.051 0.002 0.03 0.0037 0.015 Comparative Example 7 0.002 0.004 0.43 0.0049 0.016

division Manufacturing conditions Properties SRT(℃) FDT(℃) CT(℃) TS(MPa) YP(MPa) EL(%) Example 1196 911 623 775 765 20 Comparative Example 1 1196 878 634 631 591 30 Comparative Example 2 1201 805 571 678.5 613.7 29 Comparative Example 3 1206 848 591.9 640.2 568.9 31 Comparative Example 4 1216 895 450 985 931 7 Comparative Example 5 1181 911 425.6 1074 755 14 Comparative Example 6 1166 875 574.3 656.1 531.1 15 Comparative Example 7 1207 880 529.7 797.1 687 11

Referring to Tables 1 to 3, the steels of the Examples satisfying both the compositional ratios of the alloying elements and the production conditions met all of the target tensile strength, yield strength, and elongation.

The steels of Comparative Examples 1, 2 and 3 could not secure the target tensile strength (700 MPa or more) due to the lack of the titanium (Ti) content as a precipitation strengthening element.

The tensile strength and yield strength of the steel of Comparative Example 4 were secured, but the target elongation (20% or more) could not be secured due to excess carbon content and insufficient winding temperature.

The tensile strength and yield strength of the steel of Comparative Example 5 were secured, but the target elongation was not secured due to exceeding the chromium (Cr) + molybdenum (Mo) + nickel (Ni) content as a hardenability element and insufficient winding temperature.

The steel of Comparative Example 6 could not secure the target tensile strength (700 MPa) due to the lack of the precipitation strengthening element titanium (Ti) content, and could not secure the target elongation due to the excess of the carbon (C) content. .

The target tensile strength and yield strength of the steel of Comparative Example 7 were secured, but the target elongation could not be secured because the content of the hardenability element manganese (Mn) was exceeded and the coiling temperature was not reached.

As described above, according to the present invention, the economical efficiency by controlling the content of alloying elements such as carbon (C), manganese (Mn), titanium (Ti), niobium (Nb) and process conditions such as rolling temperature and coiling temperature It is possible to manufacture a high-strength hot-rolled steel sheet of 70 kg.

In the above, although the description has been mainly focused on the embodiment of the present invention, various changes or modifications can be made at the level of those skilled in the art. It can be said that such modifications and variations belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention should be judged by the claims set forth below.

Claims (4)

In weight percent, carbon (C): 0.04 to 0.07%, silicon (Si): more than 0 and 0.03% or less, manganese (Mn): 1.3 to 1.5%, phosphorus (P): more than 0 and 0.018% or less, sulfur (S) : More than 0 and less than 0.005%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.11 to 0.13%, niobium (Nb): 0.02 to 0.03%, vanadium (V): more than 0 and less than 0.01%, nitrogen ( N): more than 0 and less than 006%, contains residual iron (Fe) and other inevitable impurities,
The total content of chromium (Cr), molybdenum (Mo), and nickel (Ni) is more than 0 and 0.1% or less,
The total content of copper (Cu) and tin (Sn) is more than 0 and 0.1% or less,
Hot rolled steel sheet. According to claim 1,
The hot rolled steel sheet,
Tensile strength (TS): 700MPa or more, yield strength (YP): 550MPa or more, and elongation (EL): 20% or more,
Hot rolled steel sheet. In weight percent, carbon (C): 0.04 to 0.07%, silicon (Si): more than 0 and 0.03% or less, manganese (Mn): 1.3 to 1.5%, phosphorus (P): more than 0 and 0.018% or less, sulfur (S) : More than 0 and less than 0.005%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.11 to 0.13%, niobium (Nb): 0.02 to 0.03%, vanadium (V): more than 0 and less than 0.01%, nitrogen ( N): more than 0 and less than 006%, chromium (Cr) + molybdenum (Mo) + nickel (Ni): more than 0 0.1%, copper (Cu) + tin (Sn): more than 0 and less than 0.1%, residual iron (Fe ) And other inevitable impurities to heat the steel slab to a temperature of 1,140 ~ 1,200 ℃;
Hot-rolling the heated steel slab at 870-910°C to obtain a hot-rolled sheet material; And
Cooling the hot-rolled sheet material to an average cooling rate of 10 ~ 30 ℃ to 570 ~ 610 ℃ comprising the step of winding,
Method for manufacturing hot rolled steel sheet. According to claim 3,
The hot rolled steel sheet,
Tensile strength (TS): 700MPa or more, yield strength (YP): 550MPa or more, and elongation (EL): 20% or more,
Method for manufacturing hot rolled steel sheet.

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