KR20160150190A

KR20160150190A - Manufacturing mehtod for thick steel plate and thick steel plate thereof

Info

Publication number: KR20160150190A
Application number: KR1020150086956A
Authority: KR
Inventors: 박경갑; 이희웅
Original assignee: 현대제철 주식회사
Priority date: 2015-06-18
Filing date: 2015-06-18
Publication date: 2016-12-29

Abstract

A method for manufacturing a thick plate and an invention for a thick plate produced thereby are disclosed. The method for manufacturing a thick plate according to the present invention is a method for manufacturing a thick plate according to the present invention, which comprises the steps of: preparing a mixture of carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, Reheating a steel slab consisting of Al, Cu, Ti, Nb, V, Boron and the balance of Fe and other unavoidable impurities; Hot-rolling the reheated steel slab; And cooling the hot rolled steel slab.

Description

Technical Field [0001] The present invention relates to a method for manufacturing a thick plate,

The present invention relates to a method for manufacturing a thick plate and a thick plate produced thereby. More particularly, the present invention relates to a method for manufacturing a high-strength and high-strength thick plate and a thick plate produced thereby.

The thick plate is a thick steel plate with a thickness of 6 mm or more and is also called a post-steel plate. The steel plate is manufactured through a steelmaking process, a continuous casting process, a hot rolling process, and the like, as with a hot-rolled coil. Generally, heavy plates used in structures such as bridges and buildings are manufactured by rolling a semi-finished product in which a large amount of alloying elements are added for securing high strength and high tensile strength, or by performing heat treatment after rolling.

The quality of these thick plates varies depending on the usage environment. For example, when used in railroad bridges and harsh environments, low temperature impact toughness is required.

BACKGROUND ART [0002] The background art relating to the present invention is disclosed in Korean Patent Laid-Open Publication No. 2015-0055110 (published on May 20, 2015, entitled " Steel member and method for manufacturing the same).

According to one embodiment of the present invention, there is provided a method for manufacturing a thick plate excellent in rigidity, toughness and low temperature impact property.

According to an embodiment of the present invention, there is provided a method of manufacturing a thick plate excellent in economical efficiency.

According to an embodiment of the present invention, there is provided a thick plate manufactured by the thick plate manufacturing method.

One aspect of the present invention relates to a method for manufacturing a thick plate. In one embodiment, the thick plate manufacturing method may include 0.02 to 0.1 wt% of carbon (C), 0.4 wt% or less of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn) 0.1 to 0.5 wt% of chromium (Cr), 0.1 to 0.5 wt% of nickel (Ni), 0.1 to 0.4 wt% of molybdenum (Mo), 0.15 wt% of aluminum (Al) (V): 0.2 to 0.5% by weight, boron (V): 0.5 to 0.5% by weight, copper (Cu): 0.1 to 0.5% (B) of 0.0005 to 0.0015% by weight, and the balance of iron (Fe) and other unavoidable impurities at a slab reheating temperature of 1180 to 1230 캜; Hot-rolling the reheated steel slab at a rolling finish temperature of at least Ar3; And cooling the hot-rolled steel slab at a cooling rate of 10 ° C / s or higher.

In one embodiment, the hot rolling comprises: primary rolling the reheated steel slab in a temperature range above the recrystallization quenching temperature; And secondary rolling the primary rolled steel slab at a rolling finish temperature of at least Ar3 point.

In one embodiment, the cumulative rolling reduction during the secondary rolling may be 25% to 45%.

In one embodiment, the cooling may cool the heated steel slab to a cooling end temperature of 420 ° C to 475 ° C.

Another aspect of the present invention relates to a thick plate produced by the above thick plate manufacturing method. In one embodiment, the thick plate comprises 0.02 to 0.1 wt% carbon (C), 0.4 wt% or less silicon (Si), 1.5 to 2.3 wt% manganese (Mn) 0.1 to 0.5% by weight of nickel (Ni), 0.1 to 0.4% by weight of molybdenum (Mo), 0.15 to 0.5% by weight of aluminum (Al) (B), 0.2 to 0.5% by weight of vanadium (V), 0.2 to 0.5% by weight of boron (B), 0.1 to 0.5% by weight of copper (Cu), 0.2 to 0.4% ) Of 0.0005 to 0.0015% by weight, and the balance of iron (Fe) and other unavoidable impurities.

In one embodiment, the thick plate has a composite structure including a bainitic ferrite, a granular bainite, and a martensite, and the bainitic ferrite has an area fraction of not less than 40% Lt; / RTI >

In one embodiment, the thick plate has a tensile strength (TS) of 770 MPa or more, a yield strength (YS) of 650 MPa or more, a yield ratio (YR) of 85% or less and elongation % &Lt; / RTI >

The steel plate manufactured according to the method of the present invention has excellent rigidity, toughness and impact resistance at low temperature, and can save time and cost of the manufacturing process, thereby providing excellent economical efficiency.

1 shows a method of manufacturing a thick plate according to one embodiment of the present invention.
FIG. 2 shows the structure of the thick plate according to the embodiment of the present invention.
3 is a graph showing the results of a low-temperature impact energy test of a heavy plate manufactured according to an embodiment of the present invention and a comparative example according to the present invention.

Hereinafter, the present invention will be described in detail. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to be exemplary, self-explanatory, allowing for equivalent explanations of the present invention.

One aspect of the present invention relates to a method for manufacturing a thick plate. In the present specification, the thick plate may mean a steel plate having a thickness of 6 mm or more. 1 shows a method of manufacturing a thick plate according to one embodiment of the present invention. Referring to FIG. 1, in one embodiment, the method for manufacturing a thick plate according to the present invention includes the steps of: (S10) reheating a steel slab; (S20) hot rolling step; And (S30) a cooling step. More specifically, the thick plate manufacturing method may include: 0.02 to 0.1 wt% of carbon (C), 0.4 wt% or less of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn) 0.1 to 0.5 wt% of chromium (Cr), 0.1 to 0.5 wt% of nickel (Ni), 0.1 to 0.4 wt% of molybdenum (Mo), 0.15 wt% of aluminum (Al) (V): 0.2 to 0.5% by weight, boron (V): 0.5 to 0.5% by weight, copper (Cu): 0.1 to 0.5% (B) of 0.0005 to 0.0015% by weight, and the balance of iron (Fe) and other unavoidable impurities at a slab reheating temperature of 1180 to 1230 캜; Hot-rolling the reheated steel slab at a rolling finish temperature of at least Ar3; And cooling the hot-rolled steel slab at a cooling rate of 10 ° C / s or higher.

( S10 ) Steel slab reheating step

The above step reheats the steel slab. More specifically, the step may include: 0.02 to 0.1 weight% of carbon (C), 0.4 weight% or less of silicon (Si), 1.5 to 2.3 weight% of manganese (Mn) 0.1 to 0.5% by weight of nickel (Ni), 0.1 to 0.4% by weight of molybdenum (Mo), 0.15 to 0.5% by weight of aluminum (Al) (B), 0.2 to 0.5% by weight of vanadium (V), 0.2 to 0.5% by weight of boron (B), 0.1 to 0.5% by weight of copper (Cu), 0.2 to 0.4% ) Of 0.0005 to 0.0015% by weight and a balance of iron (Fe) and other unavoidable impurities at a slab reheating temperature of 1180 to 1230 캜.

Hereinafter, the role and content of each component contained in the steel slab will be described in detail.

Carbon (C)

The carbon (C) is added to secure the strength of the thick plate of the present invention. The carbon is included in an amount of 0.02 to 0.1% by weight based on the total weight of the steel slab. When the carbon content is less than 0.02 wt%, the fraction of the second phase structure is lowered and the strength is lowered. When the carbon content is more than 0.1 wt%, the strength of the steel is increased but the impact resistance and weldability are deteriorated at low temperatures.

silicon( Si )

The silicon (Si) is included for the purpose of deoxidizing the oxygen of the steel of the present invention and strengthening the solid solution. The silicon is contained in an amount of 0.4% by weight or less based on the total weight of the steel slab. Within the above range, deoxidation effect and solid solution strengthening effect can be excellent. When the silicon is contained in an amount exceeding 0.4 wt%, the non-metallic inclusions are excessively formed on the surface of the steel to deteriorate toughness. For example, 0.01 to 0.4% by weight.

manganese( Mn )

The manganese (Mn) is an austenite stabilizing element and serves to improve the strength and toughness by reducing the Ar3 point to enlarge the control rolling temperature region to make the crystal grains by rolling finer. The manganese is contained in an amount of 1.5 to 2.3% by weight based on the total weight of the steel slab. When the content of manganese is less than 1.5% by weight, the fraction of the second phase structure is lowered and it may be difficult to secure the strength. When the manganese is contained in an amount exceeding 2.3 wt%, the sulfur dissolved in the steel precipitates into MnS, which lowers the impact resistance at low temperatures.

In (P)

The phosphorus (P) contributes partly to the improvement of strength but is a representative element that lowers the impact toughness at low temperature. The lower the content is, the better. The phosphorus is contained in an amount of not more than 0.01% by weight based on the total weight of the steel slab. If the content of phosphorus is more than 0.01% by weight, low-temperature impact toughness may be lowered. For example, from 0.0001 to 0.01% by weight.

Sulfur (S)

The sulfur (S) forms MnS or the like to reduce the effective manganese content and cause surface defects by the MnS. The sulfur is contained in an amount of not more than 0.01% by weight based on the total weight of the steel slab. When the sulfur is contained in an amount exceeding 0.01 wt%, low-temperature impact toughness may be lowered. For example, from 0.0001 to 0.01% by weight.

chrome( Cr )

The chromium (Cr) is an effective element added to secure strength. In addition, the chromium serves to increase the hardenability. The chromium is contained in an amount of 0.1 to 0.5% by weight based on the total weight of the steel slab. When the chromium content is less than 0.1 wt%, the effect of the addition can not be exhibited properly. If the amount of chromium exceeds 0.5% by weight, the weldability and HAZ toughness may be lowered.

nickel( Ni )

The nickel (Ni) is included for the purpose of refining the crystal grains and strengthening the base by being dissolved in austenite and ferrite, and for improving the low temperature impact toughness. The nickel is contained in an amount of 0.1 to 0.5% by weight based on the total weight of the steel slab. When the amount of the nickel is less than 0.1 wt%, the nickel addition effect can not be exhibited properly. If the nickel content is more than 0.5% by weight, it may induce a red-hot brittleness.

molybdenum( Mo )

The molybdenum (Mo) contributes to improvement of strength and toughness, and is included for the purpose of ensuring stable strength at room temperature or high temperature. The molybdenum is contained in an amount of 0.1 to 0.4% by weight based on the total weight of the steel slab. When the content of the molybdenum is less than 0.1 wt%, the effect of adding molybdenum is insignificant, and when it exceeds 0.4 wt%, the weldability may be deteriorated.

aluminum( Al )

The aluminum (Al) acts as a deoxidizer to remove oxygen in the steel. The aluminum is contained in an amount of 0.15 to 0.5% by weight based on the total weight of the steel slab. When the content of aluminum is less than 0.15% by weight, the effect of addition is insignificant. When the content of aluminum exceeds 0.5% by weight, Al ₂ O ₃ , which is a nonmetallic inclusion, is formed.

Copper( Cu )

The copper (Cu) is included together with the nickel for the purpose of improving the hardenability of the steel and the impact resistance at low temperature. The copper is contained in an amount of 0.1 to 0.5% by weight based on the total weight of the steel slab. If the amount of copper is less than 0.1 wt%, the effect of adding copper is insufficient. If the amount of copper exceeds 0.5 wt%, the amount of copper exceeds the solubility limit, and therefore, the copper does not contribute to the increase in strength.

titanium( Ti )

The titanium (Ti) is included for the purpose of improving the toughness and strength of steel by reducing the austenite grain growth by welding TiC and TiN precipitates having high stability at high temperature to refine the texture of the welded part. The titanium is contained in an amount of 0.2 to 0.4% by weight based on the total weight of the steel slab. When the amount of titanium is less than 0.2% by weight, aging hardening may occur due to the residual solid carbon and solid nitrogen remaining without precipitation. When the titanium is contained in an amount exceeding 0.4% by weight, coarse precipitates are produced to lower the low-temperature impact properties of the steel, and the manufacturing cost can be increased without any additional effect.

Niobium ( Nb )

The niobium (Nb) is included for the purpose of forming carbide by binding with carbon at a high temperature. The niobium carbide improves the strength of the steel and the low temperature toughness by refining the grain by inhibiting grain growth during rolling. The niobium is contained in an amount of 0.2 to 0.7% by weight based on the total weight of the steel slab. When the content of niobium is less than 0.2% by weight, the effect of adding niobium can not be exhibited properly. When the content of niobium exceeds 0.7 wt%, the weldability of the steel is deteriorated. The strength and the low temperature toughness according to the increase of the niobium content are not improved any more, but exist in a solid state in the ferrite and may lower the impact toughness. .

Vanadium (V)

The vanadium (V) is included for the purpose of improving the strength of steel through precipitation strengthening effect by precipitate formation. The vanadium is contained in an amount of 0.2 to 0.5% by weight based on the total weight of the steel slab. When the vanadium content is less than 0.2% by weight, it may be difficult to exhibit the above effects. If the vanadium content exceeds 0.5% by weight, the low-temperature impact toughness may be deteriorated.

Boron (B)

The boron (B) is a strong minerals element and is included for the purpose of preventing segregation of the phosphorus to improve strength. In addition, when segregation of phosphorus occurs, secondary processing brittleness may occur. Therefore, boron may be added to block the segregation of phosphorus to increase the resistance to processing brittleness. The boron is contained in an amount of 0.0005 to 0.0015% by weight based on the total weight of the steel slab. When the boron content is less than 0.0005% by weight, the addition amount is insignificant, and the above effect can not be exhibited properly. If the boron content exceeds 0.0015% by weight, inclusion of boron oxide may cause a problem of inhibiting the surface quality of the steel.

In one embodiment, the steel slab may be reheated at a slab reheating temperature (SRT) of 1180 ° C to 1230 ° C. When the steel slab reheating temperature (SRT) is less than 1,180 占 폚, the reheating temperature is low and the rolling load becomes large. Since the NbC-based NbC precipitates can not reach the solidification temperature, they can not be precipitated as fine precipitates upon hot rolling, The austenitic grains can be rapidly coarsened. If the steel slab reheating temperature (SRT) exceeds 1230 DEG C, the austenite grain size increases and it is difficult to secure the strength and low temperature toughness of the thick plate, and the manufacturing cost of the thick plate may increase due to the excessive heating process. In one embodiment, the reheating time of the steel slab may be from 30 minutes to 4 hours. In this range, the components segregated at the time of casting are sufficiently reused, and the surface quality of the steel plate can be excellent.

( S20 ) Hot rolling step

The step is a step of hot-rolling the reheated steel slab. At this time, the finish rolling temperature (FDT) during the hot rolling of the reheated steel slab may be made at a temperature of Ar3 point or higher. When the finish rolling temperature is hot-rolled at a temperature lower than the Ar3 point, anomaly reverse rolling may occur and non-uniform structure may be formed to deteriorate low-temperature impact toughness.

In one embodiment, the hot-rolling may be performed at a finishing rolling temperature of 650 to 880 DEG C for the reheated steel slab. When the steel slab material is hot-rolled at the finish rolling temperature, it is possible to prevent the occurrence of a coarse-grained structure due to an anomaly reverse rolling and ensure low-temperature impact toughness.

In one embodiment, the hot rolling comprises: primary rolling the reheated steel slab in a temperature range above a recrystallization stop temperature (RST); And secondary rolling the primary rolled steel slab at a rolling finish temperature of at least Ar3 point.

In one embodiment, the primary rolling may be at 950 ° C to 1050 ° C. A sufficient reduction rate can be ensured in the primary rolling at the above temperature. For example, 1000 ° C to 1050 ° C.

In one embodiment, the cumulative rolling reduction (cumulative rolling reduction of the non-recrystallized region) during the secondary rolling can be from 25% to 45%. In the secondary rolling at the cumulative rolling reduction in the above range, the microstructure can be ensured and the strength and impact toughness of the present invention can be excellent.

( S30 ) Cooling step

The step is cooling the hot rolled steel slab. In one embodiment, the step cools the hot rolled steel slab at a cooling rate of at least 10 [deg.] C / s. And the resistance reduction characteristic can be realized when cooling at the cooling rate. If the cooling rate is less than 10 ° C / s, the crystal grain phase is promoted, and it may be difficult to secure the tensile strength and yield ratio of the present invention. For example, 10 [deg.] C / s to 50 [deg.] C / s.

In one embodiment, cooling may cool the hot rolled steel slab to a cooling end temperature of 420 ° C to 475 ° C. When cooling at the cooling end temperature, the tensile strength and yield ratio of the present invention can be easily secured.

Another aspect of the present invention relates to a thick plate produced by the above thick plate manufacturing method. The thick plate preferably contains 0.02 to 0.1 wt% of carbon (C), 0.4 wt% or less of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn), 0.01 wt% or less of phosphorus (P) (Al): 0.15 to 0.5 wt.%, And the aluminum (Al): 0.1 to 0.5 wt.%, (V): 0.2 to 0.5% by weight, boron (V): 0.5 to 0.5% by weight, copper (Cu): 0.1 to 0.5% B) of 0.0005 to 0.0015% by weight, and the balance of iron (Fe) and other unavoidable impurities.

For example, 0.02 to 0.1 wt% of carbon (C), 0.01 to 0.4 wt% of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn), 0.0001 to 0.01 wt% of phosphorus (P) 0.1 to 0.5% by weight of nickel (Ni), 0.1 to 0.4% by weight of molybdenum (Mo), 0.15 to 0.5% by weight of aluminum (Al) 0.1 to 0.5% by weight of copper, 0.2 to 0.4% by weight of titanium, 0.2 to 0.7% by weight of niobium, 0.2 to 0.5% by weight of vanadium, 0.0005 to 0.0015% by weight, and balance iron (Fe) and other unavoidable impurities.

The thick plate has a composite structure including a bainitic ferrite, a granular bainite and a martensite, wherein the bainitic ferrite has an area fraction of not less than 40% have. For example, it may have an area fraction of 40% to 85%. The martensite may have an area fraction of 0.1% to 30%. The particulate bainite may have an area fraction of 0.1% to 30%.

That is, the final microstructure of the thick plate may include martensite and a granular bainite phase in a bainitic ferrite base.

In one embodiment, the tensile strength (TS) of the thick plate may be 770 MPa or more. For example, 800 to 950 MPa.

The yield strength (YS) of the thick plate may be 650 MPa or more. For example, 650 to 770 MPa.

The yield ratio (YR) of the thick plate may be less than 85%. For example from 70% to 85%.

The elongation (El) of the thick plate may be 16% or more. For example, 16 to 20%.

In one embodiment, the impact absorption energy of the plate at -40 DEG C may be greater than or equal to 47J. For example, it may be 47J to 110J.

The steel plate manufactured by the method of the present invention can be manufactured with excellent tensile strength and yield strength without applying QT (Quenching & Tempering) or QLT (Quenching & Lamellaizing & Tempering) heat treatment, Therefore, the economic effect can be excellent.

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Example

0.05 wt% of carbon (C), 0.35 wt% of silicon (Si), 2.0 wt% of manganese (Mn), 0.005 wt% of phosphorus, 0.005 wt% of sulfur, chromium (Cr) 0.3 weight% of nickel, 0.4 weight% of nickel, 0.2 weight% of molybdenum, 0.2 weight% of aluminum, 0.3 weight% of copper, 0.3 weight of titanium, 0.3 weight% of niobium A slab composed of 0.4% by weight of iron (Nb), 0.3% by weight of vanadium (V), 0.001% by weight of boron (B) and iron and other unavoidable impurities was reheated at a slab reheating temperature of 1191 ° C.

The reheated steel slab is firstly rolled in a temperature range (1000 ° C to 1050 ° C) above the recrystallization stop temperature (RST), and the primary rolled steel slab is subjected to a rolling reduction at a rolling finish temperature of 868 ° C 36%, and subjected to hot rolling.

The hot-rolled steel slab was cooled to a cooling end temperature of 423 캜 at a cooling rate of 11 캜 / s to prepare a thick plate having a thickness of 80 mm.

Comparative Example

A thick plate was produced in the same manner as in the above example, except that the conditions of reheating temperature, cumulative rolling reduction, rolling completion temperature, cooling rate and cooling termination temperature in Table 1 were applied.

The tensile strength (TS), yield strength (YS), elongation (El), yield ratio (YR = YS / TS) of the steel plate specimens prepared according to the above- And the results are shown in Table 2 below.

FIG. 2 is a view illustrating a structure of a thick plate according to an embodiment of the present invention in a thickness direction. Referring to FIG. 2, it was found that the granular bainite structure which adversely affects the physical properties of the thick plate of the embodiment appears only at a depth of 30 mm or more. In addition, it was found that the fraction of martensite phase was large and the bainite of lath type was maintained to a deep thickness section.

Referring to Table 2, the steel plate of the present embodiment exhibited uniform distribution of the structure and excellent grain-reducing effect, tensile strength (TS) of 770 MPa or more, yield strength (YS) of 650 MPa or more, yield It was found that the yield ratio (YR) was 85% or less and the elongation (El) was 16% or more. On the other hand, in the case of the thick plate of the comparative example in which the cooling rate different from that of the present invention was applied, the tensile strength and the yield strength were lowered.

FIG. 3 is a graph showing the results of low-temperature impact absorption energy test of a heavy plate manufactured according to an embodiment of the present invention and a comparative example according to the present invention. Referring to FIG. 3, it can be seen that the impact strength of the comparative example of the present invention, which is different from that of the present invention, is lower than that of the embodiment of the present invention.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

(P): 0.01 wt% or less, sulfur (S): 0.01 wt% or less, carbon (C): 0.02 to 0.1 wt% 0.1 to 0.5 wt% of chromium (Cr), 0.1 to 0.5 wt% of nickel (Ni), 0.1 to 0.4 wt% of molybdenum (Mo), 0.15 to 0.5 wt% of aluminum (Al) (V): 0.2 to 0.5 wt.%, Boron (B): 0.0005 to 0.0015 wt.%, And Reheating the steel slab composed of the remaining amount of Fe and other unavoidable impurities at a slab reheating temperature of 1180 to 1230 캜;
Hot-rolling the reheated steel slab at a rolling finish temperature of at least Ar3; And
And cooling the hot-rolled steel slab at a cooling rate of 10 DEG C / s or more.

The method according to claim 1,
The hot-
Firstly rolling the reheated steel slab in a temperature region above a recrystallization stop temperature; And
And secondarily rolling said primary rolled steel slab at a rolling finish temperature of at least Ar3 point.

3. The method of claim 2,
Wherein the cumulative rolling reduction during the secondary rolling is 25% to 45%.

The method according to claim 1,
Wherein the cooling is performed by cooling the heated steel slab to a cooling end temperature of 420 ° C to 475 ° C.

(P): 0.01 wt% or less, sulfur (S): 0.01 wt% or less, carbon (C): 0.02 to 0.1 wt% 0.1 to 0.5 wt% of chromium (Cr), 0.1 to 0.5 wt% of nickel (Ni), 0.1 to 0.4 wt% of molybdenum (Mo), 0.15 to 0.5 wt% of aluminum (Al) (V): 0.2 to 0.5 wt.%, Boron (B): 0.0005 to 0.0015 wt.%, And Plates made of iron (Fe) and other unavoidable impurities.

6. The method of claim 5,
Wherein the thick plate has a composite structure in which the final structure includes bainitic ferrite, granular bainite, and martensite.

6. The method of claim 5,
The thick plate is characterized by having a tensile strength (TS) of 770 MPa or more, a yield strength (YS) of 650 MPa or more, a yield ratio (YR) of 85% or less and an elongation (El) of 16% Plate.