KR20160014998A - Steel sheet and method of manufacturing the same - Google Patents

Steel sheet and method of manufacturing the same Download PDF

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Publication number
KR20160014998A
KR20160014998A KR1020140097339A KR20140097339A KR20160014998A KR 20160014998 A KR20160014998 A KR 20160014998A KR 1020140097339 A KR1020140097339 A KR 1020140097339A KR 20140097339 A KR20140097339 A KR 20140097339A KR 20160014998 A KR20160014998 A KR 20160014998A
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South Korea
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steel sheet
less
present
weight
strength
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KR1020140097339A
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Korean (ko)
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강의구
강동훈
김규태
박기정
윤동현
이태호
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현대제철 주식회사
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Publication of KR20160014998A publication Critical patent/KR20160014998A/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Abstract

Disclosed are steel sheet with high strength and high toughness of which a tensile strength is 760-895 MPa; and a manufacturing method thereof. According to the present invention, the steel sheet comprises: 0.05-0.07 wt% of C; 0.2-0.3 wt% of Si; 1.4-1.6 wt% of Mn; 0.01 wt% or less of P; 0.001 wt% or less of S; 0.01-0.04 wt% of S_Al; 0.05-0.15 wt% of Cu; 0.055-0.065 wt% of Nb; 0.1 wt% or less of Ni; 0.1-0.2 wt% of Cr; 0.15-0.25 wt% of Mo; 0.01-0.02 wt% of Ti; 0.0005-0.0015 wt% of B; 0.005 wt% or less of N; 0.002 wt% or less of O; and the remainder consisting of Fe and inevitable impurities. The steel sheet is characterized by a fact that the final fine structure includes a composite structure having acicular ferrite, lath bainite, and martensite austenite constituent.

Description

Technical Field [0001] The present invention relates to a steel sheet and a method of manufacturing the steel sheet.

The present invention relates to a steel sheet and a method of manufacturing the same, and more particularly, to a steel sheet capable of securing a high strength and high toughness with a tensile strength of 760 to 895 MPa through control of alloy components and process conditions, and a method of manufacturing the same.

Generally, in order to produce a steel sheet having a tensile strength (TS) of 800 MPa, a quenching and tempering (QT) heat treatment is carried out after a hot rolling step including a slab reheating step, a hot rolling step and a cooling step.

When the QT heat treatment is performed after the hot rolling process, the time required for the process is prolonged and the productivity may be lowered.

In addition, when the order quantity is larger than QT (Quenching & Tempering) facility capacity, the production load easily occurs and the manufacturing unit price also increases.

A related prior art document is Korean Patent Registration No. 10-0345716 (issued on September 18, 2002).

An object of the present invention is to provide a method of manufacturing a steel sheet having high tensile strength of 760 to 895 MPa and high toughness through control of alloy components and process conditions.

Another object of the present invention is to provide a rubber composition which is produced by the above method and has a tensile strength (TS) of 760 to 895 MPa, a yield strength (YS) of 690 MPa or more, an elongation (EL) of 16% Of the steel sheet.

(A) 0.05 to 0.07% of C, 0.2 to 0.3% of Si, 1.4 to 1.6% of Mn, and 0.01% or less of P in weight% of the steel sheet according to the present invention. , S: 0.001% or less, Sol. 0.1 to 0.2% of Cr, 0.1 to 0.25% of Mo, 0.01 to 0.02% of Ti, 0.01 to 0.02% of Ti, 0.01 to 0.04% of Al, 0.05 to 0.15% of Cu, 0.055 to 0.065% of Nb, Reheating the slab plate made of 0.0005 to 0.0015%, N: 0.005% or less, O: 0.002% or less, and the balance iron (Fe) and unavoidable impurities to SRT: 1150 to 1250 占 폚; (b) subjecting the reheated plate to finishing hot rolling under the conditions of FRT (Finishing Rolling Temperature): 850 to 900 ° C; And (c) cooling the hot-rolled plate to a finishing cooling temperature (FCT) of 450 to 500 ° C by a direct quenching (DQ) method.

According to another aspect of the present invention, there is provided a steel sheet comprising 0.05 to 0.07% of C, 0.2 to 0.3% of Si, 1.4 to 1.6% of Mn, 0.01% or less of P, 0.001 % Or less, Sol. 0.1 to 0.2% of Cr, 0.1 to 0.25% of Mo, 0.01 to 0.02% of Ti, 0.01 to 0.02% of Ti, 0.01 to 0.04% of Al, 0.05 to 0.15% of Cu, 0.055 to 0.065% of Nb, (Fe) and unavoidable impurities, and the final microstructure is composed of at least one material selected from the group consisting of accicular ferrite, lath bainite, And a composite structure containing martensite austenite constituent (MA).

The steel sheet and the manufacturing method thereof according to the present invention can reduce the manufacturing cost by having high strength and high toughness by controlling the fractions of the acicular ferrite, rath bainite and ground martensite according to the accelerated cooling control of the direct quenching (DQ) Productivity can be improved.

Therefore, the steel sheet and the manufacturing method thereof according to the present invention have tensile strength (TS) of 760 to 895 MPa, yield strength (YS) of 690 MPa or more, elongation (EL) of 16% Of the steel sheet.

FIG. 1 is a process flow chart showing a method of manufacturing a steel sheet according to an embodiment of the present invention.
FIG. 2 is a graph showing the results of low temperature impact characteristics test on the specimen according to Example 1. FIG.
3 is a photograph showing the final microstructure of the specimen according to Comparative Example 1. Fig.
4 is a photograph showing the final microstructure of the specimen according to Example 1. Fig.

The features of the present invention and the method for achieving the same will be apparent from the accompanying drawings and the embodiments described below. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms. The present embodiments are provided so that the disclosure of the present invention is complete and that those skilled in the art will fully understand the scope of the present invention. The invention is only defined by the description of the claims.

Hereinafter, a steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Steel plate

The steel sheet according to the present invention is intended to have a tensile strength (TS) of 760 to 895 MPa, a yield strength (YS) of 690 MPa or more, an elongation (EL) of 16% or more and an impact absorption energy of 180 J or more at -20 캜 do.

For this, the steel sheet according to the present invention comprises 0.05 to 0.07% of C, 0.2 to 0.3% of Si, 1.4 to 1.6% of Mn, 0.01% or less of P, 0.001% or less of S, 0.1 to 0.2% of Cr, 0.1 to 0.25% of Mo, 0.01 to 0.02% of Ti, 0.01 to 0.02% of Ti, 0.01 to 0.04% of Al, 0.05 to 0.15% of Cu, 0.055 to 0.065% of Nb, 0.0005 to 0.0015%, N: 0.005% or less, O: 0.002% or less, and the balance of iron (Fe) and unavoidable impurities.

At this time, the steel sheet has a composite structure in which the final microstructure includes accicular ferrite, lath bainite and martensite austenite constituent (MA), and the cross-sectional area ratio of the acicular ferrite 70-80% of the texture, 15-25% of the rath bainite texture and 5-15% of the martensite texture.

Hereinafter, the role and content of each component included in the steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is added to ensure strength.

The carbon (C) is preferably added in an amount of 0.05 to 0.07% by weight based on the total weight of the steel sheet according to the present invention. When the content of carbon (C) is less than 0.05% by weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 0.07% by weight, the strength of the steel increases but the impact resistance and weldability at low temperatures are deteriorated.

Silicon (Si)

Silicon (Si) acts as a deoxidizer in the steel and contributes to securing strength.

The silicon (Si) is preferably added in an amount of 0.2 to 0.3% by weight based on the total weight of the steel sheet according to the present invention. When the content of silicon (Si) is less than 0.2% by weight, the effect of addition is insufficient. On the contrary, when the content of silicon (Si) exceeds 0.3% by weight, the toughness and weldability of the steel deteriorate.

Manganese (Mn)

Manganese (Mn) is an element useful for improving strength without deteriorating toughness.

The manganese (Mn) is preferably added at a content ratio of 1.4 to 1.6% by weight based on the total weight of the steel sheet according to the present invention. When the content of manganese (Mn) is less than 1.4% by weight, the effect of addition thereof can not be exhibited properly. On the other hand, when the content of manganese (Mn) exceeds 1.6% by weight, the sulfur dissolved in the steel precipitates into MnS, which lowers impact toughness at low temperatures.

In (P)

Phosphorous (P) is added to inhibit cementite formation and increase strength.

However, when the content of phosphorus (P) exceeds 0.01% by weight of the total weight of the steel sheet according to the present invention, the weldability is deteriorated and the slab center segregation may cause the final material deviation have. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.01% by weight or less based on the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) reacts with manganese (Mn) to form precipitates of fine MnS to improve processability.

However, if the content of sulfur (S) exceeds 0.001% by weight of the total weight of the steel sheet according to the present invention, the content of sulfur (S) dissolved in the steel sheet may be too high to significantly reduce ductility and moldability, . Therefore, the content of sulfur (S) is preferably limited to 0.001% by weight or less based on the total weight of the steel sheet according to the present invention.

Soluble Aluminum (S_Al)

Soluble aluminum (S_Al) is used as a deacidification material and at the same time it plays a role of suppressing precipitation of cementite and stabilizing austenite like silicon (Si) and enhancing strength.

The soluble aluminum (S_Al) is preferably added in a content ratio of 0.01 to 0.04% by weight based on the total weight of the steel sheet according to the present invention. When the content of soluble aluminum (S_Al) is less than 0.01% by weight, it is difficult to expect an austenite stabilizing effect. On the contrary, when the content of soluble aluminum (S_Al) exceeds 0.04% by weight, there is a problem that the nozzle clogging problem may occur at the time of steel making, and hot brittleness occurs due to Al oxide or the like during casting, and cracking and ductility are deteriorated.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve the hardenability of the steel and the impact resistance at low temperatures.

The copper (Cu) is preferably added in an amount of 0.05 to 0.15% by weight based on the total weight of the steel sheet according to the present invention. When the content of copper (Cu) is less than 0.05% by weight, the effect of adding copper can not be exhibited properly. On the contrary, when the content of copper (Cu) exceeds 0.15% by weight, it exceeds the solubility limit and does not contribute to the increase in strength.

Niobium (Nb)

Niobium (Nb) is one of the elements that have a great influence on the strength. It plays a role of precipitating Nb (C, N), which is carbonitride precipitate, in the steel, or enhancing the strength through solid solution strengthening in Fe. In particular, niobium (Nb) increases the recrystallization temperature by raising the recrystallization temperature during hot rolling, thereby increasing the non-recrystallized reverse pressurization, securing a sufficient rolling region by suppressing the effect of increasing the γ → α transformation temperature due to Mn reduction, In addition, the crystal grains of the final microstructure are refined to greatly improve low temperature toughness. At this time, the Nb-based precipitates precipitated in the crystallization zone are completely dissolved by reheating at a high temperature of 1200 ° C or higher, and then precipitated finely during hot rolling to effectively increase the strength of the steel.

The niobium (Nb) is preferably added in an amount of 0.055 to 0.065 wt% of the total weight of the steel sheet according to the present invention. When the content of niobium (Nb) is less than 0.055% by weight, it may be difficult to exhibit the above-mentioned effects properly. On the other hand, when the content of niobium (Nb) is more than 0.065% by weight, the solubility of niobium (Nb) decreases with an increase in carbon (C) content so that niobium (Nb) There is a concern.

Nickel (Ni)

In the present invention, nickel (Ni) is refined in crystal grains and solidified in austenite and ferrite to strengthen the matrix. In particular, nickel (Ni) is an effective element for improving the low-temperature impact toughness.

However, when a large amount of nickel (Ni) is added in an amount exceeding 0.1% by weight, the weldability is deteriorated and there is a problem of causing redispersibility brittleness. Therefore, it is preferable that the nickel (Ni) is added at a content ratio of 0.1 wt% or less based on the total weight of the steel sheet according to the present invention.

Chromium (Cr)

Chromium (Cr) is an effective element added to secure strength. In addition, the chromium (Cr) serves to increase the hardenability.

It is preferable that chromium (Cr) is added in a content ratio of 0.1 to 0.2% by weight based on the total weight of the steel sheet according to the present invention. If the content of chromium (Cr) is less than 0.1% by weight, the effect of addition thereof can not be exhibited properly. On the contrary, when the content of chromium (Cr) exceeds 0.2% by weight, the weldability and the heat affected zone (HAZ) toughness are lowered.

Molybdenum (Mo)

Molybdenum (Mo) is a substitutional element and improves the strength of steel by solid solution strengthening effect. In addition, molybdenum (Mo) serves to improve the hardenability of the steel.

The molybdenum (Mo) is preferably added at a content ratio of 0.15 to 0.25% by weight based on the total weight of the steel sheet according to the present invention. If the content of molybdenum (Mo) is less than 0.15% by weight, the above effect can not be exhibited properly. On the other hand, when the content of molybdenum (Mo) exceeds 0.25% by weight, there is a problem of raising the manufacturing cost without further effect.

Titanium (Ti)

Titanium (Ti) has the effect of improving the toughness and strength of steel by reducing the austenite grain growth by welding Ti (C, N) precipitates with high stability at high temperatures, thereby finishing the welded structure.

The titanium (Ti) is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the steel sheet according to the present invention. If the content of titanium (Ti) is less than 0.01% by weight, it may be difficult to exhibit the above effect properly. On the other hand, when a large amount of titanium (Ti) is added in an amount exceeding 0.02% by weight, coarse precipitates are produced, which lowers the low-temperature impact properties of the steel and raises manufacturing costs without further effect of addition.

Boron (B)

Boron (B) is a strong incipient element, which plays a role in blocking segregation of phosphorus (P) and improving strength. If segregation of phosphorus (P) occurs, secondary processing brittleness may occur, so boron (B) is added to block segregation of phosphorus (P) to increase resistance to process embrittlement.

The boron (B) is preferably added in an amount of 0.0005 to 0.0015% by weight based on the total weight of the steel sheet according to the present invention. When the content of boron (B) is less than 0.0005 wt%, the amount of boron (B) is insufficient, so that the above effect can not be exhibited properly. On the other hand, if the boron (B) content exceeds 0.0015 wt% and the boron (B) is added in an excess amount, the surface quality of the steel may be deteriorated due to the formation of boron oxide.

Nitrogen (N)

Nitrogen (N) combines with niobium (Nb) to form carbonitride, thereby increasing the grain size. However, when added in large quantities, the amount of dissolved nitrogen is increased to lower the impact characteristics and elongation of the steel and greatly deteriorate the toughness of the welded portion.

Accordingly, the nitrogen (N) is limited to 0.005% by weight or less based on the total weight of the steel sheet according to the present invention.

Oxygen (O)

Oxygen (O) is an unavoidable impurity in the present invention, and when it is contained in an amount exceeding 0.002% by weight, deterioration of the purity of the steel is deteriorated and the elongation rate is deteriorated. Therefore, the oxygen content is limited to 0.002% by weight or less based on the total weight of the steel sheet according to the present invention.

On the other hand, the steel sheet according to the present invention is characterized in that the steel sheet contains carbon (C), silicon (Si), manganese (Mn), copper (Cu), nickel (Ni), chromium (Cr) (Mo) and boron (B).

[C] + [Mn / 6] + [(Cu + Ni) / 15] + [(Cr + Mo) / 5]

Cr / 20] + [Mo / 15] + [5B]? 0.25 [C / 20] + [Ni / 60] + [Cr /

(Where [] is the weight percentage of each element)

At this time, when the carbon equivalent (Ceq) exceeds 0.40 or the welding crack susceptibility index (Pcm) exceeds 0.25, there is a high possibility that cracks are generated in the welded portion, so that carbon (C), silicon Mn, Cu, Ni, Cr, Mo, and B is controlled to be low.

Steel plate manufacturing method

FIG. 1 is a process flow chart showing a method of manufacturing a steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, a steel sheet manufacturing method according to an embodiment of the present invention includes a slab reheating step (S110), a hot rolling step (S120), and a cooling step (S130). At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the step to derive effects such as reuse of precipitates.

In the method for producing a steel sheet according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process preferably contains 0.05 to 0.07% of C, 0.2 to 0.3% of Si, 1.4 to 1.6% of Mn, , S: 0.001% or less, Sol. 0.1 to 0.2% of Cr, 0.1 to 0.25% of Mo, 0.01 to 0.02% of Ti, 0.01 to 0.02% of Ti, 0.01 to 0.04% of Al, 0.05 to 0.15% of Cu, 0.055 to 0.065% of Nb, 0.0005 to 0.0015%, N: 0.005% or less, O: 0.002% or less, and the balance of iron (Fe) and unavoidable impurities.

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1150 to 1250 ° C. When the slab reheating temperature (SRT) is less than 1150 DEG C, there is a problem that the reheating temperature is too low to increase the rolling load. In addition, since the Nb-based precipitates do not reach the solid solution temperature, they can not be precipitated as fine precipitates during hot rolling, and the austenite grain growth can not be suppressed, and the austenite grains are rapidly concentrated. On the other hand, when the slab reheating temperature exceeds 1250 deg. C, there is a problem that the austenite grains are rapidly coarsened and it is difficult to secure strength and low temperature toughness of the steel to be produced.

Hot rolling

In the hot rolling step (S120), the reheated plate is subjected to finishing hot rolling under the conditions of FRT (Finishing Rolling Temperature): 850 to 900 ° C.

At this time, if the finish hot rolling temperature (FRT) is lower than 850 占 폚, there may arise a problem such that blistering due to abnormal reverse rolling occurs. On the other hand, when the finish hot rolling temperature (FRT) exceeds 900 ° C, the austenite grains are coarsened and the ferrite grains can not be sufficiently refined after the transformation, which may make it difficult to secure the strength.

Cooling

In the cooling step (S120), the hot-rolled plate is cooled by a direct quenching (DQ) method to a temperature of 450 to 500 ° C. by a finishing cooling temperature (FCT) method. At this time, in order to improve productivity, the present invention preferably performs rapid cooling to 450 to 500 ° C at a rapid cooling rate of 25 to 30 ° C / sec.

When the finishing cooling temperature (FCT) is less than 450 ° C, a large amount of low-temperature transformed structure is formed and the low-temperature toughness is deteriorated. On the other hand, when the cooling end temperature (FCT) exceeds 500 ° C, there is a problem that strength is lowered due to formation of coarse microstructure.

In addition, when the cooling rate is less than 25 DEG C / sec, crystal grain growth is promoted and it may be difficult to secure strength. On the other hand, when the cooling rate exceeds 30 DEG C / sec, the fraction of the low-temperature structure is increased to increase the strength, but the low temperature toughness is deteriorated.

The steel sheet manufactured in the above steps S110 to S130 controls the texture fraction of the acicular ferrite, the rath bainite and the ground martensite according to the accelerated cooling control by the direct quenching (DQ) method, thereby obtaining a steel sheet having high strength and high toughness The manufacturing cost can be reduced and the productivity can be improved.

Therefore, the steel sheet produced by the method according to the present invention has tensile strength (TS) of 760 to 895 MPa, yield strength (YS) of 690 MPa or more, elongation (EL) of 16% Or more.

In addition, the steel sheet produced by the method according to the present invention is characterized in that the addition amount of nickel (Ni) and molybdenum (Mo) is controlled to be strictly low and crystal grain refinement is effected by adding niobium (Nb) The weldability can be improved by designing the low carbon alloy to have Ceq of not more than 0.45 and Pcm of not more than 0.25.

Example

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.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

Specimens according to Examples 1 to 2 and Comparative Examples 1 and 2 were prepared with the compositions of Tables 1 and 2 and the process conditions of Table 3.

[Table 1] (unit:% by weight)

Figure pat00001

[Table 2] (unit:% by weight)

Figure pat00002

[Table 3]

Figure pat00003

2. Evaluation of mechanical properties

Table 4 shows the evaluation results of the seasonality and the low-temperature impact characteristics of the specimens prepared according to Examples 1 to 2 and Comparative Examples 1 and 2, Fig.

[Table 4]

Figure pat00004

(TS): 760 to 895 MPa, yield strength (YS): 690 MPa or more, elongation (EL): 16 corresponding to the target value in the case of the specimens according to Examples 1 and 2, % And an impact absorption energy at -20 캜: 180J or more.

On the other hand, the tensile strength (TS), the yield strength (YS) and the elongation (EL) of the specimens according to Comparative Examples 1 and 2 satisfied the target values. However, in spite of the QT heat treatment, And the shock absorption energy values at -20 ℃ were only 85J and 78J, which were below the target values.

Fig. 3 is a photograph showing the final microstructure of the specimen according to Comparative Example 1, and Fig. 4 is a photograph showing the final microstructure of the specimen according to Example 1. Fig.

As shown in FIG. 3, it can be seen that the specimen according to Comparative Example 1 has a full martensite structure by carrying out QT (Quenching & Tempering).

On the other hand, as shown in FIG. 4, the specimen according to Example 1 is subjected to DQ (Direct Quenching) without performing tempering, whereby the final microstructure is made of accicular ferrite, lath bainite, and martensite austenite constituent (MA). At this time, the specimen according to Example 1 was found to have 73.6% of acicular ferrite structure, 18.5% of rath bainite structure and 7.9% of ground martensite structure as cross-sectional area ratios as a result of tissue fraction measurement.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Hot rolling step
S130: cooling step

Claims (7)

(a) 0.05 to 0.07% of C, 0.2 to 0.3% of Si, 1.4 to 1.6% of Mn, 0.01% or less of P, 0.001% or less of S, 0.01 to 0.04% of S_Al, 0.1 to 0.25% of Mo; 0.1 to 0.25% of Mo; 0.01 to 0.02% of Ti; 0.0005 to 0.0015% of B; 0.005% or less of N; , O: 0.002% or less, and the balance of iron (Fe) and unavoidable impurities to SRT: 1150 to 1250 占 폚;
(b) subjecting the reheated plate to finishing hot rolling under the conditions of FRT (Finishing Rolling Temperature): 850 to 900 ° C; And
(c) cooling the hot-rolled plate to a finishing cooling temperature (FCT) of 450 to 500 ° C. by a direct quenching (DQ) method.
The method according to claim 1,
In the step (a)
The slab plate
(Si), manganese (Mn), copper (Cu), nickel (Ni), chrome (Cr), molybdenum (Mo), and boron (B) in a range satisfying the following equations (1) Wherein the steel sheet is a steel sheet.

[C] + [Mn / 6] + [(Cu + Ni) / 15] + [(Cr + Mo) / 5]
Cr / 20] + [Mo / 15] + [5B]? 0.25 [C / 20] + [Ni / 60] + [Cr /
(Where [] is the weight percentage of each element)
The method according to claim 1,
In the step (c)
The cooling
At a rate of 25 to 30 DEG C / sec.
0.01 to 0.04% of S, 0.01 to 0.04% of S_Al, 0.01 to 0.04% of S, 0.01 to 0.04% of S, 0.01 to 0.04% of Cu, 0.1 to 0.2% of Cr, 0.1 to 0.25% of Mo, 0.01 to 0.02% of Ti, 0.0005 to 0.0015% of B, 0.005% or less of N, 0.005 to 0.005% of N, 0.002% or less and the balance of iron (Fe) and unavoidable impurities,
Wherein the final microstructure has a composite structure including accicular ferrite, lath bainite and martensite austenite constituent (MA).
5. The method of claim 4,
The steel sheet
(Si), manganese (Mn), copper (Cu), nickel (Ni), chrome (Cr), molybdenum (Mo), and boron (B) in a range satisfying the following equations (1) Wherein the steel sheet comprises a steel sheet.

[C] + [Mn / 6] + [(Cu + Ni) / 15] + [(Cr + Mo) / 5]
Cr / 20] + [Mo / 15] + [5B]? 0.25 [C / 20] + [Ni / 60] + [Cr /
(Where [] is the weight percentage of each element)
5. The method of claim 4,
The steel sheet
Wherein the cross-sectional area ratio is 70 to 80% of an acicular ferrite structure, 15 to 25% of a rath bainite structure and 5 to 15% of a road martensite structure.
5. The method of claim 4,
The steel sheet
A tensile strength (TS) of 760 to 895 MPa, a yield strength (YS) of 690 MPa or more, an elongation (EL) of 16% or more and an impact absorption energy of 180 J or more at -20 캜.
KR1020140097339A 2014-07-30 2014-07-30 Steel sheet and method of manufacturing the same KR20160014998A (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN108950380A (en) * 2018-06-11 2018-12-07 南京钢铁股份有限公司 A kind of Q690GJ building sheet and preparation method thereof
KR20210098514A (en) * 2018-12-11 2021-08-10 닛폰세이테츠 가부시키가이샤 High-strength steel sheet with excellent formability and impact resistance, and method for manufacturing high-strength steel sheet with excellent formability and impact resistance
CN114790530A (en) * 2021-01-26 2022-07-26 宝山钢铁股份有限公司 High-plasticity ultrahigh-strength steel plate and manufacturing method thereof

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN108950380A (en) * 2018-06-11 2018-12-07 南京钢铁股份有限公司 A kind of Q690GJ building sheet and preparation method thereof
KR20210098514A (en) * 2018-12-11 2021-08-10 닛폰세이테츠 가부시키가이샤 High-strength steel sheet with excellent formability and impact resistance, and method for manufacturing high-strength steel sheet with excellent formability and impact resistance
CN114790530A (en) * 2021-01-26 2022-07-26 宝山钢铁股份有限公司 High-plasticity ultrahigh-strength steel plate and manufacturing method thereof
CN114790530B (en) * 2021-01-26 2024-03-08 宝山钢铁股份有限公司 High-plasticity ultrahigh-strength steel plate and manufacturing method thereof

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