KR20150101735A

KR20150101735A - Steel sheet and method of manufacturing the same

Info

Publication number: KR20150101735A
Application number: KR1020140023380A
Authority: KR
Inventors: 강의구; 강동훈; 김규태; 박기정; 윤동현; 이태호
Original assignee: 현대제철 주식회사
Priority date: 2014-02-27
Filing date: 2014-02-27
Publication date: 2015-09-04

Abstract

Disclosed are steel sheet and a manufacturing method thereof, wherein high strength and high ductility can be ensured by adjusting a texture fraction of tempered martensite and tempered bainite via an introduction of a direct quenching and tempering (DQT) process. According to the present invention, the method to manufacture a steel sheet comprises the steps of: (a) conducting, at a finish rolling temperature of 900-950°C, the finish hot rolling of a slab sheet composed of 0.10-0.13 wt% of C, 0.2-0.3 wt% of Si, 0.7-0.9 wt% of Mn, 0.02 wt% or less of P, 0.005 wt% or less of S, 0.03-0.07 wt% of Al, 0.4-0.5 wt% of Cr, 0.9-1.0 wt% of Ni, 0.4-0.5 wt% of Mo, 0.15-0.25 wt% of Cu, 0.02 wt% or less of Ti, 0.035-0.045 wt% of V, 0.0005-0.0030 wt% of B, and the remainder consisting of Fe and inevitable impurities; (b) cooling the hot rolled sheet to a temperature of 250-300°C by conducting direct quenching (DQ); and (c) tempering the cooled sheet at a temperature of 630-700°C.

Description

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

[0001] The present invention relates to a steel sheet and a method of manufacturing the steel sheet, and more specifically, by applying a direct quenching & tempering (DQT) process, it is possible to secure a high strength and high toughness by controlling a tissue fraction of tempered martensite and tempered bainite 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).

It is an object of the present invention to provide a method of manufacturing a steel sheet having high strength and high toughness by controlling the texture fraction of tempered martensite and tempered bainite by applying a Direct Quenching & Tempering (DQT) process.

Another object of the present invention is to provide a steel sheet produced by the above method and having a tensile strength (TS) of 760 to 895 MPa, a yield strength (YS) of 690 MPa or more, and an elongation (EL) of 18% or more.

(A) 0.10 to 0.13% of C, 0.2 to 0.3% of Si, 0.7 to 0.9% of Mn, and 0.02% or less of P in weight% of the steel sheet according to the present invention. , S: not more than 0.005%, Al: 0.03 to 0.07%, Cr: 0.4 to 0.5%, Ni: 0.9 to 1.0%, Mo: 0.4 to 0.5%, Cu: 0.15 to 0.25% 0.015 to 0.045%, B: 0.0005 to 0.0030%, and the balance of iron (Fe) and unavoidable impurities to a finishing rolling temperature (FRT) of 900 to 950 占 폚; (b) subjecting the hot-rolled sheet to direct quenching (DQ) to cool the sheet to a temperature of 250 to 300 캜; And (c) tempering the cooled plate at a temperature of 630 to 700 ° C.

According to another aspect of the present invention, there is provided a steel sheet comprising 0.10 to 0.13% of C, 0.2 to 0.3% of Si, 0.7 to 0.9% of Mn, 0.02% or less of P, 0.005 Ti: not more than 0.02%, V: 0.035 to 0.045%, Ti: not more than 0.02%, and V: not more than 0.03% , B: 0.0005 to 0.0030% and the balance of iron (Fe) and inevitable impurities, and the final microstructure has a composite structure of tempered lath martensite and temperd lath bainite , And the tempered martensite structure has a sectional area ratio of 70 to 80%.

The steel sheet according to the present invention and its manufacturing method can reduce the manufacturing cost and improve the productivity by applying the DQT (Direct Quenching & Tempering) process. In addition, since the addition of vanadium (V) .

The steel sheet and the manufacturing method thereof according to the present invention are characterized by having tensile strength (TS) of 760 to 895 MPa, yield strength (YS) of 690 MPa or more, elongation (EL) of 18% Or more.

FIG. 1 is a process flow chart showing a method of manufacturing a steel sheet according to an embodiment of the present invention.
2 is a graph showing the results of low-temperature impact characteristics test on the specimens according to Examples 1 to 3.

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 18% or more and an impact absorption energy of 200 J or more at -40 캜 do.

The steel sheet according to the present invention preferably contains 0.10 to 0.13% of C, 0.2 to 0.3% of Si, 0.7 to 0.9% of Mn, 0.02% or less of P, 0.005% or less of S, 0.15 to 0.25% of Cu, 0.02% or less of Ti, 0.035 to 0.045% of V, 0.0005 to 0.0030% of B, 0.04 to 0.5% of Cr, 0.9 to 1.0% And the balance iron (Fe) and unavoidable impurities.

Also, the steel sheet has a composite structure of tempered lath martensite and tempered lath bainite, wherein the tempered rath- terite structure has a cross-sectional area ratio of 70 to 80 %. &Lt; / RTI >

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 a content ratio of 0.10 to 0.13% 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 0.10% by weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 0.13% by weight, the strength of the steel is increased 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 is preferably added in a content ratio of 0.7 to 0.9% 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 0.7% by weight, the effect of addition thereof can not be exhibited properly. On the other hand, when the content of manganese (Mn) exceeds 0.9% 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 0.02% 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 0.02% 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, when the content of sulfur (S) exceeds 0.005% by weight of the total weight of the steel sheet according to the present invention, the content of sulfur (S) is too large, so that ductility and formability may be significantly lowered, . Therefore, the content of sulfur (S) is preferably limited to 0.005% by weight or less based on the total weight of the steel sheet according to the present invention.

Aluminum (Al)

Aluminum (Al) acts as a deoxidizer to remove oxygen in the steel.

The aluminum (Al) is preferably added in an amount of 0.03 to 0.07% by weight based on the total weight of the steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.03 wt%, the deoxidizing effect described above can not be exhibited properly. On the contrary, when the content of aluminum (Al) exceeds 0.07% by weight, it is difficult to perform, resulting in a decrease in productivity and formation of a compound causing a pinning effect such as Al ₂ O ₃ , .

Chromium (Cr)

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

Cr is preferably added at a content ratio of 0.4 to 0.5% by weight based on the total weight of the steel sheet according to the present invention. When the content of chromium (Cr) is less than 0.4% by weight, the effect of the addition can not be exhibited properly. On the contrary, when the content of chromium (Cr) exceeds 0.5% by weight, the weldability and the heat affected zone (HAZ) toughness are lowered.

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.

The nickel (Ni) is preferably added at a content ratio of 0.9 to 1.0 wt% of the total weight of the steel sheet according to the present invention. If the content of nickel (Ni) is less than 0.9% by weight, the effect of adding nickel can not be exhibited properly. On the contrary, when the content of nickel (Ni) exceeds 1.0% by weight and is added in a large amount, there arises a problem of inducing the redispersible brittleness.

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 in a content ratio of 0.4 to 0.5% 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 0.4% by weight, the above effects can not be exhibited properly. On the contrary, when the content of molybdenum (Mo) exceeds 0.5% by weight, there arises a problem of raising the manufacturing cost without any further effect.

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 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 copper (Cu) is less than 0.15% by weight, the effect of adding copper can not be exhibited properly. On the contrary, when the content of copper (Cu) exceeds 0.25% by weight, it exceeds the solubility limit and does not contribute to further increase in strength, and there is a problem of inducing heat-induced brittleness.

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.

However, when titanium is added in a large amount exceeding 0.02 wt%, coarse precipitates are formed, which lowers the low-temperature impact properties of the steel and raises manufacturing costs without further effect of addition. Therefore, it is preferable that titanium (Ti) is added at a content ratio of 0.02% by weight or less based on the total weight of the steel sheet according to the present invention.

Vanadium (V)

Vanadium (V) is an element with high carbon affinity and plays a role of improving the strength of steel through precipitation strengthening effect by formation of microstructure during tempering.

The vanadium (V) is preferably added in an amount of 0.035 to 0.045% by weight based on the total weight of the steel sheet according to the present invention. When the content of vanadium (V) is less than 0.035% by weight, the precipitation strengthening effect due to vanadium addition is insufficient. On the contrary, when the content of vanadium (V) exceeds 0.045% by weight, the low-temperature impact toughness is deteriorated.

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 0.0030% 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 0.0030 wt%, boron oxide may be formed to deteriorate the surface quality of the steel.

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 hot rolling step (S110), a cooling step (S120), and a tempering step (S130).

Although not shown in the drawings, the hot-rolled steel sheet manufacturing method according to the embodiment of the present invention may further include a slab reheating step (not shown) performed before the hot-rolling step S110. At this time, the slab reheating step is not necessarily performed, but it is more preferable to carry out the step of re-heating the precipitate.

In the steel sheet manufacturing method according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process is composed of 0.10 to 0.13% of C, 0.2 to 0.3% of Si, 0.7 to 0.9% of Mn, 0.02% or less of P , S: not more than 0.005%, Al: 0.03 to 0.07%, Cr: 0.4 to 0.5%, Ni: 0.9 to 1.0%, Mo: 0.4 to 0.5%, Cu: 0.15 to 0.25% 0.035 to 0.045%, B: 0.0005 to 0.0030%, and the balance of iron (Fe) and unavoidable impurities.

In the slab reheating step, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1100 to 1250 ° C. When the slab reheating temperature (SRT) is less than 1100 ° 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 (S110), the reheated slab sheet is finishing hot-rolled under the condition of FRT (Finishing Rolling Temperature): 900 to 950 占 폚.

At this time, if the finish hot rolling temperature (FRT) is lower than 900 캜, there may occur problems such as blistering due to abnormal reverse rolling. On the other hand, when the finish hot rolling temperature (FRT) exceeds 950 ° C, the austenite grains are coarsened and the ferrite grain refinement after the transformation is not sufficiently performed, which may make it difficult to secure strength.

Cooling

In the cooling step (S120), the hot rolled plate is subjected to direct quenching (DQ) and cooled to 250 to 300 deg. At this time, in order to secure a low-temperature structure, it is preferable to perform rapid cooling at a rapid cooling rate of 15 to 20 ° C / sec and a low cooling termination temperature of 250 to 300 ° C.

When the finishing cooling temperature (FCT) is less than 250 ° 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 300 캜, there is a problem that the strength is lowered due to the formation of coarse microstructure.

In addition, when the cooling rate is less than 15 ° C / sec, crystal grain growth is promoted and it may be difficult to secure strength. On the other hand, when the cooling rate exceeds 20 DEG C / sec, the fraction of the low-temperature structure is increased and the strength is increased, but the low-temperature toughness is rapidly lowered.

Tempering

In the tempering step S130, the cooled plate is tempered at a temperature of 630 to 700 ° C.

By this tempering, the hard martensite and bainite structure are softened, and the residual austenite that is organicly transformed to form martensite is decomposed to secure high toughness. Through this, the final microstructure has a composite structure including tempered rath martensite and tempered rath bainite.

At this time, if the tempering temperature is less than 250 ° C, softening of the low-temperature structure is not sufficient, and it may be difficult to secure toughness. On the other hand, when the tempering temperature exceeds 300 캜, the low-temperature structure becomes too soft and it may be difficult to secure the strength.

In this step, the tempering is preferably carried out for 1 to 30 minutes.

When the tempering time is less than 1 minute, the softening of the cold tissue is not sufficient. On the other hand, if the tempering time exceeds 30 minutes, the low temperature structure becomes too soft due to excessive tempering, which makes it difficult to secure strength, and there is a problem that the stretch flangeability is deteriorated by precipitation of cementite.

The steel sheet manufactured in the above steps S110 to S130 is subjected to a quenching & tempering (QT) process after the cooling step by applying a DQT (Direct Quenching & Tempering) process for rapid cooling and tempering after the hot rolling step The process can be simplified, the manufacturing cost can be reduced, and the productivity can be improved.

In addition, the steel sheet produced by the above-mentioned method has a stable mechanical property due to the effect of precipitation strengthening during tempering by the addition of vanadium (V), so that the final microstructure is formed by tempered lath martensite and tempered lath bainite and a tempered lath bainite structure, wherein the tempered rathsite structure has a cross sectional area ratio of 70 to 80%.

As a result, the steel sheet 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 18% or more and impact absorption energy of 200 J or more at -40 캜 .

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

The specimens of Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the compositions of Tables 1 and 2 and the process conditions of Table 3. At this time, in the case of the hot-rolled samples according to Examples 1 to 3 and Comparative Examples 1 and 2, ingots having respective compositions were prepared and reheated, hot rolled, cooled and tempered using a rolling simulation tester. Thereafter, tensile test and low-temperature impact test were carried out on the specimens according to Examples 1 to 3 and Comparative Examples 1 and 2.

[Table 1] (unit:% by weight)

[Table 2] (unit:% by weight)

[Table 3]

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 3 and Comparative Examples 1 and 2.

[Table 4]

Tensile strength (TS): 760 to 895 MPa, yield strength (YS): 690 MPa or more, elongation (EL): 18 corresponding to the target value in the case of the specimens according to Examples 1 to 3 % And the impact absorption energy at -40 DEG C: not less than 200J.

On the other hand, in the case of the specimens according to Comparative Examples 1 and 2, the mechanical properties were similar to those of the specimen according to Example 1, but the impact absorption energy values at -40 ° C were measured to be 106 J and 97 J, respectively.

2 is a graph showing the results of low-temperature impact characteristics test on the specimens according to Examples 1 to 3.

As shown in FIG. 2, it can be seen that, in the case of the specimens according to Examples 1 to 3, the low-temperature impact characteristic at 0 ° C or less is all over 200 J regardless of the temperature change.

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: Hot rolling step
S120: cooling step
S130: Tempering step

Claims

(a) 0.10 to 0.13% of C, 0.2 to 0.3% of Si, 0.7 to 0.9% of Mn, 0.02% or less of P, 0.005% or less of S, 0.03 to 0.07% (Fe), 0.5 to 0.5% of Cr, 0.9 to 1.0% of Ni, 0.4 to 0.5% of Mo, 0.15 to 0.25% of Cu, 0.02 to 0.02% of Ti, 0.035 to 0.045% of V, 0.0005 to 0.0030% of B, And Finish Rolling Temperature (FRT): 900 to 950 占 폚;
(b) subjecting the hot-rolled sheet to direct quenching (DQ) to cool the sheet to a temperature of 250 to 300 캜; And
(c) tempering the cooled plate at a temperature of 630 to 700 ° C.

The method according to claim 1,
In the step (b)
The cooling
At a rate of 15 to 20 占 폚 / sec.

The method according to claim 1,
In the step (c)
The tempering
Wherein the heat treatment is performed for 10 to 60 minutes.

0.10 to 0.13% of C, 0.2 to 0.3% of Si, 0.7 to 0.9% of Mn, 0.02% or less of P, 0.005% or less of S, 0.03 to 0.07% of Al, 0.4 to 0.5% of Cr, (Fe) and unavoidable impurities (Fe) are contained in an amount of 0.1 to 1.0% of Ni, 0.4 to 0.5% of Mo, 0.15 to 0.25% of Cu, 0.02 to 0.25% of Ti, Lt; / RTI >
Characterized in that the final microstructure has a composite structure of tempered lath martensite and tempered lath bainite and the tempered rathsite has a cross sectional area ratio of 70 to 80% Steel plate.

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 and an elongation (EL) of 18% or more.

5. The method of claim 4,
The steel sheet
And an impact absorption energy at -40 DEG C: not less than 200J.