KR20240095963A - Ultra high-strength cold rolled steel sheet having excellent elongation and hole expension ratio and method for manufacturing the same - Google Patents

Abstract

The present invention provides an ultra-high strength cold-rolled steel sheet with excellent elongation and hole expandability and a method for manufacturing the same.

Description

Ultra-high-strength cold-rolled steel sheet with excellent elongation and hole expandability and manufacturing method thereof

The present invention relates to an ultra-high-strength cold-rolled steel sheet with excellent elongation and hole expandability and a method of manufacturing the same.

Recently, the automobile industry is focusing on reducing vehicle body weight and ensuring crash safety in order to improve fuel efficiency and safety of vehicles along with greenhouse gas emissions regulations due to global warming. Accordingly, the demand for securing manufacturing technology for ultra-high strength steel plates is increasing.

Automotive parts using ultra-high-strength steel plates require not only strength but also excellent elongation and hole expandability for forming parts, including formability and weldability. In general, as the strength of steel sheets increases, press formability deteriorates. To overcome this, a method of using TRIP (TRansformation Induced Plasticity) steel utilizing retained austenite is used.

Patent Publication No. 2017-7015003

One aspect of the present invention is to provide an ultra-high-strength cold-rolled steel sheet with excellent elongation and hole expandability and a method for manufacturing the same.

Another aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet that not only has excellent elongation and hole expandability, but also has excellent hydrogen embrittlement resistance and a method of manufacturing the same.

The object of the present invention is not limited to the above-mentioned matters. The additional problems of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional problems of the present invention from the content described in the specification of the present invention.

One embodiment of the present invention is,

In weight percent, C: 0.05~0.4%, Si: 0.1~3.0%, Al: 0.005~3.0%, Mn: 1.0~4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001~0.5%, B: 0.0001~0.003%, Nb: 0.001~0.05%, Ti: 0.001~0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), including the balance Fe and other unavoidable impurities,

As microstructure, in area%, ferrite: 30% or less (including 0%), retained austenite: more than 10% and less than 25%, tempered martensite: more than 40% and less than 80%, bainite: 40% or less (0 % included), contains 5% or less of fresh martensite (including 0%),

A cold-rolled steel sheet is provided where the value defined by the following relational expression 1 satisfies a value greater than 145 and less than 160.

[Relationship 1]

573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73× [Ti] - 68×[Nb] + 100×[B]

(In equation 1 above, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B ] indicates the weight percent content for each element in parentheses.)

In addition, another aspect of the present invention is,

In weight percent, C: 0.05-0.4%, Si: 0.1-3.0%, Al: 0.005-3.0%, Mn: 1.0-4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001-0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), reheating the steel slab, which contains the balance Fe and other inevitable impurities, and whose value defined by the following relational expression 1 satisfies more than 145 and less than 160;

Obtaining a hot rolled steel sheet by performing final hot rolling on the reheated steel slab at 830 to 980°C;

Winding the hot rolled steel sheet at 450 to 700°C;

Cold rolling the coiled hot rolled steel sheet;

Continuously annealing the cold rolled steel sheet at a temperature of 800 to 900°C to meet a dew point temperature of -45°C or lower;

Primary cooling the continuously annealed steel sheet at an average cooling rate of less than 10°C/s to a primary cooling end temperature of 550 to 650°C;

Secondary cooling the primary cooled steel sheet at an average cooling rate of 10°C/s or more to a secondary cooling end temperature of 150 to 400°C; and

It provides a method of manufacturing a cold rolled steel sheet including the step of heat treating the secondary cooled steel sheet in the range of 350 to 480°C.

[Relationship 1]

573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73× [Ti] - 68×[Nb] + 100×[B]

(In equation 1 above, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B ] indicates the weight percent content for each element in parentheses.)

According to one aspect of the present invention, an ultra-high-strength cold-rolled steel sheet with excellent elongation and hole expandability and a method for manufacturing the same can be provided.

According to another aspect of the present invention, it is possible to provide an ultra-high strength cold rolled steel sheet that not only has excellent elongation and hole expandability, but also has excellent hydrogen embrittlement resistance and a method for manufacturing the same.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

Hereinafter, embodiments of the present invention will be described. Of course, various modifications to the following examples can be made by those skilled in the art without departing from the scope of the present invention. The following examples are for understanding of the present invention, and the scope of the present invention should not be limited to the following examples, but should be determined by the claims described below as well as their equivalents.

Meanwhile, the terms used in this specification are for describing the present invention and are not intended to limit the present invention. Additionally, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary.

The meaning of “including” used in the specification specifies a configuration and does not exclude the presence or addition of another configuration.

Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in the dictionary are interpreted to have meanings consistent with related technical literature and current disclosure.

In the prior art, a TRIP steel sheet incorporating retained austenite is disclosed to ensure excellent formability of ultra-high strength steel with a tensile strength of 1470 MPa or higher. However, in order to satisfy high formability at ultra-high strength, a large amount of Si and Al must be added, and as the Si content increases, the possibility of LME (Liquid Metal Embrittlement) occurring during spot welding increases and the manufacturing cost increases.

In particular, when Al, which has a similar effect to Si, is added instead to prevent the occurrence of LME, the load during hot rolling is increased by increasing the transformation temperature of the steel as well as increasing the manufacturing cost. In addition, since a high SS (Soaking Section) temperature is required in Q&P (Quenching & Partitioning) steel, which requires single-phase annealing by increasing the Ac3 temperature, the invention cannot be implemented with current equipment and equipment, and changing both equipment and equipment is costly. This requires an excessive amount of time, and furthermore, there is a problem in that equipment and equipment also need to be newly developed.

Accordingly, it is necessary to develop ultra-high-strength steel with a tensile strength of 1470MPa that has excellent elongation and hole expansion under annealing heat treatment conditions at a workable level while limiting the amount of C, Si, and Al added to the steel sheet, but this has not been reported to date. .

In order to improve local formability, it is effective to reduce the hardness variation between the microstructures that make up the steel. As an industrial local formability evaluation, a widely performed test is the Hole Expansion Ratio (HER) measurement. Hole expandability (HER) involves fixing a specimen with a hole with a diameter of 10 mm with a punch to a die, and expanding the hole by pushing it up with a conical punch to expand the entire thickness. By measuring the diameter of the expanded hole at the time a penetrating crack occurs, a value such as the following relational expression A is obtained. Detailed hole expandability evaluation criteria are based on ISO 16630 regulations.

[Relational Expression A]

λ(HER) = (df-do)/do

(In the above relational expression A, do represents the initial diameter of the hole, and df represents the diameter of the hole at the time of thickness fracture.)

Accordingly, the present inventors precisely set the alloy composition, structure fraction, and manufacturing conditions in order to manufacture steel under conditions suitable for mass production while securing ultra-high strength with a tensile strength of 1470 MPa or higher, and at the same time ensuring excellent elongation and hole expandability. It was discovered that this problem could be solved through control, and the present invention was completed.

Hereinafter, an ultra-high-strength cold-rolled steel sheet with excellent elongation and hole expandability according to an embodiment of the present invention and a method for manufacturing the same will be described.

First, the alloy composition of the cold rolled steel sheet according to an embodiment of the present invention will be described. The content of the alloy composition mentioned below refers to weight percent.

C: 0.05~0.4%

Carbon (C) is an element that secures the strength of steel through solid solution strengthening and precipitation strengthening, and is an effective element for securing high elongation by stabilizing retained austenite. If the C content is less than 0.05%, a tensile strength of 1500 MPa cannot be obtained, and if it exceeds 0.4%, a steel sheet cannot be manufactured by cold rolling. Therefore, the appropriate C content is 0.05 to 0.4%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the C content may be 0.2%, or the upper limit of the C content may be 0.4%.

Si: 0.1~3.0%

Silicon (Si) is a useful element for increasing the strength of steel sheets through solid solution strengthening and precipitation hardening. Because it suppresses the formation of cementite, it has the effect of promoting C enrichment in austenite, and is an essential element for increasing the strength and elongation of steel by generating retained austenite after annealing. If the Si content is less than 0.1%, no residual austenite remains and uniform elongation cannot be obtained. On the other hand, when the Si content exceeds 3.0%, the physical properties of the weld zone deteriorate due to LME cracking, and the surface properties and plating properties of the steel deteriorate. Therefore, the Si content is preferably in the range of 0.1 to 3.0%. In terms of further improving the above-mentioned effect, the lower limit of the Si content may be 0.3%, 0.45%, or 0.5%. Likewise, in terms of further improving the above-mentioned effect, the upper limit of the Si content may be 2.5.

Al: 0.005~3.0%

Aluminum (Al) is an element that has a deoxidizing effect on molten steel, and, similar to Si, has the effect of improving the stability of austenite and is effective in increasing elongation.

If the Al content is less than 0.005%, deoxidation of the steel material is not sufficiently achieved and the cleanliness of the steel material is impaired. On the other hand, if the Al content is too excessive, exceeding 3.0%, the transformation temperature increases significantly, the fraction of ferrite increases, and ultra-high strength cannot be achieved. Therefore, in the present invention, the Al content is set to 0.005 to 3.0%. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the Al content may be 0.01%, or 0.02%. Likewise, in terms of further improving the above-mentioned effect, the upper limit of the Al content may be 2.0% or 1.0%.

Mn: 1.0~4.0%

Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 1.0%, it becomes difficult to secure strength. On the other hand, when the Mn content exceeds 4.0%, the bainite transformation speed is slowed, so too much fresh martensite is formed, and it becomes difficult to obtain high hole expansion. In addition, a band structure is formed due to segregation of Mn, which impairs the material uniformity and formability of the material. Therefore, the Mn content is controlled to 1.0-4.0%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Mn content may be 1.5%, or the upper limit of the Mn content may be 3.5%.

Cr: 1.5% or less (including 0%)

Chromium (Cr) is an element effective in improving strength. It suppresses the formation of carbides and makes it easy to secure retained austenite. On the other hand, when the Cr content exceeds 1.5%, local corrosiveness becomes poor and surface oxides are formed, impairing phosphate treatment properties. Therefore, the Cr content is controlled to 1.5% or less (including 0%). Meanwhile, in terms of further improving the above-mentioned effect, the upper limit of the Cr content may be 1.0%.

Mo: 0.001~0.5%

Molybdenum (Mo) improves the stability of Fe carbide, and precipitates due to Mo improve hydrogen embrittlement resistance. In order to ensure the above-mentioned effect, it is necessary to add Mo in an amount of 0.001% or more. On the other hand, when the Mo content exceeds 0.5%, phase transformation is suppressed, making it difficult to introduce a bainite structure, and as an expensive element, the economic feasibility of the steel sheet deteriorates. Therefore, in the present invention, the Mo content is in the range of 0.001 to 0.5%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Mo content may be 0.07%, or the upper limit of the Mo content may be 0.495%.

B: 0.0001~0.003%

Boron (B) strengthens grain boundaries and suppresses ferrite transformation during cooling after annealing. In order to obtain the above-mentioned effect, the B content is set to 0.0001% and the addition amount is set to 0.0001% or more. On the other hand, when the B content exceeds 0.003%, hot rolling properties are reduced and B is excessively accumulated on the surface, thereby impairing plating properties. Therefore, in the present invention, the B content is set to 0.0001 to 0.003%. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the B content may be 0.0005%, or the upper limit of the B content may be 0.0025%.

Nb: 0.001~0.05%

Neobium (Nb) creates alloy carbides and contributes to strength improvement through precipitation strengthening and structure refinement. In order to obtain the above-mentioned effect, the Nb content is set to 0.001% or more. On the other hand, when the Nb content exceeds 0.05%, recrystallization is delayed due to local grain fixation, thereby impairing the uniformity of the structure. Therefore, in the present invention, the Nb content is set to 0.001 to 0.05%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Nb content may be 0.015%, or the upper limit of the Nb content may be 0.03%.

Ti: 0.001~0.05%

Titanium (Ti) is an element that combines with C or N to form fine precipitates and refine old austenite grains to improve strength and hydrogen embrittlement resistance.

When less than 0.001% of Ti is added, it is difficult to achieve strength improvement and structure refinement effects. On the other hand, if the Ti content exceeds 0.05%, castability is impaired due to excessive formation of TiN, and recrystallization is delayed due to local grain fixation, thereby impairing the uniformity of the structure. Therefore, the Ti content is preferably in the range of 0.001 to 0.05%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Ti content may be 0.015%, or the upper limit of the Ti content may be 0.03%.

P: 0.04% or less (excluding 0%)

Phosphorus (P) is contained as an impurity and segregates at grain boundaries to reduce toughness. Therefore, it is desirable to control the content as low as possible. If P is added excessively, the toughness of the steel deteriorates. Therefore, in the present invention, it is preferable to limit the upper limit to 0.04% to prevent this. However, considering the case where it is inevitably mixed as an impurity during the manufacturing process, 0% is excluded from the above P content. Meanwhile, in terms of further improving the above-described effect, the lower limit of the P content may be 0.002%, or the upper limit of the P content may be 0.0173%.

S: 0.01% or less (excluding 0%)

Sulfur (S), like P, is contained as an impurity in steel. S combines with Mn to form inclusions, which reduces hole expandability and may also reduce weldability and hot rolling properties, so it is advantageous to control it as low as possible. Considering that S content is inevitably included, it is desirable to exclude 0%, but limit the upper limit to 0.01% or less. Meanwhile, in terms of further improving the above-described effect, the lower limit of the S content may be 0.0009%, or the upper limit of the S content may be 0.005%.

N: 0.01% or less (excluding 0%)

In the present invention, nitrogen (N) is included in steel as an impurity, and it is advantageous to control its content as low as possible. Therefore, the lower limit of N content excludes 0% (i.e. exceeds 0%), taking into account cases where N is inevitably included. However, it is desirable to limit the upper limit of N content to 0.01%. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the N content may be 0.0005%. Likewise, in terms of further improving the above-described effect, the upper limit of the N content may be 0.007%, 0.006%, or 0.0052%.

In addition to the steel composition described above, the remainder may include Fe and inevitable impurities. Inevitable impurities can be unintentionally mixed in the normal steel manufacturing process, so they cannot be completely excluded, and any engineer in the normal steel manufacturing field can easily understand their meaning. Additionally, the present invention does not completely exclude the addition of compositions other than the steel compositions mentioned above.

According to one embodiment of the present invention, although not particularly limited, the cold rolled steel sheet is optionally selected from the group consisting of Cu: 0.1% or less (including 0%), Ni: 0.1% or less (including 0%), It may further include one or more selected types.

Cu: 0.1% or less (including 0%), Ni: 0.1% or less (including 0%)

Copper (Cu) and nickel (Ni) are elements that increase the strength of steel. The above elements are elements that increase the strength and hardenability of steel, but if added in excessive amounts, they can exceed the target strength grade, and since they are expensive elements, from an economic standpoint, the upper limit is set to 0.1% or less each. It is desirable to limit it. On the other hand, since Cu and Ni act as solid solution strengthening elements, when adding one or more of Cu and Ni, the solid solution strengthening effect may be minimal if less than 0.03% is added, so it is preferable to add 0.03% or more of each. .

According to one embodiment of the present invention, although not particularly limited, the cold rolled steel sheet may optionally further include V: 0.05% or less (including 0%).

V: 0.05% or less (including 0%)

Vanadium (V) can increase the strength of steel even with the addition of a small amount, but its effect on improving elongation is not significant, so it is desirable to control its content to 0.05% or less. The content of V is more preferably 0.04% or less, and even more preferably 0.03% or less, considering elongation.

The microstructure of the cold rolled steel sheet according to an embodiment of the present invention is, in area%, ferrite: 30% or less (including 0%), retained austenite: more than 10% and less than 25%, tempered martensite: more than 40% 80 % or less, bainite: 40% or less (including 0%), and fresh martensite: 5% or less (including 0%).

Although not particularly limited, according to one embodiment of the present invention, the purpose of the cold rolled steel sheet is to secure excellent formability even at a high tensile strength of 1470 MPa class, and in particular, in order to obtain high local formability, control of additive elements and The hardness difference between the microstructure phases that make up the steel plate must be reduced. In the present invention, by controlling the alloy composition so that the value defined by the following relational equation 1 satisfies more than 145 and less than 160 while satisfying the above-described alloy composition under typical annealing heating conditions, an austenite single phase is obtained and the ferrite fraction is reduced to 30% or less. You can keep it low. However, when the ferrite fraction exceeds 30%, the yield strength decreases and hole expandability deteriorates. Meanwhile, in terms of securing high yield strength and excellent hole expandability, more preferably, the upper limit of the ferrite fraction may be 10%, and even more preferably, the upper limit of the ferrite fraction may be 7%.

[Relational Expression 1]

573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73× [Ti] - 68×[Nb] + 100×[B]

(In equation 1 above, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B ] indicates the weight percent content for each element in parentheses.)

By adjusting the value defined by the above equation 1 to exceed 145 and be less than 160, excessive formation of the soft ferrite phase can be avoided, but if bainite, which is the softest phase next to ferrite, is not sufficiently introduced, the ductility of the steel material cannot be secured. can be difficult. Meanwhile, in terms of further improving the above-described effect, the lower limit of the value defined by equation 1 may be 146, or the upper limit of the value defined by equation 1 may be 159.

In addition, although not particularly limited, according to one embodiment of the present invention, the value defined by the following relational expression 2 may satisfy more than 155 and less than 175.

[Relational Expression 2]

630×[C] - 23×[Si]　+ 410×[P]　- 150×[Cr]-15×[Ni] + 300×[B]

(In the above relational equation 2, [C], [Si], [P], [Cr], and [Ni] represent the weight percent content for each element in parentheses.)

The cold-rolled steel sheet according to the present invention contains tempered martensite in a range of more than 40% to 80% and residual austenite in a range of more than 10% to 25%, bainite: 40% or less (including 0%), and fresh marten. Includes less than 5% of sites (including 0%). If the difference in hardness between these main phases is large, hole expandability may deteriorate, and if retained austenite is insufficient, elongation may decrease. Therefore, a method for reducing the hardness difference between phases and securing the retained austenite fraction is needed.

Accordingly, as a result of careful examination, the present inventors have found that it is possible to manufacture ultra-high strength cold-rolled steel sheets with excellent elongation and hole expansion properties by controlling the components of the added elements and the ratio of the main structure in the appropriate range in relational equations 1 and 2. Found it.

Specifically, the cold rolled steel sheet according to the present invention preferably contains 30% or less (including 0%) of ferrite. When the area fraction of ferrite exceeds 30%, it is difficult to secure high hole certainty as the interphase hardness between soft ferrite and hard columnar tempered martensite and bainite increases.

Retained austenite is preferably greater than 10% and less than 25%. If it is less than 10%, it becomes difficult to secure elongation due to lack of retained austenite. If it exceeds 25%, phase transformation must be performed at a high temperature, and although it has the effect of further improving elongation, the strength of the target level (above 1470 MPa) cannot be obtained due to the lack of tempered martensite fraction.

Bainite is preferably 40% or less. If it exceeds 40%, the strength and elongation decrease due to a lack of tempered martensite and retained austenite, and as the bainite fraction decreases, the desired elongation cannot be obtained due to the decrease in the retained austenite fraction. At this time, the lower limit of the bainite fraction may include 0%, exceed 0%, or be 7.4%.

In addition, although not particularly limited, according to one embodiment of the present invention, the material and tissue fraction of the cold rolled steel sheet can be controlled so that the value defined by the following relational expression 3 satisfies 40 or more.

[Relational Expression 3]

[P-El] ² + [RA]

(In equation 3 above, [P-El] represents the post-elongation value of the uniaxial tensile test, and [RA] represents the retained austenite fraction (area %).)

P-El refers to the difference between T-El and U-El, and is a value indicating local formability. In addition, the retained austenite fraction can be controlled so that the value defined by Equation 3 in correlation with elongation satisfies 40 or more. Because of this, ultra-high strength steel sheets with excellent elongation and hole expansion properties can be manufactured even under normal annealing conditions.

In addition, although not particularly limited, according to one embodiment of the present invention, the following relational expression 4 can be satisfied, and through this, a cold-rolled steel sheet excellent in hydrogen embrittlement resistance can be secured.

[Relational Expression 4]

0.3ppm ≤ _IH2

(In the relational equation 4, I _H2 represents the critical hydrogen amount for fracture of the cold rolled steel sheet.)

Meanwhile, the cold-rolled steel sheet of the present invention may have a hot-dip galvanized layer formed on at least one side. In the present invention, there is no particular limitation on the composition of the hot-dip galvanized layer, and any hot-dip galvanized layer commonly applied in the technical field can be preferably applied to the present invention. Additionally, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with some alloy components of the steel sheet.

Hereinafter, a method for manufacturing an ultra-high-strength cold-rolled steel sheet with excellent elongation and hole expandability according to one aspect of the present invention will be described. However, this does not mean that the cold rolled steel sheet of the present invention must be manufactured only by the following manufacturing method.

According to one embodiment of the present invention, the method of manufacturing the cold rolled steel sheet is performed in the order of reheating, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, heat treatment, etc. of steel having the above-described alloy composition. .

[Reheating of steel slabs]

The method of manufacturing a slab for hot rolling is not limited, and as an example, a continuously cast slab may be used, and as another example, a slab manufactured using a thin slab caster or the like may be used. Additionally, hot rolling may be performed immediately after continuous casting. When reheating the slab, the reheating temperature is preferably 1150 to 1250°C. If the heating temperature is less than 1150°C, the finish rolling temperature tends to be less than 850°C and the rolling load increases. From the viewpoint of manufacturing cost, the heating temperature is preferably less than 1250°C.

[Hot rolling]

Thereafter, the reheated slab is subjected to final hot rolling at 830 to 980°C to obtain a hot rolled steel sheet. If the finishing hot rolling temperature (hereinafter also referred to as 'FDT') is less than 830°C, the rolling load is large, shape defects increase, and productivity deteriorates. On the other hand, if the finishing hot rolling temperature exceeds 980°C, the surface quality deteriorates due to an increase in oxides due to excessively high temperature work. Therefore, it is preferable that the finishing hot rolling temperature ranges from 830 to 980°C. The lower limit of the finishing hot rolling temperature is more preferably 880°C. The upper limit of the finish hot rolling temperature is more preferably 950°C, and even more preferably 930°C.

[Winding]

The hot rolled steel sheet obtained by the above hot rolling is wound at 450 to 700°C. If the coiling temperature (hereinafter also referred to as 'CT') exceeds 700°C, thick hot rolling internal oxidation on the surface of the steel sheet may occur and pickling properties may be reduced. In order to improve toughness by refining the effective grain diameter and improve hole expansion by uniformizing retained austenite, the lower limit of the coiling temperature is set to 450°C. Meanwhile, from the viewpoint of further improving the above-mentioned effect, the lower limit of the coiling temperature is more preferably 480°C, and even more preferably 500°C. Likewise, the upper limit of the coiling temperature is more preferably 670°C, and even more preferably 640°C. Meanwhile, although there is no particular limitation, cooling may be performed at an average cooling rate of 10 to 100° C./s to the coiling temperature after the above-mentioned finish hot rolling. If the average cooling rate is less than 10℃/s, hot rolling productivity may decrease and a problem may arise in which a cooling medium with low cooling ability must be deliberately adopted during actual production, and if it exceeds 100℃/s, the temperature deviation inside the steel sheet may occur. It may become uneven, causing problems such as poor shape and excessively high strength of the steel plate.

[Cold rolling]

The coiled hot rolled steel sheet is cold rolled. During the cold rolling, the cold rolling reduction rate may be 30 to 60%. If the cold rolling reduction rate is less than 30%, it may be difficult to secure the target thickness accuracy and correction of the shape of the steel plate may become difficult. On the other hand, if the cold rolling reduction ratio exceeds 60%, the possibility of cracks occurring at the edge of the steel sheet increases, and the cold rolling load may become excessively large. Therefore, it is preferable that the cold rolling reduction ratio is in the range of 30 to 60%.

[Continuous annealing]

The cold rolled steel sheet is continuously annealed at a temperature of 800 to 900°C to satisfy a dew point temperature of -45°C or lower. The continuous annealing step is to heat the steel sheet to the austenite single phase region to form austenite close to 100% and use it for subsequent phase transformation. If the continuous annealing temperature (hereinafter also referred to as 'SS') is less than 800°C, sufficient recrystallization and austenite transformation do not occur, making it impossible to secure the desired martensite and bainite fractions after annealing. On the other hand, if the continuous annealing temperature exceeds 900°C, productivity may decrease, coarse austenite may be formed and the material may deteriorate, and surface quality such as peeling of the plating material may deteriorate. Additionally, the continuous annealing can be performed in a continuous alloying hot dip plating furnace.

Meanwhile, during the continuous annealing, it is preferable to control the atmosphere in the continuous annealing furnace with a gas consisting of 95% or more nitrogen and the balance hydrogen in terms of volume percent. If the nitrogen fraction is less than 95% and the hydrogen ratio is not increased accordingly, an oxidizing atmosphere is formed in the furnace, oxides are formed on the surface of the steel sheet, and the surface quality deteriorates, and the hydrogen ratio increases. Process difficulties, such as risk of explosion, increase.

[Primary cooling]

Thereafter, the continuously annealed steel sheet is cooled to a primary cooling end temperature of 550 to 650°C (hereinafter referred to as 'SCS') at a temperature of less than 10°C/s (more preferably, between 1°C/s and less than 10°C/s). Primary cooling is performed at an average cooling rate of .

The primary cooling end temperature can be defined as the point at which secondary cooling (quick cooling) is initiated by additionally applying quenching equipment that was not applied in primary cooling. If the cooling process is divided into primary and secondary cooling and carried out in stages, the temperature distribution of the steel sheet can be made uniform in the slow cooling stage, the final temperature and material deviation can be reduced, and the necessary phase composition can be obtained. If the primary cooling end temperature is less than 550°C, the bainite fraction becomes excessively high, and it is difficult to cool down to below 550°C at a cooling rate of less than 10°C/s due to the actual equipment length. In other words, cooling below 550°C may require changes to equipment or additional installation, which may result in additional costs. In addition, if the primary cooling end temperature exceeds 650°C, the required cooling temperature up to the secondary cooling end temperature increases, resulting in poor shape of the steel sheet and a lower bainite fraction compared to the target level.

[Secondary cooling]

Thereafter, the primarily cooled steel sheet is secondary cooled at an average cooling rate of 10°C/s or more to a secondary cooling end temperature of 150 to 400°C (hereinafter referred to as 'RCS'). The secondary cooling end temperature is set to be below the Ms temperature of the steel sheet, so that martensite transformation occurs during cooling, and this martensite ultimately becomes a tempered martensite phase through the post-process reheating step. If the secondary cooling end temperature is less than 150°C, the initial martensite transformation amount is too large, resulting in excessively high yield strength and poor formability. On the other hand, if the secondary cooling end temperature exceeds 400°C, martensite is not generated during cooling, and ultimately, a large amount of fresh martensite is generated, which may exceed the appropriate tensile strength and has high yield strength and hole expandability. hard to get If the secondary cooling rate is less than 10°C/s, even if the target secondary cooling end temperature is reached, high-temperature phase transformation occurs during cooling, making it impossible to obtain the target martensite fraction and high strength. More preferably, in terms of improving the above-mentioned effect, the lower limit of the secondary cooling rate may be 20°C/s, and the upper limit of the secondary cooling rate may be 60°C/s.

As mentioned above, the secondary cooling may additionally apply a quenching facility that was not applied in the primary cooling, and the present invention does not specifically limit the type of the quenching facility, but a hydrogen quenching facility is a preferred example. Available. More specifically, the hydrogen quenching facility can use a gas consisting of 50 to 80% hydrogen and the balance nitrogen by volume. If the hydrogen fraction exceeds 80%, there may be a disadvantage in that it becomes difficult to manage equipment such as explosion control, and if it is less than 50%, there may be a disadvantage in that it becomes difficult to utilize the efficient heat transfer characteristics of hydrogen, a light element. .

[Heat treatment]

Thereafter, the secondary cooled steel sheet is reheated to 350-480°C. Through the above process, interphase carbon distribution and additional bainite phase transformation necessary for stabilization of retained austenite are obtained. In the present invention, the end point temperature of the heating section is referred to as reheating temperature (hereinafter also referred to as 'RHS') for convenience. If the reheating temperature is less than 350°C, the strength becomes excessively high and the elongation deteriorates. On the other hand, when the reheating temperature exceeds 480°C, the austenite phase is not transformed and remains, but becomes fresh martensite during final cooling, impairing hole expandability and elongation. Meanwhile, the so-called nose temperature at which bainite transformation is most active is about 400 to 420 degrees Celsius. In consideration of this, the lower limit of the reheating temperature is more preferably 410°C, or the upper limit of the reheating temperature is more preferably 440°C.

In addition, in one embodiment of the present invention, after the reheating step, if necessary, the reheated steel sheet may be additionally subjected to hot dip galvanizing, alloyed hot dip galvanizing, and temper rolling processes. Specifically, it may further include plating the reheated steel sheet in a zinc plating bath at 450 to 470°C.

In addition, an embodiment of the present invention may further include, if necessary, alloying heat treatment of the plated steel sheet at a temperature in the range of 470 to 550°C. The alloying heat treatment is to obtain an appropriate alloying level, and the temperature is determined according to the surface condition of the steel sheet. By controlling the surface condition of the steel, the alloying heat treatment temperature should not exceed 550°C to prevent softening and residual steel sheet due to excessive tempering. It can prevent the loss of austenite. Meanwhile, in order to quickly proceed with alloying, the alloying heat treatment temperature is preferably higher than the hot-dip galvanizing temperature, so the lower limit is controlled to 470°C. In addition, after the alloying heat treatment, in order to correct the shape of the steel sheet and adjust the yield strength, the step of cooling the alloyed heat-treated steel sheet to room temperature and then temper rolling at a reduction rate of less than 1% may be further included.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating and embodying the present invention and are not intended to limit the scope of the present invention. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

(Example)

After preparing a slab with an alloy composition shown in Tables 1 to 2, it is reheated at 1180 to 1220°C, and then hot rolled, coiled, cold rolled, continuously annealed, primary cooled, and secondary cooled under the conditions shown in Tables 3 to 4 below. , heat treatment was performed to manufacture cold rolled steel sheets. Meanwhile, during the continuous annealing, the atmosphere of the continuous annealing furnace was controlled as a gas consisting of 95% nitrogen and the balance hydrogen in terms of volume percentage, and the dew point temperature at a temperature of 800 to 900 ° C was controlled to -45 ° C.

The results of evaluating the tensile properties, elongation, and hole expandability of the cold-rolled steel sheet manufactured in this way are shown in Table 5 below. Tensile strength (TS), yield strength (YS), and elongation (EL) were measured through a tensile test in the direction perpendicular to rolling. The gauge length was 50 mm and the width of the tensile specimen was 25 mm. used. Hole expandability (HER) was measured according to the ISO 16330 standard, and the hole was sheared with a clearance of 12% using a 10 mm diameter punch.

In addition, to evaluate hydrogen embrittlement resistance, an experiment was conducted by immersing the steel in a 0.1N HCl solution for 120 hours under an applied stress of 80% of the tensile strength (TS) after 4-point bending, to determine which steel type did not fracture after 120 hours of immersion. The rupture critical hydrogen amount was measured and shown in Table 5 below. At this time, cases where fracture occurred after immersion for 120 hours were evaluated as 'X', meaning that the hydrogen embrittlement resistance standard was not met, and cases where fracture did not occur were evaluated as '○'.

In addition, the results of measuring the microstructure of the cold rolled steel sheet manufactured above and the calculation results of relational equation 3 used in the present invention are shown in Table 5.

division Alloy composition (% by weight) Steel grade C Si Mn P S Al Cr Ni Mo A 0.313 2.02 2.48 0.010 0.0025 0.0320 0.00 0.00 0.001 B 0.329 1.96 2.47 0.0090 0.0026 0.0421 0.00 0.00 0.001 C 0.342 1.92 2.46 0.0084 0.0026 0.0480 0.00 0.096 0.001 D 0.334 1.97 2.47 0.0098 0.0025 0.0424 0.00 0.00 0.106 E 0.354 1.89 2.65 0.0108 0.0040 0.0461 0.00 0.00 0.001 F 0.347 1.89 2.65 0.0102 0.0042 0.0458 0.00 0.00 0.001 G 0.354 1.88 2.67 0.0097 0.0042 0.0434 0.00 0.00 0.494

division Alloy composition (% by weight) steel grade Ti Nb Cu B N Relation 1 Relation 2 A 0.02 0.000 0.000 0.0018 0.0044 146.05 155.37 B 0.0212 0.000 0.118 0.0021 0.0035 156.06 166.51 C 0.0221 0.000 0.000 0.0021 0.0025 159.36 173.93 D 0.0230 0.0182 0.000 0.0022 0.0037 148.96 169.79 E 0.0211 0.000 0.000 0.000 0.0051 178.04 183.98 F 0.0193 0.0221 0.000 0.000 0.0048 172.69 179.32 G 0.0206 0.000 0.000 0.000 0.0054 138.36 183.76

division steel grade hot acting
thickness
[㎜] cold rolling
thickness
[㎜] Reduction rate
[%] FDT
[℃] CT
[℃] Invention Example 1 A 2.4 1.4 42 905 563 Invention Example 2 B 2.4 1.4 42 899 572 Invention Example 3 C 2.4 1.4 42 921 515 Invention Example 4 D 2.4 1.4 42 864 603 Comparative Example 1 E 2.4 1.4 42 915 595 Comparative Example 2 F 2.4 1.4 42 896 612 Comparative Example 3 G 2.4 1.4 42 933 544

division SS
[℃] SCS
[℃] RCS
[℃] RHS
[℃] OAS
[℃] FCS
[℃] Invention Example 1 874 613 195 404 356 149 Invention Example 2 873 605 203 410 360 152 Invention Example 3 871 624 206 409 359 151 Invention Example 4 894 601 224 405 355 150 Comparative Example 1 878 611 209 412 368 154 Comparative Example 2 877 613 205 408 352 150 Comparative Example 3 871 607 208 406 355 153

division Microstructure (area%) YS TS El PEl HER I _H2 * Hydrogen embrittlement resistance Relation 3 F γ TM FM B [MPa] [MPa] [%] [%] [%] [ppm] [○/X] Invention Example 1 0 14.61 70.59 3.3 11.5 1257 1542 16.8 5.3 25.2 0.388 ○ 42.7 Invention Example 2 0 17.74 70.76 3.3 8.2 1271 1543 17.7 5.6 22 0.324 ○ 49.1 Invention Example 3 0 17.74 70.86 4.0 7.4 1263 1545 17.6 5.4 22.1 0.357 ○ 46.9 Invention Example 4 0 18.9 69.88 3.5 7.9 1192 1535 18 5 20.9 0.316 ○ 43.9 Comparative Example 1 0 17.94 65.43 7.3 9.3 1193 1532 18 4.6 7.9 0.205 X 39.1 Comparative Example 2 0 17.96 63.44 8.1 10.5 1181 1538 17.4 4.2 7.1 0.233 X 35.6 Comparative Example 3 0 17.39 66.71 6.4 9.5 1184 1635 16 3.1 12.4 0.163 X 27

I _H2 *: Critical hydrogen amount for fracture

F: Ferrite area ratio

γ: Retained austenite area ratio

TM: tempered martensite area ratio

FM: Fresh martensite area ratio

B: Bainite area ratio

In the case of the invention examples that meet the alloy composition and manufacturing conditions of the present invention, compared to the comparative examples, by having higher tensile strength (TS), yield strength (YS), hole expandability (HER), and elongation (El) , it was confirmed that it had excellent elongation and hole expandability, while also having ultra-high strength characteristics.

On the other hand, in the case of comparative examples that do not meet one or more of the alloy composition and manufacturing conditions of the present invention, one or more properties selected from tensile strength (TS), yield strength (YS), hole expandability (HER), and elongation (El) This inferiority was confirmed.

Claims (6)

In weight percent, C: 0.05-0.4%, Si: 0.1-3.0%, Al: 0.005-3.0%, Mn: 1.0-4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001-0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), including the balance Fe and other unavoidable impurities,
As microstructure, in area%, ferrite: 30% or less (including 0%), retained austenite: more than 10% and less than 25%, tempered martensite: more than 40% and less than 80%, bainite: 40% or less (0%) % inclusive), contains 5% or less of fresh martensite (including 0%),
A cold-rolled steel sheet whose value defined by the following relational expression 1 satisfies more than 145 and less than 160.
[Relationship 1]
573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73× [Ti] - 68×[Nb] + 100×[B]
(In equation 1 above, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B ] indicates the weight percent content for each element in parentheses.)
According to claim 1,
A cold-rolled steel sheet that satisfies the value defined by the following relational expression 2 of more than 155 and less than 175.
[Relational Expression 2]
630×[C] - 23×[Si] + 410×[P] - 150×[Cr]-15×[Ni] + 300×[B]
(In Equation 2 above, [C], [Si], [P], [Cr], and [Ni] represent the weight percent content for each element in parentheses.)
According to claim 1,
A cold-rolled steel sheet that satisfies the value defined by the following relational expression 3 of 40 or more.
[Relational Expression 3]
[P-El] ² + [RA]
(In equation 3 above, [P-El] represents the post-elongation value of the uniaxial tensile test, and [RA] represents the retained austenite fraction %.)
According to claim 1,
A cold rolled steel sheet that satisfies the following relational expression 4.
[Relational Expression 4]
0.3ppm ≤ _IH2
(In the relational equation 4, I _H2 represents the critical hydrogen amount for fracture of the cold rolled steel sheet.)
In weight percent, C: 0.05-0.4%, Si: 0.1-3.0%, Al: 0.005-3.0%, Mn: 1.0-4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001-0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), reheating the steel slab, which contains the balance Fe and other inevitable impurities, and whose value defined by the following relational expression 1 satisfies more than 145 and less than 160;
Obtaining a hot rolled steel sheet by performing final hot rolling on the reheated steel slab at 830 to 980°C;
Winding the hot rolled steel sheet at 450 to 700°C;
Cold rolling the coiled hot rolled steel sheet;
Continuously annealing the cold rolled steel sheet at a temperature of 800 to 900°C to meet a dew point temperature of -45°C or lower;
Primary cooling the continuously annealed steel sheet at an average cooling rate of less than 10°C/s to a primary cooling end temperature of 550 to 650°C;
Secondary cooling the primary cooled steel sheet at an average cooling rate of 10°C/s or more to a secondary cooling end temperature of 150 to 400°C; and
A method of manufacturing a cold-rolled steel sheet, comprising: heat-treating the secondary cooled steel sheet in the range of 350 to 480°C.
[Relational Expression 1]
573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73× [Ti] - 68×[Nb] + 100×[B]
(In equation 1 above, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B ] indicates the weight percent content for each element in parentheses.)
According to claim 5,
The continuous annealing step is a method of manufacturing a cold-rolled steel sheet in which the atmosphere in the continuous annealing furnace is controlled with a gas consisting of 95% or more nitrogen and the balance hydrogen, in percent by volume.