EP4640894A1

EP4640894A1 - Cold-rolled steel sheet and method for manufacturing same

Info

Publication number: EP4640894A1
Application number: EP23907508.8A
Authority: EP
Inventors: Chang-Hyo Seo; Young-Roc Im; Sang-Hyun Kim; Ki-Taek Jung
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-12-05
Publication date: 2025-10-29
Also published as: WO2024136222A1; KR20240098246A; CN120303433A; MX2025006369A; JP2025541886A; EP4640894A4

Abstract

The present invention relates to an ultra-high-strength cold-rolled steel sheet having a tensile strength of 1470 MPa, which is mainly used for automobile crashworthy and structural members, and to a method for manufacturing same.

Description

Technical Field

The present disclosure relates to an ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa, mainly used for automobile collision and structural members, and a method for manufacturing the same.

Background Art

Recently, a steel sheet for automobiles is required to have higher strength to improve fuel efficiency and durability due to various environmental regulations and energy use regulations. In particular, as impact safety regulations for automobiles have recently expanded, high-strength steel having an excellent yield strength is being adopted for use in structural members such as a member, a seat rail, a pillar, and the like to improve impact resistance of a vehicle body. The structural member has a characteristic which is advantageous for impact energy absorption capability as a yield strength is higher than a tensile strength, that is, a yield ratio (tensile strength/yield strength) is higher. However, as the strength of the steel sheet generally increases, elongation may decrease, which may cause a problem of reduced formability. Therefore, there is a need to develop a material that can compensate for this.
Typically, methods for strengthening steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening, etc. However, among such methods, solid solution strengthening and strengthening by grain refinement have the disadvantage that it is very difficult to manufacture high-strength steel having a tensile strength of 490 MPa or more.
Meanwhile, precipitation strengthening-type high-strength steel is a technology that secures strength by strengthening a steel sheet by precipitating carbides and nitrides by adding carbide and nitride forming elements such as Cu, Nb, Ti, and V, or by refining grains through suppressing grain growth by fine precipitates. The technology has the advantage that can easily obtain high strength at low manufacturing costs, but has the disadvantage that high-temperature annealing should be performed to secure ductility by causing recrystallization because a recrystallization temperature rises rapidly due to fine precipitates. In addition, precipitation strengthening steel, which is strengthened by precipitating carbides and nitrides in a ferrite matrix, has the problem that it is difficult to obtain high-strength steel having a strength of 600 MPa or more.
Meanwhile, various types of transformation strengthening-type high-strength steels have been developed, including ferrite-martensite dual phase steel in which hard martensite is included in a ferrite matrix, transformation induced plasticity (TRIP) steel using transformation-induced plasticity of retained austenite, and complexed phase (CP) steel composed of a structure of ferrite and hard bainite or martensite. However, when the tensile strength that can be implemented in such an advanced high strength steel is 1500 MPa, elongation thereof is limited to about 8%. In addition, hot press forming steel, which secures a final strength by rapid cooling through direct contact with a die which is formed at high temperature and then cooled by water, is gaining attention for an application to structural members to secure collision safety. However, the application thereto is not greatly expanded due to the excessive investment costs in facilities and high heat treatment and process costs.
Recently, in order to further improve passenger safety during a collision, seat parts of a vehicle are being strengthened and made lighter at the same time. These parts are manufactured using two methods: roll forming and press forming. Here, seat parts are parts connecting passengers to a vehicle body, and should support passengers with high stress to prevent the passengers from being thrown out of the vehicle during a collision. To this end, high yield strength and yield ratio are required. In addition, most of the processed parts are parts requiring strength flangeability, so the application of a steel material having an excellent hole expansion ratio is required.
Meanwhile, Patent Document 1 ( Japanese Patent Publication No. 3729108 ) discloses a high-strength cold-rolled steel sheet having martensite as a single-phase structure and having a tensile strength of 880 to 1170 MPa by optimizing the composition and heat treatment conditions of the steel sheet. In addition, Patent Document 2 ( Japanese Patent Application Laid-Open No. 2005-272954 ) discloses a method for manufacturing a high-strength steel sheet. In Patent Document 2, a steel sheet in which a volume ratio of a low-temperature transformation phase comprised of martensite and retained austenite accounts for 90% or more of the total metal structure is controlled by heating and maintaining the steel sheet in a dual-phase region to have a structure of fine ferrite and austenite including a lath of the low-temperature transformation phase, and then, through subsequent cooling, to ultimately have a metal structure in which ferrite and the low-temperature transformation phase are finely dispersed in the lath phase. These patent documents claim that high yield strength may be obtained without a water cooling treatment, but there are disadvantages such as greatly reduced ductility or reduced stretch flangeability due to the occurrence of large amounts of austenite in steel.

(Patent Document 1) Japanese Patent Publication No. 3729108
(Patent Document 2) Japanese Patent Publication No. 2005-272954

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide an ultra-high strength cold-rolled steel sheet having excellent formability and a method for manufacturing the same.
An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

According to an aspect of the present disclosure, provided is a cold-rolled steel sheet,

the cold-rolled steel sheet including by weight%: 0.1 to 0.3% of carbon (C), 2.0% or less (excluding 0%) of silicon (Si), 1.5 to 3.0% of manganese (Mn), 1.2% or less (excluding 0%) of chromium (Cr), 0.03 to 0.25% of molybdenum (Mo), 0.1% or less (excluding 0%) of aluminum (Al), 0.001 to 0.015% of phosphorus (P), 0.001 to 0.01% of sulfur (S), 0.001 to 0.01% of nitrogen (N), 0.001 to 0.005% of boron (B), and a balance of Fe and other unavoidable impurities,
wherein a microstructure includes by area%, 75 to 90% of a total content of bainite and tempered martensite, 10% or less (excluding 0%) of retained austenite, and a balance of fresh martensite,
wherein the following Relational Expressions 1 and 2 are satisfied. 1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

In Relational Expression 1 above, [C], [Si], [Mn], [Cr], [Mo] and [B] represent a weight% content for each element in parentheses. $106 \leq ([Cr] + [Mo]) \times [Si] / [Al] \leq 275$
In Relational Expression 2 above, [Cr], [Mo], [Si] and [Al] represent a weight% content for each element in parentheses.
The microstructure may include by area%, 4.8 to 9.6% of retained austenite.
The microstructure may include by area%, 81 to 89% of a total content of bainite and tempered martensite.
According to another aspect of the present disclosure, provided is a method for manufacturing a cold-rolled steel sheet,

the method including: reheating a slab including by weight%, 0.1 to 0.3% of carbon (C), 2.0% or less (excluding 0%) of silicon (Si), 1.5 to 3.0% of manganese (Mn), 1.2% or less (excluding 0%) of chromium (Cr), 0.03 to 0.25% of molybdenum (Mo), 0.1% or less (excluding 0%) of aluminum (Al), 0.001 to 0.015% of phosphorus (P), 0.001 to 0.01% of sulfur (S), 0.001 to 0.01% of nitrogen (N), 0.001 to 0.005% of boron (B), and a balance of Fe and other unavoidable impurities, wherein the slab is satisfying the following Relational Expressions 1 and 2,
obtaining a hot-rolled steel sheet by final hot-rolling the above-mentioned reheated slab at Ar3 to Ar3+50°C ;
coiling the hot-rolled steel sheet at a temperature within a range of 500 to 750°C;
cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet at a temperature within a range of 800 to 900°C;
primarily cooling the continuously annealed cold-rolled steel sheet to a primary cooling end temperature within a range of 650 to 700°C at an average cooling rate of 1 to 10°C/s;
secondarily cooling the primarily-cooled cold-rolled steel sheet to a secondary cooling end temperature at an average cooling rate of more than 10°C/s and 20°C/s or less; and
over-aging heat treating the secondarily-cooled cold-rolled steel sheet at a temperature within a range of 250 to 350°C. 1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

In Relational Expression 1 above, [C], [Si], [Mn], [Cr], [Mo] and [B] represent a weight% content for each element in parentheses. $106 \leq ([Cr] + [Mo]) \times [Si] / [Al] \leq 275$
In Relational Expression 2 above, [Cr], [Mo], [Si] and [Al] represent a weight% content for each element in parentheses.
The manufacturing method may satisfy the following Relational Expression 3. $8 \leq 3.1 \times ([SS] - [Ac 1]) + 2.2 \times ([RCS] - [Ms]) \leq 180$
In Relational Expression 3 above, [Ac1] is a value defined by the following Relational Expression 4, [RCS] represents a secondary cooling end temperature (°C), and [Ms] represents a martensite transformation start temperature (°C). $[Ac 1] = 723 - 10.7 \times [Mn] - 16.9 \times [Ni] + 29.1 \times [Si] + 16.9 \times [Cr]$
In Relational Expression 4 above, [Mn], [Ni], [Si] and [Cr] represent a weight% content for each element in parentheses.
The method for manufacturing a cold-rolled steel sheet may further include skin pass rolling the over-aging heat-treated cold-rolled steel sheet in the range of 0.1 to 1.0%.

Advantageous Effects of Invention

As set forth above, according to an aspect of the present disclosure, an ultra-high strength cold-rolled steel sheet having excellent formability and a method for manufacturing the same may be provided.
The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and may be more easily understood through descriptions of specific embodiments of the present disclosure.

Brief description of drawings

FIG. 1 illustrates a photograph of a microstructure of a specimen obtained from Inventive Example 1, taken using a scanning electron microscope (SEM).

Best Mode for Invention

Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure may be modified in various other forms, and the scope of the present disclosure is not limited to the embodiments described below. In addition, the embodiments of the present disclosure are provided to more completely explain the present disclosure to a person having average knowledge in the art.
Meanwhile, the terms used herein are intended to describe specific embodiments and are not intended to limit the present disclosure. For example, the singular forms used herein include the plural forms unless the relevant definition clearly indicates a meaning contrary thereto. In addition, the term "comprising" as used in the specification means specifying a configuration, and does not exclude the presence or addition of other configurations.
As mentioned above, in order to manufacture a steel material having an excellent hole expansion ratio (HER) of 20% or more and elongation of 10% or more in steel suggested in the present disclosure, microstructure control is very important. A technology for securing a uniform structure is required to simultaneously increase stretch flangeability and elongation. A structure with the highest strength among low-temperature structures is generally martensite, and as previously known, the easiest method to form martensite is to maintain the steel material for a period of time, sufficient to form austenite during annealing, and then cooled by water and tempered. However, since the water cooling method may cause productivity degradation due to problems such as material deviation and shape defects, the present disclosure was intended to secure martensite by controlling alloying elements. That is, it is a technology to secure martensite even at low cooling speeds by adding a certain amount or more of hardenable elements such as Mn and Cr. However, this method can cause problems such as weldability degradation due to the addition of high alloying elements. Therefore, in the present disclosure, a carbon content, having the greatest influence on weldability, was minimized. In steel of the present disclosure, the carbon content was limited to 0.3% or less.
In order to secure a high yield ratio under cooling conditions such as that of in steel of the present disclosure, alloying elements should be added in as large a quantity as possible. However, such attempts cause additional problems such as deterioration in weldability and an increase in hot-rolled strength, so a solution thereof is needed. Accordingly, the inventors of the present disclosure have discovered that the stretch flangeability and yield ratio suggested by steel of the present disclosure may be satisfied when the size of martensite, and nano precipitates are controlled without adding excessive alloying elements through various studies, thereby completing the present disclosure. Hereinafter, alloy compositions and microstructural characteristics for securing an ultra-high strength required in the present disclosure, as well as a hole expansion ratio of 20% or more and elongation of 10% or more are described in detail.
First, the reasons for adding alloy components of the cold-rolled steel sheet of the present disclosure and the reasons for limiting a content thereof will be described in detail. It should be noted that the content of each component described below is based on weight% unless otherwise specifically stated.

Carbon (C): 0.1 to 0.3%

In steel, carbon (C) is a very important element added to strengthen a transformation structure. Carbon promotes high strength and promotes the formation of martensite in a transformation structure steel. When a content of C is less than 0.1%, it is very difficult to secure the strength of martensite suggested in the present disclosure, so the content of carbon is set to 0.1% or more. Meanwhile, as the content of carbon increases, an amount of martensite in steel increases. However, when the content of carbon exceeds 0.3%, the strength of martensite may increase, but a difference in strength with ferrite with a low carbon concentration may increase. Such a difference in strength causes stretch flangeability to deteriorate, since fracture easily occurs at an interface between phases when stress is applied. In addition, the weldability is deteriorated, so weld defects occur when processing parts. The content of carbon may be preferably 0.10 to 0.30%.

Silicon (Si): 2.0% or less (excluding 0%)

In steel, silicon (Si) promotes ferrite transformation and increases a carbon content in untransformed austenite, forming a composite structure of ferrite and martensite, thereby hindering an increase in strength of martensite. In addition, since it is preferable to limit the possible addition of Si because it not only causes surface scale defects in terms of surface characteristics but also reduces phosphatability, in the present disclosure, a content of Si is controlled to be 2.0% or less. However, considering the case in which Si is inevitably included, 0% is excluded as a lower limit of the content of Si. Preferably, the content of Si may be 2.00% or less.

Manganese (Mn): 1.5 to 3.0%

In steel, manganese (Mn) is an element which refines particles without damaging ductility, completely precipitates sulfur in steel as MnS, prevents hot brittleness caused by the formation of FeS, and strengthens steel, and simultaneously serves to lower a critical cooling rate at which a martensite phase is formed, making it easier to form martensite. When a content of Mn is less than 1.5%, it is difficult to secure the strength targeted by the present disclosure. On the other hand, when the content of Mn exceeds 3.0%, there is a high possibility that problems such as weldability and hot rolling properties may occur, so the content of Mn is set to be in the range of 1.5 to 3.0%. Preferably, the content of Mn may be 1.50 to 3.00%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Mn may be 2.0%, or an upper limit of the content of Mn may be 2.9%.

Phosphorous (P): 0.001 to 0.015%

In steel, phosphorous (P) is a substitutional alloying element with the greatest solid solution strengthening effect, which improves in-plane anisotropy and improves strength. Therefore, when a content of P is less than 0.001%, not only may the above-described effect not be secured, but it also may cause a problem of manufacturing costs. On the other hand, when the content of P is excessively added to exceed 0.015%, press formability may deteriorate and brittleness of steel may occur, so the content of P is set to be in the range of 0.001 to 0.015%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of P may be 0.003%, or an upper limit of the content of P may be 0.014%.

Sulfur (S): 0.001 to 0.01%

In steel, sulfur (S) is an impurity element which impairs ductility and weldability of a steel sheet. Therefore, when a content of S exceeds 0.01%, there is a high possibility that the ductility and weldability of the steel sheet may be impaired, so the content of S is set to 0.01% or less. On the other hand, considering the case in which S is inevitably included, a lower limit of the content of S is set to 0.001%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the content of S may be 0.002%, or an upper limit of the content of S may be 0.009%.

Aluminum (Al): 0.1% or less (excluding 0%)

In steel, soluble aluminum (Al) is an effective component which combines with oxygen to perform deoxidation and distribute carbon in ferrite to austenite, like Si, thereby improving martensite hardenability. Therefore, in order to secure the above-described Al effect, 0% is excluded as a lower limit of a content of Al. However, when the content of Al exceeds 0.1%, the effect is saturated and the manufacturing cost increases, so the content of Al is set to 0.1% or less. Preferably, the content of Al may be 0.10% or less. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Al may be 0.001%, or an upper limit of the content of Al may be 0.09%.

Nitrogen (N): 0.001 to 0.01%

In steel, nitrogen (N) is an effective component in stabilizing austenite. When a content of N exceeds 0.01%, the risk of cracks occurring during continuous casting due to AlN formation, or the like, greatly increases, so it is preferable to limit an upper limit of the content of N to 0.01%. In addition, considering the case in which N is inevitably included, a lower limit of the content of N is set to 0.001%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the content of N may be 0.002%, or an upper limit of the content of N may be 0.009%.

Chromium (Cr): 1.2% or less (excluding 0%)

In steel, chromium (Cr) is a component, added to improve the hardenability of steel and secure high strength, and in the present disclosure, Cr is an element which plays a very important role in forming martensite, a low-temperature transformation phase. In order to secure the above-described effect, 0% is excluded as a lower limit of a content of Cr. However, when the content of Cr exceeds 1.2%, not only will the effect be saturated, but an excessive increase in hot-rolled strength will also cause a problem of deterioration in cold rollability, so the content of Cr is set to 1.2% or less. Preferably, the content of Cr may be 1.20% or less. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Cr may be 0.01%, or an upper limit of the content of Cr may be 1.19%.

Boron (B): 0.001 to 0.005%

In steel, boron (B) is a component which delays the transformation of austenite into pearlite during a cooling process during annealing, and is added as an element which suppresses the formation of ferrite, and promotes the formation of martensite. However, when the content of B is less than 0.001%, it is difficult to obtain the above-described effect, and when the content of B exceeds 0.005%, deterioration in costs may occur due to excessive alloy iron, so the content of B is set to 0.001 to 0.005%.

Molybdenum (Mo): 0.03 to 0.25%

Molybdenum (Mo) is an element added to secure strength and hardenability, and when added together with Ti, Mo forms carbides together with Ti. In order to obtain a structure strengthening effect due to the formation of such carbides, the content of Mo should be at least 0.03%. However, Mo is an expensive element, so when Mo is added excessively, it will not only result in poor economic feasibility, but also delay the phase transformation too much, which may induce the formation of fresh martensite. Therefore, the content of Mo is set to 0.25% or less. Preferably, the content of Mo may be 0.030 to 0.250%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Mo may be 0.04%, or an upper limit of the content of Mo may be 0.24%.
In addition, although not particularly limited, according to an aspect of the present disclosure, the cold-rolled steel sheet may optionally further include at least one element of Ti and Nb, and Ti and Nb are elements which is effective in increasing the strength of the steel sheet and refining the crystal grains by nano-precipitates, in steel. In the present disclosure, when Ti and Nb are added, the content of Ti may be 0.01 to 0.08%, or the content of Nb may be 0.01 to 0.05%. Meanwhile, when Ti and Nb are added in large quantities as in the present disclosure, Ti and Nb combine with carbon to form very fine nano-precipitates. Such nano-precipitates strengthen a matrix structure and play a role in reducing the difference in hardness between phases.
The remaining component of the present disclosure is iron (Fe). However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
Next, the cold-rolled steel sheet according to the present disclosure includes a microstructure including by area %, 75 to 90% of a total content of bainite and tempered martensite, and 10% or less (excluding 0%) of retained austenite, and a balance of fresh martensite.
In the present disclosure, the total content of bainite and tempered martensite, which are transformed structures, should be controlled to 75% or more and 90% or less, and an area fraction of retained austenite should be controlled to 10% or less. In order to increase the hole expansion ratio (HER) and the yield ratio (YR), the higher a fraction of the transformed structure as possible, the better it is, but considering the elongation, it is recommended to control the fraction to 90% or less. As in the present disclosure, when the carbon content is low, such as 0.3% or less, when alloying elements are added considering weldability and hot rolling strength, there is a limit to an increase in the strength of martensite produced. That is, if sufficient carbon is not included in the martensite, there is a limit to the increase in strength. However, the inventors of the present disclosure have discovered that a cold-rolled steel sheet having the ultra-high strength targeted by the present disclosure even though the carbon content is as low as 0.3% or less may be provided.
Meanwhile, in terms of further improving the above-described effect, a lower limit of the area fraction of retained austenite may be 4.8%, or an upper limit of the area fraction of retained austenite may be 9.6%. Alternatively, a lower limit of the total area fraction of bainite and tempered martensite may be 81%, or an upper limit of the total area fraction of bainite and tempered martensite may be 89%.
In addition, the cold-rolled steel sheet according to the present disclosure satisfies the following Relational Expressions 1 and 2. That is, in the present disclosure, in order to secure strength and elongation, in order to secure a hole expansion ratio (HER) of at least 20% and elongation (El) of at least 10% under conditions of securing a certain ductility through numerous experiments targeting a steel material present in the component range proposed in steel of the present disclosure, it was confirmed that the steels manufactured in this case have very important components and manufacturing conditions, and that it is important that these factors satisfy the following Relational Expressions 1 and 2. 1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2
In Relational Expression 1 above, [C], [Si], [Mn], [Cr], [Mo] and [B] represent a weight% content for each element in parentheses. $106 \leq ([Cr] + [Mo]) \times [Si] / [Al] \leq 275$
In Relational Expression 2 above, [Cr], [Mo], [Si] and [Al]represent a weight% content for each element in parentheses.
Hereinafter, a method for manufacturing a cold-rolled steel sheet according to the present disclosure is specifically described.
After reheating the slab comprised of the components using the alloy design method, hot rolling is performed. Finish rolling is preferably performed so that an outlet temperature is between Ar3 to Ar3+50°C in the hot rolling. That is, when the outlet temperature is lower than Ar3 in the finish rolling, there is a high possibility that hot deformation resistance may be rapidly increased, and also, top, tail, and edge portions of a hot-rolled coil become a single-phase region to increase in-plane anisotropy, so that formability may be deteriorated. However, when the outlet temperature exceeds Ar3+50°C in the finish rolling, not only will an excessively thick oxide scale occur, but there is a high possibility that the microstructure of the steel sheet will become coarse. Meanwhile, since the Ar3 may be obtained by a method commonly known in the relevant technical field, it is not particularly limited. In addition, more specifically, the finish rolling may be performed in the temperature range of 880 to 920°C.
After the finish rolling in the hot rolling is completed, coiling is performed at a temperature within a range of 500 to 750°C. When a coiling temperature is lower than 500°C, excessive martensite or bainite is generated, which causes excessive strength increase of the hot-rolled steel sheet, which may cause manufacturing problems such as shape defects due to a load during cold rolling. On the other hand, when the coiling temperature exceeds 750°C, the acidity deteriorates due to an increase in surface scales, so it is preferable to limit the coiling temperature to a temperature within a range of 500 to 750°C.
The hot-rolled steel sheet manufactured in the manner described above is pickled and then cold rolled to obtain a cold-rolled steel sheet.
The cold-rolled steel sheet obtained in this manner is continuously annealed at a continuous annealing temperature (SS) of 800 to 900°C. When the continuous annealing temperature is low, a large amount of ferrite is generated, making it impossible to secure YS and TS. On the other hand, when the continuous annealing temperature is too high, a packet size of martensite produced during cooling increases due to the increase in the austenite grain size resulting from high-temperature annealing, making it difficult to secure the properties desired in the present disclosure.
The continuously annealed cold-rolled steel sheet is subjected to primary cooling to a primary cooling end temperature within a range of 650 to 700°C at an average cooling rate of 1 to 10°C/s. The primary cooling is performed to suppress ferrite transformation and transform most austenite into martensite.
Next, the primarily-cooled cold-rolled steel sheet is subjected to secondary cooling to the second cooling end temperature (RCS) at an average cooling rate of more than 10°C/s and 20°C/s or less, and the secondarily-cooled cold-rolled steel sheet is subjected to over-aging treatment of maintaining and heat treating the same at a temperature within a range of 250 to 350°C. Such a secondary cooling end temperature (RCS) is a very important temperature condition for securing a coil shape in a width direction and longitudinal direction, and securing high YR and high HER. When the cooling end temperature is low, the yield strength and tensile strength may increase simultaneously due to an excessive increase in the amount of martensite during overaging, and the ductility may deteriorate significantly. In particular, shape deterioration due to rapid cooling occurs, so that deterioration in workability is expected when processing automotive parts. Meanwhile, when the secondary end temperature is too high, austenite generated during annealing may not be transformed into martensite, and bainite and granular bainite, etc which are high-temperature transformation phases, are generated, causing a problem in which the yield strength rapidly deteriorates. The occurrence of such a structure is accompanied by a decrease in yield ratio and deterioration in a hole expansion ratio, making it impossible to manufacture high-yield ratio high-strength steel having excellent stretch flangeability proposed in the present disclosure.
According to an aspect of the present disclosure, a method for manufacturing the cold-rolled steel sheet may be managed to satisfy the following Relational Expression 3. By satisfying the following Relational Expression 3, a cold-rolled steel sheet having excellent strength and hole expansion ratio may be effectively provided. $8 \leq 3.1 \times ([SS] - [Ac 1]) + 2.2 \times ([RCS] - [Ms]) \leq 180$
In Relational Expression 3 above, [Ac1] is a value defined by the following Relational Expression 4, [RCS] represents a secondary cooling end temperature (°C), and [Ms] represents a martensite transformation start temperature (°C). $[Ac 1] = 723 - 10.7 \times [Mn] - 16.9 \times [Ni] + 29.1 \times [Si] + 16.9 \times [Cr]$
In Relational Expression 4 above, [Mn], [Ni], [Si] and [Cr] represent a weight% content for each element in parentheses.
In this case, [Ms] refers to a value defined by the following Relational Expression 5. $Ms = 539 - 423 \times [C] - 30.4 \times [Mn] - 12.1 \times [Cr] - 7.5 \times [Mo]$
In Relational Expression 5 above, [C], [Mn], [Cr] and [Mo]represent a weight% content for each element in parentheses.
Skin pass rolling is performed on the over-aging heat-treated cold-rolled steel sheet described above in the range of 0.1 to 1.0%. When a normal transformation structure steel is skin pass rolled, there is almost no increase in tensile strength, and at least 50 MPa or more in yield strength increases. If a rolling ratio of the skin pass rolling is less than 0.1%, it is very difficult to control the shape of ultra-high strength steel such as in steel of the present disclosure, and if the rolling ratio of the skin pass rolling is performed exceeding 1.0%, operability becomes significantly unstable due to a high elongation operation, so the value is set to 0.1 to 1.0%.

Mode for Invention

Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

(Example)

A steel slab having the composition described in Table 1 below was vacuum melted, heated in a heating furnace at a reheating temperature of 1200°C for 1 hour, hot-rolled, and then coiled. The temperature conditions during hot rolling were as shown in Table 2 below. More specifically, hot rolling was finished in the temperature range of 880 to 920°C to satisfy the range of Ar3 to Ar3+50°C based on Ar3 or higher for each example, and the coiling temperature was controlled to 500 to 680°C. Pickling was performed using a hot-rolled steel sheet and cold rolling was performed. For the cold-rolled steel sheet thus obtained, continuous annealing was performed at a continuous annealing temperature (SS) under the conditions described in Table 2 below, followed by primary cooling to a primary cooling end temperature of 650 to 700°C at an average cooling rate of 5°C/s, and then secondary cooling to a secondary cooling end temperature (RCS) was performed at an average cooling rate of 15°C/s. Next, the secondarily-cooled cold-rolled steel sheet was subjected to an over-aging heat treatment at a temperature within a range of 250 to 350°C, and a skin pass rolling ratio was finally fixed at 0.2%.
A total content of bainite and tempered martensite, and an area fraction of retained austenite and fresh martensite for each steel sheet manufactured according to the change in each steel component and annealing condition were measured and shown in Table 3 below.

In addition, JIS No. 5 tensile test specimens were produced, and a yield strength (YS), a tensile strength (TS), elongation (El), and hole expansion ratio (HER) were measured according to JIS standards, and the results were shown in Table 3 below together with Comparative Examples. Meanwhile, for the expansion ratio, when D_o is an initial hole diameter (mm) and Dh is a hole diameter after fracture (mm), it was calculated according to the following Equation.

HER (%) = (D_{h} - D_{o}) / D_{o} \times 100

[Table 1]

Division	Composition (wt%)
Division	C	Si	Mn	Cr	Mo	B	Al	P	S	N
Inventiv e Steel 1	0.245	0.61	2.48	0.32	0.048	0.0014	0.0021	0.009	0.003	0.004
Inventiv e Steel 2	0.212	0.8	2.68	0.63	0.09	0.0021	0.0021	0.011	0.004	0.005
Inventiv e Steel 3	0.252	1. 48	2.4	0.05	0.1	0.0018	0.0018	0.011	0.004	0.005
Comparat ive Steel 1	0.331	0.38	2.69	0.53	0.12	0.0022	0.025	0.011	0.003	0.005
Comparat ive Steel 2	0.18	0.65	2.45	0.21	0.07	0.0017	0.025	0.012	0.003	0.005

[Table 2]

Division	Relational Expression 1	Relational Expression 2	Ar3 [°C]	Ac1 [°C]	Ms [°C]
Inventive Steel 1	1.01	106.90	838.30	719.62	356
Inventive Steel 2	1. 19	274.29	855.13	728.25	360
Inventive Steel 3	1.19	123.33	877.40	741.23	358
Comparativ e Steel 1	1.22	9.88	813.97	714.23	310
Comparativ e Steel 2	0.96	7.28	855.13	719.25	385

Ac1 = 723 - 10.7×[Mn] - 16.9×[Ni] + 29.1×[Si] + 16.9×[Cr]

[Table 3]

Steel type	Division	SS [°C]	RCS [°C]	Relati onal Expres sion 3	YS [MPa]	TS [MPa]	El [%]	HER [%]	Retai ned r [%]	B+TM [%]
Inventi ve Steel 1	Inventiv e Example 1	852	248	173	1141	1531	10.6	27	6.4	84
	Inventiv e Example 2	847	199	14	1185	1504	10.3	35	5.2	88
	Comparat ive Example 1	859	162	5	1221	1612	8	22	2.7	93
	Comparat ive Example 2	799	157	-192	1143	1623	8	15	4.2	73
	Comparat ive Example 3	857	298	298	982	1582	9	18	11.1	79
Inventi ve Steel 2	Inventiv e Example 3	852	248	137	1175	1531	10.6	29	7.2	81
	Inventiv e Example 4	849	196	14	1221	1547	10.1	37	4.8	89
	Comparat ive Example 4	855	152	-65	1320	1621	8	23	3.1	93
Inventi ve Steel 3	Inventiv e Example 5	849	242	79	1154	1543	11.1	31	8.7	85
	Inventiv e Example 6	845	274	137	1116	1511	10.9	27	9.6	82
	Comparat ive Example 5	851	302	217	965	1612	10.2	18	10.8	78
Compara tive Steel 1	Comparat ive Example 6	847	251	282	1234	1611	8	16	9.1	83
Compara tive Steel 2	Comparat ive Example 7	855	258	141	1050	1221	14	32	7.2	81

r: Austenite, B: Bainite, TM: Tempered Martensite [a balance of FM (fresh martensite)]

As can be seen from the experimental results in Table 3 above, in the case of Inventive Examples 1 to 6 satisfying the alloy composition and manufacturing conditions of the present disclosure, it was confirmed that not only was a tensile strength as high as 1470 MPa or more, which has ultra-high strength, but also a yield strength was excellent, and elongation and expansion ratio were also excellent.
On the other hand, in the case of Comparative Examples 1 to 7 not satisfying at least one of the alloy composition and manufacturing conditions of the present disclosure, it was confirmed that at least one of the properties of a tensile strength, a yield strength, elongation, and hole expansion ratio was inferior to that of the Inventive Examples above.

Claims

A cold-rolled steel sheet, comprising by weight%:
0.1 to 0.3% of carbon (C), 2.0% or less (excluding 0%) of silicon (Si), 1.5 to 3.0% of manganese (Mn), 1.2% or less (excluding 0%) of chromium (Cr), 0.03 to 0.25% of molybdenum (Mo), 0.1% or less (excluding 0%) of aluminum (Al), 0.001 to 0.015% of phosphorus (P), 0.001 to 0.01% of sulfur (S), 0.001 to 0.01% of nitrogen (N), 0.001 to 0.005% of boron (B), and a balance of Fe and other unavoidable impurities,

wherein a microstructure includes by area%, 75 to 90% of a total content of bainite and tempered martensite, 10% or less (excluding 0%) of retained austenite, and a balance of fresh martensite,

wherein the following Relational Expressions 1 and 2 are satisfied, 1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

in Relational Expression 1 above, [C], [Si], [Mn], [Cr], [Mo] and [B] represent a weight% content for each element in parentheses. $106 \leq ([Cr] + [Mo]) \times [Si] / [Al] \leq 275$

in Relational Expression 2 above, [Cr], [Mo], [Si] and [A1] represent a weight% content for each element in parentheses.
The cold-rolled steel sheet of claim 1, wherein the microstructure includes by area%, 4.8 to 9.6% of retained austenite.
The cold-rolled steel sheet of claim 1, wherein the microstructure includes by area%, 81 to 89% of a total content of bainite and tempered martensite.
A method for manufacturing a cold-rolled steel sheet comprising:
reheating a slab including by weight%, 0.1 to 0.3% of carbon (C), 2.0% or less (excluding 0%) of silicon (Si), 1.5 to 3.0% of manganese (Mn), 1.2% or less (excluding 0%) of chromium (Cr), 0.03 to 0.25% of molybdenum (Mo), 0.1% or less (excluding 0%) of aluminum (Al), 0.001 to 0.015% of phosphorus (P), 0.001 to 0.01% of sulfur (S), 0.001 to 0.01% of nitrogen (N), 0.001 to 0.005% of boron (B), and a balance of Fe and other unavoidable impurities, wherein the slab is satisfying the following Relational Expressions 1 and 2,

obtaining a hot-rolled steel sheet by final hot-rolling the above-mentioned reheated slab at Ar3 to Ar3+50°C;

coiling the hot-rolled steel sheet at a temperature within a range of 500 to 750°C;

cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;

continuously annealing the cold-rolled steel sheet at a temperature within a range of 800 to 900°C;

primarily cooling the continuously annealed cold-rolled steel sheet to a primary cooling end temperature within a range of 650 to 700°C at an average cooling rate of 1 to 10°C/s;

secondarily cooling the primarily-cooled cold-rolled steel sheet to a secondary cooling end temperature at an average cooling rate of more than 10°C/s and 20°C/s or less; and

over-aging heat treating the secondarily-cooled cold-rolled steel sheet at a temperature within a range of 250 to 350°C, 1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

in Relational Expression 1 above, [C], [Si], [Mn], [Cr], [Mo] and [B] represent a weight% content for each element in parentheses $106 \leq ([Cr] + [Mo]) \times [Si] / [Al] \leq 275$

in Relational Expression 2 above, [Cr], [Mo], [Si] and [Al] represent a weight% content for each element in parentheses.
The method for manufacturing a cold-rolled steel sheet of claim 4, wherein the method is satisfying the following Relational Expression 3, $8 \leq 3.1 \times ([SS] - [Ac 1]) + 2.2 \times ([RCS] - [Ms]) \leq 180$ in Relational Expression 3 above, [Ac1] is a value defined by the following Relational Expression 4, [RCS] represents a secondary cooling end temperature (°C), and [Ms] represents a martensite transformation start temperature (°C) $[Ac 1] = 723 - 10.7 \times [Mn] - 16.9 \times [Ni] + 29.1 \times [Si] + 16.9 \times [Cr]$ in Relational Expression 4 above, [Mn], [Ni], [Si] and [Cr] represent a weight% content for each element in parentheses.
The method for manufacturing a cold-rolled steel sheet of claim 4, further comprising:
skin pass rolling the over-aging heat-treated cold-rolled steel sheet in the range of 0.1 to 1.0%.