WO2024136222A1 - Cold-rolled steel sheet and method for manufacturing same - Google Patents

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

Cold rolled steel sheet and manufacturing method thereof

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

Recently, automobile steel sheets are required to have higher strength in order to improve fuel efficiency and durability due to various environmental and energy use regulations. In particular, as automobile impact safety regulations have recently spread, high-strength steel with excellent yield strength is being used in structural members such as members, seat rails, and pillars to improve the impact resistance of car bodies. The structural member has an advantageous feature in absorbing impact energy as the yield strength relative to the tensile strength is higher, that is, the higher the yield ratio (tensile strength/yield strength) is. However, in general, as the strength of the steel sheet increases, the elongation decreases, which causes the problem of reduced forming processability, so the development of materials that can complement this problem is required.

Typically, methods for strengthening steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening. However, among the above methods, strengthening by solid solution strengthening and grain refinement has the disadvantage that it is very difficult to manufacture high-strength steel with a tensile strength of 490 MPa or higher.

On the other hand, precipitation-strengthened high-strength steel strengthens the steel sheet by precipitating carbon and nitrides by adding carbon and nitride forming elements such as Cu, Nb, Ti, and V, or refines the grains by suppressing grain growth by fine precipitates. It is a technique to secure strength. The above technology has the advantage of easily obtaining high strength at a low manufacturing cost, but has the disadvantage of requiring high-temperature annealing to cause sufficient recrystallization and ensure ductility, as the recrystallization temperature rises rapidly due to fine precipitates. In addition, precipitation strengthened steel, which is strengthened by precipitating carbon and nitride in the ferrite matrix, has a problem in that it is difficult to obtain high strength steel of 600 MPa or higher.

Meanwhile, transformation-strengthened high-strength steels include ferrite-martensite dual phase steels containing hard martensite in a ferrite matrix, TRIP (Transformation Induced Plasticity) steels using transformation-induced plasticity of retained austenite, or ferrite and Various CP (Complexed Phase) steels composed of hard bainite or martensite structures have been developed. However, when the tensile strength that can be achieved with this advanced high strength steel is 1500 MPa, the elongation rate is limited to about 8%. In addition, for application to structural members to ensure crash safety, hot press forming steel, which secures final strength by rapid cooling through direct contact with a water-cooled die after forming at high temperature, is receiving attention. The expansion of application is not large due to excessive facility investment costs and high heat treatment and processing costs.

Recently, in order to improve the safety of passengers in the event of a collision, increased strength and weight reduction of vehicle seat parts are being carried out simultaneously. These parts are manufactured by two methods: roll forming as well as press forming. The seat part is a part that connects the passenger and the vehicle body and must support high stress to prevent the passenger from being thrown out in the event of a collision. For this purpose, high yield strength and yield ratio are required. In addition, as most of the parts being processed require elongation flange properties, the application of steel materials with excellent hole expansion properties is required.

Meanwhile, Patent Document 1 (Japanese Patent Publication No. 3729108) discloses a high-tensile cold-rolled steel sheet with a martensitic single-phase structure and a tensile strength of 880 to 1170 MPa by optimizing the composition and heat treatment conditions of the steel sheet. In addition, in Patent Document 2 (Japanese Patent Laid-Open No. 2005-272954), a steel sheet in which the volume ratio of the low-temperature transformation phase consisting of martensite and retained austenite accounts for more than 90% of the total metal structure is heated and maintained in the two-phase region, thereby forming the low-temperature transformation phase. A method of manufacturing a high-strength steel sheet is disclosed, in which the structure is controlled by fine ferrite and austenite including laths, and by subsequent cooling, the final metal structure is formed in which ferrite and low-temperature transformation phase are finely dispersed on the laths. These patents claim that high yield strength can be obtained without water cooling, but there are disadvantages such as greatly deteriorating ductility or deterioration of stretch flangeability due to the generation of a large amount of austenite in the steel.

(Patent Document 1) Japanese Patent Publication No. 3729108

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

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

The object of the present invention is not limited to the above-described content. Anyone skilled in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the content throughout the present invention specification.

One aspect of the present invention is,

In weight%, C: 0.1~0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5~3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03~0.25%, Al : 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, including the balance Fe and other impurities,

Microstructure, in area%, includes the sum of bainite and tempered martensite: 75-90%, retained austenite: 10% or less (excluding 0%), and the balance fresh martensite;

Provided is a cold rolled steel sheet that satisfies the following relations 1 and 2.

[Relationship 1]

1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

(In Equation 1 above, [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.)

[Relational Expression 2]

106 ≤ ([Cr] + [Mo])×[Si]/[Al] ≤275

(In Equation 2 above, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

The microstructure may include 4.8 to 9.6% of retained austenite in area percent.

The microstructure may include a total of 81 to 89% of bainite and tempered martensite in area percent.

In addition, another aspect of the present invention is,

In weight%, C: 0.1~0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5~3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03~0.25%, Al : 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, including the balance Fe and other impurities, and Reheating the slab satisfying relations 1 and 2;

Obtaining a hot rolled steel sheet by performing final hot rolling on the reheated slab at Ar3~Ar3+50°C;

Winding the hot rolled steel sheet at 500 to 750°C;

Obtaining a cold rolled steel sheet by cold rolling the coiled hot rolled steel sheet;

Continuously annealing the cold rolled steel sheet at 800 to 900°C;

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

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

It provides a method of manufacturing a cold-rolled steel sheet, including the step of over-aging heat treatment of the secondary cooled cold-rolled steel sheet at 250 to 350°C.

[Relationship 1]

1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

(In Equation 1 above, [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.)

[Relational Expression 2]

106 ≤ ([Cr] + [Mo])×[Si]/[Al] ≤275

(In Equation 2 above, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

The manufacturing method can satisfy the following relational equation 3.

[Relational Expression 3]

8 ≤ 3.1×([SS]-[Ac1]) + 2.2×([RCS]-[Ms])≤ 180

(In the above relational equation 3, [Ac1] is a value defined by the following relational equation 4, [RCS] represents the secondary cooling end temperature (°C), and [Ms] represents the martensite transformation start temperature (°C). )

[Relational Expression 4]

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

(In equation 4 above, [Mn], [Ni], [Si], and [Cr] represent the weight percent content for each element in parentheses.)

The step of skin-pass rolling the over-aging heat-treated cold-rolled steel sheet in the range of 0.1 to 1.0% may be further included.

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

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.

Figure 1 shows a photograph of the microstructure of a specimen obtained in Inventive Example 1 using a scanning electron microscope (SEM).

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

Meanwhile, the terms used in this specification are for describing specific embodiments and are not intended to limit the present invention. For example, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary. Additionally, the meaning of “including” used in the specification is to specify a configuration and not to exclude the presence or addition of another configuration.

As mentioned earlier, microstructure control is very important in order to manufacture steel with an excellent hole expansion ratio (HER) of 20% or more and an elongation of 10% or more, as suggested by the present invention steel. A method of simultaneously increasing stretch flangeability and elongation requires a technique to secure a uniform structure. In general, the structure with the highest strength among low-temperature structures is martensite, and as is well known, the easiest way to manufacture martensite is to anneal it for a time to sufficiently form austenite, then cool it with water, and then temper it. It is to be processed. However, since the water cooling method may cause productivity deterioration due to problems such as material deviation and shape defect, the present invention sought to secure martensite by controlling alloy elements. In other words, it is a technology that secures martensite even at low cooling rates by adding a certain amount of hardenable elements such as Mn and Cr. However, this method may cause problems such as deterioration of weldability due to the addition of high alloy elements. Therefore, in the present invention, an attempt was made to minimize the carbon content, which has the greatest impact on weldability. In the present invention steel, the carbon content was limited to 0.3% or less.

In order to secure a high yield ratio under cooling conditions such as the steel of the present invention, alloying elements must be added as much as possible. However, these attempts cause additional problems such as deterioration of weldability and increase in hot rolling strength, so solutions are needed. Accordingly, through various studies by the present inventors, it was discovered that the elongation flangeability and yield ratio suggested by the present invention steel can be satisfied when the size and nano-precipitates of martensite are controlled without excessive addition of alloy elements, and the present invention was completed. . Below, the alloy composition and microstructural characteristics to ensure hole expandability of 20% or more and elongation of 10% or more while having the ultra-high strength required by the present invention will be described in detail.

First, the reason for adding the alloy component and the reason for limiting the content of the cold rolled steel sheet of the present invention will be explained in detail. It is important to note that the content of each ingredient described below is based on weight percent unless specifically mentioned.

C: 0.1~0.3%

Among steels, carbon (C) is a very important element added to strengthen the transformed structure. Carbon promotes high strength and promotes the formation of martensite in transformed steel. If the carbon content is less than 0.1%, it is very difficult to secure the strength of martensite proposed in the present invention, so the carbon content is set to 0.1% or more. Meanwhile, as the carbon content increases, the amount of martensite in the steel increases. However, when the carbon content exceeds 0.3%, the strength of martensite increases, but the strength difference with ferrite with a low carbon concentration increases. This difference in strength reduces the stretch flangeability because fracture easily occurs at the interface between phases when stress is applied. In addition, due to poor weldability, welding defects occur when processing parts. Preferably it may be 0.10 to 0.30%.

Si: 2.0% or less (excluding 0%)

In steel, silicon (Si) promotes ferrite transformation and increases the carbon content in untransformed austenite to form a composite structure of ferrite and martensite, which hinders the increase in the strength of martensite. In addition, it is desirable to limit the addition as much as possible because it not only causes surface scale defects in relation to surface characteristics but also reduces chemical treatment properties, so in the present invention, the Si content is controlled to 2.0% or less. However, considering unavoidable inclusion, 0% is excluded as the lower limit of Si content. Preferably it may be 2.00% or less.

Mn: 1.5~3.0%

In steel, manganese (Mn) is an element that refines the particles without damaging ductility and completely precipitates sulfur in the steel into MnS, preventing hot embrittlement caused by the formation of FeS and strengthening the steel. At the same time, critical cooling where martensite phase is obtained It plays a role in lowering the speed, making it easier to form martensite. If the Mn content is less than 1.5%, it is difficult to secure the strength targeted by the present invention, while if it exceeds 3.0%, problems such as weldability and hot rolling are likely to occur. Therefore, the Mn content is 1.5%. It is set in the range of ~3.0%. Preferably it may be 1.50 to 3.00%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Mn content may be 2.0%, or the upper limit of the Mn content may be 2.9%.

P: 0.001~0.015%

Among steels, phosphorus (P) is a substitutional alloy element with the greatest solid solution strengthening effect and plays a role in improving in-plane anisotropy and enhancing strength. Therefore, if the P content is less than 0.001%, not only can the above-mentioned effects not be secured, but it also causes problems with manufacturing costs. On the other hand, if the P content is excessively added to exceed 0.015%, press formability may deteriorate and brittleness of the steel may occur, so the P content is set at 0.001 to 0.015%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the P content may be 0.003%, or the upper limit of the P content may be 0.014%.

S: 0.001~0.01%

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

Al: 0.1% or less (excluding 0%)

Among steels, soluble aluminum (Al) is an effective ingredient in improving martensite hardenability by combining with oxygen to deoxidize and, like Si, distributing carbon in ferrite to austenite. Therefore, in order to secure the above-mentioned Al effect, 0% is excluded as the lower limit of the Al content. However, if the Al content exceeds 0.1%, the effect is saturated and the manufacturing cost increases, so the Al content is set to 0.1% or less. Preferably it may be 0.10% or less. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Al content may be 0.001%, or the upper limit of the Al content may be 0.09%.

N: 0.001~0.01%

In steel, nitrogen (N) is an ingredient that effectively stabilizes austenite. If it exceeds 0.01%, the risk of cracks occurring during playing through AlN formation, etc. increases significantly, so the upper limit is limited to 0.01%. It is desirable. In addition, considering cases where it is inevitably included, the lower limit of N content is set to 0.001%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the N content may be 0.002%, or the upper limit of the N content may be 0.009%.

Cr: 1.2% or less (excluding 0%)

Among steels, chromium (Cr) is a component added to improve the hardenability of steel and ensure high strength, and in the present invention, it is an element that plays a very important role in forming martensite, a low-temperature transformation phase. To ensure the above-mentioned effect, 0% is excluded as the lower limit of Cr content. However, if the Cr content exceeds 1.2%, not only is the effect saturated, but an excessive increase in hot rolling strength also causes the problem of deterioration of cold rolling properties, so the Cr content is set to 1.2% or less. Preferably it may be 1.20% or less. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Cr content may be 0.01%, or the upper limit of the Cr content may be 1.19%.

B: 0.001~0.005%

In steel, B is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is added as an element that suppresses the formation of ferrite and promotes the formation of martensite. However, if the content of B is less than 0.001%, it is difficult to obtain the above-mentioned effect, and if it exceeds 0.005%, cost deterioration occurs due to excessive iron alloy, so the content is set at 0.001 to 0.005%.

Mo: 0.03~0.25%

Molybdenum (Mo) is an element added to ensure strength and hardenability, and when added together with Ti, it forms carbide with Ti. In order to obtain the structure strengthening effect due to the formation of carbides, the Mo content must be added at 0.03% or more. However, Mo is an expensive element, and if it is added too excessively, it not only deteriorates economic efficiency, but also delays phase transformation too much, causing fresh martensite formation, so the Mo content is set to 0.25% or less. Preferably, it may be 0.030 to 0.250%. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the Mo content may be 0.04%, or the upper limit of the Mo content may be 0.24%.

In addition, although it is not particularly limited, according to one aspect of the present invention, the cold rolled steel sheet may optionally further include one or more elements of Ti and Nb, and Ti and Nb are among the steels, which increase the strength of the steel sheet and increase the strength of the steel sheet. It is an element effective in grain refinement by precipitates. In the present invention, when adding Ti or Nb, the Ti content can be in the range of 0.01 to 0.08%, or the Nb content can be in the range of 0.01 to 0.05%. On the other hand, when Ti and Nb are added in large quantities as in the present invention, they combine with carbon to form very fine nano-precipitates. These nano-precipitates serve to strengthen the matrix structure and reduce the difference in hardness between phases.

In addition to the above composition, the remainder is Fe. However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the art, all of them are not specifically mentioned in this specification, but representative impurities are mentioned as follows.

Next, the cold-rolled steel sheet according to the present invention has, as a microstructure and area percent, a total of bainite and tempered martensite: 75 to 90%, retained austenite: 10% or less (excluding 0%), and the remainder fresh. Contains martensite.

In the present invention, the total of bainite and tempered martensite, which are transformed structures, should be controlled to 75% or more and 90% or less, and retained austenite should be controlled to 10% or less. In order to increase the pore expandability (HER) and yield ratio (YR), the higher the transformed tissue fraction, the better. However, considering the elongation, it is better to control it to 90% or less. As in the present invention, the carbon content is kept to 0.3% or less. In low cases, when alloy elements are added considering weldability and hot rolling strength, there is a limit to the increase in the strength of the martensite produced. In other words, if sufficient carbon is not included in martensite, there is a limit to the increase in strength. However, the present inventors are able to provide a cold-rolled steel sheet having ultra-high strength at the level desired in the present invention, even though the carbon content is as low as 0.3% or less.

Meanwhile, in terms of further improving the above-described effect, the lower limit of the retained austenite area ratio may be 4.8%, or the upper limit of the retained austenite area ratio may be 9.6%. Alternatively, the lower limit of the total area ratio of the bainite and tempered martensite may be 81%, or the upper limit of the total area ratio of the bainite and tempered martensite may be 89%.

In addition, the cold rolled steel sheet according to the present invention satisfies the following relational expressions 1 and 2. In other words, in the present invention, in order to obtain elongation while securing strength, the hole expandability (HER) under the condition of securing a certain ductility is at least 20% through numerous experiments on steels within the composition range presented in the steel of the present invention. In order to secure more than 10% of elongation (El), it was confirmed that the composition and manufacturing conditions of the steels manufactured at this time are very important, and it is important for these factors to satisfy the relational equations 1 and 2 below.

[Relationship 1]

1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

(In Equation 1 above, [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.)

[Relational Expression 2]

106 ≤ ([Cr] + [Mo])×[Si]/[Al] ≤275

(In Equation 2 above, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

Hereinafter, the manufacturing method of a cold rolled steel sheet according to the present invention will be described in detail.

After reheating the slab composed of the ingredients using the above-described alloy design method, hot rolling is performed. Finish rolling in hot rolling is preferably performed so that the exit temperature is between Ar3 and Ar3+50°C. In other words, if the temperature at the exit side of the finish rolling is less than Ar3, there is a high possibility that the hot deformation resistance will increase rapidly, and the top, tail, and edge of the hot rolled coil will become a single phase region, resulting in an increase in in-plane anisotropy and Formability deteriorates. However, if the exit temperature of finish rolling exceeds Ar3+50°C, not only will too thick oxidation scale occur, but there is a high possibility that the microstructure of the steel sheet will become coarse. Meanwhile, the Ar3 can be obtained by a method commonly known in the art, so it is not particularly limited. Additionally, more specifically, the finish rolling may be performed in the temperature range of 880 to 920°C.

After completing the hot finishing rolling, it is wound at 500 to 750°C. If the coiling temperature is less than 500°C, excessive martensite or bainite is generated, causing an excessive increase in the strength of the hot rolled steel sheet, which may cause manufacturing problems such as shape defects due to load during cold rolling. On the other hand, if the coiling temperature exceeds 750°C, the pickling properties deteriorate due to an increase in surface scale, so it is preferable to limit the coiling temperature to 500-750°C.

The hot-rolled steel sheet manufactured in the above manner is pickled and then cold-rolled to obtain a cold-rolled steel sheet.

The cold rolled steel sheet obtained in this way is continuously annealed at 800 to 900°C, which is the continuous annealing temperature (SS). If the continuous annealing temperature is low, ferrite is generated in large quantities, making it impossible to secure YS and TS. On the other hand, if the continuous annealing temperature is too high, the martensite packet size produced during cooling increases due to the increase in austenite grain size due to high-temperature annealing, making it difficult to secure the desired physical properties in the present invention.

The continuously annealed cold-rolled steel sheet is subjected to primary cooling at an average cooling rate of 1 to 10°C/s until the primary cooling end temperature is 650 to 700°C. The primary cooling is to suppress ferrite transformation and transform most of the austenite into martensite.

Subsequently, the primary cooled cold-rolled steel sheet is subjected to secondary cooling at an average cooling rate of more than 10°C/s and less than 20°C/s to the secondary cooling end temperature (RCS), and the secondary cooled cold-rolled steel sheet is cooled at 250 °C/s. Over-aging treatment is performed by maintaining heat treatment at ~350°C. This secondary cooling end temperature (RCS) is a very important temperature condition for securing high YR and high HER along with securing the shape of the coil in the width and longitudinal directions. If the cooling end temperature is low, the amount of martensite may be excessive during overaging treatment. With this increase, the yield strength and tensile strength increase simultaneously and the ductility deteriorates significantly. In particular, shape deterioration occurs due to rapid cooling, which is expected to result in workability deterioration when processing automobile parts. On the other hand, if the secondary end temperature is too high, the austenite generated during annealing cannot be transformed into martensite, and high-temperature transformation phases such as bainite and granular bainite are generated, causing a rapid deterioration in yield strength. . The occurrence of such a structure is accompanied by a decrease in the yield ratio and a deterioration of the hole expandability, making it impossible to manufacture the high-yield ratio high-strength steel with excellent elongation flangeability proposed in the present invention.

According to one aspect of the present invention, the method of manufacturing the cold rolled steel sheet can be managed to satisfy the following relational equation 3. By satisfying the following relational expression 3, it is possible to effectively provide a cold rolled steel sheet that is excellent in strength and hole expandability at the same time.

[Relational Expression 3]

8 ≤ 3.1×([SS]-[Ac1]) + 2.2×([RCS]-[Ms])≤ 180

(In the above relational equation 3, [Ac1] is a value defined by the following relational equation 4, [RCS] represents the secondary cooling end temperature (°C), and [Ms] represents the martensite transformation start temperature (°C). )

[Relational Expression 4]

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

(In equation 4 above, [Mn], [Ni], [Si], and [Cr] represent the weight percent content for each element in parentheses.)

At this time, [Ms] refers to a value defined by the following relational equation 5.

[Relational Expression 5]

Ms = 539 - 423×[C] - 30.4×[Mn] - 12.1×[Cr] - 7.5×[Mo]

(In Equation 5 above, [C], [Mn], [Cr], and [Mo] represent the weight percent content for each element in parentheses.)

Skin pass rolling is performed on the above-described overaging heat-treated cold rolled steel sheet in the range of 0.1 to 1.0%. Typically, when skin-pass rolling transformed steel, the yield strength increases by at least 50Mpa or more with little increase in tensile strength. If the rolling rate 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 the present invention steel, and if the rolling rate of the skin pass rolling exceeds 1.0%, the operability is greatly reduced due to high elongation work. Since it becomes unstable, set the value to 0.1 to 1.0%.

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 the present invention by way of example 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)

Steel slabs having the composition shown in Table 1 below were vacuum melted, heated in a heating furnace at a reheating temperature of 1200°C for 1 hour, hot rolled, and then wound. The temperature conditions during hot rolling are as shown in Table 2 below. Based on Ar3 or higher for each example, hot rolling was completed in the temperature range of 880~920℃ to satisfy the range of Ar3~Ar3+50℃, and the coiling temperature was 500~50℃. It was controlled at 680°C. Pickling was performed using hot-rolled steel sheets, followed by cold rolling. The cold rolled steel sheet obtained in this way is subjected to continuous annealing at a continuous annealing temperature (SS) under the conditions shown in Table 2 below, and then cooled to a primary cooling end temperature of 650 to 700°C at an average cooling rate of 5°C/s. Primary cooling was performed, followed by secondary cooling to the secondary cooling end temperature (RCS) at an average cooling rate of 15°C/s. Next, the secondary cooled cold rolled steel sheet was subjected to overaging heat treatment maintained at 250 to 350° C., and the final skin pass rolling rate was fixed at 0.2%.

The total of bainite and tempered martensite, and the area ratio of retained austenite and fresh martensite for each steel sheet manufactured according to changes in each steel component and annealing condition were measured and are shown in Table 3 below.

In addition, a JIS No. 5 tensile test specimen was produced and the yield strength (YS), tensile strength (TS), elongation (El), and hole expandability (HER) were measured according to the JIS standard, and the results were presented along with comparative examples. It is shown in Table 3 below. Meanwhile, in the case of hole expansion, D _o is the initial hole diameter (mm), and D _h is the hole diameter after fracture (mm), and it was calculated according to the following equation.

HER(%)= (D _h -D _o )/D _o ×100

division	Composition (wt%)
	C	Si	Mn	Cr	Mo	B	Al	P	S	N
Invention lecture 1	0.245	0.61	2.48	0.32	0.048	0.0014	0.0021	0.009	0.003	0.004
Invention lecture 2	0.212	0.8	2.68	0.63	0.09	0.0021	0.0021	0.011	0.004	0.005
Invention lecture 3	0.252	1.48	2.4	0.05	0.1	0.0018	0.0018	0.011	0.004	0.005
Comparison lecture 1	0.331	0.38	2.69	0.53	0.12	0.0022	0.025	0.011	0.003	0.005
Comparison lecture 2	0.18	0.65	2.45	0.21	0.07	0.0017	0.025	0.012	0.003	0.005

division	Relation 1	Relation 2	Ar3 [°C]	Ac1 [℃]	Ms [°C]
Invention lecture 1	1.01	106.90	838.30	719.62	356
Invention lecture 2	1.19	274.29	855.13	728.25	360
Invention lecture 3	1.19	123.33	877.40	741.23	358
Comparison lecture 1	1.22	9.88	813.97	714.23	310
Comparison lecture 2	0.96	7.28	855.13	719.25	385

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

steel grade	note	SS [℃]	RCS [℃]	Relation 3	YS [MPa]	TS [MPa]	El [%]	HER [%]	residual r [%]	B+TM [%]
Invention lecture 1	Invention Example 1	852	248	173	1141	1531	10.6	27	6.4	84
	Invention Example 2	847	199	14	1185	1504	10.3	35	5.2	88
	Comparative Example 1	859	162	5	1221	1612	8	22	2.7	93
	Comparative Example 2	799	157	-192	1143	1623	8	15	4.2	73
	Comparative Example 3	857	298	298	982	1582	9	18	11.1	79
Invention lecture 2	Invention Example 3	852	248	137	1175	1531	10.6	29	7.2	81
	Invention Example 4	849	196	14	1221	1547	10.1	37	4.8	89
	Comparative Example 4	855	152	-65	1320	1621	8	23	3.1	93
Invention lecture 3	Invention Example 5	849	242	79	1154	1543	11.1	31	8.7	85
	Invention Example 6	845	274	137	1116	1511	10.9	27	9.6	82
	Comparative Example 5	851	302	217	965	1612	10.2	18	10.8	78
Comparative lecture 1	Comparative Example 6	847	251	282	1234	1611	8	16	9.1	83
Comparison lecture 2	Comparative Example 7	855	258	141	1050	1221	14	32	7.2	81

r: Austenite, B: Bainite, TM: Tempered martensite [Remaining FM (fresh martensite)]

As can be seen from the experimental results in Table 3, in the case of Invention Examples 1 to 6 that meet the alloy composition and manufacturing conditions of the present invention, the tensile strength was super high at 1470 MPa or higher, and the yield strength was also excellent. , it was confirmed that elongation and hole expandability were also excellent.

On the other hand, in the case of Comparative Examples 1 to 7, which do not meet one or more of the alloy composition and manufacturing conditions of the present invention, one or more characteristics of tensile strength, yield strength, elongation, and hole expansion are inferior to the above-described invention examples. Confirmed.

Claims (6)

1. In weight%, C: 0.1~0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5~3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03~0.25%, Al : 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, including the balance Fe and other impurities,

   Microstructure, in area%, includes the sum of bainite and tempered martensite: 75-90%, retained austenite: 10% or less (excluding 0%), and the balance fresh martensite;

   A cold rolled steel sheet that satisfies the following relations 1 and 2.

   [Relationship 1]

   1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

   (In Equation 1 above, [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.)

   [Relational Expression 2]

   106 ≤ ([Cr] + [Mo])×[Si]/[Al] ≤275

   (In Equation 2 above, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)
2. According to claim 1,

   Cold-rolled steel sheet containing, as microstructure and area %, retained austenite: 4.8 to 9.6%.
3. According to claim 1,

   Cold-rolled steel sheet containing, as microstructure and area %, the sum of bainite and tempered martensite: 81 to 89%.
4. In weight%, C: 0.1~0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5~3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03~0.25%, Al : 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, including the balance Fe and other impurities, and Reheating the slab satisfying relations 1 and 2;

   Obtaining a hot rolled steel sheet by performing final hot rolling on the reheated slab at Ar3~Ar3+50°C;

   Winding the hot rolled steel sheet at 500 to 750°C;

   Obtaining a cold rolled steel sheet by cold rolling the coiled hot rolled steel sheet;

   Continuously annealing the cold rolled steel sheet at 800 to 900°C;

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

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

   A method of manufacturing a cold-rolled steel sheet, comprising: over-aging heat treatment of the secondary cooled cold-rolled steel sheet at 250 to 350°C.

   [Relationship 1]

   1.0 ≤ [C] + (1.3×[Si]+[Mn])/6 + ([Cr]+1.2×[Mo])/5 + 100×[B] ≤ 1.2

   (In Equation 1 above, [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.)

   [Relational Expression 2]

   106 ≤ ([Cr] + [Mo])×[Si]/[Al] ≤275

   (In Equation 2 above, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)
5. According to claim 4,

   A method of manufacturing a cold rolled steel sheet that satisfies the following relational expression 3.

   [Relational Expression 3]

   8 ≤ 3.1×([SS]-[Ac1]) + 2.2×([RCS]-[Ms])≤ 180

   (In the above relational equation 3, [Ac1] is a value defined by the following relational equation 4, [RCS] represents the secondary cooling end temperature (°C), and [Ms] represents the martensite transformation start temperature (°C). )

   [Relational Expression 4]

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

   (In equation 4 above, [Mn], [Ni], [Si], and [Cr] represent the weight percent content for each element in parentheses.)
6. According to claim 4,

   A method of manufacturing a cold-rolled steel sheet, further comprising performing skin pass rolling on the over-aging heat-treated cold-rolled steel sheet in a range of 0.1 to 1.0%.

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