Patent Description:
In order to meet the contradictory goals of reducing the weight of automobile steel plates and securing collision safety for passenger safety, as well as preserving the global environment, various automobile steel plates such as dual phase (DP) steel, transformation induced plasticity (TRIP) steel, complex phase (CP) steel, etc, are being developed. However, the tensile strength that can be implemented in such advanced high strength steel is limited to about the <NUM> Mpa level. Hot press formed steel, which secures final strength by rapid cooling through a direct contact with the die, has been highlighted for application to a structural member to secure the collision safety, but an expansion of application may not be high, due to excessive investment costs in equipment and high heat-treatment and process costs.

Compared to general press forming and hot press forming, a roll forming method having high productivity is a method of manufacturing a complex shape through multi-stage roll forming, and its application to forming parts of ultra-high strength materials having low elongation is expanding. It is mainly produced in a continuous annealing furnace equipped with a water cooling facility, and a microstructure represents a tempered martensitic structure tempering martensite. There is a disadvantage in that the shape quality may be inferior due to temperature deviation in a width direction and a length direction when water is cooled, thereby deteriorating workability and material deviation by location when applying roll forming. Therefore, there is a need to devise an alternative to the rapid cooling method through water cooling.

As an ultra-high-strength steel manufacturing technology with excellent shape, there is a manufacturing method of an ultra-high strength cold-rolled steel sheet with improved shape quality while having strength of <NUM> GPa or higher in Patent Document <NUM>, and the shape quality is secured by limiting ΔT and alloying components during quenching in an annealing furnace. In addition, in the case of Patent Document <NUM> provides a manufacturing method of a cold-rolled steel sheet obtaining high strength and high ductility utilizing tempered martensite at the same time and having an excellent plate shape after continuous annealing, as there may be a possibility of causing dents in a furnace due to a high Si content.

In addition, Patent Document <NUM> provides a manufacturing method that realizes a tensile strength of <NUM> MPa using a water cooling method, but the thickness is limited to <NUM> or less, and in Patent Document <NUM>, there is a still a problem of the shape quality deterioration and material deviation by location, which are disadvantages of martensitic steel using the existing water cooling method.

Patent Document <NUM> relates to a high-strength galvanized steel sheet and a method for manufacturing the steel sheet. However, D1 comprises a second cooling process in which, after having performed cooling to a temperature equal to or higher than the Ms temperature at an average cooling rate of <NUM>/s or more, cooling is performed to a temperature of <NUM> or lower at an average cooling rate of <NUM>/s or more. Therefore, D1 does not secure good shape quality due to a low cooling finish temperature.

An aspect of the present invention is to provide an ultrahigh strength cold-rolled steel sheet having excellent shape quality and a manufacturing method thereof.

Another aspect of the present invention is to provide a manufacturing method of the ultrahigh strength cold-rolled steel sheet having excellent shape quality.

According to an aspect of the present disclosure, a cold-rolled steel sheet having superior shape quality compared to martensitic steel produced by utilizing water cooling as well as having ultra-strength of tensile strength of <NUM> MPa or more by utilizing a conventional continuous annealing furnace in which a slow cooling section is present can be provided.

An aspect of the present disclosure is to provide an ultra-high strength cold-rolled steel sheet having excellent shape quality without generating waves in a width direction and a length direction caused by rapid cooling by utilizing an existing water-cooling facility and a manufacturing method including the same.

Hereinafter, an ultra-high strength cold-rolled steel sheet according to a preferred aspect of the present disclosure will be described.

According to a naspect of the present disclosure, an ultrahigh strength cold-rolled steel sheet includes, in percentage by weight: C: <NUM> to <NUM>%; Si: <NUM>% or less (excluding <NUM>) ; Mn: <NUM>. <NUM> to <NUM>%; P: <NUM>% or less (excluding <NUM>) ; S: <NUM>% or less (excluding <NUM>); Al: <NUM>% or less (excluding <NUM>); Cr: <NUM>% or less (excluding <NUM>); Ti: <NUM>/<NUM>*[N]to <NUM>% or less; Nb: <NUM>% or less (excluding <NUM>); B: <NUM>% or less (excluding <NUM>); N: <NUM>% or less (excluding <NUM>); and a balance of Fe and other unavoidable impurities.

Carbon (C) is a component required to secure martensite strength, and should be added at least <NUM>% or more. However, if a content thereof exceeds <NUM>%, weldability becomes inferior, so an upper limit thereof is limited to <NUM>%. Therefore, the content of C is <NUM> to <NUM>%, and preferably <NUM> to <NUM>%.

Silicon (Si) is a ferrite stabilizing element and has a disadvantage of weakening strength by promoting ferrite generation during slow cooling after annealing in an ordinary continuous annealing furnace in which a slow cooling section exists. As in the present disclosure, when a large amount of Mn is added for suppressing phase transformation, the content thereof is limited to <NUM>% or less (excluding <NUM>) because there is a risk of causing dent defects due to surface concentration and oxidation by Si during annealing. The content of Si is preferably <NUM>% or less.

Manganese (Mn) in steel is an element that inhibits ferrite formation and facilitates austenite formation. When a content of Mn is less than <NUM>%, ferrite is easily generated during slow cooling, and when a content of Mn exceeds <NUM>%, bands are formed due to segregation and a cost of ferroalloy is increased due to excessive alloy inputs during converter operation, so the content thereof is limited to <NUM> to <NUM>%. The content of Mn is preferably <NUM> to <NUM>%.

Phosphorus (P) in steel is an impurity element, and if a content thereof exceeds <NUM>%, weldability decreases, a risk of brittleness of the steel increases, and a possibility of causing dent defects increases, so an upper limit thereof is limited to <NUM>%. The content of P is preferably <NUM>% less.

Sulfur (S), like P, is an impurity element in steel, and is an element that inhibits the ductility and weldability of the steel sheet. When a content thereof exceeds <NUM>%, there is a high possibility of inhibiting the ductility and weldability of the steel sheet, so an upper limit thereof is limited to <NUM>%. The content of S is preferably <NUM>% or less.

Aluminum (Al) is an alloy element that expands a ferrite region. When utilizing the continuous annealing process in which slow cooling is present as in the present disclosure, it promotes ferrite formation, and it is possible to deteriorate high-temperature hot rollability due to AlN formation, so a content of aluminum (Al) is limited to <NUM>% or less (excluding <NUM>). The content of Al is preferably <NUM>% or less.

Chromium (Cr) is an alloy element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation, and has the advantage of suppressing ferrite formation when utilizing a continuous annealing process in which slow cooling is present, as in the present disclosure, but when it exceeds <NUM>%, since costs of ferroalloy increase due to excessive amounts of alloy input, the content thereof is limited to <NUM>% or less (excluding <NUM>).

Titanium (Ti) is a nitride forming element and precipitates TiN in the steel by scavenging N. To this end, it is necessary to add <NUM>/<NUM> * [N] or more in a chemical equivalent. When Ti is not added, it is necessary to add it because it is concerned about cracks generation during continuous casting due to AlN formation, and if Ti exceeds <NUM>%, a strength of martensite is reduced due to additional carbide precipitation in addition to removal of soluble N, so the content of titanium (Ti) is limited to <NUM>/<NUM> * [N] to <NUM>%.

Niobium (Nb) is an element that segregates at an austenite grain boundary and suppresses coarsening of austenite grains during annealing heat treatment, so it is necessary to add it. When it exceeds <NUM>%, a cost of ferroalloy due to excessive amounts of alloy input increases, so a content of niobium (Nb) is limited to <NUM>% or less (excluding <NUM>). The content of Nb is preferably <NUM>% or less.

Boron (B) is a component that inhibits ferrite formation, and has an advantage of suppressing the ferrite formation upon cooling after annealing. When the content of B exceeds <NUM>%, the ferrite formation may be promoted by precipitation of Fe23(C,B)<NUM>, so a content of boron (B) is limited to <NUM>% or less (excluding <NUM>). The content of B is preferable to be <NUM>%.

When nitrogen (N) exceeds <NUM>%, a risk of crack generation during continuous casting through AlN formation, or the like is greatly increased, so the upper limit thereof is limited to <NUM>%.

A balance consists of Fe and other unavoidable impurities.

The ultrahigh strength cold-rolled steel sheet according to a aspect of the present disclosure, wherein a microstructure consists of <NUM>% or more (including <NUM>%) of martensite, and one or two kinds of <NUM>% or less (including <NUM>%) of ferrite and bainite.

The martensite is a structure that increases strength, and its fraction is <NUM>% or more. The fraction of martensite may be <NUM>%.

The ferrite and bainite are unfavorable structures in terms of tensile strength, and ferrite or bainite phases are likely to be mixed in the continuous annealing process in a method of manufacturing martensitic steel by delaying transformation by using hardenable elements such as Mn, C, and the like, not in a manufacturing process of martensitic steel by a rapid cooling method. Accordingly, in the present disclosure, the fraction of one or two kinds of ferrite and bainite is limited to <NUM>% or less. The ferrite and bainite may not be included.

The ultrahigh strength cold-rolled steel sheet according to a aspect of the present disclosure has excellent shape quality without generating waves in a width direction and a longitudinal direction, and has a tensile strength of <NUM> MPa or more.

The cold-rolled steel sheet has a wave height (ΔH) of <NUM> or less in an edge portion after cutting a steel plate to a size of <NUM> in a longitudinal direction.

Hereinafter, a manufacturing method of an ultrahigh strength cold-rolled steel sheet according to another aspect of the present disclosure will be described.

According to another aspect of the present disclosure, a manufacturing method of an ultrahigh strength cold-rolled steel sheet includes operations of:.

First, a slab satisfying the above-described composition is heated to a temperature range of <NUM> to <NUM>. When the heating temperature is less than <NUM>, a problem in which a hot rolling load increases rapidly occurs, and when the heating temperature exceeds <NUM>, an amount of surface scale increases, which may lead to loss of materials. Therefore, the slab heating temperature is limited to <NUM> to <NUM>.

The heated steel slab is hot-rolled under a finish hot rolling temperature condition of Ar<NUM> or higher to obtain a hot-rolled steel sheet. Here, Ar<NUM> means the temperature at which ferrite starts to appear when austenite is cooled.

When the finishing hot rolling temperature is less than Ar<NUM>, second-phase region of ferrite + austenite or ferrite region rolling is formed, resulting in a mixed structure, and there is concern about malfunction due to fluctuation of a hot rolling load, so it is desirable that the finish hot rolling temperature is limited to Ar<NUM> or higher. The finish hot rolling temperature is <NUM> to <NUM>.

The hot-rolled steel sheet is wound at a temperature of <NUM> or lower.

When a coiling temperature exceeds <NUM>, an oxide film on a surface of the steel sheet may be excessively generated, which may cause defects, so the coiling temperature is limited to <NUM> or less. There may be a problem in which the lower the coiling temperature, the higher the strength of the hot-rolled steel sheet, and the lower the rolling load of the cold rolling, which is a post process, but a lower limit thereof is not limited because the problem is not a factor that makes actual production impossible. More preferably, the coiling temperature is <NUM> or less.

The hot-rolled steel sheet manufactured as described above is cold rolled to obtain a cold-rolled steel sheet. During the cold rolling, a reduction ratio is <NUM> to <NUM>%.

Before the cold rolling, pickling treatment may be performed.

The cold-rolled steel sheet manufactured as described above is annealing heat treated in a temperature range of <NUM> to <NUM>.

The annealing heat treatment may be performed by a continuous annealing method.

When the annealing temperature is less than <NUM>, there is a concern in material deviation due to a drop in strength by formation large amounts of ferrite and generation of temperature gradient of top and end portions of an invention coil during connection with other steel types annealed in <NUM> or higher. Meanwhile, if the annealing temperature exceeds <NUM>° C, production may be difficult due to deterioration of durability of the continuous annealing furnace.

Therefore, the annealing temperature is limited to <NUM> to <NUM>.

The cold-rolled steel sheet which is annealing heat-treated as described above is primarily cooled to a primary cooling end temperature of <NUM> to <NUM> at a cooling rate of <NUM>/sec or less.

In general, in the case of a continuous annealing furnace including a slow cooling section, there is a slow cooling section of <NUM> to <NUM> after annealing, and there is a disadvantage that it is difficult to manufacture ultrahigh strength steel by transforming a soft phase such as ferrite by slow cooling at a high-temperature after annealing. For example, when a slow cooling section of <NUM> exists in the continuous annealing furnace, when a mailing speed of a thin steel sheet is <NUM> per minute, a time maintained in the slow cooling section means <NUM> seconds (sec). In addition, for example, when the annealing temperature is <NUM> and a last temperature in the slow cooling section is <NUM>, a cooling rate in the slow cooling section is very low at <NUM> per second (sec), so it is very likely that a soft phase such as ferrite is generated. After annealing, the cooling rate is limited to <NUM>/sec or less because an additional cooling device must be introduced to secure the slow cooling rate to be higher than <NUM>/sec.

Secondary cooling (rapid cooling section cooling) operation.

The cold-rolled steel sheet that is primarily cooled as described above is secondarily cooled to a secondary cooling end temperature (RCS) of <NUM> to <NUM> at a cooling rate of <NUM>/sec or higher.

When the secondary cooling end temperature (RCS) is less than <NUM> ° C, there may be a problem in which a yield strength and tensile strength simultaneously increase due to excessive increase in an amount of martensite during over-aging treatment, and ductility is very deteriorated, and in particular, deterioration in workability during roll forming due to shape deterioration due to rapid cooling, so it is limited to <NUM> or higher.

The secondary cooling end temperature (RCS) is <NUM> to <NUM>.

During the secondary cooling, the cooling rate is limited to <NUM>/sec or higher to improve productivity.

The more preferable secondary cooling rate is <NUM> to <NUM>/sec.

The C, Mn and Cr and the secondary cooling end temperature (RCS) should satisfy the following Relational Expression <NUM>. <MAT> (Here, C, Mn and Cr represent a content of each component in weight by percent, and RCS represents a secondary cooling end temperature).

A problem in which bainite, or the like, which is a high-temperature transformation phase, is generated according to the secondary cooling end temperature (RCS), which is a temperature, lower than that of the slow cooling section, so austenite generated during annealing cannot be transformed into martensite, resulting in a sharp deterioration in tensile strength and yield strength, occurs.

In order to obtain a tensile strength of <NUM> MPa or more by reducing the generation of ferrite in a general continuous annealing furnace in which the slow cooling section is present and suppressing the generation of bainite, or the like, which is a high-temperature transformation phase during cooling, the C, Mn, and Cr and the secondary cooling end temperature (RCS) must satisfy the above Relational Expression <NUM>.

According to the manufacturing method of the ultrahigh strength cold-rolled steel sheet according to another preferred aspect of the present disclosure, an ultrahigh strength cold-rolled steel sheet having excellent shape quality without generating waves in a width direction and a longitudinal direction, and having a tensile strength of <NUM> MPa or more are manufactured.

Hereinafter, the present disclosure will be described in more detail through embodiments. The present disclosure is not limited to the following embodiments.

After vacuum melting steel having a composition of Table <NUM> with an ingot of <NUM>, a hot-rolled slab was prepared through sizing rolling.

By utilizing this, after maintaining the slab at a temperature of <NUM> ° C for <NUM> hour, finish rolling at <NUM> ° C, charged in a furnace preheated to <NUM> ° C, maintained for <NUM> hour, and then hot rolling coiling was simulated by furnace cooling. After pickling it, cold rolling at a <NUM>% reduction rate, followed by annealing heat treatment at <NUM> ° C, followed by slow cooling to <NUM> at a cooling rate of <NUM>/sec, followed by cooling at <NUM>/sec, which is a conventional cooling rate, to the RSC temperature (secondary cooling end temperature) in Table <NUM>, and subjected to over-aging heat-treatment to manufacture a steel sheet.

The mechanical properties and shape quality of the steel sheet were measured, and the results are shown in Table <NUM> below.

Here, the shape quality is shown by measuring a wave height(ΔH) in an edge portion after cutting a steel sheet to a size of <NUM> in a longitudinal direction, as shown in <FIG>.

In Table <NUM> below, it represents that RCS: a secondary cooling end temperature, M: martensite, TM: tempered martensite, B: bainite, F: ferrite, TS: tensile strength, YS: yield strength, and El: elongation.

Meanwhile, a microstructure was observed for Inventive Example <NUM> and Comparative Example <NUM>, and Inventive Example <NUM> was shown in <FIG> and Comparative Example <NUM> was shown in <FIG>.

As shown in Table <NUM> and Table <NUM>, Comparative Example <NUM>, Comparative Example <NUM>, and Comparative Example <NUM> illustrate a steel type in which the content of Mn is outside of the scope of the present disclosure, and it can be seen that the Comparative Example <NUM>, Comparative Example <NUM>, and Comparative Example <NUM> have a low tensile strength of 1700MPa or less, and in particular, the Comparative steel <NUM>, which has a very low amount of Mn, has a very low strength that the tensile strength is less than 1200Mpa. In particular, in the case of Comparative Example <NUM>, as shown in <FIG>, it can be seen that a fraction of ferrite and bainite is high.

On the other hand, Comparative Example <NUM> illustrates a steel type that satisfies the components and component ranges of the present disclosure, but does not satisfy the Relational expression <NUM> (<NUM> [C] + <NUM> [Mn] + <NUM> [Cr]-<NUM> [RCS]> <NUM>), and in the case of Comparative Example <NUM>, the secondary cooling end temperature is <NUM>, and a tensile strength is <NUM> MPa or less, as shown in Table <NUM>. Meanwhile, in the case of Inventive Example <NUM>, the secondary cooling end temperature is <NUM>, which satisfies Relational Expression <NUM>, and represents a tensile strength of <NUM> MPa or more.

In the case of the Inventive Examples <NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>, and <NUM>, as shown in Table <NUM>, it can be seen that not only shows tensile strength of <NUM> MPa or more, but also has a low wave height of <NUM> or less, even under continuous annealing operation conditions including slow cooling by including Relational Expression <NUM> (<NUM>[C] + <NUM>[Mn] + <NUM>[Cr] - <NUM>[RCS] > <NUM>).

As shown in <FIG>, in the case of Inventive Example <NUM>, a main phase is martensite and contains a small amount (less than <NUM>%) of ferrite and bainite. It is determined that such a second phase transformation-appears in the slow cooling and over-aging, which are essential in the ordinary continuous annealing furnace.

Claim 1:
An ultrahigh strength cold-rolled steel sheet, comprising, in percentage by weight: C: <NUM> to <NUM>%; Si: <NUM>% or less excluding <NUM>%; Mn: <NUM> to <NUM>%; P: <NUM>% or less excluding <NUM>%; S: <NUM>% or less excluding <NUM>%; Al: <NUM>% or less excluding <NUM>%; Cr: <NUM>% or less excluding <NUM>%; Ti: <NUM>/<NUM>*[N]to <NUM>% or less; Nb: <NUM>% or less excluding <NUM>%; B: <NUM>% or less excluding <NUM>%; N: <NUM>% or less excluding <NUM>%; and a balance of Fe and other unavoidable impurities, wherein a microstructure consists of <NUM>% to <NUM>% of martensite, and one or two kinds of <NUM>% to <NUM>% of ferrite and bainite, and wherein the cold-rolled steel sheet has a tensile strength of <NUM> MPa or more, and wherein the cold-rolled steel sheet has a wave height ΔH of <NUM> or less in an edge portion after cutting a steel sheet to a size of <NUM> in a longitudinal direction as disclosed in the description.