KR20110084545A - Steel sheet, surface-treated steel sheet, and method for producing the same - Google Patents

Abstract

The high-strength steel sheet having a tensile strength of 590 MPa or more and excellent bendability includes C: 0.03 to 0.20%, Si: 0.005 to 2.0%, Mn: 1.2 to 3.5%, P ≦ 0.1%, S ≦ 0.01%, and sol. Al: 0.001-1.0%, N≤0.01% and Bi: 0.0001-0.05%, and Ti: ≤0.3%, Nb: ≤0.3%, V: ≤0.3%, Cr: ≤1%, Mo: ≤ Has a chemical composition containing 1%, Cu: ≤1%, Ni: ≤1%, Ca: ≤0.01%, Mg: ≤0.01%, REM: ≤0.01%, Zr: ≤0.01% and B: ≤0.01% The Mn segregation ratio (Mn _max / Mn _av ) calculated from the average Mn concentration (Mn _av ) and the maximum Mn concentration (Mn _max ) at the (1/20) depth position of the plate thickness from the steel sheet surface is less than 1.30.

Description

Steel plate and surface treatment steel plate and manufacturing method thereof {STEEL SHEET, SURFACE-TREATED STEEL SHEET, AND METHOD FOR PRODUCING THE SAME}

TECHNICAL FIELD This invention relates to a steel plate and a surface-treated steel plate, and their manufacturing method. More specifically, the present invention relates to a high strength steel sheet and a surface treated steel sheet suitable as a raw material such as an automobile reinforcing member or an automobile seat member, and a manufacturing method thereof.

In the automotive industry, high-strength steel sheets excellent in workability are attracting more and more attention because they are effective for weight reduction which leads to improvement of vehicle safety and fuel economy. In recent years, the score of the member manufactured with the high strength steel plate per vehicle is increasing. For this reason, a very high level of bendability is also required for high strength steel sheets having a tensile strength of 590 MPa or more. In particular, members having small bending radii such as seat rails and side seals of automobiles require strict bendability and new high strength.

For the purpose of improving bendability, a technique for controlling the structure of the high strength steel sheet has been conventionally adopted. Patent Document 1 discloses a high strength steel sheet in which the hardness difference between the low hardness ferrite phase around the bainite and martensite phase is reduced by lowering the hardness of the high hardness bainite or martensite phase. It is. Patent Documents 2 and 3 disclose a cold-rolled steel sheet and a hot-dip galvanized steel sheet having both high strength and stretch flange formability requiring local strain as well as bendability, by minimizing the crystal grains of ferrite.

The high strength steel sheet generally contains a large amount of Mn in order to achieve high strength. Mn tends to segregate in the steel. For this reason, the chemical composition of a high strength steel plate fluctuates locally by segregation of Mn. Due to the local fluctuation of this chemical composition, a nonuniform structure is formed in a high strength steel sheet. For this reason, as disclosed in Patent Document 1, it is practically very difficult to precisely control the hardness of the ferrite phase and the bainite and martensite phases throughout the high strength steel sheet.

1: is explanatory drawing which showed the surface property after bending deformation of a high strength steel plate. When a nonuniform structure is formed in a high strength steel plate, as shown in FIG. 1, the outstanding unevenness | corrugation which can be observed even by a naked eye appears on the surface of the processed part of a high strength steel plate. Since this unevenness encourages non-uniform deformation during bending, cracking is caused in the processed portion, and the bendability is deteriorated. In addition, even when cracking does not occur, since the unevenness existing in the processing portion remains in the member made of this high strength steel sheet, the collision characteristic of this member is deteriorated.

In addition, since the transformation phenomenon is locally changed due to segregation of Mn, the grain size of the high strength steel sheet becomes nonuniform. For this reason, the method disclosed by patent document 2, 3 cannot improve the bendability of a high strength steel plate. In particular, the steel sheets disclosed in Patent Literatures 1 to 3 have a steel composition containing a large amount of Mn and Ni, which tend to segregate in the steel, and therefore, for the reasons described above, there is a concern that deterioration of the bendability and the collision characteristics of the members are deteriorated.

Ultimate techniques such as single-phase tissue have been proposed in terms of tissue uniformity. Patent Document 4 discloses a high strength cold rolled steel sheet having improved bendability by using martensite single phase structure as an ultimate uniform structure. However, when the steel structure is made of martensite single phase structure, the flatness of the high-strength steel sheet is impaired, so that it is difficult to use it as a raw material of a member for automobiles that requires high dimensional accuracy.

Patent Literature 5 discloses a thin steel sheet having a high hole expansion ratio and strength by using a matrix as a ferrite single phase structure. When producing a high strength cold rolled steel sheet or a high strength hot dip galvanized steel sheet based on the technique disclosed in this document, it is necessary to perform cold rolling and annealing in order to improve the surface roughness and the sheet thickness precision of the product. Since the steel composition disclosed contains a large amount of carbonitride-forming elements, the recrystallization temperature of the steel increases, so annealing must be performed at a high temperature of Ac ₃ or more. As a result of the annealing at a high temperature, coarsening of the precipitates proceeds, and the strength cannot be increased. In addition, the grain size also becomes nonuniform, and the bendability cannot be improved.

Therefore, as mentioned above, even if it contains a large amount of Mn for high strength, what is mutually contradictory is necessary in order to make both the bending property of a steel plate and the high strength improvement compatible.

A technique for resolving segregation itself, which is the origin of non-uniform tissue, by diffusion is also proposed. Patent Document 6 discloses a heat treatment method for steel materials in which segregation is diffused by performing homogenization in which steels are kept at a high temperature of 1250 ° C or higher for a long time of 10 hours or longer. However, this method cannot completely eliminate segregation. For this reason, uneven structure is formed by segregation, unevenness | corrugation of a process part is not removed, and bending property cannot fully be improved.

Patent Literatures 7 and 8 disclose continuous casting under the condition of cooling the liquid crystal from the liquidus temperature to the solidus temperature with an average cooling rate at the position of (1/4) ts of the thickness ts of the slab being 100 ° C / min or more. By performing the above, a hot-dip galvanized steel sheet excellent in hole enlargement property in which segregation is reduced is disclosed. However, since the above cooling rate can only be achieved with a thin slab having a thickness of 30 to 70 mm, this technique cannot be applied to continuous casting of a conventional slab having a thickness of 200 to 300 mm.

Japanese Patent Laid-Open No. 62-13533 Japanese Unexamined Patent Publication No. 2004-211126 Japanese Unexamined Patent Publication No. 2004-250774 Japanese Unexamined Patent Publication No. 2002-161336 Japanese Unexamined Patent Publication No. 2002-322539 Japanese Unexamined Patent Publication No. 4-191322 Japanese Unexamined Patent Publication No. 2007-70649 Japanese Unexamined Patent Publication No. 2007-70659

An object of the present invention is to provide a steel sheet and a surface-treated steel sheet having a tensile strength of 590 MPa or more and excellent in bendability, and a method for producing them.

According to the present invention, "excellent bendability" means that the minimum bending radius at which the bending bend axis is not broken in the 180 ° bending test in the rolling direction is 1.0t or less, and the bending radius in the same bending direction This means that no irregularities appear on the surface of the workpiece visually after a 90 ° V bending of 1.0 t. Therefore, the bending property in this specification is evaluated by visual observation of the physical property of a steel plate and the member produced by the bending process from this steel plate, unless there is particular notice. In the case of using the steel sheet according to the present invention as a raw material for a sheet rail requiring strict bendability, the minimum bending radius in the 180 ° bending test is 0.5t or less, and visually after 90 ° V bending at a bending radius of 0.5t It is preferable that unevenness | corrugation does not appear on the surface of a process part.

In the high-strength steel sheet, the present invention can achieve a desired Mn concentration distribution by optimizing the chemical composition and the manufacturing conditions, thereby suppressing the formation of non-uniform structure caused by segregation of Mn, thereby making it a uniform structure. Based on the knowledge that a high strength steel sheet having excellent bendability with a tensile strength of 590 MPa or more can be produced.

The present invention, C, Si, Mn, P, S, sol. Contents of Al, N, Bi, Ti, Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, Zr and B ("%" regarding chemical composition "Mass%" means C: 0.03-0.20%, Si: 0.005-2.0%, Mn: 1.2-3.5%, P≤0.1%, S≤0.01%, sol. Al: 0.001-1.0%, N≤0.01%, Bi: 0.0001-0.05%, Ti: 0-0.3%, Nb: 0-0.3%, V: 0-0.3%, Cr: 0-1%, Mo: 0 -1%, Cu: 0-1%, Ni: 0-1%, Ca: 0-0.01%, Mg: 0-0.01%, REM: 0-0.01%, Zr: 0-0.01%, and B: 0-0% Mn segregation ratio (Mn _max / Mn _av ) calculated from the average Mn concentration (Mn _av ) and the maximum Mn concentration (Mn _max ) at the (1/20) depth position of the plate thickness from the steel plate surface with a chemical composition of 0.01%. ) Is less than 1.30.

In one suitable aspect of the steel sheet according to the present invention, the chemical composition contains at least one selected from the following (a) to (d):

(a) one or two or more selected from the group consisting of Ti: 0.003-0.3%, Nb: 0.003-0.3%, and V: 0.003-0.3%;

(b) Cr: 0.01% to 1%, Mo: 0.01% to 1%, Cu: 0.01% to 1%, and Ni: 0.01% to 1%;

(c) at least one selected from the group consisting of Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, REM: 0.0001 to 0.01%, and Zr: 0.0001 to 0.01%; And

(d) B: 0.0003 to 0.01%.

In another aspect, the present invention is a surface-treated steel sheet comprising the steel sheet and a plating layer formed on at least one surface of the steel sheet.

From another viewpoint, this invention is equipped with the following process (A)-(C), The manufacturing method of the steel plate characterized by the above-mentioned.

(A) Continuous casting process of casting molten steel which has the above-mentioned chemical composition into slab of 200-300 mm thickness by making solidification rate in the 10 mm depth position from the surface into 100-1000 degreeC / min;

(B) a rolling step including hot rolling and cold rolling, wherein the slab obtained in the continuous casting step is hot rolled to form a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled to form a cold rolled steel sheet; And

(C) A continuous annealing step of subjecting the cold rolled steel sheet obtained in the rolling step to recrystallization annealing at a temperature range of 750 to 950 ° C.

Moreover, from another viewpoint, this invention is the manufacturing method of the surface-treated steel plate characterized by performing the plating process which forms a plating layer in the at least single side | surface on the steel plate obtained by the manufacturing method mentioned above.

According to the present invention, a high strength steel sheet having a strength of 590 MPa or more and excellent in bendability can be obtained. The steel sheet according to the present invention can be widely used in various industrial fields, particularly in the automobile field.

1 is an explanatory view showing the surface properties after bending deformation.

The chemical composition of the steel plate which concerns on this invention is as follows.

(C: 0.03-0.20%)

C contributes to improving the strength of the steel sheet. C content is made into 0.03% or more in order to make tensile strength of a steel plate 590 Mpa or more. If C content is more than 0.20%, weldability will deteriorate. For this reason, C content is made into 0.20% or less. C content is preferably 0.05% or more in order to easily obtain a tensile strength of 980 MPa or more.

(Si: 0.005-2.0%)

When Si is contained 0.005% or more, the strength of the steel sheet can be improved without deteriorating the bendability too much. If the Si content is more than 2.0%, the chemical conversion treatability deteriorates in the case of the non-plated steel sheet, and the wettability, alloying processability and plating adhesion of the plating deteriorate in the case of the hot dip galvanized steel sheet. For this reason, Si content is made into 0.005% or more and 2.0% or less.

When Si content is more than 1.5%, the oxide containing Si is formed in the steel plate surface, and surface property may deteriorate. For this reason, Si content becomes like this. Preferably it is 1.5% or less. In addition, due to the limitation of the manufacturing process, in the case of producing a hot-dip galvanized steel sheet whose strength is not as easy as that of the cold rolled steel sheet, when the Si content is 0.4% or more, a tensile strength of 980 MPa or more can be easily obtained. . For this reason, Si content becomes like this. Preferably it is 0.4% or more.

(Mn: 1.2 to 3.5%)

Mn contributes to the strength improvement of the steel sheet. 1.2% or more of Mn content is necessary in order to make tensile strength of a steel plate 590 Mpa or more. When the Mn content is more than 3.5%, not only the melting and refining of the steel in the converter becomes difficult, but also the weldability is deteriorated. For this reason, Mn content is made into 1.2% or more and 3.5% or less. Although Mn promotes nonuniform structure of steel, as described later, by containing Bi, this adverse effect of Mn is alleviated and the structure becomes uniform, so that deterioration of bendability is suppressed and the strength is improved. In addition, Mn content becomes like this. Preferably it is 1.8% or more in order to make tensile strength 980 Mpa or more.

(P≤0.1%)

P is an impurity generally contained inevitable. However, P is also a solid solution hardening element and is effective for reinforcing the steel sheet, and thus may be actively contained. However, weldability will deteriorate when P content is more than 0.1%. For this reason, P content is made into 0.1% or less. P content is preferably 0.003% or more in order to reinforce the steel sheet more reliably.

(S≤0.01%)

S is an impurity inevitably contained in steel. S content is so preferable that it is low from a viewpoint of bendability and weldability. For this reason, S content is made into 0.01% or less. S content becomes like this. Preferably it is 0.005% or less, More preferably, it is 0.003% or less.

(sol.Al: 0.001-1.0%)

Al is contained in steel for deoxidation of steel. Al acts effectively to improve the product yield of carbonitride-forming elements such as Ti. Necessary sol. Al content is 0.001% or more. sol. If the Al content is more than 1.0%, the weldability deteriorates, and the oxide inclusions increase, so that the surface properties deteriorate. Because of this, sol. Al content is made into 0.001% or more and 1.0% or less. Also, sol. Al content becomes like this. Preferably it is 0.01% or more and 0.2% or less.

(N≤0.01%)

N is an impurity contained inevitable in steel. Since N content is so preferable that it is low from a viewpoint of bendability, it is made into 0.01% or less. N content becomes like this. Preferably it is 0.006% or less.

(Bi: 0.0001 to 0.05%)

Bi plays an important role in this invention. When steel contains Bi, the solidification structure of a slab becomes fine, even if steel contains a large amount of Mn, the structure of a steel plate will become uniform, and the deterioration of the bendability is suppressed. Therefore, in order to ensure desired bendability, Bi content of 0.0001% or more is required. However, when Bi content is more than 0.05%, hot workability will deteriorate and hot rolling will become difficult. For this reason, Bi content is made into 0.0001% or more and 0.05% or less. In order to further improve bendability, Bi content becomes like this. Preferably it is 0.0010% or more.

(1 or 2 or more selected from the group consisting of Ti≤0.3%, Nb≤0.3%, and V≤0.3% or less)

Ti, Nb, and V are all arbitrary elements which contribute to the strength improvement of a steel plate and can be contained in steel as needed. It is effective for steel to contain 1 type, or 2 or more types of Ti, Nb, and V in order to ensure the tensile strength of 980 Mpa or more. In order to acquire this effect more reliably, it is preferable to contain 0.003% or more of any 1 or more types of elements among Ti, Nb, and V. If the content of Ti, Nb and V is more than 0.3%, inclusions containing Ti, Nb or V increase, so that the surface properties of the steel sheet deteriorate. For this reason, when containing at least 1 sort (s) of Ti, Nb, and V, each content is made into 0.3% or less.

(1 or 2 or more selected from the group consisting of Cr ≦ 1%, Mo ≦ 1%, Cu ≦ 1%, and Ni ≦ 1%)

Cr, Mo, Cu, and Ni are all optional elements which can be contained in steel as needed, since they all contribute to the strength improvement of a steel plate. The steel containing one or two or more of Cr, Mo, Cu, and Ni is used to produce a cold rolled steel sheet using a cooling stop temperature of continuous annealing at 300 ° C or higher and 420 ° C or lower, or a hot dip galvanized steel sheet. In the case of manufacturing, it is effective in ensuring the tensile strength of 980 Mpa or more. In order to acquire this effect more reliably, it is preferable to contain at least 1 sort (s) of Cr, Mo, Cu, and Ni 0.01% or more. However, when the content of Cr, Mo, Cu, and Ni is more than 1%, the above effects are saturated and economically wasteful, and since the hot rolled steel sheet which has finished hot rolling becomes hard, It becomes difficult to cold-roll. For this reason, when containing at least 1 sort (s) of Cr, Mo, Cu, and Ni, each content is made into 1% or less.

(Ca≤0.01%, Mg≤: 0.01%, REM≤0.01%, and Zr≤0.01%, one or two or more selected from the group consisting of)

Ca, Mg, REM, and Zr all contribute to steel inclusion control, in particular, to finely disperse inclusions, and further improve the bendability of the steel sheet, and thus are arbitrary elements that can be contained in the steel as necessary. However, when Ca, Mg, REM and Zr are excessively contained, the surface property of a steel plate will deteriorate. Therefore, when containing 1 or more types of Ca, Mg, REM, and Zr, each content is made into 0.01% or less. In order to acquire the said effect more reliably, it is preferable to make content of Ca, Mg, REM, and Zr into 0.0001% or more.

(B≤0.01%)

B is an arbitrary element which not only contributes to the bendability improvement of the steel sheet but also is effective for securing a tensile strength of 980 MPa or more in the case of producing a hot-dip galvanized steel sheet. However, when B content is more than 0.01%, a hot rolled sheet steel will become hard and it will become difficult to cold-roll to this hot rolled sheet steel. For this reason, it is preferable to make B content into 0.01% or less. When it contains B, in order to acquire the said effect more reliably, it is preferable to make B content into 0.0003% or more.

Remainder other than the component mentioned above consists essentially of Fe and an impurity.

(Mn segregation ratio <1.30)

The steel sheet concerning this invention has a specific Mn distribution. In other words, the Mn distribution of the steel sheet satisfies a condition in which the Mn segregation ratio (Mn _max / Mn _av ) is less than 1.30. Mn segregation ratio (Mn _max / Mn _av ) is the average Mn, which is obtained by analyzing an area not containing MnS with an EPMA (Electron Probe Micro Analyzer) at the (1/20) depth position of the plate thickness from the steel plate surface. It is calculated as the ratio of the maximum Mn concentration (Mn _max ) to the concentration (Mn _av ). The Mn segregation ratio of less than 1.30 is an index of the uniform structure. As a result, the bendability of the steel sheet is improved, and irregularities in the bent portion are less likely to occur. The chemical composition of the steel contains Bi, and, as described later, by setting the casting speed to a predetermined condition, the Mn segregation ratio of the steel sheet can be made less than 1.30. The Mn segregation ratio is preferably less than 1.20 in order to further improve bendability.

The bendability of the steel sheet is influenced by the Mn distribution in the steel sheet surface layer portion. This is because the deformation in the bending process is larger in the steel plate surface layer portion than in the sheet thickness center of the steel sheet, and the bending property is governed by the deformation ability of the steel plate surface layer portion. However, there is a possibility that the correct Mn distribution cannot be measured on the surface of the steel sheet and immediately below it due to the influence of the surface oxidation of the steel sheet. Therefore, in this invention, Mn concentration is measured in said depth position near the surface of a steel plate, and Mn segregation ratio is calculated | required. The analysis by EPMA, as shown in the Example, measures about the area which can fully evaluate the local fluctuation | variation of Mn distribution, such as a rectangular area of 500 micrometers in a rolling direction and a total of 4 mm in a direction orthogonal to a rolling direction. It is preferable.

(Plating Layer)

The steel sheet according to the present invention can also be used as a surface-treated steel sheet by forming a plating layer on the surfaces of one or both surfaces thereof for the purpose of improving corrosion resistance.

The plating layer to be formed may be an electroplating layer or a hot dip layer. As an electroplating layer, an electrogalvanization layer, an electro Zn-Ni alloy plating layer, etc. are illustrated. As a hot dip galvanizing layer, a hot dip galvanizing layer, an alloying hot dip galvanizing layer, a hot dip aluminum plating layer, a hot dip Zn-Al alloy plating layer, a hot dip Zn-Al-Mg alloy plating layer, a hot dip Zn-Al-Mg-Si alloy plating layer, etc. are illustrated. The adhesion amount (or thickness) of these plating layers should just be generally employ | adopted for this kind of plated steel plate. If desired, two or more plating layers may be provided.

Next, the suitable manufacturing method of the steel plate which concerns on this invention is demonstrated.

[Continuous Casting Process]

The molten steel which has the chemical composition mentioned above is melted by well-known solvent methods, such as a converter and an electric furnace. This molten steel is cast continuously into a slab having a thickness of 200 to 300 mm at a solidification rate of 100 to 1000 ° C / min at a depth of 10 mm from the surface of the slab.

(Solidification rate: 100 to 1000 ° C / min)

If the solidification rate at a depth of 10 mm from the slab surface in the continuous casting process is less than 100 ° C./min, the dendrite primary arm spacing at the (1/20) depth position of the slab thickness cannot be refined from the slab surface, Segregation of Mn is not prevented sufficiently, and the bendability of the steel sheet may not be improved. If the solidification rate exceeds 1000 ° C / min, the surface cracks of the slab may be caused. Therefore, this solidification rate is made into 100 degrees C / min or more and 1000 degrees C / min or less.

(Thickness of slab: 200 ~ 300mm)

When the thickness of the slab is less than 200 mm, it becomes difficult to secure a total reduction ratio of 99.0% or more in hot rolling and cold rolling described later. When the thickness of the slab exceeds 300 mm, it becomes difficult to secure an Mn segregation ratio of less than 1.30 at the (1/20) depth position of the plate thickness from the steel plate surface. Therefore, the thickness of the slab is made 200 mm or more and 300 mm or less.

[Rolling process]

In the slab obtained by the continuous casting step, hot rolling is performed to form a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled to form a cold rolled steel sheet.

Preferably, the slab obtained by this continuous casting process is subjected to a homogenization treatment to be maintained at a temperature range of 1200 to 1350 ° C. for 20 minutes or more, and then finishing temperature: 800 to 950 ° C., winding Temperature: 400-750 degreeC hot rolling is used as a hot rolled sheet steel, cold rolled to this hot rolled sheet steel, and it is cold rolled sheet steel, and the total rolling reduction in hot rolling and cold rolling is 99.0% or more.

(Homogenization treatment temperature: 1200 to 1350 ° C., homogenization treatment time: 20 minutes or more)

By maintaining the slab provided for hot rolling in the temperature range of 1200 degreeC or more for 20 minutes or more, the nonuniform structure resulting from segregation of Mn is further eliminated, and the bendability of a steel plate further improves. It is preferable that homogenization treatment temperature is 1350 degrees C or less from a viewpoint of suppressing scale loss, preventing damage to a heating furnace, and improving productivity.

The homogenization treatment time is more preferably 1.0 hours or more and 3 hours or less. When the homogenization time is 1.0 hours or more, the Mn segregation ratio can be made less than 1.20, and the bendability of the steel sheet can be further improved. When the homogenization treatment time is 3 hours or less, scale loss can be suppressed and productivity can be improved, leading to a reduction in manufacturing cost.

(Finishing temperature: 800 to 950 ° C)

When the finishing temperature of hot rolling is 800 degreeC or more, the deformation resistance at the time of hot rolling can be made small, and operation can be performed more easily. When finishing temperature is 950 degrees C or less, the flaw by a scale can be suppressed more reliably and a favorable surface property can be ensured.

(Winding temperature: 400 degreeC-750 degreeC)

When the winding temperature in hot rolling is 400 degreeC or more, generation | occurrence | production of hard bainite and martensite is suppressed, and subsequent cold rolling can be performed easily. Moreover, when a coiling temperature is 750 degreeC or less, oxidation of the steel plate surface is suppressed and favorable surface property can be ensured.

In the hot rolling step, preferably, the bar bar after the rough rolling and before the finish rolling is heated by induction heating or the like, and the temperature of the rough bar is equalized over its entire length. Thereby, the characteristic variation of a steel plate can be suppressed.

(Total rolling reduction in hot rolling and cold rolling: 99.0% or more)

The hot rolled steel sheet obtained by the above hot rolling step is usually subjected to cold rolling after a descaling process by a conventional method such as acid washing, to be cold rolled steel sheet. It is preferable to make the total rolling reduction in hot rolling and cold rolling at this time into 99.0% or more. Here, the total reduction ratio is calculated by the following equation.

Total rolling reduction (%) = {1- (plate thickness of cold rolled steel sheet) / (plate thickness of slab provided for hot rolling)} × 100

The surface unevenness of the processed portion generated after bending of the steel sheet is not only the Mn segregation ratio but also the thickness of the Mn thickened band in which the Mn segregation portion generated by the solidification segregation becomes a band-shaped region in the rolling direction in the subsequent rolling process. The thickness of the direction is also affected. By reducing the thickness of this Mn thickened strip, the surface unevenness after processing can be suppressed more reliably, and the bendability of a steel plate can be improved by this. For this reason, it is effective to make total reduction rate 99.0% or more.

The reduction ratio of cold rolling is preferably 30% or more in order to make the structure of the steel sheet after continuous annealing uniform. Moreover, it is preferable from the viewpoint of ensuring the flatness of a steel plate to correct shape by performing rolling of the hardness of 5% or less of rolling reduction before or after an acid wash. In addition, since the acid cleaning property is improved and the removal of the surface thickening element is accelerated by performing the rolling of this hardness before acid cleaning, the plating adhesion is improved in the case of a hot-dipped steel sheet, and in the case of a cold rolled steel sheet, the surface properties are improved. Is improved.

[Continuous Annealing Process]

Continuous annealing is performed on the cold rolled steel sheet obtained by the rolling process including the above hot rolling and cold rolling. It is preferable to make annealing temperature 750 degreeC or more and 950 degrees C or less. It is preferable from a viewpoint of productivity that the temperature increase rate to recrystallization annealing temperature is 1 degree-C / sec or more.

(Annealing temperature: 750-950 degreeC)

If the annealing temperature is 750 ° C or higher, the remaining of the unrecrystallized structure is suppressed and a uniform structure can be reliably obtained, so that the bendability is further improved. Moreover, when annealing temperature is 950 degreeC or less, the damage of annealing furnace is suppressed and productivity improves.

The annealing time is preferably set to 10 seconds or more in order to completely remove the unrecrystallized structure and to secure a good bendability. The annealing time is preferably 300 seconds or less from the viewpoint of productivity.

In the cooling after annealing, the average cooling rate from 650 ° C to 550 ° C is preferably 5 ° C / sec or more in order to secure high tensile strength of 590 MPa or more while suppressing the addition of alloying elements connected at high cost.

It is preferable to perform temper rolling on the steel plate after annealing (after plating if hot dip galvanizing is performed). By performing temper rolling, it becomes possible to suppress generation | occurrence | production of yield point, and to prevent pressing and galling at the time of a press. It is preferable that elongation rate of temper rolling is 0.05% or more and 1% or less.

[Plating Process]

In the case of performing hot dip galvanizing on the surface of the steel sheet, in the annealing step, it is preferable to stop the cooling after annealing at 460 ° C. or higher and 550 ° C. or lower, and to immediately immerse the annealed steel sheet in a molten plating bath to perform plating continuously. Do. If the cooling stop temperature at this time is less than 460 degreeC, the heat_generation | fever at the time of immersion of a plating bath may become large, and plating operation may become difficult. Even when a cooling stop temperature exceeds 550 degreeC, plating operation may become difficult.

The hot dip galvanizing may be performed by a conventional method. For example, a steel plate is immersed in the hot-dip galvanizing bath of 410 degreeC or more and 490 degrees C or less, and plating amount is controlled by a gas wiping nozzle etc. immediately after leaving a plating bath.

You may perform alloying process after immersing in a hot dip galvanizing bath. When performing alloying process, it is preferable that alloying process temperature is 460 degreeC or more and 600 degrees C or less. If the alloying treatment temperature is less than 460 ° C, an unalloyed portion is generated and the surface properties of the steel sheet tend to deteriorate. When alloying process temperature is more than 600 degreeC, powdering of a plating film will arise easily.

When manufacturing an electroplated steel plate, what is necessary is just to electroplate the steel plate cooled after annealing after a suitable surface adjustment process by a conventional method.

After hot-dip plating or electroplating, depending on the application, the obtained plated steel sheet may be subjected to well-known post-treatment (for example, chemical conversion treatment, lubrication treatment, etc.).

According to the manufacturing method which concerns on this invention, the Mn segregation ratio in the (1/20) depth position of plate | board thickness from the steel plate surface is less than 1.30, and it can easily manufacture the steel plate and surface-treated steel plate excellent in high bending strength. .

&Lt; Example 1 >

The steel which has the chemical composition shown in Table 1 was melted in the converter. Next, the slab with a thickness of 245 mm was produced by continuous casting so that the solidification speed in the 10 mm depth position from the slab surface might become the conditions shown in Table 2.

The slab was subjected to hot rolling under the conditions shown in Table 2, followed by acid washing and cold rolling under the conditions shown in Table 2, thereby obtaining a cold rolled steel sheet having a sheet thickness of 1.2 mm.

The test material for heat treatment was extract | collected from the obtained cold rolled sheet steel, and as shown in Table 3, heat processing corresponded to the heat pattern in a continuous annealing installation or a continuous hot dip galvanizing installation.

The cold rolled steel sheet for publication (heat-treated under the conditions shown in Table 3) obtained under various manufacturing conditions was analyzed by EPMA to investigate the Mn distribution. Moreover, about this cold-rolled steel sheet for a test, the bending test which the tensile test and the bending ridge | line become a rolling direction was done, and the mechanical characteristic was evaluated.

(Experimental method)

(Average solidification rate)

The cross section of the obtained slab was etched with picric acid, and five dendrite secondary arm intervals λ (µm) were measured at a position 10 mm inward from the slab skin, and the liquid phase of the slab was determined based on the following equation. The cooling rate A (° C / min) within the line temperature to the solidus line temperature was calculated.

λ = 710 × A ^-0.39

(EPMA analysis)

The rolling surface of each cold rolled steel sheet for grinding was grinded and buff polished, and the analysis sample which appeared the analysis surface of the (1/20) depth position of plate | board thickness from the surface was produced, and Mn distribution was investigated by EPMA. A region not containing MnS was selected, and the beam diameter was 10 µm, and an area of 500 µm in the rolling direction and a total area of 4 mm in the direction perpendicular to the rolling direction was measured, and was perpendicular to the rolling direction averaged at 500 µm width. The Mn concentration distribution of the direction was analyzed. From the obtained Mn concentration distribution, Mn segregation ratio (Mn _max / Mn _av ) was calculated from the average Mn concentration (Mn _av ) and the maximum Mn concentration (Mn _max ).

(Tension test)

From each cold rolled steel sheet for test, a JIS No. 5 tensile test piece was taken from a direction perpendicular to the rolling direction, and the tensile strength (TS) was measured.

(Bending test)

From each cold rolled steel sheet for each test, a bending test piece (width 40 mm x length 100 mm x plate thickness 1.2 mm) in which the direction perpendicular to the rolling direction was the same as the lengthwise direction was taken so that the bending ridge line became the rolling direction. The 180 degree bending test (bending radius 1.0t) which sandwiched the steel plate of thickness 2.4mm was implemented, and the presence or absence of the crack was visually confirmed. About the cold rolled steel plate which was not broken, the 180 degree bending test (bending radius 0.5t) which sandwiched the steel plate of thickness 1.2mm was also performed with respect to the bending test piece collected similarly to the above, and confirmed the presence or absence of the crack similarly. Also in this test, about the cold-rolled steel plate which is not broken, the 180 degree bending test (tight bending test, bending radius 0t) which did not sandwich a steel plate was also performed, and the presence or absence of the crack was similarly confirmed.

By dividing the plate thickness of the steel plate sandwiched by two times (2.4 mm) the plate thickness of the bending test piece, the bending radius specified by the plate thickness t is obtained, and the minimum bending radius when no crack is recognized after the test. (Indicated by R _min in Table 4) was obtained. When a bending radius generate | occur | produced in 1.0 t, the minimum bending radius was set to 1.0t.

(Surface property after bending deformation)

From each cold-rolled steel sheet having a minimum bending radius of 1.0t or less in the bending test, bending test pieces (40 mm in width x 60 mm in length x plate thickness 1.2 in which the direction perpendicular to the rolling direction coincides with the longitudinal direction so that the bending ridge line becomes the rolling direction). mm) was taken. The test piece was press-fitted with a 90 ° punch having a radius of 1.2 mm at the tip to conduct a 90 ° V bending test (bending radius 1.0t), and visually confirmed the presence or absence of irregularities on the surface. Evaluation of the surface property made the thing with unevenness | defect and the thing without being good. The cold-rolled steel sheet whose surface properties were good and the minimum bending radius in the bending test was 0.5 t or less was obtained by press-fitting with a 90 ° punch having a radius of 0.6 mm at the tip of the test piece taken as described above. The 90 degree V bending test (bending radius 0.5t) was also performed and the presence or absence of the unevenness | corrugation of the surface was visually confirmed. Evaluation of surface property is the same as the above.

(Explanation of test result)

These results are shown in Table 4.

Test materials Nos. 1 to 3, 7, 8, 10 to 17, 20 to 27, 29 to 31, 33 to 40 and 42 in Table 4 are steel sheets of the examples of the present invention satisfying all the conditions of the present invention.

On the other hand, since the solidification speed in 10 mm-depth position from the surface of the test material No. 4 and 18 was below the lower limit prescribed | regulated by this invention, Mn segregation ratio will be more than 1.30, and bendability This bad or bad surface property after bending deformation became bad.

Since Test Material Nos. 6, 19, and 41 did not contain Bi, the Mn segregation ratio exceeded 1.30, resulting in poor bendability or poor surface properties after bending deformation.

As the specimens Nos. 5, 9, 28, and 32, the C content or the Mn content were below the lower limit specified in the present invention, the desired tensile strength could not be obtained.

As for the steel plates of this invention example, the tensile strength was 590 Mpa or more, and the bending property and the surface property after bending deformation were favorable. In particular, steel sheets 1, 7, 8, 10-17, 20-24, 26, 27, 30, 31, 33, and 35-40 are in the range of 0.0010% or more and 0.05% or less in which the Bi content is the above-mentioned preferred range. Since the homogenization treatment temperature and the homogenization treatment time are in the above-mentioned preferred range of 1200 ° C or more, 1350 ° C or less, 1.0 hour or more and 3 hours or less, and the Mn segregation ratio is less than 1.20, the tensile strength is 590 MPa or more. And bendability was excellent.

Claims (8)

C, Si, Mn, P, S, sol. The contents of Al, N, Bi, Ti, Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, Zr and B are in mass%, C: 0.03-0.20%, Si: 0.005-2.0% , Mn: 1.2 to 3.5%, P ≦ 0.1%, S ≦ 0.01%, sol. Al: 0.001-1.0%, N≤0.01%, Bi: 0.0001-0.05%, Ti: 0-0.3%, Nb: 0-0.3%, V: 0-0.3%, Cr: 0-1%, Mo: 0 -1%, Cu: 0-1%, Ni: 0-1%, Ca: 0-0.01%, Mg: 0-0.01%, REM: 0-0.01%, Zr: 0-0.01%, and B: 0 Mn segregation ratio (Mn _max / Mn) having a chemical composition of ˜0.01% and calculated from the average Mn concentration (Mn _av ) and the maximum Mn concentration (Mn _max ) at the (1/20) depth position of the plate thickness from the steel plate surface. _av ) is less than 1.30. The method according to claim 1,
A steel sheet, wherein the chemical composition contains, in mass%, one or two or more selected from the group consisting of Ti: 0.003-0.3%, Nb: 0.003-0.3%, and V: 0.003-0.3%. The method according to claim 1 or 2,
The chemical composition contains, in mass%, one or two or more selected from the group consisting of Cr: 0.01 to 1%, Mo: 0.01 to 1%, Cu: 0.01 to 1%, and Ni: 0.01 to 1%. , Grater. The method according to any one of claims 1 to 3,
The chemical composition contains, in mass%, one or two or more selected from the group consisting of Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, REM: 0.0001 to 0.01%, and Zr: 0.0001 to 0.01%. , Grater. The method according to any one of claims 1 to 4,
A steel sheet, wherein the chemical composition contains 0.0003 to 0.01% by weight of B. The steel plate of any one of Claims 1-5, and the plating layer formed in the surface of the at least single side | surface of this steel plate are provided, The surface-treated steel plate characterized by the above-mentioned. The manufacturing method of the steel plate containing following process (A)-(C):
(A) Continuous casting in which molten steel having the chemical composition according to any one of claims 1 to 5 is cast into a slab having a thickness of 200 to 300 mm at a solidification rate of 100 to 1000 ° C / min at a position of 10 mm depth from the surface. fair;
(B) a rolling step including hot rolling and cold rolling, wherein the slab obtained by the continuous casting step is hot rolled to form a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled to form a cold rolled steel sheet; And
(C) A continuous annealing step of subjecting the cold rolled steel sheet obtained in the rolling step to recrystallization annealing at a temperature range of 750 to 950 ° C. The steel plate obtained by the manufacturing method of Claim 7 is given the plating process which forms a plating layer in the surface of at least one side, The manufacturing method of the surface-treated steel plate characterized by the above-mentioned.

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