KR101683406B1 - Steel plate - Google Patents

Abstract

This steel sheet has a low deformation forming portion which has a degree of cleanliness of 0.08% or less in a metallic structure, a value of? Of a Mn of 1.6 or less and undergone plastic deformation of 5% or less in hot forming, The difference in average hardness? Hv after the hot forming in the highly deformed molded part subjected to deformation is 40 or less.

Description

Steel plate {STEEL PLATE}

The present invention relates to a steel sheet (hot-formed steel sheet) suitable for applications in which quenching is performed simultaneously with hot forming or immediately after hot forming, such as hot pressing. More specifically, the present invention relates to a method for manufacturing a molded article, which is capable of suppressing deformation organic ferrite transformation in a molded part, for example, even when hot forming accompanied by high- To a hot-formed steel sheet having uniform hardness after hot forming, excellent toughness, and reduced toughness and anisotropy after hot forming.

The present application claims priority based on Japanese patent application No. 2012-187959 filed on August 28, 2012, the contents of which are incorporated herein by reference.

2. Description of the Related Art In recent years, in the field of automotive steel sheets, the application of high strength steel sheets with high tensile strength has been expanding to improve fuel economy and impact resistance of automobiles. Generally, when the steel sheet has a high strength, the press formability is deteriorated. Therefore, it is difficult to manufacture a product having a complicated shape with application of a high-strength steel sheet. Concretely, since the ductility is lowered with the increase in the strength of the steel sheet, breakage occurs at a portion having a high degree of workability, and springback or wall warpage increases with increase in strength of the steel sheet. As a result, there arises a problem that the dimensional accuracy of the processed member deteriorates. Therefore, it is not easy to produce a product having a complicated shape by press molding using a steel sheet having a high strength, particularly a tensile strength of 780 MPa or more.

If the forming is carried out by roll forming rather than press forming, a certain degree of processing can be performed also for a high strength steel sheet. However, in the roll forming, there is a restriction that it is applicable only to the processing of a member having a uniform cross section in the longitudinal direction, and the degree of freedom of the member shape is remarkably limited.

Therefore, as a technique for press molding a hard press forming material such as a high strength steel sheet, for example, Patent Document 1 discloses a hot forming (e.g., hot press) technique for heating a material provided for forming and then molding . This technique is a technique for obtaining a molded member having high strength by quenching after softening is performed on a soft steel sheet before and after molding, securing good moldability at the time of molding by performing quenching at the same time or immediately after molding . According to this technique, a structure containing mainly martensite can be obtained after quenching, and a molded member having excellent local deformation and toughness can be obtained, as compared with the case of using a high-strength steel sheet having a structure containing a warp-like structure.

At present, the hot press as described above is being applied to a member having a relatively simple shape, and is expected to be applied to a member subjected to more rigid molding such as burring molding in the future. However, when applied to a member subjected to more rigid molding, deformation organic ferrite transformation occurs in the high-strain forming section, and it is feared that the hardness of the member after hot forming is locally lowered.

In order to suppress such modified organic ferrite transformation, the hot forming may be performed in a higher temperature region. However, the increase in the hot forming temperature leads to a decrease in productivity, an increase in manufacturing cost, deterioration of the surface property, and the like, so that application to mass production technology is not easy. For example, in Patent Document 1, it is described that pressing is performed at 850 占 폚 or higher. In actual hot pressing, a steel sheet heated at about 900 占 폚 in a heating furnace or the like is extracted from a heating furnace, The temperature drop to 850 DEG C or less may occur. In such a case, it is difficult to suppress transformation organic ferrite transformation in the molding.

From the viewpoint of increasing the productivity of the hot press and enhancing the material stability in the member after molding, Patent Document 2 discloses a hot press high strength steel sheet having excellent productivity, which can omit the step of cooling the steel sheet by heat leakage in the press die A method of manufacturing a forcing member is disclosed. The method disclosed in Patent Document 2 is a very good invention, but it is necessary to contain a large amount of an element having an effect of improving quenching such as Mn, Cr, Cu, and Ni in the steel. Therefore, the technique of Patent Document 2 has a problem that the cost is increased. Further, in the member manufactured using the technique of Patent Document 2, deterioration of toughness and anisotropy of toughness caused by inclusions (mainly MnS) elongated in the rolling direction due to various inclusions present are concerned. Since the actual member performance is governed by the characteristics of the low-temperature side, if the anisotropy of toughness is present, the original base member characteristics can not be sufficiently exhibited. For example, anisotropy reduction of toughness can be achieved by controlling the shape of the elongated inclusions by the Ca treatment as described in Patent Document 3. However, in this case, although the toughness value in the direction in which the toughness is lowered is improved, the amount of the interposition member in the member itself is increased, so that the value of the other directional toughness is lowered.

Japanese Patent Application Laid-Open No. 2002-102980 Japanese Patent Application Laid-Open No. 2006-213959 Japanese Patent Application Laid-Open No. 2009-242910

As described above, in the prior art, the hot press has been applied to a member having a relatively simple shape. Therefore, it is possible to obtain a member after hot forming due to the deformation of the deformed organic ferrite in the highly deformed forming portion (the hot rolled steel sheet after the hot forming step, which may occur when considering application to a member subjected to stricter molding such as burring molding) ) Have not been studied to date, such as a decrease in the local hardness of the film, anisotropy of the toughness and a decrease in the toughness value.

It is an object of the present invention to solve the above problems, that is, even when hot forming accompanied by high strain forming is performed, the deformation organic ferrite transformation in the molding portion is suppressed, and the hardness after hot forming is uniform (the difference in hardness is small) A steel sheet for hot forming having excellent toughness after hot forming and low toughness and anisotropy.

The present inventors have conducted intensive studies in order to solve the above problems.

As a result, even when hot forming accompanied by high strain forming is carried out by controlling the chemical composition, interposition quantity and center segregation of the steel sheet, the modified organic ferrite transformation is suppressed, and the hardness is uniform after hot forming, It is possible to obtain a hot-for-molding steel sheet having excellent toughness after molding and low toughness and anisotropy. In the following description, there is a case where the hardness is uniform and the hardness distribution is stable.

The gist of the present invention based on the new discovery is as follows.

(1) A steel sheet according to one aspect of the present invention has a chemical composition of 0.18% to 0.275% of C, 0.02% to 0.15% of Si, 1.85% to 2.75% of Mn, P: not more than 0.1%, S: not more than 0.0035%, N: not more than 0.01%, Ni: 0 to 0.15%, Cu: 0 to 0.5%, Cr: 0.05 to 1.00%, B: 0.0005 to 0.01% 0.05%, Ti: 0 to 0.1%, Nb: 0 to 0.2%, the balance being Fe and impurities, the degree of cleanliness in the metal structure being not more than 0.08% ? of 1.6 or less and a difference in average hardness? Hv after hot forming between the low-strain forming portion subjected to the plastic deformation of not more than 5% and the high-strain forming portion subjected to the plastic deformation of not less than 20% Features, steel plate.

(2) The steel sheet according to the above (1), wherein the chemical composition contains, in mass%, one kind selected from the group consisting of 0.02 to 0.15% Ni and 0.003 to 0.05% Or may further contain two species.

(3) The steel sheet according to the above (1) or (2), wherein the chemical composition contains, in mass%, 0.005 to 0.1% of Ti and 0.005 to 0.2% of Nb, May be further contained.

(4) In the steel sheet according to any one of (1) to (3), a plating layer may further be provided on the surface of the steel sheet.

According to the above aspect of the present invention, even when hot forming accompanied by high strain forming such as burring molding is carried out, deformation organic ferrite transformation in the molding portion is suppressed, so that it has a stable hardness distribution after hot forming, A steel sheet having excellent toughness after molding and low toughness and anisotropy can be obtained. This steel sheet is suitable as a material for a mechanical structural member such as a body structural member of an automobile, a lower body member, etc., and therefore, the present invention is industrially advantageous.

The hot forming may be carried out according to a conventional method. For example, the material steel sheet is heated to a temperature of not less than Ac ₃ point (about 800 ° C), Ac ₃ point + 200 ° C or less, maintained at 0 second or more and 600 seconds or less, It is possible to maintain at least 5 seconds at the point. At this time, the heating method can be appropriately selected, and in the case of rapid heating, energization heating and high-frequency heating can be performed. In normal heating, furnace heating set at a heating temperature may be used. Since it is air-cooled during the conveyance to the press machine, if the conveyance time becomes long, there is a possibility that the ferrite transformation occurs until the start of the press and softens. Therefore, the conveying time is preferably 15 seconds or less. In order to prevent the mold temperature from rising, the mold may be cooled. In this case, the cooling method may be carried out by cooling the mold as required, for example, by performing cooling piping in the mold and flowing a coolant.

Hereinafter, a steel sheet according to an embodiment of the present invention (which may be referred to as a steel sheet according to the present embodiment) will be described in more detail. In the following description, the percentages regarding the chemical composition of the steel sheet are all% by mass.

1. Chemical composition

(1) C: 0.18% to 0.275%

C is an important element for increasing the quenching of the steel, determining the strength after quenching, and controlling the local ductility and toughness after hot forming. Further, since C is an austenite generating element, it has an effect of suppressing deformation organic ferrite transformation at the time of high strain forming and facilitating obtaining a stable hardness distribution in a member after hot forming. However, when the C content is less than 0.18%, it is difficult to secure a tensile strength of 1100 MPa or more, which is a preferable strength after quenching, and the effect of obtaining a stable hardness distribution due to the above action can not be obtained. On the other hand, when the C content exceeds 0.275%, the local ductility and toughness are lowered. Therefore, the C content is 0.18% to 0.275%. The preferable upper limit of the C content is 0.26%, and the more preferable upper limit is 0.24%.

(2) Si: 0.02% to 0.15%

Si is an element which improves the quenching property and improves the scale adhesion after hot forming. However, when the Si content is less than 0.02%, the above effect may not be sufficiently obtained. Therefore, the lower limit of the Si content is set at 0.02%. The lower limit is preferably 0.03%. On the other hand, if the Si content exceeds 0.15%, the heating temperature required for the austenite transformation at the time of hot forming becomes remarkably high. For this reason, the cost required for the heat treatment may be increased, or quenching may become insufficient due to insufficient heating. Since Si is a ferrite generating element, if the Si content is excessively high, deformation organic ferrite transformation tends to occur at the time of high strain forming, locally hardness is lowered in a member after hot forming, and a stable hardness distribution is obtained . In addition, a large amount of Si may cause unplated plating due to a decrease in wettability in the case of performing the hot-dip plating treatment. Therefore, the upper limit of the Si content is set to 0.15%.

(3) Mn: 1.85% to 2.75%

Mn is an effective element for increasing the quenching of the steel and for stably securing the strength of the steel after quenching. Further, since Mn is an austenite generating element, it is easy to suppress the deformation organic ferrite transformation at the time of high strain forming, and to obtain a stable hardness distribution in a member after hot forming. However, when the Mn content is less than 1.85%, the above effect may not be sufficiently obtained. Therefore, the lower limit of the Mn content is set to 1.85%. On the other hand, when the Mn content exceeds 2.75%, the above effect is saturated, and toughness deterioration occurs after quenching. Therefore, the upper limit of the Mn content is 2.75%. The preferable upper limit of the Mn content is 2.5%.

(4) sol.Al: 0.0002% to 0.5%

Al is an element that deoxidizes molten steel to make it stronger. If the sol.Al content is less than 0.0002%, deoxidation is not sufficient. Therefore, the lower limit of the sol.Al content is 0.0002%. Further, Al is an effective element for enhancing the quenching of the steel sheet and securing the strength after quenching stably, so that Al may be contained positively. However, if it is contained in an amount exceeding 0.5%, the effect is not only saturated but also leads to an increase in cost. Therefore, the upper limit of the Al content is set at 0.5%.

The term sol.Al means acid-soluble Al, and the content thereof does not include the amount of Al contained in Al ₂ O ₃ or the like which is not dissolved in an acid.

(5) Cr: 0.05% to 1.00%

Cr is an element that increases the quenching of a steel. Further, since Cr is an austenite generating element, it is easy to suppress the transformation organic ferrite transformation at the time of high strain forming, and to obtain a stable hardness distribution in a member after hot forming. However, when the Cr content is less than 0.05%, the above effects may not be sufficiently obtained. Therefore, the lower limit of the Cr content is set to 0.05%. A preferable lower limit is 0.1%, and a more preferable lower limit is 0.2%. On the other hand, when the Cr content exceeds 1.00%, Cr is concentrated in the carbide in the steel. As a result, solidification of the carbide in the heating step in the case of being provided for hot forming is delayed, and the quenching property is lowered. Therefore, the upper limit of the Cr content is 1.00%. The preferred upper limit of the Cr content is 0.8%.

(6) B: 0.0005% to 0.01%

B is an effective element for increasing the quenching of the steel and securing the strength after quenching in a stable manner. However, when the B content is less than 0.0005%, the above effects may not be sufficiently obtained. Therefore, the lower limit of the B content is 0.0005%. On the other hand, when the B content exceeds 0.01%, the above effect is saturated and toughness of the quenched portion is deteriorated. Therefore, the upper limit of the B content is set to 0.01%. The preferred upper limit of the B content is 0.005%.

(7) P: not more than 0.1%

P is an element generally contained as an impurity. However, since it has an action of enhancing the quenching of the steel and securing the strength of the steel after quenching in a stable manner, it may be actively contained. However, if the P content exceeds 0.1%, the toughness deteriorates remarkably. Therefore, the P content is limited to 0.1%. The preferred upper limit of the P content is 0.05%. The lower limit of the P content is not particularly limited, but an excessive reduction in the P content results in a remarkable increase in cost. Therefore, the lower limit of the P content may be 0.0002%.

(8) S: not more than 0.0035%

S is an element contained as an impurity. In particular, MnS is formed, which is a major cause of the decrease in toughness and anisotropy of toughness. If the S content exceeds 0.0035%, deterioration of toughness becomes remarkable, so the S content is limited to 0.0035%. The lower limit of the S content is not particularly limited, but excessive reduction of the S content leads to remarkable increase in cost. Therefore, the lower limit of the S content may be set to 0.0002%.

(9) N: 0.01% or less

N is an element contained as an impurity. When the N content exceeds 0.01%, a coarse nitride is formed in the steel, thereby remarkably deteriorating local strain and toughness. Therefore, the N content is limited to 0.01%. The lower limit of the N content is not particularly limited, but excessive reduction of the N content results in remarkable increase in cost. Therefore, the lower limit of the N content may be 0.0002%. A more preferred lower limit of the N content is 0.0008% or more.

In addition to the above elements, the steel sheet according to the present embodiment may contain the following optional elements. These elements are not necessarily included. Therefore, the lower limit of the content is not particularly limited, and the lower limit thereof is 0%.

(10) Ni: 0.15% or less, Cu: 0.05% or less

Ni and Cu are effective elements for enhancing the quenching of the steel and securing the strength after quenching stably. Therefore, one or two of these elements may be contained. However, even if any one element is contained in excess of the upper limit value, the above effect becomes saturated and becomes costly disadvantageous. Therefore, the content of each element is as described above. Preferably, the Ni content is 0.10% or less and the Cu content is 0.03% or less. In order to obtain the above effect more reliably, it is preferable to contain one or two kinds selected from the group consisting of 0.02% or more of Ni and 0.003% or more of Cu.

(11) Ti: not more than 0.1%, Nb: not more than 0.2%

Ti and Nb are elements that reduce recrystallization and form fine carbides to inhibit grain growth and provide austenite lips as fine grains when the steel sheet is heated to a temperature of Ac ₃ or higher and provided for hot forming. When the austenite grains become fine, the toughness of the hot-formed member is greatly improved. Further, Ti is preferentially bonded to N in the steel to generate TiN, and B is prevented from being consumed by precipitation of BN. As a result, by including Ti, the quenching property by B can be increased. In order to obtain the above effect, one or two kinds of these elements may be contained. However, if any one element is contained in excess of the upper limit, the precipitation amount of TiC or NbC is increased to consume C, and the strength after quenching may be lowered. Therefore, the content of each element is as described above. Preferably, the upper limit of the Ti content is 0.08% and the upper limit of the Nb content is 0.15%. In order to more reliably obtain the above effect, it is preferable to contain one or two kinds selected from the group consisting of Ti: 0.005% or more and Nb: 0.005% or more.

Remaining portions other than the above are Fe and impurities. Impurities refer to raw materials such as ores and scrap, and those incorporated in a manufacturing environment.

The steel sheet according to the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, or an annealed hot-rolled steel sheet or an annealed cold-rolled steel sheet which is annealed to a hot-rolled steel sheet or a cold-rolled steel sheet.

2. Metal structure

(1) Cleanliness: Less than 0.08%

The degree of cleanliness in the present embodiment is defined as the sum of arithmetic calculations of the A-based (sulfide based), B-based (alumina based), and C-based (silicate based) intercalated materials included in the steel sheet specified in JIS G0555. As the amount of interposition increases, propagation of cracks becomes easier, leading to deterioration of toughness and anisotropy of toughness. Therefore, the upper limit of cleanliness is set at 0.08%. The preferred upper limit is 0.04%. In the steel sheet according to the present embodiment, MnS, which is an A-based inclusion, is a cause of a decrease in anisotropy of main toughness. Therefore, it is particularly preferable that the A-based inclusions are 0.06% or less. More preferably, the A-based inclusions are 0.03% or less.

It is preferable that the degree of cleanliness is low, but the lower limit may be 0.003% or 0.005% from the viewpoint of cost.

(2) Mn segregation degree?: Not more than 1.6

Mn tends to be segregated near the center of the thickness of the steel sheet during casting. When this center segregation is large, inclusions such as MnS are concentrated on the segregation portion, resulting in lowering of toughness and anisotropy of toughness. Further, the martensite generated in the segregation portion at the time of quenching is hard, and toughness is deteriorated. Further, due to the interaction between Mn and P, the P segregation is also increased in the Mn segregation portion, which causes toughness deterioration. Therefore, the Mn segregation degree? Represented by the following formula 1 is set to 1.6 or less. It is preferable that the Mn stiffness? Is close to 1.0 (that is, without segregation), but from the viewpoint of cost, the lower limit may be 1.03 or 1.05.

[Formula 1]

3. Plating layer

A surface-treated steel sheet may be formed on the surface of the hot-formed steel sheet according to the present invention by forming a plating layer for the purpose of improving corrosion resistance and the like. Even if a plating layer is provided, the effect of the present embodiment is not impaired. The plating layer may be an electroplating layer or a molten plated layer. Examples of the electroplating layer include an electro-galvanized layer, an electro-Zn-Ni alloy plating layer, and the like. Examples of the hot-dip coating layer include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, a hot-dip galvanized layer, a hot-rolled Zn-Al alloy layer, a hot-rolled Zn-Al-Mg alloy layer and a hot rolled Zn-Al-Mg-Si alloy layer. The plating adhesion amount is not particularly limited and may be within a general range.

4. Manufacturing Method

Next, a typical manufacturing method of the hot-formed steel sheet according to the present invention will be described. By using the manufacturing method including the following steps, the steel sheet according to the present embodiment can be easily obtained.

(1) Continuous casting process (S1)

The molten steel having the above-mentioned chemical composition is formed into a steel strip (slab) by a continuous casting method. In this continuous casting step, the molten steel temperature is set to be 5 ° C or more higher than the liquidus temperature, the molten steel casting amount per unit time is set to 6 ton / min or less, and the center segregation reduction treatment is performed before the casting pieces completely solidify .

If the amount of casting (casting speed) per unit time of molten steel during continuous casting exceeds 6 t / min, the flow of molten steel in the mold is rapid, so inclusions are easily replenished and inclusions in the slab are increased. If the molten steel temperature is lower than 5 占 폚 from the liquidus temperature, the viscosity of the molten steel increases and the inclusions are less likely to float, thereby increasing the amount of intervening material in the steel and decreasing the cleanliness (increasing the value). When molten steel is continuously cast, it is more preferable that the temperature of the molten steel is set to 8 ° C or more higher than the liquidus temperature and the casting amount is set to 5 ton / min or less.

As the center segregation reduction treatment, for example, the concentrated portion can be relieved or discharged by performing electromagnetic stirring or non-solidified layer pressing on the uncoagulated layer before the casting piece is completely solidified.

(2) Slab homogenization treatment process (S2)

The slab homogenization treatment in which the slab is heated to 1150 캜 to 1350 캜 and held for 10 to 50 hours may be performed as the segregation reduction process after the slab is completely solidified. By performing the slab homogenization treatment under the above conditions, the degree of segregation can be further reduced. The preferable upper limit of the heating temperature is 1300 占 폚, and the upper limit of the preferable holding time is 30 hours.

(3) Hot rolling step (S3) - Cooling step (S4) - Winding step (S5)

The steel strip obtained by performing the above-described continuous casting process and the slab homogenizing process as necessary is heated to 1050 to 1350 占 폚 and then hot-rolled to obtain a steel sheet. The hot rolled steel sheet is held for 5 seconds to 20 seconds in the temperature range. After the holding, the steel sheet is cooled to a temperature range of 400 ° C to 700 ° C by water cooling. Subsequently, the cooled steel sheet is wound.

The steel strip may contain a non-metallic inclusion which causes deterioration of toughness and local deformation of the member after the steel sheet is quenched. Therefore, it is preferable to sufficiently solidify these nonmetallic inclusions when providing the steel strip to hot rolling. With respect to the steel having the above-mentioned chemical composition, the solidification of the non-metallic inclusion is promoted by setting the temperature to 1050 占 폚 or higher in the hot rolling. Therefore, it is preferable that the temperature of the steel strip provided for hot rolling is 1050 DEG C or higher. The temperature of the steel strip to be provided for hot rolling may be 1050 占 폚 or higher, and the steel strip having a temperature lower than 1050 占 폚 may be heated to 1050 占 폚 or higher.

In the case of transformation from the as-processed austenite state after finish rolling, the rolled aggregate remains, which causes anisotropy in the final product. Therefore, it is preferable that the steel sheet is subjected to the transformation from the recrystallized austenite, and the steel sheet is maintained in the temperature region for more than 5 seconds after completion of rolling. In order to maintain the production line for more than 5 seconds, for example, the cooling zone after finish rolling may be carried without water cooling.

By setting the coiling temperature at 400 占 폚 or higher, the ferrite area ratio in the metal structure can be increased. When the ferrite area ratio is high, the strength of the hot-rolled steel sheet is suppressed, and when cold rolling is performed in a subsequent step, the load control, the steel sheet flatness and plate thickness control become easy, and the manufacturing efficiency becomes high. Therefore, the coiling temperature is preferably 400 DEG C or higher.

On the other hand, when the coiling temperature is 700 占 폚 or less, the scale growth after winding is suppressed, and the occurrence of scale damage is suppressed. Further, deformation due to the own weight of the coil after winding is suppressed, and generation of scratches on the coil surface due to this deformation is suppressed. Therefore, the coiling temperature is preferably 700 DEG C or less. In addition, in the above-mentioned deformation, when untransformed austenite remains after winding, and when the untransformed austenite undergoes ferrite transformation after winding, the winding tension of the coil is lost by volume expansion due to ferrite transformation and subsequent thermal shrinkage Occurs.

(4) Acid cleaning step (S6)

The steel sheet after the winding step may be pickled. Acid cleaning may be carried out according to a conventional method. The skin pass rolling may be performed before the acid cleaning or after the acid cleaning to promote the flatness correction and the scale peeling promotion, so that the effect of the present embodiment is not affected. The elongation at the time of performing the skin pass rolling is not particularly specified, but may be set at 0.3% or more and less than 3.0%, for example.

(5) Cold rolling step (S7)

The pickled steel sheet obtained by the acid pickling step may be subjected to cold rolling if necessary. The cold rolling may be carried out according to a conventional method. The reduction ratio of the cold rolling may be within a normal range, and is generally 30% to 80%.

(6) Annealing step (S8)

The hot-rolled steel sheet obtained in the winding step (S5) or the cold-rolled steel sheet obtained in the cold rolling step (S7) can be annealed at 700 ° C to 950 ° C, if necessary.

By applying annealing to the hot-rolled steel sheet and the cold-rolled steel sheet in a temperature region of 700 占 폚 or more, it is possible to reduce the influence of hot rolling conditions and further stabilize the characteristics after quenching. With regard to the cold-rolled steel sheet, the steel sheet is softened by recrystallization, and the workability before hot-forming can be improved. Therefore, when the hot-rolled steel sheet or the cold-rolled steel sheet is annealed, it is preferable to maintain it in a temperature region of 700 ° C or more.

On the other hand, by setting the annealing temperature to 950 占 폚 or lower, the cost required for annealing can be suppressed and high productivity can be ensured. Further, since the assembly of the structure can be suppressed, better toughness can be ensured after quenching. Therefore, in the case of annealing the hot-rolled steel sheet or the cold-rolled steel sheet, it is preferable to maintain the temperature in the temperature range of 950 ° C or lower.

The cooling after annealing in the case of performing the annealing is preferably carried out at an average cooling rate of from 3 캜 / sec to 20 캜 / sec up to 550 캜. By setting the average cooling rate to 3 DEG C / sec or more, the generation of coarse pearlite and coarse cementite can be suppressed and the characteristics after quenching can be improved. Further, by setting the average cooling rate to 20 DEG C / sec or less, it is easy to stabilize the material.

(7) Plating step (S9)

When a plating layer is formed on the surface of a steel sheet to form a plated steel sheet, both electroplating and hot-dip plating may be carried out according to a conventional method. In the case of hot-dip galvanizing, a continuous hot-dip galvanizing facility may be used and the above-described annealing process and a continuous plating process may be performed in the facility, or the plating process may be performed independent of the annealing process. The hot-dip galvanizing may also be performed by alloying and hot-dip galvanizing. When the alloying treatment is carried out, it is preferable to set the alloying treatment temperature to 480 캜 to 600 캜. By controlling the alloying treatment temperature to 480 DEG C or more, it is possible to suppress unevenness of the alloying treatment. In addition, by setting the alloying treatment temperature to 600 占 폚 or less, it is possible to suppress the production cost and ensure high productivity. After hot dip galvanizing, skin pass rolling may be carried out as needed for flattening. The elongation of the skin pass rolling can be determined according to the ordinary method.

The amount of inclusions and the degree of segregation in the present steel sheet are almost determined in the process up to the hot rolling and do not substantially change before and after the hot forming. Therefore, if the chemical composition, the amount of interposition (degree of cleanliness), and the degree of segregation of the steel sheet before hot forming satisfy the range of the present embodiment, hot press members produced by hot pressing also satisfy the range of this embodiment .

Example

Steels having the chemical compositions shown in Table 1 were dissolved in a test converter and continuous casting was carried out in a continuous casting machine for testing. As shown in Table 2, in the continuous casting step, the casting speed and the molten steel heating temperature difference (molten steel temperature-liquidus temperature) were variously changed during casting. Further, in the slab solidification process, electron stirring was performed. Further, in the final solidification portion of the slab, the center segregation portion was discharged by the uncoagulated layer depression (extrusion) in which the upper and lower pairs of rolls in the continuous casting machine were narrowed. As a comparison, some slabs not subjected to electromagnetic stirring and / or extrusion (center segregation reduction treatment) were also prepared. Thereafter, the slab homogenization treatment was carried out at 1300 ° C for 20 hours. Some omit the slab homogenization treatment. The slab thus prepared was subjected to hot rolling, and then cooled and taken up to obtain a hot-rolled steel sheet having a thickness of 5.0 mm or 2.9 mm. The hot rolling conditions at this time were as follows: the heating temperature of the slab was 1250 ° C, the rolling start temperature was 1150 ° C, the rolling finish temperature was 900 ° C, and the coiling temperature was 650 ° C. The hot rolling was carried out by multi-pass rolling, and the holding was performed for 10 seconds after the end of the rolling. Cooling after hot rolling was performed by water cooling. For comparison, some were not maintained.

In addition, the casting speed differs between the actual equipment production facility and the continuous casting machine for testing used in the present embodiment in size. Therefore, in Table 2, a value converted into the casting speed in the actual equipment production facility is described in consideration of the size factor. The heating temperature difference in Table 2 is a value obtained by subtracting the liquidus temperature from the molten steel temperature.

The obtained hot-rolled steel sheet was subjected to pickling treatment according to a conventional method to obtain an acid-washed steel sheet. The acid-cleaned steel sheet having a plate thickness of 5.0 mm was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 2.9 mm. Some hot rolled steel sheets were electroplated. Some of the cold-rolled steel sheets were subjected to recrystallization annealing (annealing temperature: 800 DEG C, annealing time: 60 seconds) in the continuous annealing equipment, and electro galvanizing was performed thereafter. Further, a part of the hot-rolled steel sheet and the cold-rolled steel sheet was subjected to annealing (annealing temperature 800 캜, annealing time 60 seconds) and hot-dip galvanizing in a continuous hot-dip galvanizing facility. The temperature of the hot-dip galvanizing bath was set to 460 ° C, and a part of the hot-dip galvanizing bath was subjected to alloying treatment at 540 ° C for 20 seconds to obtain a hot-dip galvanized steel sheet and a galvannealed steel sheet.

The steel sheet thus prepared was used as a blank material and subjected to hot press forming using a hot press testing apparatus. The steel sheet punched with a blank size of 150 mm x 150 mm and a punching hole diameter of 36 mm (clearance of 10%) was heated in a heating furnace until the surface temperature of the steel sheet reached 900 DEG C and maintained at that temperature for 4 minutes And then taken out from the heating furnace. Thereafter, the resultant was cooled by cooling until the temperature reached 750 ° C, and the hot burring was carried out at the time when the temperature reached 750 ° C, and maintained at the bottom dead center for one minute. The conditions for hot burring molding are as follows.

Punch shape: cone,

Punch diameter: 60 mm,

Press speed: 40 mm / sec,

Cooling after the molding was carried out by cooling the mold by keeping it at the bottom dead center for one minute.

(A high-strain forming portion subjected to plastic deformation at 20% or more) and a flange portion (a low-strain forming portion having a plastic deformation amount of 5% or less) with respect to a section parallel to the rolling direction of the hot- The hardness at the 1/4 depth position was measured with a Vickers hardness meter. The measured load was 98 kN. The measurement method was in accordance with JIS Z2244. This hardness measurement was carried out five times in total while moving at the same plate thickness position at a pitch of 200 mu m. The average value of the five Vickers hardness values obtained for each member was obtained, and the average hardness (Hv) was obtained. (DELTA Hv = [flange portion Hv] - [burring portion Hv]) between the average hardness of the burring portion and the average hardness of the flange portion was determined. When? Hv was 40 or less, The results of the investigation of the hardness are shown in Table 3.

In addition, the deformation amount was obtained by measuring the plate thickness at each position of the processed steel sheet, and calculating the reduction amount of the plate thickness after processing with respect to the plate thickness before processing.

In addition, the produced steel sheet was used as a specimen, and the toughness value (maximum value of toughness) and anisotropy of toughness were investigated.

The survey was conducted as follows. First, the 2.9 mm steel sheet was heated in the heating furnace until the surface temperature of the steel sheet reached 900 DEG C, held at that temperature for 4 minutes, and then taken out from the heating furnace. Subsequently, the mold was cooled by cooling until the temperature reached 750 ° C, and the mold was sandwiched from the top and bottom at the time of reaching 750 ° C for 1 minute. Thereafter, the front and back surfaces from the specimen were ground to a thickness of 2.5 mm. A Charpy impact test sample was taken such that the longitudinal direction of the sample was in the rolling direction and in the direction perpendicular to the rolling direction. At this time, the notch was a V notch with a depth of 2 mm. The test temperature was set at room temperature and an impact test was carried out in accordance with JIS Z 2242. The ratio of the impact value (absorption energy / cross-sectional area) in the rolling direction and the impact value in the direction perpendicular to the rolling direction was used as an index of anisotropy.

The results are shown in Table 3. As a result of the test, when the impact value in the longitudinal rolling direction was 70 J / cm 2 or more and the impact value ratio was 0.65 or more, the property was judged to be good.

The cleanliness of the steel sheet was investigated in accordance with JIS G0555. For each steel sheet of each test number, the specimens were cut from five places, and the degree of cleanliness was measured at each position of 1/8, 1/4, 1/2, 3/4, and 7/8 of the plate thickness Respectively. Among the results at each plate thickness position, the highest degree of cleanliness was defined as the degree of cleanliness of the disclosed material. The cleanliness was the sum of A, B and C inclusions.

The Mn segregation was determined by analyzing the component surface of Mn by EPMA. For each steel sheet of each test number, the specimens were cut out from five places, and the average value of Mn segregation degree of each field was adopted at a magnification of 500 times at 1/4 and 1/2 of the plate thickness Respectively.

In Test Nos. 16 to 19, 21 and 22, the average hardness of the burring portion, which is the high strain portion, was remarkably lowered and the value of? Hv was increased to 41 to 99 as compared with the average hardness of the flange portion. This is because burring portions are softened by the modified organic ferrite transformation caused by deburring. In such a case, the produced hot-rolled products are locally different in hardness, so that the strength of the molded product is not uniform, and the low-strength part is partially obtained, so that the reliability of the product is impaired.

Since the chemical compositions, the degree of cleanliness, or the degree of segregation are out of the scope of the present invention in Test Nos. 4, 8, 10, 12, 15, 18, 20, 23 and 24, The rain was not enough.

On the other hand, the steel sheet having the chemical composition of the present invention has ΔHv of -4 to 24, regardless of the presence or absence of the cold rolling process, the presence or absence of the annealing process, and the plating type, and the difference between the average hardness of the flange portion and the average hardness of the burring portion is small , And the stability of hardness and strength at the time of high strain molding was excellent.

In addition, a sufficient value was exhibited for the anisotropy of toughness and toughness after hot rolling.

The steel sheet of the present invention has a stable hardness distribution after hot forming because deformation of the deformed organic ferrite in the forming portion is suppressed even when hot forming accompanied by high strain forming such as burring molding is performed, A steel sheet excellent in toughness and low in toughness and anisotropy can be obtained. This steel sheet is suitable as a material for a mechanical structural member such as a body structural member of an automobile, a lower body member, etc., and therefore, the present invention is industrially advantageous.

Claims (5)

Chemical composition, in% by mass,
C: 0.18% to 0.275%,
Si: 0.02% to 0.15%,
Mn: 1.85% to 2.75%
sol.Al: 0.0002% to 0.5%,
Cr: 0.05% to 1.00%
B: 0.0005% to 0.01%,
P: not more than 0.1%
S: 0.0035% or less,
N: 0.01% or less,
Ni: 0 to 0.15%
Cu: 0 to 0.05%
Ti: 0 to 0.1%
Nb: 0 to 0.2%
≪ / RTI >
The balance being Fe and impurities,
The degree of cleanliness in the metal structure is 0.08% or less,
?, Which is the degree of segregation of Mn represented by the following formula 1, is 1.6 or less,
The average hardness difference? Hv after hot forming of the low deformation forming portion subjected to the plastic deformation of 5% or less and the high deformation forming portion subjected to the plastic deformation of 20% or more in the hot forming is 40 or less,
The ratio of the impact value in the rolling direction to the impact value in the direction perpendicular to the rolling direction is 0.65 or more,
Wherein the cleanliness degree is defined as the sum of arithmetic calculations of the A-system, B-system and C-system intervening materials included in the steel sheet specified in JIS G0555.
[Formula 1]

The method according to claim 1,
Characterized in that the above chemical composition further contains one or two selected from the group consisting of 0.02 to 0.15% Ni and 0.003 to 0.05% of Cu, in mass%, instead of a part of Fe. Steel plate. 3. The method according to claim 1 or 2,
Characterized in that the above chemical composition further contains one or two selected from the group consisting of 0.005 to 0.1% of Ti and 0.005 to 0.2% of Nb, in mass%, instead of a part of the Fe. Steel plate. 3. The method according to claim 1 or 2,
The steel sheet according to claim 1, further comprising a plating layer on the surface of the steel sheet. The method of claim 3,
The steel sheet according to claim 1, further comprising a plating layer on the surface of the steel sheet.