CN104583445B - steel plate - Google Patents

Abstract

The purity of the metal structure of the steel sheet of the present invention is 0.08% or less, α which is the degree of Mn segregation is 1.6 or less, the low-strain formed part subjected to plastic strain of 5% or less during hot forming, and the high-strain section subjected to plastic strain of 20% or more. The average hardness difference ΔHv after the thermoforming of the strain formed portion is 40 or less.

Description

steel plate

technical field

The present invention relates to a steel sheet (steel sheet for hot forming) that is preferable for applications such as hot pressing that are quenched simultaneously with hot forming or immediately after hot forming. In more detail, the present invention relates to suppression of strain-induced ferrite phase in a formed part, for example, even when hot forming accompanied by high-strain forming (forming in which the formed part is subjected to a plastic strain of 20% or more) is carried out. A steel sheet for thermoforming that has uniform hardness after thermoforming, excellent toughness, and less anisotropy of toughness after thermoforming.

This application claims priority based on Japanese Patent Application No. 2012-187959 filed in Japan on August 28, 2012, the content of which is incorporated herein.

Background technique

In recent years, in the field of steel sheets for automobiles, the range of application of high-strength steel sheets with high tensile strength has expanded due to improvements in fuel consumption and crash resistance of automobiles. In general, when the strength of a steel sheet increases, the press formability decreases. Therefore, with the application of high-strength steel sheets, it becomes difficult to manufacture products of complicated shapes. Specifically, since the ductility decreases as the strength of the steel sheet increases, fracture occurs at a portion with a high degree of workability, or springback or sidewall warpage increases as the strength of the steel sheet increases. As a result, problems such as degradation of the dimensional accuracy of the processed member occur. Therefore, it is not easy to manufacture a product having a complicated shape by press forming using a high-strength steel sheet having a tensile strength of 780 MPa or higher.

High-strength steel sheets can also be processed to some extent if they are formed by roll forming instead of press forming. However, in roll forming, there is a constraint that it can only be applied to processing of members having the same cross-section in the length direction, and the degree of freedom of member shape is significantly limited.

Therefore, as a technique for press-forming difficult-to-press-form materials such as high-strength steel sheets, for example, Patent Document 1 discloses a thermoforming (for example, hot pressing) technique in which a material to be formed is heated and then formed. This technology is to quench the soft steel plate before forming at the same time as forming or immediately after forming, so as to ensure good formability during forming, and after forming on this basis, obtain a formed member with high strength by quenching Technology. According to this technology, a structure mainly composed of martensite can be obtained after quenching, and a molded member with excellent local deformability and toughness can be obtained compared with the case of using a high-strength steel plate having a structure composed of a multiphase structure.

At present, the above-mentioned hot pressing promotes application to members having a relatively simple shape, and application to members subjected to more severe molding such as fin removal molding is expected in the future. However, when applied to a member subjected to more severe forming, strain-induced ferrite transformation occurs in a high-strain formed part, and the hardness of the member after hot forming may be locally lowered.

In order to suppress this strain-induced ferrite transformation, it is sufficient to carry out hot forming in a higher temperature region. However, an increase in the thermoforming temperature leads to a reduction in productivity, an increase in manufacturing cost, a deterioration in surface properties, etc., so it is difficult to apply it to mass production technology. For example, Patent Document 1 describes that pressing is performed at 850°C or higher, but in actual hot pressing, there are cases where a steel plate heated to about 900°C in a heating furnace or the like is extracted from the heating furnace and transported. And while putting it into the press machine, the temperature may drop below 850°C. In this case, it is difficult to suppress the strain-induced ferrite transformation in forming.

From the standpoint of improving the productivity of hot pressing and improving the stability of the material inside the molded member, Patent Document 2 discloses a hot pressing machine capable of omitting the cooling process of the material by heat dissipation in the press mold and having excellent productivity. Method of manufacturing strength steel members. The method disclosed in Patent Document 2 is a very excellent invention, but it is necessary to contain a large amount of elements having an effect of improving hardenability such as Mn, Cr, Cu, and Ni in steel. Therefore, the technique of Patent Document 2 has a problem of cost increase. In addition, in the member manufactured using the technique of Patent Document 2, there is a possibility that deterioration of toughness due to the presence of various inclusions and inclusions (mainly MnS) extending in the rolling direction may occur. Anisotropy of toughness. Since the actual component performance is dominated by the characteristics of the low toughness side, when there is anisotropy in toughness, the original base material characteristics cannot be fully exhibited. For example, as described in Patent Document 3, the anisotropy of toughness can be reduced by controlling the morphology of elongated inclusions by Ca treatment. However, in this case, there is a problem that although the toughness value increases in the direction in which the toughness decreases the most, the amount of inclusions in the member itself increases, so that other directional toughness values decrease.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2002-102980

Patent Document 2: Japanese Patent Laid-Open No. 2006-213959

Patent Document 3: Japanese Patent Laid-Open No. 2009-242910

Contents of the invention

The problem to be solved by the invention

As mentioned above, hot pressing in the prior art has remained limited to components with simpler shapes. Therefore, for thermoformed components (after thermoformed The technical issues of local decrease in hardness, anisotropy of toughness, and decrease in toughness value of the steel plate in the process) have not been discussed before.

The object of the present invention is to provide a solution that suppresses the strain-induced ferrite transformation in the formed part and provides uniform hardness (small difference in hardness) ), a steel sheet for thermoforming with excellent toughness after thermoforming and small anisotropy of toughness.

means to solve the problem

The inventors of the present invention conducted intensive studies to solve the above-mentioned problems.

As a result, a new discovery was made: by controlling the chemical composition, amount of inclusions, and center segregation of the steel plate, even when hot forming accompanied by high strain forming is performed, the strain-induced ferrite transformation is suppressed, and the hardness after hot forming can be obtained. A steel sheet for hot forming that is uniform, has excellent toughness after hot forming, and has small anisotropy of toughness. In addition, uniform hardness may be referred to as stable hardness distribution in the following description.

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

(1) The steel sheet according to one aspect of the present invention relates to a steel sheet characterized in that the chemical composition contains, in 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: 0.1% or less, 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%, and the remainder is Fe and impurities; the purity of the metal structure is 0.08% or less; α is 1.6 or less; the difference ΔHv in the average hardness after thermoforming of the low-strain molded part subjected to plastic strain of 5% or less during thermoforming and the high-strain molded part subjected to plastic strain of 20% or more is 40 or less.

α = (Maximum Mn concentration in mass % at center of plate thickness of the above-mentioned steel sheet) / (Average Mn concentration in mass % at a depth of 1/4 of the plate thickness from the surface of the above-mentioned steel plate)

formula a

(2) In the steel sheet described in the above (1), the above chemical composition may further replace a part of the above Fe, and may contain Ni: 0.02% to 0.15% and Cu: 0.003% to 0.05% in mass % from the group consisting of 1 or 2 types of.

(3) In the steel sheet described in the above (1) or (2), the above-mentioned chemical composition may further replace a part of the above-mentioned Fe, and contain in mass % selected from the group consisting of Ti: 0.005% to 0.1% and Nb: 0.005% to 0.2%. 1 or 2 types in the group that constitutes.

(4) The steel sheet according to any one of (1) to (3) above may further have a plated layer on the surface of the steel sheet.

Invention effect

According to the above-mentioned aspect of the present invention, even when heat forming accompanied by high strain forming such as fin removal forming is carried out, the strain-induced ferrite transformation of the formed part can be suppressed, so that a stable steel structure after heat forming can be obtained. A steel plate having excellent hardness distribution and toughness after hot forming and low toughness anisotropy. The steel sheet is suitable as a raw material for mechanical structural members such as automobile body structural members and vehicle running members, for example, and thus the present invention is extremely useful industrially.

In addition, thermoforming may be performed according to a conventional method. For example, the raw material steel plate can be heated to a temperature of Ac ₃ point or more (about 800°C) and Ac ₃ point+200°C, hold it for 0 seconds or more and 600 seconds or less, and transfer it to a press machine for press forming, and perform it at the bottom dead center. Hold for more than 5 seconds. At this time, the heating method can be appropriately selected, and in the case of rapid heating, electric heating or high-frequency heating can be performed. In addition, for normal heating, furnace heating set to a heating temperature or the like can be used. Air cooling is performed during the transfer to the press machine, so if the transfer time becomes longer, ferrite transformation may occur before pressing starts and softening may occur. Therefore, the transfer 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 any cooling method as necessary, such as cooling piping in the mold, flowing a refrigerant, or the like.

detailed description

The steel sheet according to one embodiment of the present invention (may be referred to as the steel sheet of this embodiment) will be described in more detail below. In the following description, all the % related to the chemical composition of the steel sheet are % by mass.

1. Chemical composition

(1) C: 0.18% to 0.275%

C is an important element for improving the hardenability of steel, determining the strength after quenching, and controlling the local ductility and toughness after hot forming. In addition, C is an austenite-forming element, and therefore has the effect of suppressing strain-induced ferrite transformation during high-strain forming, and making it easy to obtain a stable hardness distribution in the hot-formed member. However, when the C content is less than 0.18%, it is difficult to ensure 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-mentioned effect cannot be obtained. On the other hand, when the C content exceeds 0.275%, local ductility and toughness decrease. Therefore, the C content is made 0.18% to 0.275%. A preferable upper limit of the C content is 0.26%, and a more preferable upper limit is 0.24%.

(2) Si: 0.02% to 0.15%

Si is an element that improves the scale adhesion after hot forming while improving the hardenability. However, when the Si content is less than 0.02%, the above effects may not be sufficiently obtained. Therefore, the lower limit of the Si content is made 0.02%. A preferable lower limit is 0.03%. On the other hand, when the Si content exceeds 0.15%, the heating temperature required to transform austenite during hot forming becomes significantly high. Therefore, the cost required for heat treatment may increase, or quenching may become insufficient due to insufficient heating. In addition, Si is a ferrite-forming element. Therefore, when the Si content is too high, strain-induced ferrite transformation is likely to occur during high-strain forming, and the hardness of the hot-formed member is locally reduced. A stable hardness distribution cannot be obtained. Furthermore, when a large amount of Si is contained, non-plating may occur due to a decrease in wettability at the time of hot-dipping treatment. Therefore, the upper limit of the Si content is 0.15%.

(3) Mn: 1.85% to 2.75%

Mn is an element effective for improving the hardenability of steel and stably securing the strength of the quenched steel. In addition, since Mn is an austenite-forming element, strain-induced ferrite transformation during high-strain forming is suppressed, and it is easy to obtain a stable hardness distribution in the hot-formed member. However, when the Mn content is less than 1.85%, the above effects may not be sufficiently obtained. Therefore, the lower limit of the Mn content is made 1.85%. On the other hand, when the Mn content exceeds 2.75%, the above-mentioned effect is saturated, and the toughness after quenching deteriorates instead. Therefore, the upper limit of the Mn content is made 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 and completes steel. When the sol.Al content is less than 0.0002%, deoxidation is insufficient. Therefore, the lower limit of the sol.Al content is made 0.0002%. Furthermore, Al is also an element effective in improving the hardenability of the steel sheet and stably ensuring the strength after quenching, so it can be positively contained. However, even if it is contained in excess of 0.5%, not only the effect is saturated, but also the cost increases. Therefore, the upper limit of the Al content is made 0.5%.

In addition, sol.Al represents acid-soluble Al, and the amount of Al contained in acid-insoluble Al ₂ O ₃ and the like is not included in the content.

(5) Cr: 0.05% to 1.00%

Cr is an element that improves the hardenability of steel. In addition, since Cr is an austenite-forming element, strain-induced ferrite transformation during high-strain forming is suppressed, and it is easy to obtain a stable hardness distribution in the hot-formed member. 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 made 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 concentrates in carbides in the steel. As a result, solid solution generation of carbides in the heating step at the time of hot forming is delayed, and hardenability is lowered. Therefore, the upper limit of the Cr content is made 1.00%. The preferable upper limit of the Cr content is 0.8%.

(6) B: 0.0005% to 0.01%

B is an element effective in improving the hardenability of steel and stably ensuring the strength after quenching. 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 made 0.0005%. On the other hand, when the B content exceeds 0.01%, the above-mentioned effect is saturated, which in turn leads to deterioration of the toughness of the quenched portion. Therefore, the upper limit of the B content is made 0.01%. The preferable upper limit of the B content is 0.005%.

(7) P: less than 0.1%

P is generally an element contained as an impurity. However, since it has the effect of improving the hardenability of steel and stably securing the strength of steel after quenching, it can be contained positively. However, when the P content exceeds 0.1%, the toughness significantly deteriorates. Therefore, the P content is limited to 0.1%. The preferable upper limit of the P content is 0.05%. The lower limit of the P content does not need to be particularly limited, but an excessive reduction of the P content leads to a significant cost increase. Therefore, the lower limit of the P content can be made 0.0002%.

(8) S: 0.0035% or less

S is an element contained as an impurity. In addition, in particular, the formation of MnS becomes a main cause of a decrease in toughness and anisotropy of toughness. When 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 does not need to be particularly limited, but an excessive reduction of the S content leads to a significant cost increase, so the lower limit of the S content can be made 0.0002%.

(9) N: 0.01% or less

N is an element contained as an impurity. When the N content exceeds 0.01%, coarse nitrides are formed in the steel, and local deformability and toughness are remarkably deteriorated. Therefore, the N content is limited to 0.01%. The lower limit of the N content does not need to be particularly limited, but an excessive reduction of the N content leads to a significant cost increase. Therefore, the lower limit of the N content may be 0.0002%. A more preferable lower limit of the N content is 0.0008% or more.

In addition to the above elements, the steel sheet of the present embodiment may contain any of the following elements. These elements do not necessarily need to be contained. Therefore, the lower limit of the content does not need to be particularly limited, and their lower limit is 0%.

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

Ni and Cu are elements effective in improving the hardenability of steel and stably ensuring the strength after quenching. Therefore, one or two of these elements may be contained. However, even if any of the elements is contained in excess of the above-mentioned upper limit, the above-mentioned effect is saturated and it becomes disadvantageous in terms of cost. Therefore, the content of each element is as 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 selected from the group consisting of Ni: 0.02% or more and Cu: 0.003% or more.

(11) Ti: 0.1% or less, Nb: 0.2% or less

Ti and Nb are elements that suppress recrystallization and further form fine carbides when the steel sheet is heated to the Ac ₃ point or higher for hot forming, suppress grain growth, and make austenite grains finer. The toughness of thermoformed components is greatly improved when the austenite grains become finer. In addition, Ti is preferentially bonded to N in the steel to form TiN, thereby suppressing the consumption of B due to the precipitation of BN. As a result, the hardenability by B can be improved by containing Ti. In order to obtain the above effects, one or two of these elements may be contained. However, when any element is contained in excess of the above upper limit, the amount of precipitation of TiC or NbC increases, C is consumed, and the strength after quenching may decrease. Therefore, the content of each element is as above. Preferably, the upper limit of the Ti content is 0.08%, and the upper limit of the Nb content is 0.15%. In addition, in order to obtain the above effect more reliably, it is preferable to contain one or two selected from the group consisting of Ti: 0.005% or more and Nb: 0.005% or more.

The balance other than the above is Fe and impurities. Impurities are substances mixed from raw materials such as ore or slag or from the manufacturing environment.

The steel sheet of the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, or may be an annealed hot-rolled steel sheet or an annealed cold-rolled steel sheet obtained by annealing the hot-rolled steel sheet or the cold-rolled steel sheet.

2. Metal structure

(1) Purity: below 0.08%

The purity in the present embodiment is defined by the sum of arithmetic calculations of the amounts of A-type, B-type, and C-type inclusions contained in the steel sheet specified in JIS G0555. When the amount of inclusions increases, crack propagation becomes easy, resulting in deterioration of toughness and increase in anisotropy of toughness. Therefore, the upper limit of the purity is made 0.08%. A preferable upper limit is 0.04%. In the steel sheet of the present embodiment, MnS, which is an A-type inclusion, is a main factor for reducing the toughness anisotropy. Therefore, it is particularly preferable that the A-type inclusions are 0.06% or less. More preferably, the A-type inclusions are 0.03% or less.

Among them, those with low purity are preferable, but the lower limit may be 0.003% or 0.005% from the viewpoint of cost.

(2) Mn segregation degree α: below 1.6

Mn tends to segregate near the thickness center of the steel sheet during casting. When the center segregation is large, inclusions such as MnS concentrate in the segregation portion, leading to a decrease in toughness and an increase in the anisotropy of toughness. Furthermore, since the martensite formed in the segregated part during quenching is hard, the toughness deteriorates. In addition, due to the interaction between Mn and P, P segregation also increases in the Mn segregation portion, which also leads to deterioration of toughness. Therefore, the Mn segregation degree α represented by the following formula 1 is set to be 1.6 or less. The Mn segregation degree α is preferably close to 1.0 (that is, no segregation), but from the viewpoint of cost, the lower limit may be 1.03 or 1.05.

α=[Maximum Mn concentration (mass %) at the center of the plate thickness]/[Average Mn concentration (mass %) at a depth of 1/4 of the plate thickness from the surface] (Formula 1)

3. Coating

For the purpose of improving corrosion resistance or the like, a plated layer may be formed on the surface of the steel sheet for hot forming according to the present invention to obtain a surface-treated steel sheet. Even if it has a plating layer, the effect of this embodiment will not be impaired. The coating can be electroplating or hot-dip coating. As an electroplated layer, an electroplated zinc layer, an electroplated Zn-Ni alloy layer, etc. are illustrated. Examples of the hot-dip coating include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, hot-dip Plating Zn-Al-Mg-Si alloy layer, etc. The plating deposition amount is not particularly limited, and may be within a general range.

4. Manufacturing method

Next, a typical production method of the steel sheet for hot forming of the present invention will be described. The steel plate of the present embodiment can be easily obtained by using a production method including the following steps.

(1) Continuous casting process (S1)

Molten steel having the above-mentioned chemical composition is made into steel sheets (slabs) by continuous casting. In this continuous casting process, it is preferable to set the molten steel temperature to a temperature higher than the liquidus temperature by 5° C. or more, and to control the casting amount of molten steel per unit time to 6 ton/min or less, and further reduce the central segregation before the slab is completely solidified. deal with.

When the amount of molten steel poured per unit time (casting speed) exceeds 6 ton/min during continuous casting, the molten steel in the mold flows quickly, so it becomes easy to replenish inclusions and increase inclusions in the slab. Also, when the molten steel temperature is less than 5°C from the liquidus temperature, the viscosity increases, inclusions become less likely to float, the amount of inclusions in the steel increases, and the purity deteriorates (value increases). When continuously casting molten steel, it is more preferable that the temperature of the molten steel be higher than the liquidus temperature by 8° C. or more, and that the casting rate be 5 ton/min or less.

As the center segregation reduction treatment, for example, the unsolidified layer before the slab is completely solidified is electromagnetically stirred or the unsolidified layer is pressed down to relax or discharge the enriched part.

(2) Slab homogenization treatment process (S2)

As the segregation reduction treatment after the slab is completely solidified, a slab homogenization treatment of heating the slab to 1150° C. to 1350° C. and holding it for 10 hours to 50 hours may be further performed. The degree of segregation can be further reduced by performing the slab homogenization treatment under the above conditions. In addition, the preferable upper limit of the heating temperature is 1300° C., and the preferable upper limit of the holding time is 30 hours.

(3) Hot rolling process (S3) ~ cooling process (S4) ~ winding process (S5)

A steel sheet obtained by performing the above-mentioned continuous casting process and, if necessary, the slab homogenization process is heated to 1050° C. to 1350° C., and then hot-rolled to obtain a steel sheet. The hot-rolled steel sheet is kept in this temperature range for 5 seconds to 20 seconds. After holding, the steel plate is cooled to a temperature range of 400°C to 700°C by water cooling. Next, the cooled steel sheet is coiled.

The steel sheet may contain non-metallic inclusions which cause deterioration of the toughness and local deformability of the member after quenching the steel sheet. Therefore, when the steel sheet is subjected to hot rolling, it is preferable to sufficiently dissolve these non-metallic inclusions. The solid solution of the above-mentioned non-metallic inclusions is promoted by making the steel sheet of the above-mentioned chemical composition reach 1050° C. or higher when it is subjected to hot rolling. Therefore, it is preferable that the temperature of the steel sheet to be subjected to hot rolling is 1050° C. or higher. In addition, the temperature of the steel sheet to be subjected to hot rolling may be 1050°C or higher, and the temperature of the steel sheet lower than 1050°C may be heated to 1050°C or higher.

When austenite is processed and transformed after finish rolling, the as-rolled structure remains, which is the main cause of anisotropy in the final product. Therefore, it is preferable to hold the steel sheet in the temperature range for 5 seconds or more after the rolling of the steel sheet is completed in order to obtain a phase transformation derived from recrystallized austenite. In order to carry out holding|maintenance for 5 seconds or more in a manufacturing line, for example, what is necessary is just to convey without performing water cooling in the cooling zone after finish rolling.

By setting the coiling temperature to 400° C. or higher, the area ratio of ferrite in the metal structure can be increased. When the area ratio of ferrite is high, the strength of the hot-rolled steel sheet is suppressed, and when cold rolling is performed in a post-process, load control or flattening/thickness control of the steel sheet becomes easy, and manufacturing efficiency improves. Therefore, the winding temperature is preferably 400° C. or higher.

On the other hand, by setting the winding temperature to 700° C. or lower, scale growth after winding is suppressed, and occurrence of scale defects is suppressed. In addition, deformation due to the weight of the coiled material after winding is suppressed, and occurrence of scratches on the surface of the coiled material due to the deformation is suppressed. Therefore, the winding temperature is preferably 700°C or lower. In addition, the above-mentioned deformation is caused by the following reason: when the untransformed austenite remains after coiling and the ferrite transformation occurs after the untransformed austenite is coiled, the ferrite transformation causes The volume expansion and subsequent thermal contraction will lose the winding tension of the coil.

(4) Pickling process (S6)

Pickling may be performed on the steel sheet after the above-mentioned coiling step. Pickling can be performed according to a conventional method. Before or after pickling, skin pass rolling may be performed for flattening or promoting scale peeling without affecting the effect of this embodiment. The elongation ratio at the time of skin pass rolling is not particularly specified, and may be, for example, 0.3% or more and less than 3.0%.

(5) Cold rolling process (S7)

Cold rolling may also be performed on the pickled steel sheet obtained by the above-mentioned pickling process as needed. For the cold rolling method, it may be carried out according to a conventional method. The rolling reduction in cold rolling may be within a normal range, and generally, it is 30% to 80%.

(6) Annealing process (S8)

The hot-rolled steel sheet obtained in the coiling step (S5) or the cold-rolled steel sheet obtained in the cold-rolling step (S7) may be annealed at 700°C to 950°C as necessary.

By subjecting the hot-rolled steel sheet and the cold-rolled steel sheet to annealing at a temperature range of 700° C. or higher, the influence of the hot-rolling conditions can be reduced, and the properties after quenching can be further stabilized. In addition, in the cold-rolled steel sheet, the steel sheet can be softened by recrystallization, and the workability before hot forming can be improved. Therefore, when annealing a hot-rolled steel sheet or a cold-rolled steel sheet, it is preferable to keep it in a temperature range of 700° C. or higher.

On the other hand, by setting the annealing temperature to 950° C. or lower, high productivity can be secured while suppressing the cost required for annealing. In addition, since coarse graining of the structure can be suppressed, better toughness can be ensured after quenching. Therefore, when annealing a hot-rolled steel sheet or a cold-rolled steel sheet, it is preferable to keep it in a temperature range of 950° C. or lower.

Cooling after annealing when annealing is performed is preferably performed at an average cooling rate of 3°C/sec to 20°C/sec to 550°C. By setting the above-mentioned average cooling rate to 3°C/sec or more, the formation of coarse pearlite or coarse cementite can be suppressed, and the properties after quenching can be improved. Moreover, by making the said average cooling rate into 20 degrees C/second or less, it becomes easy to stabilize a material.

(7) Plating process (S9)

When forming a plated layer on the surface of a steel sheet to obtain a plated steel sheet, both electroplating and hot-dip plating may be performed according to a conventional method. In the case of hot-dip galvanizing, continuous hot-dip galvanizing equipment may be used, and the above-mentioned annealing step and the subsequent plating treatment may be performed in the equipment, or the plating treatment may be performed independently of the above-mentioned annealing step. Hot-dip galvanizing can further carry out alloying treatment to perform alloyed hot-dip galvanizing. When performing alloying treatment, the alloying treatment temperature is preferably 480°C to 600°C. By setting the alloying temperature to 480° C. or higher, alloying unevenness can be suppressed. In addition, by setting the alloying treatment temperature to 600° C. or lower, high productivity can be secured while suppressing manufacturing costs. After hot-dip galvanizing, skin pass rolling may be performed as necessary for flattening. What is necessary is just to carry out the elongation rate of skin pass rolling according to a conventional method.

The amount of inclusions and the degree of segregation in the present steel sheet are basically determined in the process before hot rolling, and there is substantially no change before and after hot forming. Therefore, as long as the chemical composition, the amount of inclusions (purity) and the degree of segregation of the steel sheet before hot forming satisfy the range of this embodiment, the hot-pressed member manufactured by hot pressing afterwards also satisfies the range of this embodiment in the same way.

Example

Steels having the chemical compositions shown in Table 1 were melted in a test converter, and continuous casting was performed using a test continuous casting machine. As shown in Table 2, in the continuous casting process, various changes were made to the casting speed and the molten steel heating temperature difference (molten steel temperature - liquidus temperature) during casting. In addition, during the solidification process of the slab, electromagnetic stirring was carried out. Furthermore, in the final solidification part of the slab, discharge of the center segregation part was carried out by unsolidified lamination (extrusion) which narrowed the distance between the upper and lower pairs of rolls in the continuous casting machine. For comparison, some slabs were produced without electromagnetic stirring and/or extrusion (center segregation reduction treatment). Thereafter, a slab homogenization treatment was performed at 1300° C. for 20 hours. In some cases, slab homogenization treatment was omitted. The slab produced in this way was hot-rolled, cooled, and coiled 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 that the heating temperature of the slab was 1250°C, the rolling start temperature was 1150°C, the rolling end temperature was 900°C, and the coiling temperature was 650°C. Hot rolling was carried out by multi-pass rolling and held for 10 seconds after the end of rolling. Cooling after hot rolling was implemented by water cooling. For comparison, a part was not maintained.

In addition, the casting speed is different in the size of the equipment in the actual production equipment and the test continuous casting machine used in this example. Therefore, Table 2 describes the values converted into casting speeds of actual production facilities in consideration of size factors. In addition, the molten steel heating temperature difference in Table 2 refers to the value obtained by subtracting the liquidus temperature from the molten steel temperature.

The obtained hot-rolled steel sheet was pickled according to a conventional method to obtain a pickled steel sheet. A pickled steel sheet having a thickness of 5.0 mm was subjected to cold rolling to obtain a cold-rolled steel sheet of 2.9 mm. Plating was performed on some hot-rolled steel sheets. Recrystallization annealing (annealing temperature: 800° C., annealing time: 60 seconds) was performed on a part of the cold-rolled steel sheets in a continuous annealing facility, and electrogalvanizing was performed on a part of them thereafter. In addition, annealing (annealing temperature: 800° C., annealing time: 60 seconds) and hot-dip galvanizing were performed on a part of the hot-rolled steel sheet and the cold-rolled steel sheet in a continuous hot-dip galvanizing facility. A hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet were obtained by setting the temperature of the hot-dip galvanizing bath to 460° C. and performing an alloying treatment at 540° C. for 20 seconds on a part.

Using the manufactured steel sheet as a test material, hot press forming was performed using a hot press tester. The steel plate that has been perforated with blank size: 150 mm square, perforated hole diameter: 36 mm (gap 10%) is heated in the heating furnace to the surface temperature of the steel plate to 900 ° C, and after being kept at this temperature for 4 minutes, it is removed from the heating furnace take out. Thereafter, it was left to cool to 750° C., and when it reached 750° C., thermal definning was performed, and it was held at the bottom dead center for 1 minute. The thermal definning molding conditions are as follows.

Perforator shape: cone,

Perforator diameter: 60mm,

Pressing speed: 40mm/sec,

Cooling after molding was carried out by cooling the mold at the bottom dead center for 1 minute.

For the cross-section parallel to the rolling direction of the hot-pressed steel sheet, the finned part (a high-strain formed part subjected to a plastic strain of 20% or more) and the flange part (plastic strain amount) were measured using a Vickers hardness tester. The hardness at the depth of 1/4 of the plate thickness of the cross-section of the low-strain molded part which is 5% or less. The measured load was 98kN. The measurement method is based on JIS Z2244. This hardness measurement was implemented a total of 5 times at the same plate thickness position while moving at intervals of 200 μm. The average value of five Vickers hardness values obtained for each member was determined as the average hardness (Hv). The difference between the average hardness of the finned portion and the flange portion (ΔHv=[flange portion Hv]−[finned portion Hv]) was obtained, and when ΔHv was 40 or less, the hardness was judged to be acceptable. Table 3 shows the results of the hardness survey.

Here, the amount of strain is obtained by measuring the plate thickness at each position of the processed steel sheet, and obtaining it from the amount of decrease in the plate thickness after processing relative to the plate thickness before processing.

Moreover, the toughness value (absolute value of toughness) and the anisotropy of toughness were investigated using the produced steel plate as a test material.

The investigation was carried out in the following manner. First, the above-mentioned 2.9 mm steel plate was heated in a heating furnace until the surface temperature of the steel plate reached 900° C., kept at this temperature for 4 minutes, and then taken out from the heating furnace. Then, it was left to cool to 750° C., and when it reached 750° C., it was clamped from the top and bottom using a flat plate mold, and held for 1 minute. Thereafter, the front and back surfaces derived from the test material were ground to a thickness of 2.5 mm. The Charpy impact test samples were collected in such a way that the longitudinal direction of the sample was at right angles to the rolling direction. At this time, the notch is a V notch with a depth of 2 mm. The test temperature was room temperature, and an impact test was performed based on JISZ 2242. The ratio of the impact value (absorbed energy/cross-sectional area) in the rolling direction to the impact value in the direction perpendicular to rolling was used as an index of anisotropy.

The results are shown in Table 3. As a result of the test, if the impact value in the longitudinal rolling direction is 70 J/cm ² or more and the impact value ratio is 0.65 or more, it is judged that the characteristics are good.

The purity of the steel sheet was investigated based on JIS G0555. For the steel plate of each test number, the test material was cut out from 5 places, and at each position of 1/8, 1/4, 1/2, 3/4 and 7/8 of the plate thickness, the point counting method was used to investigate purity. Among the results of each plate thickness position, the value with the largest purity value is taken as the purity of the test material. The purity is the sum of A series, B series and C series inclusions.

The degree of Mn segregation was obtained by performing surface analysis of Mn components using EPMA. For the steel plate of each test number, the test material was cut out from 5 places, and 10 fields of view were measured at each position of 1/4 and 1/2 of the plate thickness at a magnification of 500 times, and the Mn segregation degree of each field was used. average of.

In Test Nos. 16-19, 21, and 22, the average hardness of the high-strain deformation portion without fins was significantly lower than the average hardness of the flange portion, which was a low-strain deformation portion, and the value of ΔHv was as high as 41-99. The reason for this is that the strain-induced ferrite transformation caused by the fin removal process softened the fin removal portion. At this time, the hardness of the manufactured thermoformed product varies locally, and the strength of the molded product does not become the same, and the strength of the molded product becomes partially low, thereby impairing the reliability as a product.

In addition, in test numbers 4, 8, 10, 12, 15, 18, 20, 23, and 24, the chemical composition, purity, and segregation were outside the scope of the present invention, so the impact value in the rolling direction and/or the impact value were not as good as full.

In contrast, the steel sheet having the chemical composition of the present invention has a 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 type of plating, and the average hardness of the flange portion and the average hardness of the finned portion The difference in degree is small, and the stability of hardness and strength during high-strain molding is excellent.

In addition, sufficient values were also shown for the toughness after hot rolling and the anisotropy of toughness.

Industrial practical possibility

Even when the steel sheet of the present invention is subjected to hot forming accompanied by high strain forming such as finning forming, the strain-induced ferrite transformation in the formed part is suppressed, so that a stable hardness distribution after hot forming, A steel plate with excellent toughness after hot forming and low toughness anisotropy. The steel sheet is preferable as a raw material for mechanical structural members such as car body structural members and vehicle running members, for example, and thus the present invention is extremely useful industrially.

Claims (5)

1. a steel plate, it is characterised in that

Chemical composition contains in terms of quality %

C:0.18%～0.275%,

Si:0.02%～0.15%,

Mn:1.85%～2.75%,

Sol.Al:0.0002%～0.5%,

Cr:0.05%～1.00%,

B:0.0005%～0.01%,

Below P:0.1%,

Below S:0.0035%,

Below N:0.01%,

Ni:0～0.15%,

Cu:0～0.05%,

Ti:0～0.1%,

Nb:0～0.2%,

Remainder is Fe and impurity；

The purity of metal structure is less than 0.08%；

The α as Mn degree of segregation shown in following formula 1 is less than 1.6；

In thermoforming by less than 5% plastic strain low strain dynamic forming part with by more than 20% Plastic strain Large strain forming part described thermoforming after the difference Δ Hv of average hardness be 40 with Under,

A system, B system and C system field trash contained in the steel plate that described purity JIS G0555 specifies The summation of the algorithm calculations of amount defines,

α=maximum Mn the concentration of quality % (unit of the thickness of slab central part of described steel plate be)/(distance Described surface of steel plate be the unit of 1/4 depth location of thickness of slab be the average Mn concentration of quality %) formula 1。

Steel plate the most according to claim 1, it is characterised in that described chemical composition further with In the quality % meter group containing choosing free Ni:0.02%～0.15% and Cu:0.003%～0.05% composition 1 kind or 2 kinds with replace described Fe a part.

3. according to the steel plate described in claim 1 or claim 2, it is characterised in that described chemistry Composition contains choosing free Ti:0.005%～0.1% and Nb:0.005%～0.2% further in terms of quality % In the group constituted a kind or 2 kinds is to replace a part of described Fe.

Steel plate the most according to claim 1 and 2, it is characterised in that on the surface of described steel plate On there is coating further.

Steel plate the most according to claim 3, it is characterised in that enterprising on the surface of described steel plate One step has coating.