JP2010043323A - Hot rolled steel sheet for hot press, method for producing the same, and method for producing hot pressed steel sheet member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet for hot press which is suitable as the stock for a hot pressed steel sheet member used for automobiles and various industrial machineries and having a tensile strength of ≥980 MPa, and further can contribute to the extension of the service life of a die. <P>SOLUTION: The hot rolled steel sheet for hot press includes a steel composition containing 0.09 to 0.50% C, 0.02 to 2.0% Si, 0.3 to 3.5% Mn, 0.01 to 1.0% Cr, 0.008 to 0.10% Ti, 0.0002 to 0.0050% B, 0.005 to 0.1% Al, ≤0.10% P, ≤0.05% S and ≤0.01% N, and the balance Fe with impurities. Wherein, the surface layer part is provided with a decarburized layer with the average thickness of ≥2 μm also in a ratio of ≤5% of the sheet thickness, the number density of inclusions and precipitates with the grain size of ≥1 μm present in the surface is ≤30 pieces/mm<SP>2</SP>, and further, surface roughness Ra is ≤1.5 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet for hot pressing suitable as a material for a hot-pressed steel sheet member used as an undercarriage, chassis, reinforcing part, etc. for automobiles, a manufacturing method thereof, and a hot-pressed steel sheet member manufacturing Regarding the method.

In recent years, efforts have been made to increase the strength of steel materials and reduce the thickness of steel materials to be used in order to reduce the weight of automobiles. And as a technique for press-forming a difficult-to-press molding material such as a high-strength steel plate, a hot forming (hot press) technique in which a material to be formed is preliminarily heated and formed is employed. Several techniques have been disclosed.
Japanese Patent Laid-Open No. 2002-102980 Japanese Patent Laid-Open No. 2003-73774 Japanese Patent Laid-Open No. 2003-147499 Japanese Patent Laid-Open No. 2003-126921

  Conventionally, as disclosed in Patent Documents 1 to 4, the hot-pressed steel sheet member is a cold-rolled steel sheet having a thickness of less than 1.6 mm or a hot-dip galvanized hot-dip galvanized steel sheet for thickness reduction. Plated steel sheets or galvannealed steel sheets have been used as materials.

  However, with the progress of hot press technology and the expansion of needs for applied parts, hot pressed steel plate members are also being adopted for large parts, especially as large hot pressed steel plate members for automobiles. Have been adopted steel plates having a thickness of 1.6 mm or more for bumper reinforcements, center pillar reinforcements, and the like.

  Here, generally, about a steel plate with a plate thickness of 1.6 mm or more, a hot-rolled steel plate is often used from the viewpoint of production efficiency and cost. Therefore, it is expected that the hot-rolled steel sheet is required to be applied to a hot-press steel sheet having a large thickness.

  However, the present situation is that sufficient examination is not made about application of a hot-rolled steel sheet to a hot-press steel sheet. According to the study by the present inventors, it has been clarified that there are important problems to be solved in order to apply a hot-rolled steel sheet to a hot-press steel sheet. In other words, when a hot-rolled steel sheet is used instead of a cold-rolled steel sheet in a general cold press, the hot-rolled steel sheet has a rougher surface roughness and a sharper roughness than the cold-rolled steel sheet, so the life of the press die is short. However, in the case of hot pressing, the die life is shortened compared to the case of cold pressing because of the severe condition that the die is exposed to high temperature. Furthermore, the strength of the cold-pressed steel sheet member after the cold press only rises to some extent by work hardening caused by the cold press, but the strength of the hot-pressed steel sheet member after the hot press is significantly increased by quenching. Since the strength increases, galling at the time of taking out from the mold is likely to occur compared to the cold press. This further shortens the mold life.

  The present invention has been made in view of the above situation, and is suitable as a material for a hot-pressed steel sheet member having a tensile strength of 980 MPa or more, which is used in automobiles and various industrial machines, and further prolongs the life of the mold. An object of the present invention is to provide a hot-rolled steel sheet for hot pressing that can contribute, a manufacturing method thereof, and a manufacturing method of a hot-pressed steel sheet member.

The present inventor has intensively studied to solve the above problems. As a result, the following knowledge was obtained.
As a method for suppressing the galling when the hot-pressed steel sheet member is taken out from the mold while ensuring the desired high strength, the present inventor has newly added a sheet thickness central portion of the hot-pressed steel sheet member as in the past. The idea was to soften only the surface layer while maintaining a hard structure.

  And as a method of softening only the surface layer portion of the hot-pressed steel plate member, the surface layer portion of the hot-press steel plate, which is the material of the hot-pressed steel plate member, is decarburized, so that the hot press does not cause quenching. It was found that it is effective to do so.

  Furthermore, it has also been found that the decarburized layer formed on the surface layer portion of the steel sheet for hot pressing needs to have a certain thickness so that it does not disappear due to the progress of C diffusion in the heating process when subjected to hot pressing. . And it discovered that the thickness of such a decarburized layer could be adjusted by prescribing the winding conditions after hot rolling.

  Further, the present inventor has found that the factors that shorten the mold life in the hot press are coarse and hard inclusions and precipitates existing on the surface of the steel sheet, and the mold life of such inclusions and precipitates. It has been found that the influence on the mold becomes particularly significant in a hot press in which the mold is exposed to a high temperature. The inclusions are mixed in the continuous casting process, and the precipitates are generated in the solidification process in the continuous casting process, and these inclusions are defined by defining various conditions in the continuous casting process. It has been found that it is possible to reduce deposits and precipitates. In particular, it has also been found that the inclusions can be effectively reduced by stirring the molten steel with a moving magnetic field in the mold of a continuous casting machine.

  Furthermore, the present inventor has found that the cause of shortening the die life when using a hot-rolled steel sheet instead of the cold-rolled steel sheet is that the hot-rolled steel sheet has a rougher surface roughness and sharper roughness than the cold-rolled steel sheet. At some point, the surface roughness of such hot-rolled steel sheet is determined to be due to the scale formed in the hot rolling process and the grain boundary oxidation that occurs in the subsequent cooling and winding processes. It was found that the surface roughness of the hot-rolled steel sheet can be adjusted by prescribing the cooling conditions and the winding conditions.

This invention is made | formed based on the knowledge obtained by the said examination.
In the present invention, C: 0.09% or more and 0.50% or less (hereinafter, “%” regarding the composition means “mass%” unless otherwise specified), Si: 0.02% or more and 2.0% Hereinafter, Mn: 0.3% to 3.5%, Cr: 0.01% to 1.0%, Ti: 0.008% to 0.10%, B: 0.0002% to 0.00%. 0050% or less, Al: 0.005% or more and 0.1% or less, P: 0.10% or less, S: 0.05% or less and N: 0.01% or less, with the balance being Fe and impurities The steel sheet has a decarburized layer having an average thickness of 2 μm or more and 5% or less of the plate thickness at the surface layer portion of the steel sheet, and the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is Hot press heat, characterized in that it is 30 pieces / mm ² or less and the surface roughness Ra is 1.5 μm or less. It is a rolled steel sheet.

  In the hot-rolled steel sheet for hot pressing according to the present invention, the steel composition is replaced with a part of Fe, Nb: 0.1% or less, V: 0.5% or less, W: 0.5% or less, It is preferable to contain one or more selected from the group consisting of Mo: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less.

  In these hot-rolled steel sheets for hot pressing according to the present invention, the steel composition is replaced by a part of Fe, REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less It is preferable to contain 1 type, or 2 or more types selected from the group consisting of.

In these hot-rolled steel sheets for hot pressing according to the present invention, the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is preferably 5 / mm ² or less.
From another point of view, the present invention provides a molten steel having the above steel composition with a molten steel casting amount per unit time of 2.0 to 6.0 ton / min, and 5 mm from the surface of the slab. A slab is formed by continuous casting with an average cooling rate between the liquidus temperature and the solidus temperature of the surface layer up to the depth position of 4 ° C / second or more, and the slab is rolled at a temperature range of 1000 ° C or less. Hot-rolled steel sheet subjected to hot rolling to be completed, cooled at an average cooling rate of 5 ° C./second to 100 ° C./second, and wound in a temperature range of 580 ° C. to 750 ° C. It is a manufacturing method of the hot-rolled steel plate for a press.

In the method for producing a hot-rolled steel sheet for hot pressing according to the present invention, it is preferable to stir the molten steel in the mold of the continuous casting machine with a moving magnetic field during continuous casting.
From another point of view, the present invention is to perform hot pressing after holding the hot-rolled steel sheet for hot pressing according to the present invention in a temperature range of 850 ° C. to 1000 ° C. for 2 minutes to 15 minutes. A method for producing a hot-pressed steel sheet member, characterized by cooling to a temperature range lower than 350 ° C. at a cooling rate of 10 ° C./second or more.

  According to the present invention, a hot-rolled steel sheet for hot press that is suitable as a material for a hot-pressed steel sheet member having a tensile strength of 980 MPa or more, which is used in automobiles and various industrial machines, and that can further contribute to the extension of the life of a mold, and The manufacturing method and the manufacturing method of a hot press steel plate member can be provided.

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the hot-rolled steel sheet for hot pressing according to the present invention, the manufacturing method thereof, and the manufacturing method of the hot-pressed steel sheet member will be described below.
(A) Steel composition
C: 0.09% or more and 0.50% or less Hot pressing is a feature that softens the steel sheet for hot pressing, which is the raw material, to facilitate press forming. In addition, the steel is quenched by quenching with a press die or the like to obtain a hot-pressed steel sheet member that is a higher-strength molded product. Since the strength after quenching of steel is mainly determined by the C content, the C content is set according to the strength required for the hot-pressed steel sheet member. When the C content is less than 0.09%, it is difficult to set the tensile strength of the hot-pressed steel sheet member intended by the present invention to 980 MPa or more. Therefore, the C content is set to 0.09% or more. Preferably it is 0.18% or more. On the other hand, when the C content exceeds 0.50%, the toughness of the hot-pressed steel sheet member is significantly deteriorated. Therefore, the C content is 0.50% or less. Preferably it is 0.35% or less.

Si: 0.02% or more and 2.0% or less Si is an element effective for enhancing the hardenability of the steel sheet for hot pressing and ensuring the strength of the hot pressed steel sheet member stably. If the Si content is less than 0.02%, it is difficult to sufficiently obtain the effect of the above action. Therefore, the Si content is 0.02% or more. On the other hand, when the Si content exceeds 2.0%, a large amount of Si scale is generated on the surface of the hot-rolled steel sheet for hot pressing in the heating process when subjected to hot pressing, and Si scale is formed on the hot-pressed steel sheet member. Induces sputum. Therefore, the Si content is 2.0% or less. When alloying hot-dip galvanizing is performed on the surface of a hot-rolled steel sheet for hot pressing, the Si content is preferably 1.0% or less and 0.5% or less from the viewpoint of alloying processability. More preferably.

Mn: 0.3% or more and 3.5% or less Mn is an element that is very effective in enhancing the hardenability of the hot-pressed steel sheet and stably securing the strength of the hot-pressed steel sheet member. If the Mn content is less than 0.3%, it is difficult to sufficiently obtain the effect of the above action. Therefore, the Mn content is 0.3% or more. Preferably it is 0.8% or more. On the other hand, when the Mn content exceeds 3.5%, the toughness of the hot-pressed steel sheet member is deteriorated. Therefore, the Mn content is 3.5% or less. Preferably it is 3.0% or less.

Cr: 0.01% or more and 1.0% or less Cr is an element that is very effective in enhancing the hardenability of the hot-pressed steel sheet and stably securing the strength of the hot-pressed steel sheet member. If the Cr content is less than 0.01%, it is difficult to sufficiently obtain the effect of the above action. Therefore, the Cr content is 0.01% or more. Preferably it is 0.1% or more. On the other hand, when the Cr content exceeds 1.0%, the toughness of the hot-pressed steel sheet member is deteriorated. Therefore, the Cr content is 1.0% or less. Preferably it is 0.6% or less.

Ti: 0.008% or more and 0.10% or less Since the effect of B, which will be described later, is brought about by B in a solid solution state, it is necessary to suppress the formation of BN in the steel. Since Ti has a higher nitride forming ability than B, the inclusion of Ti makes it possible to suppress the formation of BN in the steel. In order to obtain this effect, the Ti content is set to 0.008% or more. Preferably it is 0.01% or more. On the other hand, if the Ti content exceeds 0.10%, TiN becomes coarse, and if TiN is deposited on the surface layer of the steel sheet, the TiN is very hard, which adversely affects the mold life. It causes toughness deterioration of the rolled steel sheet member. Therefore, the Ti content is 0.10% or less. Preferably it is 0.035% or less.

B: 0.0002% or more and 0.0050% or less B is an element that is very effective for enhancing the hardenability of the steel sheet for hot pressing and stably securing the strength of the hot pressed steel sheet member. When the B content is less than 0.0002%, it is difficult to sufficiently obtain the effect by the above action. Therefore, the B content is 0.0002% or more. Preferably it is 0.0003% or more. On the other hand, when the B content exceeds 0.0050%, the toughness of the hot-pressed steel sheet member is deteriorated. Therefore, the B content is set to 0.0050% or less. Preferably it is 0.0035% or less.

Al: 0.005% or more and 0.1% or less Al is added as a deoxidizing material in the steel making process, and has a function of making the steel material sound. If the Al content is less than 0.005%, it is difficult to sufficiently obtain the effect of the above action. Therefore, the Al content is 0.005% or more. Preferably it is 0.01% or more. On the other hand, if the Al content exceeds 0.1%, a large amount of oxide is formed in the steel and the toughness of the hot pressed steel sheet member is deteriorated. Therefore, the Al content is 0.1% or less. Preferably it is 0.08% or less.

P: 0.10% or less, S: 0.05% or less, N: 0.01% or less P, S, and N are elements generally contained in steel as impurities. Since it is also an element effective for enhancing the hardenability and stably securing the strength of the hot-pressed steel sheet member, it may be positively incorporated. However, if the P content exceeds 0.10%, the S content exceeds 0.05% or the N content exceeds 0.01%, the toughness of the hot pressed steel sheet member is deteriorated. Therefore, the P content is 0.10% or less, the S content is 0.05% or less, and the N content is 0.01% or less. It should be noted that excessive reduction of these impurity elements is accompanied by a significant increase in cost, so that the P content is 0.005% or more, the S content is 0.0005% or more, and the N content is 0.001% or more. It is preferable that

Nb: 0.1% or less, V: 0.5% or less, W: 0.5% or less, Mo: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less One or more of these elements selected from the above are optional elements that are effective in enhancing the hardenability of the hot-pressed steel sheet and stably ensuring the strength of the hot-pressed steel sheet member. . Therefore, you may actively contain 1 type, or 2 or more types chosen from these elements. However, Nb exceeds 0.1%, V and W each exceed 0.5%, and Mo, Cu and Ni each exceed 1.0%, and even if included, the above action The effect will be saturated and will only increase costs. Therefore, the Nb content is 0.1% or less, the V and W contents are 0.5% or less, and the Mo, Cu, and Ni contents are 1.0% or less, respectively. In order to obtain the effect of the above operation more reliably, 0.005% or more for Nb, 0.01% or more for V and W, 0.02% or more for Mo, Cu and Ni, respectively. It is preferable to contain any element.

One or more selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less, REM (rare earth element), Mg and Ca are all steel. It is an element that has an effect of refining the form of inclusions therein and is effective in preventing cracking during hot pressing by inclusions. Therefore, you may actively contain 1 type, or 2 or more types chosen from these elements. However, even if REM exceeds 0.1% and Mg and Ca exceed 0.01%, the effect of the above action is saturated, and this only increases the cost. . Therefore, the REM content is 0.1% or less, and the Mg and Ca contents are 0.01% or less, respectively. In addition, in order to acquire the effect by the said action more reliably, it is preferable that content of any element shall be 0.0005% or more.

The balance other than the above is Fe and impurities.
(B) Various properties of hot-rolled steel sheet for hot pressing <Decarburized layer>
In order to suppress die wear in hot pressing, it is effective to have a decarburized layer having an average thickness of 2 μm or more and 5% or less of the plate thickness at the surface layer portion of the hot-rolled steel sheet for hot pressing. Here, the decarburized layer is a layer having a structure in which the area ratio of ferrite existing in the steel sheet surface layer part is 95% or more, and is a layer mainly composed of ferrite with almost no carbides. And can be easily distinguished.

  When the average thickness of the decarburized layer is less than 2 μm, the effect of suppressing mold wear cannot be sufficiently obtained. This is because when C is diffused in the decarburized layer in the heating step of the steel sheet for hot pressing to be subjected to hot pressing, the decarburized layer disappears in this heating step in the thin decarburized layer having an average thickness of less than 2 μm, It is because the soft layer by decarburization may not be maintained in the surface layer part of the hot-pressed steel plate member after hot pressing. Therefore, the average thickness of the decarburized layer in the surface layer portion of the hot-rolled steel sheet for hot pressing is 2 μm or more.

  On the other hand, if the average thickness of the decarburized layer exceeds 5% of the plate thickness, the ratio of the portion to be strengthened by quenching by hot pressing becomes too small, and the desired tensile strength of the hot pressed steel plate member is obtained. It may not be possible. Therefore, the average thickness of the decarburized layer in the surface layer portion of the hot-rolled steel sheet for hot pressing is 5% or less of the sheet thickness.

  Hot pressing is a process in which the hot-rolled steel sheet for hot pressing to be supplied is once heated to the austenite single-phase region. Therefore, the internal steel structure other than the decarburized layer in the hot-rolled steel sheet for hot pressing needs to be specified. Absent. That is, any phase and structure such as ferrite, pearlite, bainite, martensite, austenite, and spheroidized cementite may be used. However, from the viewpoint of ensuring workability in blanking before being subjected to hot pressing, a steel structure of ferrite and pearlite or ferrite and spheroidized cementite is preferable.

<Inclusions and precipitates>
The number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is set to 30 pieces / mm ² or less. Here, the particle sizes of the inclusions and precipitates are equivalent circle diameters converted from the areas of the inclusions and precipitates.

The inclusion is an Al-based or Ti-based oxide, and a typical one is Al ₂ O ₃ . Further, the precipitate is a Ti-based or Al-based precipitate, and a typical one is a nitride, which is TiN or AlN. However, TiN is generated and coarsened at a high temperature. Since these inclusions and precipitates are all hard, if there are many coarse inclusions having a particle size of 1 μm or more among these inclusions and precipitates on the steel sheet surface, the wear of the mold is promoted, and the mold Life is shortened. Therefore, the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is set to 30 pieces / mm ² or less. The inclusions and precipitates having a particle size of 1 μm or more are defined because inclusions and precipitates having a particle size of less than 1 μm have a small effect on mold wear.

<Surface roughness>
The surface roughness Ra of the steel sheet is 1.5 μm or less. When the surface roughness Ra of the steel plate is more than 1.5 μm, the wear of the mold is remarkably promoted and the mold life is remarkably shortened. Therefore, the surface roughness Ra of the steel sheet is 1.5 μm or less.

<Others>
The surface of the hot-rolled steel sheet for hot pressing is subjected to surface treatment such as hot dip galvanization or alloyed hot dip galvanizing for the purpose of suppressing scale formation in the hot pressing process and improving the corrosion resistance of the hot pressed steel sheet member. May be applied. This is because such a plating film is soft, and therefore the die life is determined by the properties of the hot-rolled steel sheet that is the base material for plating.

(C) Manufacturing conditions of hot-rolled steel sheet for hot pressing In order to obtain the hot-rolled steel sheet for hot pressing according to the present invention described above, it is effective to apply the following manufacturing conditions.

(Continuous casting process)
In a continuous casting process in which molten steel is continuously cast to form a slab, the molten steel casting amount per unit time is set to 2.0 ton / min to 6.0 ton / min, and further from the surface of the slab to a depth of 5 mm. The average cooling rate between the liquidus temperature and the solidus temperature of the surface layer is set to 4 ° C./second or more.

When the molten steel casting amount per unit time is less than 2.0 tons / min, the amount of heat supplied to the mold is reduced and the claw length at the top of the solidified shell is increased, so that the mold powder is easily trapped on the surface of the slab. Therefore, the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface may exceed 30 pieces / mm ² . Therefore, the molten steel casting amount per unit time is set to 2.0 ton / min or more. The mold powder is generally composed of a component system of Al ₂ O ₃ , SiO ₂ , and CaO.

On the other hand, when the molten steel casting amount per unit time exceeds 6.0 tons / min, the molten steel flow becomes too large, and the inclusion of alumina inclusions on the slab surface due to the entrainment of mold powder tends to occur, and the steel plate surface In some cases, the number density of inclusions and precipitates having a particle diameter of 1 μm or more present in the substrate exceeds 30 pieces / mm ² . Therefore, the casting amount of molten steel per unit time is set to 6.0 tons / min or less.

Further, since the liquidus temperature and the solidus temperature are in the middle of solidification, Ti, Al, N, etc. segregate in the molten steel, and TiN and AlN start to precipitate at the solidification interface and grow. Therefore, when the average cooling rate between the liquidus temperature and the solidus temperature of the surface layer part from the surface of the slab to a depth position of 5 mm is less than 4 ° C./sec, the cooling rate is low, so that segregation of molten steel proceeds. As a result, precipitation nucleation is promoted and grain growth of the precipitate after precipitation is also promoted, and the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface may exceed 30 pieces / mm ^2. is there. Accordingly, the average cooling rate between the liquidus temperature and the solidus temperature of the surface layer portion from the surface of the slab to the position of 5 mm depth is set to 4 ° C./second or more.

  From the viewpoint of suppressing the formation of precipitates on the surface of the steel sheet, it is preferable that the average cooling rate is large. Therefore, the upper limit of the average cooling rate does not need to be specified in particular, but if the cooling rate is excessive, the slab is cracked. Therefore, it is preferable to set the temperature to 100 ° C./second or less.

In order to further reduce the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface, it is effective to stir the molten steel with a moving magnetic field in the mold of a continuous casting machine. By performing stirring of the molten steel by the moving magnetic field, the length of the claw at the top of the solidified shell is shortened, and it becomes difficult for the mold powder to be captured on the slab surface layer. In addition, even once trapped, the frequency of inclusions rising again due to the molten steel flow on the upper surface of the mold increases. Therefore, stirring the molten steel with a moving magnetic field is effective in reducing the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface, and the number density should be 5 pieces / mm ² or less. Thus, the mold life can be further improved.

  Here, the stirring of the molten steel by the moving magnetic field is preferably performed so that the flow rate at a position 20 mm from the mold is 10 cm / second or more and 100 cm / second or less. This is because inclusion trapping suppression action may not be sufficient when the flow rate is less than 10 cm /, and inclusions may increase due to the entrainment of powder when the flow rate exceeds 100 cm / second. The flow velocity can be measured using, for example, a Karman vortex flow meter.

(Hot rolling process)
The slab thus obtained is subjected to hot rolling to complete rolling in a temperature range of 1000 ° C. or less to obtain a hot-rolled steel sheet, which is cooled at an average cooling rate of 5 ° C./second or more and 100 ° C./second or less to 580 ° C. Winding is performed in a temperature range of 750 ° C. or lower.

  When the hot rolling completion temperature exceeds 1000 ° C., scale flaws frequently occur on the surface of the steel sheet, and the surface roughness Ra of the steel sheet may exceed 1.5 μm. Therefore, the hot rolling completion temperature is set to 1000 ° C. or less. The lower limit of the hot rolling completion temperature is not particularly required, but it is preferably set to 820 ° C. or higher in order to induce an operation trouble due to volume fluctuation when transformation occurs in the hot rolling process.

  Further, if the average cooling rate from completion of hot rolling to winding is less than 5 ° C./second, the time for maintaining the high temperature state is too long, so that the grain boundary oxidation proceeds significantly, and the surface roughness Ra of the steel sheet. May exceed 1.5 μm. Therefore, the average cooling rate from completion of hot rolling to winding is 5 ° C./second or more.

  On the other hand, when the average cooling rate from completion of hot rolling to winding is over 100 ° C./second, the time for maintaining the high temperature state is too short, so the average thickness of the decarburized layer formed on the surface layer portion of the steel sheet is 2 μm. May be less. Therefore, the average cooling rate from completion of hot rolling to winding is 100 ° C./second or less.

  In addition, when the coiling temperature is less than 580 ° C., the time for maintaining the high temperature state is too short, and thus the average thickness of the decarburized layer formed on the surface layer portion of the steel sheet may be less than 2 μm. Therefore, the coiling temperature is set to 580 ° C. or higher.

  On the other hand, when the coiling temperature is higher than 750 ° C., the time for maintaining the high temperature state is too long, so that the grain boundary oxidation proceeds remarkably, and the surface roughness Ra of the steel sheet may exceed 1.5 μm. Therefore, the coiling temperature is 750 ° C. or lower.

In addition, if the said cooling conditions are satisfied, you may employ | adopt the cooling called primary cooling-air cooling-secondary cooling for the cooling after hot rolling.
(D) Manufacturing method of hot-pressed steel sheet member (Heating condition: Hold for 2 minutes to 15 minutes in a temperature range of 850 ° C to 1000 ° C)
When the steel sheet for hot pressing is subjected to hot pressing, heating is performed so as to be in an austenite single phase state.

  If the heating temperature at this time is less than 850 ° C. or the heating time is less than 2 minutes, ferrite, pearlite, bainite, and cementite may remain without becoming an austenite single phase, or the solid solution of C may be uneven. For this reason, the target intensity | strength cannot be ensured about a hot press steel plate member, and the hardness fluctuation | variation in a hot press steel plate member may become remarkable. Accordingly, the heating temperature is 850 ° C. or higher, and the heating time is 2 minutes or longer. On the other hand, if the heating temperature exceeds 1000 ° C. or the heating time exceeds 15 minutes, a thick scale is formed on the surface of the steel sheet, and scale biting may occur in hot pressing, resulting in surface flaws on the hot pressed steel sheet member. is there. Moreover, since the diffusion of C becomes large, the decarburized layer in the steel plate surface layer portion disappears, and the target mold wear suppression effect may not be obtained in some cases. Therefore, the heating temperature is 1000 ° C. or less, and the heating time is 15 minutes or less.

(Cooling to a temperature range of less than 350 ° C at a cooling rate of 10 ° C / second or more after hot pressing)
When the cooling rate after hot pressing is less than 10 ° C./second, ferrite starts to be generated in the middle of hot pressing or in the subsequent cooling process, so that the intended strength of the hot pressed steel sheet member may not be ensured. Further, if the cooling is stopped at a temperature of 350 ° C. or higher, phases and structures other than martensite may be generated, and the target strength of the hot pressed steel sheet member may not be ensured. Therefore, after hot pressing, it is cooled to a temperature range of less than 350 ° C. at a cooling rate of 10 ° C./second or more. The cooling rate is preferably 20 ° C./second or more, and the upper limit of the cooling rate need not be specified. Further, the cooling completion temperature is preferably 100 ° C. or lower, and more preferably room temperature.

  In this way, according to the present embodiment, heat suitable for use as a material for hot-pressed steel sheet members having a tensile strength of 980 MPa or more, which is used in automobiles and various industrial machines, and further contributes to extending the life of the mold. A hot-rolled steel sheet for hot pressing, a manufacturing method thereof, and a manufacturing method of a hot-pressed steel sheet member are provided.

Specific examples of the present invention will be described below.
Steel having the chemical components shown in Table 1 was melted in a test converter, and continuous casting was performed with a test continuous casting machine to obtain a slab. A part of the test material was subjected to in-mold electromagnetic stirring during continuous casting. Then, after heating the obtained slab on the conditions shown in Table 2 with a test hot rolling mill, hot rolling was performed. The plate thickness was 3.2 mm. Thereafter, pickling was performed in a laboratory to obtain a hot-rolled steel sheet for hot pressing.

  The obtained hot-rolled steel sheet for hot pressing is made to reach a steel sheet surface temperature of 900 ° C. in a heating furnace, held at that temperature for 4 minutes, and taken out from the heating furnace immediately to perform the hat forming method in this example. A hot press test apparatus 3 including a copper punch 1 with a cooling device and a die 2 shown in FIG. 1, which is an explanatory diagram schematically showing, is subjected to a hot press, a quenching process, A pressed steel plate member 4 was obtained.

As shown in FIG. 1, the hot-pressed steel plate member 4 was shaped like a hat (width 100 mm × length 150 mm), and the forming height was 70 mm. Fifty steel plates manufactured under each condition were prepared and hot pressed.
1) Calculation of average solidification rate between liquidus temperature and solidus temperature at 5 mm depth from slab surface A part of the obtained slab was cut out and etched with picric acid. The solidified structure of the slab was observed using an optical microscope. The dendrite secondary arm interval at a position of 5 mm from the slab surface was measured. The measurement was carried out with n = 10 for each sample material, the cooling rate was determined at each location, and the average value was taken as the average cooling rate.
2) Evaluation of steel structure About the cross section parallel to the rolling direction of a steel plate, it etched with the nital liquid and observed the steel structure of the steel plate surface layer using the optical microscope. The magnification was set to 500 times, and observation was performed with n = 10 fields of view for each production condition. The minimum decarburized layer thickness and the maximum decarburized thickness in each field of view were measured, and the average thickness of the decarburized layer was calculated from the obtained measured value (n = 20).
3) Investigation of inclusions and precipitates The surface of the steel sheet was observed with a scanning electron microscope. The magnification was 3000 times, and n = 20 fields of view were observed for each production condition. The particle size was converted into a circle equivalent particle size from the area of each inclusion and precipitate determined by image analysis, and the diameter was used as the particle size of each inclusion and precipitate. The number of particles having a particle size of 1 μm or more was counted, and the number in unit area was converted.
4) Mechanical properties of steel plates JIS No. 5 tensile test was taken from the direction perpendicular to the rolling of each steel plate. The test method conformed to JIS Z 2241. Yield point (YP), tensile strength (TS), and total elongation (El) were measured.
5) Surface Roughness of Steel Sheet Using a surface roughness meter, the steel sheet surface roughness (Ra) was measured in the rolling direction and the direction perpendicular to the rolling direction of each steel sheet. For each steel plate, Ra was measured for n = 5 in the rolling direction and n = 5 in the direction perpendicular to the rolling, and an average value was obtained by arithmetic calculation.
6) Evaluation of die wear The die wear was evaluated by hot pressing 50 hot-pressed steel plates manufactured under the same conditions, and then removing the punch 1 shown in FIG. Roughness Ry was measured. The difference between Ry before use of the mold and Ry after use of the mold that was hot-pressed 50 sheets was defined as the amount of wear.
7) Hardness of hot-pressed steel sheet member The hat-shaped hardness of the hot-pressed steel sheet member 1 shown in FIG. 1 was measured using a Vickers hardness meter. The load was 98 kN (10 kgf). The vertical wall portion of the hat-shaped portion was cut out, the cross-sectional hardness at a ¼ part thickness was measured at n = 5, and the average value was calculated by arithmetic calculation.

  The results are summarized in Table 3.

  The test materials H1 to H8 according to the present invention were good because the mold wear amount Ry was 6 μm or less and the mold wear was small. In particular, H1, H6, and H8 subjected to in-mold electromagnetic stirring have a smaller number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface, and the mold wear amount Ry is 3 μm or less. Mold wear was very small and good.

Specimen No. H9 to H17 are comparative examples that do not meet any of the conditions defined in the present invention.
The specimen H9 had a molten steel casting rate per unit time of 1.8 tons / minute, which was outside the scope of the present invention. Therefore, the amount of heat of the mold is reduced, inclusions are easily trapped in the solidified shell, the number density of inclusions and precipitates having a particle size of 1 μm or more existing on the steel sheet surface is increased to 35 / mm ^2, and the amount of die wear is increased. Ry deteriorated to 10 μm.

The specimen H10 had a molten steel casting amount per unit time of 6.2 tons / minute, which was outside the scope of the present invention. Therefore, the molten steel flow in the mold becomes excessive, and the inclusion of alumina inclusions on the slab surface layer due to the entrainment of mold powder is likely to occur, and the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is 38 pieces. / Mm ² and the mold wear amount Ry deteriorated to 12 μm.

In the test material H11, the average cooling rate between the liquidus temperature to the solidus temperature of the surface layer portion from the surface of the slab to the 5 mm depth position was 3 ° C./second, which was outside the scope of the present invention. Therefore, the segregation of molten steel proceeds to promote the formation of precipitation nuclei and the grain growth of the precipitates after precipitation, and the number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is 50 / and increased mm ^2, die wear amount Ry is deteriorated with 14 [mu] m.

  The sample material H12 had a hot rolling completion temperature of 1020 ° C., which was outside the scope of the present invention. Therefore, scale flaws frequently occurred on the surface of the steel plate, the surface roughness Ra of the steel plate became 1.8 μm, and the mold wear amount Ry deteriorated to 12 μm.

  The sample material H13 had an average cooling rate from completion of hot rolling to winding of 4 ° C./second, which was outside the scope of the present invention. Therefore, grain boundary oxidation at the surface layer of the steel sheet became significant, the surface roughness Ra of the steel sheet became 1.8 μm, and the mold wear amount Ry deteriorated to 13 μm.

  In the test material H14, the average cooling rate from completion of hot rolling to winding was 120 ° C./second, which was outside the scope of the present invention. Therefore, the average thickness of the decarburized layer formed on the surface layer portion of the steel sheet became 0 μm, and the mold wear amount Ry deteriorated to 16 μm.

  The test material H15 had a winding temperature of 560 ° C., which was outside the scope of the present invention. Therefore, the average thickness of the decarburized layer formed on the surface layer portion of the steel sheet became 1 μm, and the mold wear amount Ry deteriorated to 15 μm.

  The test material H16 had a winding temperature of 760 ° C., which was outside the scope of the present invention. Therefore, grain boundary oxidation at the surface layer of the steel sheet became prominent, the surface roughness Ra of the steel sheet became 1.7 μm, and the die wear amount deteriorated to 12 μm.

  Furthermore, the sample material H17 had a Mn content of 0.2%, which was outside the scope of the present invention. Therefore, hardenability became inadequate, and the hardness of the hot-pressed steel sheet member was as low as 200 Hv.

  The steel sheet for hot pressing according to the present invention is suitable as a material for a hot-pressed steel sheet member, and the hot-pressed steel sheet member is a structural member used for automobiles and various industrial machines, particularly automobile members and undercarriages. It is suitable as a structural member represented by parts. In addition, it is possible to extend the life of dies used for hot pressing. Therefore, there is a remarkable industrial effect.

It is explanatory drawing which shows typically the hat-molding method in an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Punch 2 Dies 3 Hot press test equipment 4 Hot press steel plate member

Claims (7)

In mass%, C: 0.09 to 0.50%, Si: 0.02 to 2.0%, Mn: 0.3 to 3.5%, Cr: 0.01 to 1.0%, Ti: 0.008 to 0.10%, B: 0.0002 to 0.0050%, Al: 0.005 to 0.1%, P: 0.10% or less, S: 0.05% or less, and N: 0 .01% or less, the balance having a steel composition consisting of Fe and impurities,
The surface layer of the steel sheet has a decarburized layer having an average thickness of 2 μm or more and 5% or less of the plate thickness,
The number density of inclusions and precipitates having a particle diameter of 1 μm or more present on the steel sheet surface is 30 pieces / mm ² or less,
A hot-rolled steel sheet for hot pressing, having a surface roughness Ra of 1.5 μm or less.   The steel composition is mass% in place of part of Fe, Nb: 0.1% or less, V: 0.5% or less, W: 0.5% or less, Mo: 1.0% or less, Cu The hot-rolled steel sheet for hot press according to claim 1, comprising one or more selected from the group consisting of: 1.0% or less and Ni: 1.0% or less.   The steel composition may be one or more selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less, instead of part of Fe. The hot-rolled steel sheet for hot pressing according to claim 1 or 2, characterized by comprising. The hot rolling for hot pressing according to any one of claims 1 to 3, wherein the number density of inclusions and precipitates having a particle diameter of 1 µm or more present on the steel sheet surface is 5 pieces / mm ² or less. steel sheet.   The molten steel having the steel composition according to any one of claims 1 to 3 has a molten steel casting amount per unit time of 2.0 to 6.0 tons / min, and further, a depth of 5 mm from the surface of the slab. The slab is formed by continuous casting at an average cooling rate between the liquidus temperature and the solidus temperature of the surface layer up to the position of 4 ° C./second or more, and the slab is rolled in a temperature range of 1000 ° C. or less. A method for producing a hot-rolled steel sheet for hot pressing, which is rolled into a hot-rolled steel sheet, cooled at an average cooling rate of 5 to 100 ° C / second, and wound in a temperature range of 580 to 750 ° C.   The method for producing a hot-rolled steel sheet for hot pressing according to claim 5, wherein the molten steel in the mold of the continuous casting machine is stirred by a moving magnetic field during the continuous casting.   The hot-rolled steel sheet for hot pressing according to any one of claims 1 to 4 is held in a temperature range of 850 to 1000 ° C for 2 to 15 minutes, and then hot-pressed, and 10 ° C / second or more. The manufacturing method of the hot press steel plate member characterized by cooling to the temperature range below 350 degreeC with the cooling rate of.

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