JP2010095753A - Hot-rolled steel plate and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel plate which has high-strength and excellent pitting-corrosion resistance, can be manufactured inexpensively, and accordingly is most suitable for being used particularly as a raw material of such a structural member of automobiles as to be represented by a chassis, a bumper and an undercarriage component. <P>SOLUTION: The hot-rolled steel plate which has a steel composition comprising, 0.01-0.35% C, 0.01-2.0% Si, 0.1-3.0% Mn, 0.3% or less P, 0.01% or less S, 0.005-2.0% Al, 0.01% or less N, 0.01-0.25% Ti and the balance Fe with impurities; and contains 50 pieces/mm<SP>2</SP>or less by number density in total of inclusions, crystallized products and precipitates having particle sizes of 5 μm or larger, in a cross section in a sheet thickness direction of a surface layer part of the steel plate between the surface of the steel plate and a position of 50 μm deep in a sheet thickness direction. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof. In particular, the present invention is suitable for use as a material for structural members used in automobiles and various industrial machines, and particularly as a material for structural members typified by automobile suspension members, chassis, and bumper reinforcement members. The present invention relates to a hot-rolled steel sheet having a tensile strength of 390 MPa or more, which is excellent in corrosiveness, and a method for producing the same.

  In recent years, automobiles have been required to guarantee for 10 years that there are no holes due to corrosion. For this reason, excellent corrosion resistance is required for steel plates used in automobiles, and there is a strong demand for improving corrosion resistance for steel plates used for suspension members exposed to particularly severe corrosive environments. For example, in cold regions such as North America, corrosion due to salt sprayed for preventing freezing of the road surface and melting snow is remarkably promoted, and therefore further improvement in corrosion resistance is strongly desired mainly for suspension members.

  Conventionally, as one method for improving the corrosion resistance, a member that uses a bare steel plate is generally switched to use a plated steel plate centered on hot dip galvanizing. From such a background, a number of plated steel sheets having excellent corrosion resistance have been developed. For example, Patent Document 1 discloses hot galvanized steel having excellent workability by performing hot dip galvanizing without cold rolling using a hot rolled steel sheet as a base plate. A method for producing a plated steel sheet is disclosed.

  However, because there are many parts that are manufactured by arc welding of individual parts in automobile undercarriage members and reinforcing members, defects such as blow holes occur during welding when using plated steel sheets as materials. There was a problem that a sound weld could not be obtained.

  On the other hand, automobiles are also desired to improve fuel efficiency from the viewpoint of global environmental protection and the like, and weight reduction is promoted by increasing the strength and thickness of the steel sheet used. The reduction in the thickness of the steel sheet accompanying this weight reduction works disadvantageously from the viewpoint of perforated corrosion resistance.

  For this reason, for example, Patent Document 2 discloses a method for producing an alloyed galvanized steel sheet that is said to exhibit excellent corrosion resistance even with thinning by improving the corrosion resistance of the plating original plate itself. However, the steel plate for automobile undercarriage manufactured by this method has a plating layer, although it is thin, it is inevitable that defects occur during arc welding. Further, it cannot be said that the proposed plating original plate has sufficient corrosion resistance to be used bare.

  By the way, with regard to increasing the strength of a steel sheet, as disclosed in, for example, Patent Document 3 and Patent Document 4, it is generally performed to add a large amount of Ti that is inexpensive but has a large strength increase with respect to the added amount. ing.

  However, when a large amount of Ti is added, coarse Ti-based crystallized substances and precipitates are generated on the steel sheet as the slab solidifies and segregates. Further, nonuniform Ti concentration occurs due to segregation of Ti. The generation of such coarse crystallized products and precipitates and the nonuniformity of the Ti concentration become the starting point of perforated corrosion and the factors that promote the progress thereof. For this reason, the perforated corrosion resistance of the Ti-added steel sheet is significantly deteriorated.

Patent Document 5 discloses a high-tensile steel plate having excellent perforated corrosion resistance, in which the Ti content is strictly controlled. However, the invention disclosed in Patent Document 5 does not take into account the above-described influence due to the addition of Ti, so it cannot be said that a high-strength steel sheet having sufficient perforated corrosion resistance is disclosed.
Japanese Patent Laid-Open No. 2-310354 JP-A-2-22416 JP-A-9-143570 Japanese Patent Laid-Open No. 10-8138 JP-A-5-195144

  The present invention has been made in view of the above-described problems of the conventional technology. For example, it is suitable as a material for an automobile undercarriage member or a reinforcing member, and has excellent perforated corrosion resistance that can withstand use under bare conditions. Another object of the present invention is to provide a hot-rolled steel sheet having a tensile strength of 390 MPa or more that is excellent in perforated corrosion resistance and a method for producing the same.

In order to solve the above-mentioned problems, the present inventors have intensively studied as follows.
From the viewpoint of cost, it is advantageous to add Ti to increase the strength of the hot-rolled steel sheet. However, when a large amount of Ti is added, coarse Ti-based crystallized substances and precipitates are generated in the steel as the slab solidifies and segregates, and Ti concentration becomes uneven due to Ti segregation. These serve as a starting point for perforated corrosion and promote the progress of perforated corrosion, so that the perforated corrosion resistance of the steel sheet is remarkably deteriorated.

  For this reason, in order to improve the perforated corrosion resistance, it is only necessary to prevent the generation of these coarse Ti-based crystallized substances and precipitates and the unevenness of the Ti concentration. However, as much Ti is added, it is difficult to completely prevent them.

  The present inventors have focused on the fact that the rate of perforated corrosion becomes the rate of occurrence of the starting point of corrosion and the progress of corrosion at the initial stage of corrosion. The idea was to improve the perforated corrosion resistance of the steel sheet by suppressing the formation of coarse Ti-based crystals and precipitates and the non-uniform Ti concentration in the steel sheet surface layer. Here, the coarse inclusions in the surface layer portion of the steel sheet have the effect of promoting perforated corrosion like the coarse Ti-based crystallized substances and precipitates.

As a result, the perforated corrosion resistance of the steel sheet is remarkably improved by suppressing the total number density of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more in the surface layer portion of the steel sheet to 50 pieces / mm ² or less. I found out.

Further, in this way, in order to make the total number density of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer part to be 50 pieces / mm ² or less, in the continuous casting process, per unit time The molten steel casting amount is 1 to 6 tons / minute, the average solidification rate in the slab surface layer is 5 ° C./second or more, and in the hot rolling process, the slab is charged into a heating furnace at a temperature of 1100 ° C. or more. It is effective to hold for 30 minutes or more, complete rolling in a temperature range of 750 ° C. or more within 10 minutes after extraction from the heating furnace, and then cool and wind at an average cooling rate of 5 ° C./second or more. I found out.

Further, the ratio (Ti _max / Ti _min ) between the maximum value (Ti _max ) and the minimum value (Ti _min ) of the mass concentration of Ti, which is an index of non-uniformity of the Ti concentration in the steel sheet surface layer, is expressed by the following formula (1 ), More preferably by satisfying the following formula (2), it has been found that the perforated corrosion resistance of the steel sheet is further improved.

1.0 ≦ (Ti _max / Ti _min ) ≦ 6.0 (1)
1.0 ≦ (Ti _max / Ti _mix ) ≦ 4.0 (2)
In order to satisfy the above formula (1), in the continuous casting process, it is effective to stir the molten steel in the mold of the continuous casting machine with a moving magnetic field, and to satisfy the above formula (2). Furthermore, it has been found that it is effective to contain Bi as a steel composition.

  Furthermore, as a result of examining from the viewpoint of suppressing the occurrence of corrosion starting point and the progress of corrosion in the early stage of corrosion, the perforated corrosion resistance is improved by reducing the surface roughness of the steel sheet and allowing the corrosion to proceed uniformly. With this in mind, it was found that the perforated corrosion resistance is further improved by setting the surface roughness Rz of the steel sheet to 15 μm or less.

The surface roughness Rz of the steel plate to 15μm or less, in a continuous casting process, the use of the continuous casting mold flux containing one or two P ₂ O ₅ and B ₂ O _3, hot It was found that the descalability in the rolling process is improved and the unevenness of the steel sheet surface due to the scale can be reduced, which is extremely effective.

The present invention has been made based on these findings.
In the present invention, C: 0.01% or more and 0.35% or less (hereinafter, “%” relating to the composition means “mass%” unless otherwise specified), Si: 0.01% or more 2 0.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.3% or less, S: 0.01% or less, Al: 0.005% or more and 2.0% or less, N: 0 .01% or less and Ti: 0.01% or more and 0.25% or less, the steel composition having the balance Fe and impurities, and the steel plate surface layer plate from the steel plate surface to the depth direction of 50 μm in the plate thickness direction A hot-rolled steel sheet having a total number density of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more in a cross section in the thickness direction of 50 pieces / mm ² or less.

  In the hot-rolled steel sheet according to the present invention, the steel composition is V: 0.5% or less, Nb: 0.1% or less, Cr: 1.0% or less, Mo: 1. It is preferable to contain one or more selected from the group consisting of 0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less.

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

In these hot-rolled steel sheets according to the present invention, the ratio (Ti _max / Ti _min ) between the maximum value (Ti _max ) and the minimum value (Ti _min ) of the mass concentration of Ti in the sheet thickness direction cross section of the steel sheet surface layer portion is as follows. It is preferable to satisfy Formula (1).

1.0 ≦ (Ti _max / Ti _min ) ≦ 6.0 (1)
In these hot-rolled steel sheets according to the present invention, the steel composition contains 0.1% or less of Bi instead of a part of Fe, and the ratio (Ti _max / Ti _min ) satisfies the following formula (2). It is preferable to do.

1.0 ≦ (Ti _max / Ti _mix ) ≦ 4.0 (2)
In these hot-rolled steel sheets according to the present invention, the surface roughness Rz is preferably 15 μm or less.

From another viewpoint, the present invention is a method for producing a hot-rolled steel sheet, comprising the following steps (A) to (C).
(A) The molten steel having the steel composition of the hot-rolled steel sheet according to the present invention described above has a molten steel casting amount per unit time of 1 to 6 tons / minute, and further, 0 in the slab thickness direction from the slab surface. .05 mm × (slab thickness (mm) / product thickness (mm)) Continuous casting process for forming a slab by a continuous casting method in which the average solidification rate in the slab surface layer at a depth position is 5 ° C./second or more;
(B) The slab is charged into a heating furnace and held at a temperature of 1100 ° C. or higher for 30 minutes or more. The slab extracted from the heating furnace is rolled in a temperature range of 750 ° C. or higher within 10 minutes from the extraction of the heating furnace. A hot rolling step in which hot rolling is performed to obtain a hot rolled steel sheet; and (C) a hot rolled steel sheet is cooled at an average cooling rate of 5 ° C./second or more after completion of hot rolling and a temperature of 700 ° C. or lower. Cooling and winding process that winds up in the area.

In the method for producing a hot-rolled steel sheet 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 in the continuous casting process.
Furthermore, in the method for producing a hot-rolled steel sheet according to the present invention, it is preferable to use a mold flux for continuous casting containing one or two of P ₂ O ₅ and B ₂ O _{3 in} the continuous casting step.

  The hot-rolled steel sheet according to the present invention has high strength and excellent perforated corrosion resistance, and can be manufactured at low cost. Therefore, it is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile chassis, bumpers and undercarriage parts.

The best mode for carrying out the hot-rolled steel sheet and the method for producing the same according to the present invention will be specifically described below.
First, the reason for limiting the steel composition of the hot rolled steel sheet according to the present invention will be described.
(A) Steel composition [C: 0.01% or more and 0.35% or less]
C is an element effective for increasing the strength. If the C content is less than 0.01%, the effect is small. Therefore, the C content is 0.01% or more. Preferably it is 0.02% or more. On the other hand, when the C content exceeds 0.35%, the weldability is significantly deteriorated. Furthermore, the workability deteriorates significantly due to an increase in the second phase such as pearlite, bainite, martensite, and retained austenite. Therefore, the C content is 0.35% or less. Preferably it is 0.25% or less.

[Si: 0.01% to 2.0%]
Si is an element effective for increasing the strength. If the Si content is less than 0.01%, the effect is small. Therefore, the Si content is 0.01% or more. Preferably it is 0.02% or more. On the other hand, if the Si content exceeds 2.0%, the chemical conversion processability is lowered, or surface defects called island-shaped scale wrinkles become remarkable. Therefore, the Si content is 2.0% or less. Preferably it is 1.5% or less.

[Mn: 0.1% to 3.0%]
Mn is an element effective for increasing the strength. If the Mn content is less than 0.1%, the effect is small. Therefore, the Mn content is 0.1% or more. Preferably it is 0.5% or more. On the other hand, if the Mn content exceeds 3.0%, the weldability is significantly deteriorated. Therefore, the Mn content is 3.0% or less. Preferably it is less than 2.5%.

[P: 0.3% or less]
P is an undesirable element that degrades toughness. Therefore, the P content is 0.3% or less. Preferably it is 0.1% or less, More preferably, it is 0.05% or less.

[S: 0.01% or less]
S is an undesirable element that reduces toughness. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.004% or less.

[Al: 0.005% to 2.0%]
Al is an element effective for promoting the formation of ferrite and improving workability. If the Al content is less than 0.005%, the effect is small. 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 2.0%, the weldability deteriorates remarkably. Therefore, the Al content is 2.0% or less. Preferably it is 1.0% or less.

[N: 0.01% or less]
N combines with Ti to form a nitride. If the N content exceeds 0.01%, coarse TiN precipitates and the toughness deteriorates significantly. Therefore, the N content is 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less.

[Ti: 0.01% or more and 0.25% or less]
Ti is an important element in the present invention. Although it is a relatively inexpensive element, the steel sheet can be effectively strengthened by precipitation strengthening. The effect is small when the Ti content is less than 0.01%. Therefore, the Ti content is 0.01% or more. Preferably it is 0.02% or more. On the other hand, if the Ti content exceeds 0.25%, a large amount of coarse Ti-based crystallized substances and Ti-based carbonitrides are generated, and the perforated corrosion resistance and toughness are significantly deteriorated. Therefore, the Ti content is 0.25% or less. Preferably it is 0.20% or less.

Next, arbitrary elements will be described.
[V: 0.5% or less, Nb: 0.1% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B : One or more selected from the group consisting of 0.01% or less]
V, Nb, Cr, Mo, Cu, Ni and B are all optional elements that contribute to high strength by improving hardenability. Therefore, the intensity | strength of a steel plate can be raised further by containing these arbitrary elements. These optional elements do not lose their functions even if they contain two or more elements in combination. However, 0.5% for V, 0.1% for Nb, 1.0% for Cr, Mo, Cu and Ni, and 0.01% for B may be included. The above effect is saturated and the cost is unnecessarily high. Therefore, when these elements are contained, V: 0.5% or less, Nb: 0.1% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% Hereafter, it is good to contain 1 type or 2 types or more as Ni: 1.0% or less and B: 0.01% or less.

  In this case, the above effect can be obtained more reliably by containing 0.001% or more of V, Nb, Cr, Mo, Cu and Ni, and 0.0001% or more of B. Therefore, it is preferable to set these as the lower limit.

[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, Mg, and Ca are all elements that have the effect of spheroidizing inclusions such as sulfides and oxides and suppressing the reduction in ductility due to these inclusions. Therefore, the inclusion of these elements can further improve the ductility of the steel sheet, and even when two or more elements are contained, the respective actions are not lost.

  However, even if the content of REM exceeds 0.1% and that of Mg and Ca exceeds 0.01%, the above effects are saturated, and the cost is unnecessarily high. Therefore, when these elements are contained, REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less are preferably contained. In this case, the above effect can be obtained more reliably by containing 0.0001% or more of REM and 0.0001% or more of Mg and Ca. It is preferable.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.

[Bi: 0.1% or less]
Bi becomes a solidification nucleus in the solidification process of the molten steel, and has an effect of reducing the dendrite arm interval. As a result, there is an effect of suppressing the concentration of components between dendrite trees, in other words, segregation. In the case of containing an element that easily segregates, such as Ti, it is particularly effective in suppressing segregation. Therefore, Bi is an important component when suppressing the segregation of Ti in the present invention. However, if the Bi content exceeds 0.1%, inclusions may be formed to deteriorate workability. Therefore, when Bi is contained, its content is 0.1% or less. In order to obtain the above effect more reliably, the Bi content is preferably 0.0001% or more.

Compositions other than the above are Fe and impurities.
(B) Inclusion, crystallized substance and precipitate In the Ti-added hot-rolled steel sheet, in order to ensure good perforated corrosion resistance, the steel sheet surface layer plate from the steel sheet surface to the depth direction of 50 μm depth The total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the cross section in the thickness direction is 50 pieces / mm ² or less.

  Here, the inclusions, crystallizations and precipitates in the steel sheet surface layer part from the steel sheet surface to the 50 μm depth position in the plate thickness direction are defined in order to suppress the perforated corrosion in the generation of corrosion origin and in the initial stage of corrosion. This is because it is important to suppress the progress of corrosion, and inclusions, crystallized substances, and precipitates in the surface layer portion of the steel sheet have a great influence on the occurrence of the corrosion starting point of perforated corrosion and the corrosion progress in the initial stage of corrosion.

That is, if there are coarse inclusions, crystallized substances, and precipitates, corrosion is likely to occur and proceed at the interface between them and Fe, and therefore, inclusions having a grain size of 5 μm or more in the cross section in the thickness direction of the steel sheet surface layer portion. When the total number density of the product, the crystallized product and the precipitate exceeds 50 / mm ² , not only the corrosion starting point of the perforated corrosion increases, but also the corrosion progress in the initial stage of corrosion is promoted.

Therefore, the total number density of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more in the cross section in the plate thickness direction of the steel sheet surface layer portion is set to 50 pieces / mm ² or less.
Since the perforation corrosion resistance is improved as the number density is lower, it is not necessary to define the lower limit of the number density from the viewpoint of perforation corrosion resistance. However, since the reduction of inclusions, crystallized substances, and precipitates in the surface layer of the steel sheet is accompanied by a decrease in operation efficiency and an increase in cost, the number density is set to 0.01 pieces / mm ² or more from this viewpoint. It is preferable.

The inclusions are mainly alumina inclusions, CaO inclusions, and SiO ₂ inclusions captured by the slab in the continuous casting process. The crystallized substances and precipitates are TiN or Ti carbonitriding. For example, Ti (N, C) is a material.
(C) Ti concentration distribution The ratio (Ti _max / Ti _min ) of the maximum value (Ti _max ) and the minimum value (Ti _min ) of the mass concentration of Ti in the cross section in the plate thickness direction of the steel sheet surface layer is expressed by the following formula (1) Is preferably satisfied.

1.0 ≦ (Ti _max / Ti _min ) ≦ 6.0 (1)
If the Ti concentration distribution is not uniform, the corrosion of the steel sheet does not proceed uniformly, and the corrosion proceeds selectively in the portion where the Ti concentration is low. For this reason, if the Ti concentration distribution in the steel plate surface layer portion is not uniform, the progress of corrosion in the initial stage of perforated corrosion is promoted, and the perforated corrosion resistance is deteriorated.

By setting the ratio (Ti _max / Ti _min ) to 6.0 or less, it is possible to suppress the deterioration of the perforated corrosion resistance due to the nonuniformity of the Ti concentration distribution. Therefore, it is preferable that the above formula (1) is satisfied with respect to the ratio (Ti _max / Ti _min ). More preferably, the ratio (Ti _max / Ti _min ) satisfies the following formula (2).

1.0 ≦ (Ti _max / Ti _min ) ≦ 4.0 (2)
(D) Steel plate surface roughness It is preferable that the surface roughness Rz of a steel plate shall be 15 micrometers or less. When the surface roughness of the steel sheet is rough, particularly when Rz exceeds 15 μm, the progress of corrosion in the initial stage of perforation corrosion becomes significant. This is because when the unevenness on the surface of the steel sheet becomes large, the corrosive liquid tends to accumulate in the concave portion, and the progress of perforated corrosion resistance is promoted. Since the perforated corrosion resistance is improved as the surface roughness of the steel sheet is smaller, it is not necessary to define the lower limit of the surface roughness Rz of the steel sheet from the viewpoint of perforated corrosion resistance. However, when the surface roughness Rz of the steel sheet is less than 4 μm, the influence of the surface roughness of the steel sheet on the perforated corrosion resistance is almost eliminated, so that the surface roughness Rz of the steel sheet should be 4 μm or more in terms of operational efficiency and cost. It is preferable from the viewpoint.
(E) Metal structure Since this invention aims at improving the perforated corrosion resistance accompanying the addition of Ti, the metal structure is not particularly defined. What is necessary is just to select a suitable metal structure suitably according to a desired characteristic.

  For example, when emphasizing stretch workability, it is preferable to use a structure mainly composed of ferrite, and pearlite, bainite, austenite, martensite, etc. may be appropriately selected as the second phase in order to obtain a desired strength.

Further, when emphasizing hole-expanding workability, stretch-flange workability, and bending workability, it is preferable to select a single phase structure of ferrite or bainite or a structure having a small hardness difference from ferrite, such as bainite.
(F) Manufacturing conditions To make the total number density of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more in the cross section in the plate thickness direction of the steel sheet surface layer part 50 pieces / mm ² or less,
In the continuous casting step, the molten steel having the above steel composition has a molten steel casting amount per unit time of 1 to 6 tons / minute, and further, 0.05 mm × (slab thickness (in the slab thickness ( mm) / product thickness (mm)) The slab is formed by a continuous casting method in which the average solidification rate in the slab surface layer at the depth position is 5 ° C./second or more,
Next, in the hot rolling process, the obtained slab is charged into a heating furnace and held at a temperature of 1100 ° C. or higher for 30 minutes or more, and the slab extracted from the heating furnace is 750 within 10 minutes from the heating furnace extraction. Hot-rolled steel sheet is obtained by hot rolling to complete rolling in a temperature range of ℃ or higher,
Furthermore, in the cooling and winding process, it is effective to cool the obtained hot-rolled steel sheet at an average cooling rate of 5 ° C./second or more after completion of hot rolling and wind it in a temperature range of 700 ° C. or less. .

  If the molten steel casting amount per unit time in the continuous casting process exceeds 6 tons / minute, the flow of molten steel becomes excessive, so that inclusions are easily trapped near the slab surface layer, and large inclusions in the steel sheet surface layer portion increase. End up. On the other hand, when the molten steel casting amount per unit time is less than 1 ton / min, the temperature drop in the continuous casting process becomes large, so that the claw at the tip of the solidified shell extends or the inclusions are difficult to float, and the intervening A thing becomes easy to be caught near the slab surface layer, and a large-sized inclusion in a steel plate surface layer part will increase. Therefore, the molten steel casting amount per unit time in the continuous casting process is preferably 1 ton / min to 6 ton / min.

  In addition, when the average solidification rate in the slab surface layer portion at a depth of 0.05 mm × (slab thickness (mm) / product thickness (mm)) in the slab thickness direction from the slab surface is less than 5 ° C./second, the liquidus temperature to TiN that crystallizes between the solidus temperatures becomes coarse, and large crystallized substances in the surface layer of the steel sheet increase. Therefore, the average solidification rate is preferably 5 ° C./second or more. The larger the average solidification rate is, the more the generation of large crystallization products in the steel sheet surface layer portion can be suppressed, so there is no need to define the upper limit of the average solidification rate. However, if the average solidification rate is excessively high, the possibility of inducing surface cracking of the slab increases. From this viewpoint, the average solidification rate is preferably 100 ° C./second or less.

  Here, the “average solidification rate in the slab surface layer portion” means a pitch of 1 mm in the slab thickness direction from the slab surface to 0.05 mm × (slab thickness (mm) / product thickness (mm)) depth position in the slab thickness direction. It is the average of the coagulation rate measured in (1). The “solidification rate” is a cooling rate between the liquidus temperature and the solidus temperature when the molten steel solidifies, and can be obtained from the dendrite secondary arm interval at each measurement position.

  In the hot rolling process, when the temperature at which the slab is charged and held in the heating furnace is less than 1100 ° C. or when the holding time is less than 30 minutes, it precipitates in the slab stage after solidification after continuous casting. The re-dissolution of coarse Ti (N, C) does not proceed sufficiently, and large precipitates in the surface layer portion of the steel sheet increase. Therefore, in the hot rolling process, it is preferable to insert the slab into a heating furnace and hold it at a temperature of 1100 ° C. or higher for 30 minutes or more.

  As the holding temperature is higher and the holding time is longer, the re-solution of large precipitates in the steel sheet surface layer portion can be promoted, so there is no need to define the upper limit of the holding temperature or the holding time. However, if the holding temperature is too high, the operating cost increases, and if the holding time is too long, the operating efficiency is lowered. Therefore, from such a viewpoint, it is preferable that the holding temperature is 1320 ° C. or less and the holding time is 10 hours or less.

  In the hot rolling process, the time from extraction of the heating furnace to the completion of hot rolling is over 10 minutes, the hot rolling completion temperature is less than 750 ° C., or the hot rolling in the cooling and winding process When the average cooling rate from completion to winding is less than 5 ° C./second or the winding temperature is over 700 ° C., Ti (N, C) in which the slab is charged into the heating furnace and re-dissolved. ) Reprecipitates and becomes coarse, and coarse precipitates in the surface layer portion of the steel sheet increase.

  Therefore, the slab extracted from the heating furnace is subjected to hot rolling that completes rolling in a temperature range of 750 ° C. or more within 10 minutes after extraction from the heating furnace to obtain a hot-rolled steel sheet. It is preferable to cool at an average cooling rate of 5 ° C./second or more after completion of the hot rolling and wind up in a temperature range of 700 ° C. or less.

  Here, since the reprecipitation of Ti (N, C) can be suppressed as the time from the heating furnace extraction to the completion of hot rolling is shorter, it is not necessary to define the lower limit of the above time. However, in order to shorten the time significantly, it is necessary to increase the rolling force of the rolling mill, resulting in an increase in equipment cost. Therefore, from such a viewpoint, the time is preferably set to 30 seconds or more.

  Moreover, since the reprecipitation of Ti (N, C) can be suppressed as the hot rolling completion temperature is higher, it is not necessary to define the upper limit of the hot rolling completion temperature. However, increasing the hot rolling completion temperature increases the possibility of inducing scale flaws. Therefore, it is preferable that the hot rolling completion temperature is 1050 ° C. or lower from such a viewpoint.

  Moreover, since the reprecipitation of Ti (N, C) can be suppressed as the average cooling rate from the completion of hot rolling to the winding is larger, it is not necessary to define the upper limit of the average cooling rate. In the manufacturing process, it is usually 2000 ° C./second or less.

  The winding temperature may be appropriately selected according to a suitable second phase structure that matches the desired characteristics. For example, in order to obtain a pearlite structure as the second phase, 580 ° C. or more and 750 ° C. or less, in order to obtain a bainite structure, 350 ° C. or more and less than 580 ° C., and in order to obtain a martensite structure, room temperature or more and less than 350 ° C. may be used.

  In addition, the cooling from the completion of hot rolling to the winding may be performed as long as it is cooled to a temperature range of 700 ° C. or less at an average cooling rate of 5 ° C./second or more. Then, it may be cooled.

The ratio (Ti _max / Ti _min ) between the maximum value (Ti _max ) and the minimum value (Ti _min ) of the mass concentration of Ti in the cross section in the plate thickness direction of the steel sheet surface layer portion satisfies the following formula (1). In the continuous casting process, it is effective to stir the molten steel in the mold of the continuous casting machine with a moving magnetic field.

1.0 ≦ (Ti _max / Ti _min ) ≦ 6.0 (1)
Furthermore, in order for the ratio (Ti _max / Ti _min ) to satisfy the following formula (2), in addition to the stirring by the moving magnetic field, 0.1% or less of Bi is further included as a steel composition. It is valid.

1.0 ≦ (Ti _max / Ti _mix ) ≦ 4.0 (2)
In the continuous casting process, the component segregation of Ti in the surface portion of the slab is suppressed by stirring the molten steel in the mold of the continuous casting machine with a moving magnetic field. Thereby, the said ratio ( _Timax / _Timin ) can be reduced.

  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 5 cm / sec or more and 100 cm / sec or less. This is because if the flow rate is less than 5 cm /, the Ti component segregation suppressing action may not be sufficient, and if it exceeds 100 cm / second, inclusions may increase due to the entrainment of powder. More preferably, the flow rate is 10 cm / second or more and 60 cm / second or less. The flow velocity can be measured using, for example, a Karman vortex flow meter.

In addition to the stirring by the moving magnetic field, when Ti is contained in an amount of 0.1% or less as a steel composition, Ti component segregation in the slab surface layer portion is further suppressed. Thereby, the ratio (Ti _max / Ti _min ) can be further reduced.

In order to set the surface roughness Rz to 15 μm or less, it is effective to use a mold flux for continuous casting containing one or two of P ₂ O ₅ and B ₂ O _{3 in} the continuous casting process.

When one or two of P ₂ O ₅ and B ₂ O ₃ are contained in the mold flux, these oxides are concentrated in the firelite layer in the scale formed on the surface of the steel material at a high temperature range, Decrease melting point. Thereby, the descaling property in a hot rolling process improves, and the unevenness | corrugation of the steel plate surface resulting from a scale can be reduced.

It is preferable that the P ₂ O ₅ concentration in the mold flux is 0.5% or more and 4.0% or less, and the B ₂ O ₃ concentration is 1.0% or more and 8.0% or less. If these contents are less than the respective specified ranges, the effect of lowering the melting point of the firelite may not be sufficiently obtained, and if these contents are more than the respective specified ranges, the mold This is because the influence on the physical properties of the flux becomes remarkable, and it becomes difficult to adjust the physical properties of the mold flux.

From the viewpoint of preventing slab cracking and lubrication between the mold and the slab solidified shell, the CaO / SiO ₂ concentration ratio of the mold flux is 0.6 to 2.0 and the solidification temperature is 1050 ° C. to 1280 ° C. Is preferred.

  Thus, according to the present embodiment, a hot-rolled steel sheet having a high strength of 390 MPa or more and excellent perforated corrosion resistance can be produced at low cost, so that a material for a structural member used in automobiles and various industrial machines, In particular, a hot-rolled steel sheet suitable as a material for structural members typified by automobile chassis, bumpers and undercarriage parts is provided.

Next, the present invention will be described more specifically with reference to examples.
Molten steel having the steel composition shown in Table 1 was melted using a test converter, and a slab was formed using a test continuous casting machine.

  The solidification rate of the slab surface layer in the continuous casting process was changed by adjusting the amount of cooling water in the mold. The average cooling rate at the slab surface layer was determined by measuring the dendrite secondary arm spacing at a pitch of 1 mm from the slab surface in the slab thickness direction, calculating the solidification rate at each position, and averaging these.

As the mold flux for continuous casting, one having a solidification temperature of 1200 ° C. and a CaO / SiO ₂ concentration ratio of 1.2 was used. _Further, P ₂ O 5: 2.0% and / or _B 2 _{O 3:} as a 4.0% was _contained, was used without addition of P 2 _{O 5} and _B 2 _{O 3.}

  For some of the test materials, the molten steel in the mold of the continuous casting machine was stirred by a moving magnetic field. The molten steel was stirred by the moving magnetic field under the condition that the flow velocity at a position 20 mm from the mold was 25 cm / sec.

The slab thus obtained was hot-rolled using a test hot finish rolling mill to produce a hot-rolled steel sheet having a thickness of 2.6 mm.
Subsequently, about the obtained hot-rolled steel sheet, scale removal was performed with the pickling equipment for a test.
These production conditions are shown in Table 2.

  A JIS No. 5 tensile test piece was taken from the hot-rolled steel sheet thus obtained in the direction perpendicular to the rolling direction and subjected to a tensile test.

Further, the cross section in the plate thickness direction parallel to the rolling direction was observed, and the number of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more present in the surface layer portion of the steel sheet was counted.
The observation was performed using a scanning electron microscope with 100 fields of view at a magnification of 2000 times. Then, the total number of them was arithmetically calculated to obtain the number density (pieces / mm ² ).

  Here, the particle sizes of inclusions, crystallized substances, and precipitates are obtained by image analysis of the inclusions, crystallized substances, and precipitates, and these areas are replaced with circles, and the diameters of the circles are calculated. The value obtained by calculating was defined as the particle size.

Furthermore, the maximum value (Ti _max ) and the minimum value (Ti _min ) of the Ti mass concentration in the cross section in the plate thickness direction of the steel sheet surface layer portion were measured by GDS (glow discharge spectroscopic analysis). Measurement was performed at five different positions for each sample, and the ratio (Ti _max / Ti _min ) was calculated from the maximum value (Ti _max ) and the minimum value (Ti _min ) of the obtained Ti mass concentration.

Furthermore, Rz was measured for the roughness of the steel sheet surface in accordance with JIS B 0601.
Furthermore, each obtained hot-rolled steel sheet was subjected to an acid corrosion test.
In the corrosion test, a 70 × 150 mm test piece was cut out, the end face and the back face were sealed, and a composite corrosion test (CCT) was performed. The test conditions were as follows: a salt spray test at 35 ° C. according to JIS Z 2371 for 3 hours, a drying test at room temperature for 2 hours, and a wet test at 95% humidity at 50 ° C. for 6 hours for a total of 12 hours as 60 cycles. Cycled.

After removing the corrosion products, the maximum erosion depth (mm) and the corrosion weight loss (g) were measured to evaluate the perforated corrosion resistance.
Table 3 shows the measurement results.

Specimen No. which is steel of the present invention. Nos. 1 to 15 have a maximum number of erosion depth after CCT of 0.10 because the total number density of inclusions, crystallized substances and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer is 50 pieces / mm ² or less. The corrosion weight loss (g) after CCT was 2.0 to 7.2 g and the perforated corrosion resistance was good.

Among them, the test material No. 1 in which the molten steel was stirred by the moving magnetic field in the mold. 1, 3, 5, 7, 9, and 12 to 15 have a ratio (Ti _max / Ti _min ) in the steel sheet surface layer portion of 2.0 to 5.4, and since segregation of Ti was suppressed, maximum erosion after CCT The depth was 0.10 to 0.27 mm, the corrosion weight loss (g) was 2.0 to 4.8 g, and the perforated corrosion resistance was even better.

Further, among them, specimen No. No. 1 containing Bi and stirring the molten steel by the moving magnetic field in the mold was used. In Nos. 12 to 15, the ratio (Ti _max / Ti _min ) in the steel sheet surface layer portion was 2.0 to 3.2, and the segregation of Ti was further suppressed, so that the maximum erosion depth after CCT was 0.10 to 0. The perforated corrosion resistance was remarkably good at 17 mm and a weight loss (g) of 2.0 to 2.9 g.

Further, among them, No. 1 was cast using a continuous casting mold flux containing one or two of P ₂ O ₅ and B ₂ O ₃ . In Nos. 13 and 15, the steel plate surface roughness Rz was as small as 7 μm, the maximum erosion depth after CCT was 0.10 mm, and the corrosion weight loss (g) was 2.0 g.

On the other hand, the test material No. 16 to 31 are comparative examples.
Specimen No. In Nos. 16 and 23, the molten steel casting amount per unit time exceeded 6 tons / min during continuous casting. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.50 mm and 0.54 mm, and the corrosion weight loss (g) was 10.0 g and 11.7 g, which was poor in perforated corrosion resistance.

Specimen No. In Nos. 17 and 24, the average solidification rate in the slab surface layer portion was less than 5 ° C./second during continuous casting. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.52 mm and 0.53 mm, and the corrosion weight loss (g) was 11.2 g and 10.8 g, which was poor in perforated corrosion resistance.

Specimen No. 18 and 25 had a slab heating temperature in the heating furnace of less than 1100 ° C. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.55 mm and 0.51 mm, and the corrosion weight loss (g) was 13.1 g and 12.5 g, which was poor in perforated corrosion resistance.

Specimen No. 19 and 26 had a slab heating time in the heating furnace of less than 30 minutes. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.56 mm and 0.54 mm, and the corrosion weight loss (g) was 15.6 g and 12.1 g, which was poor in perforated corrosion resistance.

Specimen No. In Nos. 20 and 27, the time from extraction in the heating furnace to completion of hot rolling exceeded 10 minutes. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.52 mm and 0.54 mm, and the corrosion weight loss (g) was 11.4 g and 12.3 g, which was poor in perforated corrosion resistance.

Specimen No. Nos. 21 and 28 had a hot rolling completion temperature of less than 750 ° C. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.54 mm and 0.60 mm, and the corrosion weight loss (g) was 12.0 g and 14.1 g, which was poor in perforated corrosion resistance.

Specimen No. In Nos. 22 and 29, the average cooling rate from completion of hot rolling to winding was less than 5 ° C./second. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.52 mm and 0.58 mm, and the corrosion weight loss (g) was 10.6 g and 12.5 g.

Specimen No. 23 and 30 had a coiling temperature of more than 700 ° C. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.54 mm and 0.54 mm, and the corrosion weight loss (g) was 11.7 g and 11.9 g, which were poor in perforated corrosion resistance.

Furthermore, the test material No. No. 31 had a Ti content of 0.3%, which was outside the scope of the present invention. As a result, the total number density of inclusions, crystallized substances, and precipitates having a particle diameter of 5 μm or more in the steel sheet surface layer portion exceeded 50 pieces / mm ² . For this reason, the maximum erosion depth after CCT was 0.55 mm, and the corrosion weight loss (g) was 12.0 g, which was poor in perforated corrosion resistance.

  The steel sheet of the present invention has high strength and excellent perforated corrosion resistance. Therefore, it is most suitable as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile chassis, bumpers and undercarriage parts. Moreover, since it can be manufactured at a low cost, it has a remarkable industrial effect.

Claims (9)

In mass%, C: 0.01% to 0.35%, Si: 0.01% to 2.0%, Mn: 0.1% to 3.0%, P: 0.3% or less , S: 0.01% or less, Al: 0.005% or more and 2.0% or less, N: 0.01% or less, and Ti: 0.01% or more and 0.25% or less, and the balance Fe and impurities The total number density of inclusions, crystallized substances, and precipitates having a particle size of 5 μm or more in the cross section in the thickness direction of the steel sheet surface layer portion from the steel sheet surface to the depth position of 50 μm in the thickness direction is 50 A hot-rolled steel sheet having a number of pieces / mm ² or less.   The steel composition is mass% in place of a part of Fe, V: 0.5% or less, Nb: 0.1% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu The heat according to claim 1, comprising one or more selected from the group consisting of: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less. Rolled steel sheet.   The steel composition may be one selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass%, instead of a part of Fe. The hot-rolled steel sheet according to claim 1 or 2, comprising two or more kinds. The ratio (Ti _max / Ti _min ) between the maximum value (Ti _max ) and the minimum value (Ti _min ) of the mass concentration of Ti in the cross section in the plate thickness direction of the steel sheet surface layer portion satisfies the following formula (1): The hot-rolled steel sheet according to any one of claims 1 to 3.
1.0 ≦ (Ti _max / Ti _min ) ≦ 6.0 (1) The steel composition contains, in place of a part of Fe, mass% and Bi of 0.1% or less, and the maximum value (Ti _max ) of the mass concentration of Ti in the cross section in the thickness direction of the steel sheet surface layer portion. the minimum value _{(Ti min)} and the ratio _(Ti max _{/ Ti min)} is hot rolled steel sheet according to any one of claims 1, characterized by satisfying the following formula (2) to claim 3.
1.0 ≦ (Ti _max / Ti _mix ) ≦ 4.0 (2)   The hot rolled steel sheet according to any one of claims 1 to 5, wherein the surface roughness Rz is 15 µm or less. A method for producing a hot-rolled steel sheet comprising the following steps (A) to (C):
(A) The molten steel having the steel composition according to any one of claims 1 to 5, wherein the molten steel casting amount per unit time is 1 to 6 tons / minute, and the slab surface To slab thickness by 0.05mm × (slab thickness (mm) / product thickness (mm)) depth position of continuous slab by continuous casting method with average solidification rate at 5 ° C / second or more Process;
(B) The slab is charged into a heating furnace, held at a temperature of 1100 ° C. or higher for 30 minutes or more, and rolled to a slab extracted from the heating furnace in a temperature range of 750 ° C. or higher within 10 minutes from the extraction of the heating furnace. (C) The hot-rolled steel sheet is cooled at an average cooling rate of 5 ° C./second or more after the hot-rolling is completed, and 700 is obtained. Cooling and winding process for winding in a temperature range below ℃.   The method for producing a hot-rolled steel sheet according to claim 7, wherein in the continuous casting step, the molten steel in the mold of the continuous casting machine is stirred by a moving magnetic field. 9. The hot-rolled steel sheet according to claim 7, wherein a mold flux for continuous casting containing one or two of P ₂ O ₅ and B ₂ O ₃ is used in the continuous casting step. Production method.