JP5896183B2 - High-strength hot-rolled steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet suitable for use mainly in members of transportation equipment such as automobiles, home appliances, heavy machinery, steel structures, and the like, and a method for producing the same.

  In recent years, from the viewpoint of protecting the global environment and ensuring the safety of passengers, in order to reduce the weight and strength of automobile bodies, the strength and thinning of steel materials have been actively promoted. ing. An automobile body is composed of a member or a structural material obtained by processing a steel plate, and the strength thereof is usually increased as the tensile strength TS and the yield stress YS of the material steel plate are higher. Therefore, high yield stress YS is calculated | required for the steel plate used as those materials in addition to high tensile strength TS.

  High strength hot-rolled steel sheets are often applied to internal members of structures that are not directly visible from the outside, such as so-called underbody members (strength members) in automobile bodies. Such a member is required to have excellent stretch flangeability because it is often subjected to hole expansion after blanking in addition to bending and drawing by a press.

  Conventionally, as a method of increasing the strength of steel, a solid solution strengthening method in which alloy elements are added and solid solution strengthened, a structure strengthening method in which a hard transformation phase having a high dislocation density is generated and strengthened, A precipitation strengthening method for increasing the strength by dispersing precipitates, or a method for increasing the strength by combining the above methods is known.

  Among the high-strength steel sheets manufactured by these methods, a precipitation-strengthened high-strength hot-rolled steel sheet having a high yield stress is proposed in Patent Document 1, for example, as a steel structure matrix is composed of a ferrite single phase. In addition, there is a precipitation-strengthened high strength hot-rolled steel sheet in which most of the structure is polygonal ferrite and the strength is increased by combining precipitation strengthening and solid solution strengthening with precipitates centered on TiC. However, since the precipitation strengthening method disclosed in Patent Document 1 requires the addition of a large amount of Ti, coarse precipitates are likely to be generated, and the resulting strength and workability are likely to be unstable. The strength obtained is only about 735 MPa in terms of yield stress.

  Patent Documents 2 and 3 disclose a technique for stably increasing the strength of a steel sheet by precipitating fine carbides of Ti and Mo. The techniques described in these documents are intended to ensure high strength by precipitation strengthening of fine carbides while ensuring workability by making the matrix of the steel structure a ferrite single phase. However, the tensile strength of the obtained steel sheet is up to 980 MPa class, and the yield stress is only about 800 MPa at most.

  Patent Document 4 discloses that a tensile strength of 980 MPa or more can be obtained by dispersing and precipitating a composite carbide of Ti, Mo, and V in steel that is substantially a ferrite single-phase structure. Discloses that a steel sheet having a tensile strength of 1180 MPa or more can be obtained. However, in the technique disclosed in Patent Document 4, it is difficult to control the temperature so that both the composition and size of the composite carbide are matched to preferable conditions, and the desired strength and workability are stably imparted to the obtained steel sheet. There is a problem that can not be.

  Further, Patent Document 5 discloses a technique for producing a steel sheet having high strength by finely dispersing metal carbide in ferrite having an area ratio of 90% or more, and having excellent strength, ductility and stretch flangeability. However, the tensile strength of the steel sheet obtained by this technique is up to about 840 MPa, and the yield stress is naturally lower than that.

  Patent Document 6 discloses that a yield stress is 1000 MPa or more by dispersing and precipitating a composite carbide containing Ti, Mo, and V and a carbide containing only V in a steel sheet having a substantially ferrite single phase structure. A technique for obtaining a steel sheet having excellent bending characteristics is disclosed. However, it is difficult to cause both carbides to exist at the same time and to make the amount of the carbides in a preferable condition, and the desired characteristics cannot be obtained stably. Moreover, it is not possible to obtain excellent stretch flangeability simply by controlling the amount of precipitation.

Japanese Patent Laid-Open No. 06-200351 Japanese Patent No. 3637885 Japanese Patent No. 3882577 JP 2007-063668 A JP 2006-152341 A JP 2008-174805 A

  By the way, conventionally, slabs manufactured by continuous casting are once cooled to near room temperature, reheated to a predetermined temperature before hot rolling, and then subjected to hot rolling, and then cooled to near room temperature after casting. During this time, the temperature was hardly controlled. Therefore, a large amount of coarse precipitates were deposited in the slab after cooling. Conventionally, in the method of using precipitation strengthening of fine carbides to increase the strength of hot-rolled steel sheets, the size of the precipitates is mainly controlled by cooling (temperature) at the run-out table in the hot-rolling line. And coil winding temperature control. That is, in the conventional high strength method using precipitates, the carbides and other precipitates precipitated in the slab are redissolved by reheating the slab, and then the carbides are controlled by cooling control and coiling temperature control after hot rolling. Was reprecipitated finely.

However, even if the coarse precipitates precipitated in the slab are heated to the austenite single-phase region where the entire amount can be re-dissolved in theory, there is a limit to the amount that can be re-dissolved during slab heating. difficult. Therefore, in the prior art, there is a limit to increasing the strength, and it has been difficult to stably manufacture steel having a yield stress of 1000 MPa class.
Similarly, in the method of increasing the strength by refining the structure, the microstructure is controlled by using cooling control after hot rolling and coiling temperature control. However, in the prior art, since temperature control in a high temperature range from slab casting to hot rolling is not performed, the obtained steel sheet has microstructures and precipitates that are undesirable for stretch flangeability.

  The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to stably produce a high-strength hot-rolled steel sheet having a yield stress of 1000 MPa or more and excellent in stretch flangeability. And providing an advantageous manufacturing method.

  In order to solve the above-mentioned problems, the inventors have made extensive studies focusing on the influence of temperature control from casting of a slab to hot rolling on the strength and stretch flangeability of a steel plate. As a result, by controlling the temperature and time from slab casting to hot rolling appropriately, it suppresses macro- and micro-segregation of alloy elements and at the same time suppresses precipitation of coarse sulfides and nitrides. It has been found that by promoting the precipitation of carbides and optimizing the ferrite crystal grain size, a high-strength steel sheet having a yield stress of 1000 MPa or more stably and excellent in stretch flangeability can be obtained, The present invention has been developed.

The present invention based on the above findings is C: 0.08 to 0.5 mass%, Si: 0.2 mass% or less, Mn: 1.5 mass% or less, P: 0.03 mass% or less, S: 0.005 mass% or less. , Al: 0.05 mass% or less, N: 0.005 mass% or less and Ca: containing 0.001 to 0.01%, further, Ti: 0.04~2.0mass%, Nb: 0.05~ One or more selected from 3.0 mass% and Zr: 0.05 to 3.5 mass%, and Mo: 0.01 to 0.5 mass%, V: 0.01 to 1.0 mass% And W: one or more selected from 0.01 to 1.0 mass%, the following formula (1);
0.7 ≦ (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) ≦ 1.2 (1)
(The element symbol in the above formula (1) represents the content (mass%) of each element.)
Is a hot-rolled steel sheet obtained by hot rolling a steel slab having a component composition consisting of Fe and inevitable impurities, the ferritic unit having an average crystal grain size of 1 to 5 μm. It is a high-strength hot-rolled steel sheet comprising a phase and having an average grain size of carbides precipitated in the ferrite grains of less than 10 nm and a yield stress of 1000 MPa or more.

  The high-strength hot-rolled steel sheet of the present invention is characterized in that a plating layer is formed on the surface of the steel sheet obtained by hot rolling.

  The plating layer in the high-strength hot-rolled steel sheet of the present invention is a zinc-based plating layer.

  The zinc-based plating layer in the high-strength hot-rolled steel sheet of the present invention is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

  In addition, the present invention is to cool a steel slab obtained by melting and continuously casting steel having any of the above-described component compositions from 0.5 to 5 ° C / sec from the freezing point to 1200 ° C. Then, without reheating the steel slab, when the continuous casting machine has a bending correction part, within 30 minutes after passing the bending correction point, when the continuous casting machine does not have the bending correction part, the final pinch Within 30 minutes after passing through the roll, hot rolling is started to finish the finish rolling at 800 ° C. or higher, and the steel structure is wound into a coil at a temperature of 720 to 520 ° C., so that the steel structure has an average grain size of 1 to 5 μm. A method for producing a high-strength hot-rolled steel sheet, which is made of a single phase and obtains a hot-rolled steel sheet having an average particle size of carbides precipitated in the ferrite grains of less than 10 nm and a yield stress of 1000 MPa or more, is proposed.

  In the method for producing a high-strength hot-rolled steel sheet of the present invention, the ratio (R / t) of the curvature radius R (m) at the curved portion to the thickness t (m) of the steel slab is 25 or more in the production of the steel slab. It is characterized by using the continuous casting machine which is.

  Moreover, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention forms a plating layer on the surface of the steel plate obtained by the said hot rolling.

  According to the present invention, the carbide that has been coarsely precipitated in the slab before hot rolling can be refined and the microstructure can be optimized, so that the yield stress is 1000 MPa or more and stretch flangeability It is possible to stably produce and provide a high-strength hot-rolled steel sheet that is excellent in the quality.

First, the component composition of the high-strength hot-rolled steel sheet of the present invention will be described.
C: 0.08 to 0.5 mass%
C is an element essential for the formation of carbides necessary for precipitation strengthening of steel. If C is less than 0.08 mass%, the amount of precipitates is small, and it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, addition exceeding 0.5 mass% causes a decrease in weldability, carbides precipitate coarsely at a high temperature, and the strength of the obtained steel sheet decreases, and the stretch flangeability decreases. Therefore, C is set to a range of 0.08 to 0.5 mass%. Preferably it is the range of 0.1-0.4 mass%.

Si: 0.2 mass% or less Si is an element added as a deoxidizer, and has been actively used as an element for solid solution strengthening of steel. However, Si has the effect of promoting C precipitation from ferrite, and it is desirable to reduce it as much as possible in order to facilitate precipitation of coarse carbides at grain boundaries. Therefore, in the present invention, Si is set to 0.2 mass% or less. Preferably it is 0.1 mass% or less.

Mn: 1.5 mass% or less Mn is a useful element for solid solution strengthening of steel. However, Mn, like Si, is an element that inhibits macro-segregation or micro-segregation in the final solidified portion to make the structure non-uniform and precipitate carbides uniformly and finely. Moreover, since it has the effect of improving hardenability, it is also an element that generates bainite and martensite, makes the steel structure a single phase of ferrite, and makes it difficult to precipitate fine precipitates in the grains. Therefore, in this invention, Mn shall be 1.5 mass% or less. Preferably it is 1.0 mass% or less.

P: 0.03 mass% or less P, like Si, facilitates the precipitation of coarse carbides at grain boundaries, and, like Mn, it concentrates in the final solidified part during solidification to make the structure non-uniform, resulting in hole expandability and elongation. Since it is an element that reduces the flangeability, it is desirable to reduce it as much as possible. Therefore, P is set to 0.03 mass% or less. Preferably it is 0.01 mass% or less.

S: 0.005 mass% or less S forms sulfides. In particular, plate-like sulfides reduce stretch flangeability, so the smaller the S, the better. Therefore, in the present invention, the upper limit of S is set to 0.005 mass%. Preferably it is 0.004 mass% or less.

Al: 0.05 mass% or less Al is an element added as a deoxidizer for steel. However, if added excessively, the amount of non-metallic inclusions such as alumina increases, which not only adversely affects internal quality and surface quality, but also adversely affects stretch flangeability. Therefore, the upper limit of Al is 0.05 mass%.

N: 0.005 mass% or less N is a harmful element that combines with Ti, Nb, and Zr to form nitrides and consumes carbide-forming elements that contribute to precipitation strengthening. In addition, the presence of coarse nitrides reduces stretch flangeability. Therefore, the upper limit of N is set to 0.005 mass%. Preferably it is 0.004 mass% or less.

Ca: 0.01 mass% or less Ca has an effect of spheroidizing the sulfide and reducing the adverse effect on the workability of S. In order to obtain this effect, 0.001 mass% or more is preferably added. However, if added over 0.01 mass%, hole expandability and stretch flangeability are reduced. Therefore, in this invention, the addition amount of Ca shall be 0.01 mass% or less.

In addition to the above components, the high-strength hot-rolled steel sheet of the present invention is further selected from Ti, Nb, and Zr, which are carbide forming elements, from the viewpoint of stably ensuring high strength (yield stress) or It is necessary to contain 2 or more types within the following range.
Ti: 0.04 to 2.0 mass%
Ti is a useful element that forms fine charcoal / nitrides and precipitates to contribute to the strengthening of steel. However, when the Ti content is less than 0.04 mass%, it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, addition exceeding 2.0 mass% produces coarse Ti oxides, and coal / nitrides aggregate to reduce the precipitation strengthening ability, so that the desired yield stress can be obtained stably. Disappear. Therefore, when adding Ti, it is set as the range of 0.04-2.0 mass%.

Nb: 0.05 to 3.0 mass%
Nb, like Ti, is a useful element that forms charcoal / nitrides and precipitates to contribute to increasing the strength of steel. However, when the Nb content is less than 0.05 mass%, it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, when the addition exceeds 3.0 mass%, coarse Nb charcoal / nitride is generated and aggregates, and a desired yield stress cannot be stably obtained. Therefore, when adding Nb, it is set as the range of 0.05-3.0 mass%.

Zr: 0.05 to 3.5 mass%
Zr, like Ti and Nb, is a useful element that forms and precipitates carbon / nitride and contributes to increasing the strength of steel. However, when the Zr content is less than 0.05 mass%, it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, when it exceeds 3.5 mass%, coarse Zr oxide is formed, and carbon / nitride aggregates on the Zr oxide, so that a desired yield stress cannot be stably obtained. Therefore, when adding Zr, it is set as the range of 0.05-3.5 mass%.

  In addition to the above components, the high-strength hot-rolled steel sheet of the present invention further includes one or more selected from Mo, V, and W in addition to the above components from the viewpoint of securing high strength more stably. It is necessary to contain in the range of. However, Mo, V, and W have a weak carbide forming ability as compared with Ti, Nb, and Zr. When these are added, a stable fine carbide can be obtained by adding in combination with other carbide forming elements. Can be formed. Therefore, after adding one or more selected from Ti, Nb and Zr, it is necessary to add one or more of Mo, V and W within the following range. There is.

Mo: 0.01-0.5 mass%
Mo is a useful element that forms fine carbides and precipitates and contributes to increasing the strength of steel. However, if the Mo content is less than 0.01 mass%, it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, when the addition exceeds 0.5 mass%, coarse Mo oxides are formed, and carbides agglomerate to this to lower the precipitation strengthening ability, so that a yield stress of 1000 MPa or more cannot be stably obtained. Therefore, when adding Mo, it is set as the range of 0.01-0.5 mass%.

V: 0.01-1.0 mass%
V is a useful element that forms and precipitates fine carbonitrides and contributes to increasing the strength of steel. However, if the V content is less than 0.01 mass%, it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, addition exceeding 1.0 mass% produces coarse V oxide, and carbonitride aggregates to lower the precipitation strengthening ability, so that a yield stress of 1000 MPa or more can be stably obtained. Disappear. Therefore, when adding V, it is set as the range of 0.01-1.0 mass%.

W: 0.01-1.0 mass%
W is a useful element that forms and precipitates fine carbides and contributes to increasing the strength of steel. However, if the W content is less than 0.01 mass%, it is difficult to stably obtain a yield stress of 1000 MPa or more. On the other hand, addition exceeding 1.0 mass% produces coarse W oxides, and carbides agglomerate to this to lower the precipitation strengthening ability. Therefore, yield stress of 1000 MPa or more cannot be obtained stably. The yield ratio may not be 80% or more. Therefore, when adding W, it is set as the range of 0.01-1.0 mass%.

Further, the high-strength hot-rolled steel sheet of the present invention is further selected from Mo, V, and W in addition to one or more selected from Ti, Nb, and Zr from the viewpoint of securing high strength more stably. When adding more than seeds, the following formula (1);
0.7 ≦ (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) ≦ 1.2 (1)
(The element symbol in the above formula (1) represents the content (mass%) of each element. However, in the case of a component not contained, it is calculated as 0.)
It is necessary to satisfy and contain.
Here, the above formula (1) is the ratio of the total number of atoms of Ti, Nb, Zr, Mo, V, and W that form fine precipitates with respect to the number of C atoms, that is, C and the carbide-forming element. It represents the atomic ratio. If this ratio is less than 0.7, a high yield stress cannot be obtained. On the other hand, if it exceeds 1.2, carbides are coarsened and a high yield stress cannot be obtained, so the above range is set. In addition, Preferably, the value of the left side of said Formula (1) is 0.75, and the value of the right side is 1.15.

One or more selected from Cr, Hf, Ta, Be, B, Cu, Ni, Au, Ag, Co, Pt, Sb, Sn, Zn, Mg and REM: 2 mass% or less in total Cr, Hf and Ta, like Ti, Nb and Zr described above, are elements that actively form carbides in the steel and contribute to high strength. Be and B are solid solution strengthening and grain boundary strengthening. Therefore, it can be added as a strengthening element when it is desired to obtain higher strength.

  Cu is an impurity element usually mixed from scrap or the like, but is an element effective for increasing the strength of steel. Therefore, in the present invention, it is possible to allow Cu contamination to some extent, and to actively utilize scrap which is a recycling resource, and to reduce the raw material cost. In the steel sheet of the present invention, the influence on the material of Cu is small, but if excessively mixed, it may cause surface defects due to cracking due to hot brittleness during hot rolling, so the upper limit of Cu content Is preferably limited to about 0.3 mass%.

  Of the above elements, Cr, B and Cu are elements that enhance the hardenability like Mn. If the hardenability becomes too high, bainite and martensite are generated, and a ferrite single phase structure is formed. It becomes difficult to obtain and inhibits fine precipitation in ferrite grains. Therefore, these elements are preferably each or a total of 1 mass% or less.

  Ni has a small effect on the material of the steel sheet, but is an element effective in preventing hot brittleness due to the addition of Cu and improving the surface quality. Since this effect is obtained by addition of ½ or more of the Cu content, when Cu is contained, it is preferable to add Ni of ½ or more of the Cu content. However, excessive addition of Ni causes surface defects due to non-uniformity of scale, so the upper limit is preferably about 0.3 mass%.

  In addition, Au, Ag, Co, Pt, Sb, Sn, and Zn suppress the surface oxidation and nitridation, or the decarburization of the steel sheet surface layer of several tens of microns, resulting in improved fatigue characteristics and aging resistance. Therefore, it can be added as appropriate. However, Sn is preferably added in an amount of 0.005 mass% or more in order to obtain the above effect, but excessive addition causes a reduction in the toughness of the steel, so the upper limit is preferably about 0.2 mass%. .

  Further, Mg and REM can be added as appropriate because they are effective elements for improving the workability through the form control of inclusions.

  In addition, although said element added selectively is added from said viewpoint, it is preferable to restrict | limit the sum total of the addition amount of these elements to 2 mass% or less. This is because if it exceeds 2 mass%, the precipitates become coarse, making it difficult to obtain a high yield stress, and also causing a decrease in formability and an increase in alloy costs.

One or more selected from As, Cs, Pb, Se and Sr: 2 mass% or less in total As, Cs, Pb, Se and Sr are elements that are positioned as inevitable impurities in the present invention. . However, these elements tend to increase in recent years when the amount of scrap used is increasing, and a large amount of refining costs are required to remove them. However, these elements do not particularly impair the effects of the present invention as long as the total content is 2 mass% or less. Therefore, these elements are allowed within a total range of 2 mass% or less.

Next, the steel structure and precipitates of the high-strength hot-rolled steel sheet of the present invention will be described.
The high-strength hot-rolled steel sheet of the present invention is strengthened by finely precipitating carbides, and in order to achieve a high yield stress of 1000 MPa or more, the matrix of the steel structure is substantially a ferrite single phase in which fine carbides are precipitated. It is necessary to be. In a bainite or martensite structure, yield stress decreases because a large amount of movable dislocations are introduced. Further, when pearlite appears, C is consumed due to the formation of cementite, and the precipitation of fine carbides is suppressed, so that high yield stress cannot be obtained. Moreover, when it becomes a double phase, it is because favorable stretch flangeability cannot be obtained due to the difference in hardness between phases.
However, it is acceptable if the total of phases other than ferrite is about 5% or less in terms of area ratio.

  The average crystal grain size of the ferrite phase needs to be in the range of 1 to 5 μm. This is because refinement of crystal grains not only increases yield stress but also improves stretch flangeability in order to increase the effect of suppressing the progress of cracks introduced by processing. However, excessive miniaturization, on the other hand, impairs ductility and reduces stretch flangeability. Therefore, in the present invention, the average crystal grain size of the ferrite phase is set in the range of 1 to 5 μm.

  Further, the high strength hot rolled steel sheet according to the present invention needs to refine the size of the carbide precipitated in the ferrite phase to 100 nm or less in order to achieve a high strength of a yield stress of 1000 MPa or more. Here, the maximum diameter of the carbide is measured by using a transmission electron microscope (TEM) to measure the size (particle diameter) of the carbide precipitated by the extraction replica method, the thin film method, the dispersion replica method, or the like. This means a diameter corresponding to (x + 3σ) obtained by adding three times the standard deviation σ to the average value x. This is because when the maximum diameter of the carbide exceeds 100 nm, it becomes difficult to stably secure a yield stress of 1000 MPa or more.

  The high-strength hot-rolled steel sheet of the present invention is stable when the yield stress YS is 1000 MPa or more only when the above-mentioned component composition, steel structure (phase, particle size), and the size of the precipitated carbide are all satisfied. It can be ensured and excellent stretch flangeability can be ensured.

Next, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention is demonstrated.
The high-strength hot-rolled steel sheet of the present invention is a steel slab obtained by continuously melting and casting steel adjusted to the above-described component composition in a conventional refining process using a converter, electric furnace, vacuum degassing apparatus, etc. In a continuous casting machine having a bending part and a bending correction part, after cooling from the freezing point to 1200 ° C. at 5 ° C./sec or less and then reheating the slab, within 30 min after passing the bending correction point. In a continuous casting machine that does not have a straightening part, within 30 minutes after passing through the final pinch roll, hot rolling is started to finish the finish rolling at 800 ° C. or higher, and wound into a coil at a temperature of 720 to 520 ° C. To manufacture.
The steel slab may be a normal steel slab having a thickness of 100 mm or more, or a so-called thin slab having a thickness of 100 mm or less.

In addition, after continuous casting, heating the steel slab being transported to the hot rolling mill or the width direction end (edge) of the sheet bar during hot rolling with an edge heater or the like increases the temperature. Rather, it is preferable because the rolling temperature becomes uniform in the width direction and the material in the sheet width direction can be made uniform. In this case, if the temperature at the end of the slab at the start of hot rolling can be 1000 ° C. or higher, the lower limit temperature at the center of the slab thickness can be allowed even immediately above the Ar ₃ transformation point. The heating method of the edge heater may be a gas combustion method or an electromagnetic heating method such as resistance heating or induction heating.
Moreover, in order to suppress the temperature drop at the time of conveyance of a slab or a sheet bar, the conveyance route may be a tunnel path, or a heat insulating cover may be provided on the conveyance route to suppress the cooling of the edge portion. In this case, it is economical because no fuel or electricity is used.

  As described above, in order to produce a high-strength hot-rolled steel sheet having a yield stress of 1000 MPa or more, it is necessary to refine the size of carbides precipitated in the steel sheet after hot rolling. For that purpose, it is important not to deposit coarse carbides in the slab after continuous casting and before hot rolling. This is because, according to the study by the inventors, it is clear that coarse carbides form nuclei when the slab after casting is present in the δ ferrite region, and are formed by coarsening. Yes. The reason is that the solubility of elements such as Ti, Nb, and Zr that form carbides in steel is δ ferrite, which is a body-centered cubic lattice (BCC), compared to the γ-austenite phase, which is a face-centered cubic lattice (FCC). The phase is small. For this reason, if the liquid phase becomes a δ ferrite phase during slab cooling, the carbide-forming elements are rapidly supersaturated, and the carbide is likely to precipitate. In particular, high-temperature δ ferrite has a high diffusion rate, and thus is easily precipitated and coarsened in a short time.

Therefore, in order to prevent the precipitation of coarse carbonitride and stably obtain a high strength hot rolled steel sheet having a yield stress of 1000 MPa or more, the cooling rate of the slab after casting is increased, and Ti, Nb, Zr, etc. It is necessary to pass through the δ ferrite temperature range where carbides are likely to precipitate in a short time.
Therefore, in the present invention, when cooling a steel slab produced by continuous casting, it is preferable to set the average cooling rate from the δ ferrite region, that is, from the freezing point to 1200 ° C., to 0.5 ° C./sec or more. If it is less than 0.5 ° C / sec, carbide nuclei are formed in δ ferrite, and the carbide tends to be coarsened before hot rolling, so the precipitation strengthening ability after hot rolling is reduced and the yield stress of 1000 MPa or more is stable. It is difficult to secure it. Preferably, it is 1.0 ° C./sec or more.
On the other hand, when the cooling rate from the start of solidification to 1200 ° C. exceeds 5 ° C./sec, microsegregation formed during solidification remains in the steel sheet after hot rolling, and the structure in the steel sheet becomes non-uniform. The flangeability is greatly reduced. Therefore, the upper limit cooling rate is 5 ° C./sec.
In addition, the said freezing point and the temperature of 1200 degreeC in this invention are the temperature of slab thickness center part obtained by heat transfer calculation from slab surface temperature.

  In addition, according to the research by the inventors, it has been clarified that the strain applied to the continuously cast steel slab promotes the precipitation of carbides. The continuous casting machine currently used for the production of steel slabs has a vertical type that does not have a bending part and a correction part, a bending type that consists of a bending part (bending part) and a correction part, and a vertical part and a bending part and correction. There is a vertical bending mold consisting of parts. In the straightening section, strain is introduced by straightening the curved slab, but this strain promotes precipitation of carbides, and therefore precipitates in the slab (before hot rolling) and promotes coarsening. Therefore, in order to enjoy the effects of the present invention, the curvature radius R (m) of the curved portion and the slab thickness t (m) and the ratio (R / t) are 25 in order to reduce the distortion caused by the correction in the continuous casting machine. The above is preferable. More preferably, it is 28 or more.

  Further, the present invention provides a residence time in a high temperature range where carbides are likely to precipitate, specifically, in a continuous casting machine having a straightening section, a time from passing a straightening point to the start of hot rolling, and a straightening section. In a continuous casting machine that does not have, the time from passing through the final pinch roll to the start of hot rolling is limited to within 30 minutes, thereby suppressing precipitation and coarsening of carbides in a high temperature range.

  The steel slab obtained as described above is then hot-rolled without being reheated to obtain a hot-rolled sheet (high-strength hot-rolled steel sheet) having a predetermined thickness. That is, in general, in hot rolling, the slab is once cooled to near room temperature to form a cold piece, and then charged into a heating furnace and reheated to a predetermined temperature. Then, the slab after continuous casting is directly subjected to hot rolling without reheating.

  At this time, it is desirable that the slab temperature (center temperature) is not cooled below 1000 ° C. in order to secure the finish rolling finish temperature described later. However, if the finish rolling finish temperature can be ensured, it may be cooled to 900 ° C. Cooling to below 900 ° C. is not preferable because an α-ferrite phase having a small solid solubility limit of carbide-forming elements begins to appear again, and precipitation of carbide is promoted.

  Moreover, the said hot rolling needs to complete | finish finish rolling at the temperature of 800 degreeC or more. Here, the reason why the finish rolling finish temperature in the hot rolling is 800 ° C. or higher is that if it is lower than 800 ° C., carbides precipitate in the processed austenite and become coarse, and a high strength with a yield stress of 1000 MPa or more is obtained. Because it disappears. Preferably, the finish rolling end temperature is 820 ° C. or higher. However, if the finish rolling finish temperature is too high, the γ grains become coarse and fine ferrite (1 to 5 μm) cannot be obtained, so the upper limit is made 900 ° C. The finish rolling finish temperature and the coiling temperature described later are temperatures measured on the steel sheet surface.

  Moreover, in order to ensure the said finish rolling completion temperature, it is preferable that the rolling start temperature of hot rolling shall be 1000 degreeC or more. However, when the temperature at which the hot rolling is started is reduced to about 1000 ° C., the rolling load increases, and it becomes difficult to perform hot rolling including normal rough rolling and finish rolling. In such a case, a thin slab having a thickness of 100 mm or less may be employed. Further, in this case, rough rolling may be omitted. When a thin slab is employed and rough rolling is omitted, the temperature at which hot rolling is started can be set to 900 ° C. or higher. In this case, since the slab residence time at high temperature can be shortened, it is preferable also in suppressing the coarsening of carbides.

  The hot-rolled steel sheet (steel strip) having a predetermined thickness by hot rolling is then cooled and wound on a coil. The winding temperature at this time is such that carbonitrides such as Ti, Nb, and Zr are uniformly and finely precipitated in the ferrite, and the ferrite grains are finely divided by the pinning effect, thereby increasing the strength. It is necessary to set it as the range of 520-720 degreeC. When the coiling temperature is less than 520 ° C., a bainite-based structure is obtained, and a ferrite single-phase structure cannot be obtained, and fine carbides are not sufficiently precipitated in ferrite grains, so that a desired yield stress cannot be obtained. . On the other hand, when the coiling temperature exceeds 720 ° C., the precipitated carbide is coarsened and the precipitation strengthening ability is lowered, so that the desired yield specific stress cannot be obtained. In addition, coiling temperature is the temperature which measured the steel plate surface.

  The high-strength hot-rolled steel sheet of the present invention obtained as described above has a yield stress of 1000 MPa or more and excellent stretch flangeability. Moreover, this excellent characteristic of the high-strength hot-rolled steel sheet of the present invention can be maintained as it is, not only by electrogalvanization but also by hot dip galvanization with heat treatment and alloying treatment after hot dip galvanization. it can. Therefore, the high-strength hot-rolled steel sheet of the present invention can be suitably used as a material for a surface-treated steel sheet that forms a plated layer such as electrogalvanized, hot-dip galvanized, and alloyed hot-dip galvanized on the steel sheet surface. The plating layer is not limited to a zinc-based plating layer, and may of course be an Al-based, Ni-based or other metal plating layer.

  C: 0.18 mass%, Si: 0.02 mass%, Mn: 1.30 mass%, P: 0.020 mass%, S: 0.001 mass%, Al: 0.04 mass%, N: 0.003 mass%, Ca : 0.0015 mass%, Ti: 0.19 mass%, Mo: 0.3 mass% and V: 0.41 mass% ((Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12 ): 1.01), the remainder is made of steel having a component composition consisting of Fe and inevitable impurities, and the bending radius of curvature R (m) and slab thickness t (m) are measured using a vertical bending type continuous casting machine. ) In the range of 26.7 to 36.4, and after cooling from the freezing point to 1200 ° C. at the cooling rate shown in Table 1, without reheating, Hot rolled under the conditions shown in axial Table 1, it was with various thickness of the hot rolled steel sheet. Some slabs were cooled to room temperature RT and then reheated or rough rolling was omitted.

Moreover, about a part of hot-rolled steel plate obtained as mentioned above, after pickling and removing a scale, it passed through the continuous hot dip galvanizing line CGL, and annealed at 650-850 degreeC. Then, it was immersed in a hot dip galvanizing tank to obtain a hot dip galvanized steel sheet GI, or was further alloyed at 500 to 550 ° C. to obtain an alloyed hot dip galvanized steel sheet GA.
Furthermore, about a part of hot-rolled steel plate obtained as mentioned above, after pickling and removing a scale, it passed through the electrogalvanization line EGL, and the Zn-Ni type electrogalvanized steel plate EG It was.

A sample is taken from the hot-rolled steel sheet thus obtained, the steel sheet structure is confirmed using an optical microscope and a scanning electronic distance microscope (SEM), and the average grain size of the ferrite grains in the rolling direction section is determined by the line segment method. Measured with Further, a JIS No. 5 tensile test piece having the L direction as the tensile direction was taken from the above sample, a tensile test was performed in accordance with JIS Z2241, and the yield stress YS was measured.
Moreover, the hole expansion test was done and stretch flangeability was evaluated. Here, in the hole expansion test, a punch having a tip angle of 60 ° was used to push into a punched hole having a diameter of 10 mm, and the hole diameter d when the generated crack penetrated the plate thickness of the test piece was measured. The hole expansion rate (λ) was determined by the equation.
λ (%) = {(d−d ₀ ) / d ₀ } × 100
(D: hole diameter when crack penetrates plate thickness, d ₀ : initial diameter)

The results are shown in Table 1 together with the production conditions. From Table 1, No. 1 satisfying all the conditions of the present invention is obtained. The steel sheets 1 to 6 (invention examples) all have a yield stress YS of 1000 MPa or more and an excellent hole expansion ratio λ. On the other hand, No. in which any manufacturing condition deviates from the present invention. None of the steel plates 7 to 14 (comparative examples) have a yield stress of less than 1000 MPa or a good hole expansion rate.
Further, No. 1 satisfying all the conditions of the present invention. The steel plates 15 to 17 (invention examples) can maintain the above excellent characteristics as they are after electrogalvanization, after hot dip galvanization with heat treatment, and after alloying treatment. I understand that.

  The steel of the code | symbol AP which has a component composition shown in Table 2 is melted, R / t in the curvature radius R (m) of a bending part and slab thickness t (m) is used using a vertical bending type continuous casting machine. After forming a steel slab in the range of 26.7 to 36.4, after cooling from the freezing point to 1200 ° C. at the cooling rate shown in Table 3, it was hot-rolled under the conditions shown in Table 3 without reheating. And it was set as the hot-rolled steel plate of various board thickness. Some slabs were cooled to room temperature RT and then reheated or rough rolling was omitted.

  A sample is taken from the hot-rolled steel sheet thus obtained, the steel sheet structure is confirmed using an optical microscope and a scanning electronic distance microscope (SEM), and the average grain size of the ferrite grains in the rolling direction section is determined by the line segment method. Measured with Further, a JIS No. 5 tensile test piece having the L direction as the tensile direction was taken from the above sample, and a tensile test was performed in accordance with JIS Z2241, to determine the yield stress YS. Further, in the same manner as in Example 1, the hole expansion ratio λ was measured, and the stretch flangeability was evaluated.

  The results are shown in Table 3 together with the production conditions. From Tables 2 and 3, all of the steel sheets of A to I (invention examples) that satisfy all the conditions of the present invention have a yield stress of 1000 MPa or more, and an excellent hole expansion rate is obtained. On the other hand, any of the steel sheets (comparative examples) of J to P whose manufacturing conditions deviate from the present invention have a yield stress of less than 1000 MPa or a good hole expansion rate. I understand that.

Claims (8)

C: 0.08 to 0.5 mass%, Si: 0.2 mass% or less, Mn: 1.5 mass% or less, P: 0.03 mass% or less, S: 0.005 mass% or less, Al: 0.05 mass% or less N: 0.005 mass% or less and Ca: 0.001-0.01 mass%, Ti: 0.04-2.0 mass%, Nb: 0.05-3.0 mass%, and Zr: 0 One or more selected from 0.05 to 3.5 mass%, Mo: 0.01 to 0.5 mass%, V: 0.01 to 1.0 mass%, and W: 0.01 to 1 Obtained by hot-rolling a steel slab containing one or more selected from 0.0 mass%, satisfying the following formula (1), and having the balance of Fe and inevitable impurities. Fever A rolled steel sheet,
High strength hot rolling characterized in that the steel structure is composed of a ferrite single phase having an average crystal grain size of 1 to 5 μm, the average grain size of carbides precipitated in the ferrite grains is less than 10 nm, and the yield stress is 1000 MPa or more. steel sheet.
0.7 ≦ (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) ≦ 1.2 (1)
(The element symbol in the above formula (1) represents the content (mass%) of each element.) The high-strength hot-rolled steel sheet according to claim 1 , wherein a plated layer is formed on the surface of the steel sheet obtained by hot rolling. The high-strength hot-rolled steel sheet according to claim 2 , wherein the plating layer is a zinc-based plating layer. The high-strength hot-rolled steel sheet according to claim 3 , wherein the zinc-based plated layer is a hot-dip galvanized layer. The high-strength hot-rolled steel sheet according to claim 3 , wherein the zinc-based plated layer is an alloyed hot-dip galvanized layer. A steel slab obtained by melting and continuously casting steel having the component composition according to claim 1 is cooled from a freezing point to 1200 ° C. at 0.5 to 5 ° C./sec. Without reheating, when the continuous casting machine has a bending correction part, within 30 min after passing the bending correction point, and when the continuous casting machine does not have the bending correction part, within 30 min after passing the final pinch roll. , By starting hot rolling to finish the finish rolling at 800 ° C. or higher, and winding the coil at a temperature of 720 to 520 ° C.
A high-strength hot-rolled steel sheet that has a steel structure consisting of a ferrite single phase with an average crystal grain size of 1 to 5 μm, a hot-rolled steel sheet having an average grain size of carbides precipitated in the ferrite grains of less than 10 nm and a yield stress of 1000 MPa or more. Manufacturing method. The steel slab is manufactured using a continuous casting machine having a ratio (R / t) of a radius of curvature R (m) at a curved portion to a thickness t (m) of the steel slab of 25 or more. Item 7. A method for producing a high-strength hot-rolled steel sheet according to Item 6 . The method for producing a high-strength hot-rolled steel sheet according to claim 6 or 7 , wherein a plating layer is formed on a surface of the steel sheet obtained by hot rolling.