JP2009185361A - High-strength hot-rolled steel sheet and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel sheet which has tensile strength (TS) of 540-780 MPa, and is excellent in the strength uniformity wherein variance in strength is small by using an inexpensive Ti series general-using steel sheet. <P>SOLUTION: The component composition of this steel sheet contains, by mass, 0.05-0.12% C, ≤0.5% Si, 0.8-1.8% Mn, ≤0.030% P, ≤0.01% S, 0.005-0.1% Al, ≤0.01% N, 0.030-0.080% Ti and the balance Fe with inevitable impurities. And, in the structure, a polygonal-ferrite is contained in faction ratio of ≥70% and the quantity of Ti existing in the precipitation having size of <20 nm, is ≥50% of Ti* value calculated with the following formula (1)...T*=[Ti]-48/14×[N]. Wherein, [Ti] and [N] each represent the component-compositions (mass%) of Ti ans N in the steel sheet. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a high-strength hot-rolled steel sheet having a tensile strength (TS) of 540 to 780 MPa that is useful for use in automobile steel sheets and the like, and excellent in strength uniformity with small strength variations between and within the coils. It relates to the manufacturing method.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, improvement in fuel efficiency of automobiles has been demanded. In addition, in order to ensure the safety of passengers in the event of a collision, safety improvements centering on the collision characteristics of automobile bodies are also required. For this reason, both weight reduction and strengthening of the automobile body are being actively promoted. In order to satisfy the weight reduction and strengthening of the car body at the same time, it is said that it is effective to increase the material strength within the range where rigidity does not become a problem and reduce the weight by reducing the plate thickness. Are actively used in automotive parts. Since the weight reduction effect increases as the strength of the steel sheet used increases, the automotive industry tends to use, for example, a steel sheet having a tensile strength (TS) of 540 MPa or more as a structural material.

  On the other hand, many automobile parts made of steel plates are manufactured by press molding. Regarding the formability of a high-strength steel sheet, dimensional accuracy is important in addition to cracking and wrinkling, and in particular, control of the spring back is an important issue. Recently, CAE (Computer Assisted Engineering) has made the development of new cars much more efficient, and it has become impossible to make molds many times. At the same time, when the characteristics of the steel plate are input, the amount of springback can be predicted with higher accuracy. If there is variation in the amount of springback, it becomes a problem when parts are joined together, so it is necessary to make it smaller, but this requires a high-strength steel sheet with particularly low strength variation and excellent strength uniformity. Yes.

As a method for reducing the strength variation in the coil, Patent Document 1 (Japanese Patent Laid-Open No. 4-289125) discloses that after hot rolling a low-Mn steel containing Nb (Mn: 0.5% or less), after rough rolling. A method of achieving uniform strength in a coil of a high-strength hot-rolled steel sheet by winding the sheet bar into a coil shape, joining to the preceding sheet bar while rewinding, and continuously performing finish rolling. It is disclosed. Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-322541) includes a combination of Ti and Mo, which is excellent in strength uniformity with small strength variation in which very fine precipitates are uniformly dispersed. Strength hot-rolled steel sheets have been proposed.
JP-A-4-289125 JP 2002-322541 A

However, the above prior art has the following problems.
In the method described in Patent Document 1, there is a problem that the coil is divided again at the time of winding. Further, Nb addition causes an increase in cost and is economically disadvantageous. Further, although the steel sheet described in Patent Document 2 is Ti-based, expensive Mo needs to be added, resulting in an increase in cost. Furthermore, none of the patent documents considers the two-dimensional intensity uniformity in the coil plane including both the width direction and the longitudinal direction of the coil. Such intensity variation in the coil surface inevitably occurs because the coil cooling history after winding differs depending on the position, no matter how the winding temperature is controlled uniformly.

  In view of such circumstances, the present invention advantageously solves the above-mentioned problems, uses an inexpensive Ti-based general-purpose steel plate, has a tensile strength (TS) of 540 to 780 MPa, and has high strength excellent in strength uniformity with small strength variation. The object is to provide a hot-rolled steel sheet.

  As a result of diligent investigations to solve the above-mentioned problems, by controlling the chemical composition of the steel sheet, the metal structure, and the precipitation state of Ti that contributes to precipitation strengthening, there is no variation in strength over the entire surface of the hot-rolled steel sheet. The present invention succeeded in obtaining a high-strength hot-rolled steel sheet excellent in small strength uniformity.

The summary of the high-strength hot-rolled steel sheet and the method for producing the same according to the present invention, which are excellent in strength uniformity with small variations in in-plane strength, are as follows.
[1] Component composition is mass% C: 0.05-0.12%, Si: 0.5% or less, Mn: 0.8-1.8%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N : 0.01% or less, Ti: 0.030 to 0.080%, with the balance consisting of Fe and inevitable impurities, with a structure containing polygonal ferrite in a fraction of 70% or more, and in precipitates less than 20 nm in size A high-strength hot-rolled steel sheet characterized in that the amount of Ti present in is 50% or more of the value of Ti * calculated by the following formula (1).
Ti * = [Ti] −48 ÷ 14 × [N] (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively.
[2] Component composition is mass%, C: 0.05 to 0.12%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.030% or less, S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.01% or less, Ti: 0.030-0.080%, the steel slab consisting of Fe and unavoidable impurities in the remainder is heated to a heating temperature of 1150-1300 ° C and then hot at a finishing temperature of 800-950 ° C Perform finish rolling, start cooling at a cooling rate of 20 ° C./s or more within 2 seconds after the hot finish rolling, stop cooling at a temperature of 650 ° C. to 750 ° C., and continue for 2 to 15 seconds. A method for producing a high-strength hot-rolled steel sheet, which is subjected to a cooling step and then cooled again at a cooling rate of less than 100 ° C / s and wound in a coil shape in a temperature range of 550 to 650 ° C.

  According to the present invention, a high strength hot rolled steel sheet having a tensile strength (TS) of 540 to 780 MPa, it is possible to narrow the strength variation in the coil, and thereby the shape of the steel sheet during press forming can be reduced. Stabilization of freezing, component strength, and durability performance is achieved, and reliability during production and use of automobile parts is improved. Furthermore, in the present invention, the above effect can be obtained without adding an expensive raw material such as Nb, so that the cost can be reduced.

The present invention is described in detail below.
1) First, a method for evaluating strength uniformity with little variation in strength in the present invention will be described.
An example of the target steel sheet is a coil wound in a coil shape with a weight of 5 t or more and a steel sheet width of 500 mm or more. In such a case, the innermost and outermost windings at the front and rear ends in the longitudinal direction and 10 mm at both ends in the width direction are not subject to evaluation in the hot-rolled state. The strength variation is evaluated by the distribution of tensile strength measured two-dimensionally in at least 10 divisions in the longitudinal direction and at least 5 divisions in the width direction. Further, the present invention is directed to the range where the tensile strength (TS) of the steel sheet is not less than 540 MPa and not more than 780 MPa.

2) Next, the reasons for limiting the chemical composition (component composition) of steel in the present invention will be described.
The unit of element content is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.

C: 0.05-0.12%
C is an important element in the present invention together with Ti described later. C forms a carbide with Ti and is effective for increasing the strength of the steel sheet by precipitation strengthening. In the present invention, from the viewpoint of precipitation strengthening, it is preferable to contain 0.05% or more of C, and more preferably 0.06% or more. On the other hand, the content of C exceeding 0.012% tends to adversely affect good elongation and hole expandability, and the upper limit of the C content is 0.12%, preferably 0.10% or less.

Si: 0.5% or less
Si has the effect of improving ductility as well as the effect of solid solution strengthening. In order to acquire the said effect, it is effective to contain Si 0.01% or more. On the other hand, if Si is contained in excess of 0.5%, surface defects called red scales are likely to occur during hot rolling, and the surface appearance of the steel sheet may deteriorate, so the Si content is 0.5%. % Or less, and more preferably 0.3% or less.

Mn: 0.8-1.8%
Mn is effective for increasing the strength and has the effect of lowering the transformation point and making the ferrite grain size finer. Mn needs to be contained in an amount of 0.8% or more, preferably 1.0% or more. On the other hand, if it contains excessive Mn exceeding 1.8%, a low-temperature transformation phase is generated after hot rolling and the ductility is lowered, and TiC precipitation tends to become unstable, so the upper limit of Mn content is 1.8% And

P: 0.030% or less P is an element having an effect of strengthening solid solution, and has an effect of reducing scale defects caused by Si. However, if the P content exceeds 0.030%, P tends to segregate at grain boundaries, and toughness and weldability tend to deteriorate. Therefore, the upper limit of the P content is 0.030%.

S: 0.01% or less S is an impurity and causes hot cracking. In addition, S is present as an inclusion in steel and deteriorates various properties of the steel sheet, so it is necessary to reduce it as much as possible. Specifically, the S content is 0.01% or less because it is acceptable up to 0.01%.

Al: 0.005-0.1%
In addition to being useful as a deoxidizing element for steel, Al has the effect of improving the normal temperature aging resistance by fixing solute N present as an impurity. In order to exert such an effect, the Al content needs to be 0.005% or more. On the other hand, if the Al content exceeds 0.5%, high alloy costs are caused and surface defects are more likely to be induced. Therefore, the upper limit of Al content is set to 0.1%.

N: 0.01% or less N is an element that degrades aging resistance at room temperature, and is an element that is preferably reduced as much as possible. When the N content increases, the room temperature aging resistance deteriorates, and a large amount of Al or Ti is required to fix the solid solution N. Therefore, it is preferable to reduce it as much as possible, and the upper limit of the N content is 0.01%. To do.

Ti: 0.030-0.080%
Ti is an important element for strengthening steel by precipitation strengthening. In the case of the present invention, it contributes to precipitation strengthening by forming carbide together with C.
That is, in order to obtain a high-strength steel sheet having a tensile strength TS of 540 MPa or more and 780 MPa or less, it is preferable to refine the precipitate so that the precipitate size is less than 20 nm. It is also important to increase the proportion of this fine precipitate (precipitate size less than 20 nm). One reason for this is that when the size of the precipitate is 20 nm or more, it is difficult to obtain the effect of suppressing the movement of dislocations, and polygonal ferrite cannot be hardened sufficiently, so that the strength may decrease. It is done. Therefore, the size of the precipitate is preferably less than 20 nm. In the present invention, the precipitate containing fine Ti of less than 20 nm is formed by adding both Ti and C within the above range. In the present specification, precipitates containing Ti and C are collectively referred to as Ti-based carbides. Examples of Ti-based carbides include TiC and Ti ₄ C ₂ S ₂ . Further, N may be included in the carbide as a composition, or may be precipitated in combination with MnS or the like.
In the high-strength steel sheet of the present invention, it has been confirmed that Ti-based carbides are mainly precipitated in polygonal ferrite. This is presumably because the solid solubility limit of C in polygonal ferrite is small, and thus supersaturated C tends to precipitate as carbide in polygonal ferrite. For this reason, such a precipitate hardens soft polygonal ferrite, and a tensile strength (TS) of 540 MPa or more and 780 MPa or less is obtained. At the same time, Ti is a preferable element for fixing solute N because Ti is easily bonded to solute N. In that sense, 0.030% or more. However, excessive addition of Ti is not preferable because it only produces TiC, which is a coarse undissolved carbide of Ti that does not contribute to strength in the heating stage, and is uneconomical. From this viewpoint, the upper limit of Ti is set to 0.080%.
In the present invention, the balance other than the above-described components is preferably substantially composed of iron and inevitable impurities.

3) Next, the reason why the steel structure of the steel sheet of the present invention is limited will be described.
The amount of Ti in the precipitate having a structure containing polygonal ferrite in a fraction of 70% or more and a size of less than 20 nm is 50% or more of Ti * represented by the formula (1). The strength of the hot-rolled steel sheet is determined by superimposing the respective strengthening amounts by the three strengthening mechanisms of solid solution strengthening, structure strengthening or precipitation strengthening on the base strength of the steel itself. Of these, the base strength is the original strength of iron, and the solid solution strengthening is determined almost uniquely when the chemical composition is determined. Therefore, these two strengthening mechanisms are hardly involved in the strength variation in the coil. Precipitation strengthening is most closely related to strength variation, followed by structure strengthening.

  The amount of strengthening by precipitation strengthening is determined by the size and dispersion of the precipitate (specifically, the precipitate interval). Since the dispersion of the precipitate can be expressed by the amount and size of the precipitate, if the size and amount of the precipitate are determined, the strengthening amount by precipitation strengthening is determined. The structure strengthening is determined by the type of steel structure. The type of steel structure is determined by the temperature range that transforms from austenite. If the chemical composition and steel structure are determined, the amount of strengthening is determined.

4) Next, experimental facts that serve as the basis for the present invention will be described.
Steel composition with 0.08C-0.1Si-1.5Mn-0.011P-0.002S-0.017Al-0.005N as the basic composition and Ti addition of 0.04% and steel B with 0.06% in the laboratory It was made into a slab by melting. These were made into 25 mm thick sheet bars by split rolling. This was heated at 1230 ° C., hot-rolled at a finishing temperature of 880 ° C. in 5 passes, and water-cooled at a cooling rate of 25 ° C./s 1.7 seconds after finish rolling. At this time, the cooling stop temperature was variously changed between 720-520 ° C. After cooling with water, it was allowed to cool for 10 seconds and then inserted into an electric furnace at 500 to 700 ° C. to perform a winding process, and the holding time in the furnace was changed between 1 and 300 minutes. At this time, when the difference between the cooling stop temperature and the furnace temperature is 30 ° C. or more, the water cooling is performed at a cooling rate of 25 ° C./s to 30 ° C. before the furnace temperature following the standing to cool. By the above method, hot-rolled steel sheets with various Ti precipitation states and steel structures were produced. These hot-rolled steel strips were pickled and subjected to temper rolling with an elongation of 0.5%, and then a tensile specimen and a precipitate analysis sample were collected.

From the group of hot-rolled steel sheets produced as described above, the one in which the amount of Ti contained in the precipitate having a size of less than 20 nm is 50% or more of Ti * represented by the following formula (1) is extracted, and polygonal ferrite is extracted. FIG. 1 shows the results of investigating the correlation between the fraction (%) and the tensile strength TS (MPa). As can be seen from this figure, the tensile strength TS tends to decrease as the polygonal ferrite fraction increases, but at a polygonal ferrite fraction of 70% or more, the fluctuation of TS becomes small and stabilizes.
The fraction of polygonal ferrite can be determined, for example, as follows. For the portion of the steel sheet with the L cross section (cross section parallel to the rolling direction) except the surface layer of 10%, the corrosion appearance structure with 5% nital is magnified 1000 times with a scanning electron microscope (SEM). Smooth ferrite grains with grain boundary irregularities of less than 0.1 μm and no corrosion marks in the grains are defined as polygonal ferrite, and other forms of ferrite phases and different transformation phases such as pearlite and bainite Distinguish. These are color-coded on the image analysis software, and the area ratio is defined as the polygonal ferrite fraction.
Similarly, from the group of hot-rolled steel sheets manufactured as described above, the one having a polygonal ferrite fraction of 70% or more is extracted, and the precipitate of less than 20 nm in size with respect to Ti * represented by the following formula (1) is extracted. FIG. 2 shows the result of investigating the correlation between the ratio (%) of the amount of Ti contained and the tensile strength TS (MPa). As described above, precipitates with a size of less than 20 nm that contribute to precipitation strengthening are formed by the added Ti. Therefore, if the amount of Ti in the precipitates of less than 20 nm is grasped, Ti can be efficiently converted into fine precipitates. This is because it can be clarified whether or not it is deposited. As can be seen from this figure, TS shows an increasing trend with an increase in the amount of Ti contained in precipitates with a size of less than 20 nm. However, when the amount of Ti contained in the precipitates is 50% or more of Ti *, there is a fluctuation in TS. Smaller and more stable.

From the above results, the steel structure is controlled to a fractional range where polygonal ferrite is 70% or more, and the amount of Ti contained in precipitates having a size of less than 20 nm is 50% of Ti * represented by the following formula (1). If the control is performed so as to be within the above range, even if the intensity variation inevitably occurs because the cooling history of the coil after winding varies depending on the position, the resulting intensity variation is remarkably reduced and there is no practical problem. I came up with what I could do.
Ti * = [Ti] −48 ÷ 14 × [N] (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively.

  Therefore, the requirement of the present invention, that is, the amount of Ti contained in the precipitate having a structure containing polygonal ferrite in a fraction of 70% or more and having a size of less than 20 nm is Ti * represented by the above formula (1). If the amount of the steel sheet is achieved at any position of the steel sheet, even if the coil cooling history varies from position to position, the amount of strengthening of the steel sheet at each position is almost the same. The steel sheet can be excellent in strength uniformity with small strength variation.

5) Further, the amount of Ti contained in the precipitate having a size of less than 20 nm can be measured by the following method.
After the sample is electrolyzed in a predetermined amount in the electrolytic solution, the sample piece is taken out of the electrolytic solution and immersed in a solution having dispersibility. Next, the precipitate contained in the solution is filtered using a filter having a pore diameter of 20 nm. Precipitates that have passed through the filter having a pore diameter of 20 nm together with the filtrate have a size of less than 20 nm. Next, the filtrate after filtration is analyzed by appropriately selecting from inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, etc., and Ti in precipitates with a size of less than 20 nm Find the amount of.

6) Next, an example of a preferable method for producing the high-strength hot-rolled steel sheet of the present invention will be described.
The composition of the steel slab used in the production method of the present invention is the same as that of the steel sheet described above, and the reason for the limitation is also the same. The high-strength hot-rolled steel sheet of the present invention can be produced by using a steel slab having a composition within the above-described range as a raw material, and subjecting the raw material to rough rolling to obtain a hot-rolled steel sheet.

B) Heating temperature is 1150 ℃ ~ 1300 ℃
The slab heating temperature is preferably 1150 ° C. or higher in order to prevent Ti-based carbides such as TiC from becoming insoluble in the heating stage. This is because it is preferable to avoid the Ti-based carbide from becoming insoluble since it adversely affects the tensile strength of the hot-rolled steel sheet. However, since heating at an excessive temperature causes problems such as an increase in scale loss accompanying an increase in oxidized weight, the upper limit of the slab heating temperature is preferably 1300 ° C.
The steel slab heated under the above conditions is subjected to hot rolling for rough rolling and finish rolling. Here, the steel slab is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be determined according to a conventional method. From the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling, it is preferable to use a so-called sheet bar heater that heats the sheet bar.
Next, the sheet bar is finish-rolled to obtain a hot-rolled steel sheet.

B) Finishing temperature (FDT) of 800-950 ° C
When the finishing temperature is high, the grains become coarse, the moldability is lowered, and scale defects are likely to occur. On the other hand, if the temperature is less than 800 ° C., the rolling load increases, the rolling load increases, the rolling rate of austenite unrecrystallized increases, an abnormal texture develops, and this is not preferable from the viewpoint of strength uniformity. In that sense, the finishing temperature is 800 ° C. or higher and 950 ° C. or lower. Preferably it is set as 840 to 920 degreeC.
Moreover, in order to reduce the rolling load at the time of hot rolling, lubrication rolling may be performed between some or all passes of finish rolling. Lubrication rolling is effective from the viewpoint of uniform steel plate shape and uniform strength. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25. Furthermore, it is also preferable to set it as the continuous rolling process which joins the sheet bar which precedes and follows, and carries out finish rolling continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.

C) Distortion accumulated during finish rolling if more than 2 seconds elapses before starting cooling after cooling finish rolling at a cooling rate (primary cooling) of 20 ° C / s or more within 2 seconds after hot finish rolling. Is released, and even if the cooling control described later is performed, the generation of ferrite does not occur effectively, and stable precipitation of TIC is not performed. Moreover, the same phenomenon is likely to occur when the cooling rate is lower than 20 ° C./s.

D) The cooling stop in the temperature range of 650 ° C to 750 ° C and the cooling temperature in the cooling process of 2 to 15 seconds effectively deposits Ti carbide such as TiC in a short time passing through the run-out table. In order to achieve this, it is necessary to maintain for a certain time in a temperature range where the ferrite transformation proceeds most. When the cooling (holding) temperature is lower than 650 ° C., the growth rate of precipitation of Ti-based carbides is small, so that the amount of Ti-based carbides necessary for the desired strengthening amount cannot be secured. On the other hand, when the cooling temperature is higher than 750 ° C., the nucleation of precipitation is not sufficient and the growth rate is fast, so that the Ti-based carbides are sparse and coarsely distributed, so that the strengthening ability is reduced. Therefore, the cooling temperature is set to 650 ° C to 750 ° C.
When the cooling time is shorter than 2 seconds, the amount of Ti-based carbide deposited is not sufficient, and it is difficult to secure the necessary strengthening amount. On the other hand, when the cooling time is longer than 15 seconds, the Ti-based carbides are sparse and coarsely distributed, so that the strengthening ability is reduced. Therefore, the cooling time is 2 seconds to 15 seconds.

E) When the cooling rate following the cooling and cooling process is 100 ° C / s or higher again at a cooling rate of less than 100 ° C / s (secondary cooling), the controllability of the coiling temperature becomes worse and it becomes difficult to stabilize the strength. Become. Therefore, it shall be less than 100 ° C / s. The lower limit of the cooling rate is not particularly limited, but is preferably 5 ° C./s or more from the viewpoint of suppressing the coarsening of the precipitate.

F) When the coiling temperature in the temperature range of 550 to 650 ° C. is less than 550 ° C., an untransformed part is generated as a low-temperature transformation phase on the run-out table and causes variation in strength. Ductility decreases. When the coiling temperature exceeds 650 ° C, the growth of Ti-based carbides such as TiC proceeds even after coiling and is distributed sparsely and coarsely, so the strengthening ability is reduced and the cooling history after coiling is reduced. Corresponding strength variations are likely to occur. Accordingly, the winding temperature is 550 to 650 ° C.

  When the strength variation is taken into consideration in the coil, for example, precipitation of Ti carbide such as TiC mainly proceeds in the cooling stage after winding, so it is desirable to consider the cooling history of the steel sheet after winding. In particular, the precipitation of Ti-based carbides may not sufficiently proceed at the leading end and the trailing end of the coil due to rapid cooling. For this reason, in the coil front end portion and the rear end portion, when the temperature is increased with respect to the inside of the coil other than the front end portion and the rear end portion, the strength variation is further improved.

Next, examples of the present invention will be described.
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. These steel slabs were heated to 1250 ° C. and roughly rolled into sheet bars, and then hot rolled steel sheets were formed by a hot rolling process in which finish rolling under the conditions shown in Table 2 was performed.
Next, these hot-rolled steel sheets were pickled and subjected to temper rolling with an elongation of 0.5%, and then 10 mm in the end in the width direction was trimmed and removed to evaluate various properties. A steel plate was sampled from a position where the innermost and outermost windings were cut at the front end and rear end of the coil, and a dividing point obtained by dividing the inside into 20 equal parts in the longitudinal direction. Tensile specimens and precipitate analysis samples were collected from these width ends and dividing points divided into 8 in the width direction.

Tensile test specimens were taken in the direction parallel to the rolling direction (L direction) and processed into JIS No. 5 tensile specimens. A tensile test was performed at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241 to determine the tensile strength (TS). Table 2 shows the results of examining the tensile properties of the obtained hot-rolled steel sheets.
The microstructure of the L section (cross section parallel to the rolling direction) excluding the surface layer of 10% is identified by magnifying the corrosion appearance structure by nital by 5,000 times with a scanning electron microscope (SEM). The ferrite fraction was measured using image processing software by the method described above.
The quantitative determination of Ti in the precipitate having a size of less than 20 nm was carried out by the following quantitative method.
The hot-rolled steel sheet obtained as described above is cut to an appropriate size, and about 0.2 g in a 10% AA electrolyte solution (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol) has a current density of 20 mA / cm ^2. And constant current electrolysis.
After the electrolysis, remove the sample piece with deposits on the surface from the electrolyte and immerse it in an aqueous solution of sodium hexametaphosphate (500 mg / l) (hereinafter referred to as the SHMP aqueous solution) to apply ultrasonic vibration. The precipitate was peeled off from the sample piece and extracted into an aqueous SHMP solution. Next, the SHMP aqueous solution containing the precipitate was filtered using a filter with a pore size of 20 nm, and the filtrate after filtration was analyzed using an ICP emission spectrometer, and the absolute amount of Ti in the filtrate was measured. . Next, the absolute amount of Ti was divided by the electrolytic weight to obtain the amount (mass%) of Ti contained in the precipitate having a size of less than 20 nm. In addition, the electrolysis weight was calculated | required by measuring a weight with respect to the sample after deposit peeling, and subtracting from the sample weight before electrolysis. After this, the amount of Ti (mass%) contained in the precipitate having a size less than 20 nm obtained above was calculated by substituting the Ti and N contents shown in Table 1 into the formula (1). To obtain the ratio (%) of the amount of Ti contained in the precipitate having a size of less than 20 nm.

Here, among the results shown in Table 2, the fraction of polygonal ferrite, the ratio of the amount of Ti contained in precipitates with a size of less than 20 nm with respect to Ti * represented by the formula (1), and the tensile strength TS are the longitudinal center of the coil. The value at the center of the width is used as a representative value. Moreover, the steel structure conformity ratio is a ratio of points satisfying both requirements of the measured 189 points, that is, the ratio of polygonal ferrite and the ratio of Ti amount in precipitates having a size of less than 20 nm. The TS conformity rate is a ratio showing a value of 540 MPa or more out of 189 measured points. ΔTS is a value obtained by multiplying the standard deviation σ by four times from the measured TS of 189 points.
As is clear from the investigation results shown in Table 2, in all of the examples of the present invention, TS has a high strength of 540 MPa or more, and strength variation (ΔTS) in the coil surface is as small as 50 MPa or less. A good steel sheet is obtained.

  According to the present invention, it is possible to stably produce a hot-rolled steel sheet having a tensile strength (TS) of 540 MPa or more and a small variation in strength at a low cost, and has a remarkable industrial effect. For example, when the high-strength hot-rolled steel sheet of the present invention is applied to automobile parts, the amount of springback after forming in high tension and the variation in collision characteristics can be reduced, making it possible to increase the accuracy of vehicle body design, and collision safety of automobile bodies There is an effect that it can sufficiently contribute to performance and weight reduction.

It is a figure which shows the result of having investigated the correlation with the fraction (%) of polygonal ferrite, and tensile strength TS (MPa). FIG. 6 is a diagram showing the results of investigating the correlation between the ratio (%) of the amount of Ti contained in precipitates having a size of less than 20 nm with respect to Ti * and the tensile strength TS (MPa).

Claims (2)

Component composition is mass%, C: 0.05-0.12%, Si: 0.5% or less, Mn: 0.8-1.8%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01 % Or less, Ti: 0.030 to 0.080%, the remainder is composed of Fe and inevitable impurities, has a structure containing polygonal ferrite in a fraction of 70% or more, and exists in precipitates with a size of less than 20 nm A high-strength hot-rolled steel sheet characterized in that the amount of Ti to be processed is 50% or more of the value of Ti * calculated by the following formula (1).
Ti * = [Ti] −48 ÷ 14 × [N] (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively.   Component composition is mass%, C: 0.05-0.12%, Si: 0.5% or less, Mn: 0.8-1.8%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01 % Steel, Ti: 0.030-0.080%, the remainder of the steel slab consisting of Fe and inevitable impurities is heated to a heating temperature of 1150-1300 ° C and then hot-finished rolling at a finishing temperature of 800-950 ° C The cooling is started at a cooling rate of 20 ° C./s or more within 2 seconds after the hot finish rolling, the cooling is stopped at a temperature of 650 ° C. to 750 ° C., and then the cooling process is performed for 2 seconds to 15 seconds. After that, a method for producing a high-strength hot-rolled steel sheet, which is cooled again at a cooling rate of less than 100 ° C./s and wound in a coil shape in a temperature range of 550 to 650 ° C.

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