JP5453973B2 - High-strength cold-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength (TS) of 880 MPa or more, which is useful for the use of automobile frame members and the like, and a manufacturing method thereof.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, there is an urgent need to improve the fuel efficiency of automobiles, and there is a demand for weight reduction by using thinner materials. 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. The weight reduction effect increases as the steel sheet used has higher strength.

Solid solution strengthening, precipitation strengthening, transformation strengthening, etc. are widely used to increase the strength of steel sheets. Among these, precipitation strengthening is an advantageous strengthening method in terms of manufacturing cost because high strength can be obtained with a small amount of added alloy. As precipitation-strengthened steel, steel sheets with Ti, V, and Nb added are widely known, and dislocation movement is prevented by finely dispersing Ti, V, and Nb carbonitrides in the steel.
For example, Patent Document 1 proposes a manufacturing method in which a steel containing Nb and Ti, which are precipitation strengthening elements, is hot-rolled and an annealing process is provided to reduce variation in the coil.
Patent Document 2 proposes a method in which precipitates generated by controlling the cooling rate during casting are divided during cold rolling and distributed in rows.
Patent Document 3 discloses a method for producing a cold-rolled steel sheet having high strength and high workability by forming a metal structure containing a certain amount or more of retained austenite and precipitates in ferrite.

JP 2007-239097 JP 2008-308718 JP 2008-291314 A

However, the above prior art has the following problems.
In the method described in Patent Document 1, no cold rolling process is provided before the annealing process. Therefore, strengthening of the steel sheet cannot be expected because the strain energy which becomes a driving force for newly depositing precipitates is insufficient.
In the method described in Patent Document 2, since the precipitates are divided by rolling, the distribution and size of the precipitates are biased, and variations in the coil plane cannot be ignored.
In the method of Patent Document 3, by providing an annealing process at a high temperature, precipitates in the ferrite become coarse and the ferrite becomes soft, and a high yield strength cannot be obtained because the low-temperature transformation phase is utilized.
Furthermore, in any patent document, precipitation strengthening is determined by the amount and size of the precipitate, so that the precipitate is deposited to the maximum with respect to the precipitation strengthening element content, and further, coarsening of the precipitate is suppressed. However, the strengthening amount relative to the precipitation strengthening element content, that is, the precipitation efficiency relative to the precipitation strengthening element content is not considered.

  In view of such circumstances, the present invention advantageously solves the above problems, and uses an inexpensive Ti-based general-purpose steel plate without using expensive elements such as Ni, Nb, and Mo, and has a tensile strength (TS) of 880 MPa or more. An object of the present invention is to provide a high-strength cold-rolled steel sheet having a total elongation (El) of 15% or more and advantageous in production cost, and a production method thereof.

  As a result of diligent studies to solve the above problems, the amount of strengthening is large relative to the contained Ti by controlling the chemical composition of the steel sheet, the metal structure, and the precipitation state of Ti that contributes to precipitation strengthening. The present inventors have succeeded in obtaining a high-strength cold-rolled steel sheet having a high precipitation efficiency with respect to the content of the precipitation strengthening element, and have completed the present invention.

The summary of the high-strength cold-rolled steel sheet and the manufacturing method thereof according to the present invention is as follows.
[1] Component composition is mass%, C: 0.03-0.10%, Si: 0.5% or less, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01% or less, Ti: 0.035 to 0.090%, the balance is composed of Fe and inevitable impurities, the ferrite phase is contained in a fraction of 70% or more, and the aspect ratio of the ferrite grains is 10.0 or more A high-strength cold-rolled steel sheet characterized in that the amount of Ti present in a precipitate having a size of less than 20 nm is 70% or more of the value of Ti * calculated by the following formula (1).
Ti * = [Ti] −48 × [N] ÷ 14 ... Formula (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: 0.03-0.10%, Si: 0.5% or less, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01% or less, Ti: 0.035-0.090% steel slab containing Fe and unavoidable impurities in the balance, heated to 1200-1300 ° C, then hot at 800-950 ° C finishing temperature Perform finish rolling, start cooling at a cooling rate of 25 ° C./s or more within 2 seconds after the hot finish rolling, stop cooling at a temperature of 650 ° C. to 720 ° C., and continue for 2 to 30 seconds. After passing through the cooling process, it is cooled again at a cooling rate of 25 ° C / s or more, wound at a temperature of 650 ° C or less, and then cold-rolled at a rolling rate of 50% or more, 500 to 650 ° C. A method for producing a high-strength cold-rolled steel sheet, characterized by annealing at a temperature of 5 ° C.

In the present specification, “%” indicating the component of steel is “% by mass”. The high-strength cold-rolled steel sheet in the present invention is a steel sheet having a tensile strength (hereinafter sometimes referred to as TS) of 880 MPa or more. Surface treated steel sheets that have been treated are also targeted.
Further, the target characteristics of the present invention are that the total elongation (El) is 15% or more and the amount of increase in the yield point (ΔYS) due to the inclusion of the precipitation strengthening element is 7000 MPa /% or more.

  According to the present invention, the tensile strength (TS) is 880 MPa or more, the total elongation (El) is 15% or more, and the increase in the yield point (ΔYS) due to the inclusion of the precipitation strengthening element is 7000 MPa /% or more. A strength cold-rolled steel sheet is obtained. The high-strength cold-rolled steel sheet according to the present invention is capable of obtaining the maximum precipitation strengthening amount with respect to the amount of Ti contained while having high strength and high workability due to the above characteristics, and is excellent in production cost Become. Furthermore, in the present invention, the above-described effect can be obtained without containing expensive raw materials such as Nb, so that the cost can be reduced.

The present invention is described in detail below.
First, the reason for limiting the chemical component (component composition) of steel in the present invention will be described.

C: 0.03-0.10%
C is an important element in the present invention together with Ti described later. C forms carbide with Ti and is effective in increasing the strength of the steel sheet by precipitation strengthening. In the present invention, 0.03% or more of C is contained from the viewpoint of precipitation strengthening. On the other hand, if it exceeds 0.10%, the ductility is lowered and the toughness and stretch flangeability are adversely affected. Therefore, the upper limit of the C content is 0.10%, and the preferable C range is 0.035 to 0.09%.

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 preferable to contain Si 0.01% or more. On the other hand, when Si exceeds 0.5%, surface defects called red scale are likely to occur during hot rolling, which deteriorates the surface appearance of the steel sheet and adversely affects fatigue resistance and toughness. May have an effect. Therefore, the Si content is 0.5% or less. Preferably it is 0.3% or less.

Mn: 0.8-2.0%
Mn is effective for increasing the strength and has the effect of lowering the transformation point and refining the ferrite grain size, so it is necessary to contain Mn in an amount of 0.8% or more. Preferably it is 1.0% or more. On the other hand, if it contains excessive Mn exceeding 2.0%, a low-temperature transformation phase is generated after hot rolling and the ductility is lowered, or precipitation of Ti-based carbides described later tends to become unstable, resulting in a large variation in strength. Therefore, the upper limit of the Mn content is 2.0%, preferably 1.5%.

P: 0.030% or less P is an element effective for strengthening solid solution. However, if the P content exceeds 0.030%, P tends to segregate at the grain boundaries, and the 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 is present as an inclusion in the steel, and deteriorates various properties of the steel sheet such as causing hot cracking. Therefore, it must be reduced as much as possible. Specifically, the S content is 0.01% or less because it is acceptable up to 0.01%. Preferably it is 0.005% or less.

Al: 0.005-0.1%
Al is useful as a deoxidizing element for steel. In order to exert such an effect, the Al content needs to be 0.005% or more. On the other hand, the content of Al exceeding 0.1% leads to a high alloy cost and further easily induces surface defects, so the upper limit of the Al content is set to 0.1%.

N: 0.01% or less When N content increases, normal temperature aging resistance deteriorates and precipitates as coarse Ti-based nitride that contributes little to improving mechanical properties, so a large amount to fix solute N Al and Ti must be included. Therefore, it is preferable to reduce as much as possible, and the upper limit of N content is 0.01%.

Ti: 0.035-0.090%
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.
In order to obtain a high-strength steel sheet having a tensile strength TS of 880 MPa or more, it is preferable to refine the precipitate so that the size of the precipitate is less than 20 nm. It is also important to increase the proportion of the fine precipitates (precipitate size less than 20 nm). This is because 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 the ferrite may not be sufficiently hardened, so that the strength may be lowered. Therefore, the size of the precipitate is preferably less than 20 nm. And the precipitate containing fine Ti less than 20 nm is formed by containing 0.035% or more and 0.090% or less of Ti and 0.03 to 0.10% of C.
In the present invention, these 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, the Ti-based carbide may contain N as a composition, or may be precipitated in combination with Mn, S and the like.
Furthermore, in the high-strength steel sheet of the present invention, it has been confirmed that Ti-based carbides are mainly precipitated in ferrite. This is presumably because the solid solubility limit of C in ferrite is small, and supersaturated C tends to precipitate as carbides in ferrite. For this reason, soft ferrite is hardened by such precipitates, and a tensile strength (TS) of 880 MPa or more can be obtained. At the same time, Ti is easily combined with solute N, and TiN is coarse and does not contribute to strengthening. Therefore, Ti is also a preferable element for fixing solute N. From this point of view, Ti is made 0.035% or more.
However, excessive Ti content 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. Therefore, the upper limit of Ti is 0.090%.
In the present invention, the balance other than the above-described components is composed of iron and inevitable impurities.

Next, the reason why the steel structure of the high-strength cold-rolled steel sheet of the present invention is limited will be described.
The amount of Ti present in the precipitate having a ferrite phase content of 70% or more, a ferrite grain aspect ratio of 10.0 or more, and a size less than 20 nm is expressed by the following formula (1). 70% or more of the calculated Ti * value.
Ti * = [Ti] −48 × [N] ÷ 14 ... Formula (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively.

In the steel of the present invention, precipitates having a good thermal stability that are less than 20 nm in size and hardly cause coarsening in the annealing process mainly occur simultaneously with the ferrite transformation. Furthermore, when the ferrite phase fraction is less than 70%, the ductility decreases, and the target total elongation (El) of 15% or more cannot be secured. For this reason, in order to effectively precipitate, strengthen, and maintain the ductility of the precipitate with respect to the Ti content, it is necessary to have a structure including a ferrite phase in a fraction of 70% or more. The second phase structure at this time may be pearlite, martensite, bainite, or the like, but is not particularly defined.
Moreover, when examined, it turned out that the amount of Ti which precipitates at the time of completion | finish of hot rolling is 50 to 70% with respect to the amount of Ti contained. Therefore, it became clear that further precipitation strengthening can be performed on Ti in a solid solution state using the strain energy introduced by cold rolling and the thermal energy applied in the annealing process as the driving force. .
Here, there is a correlation between strain energy and the degree of extension of ferrite grains. Therefore, in order to accumulate the strain energy necessary for precipitating Ti after cold rolling, the degree of extension of ferrite grains was examined. As a result, it was found that if the ratio of the major axis to the minor axis of the ferrite grains, that is, the aspect ratio is 10.0 or more, the strain energy necessary for precipitating Ti after cold rolling can be accumulated sufficiently. In order to obtain ferrite grains having an aspect ratio of 10.0 or more, it is necessary that the cold rolling reduction rate is 50% or more.
Furthermore, the amount of strengthening by precipitation strengthening is determined by the size and dispersion of the precipitates (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. That is, for effective strengthening, it is necessary to deposit Ti as fine precipitates having a size of less than 20 nm as much as possible. As a result of the above examination, the amount of Ti present in the precipitate having a size of less than 20 nm is set to 70% or more of the value of Ti * calculated by the above equation (1).

The method for deriving the ferrite phase fraction and the ferrite grain aspect ratio is as follows. The ferrite phase fraction was obtained by photographing 10-field images of a corrosion-induced microstructure with 5% nital at a magnification of 1000 times with a scanning electron microscope at the center of the thickness of the cross section (L cross section) parallel to the rolling direction. In the image analysis software, a smooth crystal grain with no trace of corrosion is defined as ferrite, and different transformation phases such as pearlite and bainite are used. The area ratio of the ferrite phase was used as the fraction of the ferrite phase.
The aspect ratio of the ferrite grain is the L-section plate thickness center, and the corrosion appearance structure by 5% nital is observed with a scanning electron microscope at 400 times, Image-`Pro PLUS Ver. 4.0.0.11 (Media Cybernetics) Assuming an ellipse (feature equivalent ellipse) that has the same area and the same moment of inertia as the ferrite phase observed by image analysis processing, the ellipse major axis length and minor axis length were calculated for each ellipse. . The aspect ratio was the ellipse major axis length / ellipse minor axis length. The average value of the aspect ratios of the individual ferrite phases obtained by such a method was used as the aspect ratio of the ferrite phase.

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 are less than 20 nm in size. Next, the filtrate after filtration is analyzed by appropriately selecting from inductively coupled plasma (ICP) emission spectrometry, ICP mass spectrometry, atomic absorption spectrometry, etc., and precipitation with a steel composition size of less than 20 nm The amount of Ti in the product (mass% when the total composition of the sample is 100 mass%) is determined. Thereafter, the amount of Ti (mass%) contained in the precipitate obtained above with a size of less than 20 nm is divided by Ti * calculated by substituting the content of Ti and N into the following formula (1), The ratio (%) of the amount of Ti contained in precipitates of size less than 20 nm.
Ti * = [Ti] −48 × [N] ÷ 14 ... Formula (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively.

Next, an example of a preferable method for producing the high-strength cold-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 cold-rolled steel sheet of the present invention uses a steel slab having a composition in the above-described range as a raw material, and performs rough rolling on the raw material to obtain a cold-rolled steel sheet through a hot rolling process, a cold rolling process, and an annealing process. Can be manufactured.

B) As one of the purposes of heating the steel slab at a heating temperature of 1200 ° C to 1300 ° C before hot rolling, the coarse Ti-based carbide produced before continuous casting is re-dissolved in the steel. Is mentioned. When the heating temperature is lower than 1200 ° C., the solid solution state of the precipitate becomes unstable, and the amount of fine Ti-based carbide generated in the subsequent process becomes non-uniform. Therefore, the lower limit of the heating temperature is 1200 ° C. On the other hand, heating exceeding 1300 ° C has an adverse effect of increasing scale loss on the slab surface, so the upper limit is set to 1300 ° C.
Next, 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 performed according to a conventional method. Further, from the viewpoint of securing the finish rolling temperature and preventing troubles during hot rolling, it is preferable to utilize 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
If the finishing temperature is less than 800 ° C, the rolling load increases, the rolling rate increases in the austenite non-recrystallization temperature region, abnormal texture develops, and coarse precipitates due to strain-induced precipitation of Ti-based carbides. Is not preferable. On the other hand, if the finishing temperature exceeds 950 ° C., the ferrite grain size becomes coarse, and the formability deteriorates and a scale defect occurs. 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. Lubricating 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) Start cooling at a cooling rate of 25 ° C / s or more within 2 seconds after hot finish rolling (primary cooling) at a cooling rate of 25 ° C / s or more within 2 seconds after hot finish rolling. If it takes more than 2 seconds to start cooling after finish rolling, the strain accumulated during finish rolling is released, and ferrite grains become coarse and strain-induced precipitation of coarse Ti carbide occurs. . The same phenomenon is likely to occur when the cooling rate is less than 25 ° C./s.

D) Cooling is stopped in the temperature range of 650 ° C to 720 ° C, and then allowed to cool for 2 to 30 seconds.
By providing a cooling process at 650 to 720 ° C., it is possible to obtain fine precipitates having a size of less than 20 nm. Furthermore, the precipitates that precipitate with the ferrite transformation have good thermal stability and can suppress coarsening of the precipitates in the subsequent annealing step. When it is allowed to cool below 650 ° C., the growth of ferrite grains is hindered, and Ti carbide is less likely to precipitate. On the other hand, if it is allowed to cool above 720 ° C., it will lead to an adverse effect of coarsening of ferrite grains and Ti-based carbides. In addition, the minimum cooling time for obtaining a ferrite fraction of 70% or more with the steel of the present invention is 2 seconds, and cooling for more than 30 seconds decreases the strength due to the coarsening of the Ti-based carbide, and the strength within the coil plane Variations increase.
From the above, the cooling is stopped at a temperature of 650 ° C. to 720 ° C., and then allowed to cool for 2 to 30 seconds.

E) Cool again at a cooling rate of 25 ° C / s or higher (secondary cooling). Cool again at a cooling rate of 25 ° C / s or higher. In order to maintain the state of the fine Ti-based carbide stably obtained by the above-described process, a large cooling rate is required. Therefore, the lower limit of the cooling rate is 25 ° C / s.

F) Winding at a temperature of 650 ° C or less
Wind at a temperature of 650 ° C or lower. When the coiling temperature exceeds 650 ° C., the size of the precipitate becomes coarse, which causes a decrease in strength. The lower limit of the winding temperature is not particularly defined because it does not cause a variation in strength with respect to the winding temperature on the low temperature side.

G) Cold rolling is performed, and cold rolling is performed at a temperature of 500 to 650 ° C., whereby strain energy is accumulated in the steel sheet, and solid solution Ti is precipitated by using the energy as a driving force. The cold rolling at that time may be performed by a conventional method. However, in order to obtain the ferrite grain aspect ratio of 10.0 or more, the rolling rate needs to be 50% or more, and preferably 60% or more.
Next, precipitates are obtained by performing annealing at a temperature of 500 to 650 ° C. on the steel sheet into which strain energy has been introduced by cold rolling. Continuous annealing is a desirable manufacturing method because it has a higher manufacturing efficiency than batch annealing. When the annealing temperature is below 500 ° C., the diffusion rate of the precipitated elements is slow, and many precipitates cannot be obtained particularly in the continuous annealing process. On the other hand, when the annealing temperature exceeds 650 ° C., the precipitate becomes coarse and the precipitation strengthening amount decreases. Therefore, the temperature range necessary for obtaining fine precipitates and suppressing coarsening is 500 to 650 ° C. In the continuous annealing step, hot dip galvanizing or alloying hot dip galvanizing may be performed simultaneously.

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. After reheating these steel slabs at 1250 ° C, they are roughly rolled into sheet bars, subjected to finish rolling at 830 to 1000 ° C, and after finishing rolling, cooled at a cooling rate of 20 to 35 ° C / s after 1.5 to 3 seconds. The cooling was stopped at a temperature of 600 to 780 ° C., and after passing through the cooling process for 5 to 60 seconds, the cooling was again performed at a cooling rate of 20 to 110 ° C./s, and in the range of 500 to 680 ° C. A coiled hot-rolled steel sheet was obtained. Next, cold rolling was performed on the obtained hot-rolled steel sheet at a rolling rate of 46 to 62%, and continuous annealing was performed at a heating temperature of 400 to 700 ° C. to obtain a cold-rolled steel sheet. The details of the above manufacturing conditions are shown in Table 2.

  Various characteristics were evaluated with respect to the cold-rolled steel sheet obtained by the above. The sample was collected at the central portion in the plate width direction at the rear end portion excluding the outermost winding in the longitudinal direction of the coil.

  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).

  The value of ΔYS / Ti * was used as an index of precipitation strengthening amount. ΔYS is a value obtained by subtracting the yield point (or 0.2% yield strength) of steel not containing Ti from the yield point (or 0.2% yield strength) obtained from the tensile test. The yield point of steel not containing Ti is the value obtained from steel A based on steel A in Table 1. The effect of Mn, a solid solution strengthening element, was corrected using the Pickering equation. Values were used. This parameter is the amount of precipitation strengthening element that can be precipitated as fine precipitates (Ti *) (Ti * = [Ti] −48 × [N] ÷ 14 (1) where [Ti] and [N] Indicates strength against the Ti and N component compositions (mass%) of the steel sheet, respectively, and the higher the cost, the better the cost. In particular, the ratio of precipitates with a size of 20 nm to ΔYS / Ti * and Ti * values has a strong correlation. Since the steel sheet with good precipitation efficiency and fine precipitates has high ΔYS / Ti *, it can be said that the production cost is excellent because the amount of strengthening with respect to the contained precipitation strengthening element is large. In addition, the ratio of precipitates with a size of 20 nm to the Ti * value correlates with the ferrite fraction and the ferrite aspect ratio.

  The microstructure of the L-section (cross section parallel to the rolling direction) of ± 17% of the center of the plate thickness was compared to 16 fields of view where the corrosion appearance structure by Nital was expanded 400 times with a scanning electron microscope (SEM). I went.

  The ferrite fraction was determined as follows. For the portion of the steel sheet excluding the 10% surface layer of the L section (cross section parallel to the rolling direction), the corrosion appearance structure by 5% nital is magnified 100 times with a scanning electron microscope (SEM). A smooth crystal grain with no trace of corrosion remaining in the grain is defined as ferrite, and different transformation phases such as other forms of pearlite and bainite are distinguished. These are color-coded on the image analysis software, and the area ratio is used as the ferrite fraction. The ferrite grain size is a cutting method compliant with JIS G 0551, and for each photo taken at a magnification of 100 times, three vertical and horizontal lines are drawn to calculate the average grain size. The particle size was determined.

  The aspect ratio of the ferrite grain is the L-section plate thickness center, and the corrosion appearance structure by 5% nital is observed with a scanning electron microscope at 400 times, Image-`Pro PLUS Ver. 4.0.0.11 (Media Cybernetics) Assuming an ellipse (feature equivalent ellipse) that has the same area and the same moment of inertia as the ferrite phase observed by image analysis processing, the ellipse major axis length and minor axis length were calculated for each ellipse. . The aspect ratio was the ellipse major axis length / ellipse minor axis length. The average value of the aspect ratios of the individual ferrite phases obtained by such a method was used as the aspect ratio of the ferrite phase.

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 cold-rolled steel sheet obtained above is cut to an appropriate size, and about 0.2 g in a 10% AA electrolyte (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 of Ti contained in the precipitate having a size of less than 20 nm (mass% when the total composition of the sample was 100 mass%). 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.

  Table 3 shows the results of investigation on the tensile properties, microstructure, and precipitates of each cold-rolled steel sheet obtained as described above.

  As apparent from Table 3, the steel sheets of the inventive examples all have TS of 880 MPa or more, total elongation (El) of 15% or more, and ΔYS with respect to Ti * amount of 7000 MPa /% or more. A steel plate with high strength is obtained by effectively precipitating. Furthermore, it can be seen that the ferrite fraction and ductility, the amount of fine precipitates, and the amount of strengthening are closely related. From these results, in the present invention, it is possible to effectively utilize the contained precipitation strengthening element, and it is possible to produce a high-strength cold-rolled steel sheet with a low production cost.

Claims (2)

Component composition is mass%, C: 0.03-0.10%, Si: 0.5% or less, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01 % Or less, containing Ti: 0.035 to 0.090%, with the balance being Fe and inevitable impurities, including a ferrite phase in a fraction of 70% or more, and having an aspect ratio of ferrite grains of 10.0 or more, A high-strength cold-rolled steel sheet characterized in that the amount of Ti present in a precipitate having a size of less than 20 nm is 70% or more of the value of Ti * calculated by the following formula (1).
Ti * = [Ti] −48 × [N] ÷ 14 ... Formula (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively. Component composition is mass%, C: 0.03-0.10%, Si: 0.5% or less, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01 %, Ti: 0.035-0.090% steel slab with Fe and inevitable impurities remaining, heated to 1200-1300 ℃ heating temperature, then hot-finished rolling at 800-950 ℃ finishing temperature The cooling is started at a cooling rate of 25 ° C./s or more within 2 seconds after the hot finish rolling, the cooling is stopped at a temperature of 650 ° C. to 720 ° C., and then the cooling process is performed for 2 seconds to 30 seconds. After passing through, it is cooled again at a cooling rate of 25 ° C./s or more, wound at a temperature of 650 ° C. or less, then cold-rolled at a rolling rate of 50% or more, and at a temperature of 500 to 650 ° C. The amount of Ti present in the precipitate having a size of less than 20 nm is characterized by having a structure in which the ferrite phase is included in a fraction of 70% or more and the aspect ratio of the ferrite grains is 10.0 or more. ,under Method for producing a high strength cold rolled steel sheet is more than 70% of the value of Ti * calculated in (1).
Ti * = [Ti] −48 × [N] ÷ 14 ... Formula (1)
Here, [Ti] and [N] indicate the component composition (mass%) of Ti and N of the steel sheet, respectively.