JP4767652B2 - Hot-rolled steel strip for cold-rolled high-tensile steel sheet with small thickness variation after cold rolling and method for producing the same - Google Patents

Description

  The present invention relates to a hot-rolled steel strip for cold-rolled high-tensile steel sheet having a tensile strength of 590 MPa or more and a method for producing the same, and in particular, cold-rolled hot-rolled steel strip and having a tensile strength of 590 MPa or more. The present invention relates to a hot-rolled steel strip having a small thickness variation after cold rolling and a manufacturing method thereof when manufacturing a high-tensile steel plate.

  A cold-rolled high-tensile steel plate (referred to as “HITEN”) is manufactured by cold-rolling a hot-rolled steel strip and subjecting it to heat treatment as necessary.

  In recent years, the application of cold-rolled high-tensile steel sheets to structural members such as automobile members has expanded, and in particular, the demand for cold-rolled high-tensile steel sheets having a tensile strength of 590 MPa or more has increased. In order to increase the accuracy of molded parts manufactured from steel plates, cold-rolled high-tensile steel plates with high plate thickness accuracy are required by customers.

  However, when a hot-rolled steel strip, which is a raw material of a high-tensile steel plate, is cold-rolled, there arises a problem that the plate thickness varies after cold-rolling.

  Thickness control in cold rolling is conventionally performed by controlling the roll crown, reduction amount, tension, etc., but with cold rolled high-tensile steel plates, strict thickness accuracy is required only with conventional thickness control. Cannot handle enough. As a technique for manufacturing a cold-rolled high-tensile steel sheet having a small thickness variation, there is a manufacturing method for reducing the strength variation of the hot-rolled plate by increasing the uniformity of cooling after hot rolling.

  For example, in the cooling zone between the hot-rolling final pass stand and the winding, the steel plate temperature distribution in the plate width direction is within 50 ° C. at any position in the cooling zone line direction, and at any position in the cooling zone line direction. The steel strip is cooled to produce a hot-rolled steel strip so that the fluctuation width of the cooling rate at a certain position in the plate width direction of the steel plate passing through that position is within 5 ° C / second. There is a method of manufacturing a cold-rolled steel sheet having a small thickness variation by performing cold rolling using it (see, for example, Patent Document 1).

  Moreover, as a cooling method after the finish rolling of the hot-rolled steel strip, a cooling method for cooling to a uniform temperature in the width direction of the steel strip and obtaining a uniform steel strip in the width direction (for example, see Patent Document 2) There is a method of cooling the hot-rolled steel strip while masking the edge portion of the steel strip in order to prevent overcooling of the edge portion (for example, see Patent Document 3).

  Furthermore, in order to make the material in the longitudinal direction of the steel sheet uniform, there is a method of accelerating and cooling the steel sheet at a constant acceleration rate from the time when the steel sheet tip enters the cooling device to the time when the steel sheet tail end exits the cooling device. Yes (see, for example, Patent Document 4).

  However, the plate thickness variation which is a problem in the present invention occurs at a portion corresponding to the tail end portion of the hot-rolled steel strip used for the production of the cold-rolled high-tensile steel plate as shown in FIG. Because this is due to temperature fluctuations that occur later, the technology aimed at making the material in the width direction and longitudinal direction of the steel strip before winding mentioned above uniform prevents sheet thickness fluctuations during cold rolling. It is not possible.

  Therefore, when cold-rolling a hot-rolled steel strip into a cold-rolled high-strength steel sheet, it is necessary to remove (yield off) the plate thickness fluctuation part that has been generated after cold rolling so far to make a product. Is the current situation. When the yield drop occurs, the production efficiency is lowered, and therefore, a stable production technique for a high-tensile steel sheet with small thickness variation is desired.

JP 2004-276090 A JP 2003-191005 A Japanese Patent Laid-Open No. 2004-290988 JP 2005-154841 A

  Then, in view of the said present condition, this invention makes it a subject to provide the hot-rolled steel strip which can manufacture the cold-rolled high-tensile steel plate with the small board | plate thickness fluctuation | variation after cold rolling, and its manufacturing method. .

  Until now, as a countermeasure to fluctuations in sheet thickness of cold-rolled high-tensile steel sheets, it is possible to control the sheet thickness in the cold rolling process and increase the hardness of the hot-rolled steel strip, which is the raw material, by increasing the uniformity of the temperature before winding. Although there is a method of controlling, etc., none of them has led to a fundamental solution of the plate thickness fluctuation shown in FIG.

  The present inventor has investigated the mechanism of occurrence of sheet thickness fluctuations, and by controlling the structure of the hot-rolled steel strip, the sheet thickness fluctuations after cold rolling, especially the cold-rolling high tension with small fluctuations at the tail end. The invention of the hot-rolled steel strip for steel plate and the manufacturing method thereof was completed.

  The gist of the present invention is as follows.

( 1 ) The steel strip component is mass%,
C: 0.05 to 0.22%,
Si: 0.3 to 2.0%,
Mn: 1.3-3.2%
P: 0.025% or less,
S: 0.015% or less,
Al: containing 0.08% or less, the balance Ri Fe der except inevitable impurities, at least 200m below the range of the tail of the hot-rolled coil, a bainite main tissue, pearlite fraction in the tissue Is a hot-rolled steel strip for cold-rolled high-tensile steel sheet with small sheet thickness variation after cold rolling, characterized in that the difference in the longitudinal direction of the pearlite fraction is 10% or less.

( 2 ) The steel strip is further mass%,
Nb: 0.005 to 0.10% or less,
Ti: 0.03 to 0.20% or less,
V: 0.005 to 0.10% or less,
Mo: 0.02 to 0.5% or less,
Cr: 0.1 to 5.0% or less,
Co: 0.01 to 5.0% or less,
W: 0.01 to 5.0% or less,
The hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling as described in ( 1 ) above, comprising one or more of the following.

( 3 ) The steel strip is further mass%,
Mg, 1 kind or two kinds respectively 0.0005% or more Zr, small thickness variation after cold rolling according to (1) or (2), characterized in that it contains 0.05% or less Hot-rolled steel strip for cold-rolled high-tensile steel sheets.

( 4 ) The steel strip is further mass%,
Cu: 0.04 to 2.0% or less,
Ni: 0.02 to 1.0% or less,
B: 0.0003 to 0.007% or less,
The hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling according to any one of ( 1 ) to ( 3 ), characterized by containing one or more of the following.

( 5 ) The hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling according to any one of (1) to ( 4 ), wherein the tensile strength is 590 MPa or more.

( 6 ) When the steel piece is hot-rolled so that the finish rolling completion temperature (FT) is Ar3 or higher and cooled to the coiling temperature, the change over time in the steel strip temperature (T) in the cooling step is as follows. Cooling with a small variation in sheet thickness after cold rolling according to any one of (1) to ( 5 ) above, wherein cooling is performed under a condition that L calculated using formula (1) satisfies 20 μm or less. A method for producing a hot-rolled steel strip for high-tensile steel sheet.

In the above formula, t: time after finishing rolling delivery (seconds), t ₆₀₀ : finishing rolling completion time 0, time to reach 600 ° C. (seconds), T: steel strip temperature after finishing rolling (° C.) .

( 7 ) Winding in a temperature range of 400 to 600 ° C. The production of a hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling as described in ( 6 ) above Method.

  According to the structure-controlled hot-rolled steel sheet of the present invention, when the hot-rolled steel sheet is cold-rolled into a cold-rolled high-tensile steel sheet, the cold-rolled height is small in the thickness variation after cold rolling over the entire length. By using a tension steel plate, it is possible to eliminate a yield drop caused by a thickness variation of about 200 μm, which is generated at the tail end portion of the hot-rolled steel strip as in the conventional case. Further, in the present invention, the hot-rolled steel strip of steel component containing C: 0.05 to 0.22% and Si: 0.3 to 2.0%, which is likely to cause sheet thickness fluctuation at the time of cold rolling. And the board thickness fluctuation | variation after cold rolling can be suppressed effectively.

  Hereinafter, the present invention will be described in detail.

  A cold-rolled high-tensile steel sheet is manufactured by cold-rolling a hot-rolled steel strip, but a plate thickness variation as shown in FIG. 1 occurs after cold-rolling. The cause and countermeasures of the plate thickness variation are unresolved, and the product must be made while generating a drop in yield due to the removal of the plate thickness varying portion.

  Accordingly, the present inventor has intensively investigated the mechanism of occurrence of sheet thickness variation, and based on the results, completed the invention of a hot-rolled steel strip for cold-rolled high-tensile steel sheet and a method for producing the same.

  First, the actual state of occurrence of the plate thickness variation found in the present invention will be described.

  As a result of investigating the relationship between the coil length and the plate thickness variation over the entire length of the cold-rolled high-tensile steel plate in order to identify the portion where the plate-thickness variation occurs, as shown in FIG. It was found that the occurrence site of the plate thickness fluctuation occurred mainly within a range of about 200 m from the tail end of the hot-rolled steel strip. Further, when the occurrence portion of the plate thickness fluctuation is enlarged, as shown in FIG. 2, the plate thickness fluctuation occurs at substantially the same cycle (hunting pitch), and when the hunting pitch is calculated, (Hot rolled coil) It was found that it coincides with the outer peripheral pitch.

  Therefore, it can be presumed that the cause of the plate thickness variation after the cold rolling occurs during the hot rolling.

  In addition, when the relationship between the thickness and hardness of the cold-rolled high-tensile steel sheet was examined, there was a clear correlation between the hardness and the thickness variation, and the variation in hardness that occurred during hot rolling caused the variation in thickness. I found out.

  Therefore, as a result of investigating changes in hardness and structure in the longitudinal direction of the hot-rolled coil, the structure of the normal part is a bainite structure as shown in FIG. It was confirmed that a pearlite band as shown in (b) was formed, and it was found that the pearlite structure was locally softened by the formation of this pearlite structure. Therefore, in order to suppress the occurrence of pearlite texture that occurs locally in the winding coil period, we have intensively studied. As a result, it has been found that this pearlite structure is caused by a local occurrence of a slow cooling rate in the hot-rolled coil after the hot-rolled steel strip is wound.

  However, such a local change in the cooling rate is caused by, for example, a wrapper roll pressed against the coil in order to prevent loose winding of the coil or flapping of the tail end after winding, This avoidance is difficult.

  Therefore, the present inventors first examined a structure aimed at suppressing fluctuations in plate thickness. At this time, forming a pearlite structure by hot rolling causes a reduction in local deformability after annealing, so a bainite-based structure with a reduced amount of pearlite that is less susceptible to fluctuations in the cooling rate after winding is created. .

  And the structure control of the hot-rolled steel strip is performed over the entire length, and in particular, a range of at least 200 m or less from the tail end (tail) of the hot-rolled coil corresponding to the tail end portion where the sheet thickness fluctuation easily occurs after cold rolling. The bainite-based structure has a pearlite fraction of 15% or less, preferably 10% or less in the structure, and the thickness variation in cold rolling when the longitudinal variation difference of the pearlite fraction is 10% or less. It was found that can be suppressed. At this time, it is desirable that the variation difference of the pearlite fraction is 5% or less, especially for a severe requirement for the plate thickness variation. In addition, 20% or less of the ferrite structure (α) inevitably formed in the structure can be allowed.

  In the present invention, the pearlite fraction is obtained by etching the steel sheet with a nital etching solution, and then observing the structure in the range from 1/4 t to 3/4 t in the thickness center with a scanning electron microscope. Is obtained by measuring by image analysis. The fluctuation difference of the pearlite fraction is observed at 50 points or more of the central part of the steel strip at equal intervals from 10 m in the longitudinal direction of the steel strip, the pearlite fraction is measured, and the maximum and minimum values are measured. This difference was defined as the variation difference in the pearlite fraction.

  The reason why the thickness of the hot rolled coil is limited to at least 200 m from the tail end in the present invention is that the thickness variation after the cold rolling significantly occurs in the range at least 200 m from the tail end. Therefore, since at least 200 m of tissue control is required, the present invention is limited to a range of at least 200 m from the tail end of the hot-rolled coil.

  Further, the reason why the pearlite fraction in the bainite structure is limited to 15% or less is that when the pearlite fraction exceeds 15%, a soft part is formed in the steel strip, and the thickness of the steel sheet varies after cold rolling.

  Furthermore, the longitudinal variation difference of the pearlite fraction was limited to 10% or less because when the variation difference in the longitudinal direction of the pearlite fraction exceeds 10%, a soft part is generated in the steel strip, and the sheet thickness after cold rolling This is because fluctuation occurs.

  And, as a result of intensive studies in consideration of suppressing the local tissue fluctuation generated after winding by controlling the cooling pattern, the present inventors have found the relationship of the following formula (1). As shown in FIG. 4, it was found that the variation in plate thickness can be suppressed when the above condition is satisfied. The cause of this is not clear, but the ferrite transformation occurs from the portion farthest from the microsegregation of Mn during solidification, and at this time, C is discharged from the transformation structure (ferrite) and diffuses into the segregated portion of Mn. When this diffusion occurs sufficiently, a pearlite structure is likely to be formed, and when C diffusion can be suppressed, a pearlite structure is unlikely to be formed. And in order to suppress C diffusion, hot finishing temperature and the cooling rate in a high temperature range are important, and it came to find the cooling conditions whose L of Formula (1) is 20 micrometers or less.

In the above formula, t: time after finishing rolling delivery (seconds), t ₆₀₀ : finishing rolling completion time 0, time to reach 600 ° C. (seconds), T: steel strip temperature after finishing rolling (° C.) Means.

  The hot finishing temperature should be low in order to satisfy the formula (1). However, hot rolling below the Ar3 transformation point may cause meandering and the like, impairing stable operation, and unrecrystallized austenite. On the contrary, the ferrite transformation from the above promotes the formation of a band structure, causes local pearlite generation, and increases the thickness variation. For this reason, finishing temperature shall be Ar3 point or more.

  Next, it is desirable that the cooling rate in the high temperature region immediately after the hot finishing satisfies the formula (1). However, a cooling rate of 200 ° C./s or more causes an increase in manufacturing cost, a steel plate shape deteriorates, the cooling uniformity is hindered, and the plate thickness accuracy over the entire coil length is deteriorated to 200 ° C./s or less. Is desirable.

Next, as the winding temperature control, the winding temperature was lowered in order to suppress the generation of pearlite even at a low cooling speed after winding.
When the coiling temperature after cooling is increased, pearlite transformation occurs in the cooling process after coiling, so that a lot of pearlite is generated. On the other hand, when the winding temperature is lowered, the generation of pearlite is reduced in the cooling process after winding.

  However, when the coiling temperature is less than 400 ° C., martensite is generated and the strength fluctuation of the hot-rolled steel sheet is increased, which is not preferable because the thickness accuracy is deteriorated. Therefore, in the present invention, the lower limit of the coiling temperature is 400 ° C. did. On the other hand, the upper limit of the coiling temperature is 600 ° C. from the pearlite production rate, but preferably 500 ° C.

  Next, the material of the hot-rolled steel strip that can be used in the present invention will be described.

  Cold-rolled high-tensile steel sheets having various steel components having a tensile strength of 590 MPa or more (for example, 590 to 1170 MPa) are known. The hot-rolled steel strip of the present invention can be applied to the production of these cold-rolled high-tensile steel sheets while preventing fluctuations in the thickness after cold rolling.

  In particular, the steel sheet has a tensile strength of 590 MPa or more of steel types containing C: 0.05 to 0.22% by mass and Si: 0.3 to 2.0% in mass%, which are likely to cause sheet thickness fluctuation after cold rolling. It can be effectively applied to cold-rolled high-tensile steel sheets.

  That is, C is contained to ensure strength, but when C: 0.05% or more, pearlite transformation is likely to occur, and a softened part is generated in the hot-rolled steel strip, and the thickness after cold rolling Causes fluctuations. In other words, the steel slab subjected to hot rolling inevitably has Mn segregated between dendritic branches during steelmaking, and when the steel slab is hot-rolled, the Mn segregated portion is stretched into a layer and becomes a band shape. . After hot rolling, C can move freely at a high temperature, and is discharged from the ferrite by ferrite transformation, and moves to the Mn segregation portion to cause pearlite transformation in a band shape.

  When C is less than 0.05%, pearlite transformation hardly occurs. Further, when C exceeds 0.22%, the toughness and weldability are deteriorated, so the upper limit of C is set to 0.22%.

  Si is an element that improves strength and elongation, but when Si is 0.3% or more, the bainite formation rate decreases, so that C diffusion is relatively likely to occur, and fluctuations in the pearlite fraction are likely to occur. To do. For this reason, when Si is high, the thickness variation in cold rolling is caused. However, since Si exceeds 2.0%, the weldability is deteriorated, so the upper limit of Si is set to 2.0%.

  As described above, the hot-rolled steel strip for cold-rolled high-tensile steel sheets of steel types containing C: 0.05 to 0.22% and Si: 0.3 to 2.0% generates fluctuations in the pearlite fraction. It is easy to cause fluctuations in sheet thickness after cold rolling, but if the present invention is applied to this steel type, pearlite transformation can be suppressed and fluctuations in pearlite fraction are small. It can be effectively prevented.

  Other steel components contained in the cold-rolled high-tensile steel plate will be described.

Mn: 1.0-3.2%
Mn is contained to ensure strength and toughness, but if it is less than 1.0%, the effect is not sufficient, and if it exceeds 3.2%, the toughness and weldability are deteriorated, so 1.0 to 3.2. %.

P: 0.025% or less P is an ingredient inevitably contained in the steel, and is preferably a component that lowers workability and corrosion resistance. This is an acceptable range.

S: 0.015% or less S is an unavoidable component similar to P, and since it adversely affects workability, it is preferable not to contain it, but 0.015% or less is contained. Even within the allowable range.

Al: 0.08% or less Al is important as a deoxidizer. For this purpose, it is desirable to add 0.010% or more of Al. On the other hand, even if Al is added excessively, the above effect is saturated and the steel is embrittled, so the upper limit was made 0.8%. Nb, Ti, and V precipitate fine carbonitrides and strengthen the steel. Mo, Cr, Co, and W enhance the hardenability and strengthen the steel. For this purpose, Nb: 0.005% or more, Ti: 0.03% or more, V: 0.005% or more, Mo: 0.02% or more, Cr: 0.1% or more, Co: 0.0. It is necessary to contain one or more of 01% or more and W: 0.01% or more. However, Nb: more than 0.10%, Ti: more than 0.20%, V: more than 0.10%, Mo: more than 0.5%, Cr: more than 5.0%, Co: more than 5.0%, Even if W: more than 5.0% is added, the effect of increasing the strength is not only saturated but also the ductility is decreased.

Steel further, Mg, one or both of Zr, alone or in total 0.0005% or more, may contain 0.02% or less. Mg and Zr improve the local ductility and hole expansibility by controlling the shape of sulfides and oxides. For this purpose, it is necessary to add one or more 0.0005% to 2 alone or the sum of these elements. However, excessive addition deteriorates workability, so the upper limit was made 0.02%.

  Further, the steel is Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less, or 1 or 2 More than seeds can be contained. Although these elements can also improve the hardenability and increase the strength of the steel, Cu: less than 0.04%, Ni: less than 0.02%, B: less than 0.0003% has an effect of strengthening the steel. small. On the other hand, even if Cu: more than 2.0%, Ni: more than 1.0%, and B: more than 0.007%, the effect of increasing the strength is saturated and the ductility is lowered.

  Hereinafter, the present invention will be described based on examples.

  Steel strips of high-tensile steel plate components shown in Table 1 were extracted from a heating furnace at 1200 ° C., and subjected to rough rolling and finish rolling in a hot rolling line to produce a hot rolled steel strip. The finishing rolling temperature was carried out so as to be not lower than the Ar3 transformation temperature based on the steel component and lower than that as a comparative example.

The steel strip that exits the final finishing mill (FT) of the hot finishing mill is rapidly cooled by primary cooling by water cooling in the cooling zone of the run-out table, and then cooled in two stages or primary and secondary cooling. It cooled to the coiling temperature with uniform cooling speed with cooling. Subsequently, the coil was wound at the coiling temperature to obtain a hot rolled coil. Tables 2 and 3 show the hot rolling conditions.
In order to obtain the pearlite fraction of the obtained hot-rolled steel sheet, the steel tail end is etched with a nital etching solution, and then the structure in the range of the thickness 1/4 to 3/4 t is observed with a scanning electron microscope. The pearlite fraction was measured by image analysis. In addition, in order to obtain the variation difference of the pearlite fraction, 50 points of the structure were observed at regular intervals from 10 m in the longitudinal direction of the steel sheet, and the variation difference of the pearlite fraction was obtained.
The average pearlite fraction, pearlite fraction fluctuation difference, and plate thickness fluctuation amount of the hot-rolled steel sheet are shown in Tables 2 and 3 together.

  Using this hot-rolled coil, after pickling, cold rolling with a rolling reduction of 40% or more was performed in a cold rolling line to produce a cold-rolled steel sheet.

  The example shown in Table 2 is an example in which the effect of L was tested using the steel type A shown in Table 1. As shown in Table 2, the examples of the steel of the present invention satisfying the requirements of the present invention are as follows. Although the plate thickness variation after cold rolling was as small as 50 μm or less, all the comparative steels with L exceeding 20 μm had a plate thickness variation of over 210 μm and did not show good plate thickness variation.

  The example shown in Table 3 is an example in which the effects of the finish rolling temperature, the coiling temperature, and L are tested using steel types other than A shown in Table 1, and examples of the steel of the present invention that satisfies the requirements of the present invention are as follows: In all cases, the plate thickness variation was small and good. On the other hand, the sample symbol B1, which is a comparative example, has a rolling finish temperature (FT) that is too high, so that L is more than 20 μm, the pearlite fraction variation is large, and the plate thickness variation is large and defective. became.

  In addition, the sample symbol J3 having a rolling finishing temperature of Ar3 or lower had a large pearlite fraction variation difference and a large plate thickness variation. In this way, when the rolling finishing temperature is Ar3 or lower, the amorphous austenite grain boundaries are aligned in the rolling direction, and C segregates at the grain boundaries, so that the pearlite fraction tends to fluctuate and the plate thickness fluctuates greatly. It is thought that it became.

  The sample symbol F3 has a winding temperature (CT) as high as 650 ° C., so that L continues to increase after winding and becomes a very large value, the average pearlite fraction increases, the pearlite fluctuation difference is high, A large amount of thickness variation occurred. On the other hand, in the sample symbol K2 where the coiling temperature (CT) is too low as 300 ° C., the portion where the Ms point or less is locally generated causes martensitic transformation and becomes locally hard. Although it was small, the plate thickness fluctuation amount became large.

  Further, in all of the sample symbols C1, F2, and I2, L exceeded 20 μm, the variation difference of the pearlite fraction exceeded 10%, and the plate thickness variation amount was large.

  Furthermore, in the comparative steel of the sample symbol m1 for steel m having a high C content and Mn content in the steel component, since the C content is high, the production of pearlite is promoted and the temperature after winding becomes sensitive. The fluctuation of the rate was easy to occur, the fluctuation amount exceeded 10%, and the fluctuation of the plate thickness became large. And in the comparative steel of the sample symbol n1 for the steel n1 having a high Si content in the steel component, since the Si is high, the bainite formation rate is lowered, and C diffusion occurs relatively, causing fluctuations in the pearlite fraction. Therefore, the fluctuation amount of the pearlite fraction exceeded 10%, and the fluctuation amount of the plate thickness was large.

  As is clear from the above examples, in the comparative example, the thickness fluctuation amount exceeded 200 μm, but the hot-rolled steel strip for cold-rolled high-tensile steel sheet of the invention example that satisfies the requirements specified in the present invention is By cold rolling, the thickness fluctuation amount is 100 μm or less over the entire length, and in particular, a cold-rolled high-tensile steel plate with a small thickness fluctuation amount at the tail end of the steel strip can be obtained, and by removing the thickness fluctuation part No yield loss occurs.

It is a figure which shows the plate | board thickness fluctuation | variation by the side of a rolling mill at the time of cold-rolling a hot-rolled steel strip with a 2 tandem cold rolling mill (2TCM). It is the figure which expanded and showed the plate | board thickness fluctuation | variation part of FIG. It is a microscope picture of the microstructure of the softened part and normal part of a hot-rolled steel strip. It is a figure which shows the relationship between L / micrometer and plate | board thickness variation | change_quantity / micrometer.

Claims (7)

The steel strip component is mass%,
C: 0.05 to 0.22%,
Si: 0.3 to 2.0%,
Mn: 1.3-3.2%
P: 0.025% or less,
S: 0.015% or less,
Al: 0.08% or less,
Containing the balance Ri Fe der except inevitable impurities, at least 200m below the range of the tail of the hot-rolled coil, a bainite main tissue, pearlite fraction in tissues is 15% or less, A hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small variation in sheet thickness after cold rolling, wherein the difference in the longitudinal direction of the pearlite fraction is 10% or less. The steel strip is further mass%,
Nb: 0.005 to 0.10% or less,
Ti: 0.03 to 0.20% or less,
V: 0.005 to 0.10% or less,
Mo: 0.02 to 0.5% or less,
Cr: 0.1 to 5.0% or less,
Co: 0.01 to 5.0% or less,
W: 0.01 to 5.0% or less,
The hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small variation in sheet thickness after cold rolling according to claim 1, comprising one or more of the following. The steel strip is further mass%,
Mg, Zr 1, two or respectively 0.0005% or more, the thickness variation after the cold rolling according to claim 1 or claim 2, characterized in that it contains 0.05% or less Is a hot-rolled steel strip for cold-rolled high-tensile steel. The steel strip is further mass%,
Cu: 0.04 to 2.0% or less,
Ni: 0.02 to 1.0% or less,
B: 0.0003 to 0.007% or less,
One or cold plate thickness variation is less cold-rolled high strength steel sheet for hot-rolled steel strip after rolling according to any one of claims 1 to 3, characterized in that it contains two or more. The hot-rolled steel strip for cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling according to any one of claims 1 to 4 , wherein the tensile strength is 590 MPa or more. When the steel slab is hot-rolled so that the finish rolling completion temperature (FT) becomes Ar3 or more and cooled to the coiling temperature, the change over time in the steel strip temperature (T) in the cooling step is expressed by the following formula (1). The cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling according to any one of claims 1 to 5 , wherein L is calculated under a condition satisfying L of 20 µm or less. Manufacturing method for hot-rolled steel strips.

In the above formula, t: time after finishing rolling delivery (seconds), t ₆₀₀ : finishing rolling completion time 0, time to reach 600 ° C. (seconds), T: steel strip temperature after finishing rolling (° C.) . The method for producing a hot-rolled steel strip for a cold-rolled high-tensile steel sheet having a small thickness variation after cold rolling according to claim 6 , wherein winding is performed in a temperature range of 400 to 600 ° C.

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