JP2016130334A - Hot rolled steel strip, cold rolled steel strip, and production method of hot rolled steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel strip whose intensity, hardness, internal oxidation thickness are uniform over whole width and longitudinal directions of the hot rolled steel strip, a cold rolled steel strip formed by cold rolling the hot rolled steel strip, and a production method for producing the hot rolled steel strip while suppressing increase of production cost.SOLUTION: The present invention relates to the hot rolled steel strip comprising by mass%, Mn:2.0-3.0%, Si:0.01-3.0%, C:0.03-0.3%, and comprising following characteristics. The characteristics are: when Vickers hardness, tension strength, and internal oxidation thickness are measured at total 9 measurement points, in which, in a center of a longitudinal direction, a tip end and tail end of the hot rolled steel strip which is unwound from a coil, the respective width direction center and both ends of the width direction are measurement points, difference between a maximum value and a minimum value of the Vickers hardness is less than 50 HV, difference of a maximum value and a minimum value of the tension strength is less than 100 MPa, and difference between a maximum value and a minimum value of the internal oxidation thickness is less than 3 μm.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a hot-rolled steel strip that can be used for producing a cold-rolled steel strip having uniform characteristics in the longitudinal direction (rolling direction) and the width direction, and a cold-rolled cold-rolled steel strip. The present invention relates to a rolled steel strip and a method for producing the hot rolled steel strip.

  In a general hot-rolled steel strip, the transformation to ferrite + pearlite is almost completed before being wound into a coil.

  Incidentally, in recent years, steel strips having excellent mechanical properties such as strength, elongation and Young's modulus have been developed. The steel strip having excellent mechanical properties contains a large amount of reinforcing elements such as C, Si, and Mn. Since these strengthening elements delay the ferrite transformation, the transformation may not be completed or the transformation may not proceed at the time when the steel strip is wound into a coil. A steel strip containing such a strengthening element is hot-rolled and wound, and then undergoes transformation in the middle of conveyance to the next process and / or in a coil yard. At this time, since the cooling rate of the coil outer peripheral part, the coil inner peripheral part, and both ends in the width direction of the coil is high in the coil state, the coil outer peripheral part, the coil inner peripheral part, and both ends in the width direction of the coil are different from other parts after transformation. Becomes hard. This hardening results in reduced productivity, yield and quality, such as lowering of rolling speed due to increased rolling load during cold rolling, deterioration of sheet thickness accuracy, and locations that do not satisfy the required characteristics in mechanical properties such as elongation. Incurs a decline. Further, there is a problem that the hardness varies according to the winding cycle.

  In addition, when Si is contained in a large amount as a strengthening element, the scale generated during hot rolling and the base iron may react to generate an oxide, and a layer called an internal oxide layer may be formed immediately below the surface. This internal oxide layer becomes thicker as the hot-rolled steel strip is hotter and as it is held for a longer time at the same temperature. The internal oxide layer is known to cause surface defects in the final product after cold rolling or plating. For this reason, it is necessary to remove the internal oxide layer by some method. However, when considering removal by pickling, for example, if there is a difference in the thickness of the internal oxide layer depending on the position in the steel strip, it is necessary to increase the pickling weight loss to remove the thick one, and the yield decreases, There is a problem that the pickling time is prolonged and the production efficiency is lowered, and it is difficult to appropriately remove the internal oxide layer.

  Conventionally, in order to solve these problems, a technique for making the mechanical properties and surface quality of the steel strip uniform over the entire length has been developed.

  For example, Patent Document 1 discloses a technique for equalizing the hardness in the longitudinal and width directions of a steel strip by performing a tempering heat treatment before cold rolling according to the coiling temperature after hot rolling. Yes.

  Further, Patent Document 2 discloses that after the winding and holding after hot rolling, the yield during cold rolling is improved by further rewinding and excessive generation of grain boundary oxidation (referred to as an internal oxide layer in the present invention). A technique for suppressing the above is disclosed.

JP 2010-144243 A JP2013-253301A

  Moreover, in the method of patent document 1, when annealing becomes long time and a annealing becomes long time, a precipitate will generate | occur | produce in the inside of a steel plate, and there exists a subject which accompanies embrittlement starting from the precipitate. Due to the occurrence of embrittlement starting from precipitates, there is a risk that the hot-rolled steel strip will break during cold rolling, and the steel strip after hot-rolling is annealed as it is to accelerate the reaction between the scale and the steel. Therefore, the variation in the internal oxide thickness of the hot-rolled steel strip is further promoted and becomes non-uniform.

  In the method of Patent Document 2, after hot rolling, it is wound at a temperature of 750 to 600 ° C., held for 10 to 30 minutes, and then unwound while cooling at a cooling rate of 20 ° C./s or more, and further at a temperature of 550 ° C. It is a technology to rewind with. In this method, an internal oxide layer having a thickness of more than 10 μm is already generated before unwinding, and the load of the subsequent removal process is increased. In addition, cooling of the width edge proceeds, so that the internal oxide thickness varies in the width direction. Therefore, it is impossible to improve the original cold rolling property.

  This invention solves the said subject, The objective is a hot-rolled steel strip with uniform intensity | strength, hardness, and internal oxidation thickness over the whole width direction and longitudinal direction of a hot-rolled steel strip, and the hot-rolled steel strip An object of the present invention is to provide a method for producing a cold-rolled steel strip formed by cold rolling and a hot-rolled steel strip while suppressing an increase in production cost.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, the transformation curve during cooling of the steel strip, the outer periphery of the coil corresponding to the tail end of the steel strip, the central portion of the coil corresponding to the central portion of the steel strip, and the inner peripheral portion of the coil corresponding to the tip of the steel strip Based on the cooling curves of the respective parts, it was found that the above-mentioned problems can be solved by setting the winding temperature, and the present invention has been completed. More specifically, the present invention provides the following.

  [1] Hot-rolled steel containing, by mass, Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, C: 0.03 to 0.3% and having the following characteristics band.

(Characteristic)
Vickers hardness, tensile strength, and internal oxidation at a total of 9 measurement points at the center in the longitudinal direction and at both ends in the longitudinal direction of the hot rolled steel strip unwound from the coil. When the thickness is measured, the difference between the maximum value and the minimum value of Vickers hardness is less than 50 HV, the difference between the maximum value and the minimum value of tensile strength is less than 100 MPa, and the maximum value and minimum value of the internal oxide thickness The difference from the value is less than 3 μm.

  [2] Further, it contains at least one selected from Nb: 0.0005 to 0.15%, Ti: 0.0005 to 0.15%, and V: 0.0005 to 0.15%. The hot rolled steel strip according to [1].

  [3] A cold-rolled steel strip obtained by cold rolling the hot-rolled steel strip according to [1] or [2].

[4] For a steel material containing Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, and C: 0.03 to 0.3% by mass%, the finishing temperature: Finished rolling is performed at 850 to 1100 ° C. to form a steel strip, the steel strip is wound in a coil shape at a winding temperature of 400 to 700 ° C., and the steel strip after winding is cooled to a cooling stop temperature: 20 to 400 ° C. In the course of cooling, the transformation curve during cooling of the steel strip, the outer periphery of the coil corresponding to the tail end of the steel strip, the central portion of the coil corresponding to the central portion of the steel strip, and the inner periphery of the coil corresponding to the tip of the steel strip Based on the cooling curve of each part, the coiling temperature of each part is set so that the ferrite fraction of each part becomes substantially constant and the cooling rate of each part satisfies Equation 1. Manufacturing method.
(Formula 1) 0.005 ≦ cooling rate (° C./s)≦0.035
However, cooling rate = ((temperature immediately after winding-temperature after 10000 seconds after winding) / 10000).

  [5] The method for producing a hot-rolled steel strip according to [4], wherein the coil is covered with a heat insulating material from a predetermined time point immediately after coil winding up to 1000 seconds later.

  [6] The winding temperature of the coil outer peripheral portion and the coil inner peripheral portion is set to 30 to 300 ° C. higher than the winding temperature of the coil central portion, as described in [4] or [5] Manufacturing method of hot-rolled steel strip.

  [7] The method for producing a hot-rolled steel strip according to any one of [4] to [6], wherein a winding temperature of each part is adjusted by adjusting a water injection condition on the run-out table.

  According to the present invention, a hot-rolled steel strip having a uniform strength, hardness, and internal oxide thickness can be obtained throughout the width direction and the longitudinal direction.

  Moreover, if a cold-rolled steel strip is manufactured using the hot-rolled steel strip of the present invention, a cold-rolled steel strip having uniform characteristics and quality can be obtained.

  Moreover, although the manufacturing method of the hot-rolled steel strip of this invention is a manufacturing method of the hot-rolled steel strip which has said characteristic, the raise of manufacturing cost can be suppressed compared with the conventional method.

It is a figure which shows an example of an internal oxide layer. It is a figure which shows an example of a transformation curve and a cooling curve. It is a figure which shows an example of the positional relationship of a heat insulating material and a coil. It is a figure which shows measurement points, such as hardness in a hot-rolled steel strip.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment. In this specification, the hot-rolled steel strip may be referred to as “steel strip”. Further, “%” representing the component amount means “mass%”.

  First, the problems of the prior art will be briefly described.

  In a high-strength steel strip containing a large amount of Mn targeted by the present invention, unlike a general hot-rolled steel strip, the ferrite nose is on the relatively long time side in the continuous cooling transformation curve (CCT) diagram. Existing. For this reason, it is the cooling process after hot rolling that determines the structure of the steel strip. Since the coil outer peripheral portion, the coil inner peripheral portion, and the end portion in the width direction of the coil where the heat removal proceeds quickly are hardened, the rolling load increases during cold rolling and the rolling load increases. Further, cooling unevenness occurs at the outer peripheral portion of the coil, the inner peripheral portion of the coil, and the end portion in the width direction of the coil, resulting in a decrease in plate thickness accuracy and a hardened final product. These cause a reduction in manufacturing capacity and yield.

  In the middle of the coil, which corresponds to the center in the longitudinal direction and width direction of the steel strip, which is difficult to remove heat, the reaction between the steel and the hot-rolled scale proceeds, and the surface of the steel is the turtle shell along the grain boundary called the internal oxide layer. The formation of the oxide of the pattern is promoted. FIG. 1 shows an example of the internal oxide layer. When the thickness of the internal oxide layer (internal oxide thickness) is increased, it cannot be removed in a subsequent process, and the surface quality (platability, wrinkles, etc.) of the final product is deteriorated. On the other hand, when trying to obtain the same surface quality with the entire length of the steel strip, it is necessary to determine the removal conditions according to the thickest part of the internal oxide layer, and the productivity such as worsening the yield. The decline also accompanies. For example, if it is attempted to remove by pickling, it is necessary to pass through at a low speed in order to increase the amount of pickling, and also a portion that does not require dissolution (a portion having a thin internal oxide thickness) is excessively pickled and removed. The problem due to the internal oxide layer is more likely to occur as the Si content increases.

  Next, the present invention will be described.

<Hot rolled steel strip>
The hot-rolled steel strip of the present invention is mass% and contains Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, and C: 0.03 to 0.3%.

  Generally, the structure after winding a hot-rolled steel strip used for manufacturing a cold-rolled steel strip is a ferrite + pearlite structure. However, in the case of a high-strength steel strip, since an element that enhances hardenability is added, the structure after winding is often a bainite structure. Among strengthening elements that enhance hardenability, Cr and Mo are difficult to obtain stably and inexpensively, so the production of high-strength steel strips with component systems mainly containing C, Si, and Mn Is desired. The present invention is directed to a steel strip made high in strength using C, Mn, and Si.

Mn: 2.0 to 3.0%
Mn is an element that improves the tensile strength (TS) of steel by solid solution strengthening. The Mn content may be appropriately adjusted according to the desired strength. However, if the Mn content is less than 2.0%, the ferrite transformation proceeds before winding of the hot-rolled steel strip, that is, on the runout table. End up. If the ferrite transformation proceeds on the run-out table, the effect of controlling the cooling history by controlling the coiling temperature as in the present invention cannot be obtained. On the other hand, if the Mn content exceeds 3.0%, ferrite transformation does not occur unless held at elevated temperatures up to 10 ⁵ seconds the order. In this case, since it is necessary to heat the coil after winding, the ferrite fraction cannot be controlled only by controlling the winding temperature. For these reasons, the Mn content is limited to the range of 2.0 to 3.0%.

Si: 0.01-3.0%
Si increases the strength of steel and further contributes to the improvement of workability. Si is an inexpensive element. In order to obtain strength, the Si content is set to 0.01% or more. However, if it exceeds 2.5%, it causes embrittlement and also causes deterioration of the surface properties due to the occurrence of red scale and the like. Therefore, Si is made 3.0% or less.

C: 0.03-0.3%
C is an important element from the viewpoint of increasing the strength of steel and forming carbides. In order to secure desired strength and carbide content, the C content is set to 0.03% or more. On the other hand, when the C content exceeds 0.3%, the weldability is remarkably deteriorated. For this reason, the C content is desirably in the range of 0.03 to 0.3%.

Other elements Further, in order to obtain a desired strength, the hot-rolled steel strip may contain Nb, Ti, and V. Specifically, it is allowed to contain at least one of Nb: 0.15% or less, Ti: 0.15% or less, and V: 0.15% or less. Nb, Ti, and V are all elements that form carbonitrides and contribute to increasing the strength of the steel sheet by precipitation strengthening. In order to obtain such an effect, the content of 0.0005% or more is required in all cases, but even if the content exceeds 0.15%, the effect is saturated and an effect commensurate with the content can be expected. Disappear. Therefore, it is desirable to limit any element to the range of 0.0005 to 0.15%.

  Moreover, a hot-rolled steel strip may contain P: 0.10% or less, S: 0.02% or less, Al: 0.02-0.1%, N: 0.008% or less.

P: 0.10% or less P has an effect of strengthening steel. However, when P is contained excessively, weldability and toughness are reduced due to the formation of precipitates. For this reason, it is desirable that the P content be in the range of 0.10% or less.

S: 0.02% or less S adversely affects stretch flangeability and toughness. Therefore, it is desirable to reduce the S content as much as possible. However, if the S content is excessively reduced, the manufacturing cost increases. Therefore, the S content is preferably in the range of 0.02% or less.

Al: 0.02 to 0.1%
Al acts as a deoxidizer. Moreover, Al forms a precipitate and suppresses crystal grain coarsening at a high temperature. However, when Al is contained excessively, the cleanliness of the steel is lowered or the surface quality is deteriorated. Therefore, the Al content is desirably in the range of 0.02 to 0.1%.

N: 0.008% or less When N is excessively contained, weldability is lowered. However, if the N content is excessively reduced, the manufacturing cost increases. Therefore, the N content is preferably in the range of 0.008% or less.

  Moreover, the hot-rolled steel strip of the present invention further includes Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.00. Even if it contains 01-1.0%, B: 0.0003-0.003%, Ca: 0.001-0.005%, the effect of this invention is not prevented.

  Subsequently, the hot-rolled steel strip of the present invention has the following characteristics.

Vickers hardness, tensile strength, and internal oxidation at a total of 9 measurement points at the center in the longitudinal direction and at both ends in the longitudinal direction of the hot rolled steel strip unwound from the coil. When the thickness is measured, the difference between the maximum value and the minimum value of Vickers hardness is less than 50 HV, the difference between the maximum value and the minimum value of tensile strength is less than 100 MPa, and the maximum value and minimum value of the internal oxide thickness The difference from the value is less than 3 μm. In addition, in this specification, a front-end | tip and a tail end mean the front-end | tip and tail end of a steel strip at the time of winding up.

Vickers hardness To suppress the deterioration of cold-rollability due to the difference in cooling rate after winding the hot-rolled steel strip, the hardness must be constant at the base plate before cold-rolling, that is, the hot-rolled steel strip. is important. There is a close relationship between the Vickers hardness and the deformation resistance (yield strength) during cold rolling, and the hardness within a certain range indicates that the deformation resistance does not increase excessively and the rolling load is stable. In addition, the measurement conditions of Vickers hardness shall comply with JISZ2244.

  Coil outer periphery corresponding to the tail end of the hot-rolled steel strip when wound in a coil shape, coil inner peripheral portion corresponding to the tip of the hot-rolled steel strip, both ends in the width direction of the coil, coil skid of the coil There may be a case where the contact portion with the is periodically hardened. When the hardness of the hardened portion shows a value larger than that of the other portions by HV50, the rolling load becomes high and the dimensions after cold rolling are restricted. Moreover, when the variation in hardness is large, the intended mechanical properties cannot be obtained after cold rolling or cold rolling annealing, and the yield deteriorates. Further, since the periodic hardening causes gauge fluctuation, the cold rolling speed cannot be increased in order not to cause the gauge fluctuation, resulting in a decrease in productivity.

  In order to obtain stable mechanical properties over the entire steel strip without being affected by the above, and to stably roll with the cold rolling speed kept constant, the hardness is measured at the above nine measurement points. The difference between the highest and lowest values when measured needs to be less than HV50. The smaller the difference in hardness, the better.

Tensile strength (TS)
TS is known to correlate with the ferrite fraction, and the higher the ferrite fraction, the lower the TS. Therefore, the variation in TS of less than 100 MPa indicates that the ferrite fraction is the same in the portion where TS is high and the portion where TS is low, and that it is a hot-rolled steel strip having equivalent deformation resistance. If the TS difference when the hardness is measured at the above nine measurement points is 100 MPa or more, the cause of deterioration in yield due to failure to obtain desired mechanical strength and elongation after subsequent cold rolling, cold rolling annealing or plating It becomes. A more preferable TS difference is less than 50 MPa. TS is measured according to JISZ2241.

Internal oxide thickness The internal oxide thickness varies depending on the heat retention after hot rolling. The higher the temperature, and the longer the heat retention time at the same temperature, the more the oxidation reaction between the ground iron and the scale proceeds. In order to make the internal oxide thickness uniform, it is desirable to cool the coil with the same cooling history regardless of the position in the steel strip.

  In particular, the difference between the maximum value and the minimum value of the internal oxide thickness when the internal oxide thickness is measured at the above nine measurement points is less than 3 μm from the viewpoint of efficiently removing the internal oxide layer in the subsequent process and the surface quality. It is necessary to be. By making the internal oxide thickness uniform, it is possible to keep the surface quality of the final product constant.

  In particular, the central part of the steel strip that is easily retained after hot-rolling (the part corresponding to the center in the width direction and the longitudinal direction of the steel strip in the coil) and the part that is subject to heat removal (coil inner peripheral part, coil outer peripheral part and Differences in the internal oxide thickness tend to occur at both ends in the width direction of the coil. When the difference is 3 μm or more, even if the pickling is removed under the same conditions, there is a problem that the internal oxide layer partially remains or the productivity is lowered by peracid washing. According to the present invention, this problem is suppressed. The smaller the difference in internal oxide thickness, the better.

  The hot-rolled steel strip of the present invention is then subjected to hot-rolled sheet annealing and pickling as necessary, cold-rolled to the required dimensions of the final product, and in some cases appropriate cold-rolled sheet annealing, pickling, or It is plated to become a product. The steel strip obtained by the above cold rolling is the cold rolled steel strip of the present invention.

<Method for producing hot-rolled steel strip>
With respect to a steel material containing Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, and C: 0.03 to 0.3% in terms of mass%, finishing temperature: 850 to 1100 Finishing rolling at ℃ to make a steel strip, winding the steel strip into a coil shape, and cooling stop temperature: when cooling to 20 to 400 ° C, the winding temperature at the time of “winding the steel strip into a coil shape” will be described later It is characterized in that it is set by the method to do. Hereinafter, each condition will be described. For the explanation of the component composition of “a steel material containing Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, C: 0.03 to 0.3% in mass%” Since it is the same as that explained in the above-mentioned “hot rolled steel strip”, it is omitted.

Finishing temperature of hot-rolling In the production of hot-rolled steel strip, the exit temperature of the finish rolling mill shall be the austenitic region as usual, and it will be finished at the full isothermal temperature. However, if the finishing temperature exceeds 1100 ° C., the austenite grains become coarse, and the target elongation cannot be obtained in the final product. On the other hand, when the finishing temperature is lower than 850 ° C., after winding the hot-rolled steel strip, a sufficient amount of heat cannot be obtained, and particularly both end portions in the width direction of the coil become hard. Therefore, the exit side temperature of finish rolling is set to 850 to 1100 ° C, more preferably in the range of 900 to 1000 ° C.

Hot Rolling Winding Temperature The winding temperature sets the winding temperature of each part so as to satisfy Equation 1 based on the following transformation curve and cooling curve.
(Transformation curve) Transformation curve during cooling of steel strip (cooling curve) Coil outer periphery corresponding to the tail end of the steel strip, coil central portion corresponding to the central portion of the steel strip, coil corresponding to the tip of the steel strip Cooling curve of each part of the inner periphery Depending on the Mn content, the ferrite transformation start time varies, and the internal oxide thickness increases depending on the Si content. The cooling curve can be calculated from the internal oxide thickness. A method of setting the coiling temperature based on the transformation curve and the cooling curve calculated in advance is one of effective methods. Further, it is one of effective methods to experimentally obtain any curve. In the present invention, either may be used.

  As a method for deriving the cooling curve, specifically, the coiling temperature that is the cooling start temperature is variously changed for each position in the longitudinal direction of the hot-rolled steel strip (coil outer periphery, coil center, and coil inner periphery). The cooling curve is calculated in advance. In the calculation, in order to derive a cooling curve at each position, it is necessary to calculate using temperatures at at least three positions corresponding to the coil outer peripheral portion, the coil inner peripheral portion, and the coil central portion. Furthermore, since the cooling curve changes depending on the coil dimensions (inner diameter, outer diameter), the arrangement in the cooling yard, and the material, it is desirable to actually measure the coil temperature and correct the error from the calculated cooling history. The coil outer peripheral portion is a region from the tail end of the steel strip to 15 to 30% of the total length of the steel strip, and the coil inner peripheral portion is a region from the tip of the steel strip to 5 to 15% of the total length of the steel strip. Yes, the remaining part is the central part of the coil.

  FIG. 2 shows an example of a transformation curve (represented as a ferrite transformation start curve, indicated by a solid line) and a cooling curve (shown by a thick line for each part). Using the transformation curve and the cooling curve, the winding temperature of the part corresponding to each part is set so that the ferrite fraction of each part becomes substantially constant. The phrase “so that the ferrite fraction of each part becomes substantially constant” means that the ferrite fraction approaches a constant by bringing the intersection of the transformation curve and the cooling curve of each part close to each other. “The ferrite fraction is substantially constant” means that the difference between the maximum value and the minimum value of the ferrite fraction at the coil outer peripheral portion, the coil central portion, and the coil inner peripheral portion is less than 20%.

  In addition, the relationship between the internal oxide thickness and the elapsed time after winding (shown by the broken line in FIG. 2) is obtained by experiment or simulation, etc., so that the internal oxide layer thickness passes through the same range, It is more preferable to set the winding temperature for each part in the longitudinal direction. In the present invention, since the upper limit of the Si content is determined as described above, it is possible to suppress the internal oxide thickness from becoming too thick, but it is possible to sufficiently suppress the internal oxide thickness from becoming too thick and to reduce the internal oxide thickness difference. From the viewpoint of easy control, it is preferable to use the relationship between the internal oxide thickness and the elapsed time after winding in addition to the transformation curve and the cooling curve when determining the winding temperature. In addition, even if it becomes like Fig.2 (a) when the winding temperature of each part is made constant, by adjusting the winding temperature and moving a cooling curve as shown in Fig.2 (b), it is desired The hot rolled steel strip can be obtained.

  In addition, in order to reduce variations in the quality and characteristics of the steel strip as much as possible, the winding temperature of each part is set so that the cooling rate immediately after the end of winding up to 10,000 seconds later is 0.005 to 0.035 ° C./s. Set. The reason why the cooling rate needs to satisfy the above range is that the target hot-rolled steel strip contains a large amount of Mn, so that it occurs after an elapsed time of the order of 1000 seconds after the ferrite transformation is wound. . On the other hand, if the cooling rate immediately after the end of winding up to 10,000 seconds later is less than 0.005 ° C./s, it is heated too much by heat retention, causing coil collapse and excessively thick internal oxide thickness. On the other hand, when the cooling rate is 0.035 ° C./s or more, there is a high possibility that bainite transformation will occur, and it will become hard.

  In order to set the cooling rate of each part in the above range, it is effective to set the coiling temperature of the coil outer peripheral part and the coil inner peripheral part to 30 to 300 ° C. higher than the coiling temperature of the coil central part. When the winding temperature difference is less than 30 ° C., there is no change from the case of winding at the same winding temperature, and it may be difficult to make the ferrite fraction the same level. When the winding temperature difference is higher than 300 ° C., a sudden change in temperature on the run-out table causes a change in material, shape, tension, etc., which causes operational troubles during winding. Therefore, the winding temperature difference is preferably 30 to 300 ° C. More preferably, it is the range of 50-200 degreeC. In addition, when Mn content is 2.3 to 3.0%, it is necessary to make a temperature difference.

  Further, when the hot-rolled steel strip is wound, if it is wound at a temperature higher than 700 ° C., softening proceeds immediately after winding, and there is a case where a coil shape deformation called coil crushing occurs. Further, at a temperature lower than 400 ° C., the bainite transformation proceeds immediately after winding to harden, and the dimensions of the final product are greatly restricted. Therefore, the suitable range of coiling temperature is 400-700 degreeC.

  Furthermore, in order to adjust so that the winding temperature of each said part differs, the method of adjusting the water injection conditions on a run-out table about the position corresponding to each said part of the longitudinal direction of a hot-rolled steel strip is effective. That is, at the position corresponding to the coil inner peripheral part or coil outer peripheral part where the coiling temperature is increased, water injection is stopped or water injection density is decreased, and the water injection density is provided at the position corresponding to the coil central part which lowers the coiling temperature. Increase However, as a result of quenching in order to lower the coiling temperature of the steel strip center part corresponding to the coil central part, if it becomes hardened and hardens before reaching the mandrel, it breaks when the steel strip is wound up There is a risk. From the viewpoint of preventing this, it is preferable to set the difference between the winding temperature of the coil outer peripheral portion and the coil inner peripheral portion and the winding temperature of the coil central portion within 300 ° C. In addition, from the viewpoint of the cooling capacity of the run-out table, the hot rolling reduction rate, and the hot rolling speed, the above winding temperature difference is desirably within a range of 200 ° C. or less.

  Further, it is preferable to cover the coil with a heat insulating material from a predetermined time point immediately after coil winding up to 1000 seconds later. By covering with a heat insulating material, quality and characteristics in the width direction of the coil are more uniform. As a covering method, a method of covering the coil with a heat-insulating box that can cover the entire coil is preferable from the viewpoint of cost and operation. FIG. 3 shows an example of the positional relationship between the heat insulating material and the coil. At this time, in order to reduce heat removal from the gap between the ground and the heat insulating material, a box that can completely cover the coil in contact with the ground is more preferable. In addition, when 1000 seconds or more have passed immediately after coil winding, a portion that undergoes bainite transformation occurs at the outer periphery of the coil due to the effect of heat removal from the ground surface, causing periodic hardening, that is, gauge fluctuation during cold rolling. There is a case. More preferably, it is preferable to cover with a heat insulating material within 600 seconds from the end of winding. The steel strip may be covered with a heat insulating material until the entire length and width of the steel strip reach a temperature at which the metal structure can be determined. Thereafter, cooling with the heat insulating material covered, air cooling, or water cooling may be performed. The temperature at which the metal structure is determined is in the range of 200 to 400 ° C., but strictly speaking, since it varies depending on the components, it is preferable to confirm in advance experimentally.

  Moreover, the cooling stop temperature should just be about room temperature, and is specifically the range of 20-400 degreeC. When transporting or transporting to the next step such as cold rolling, it is preferable to cool the hot-rolled steel strip so that the temperature is about 20 ° C. to less than 200 ° C. In addition, since it is considered that the metal structure of the hot-rolled steel strip of the present invention is determined at around 400 ° C., the effect of the present invention is not affected if the temperature of the cooled hot-rolled steel strip is lower than that. . In addition, what is necessary is just to confirm by experiment as needed about what cooling temperature it has no influence on a metal structure and it can be said that it does not produce the influence of this invention.

  The Example of this invention is shown. The steel materials shown in Table 1 were rolled to a width of 1200 mm and a plate thickness of 2 to 3 mm with a continuous hot rolling mill, and wound on a mandrel having a diameter of 760 mm. Table 2 shows the winding temperature of each part, the cooling rate at each part, and the time required from winding to covering with a heat-resistant material. When the difference between the maximum value and the minimum value of the coiling temperature is less than 10 ° C., it is assumed that the coil is wound at a constant temperature, and the cooling rate control is considered unnecessary. The cooling stop temperature was 50 to 200 ° C. The specific adjustment of the coiling temperature was performed by adjusting the water injection conditions on the run-out table.

  Here, with respect to Samples 14, 20, 24, and 27, which are examples of the invention, the winding temperature of each part was adjusted so that the ferrite fraction of each part was substantially constant. Specifically, using the component steel ingots of the target steel, a hot rolling experiment was performed, the coiling temperature was changed arbitrarily after the hot rolling was completed, the time was held for an arbitrary time, and after water cooling, each condition (winding temperature and The metal structure was observed for each holding time condition). The ferrite fraction was derived by microstructure observation and used as a transformation curve. Furthermore, the cooling curve of each part of the steel strip after winding was derived by calculation from the width, weight, and inner diameter at the time of winding. Based on the transformation curve and the cooling curve, the winding temperature of each part was adjusted so that the ferrite fraction became substantially constant.

  FIG. 4 shows a schematic diagram of the hot-rolled steel strip. FIG. 4 shows the measurement points. The Vickers hardness at each measurement point 1 to 9 in FIG. 4 was measured according to JISZ2244. Further, a JIS No. 5 tensile test piece was taken from the same portion in the rolling direction, and TS was measured in accordance with JISZ2241. In any case, the case where the difference between the maximum value and the minimum value was less than HV50 or less than 100 MPa was regarded as acceptable (◯).

  In addition, the internal oxide thickness was measured based on this photographed image by cutting a section in the C direction (direction perpendicular to the rolling direction) at the same location in FIG. . In addition, the location where the turtle-shell-shaped black line has arisen like FIG. 1 was made into the internal oxide layer. The case where the difference between the maximum value and the minimum value of the thickness was less than 3 μm was determined as pass (◯).

  Sample No. In 1, the hot rolling conditions are within the range within the present invention, but the distribution in the width direction cannot be eliminated because the component composition of the steel is not within the range of the present invention.

  Sample No. 2 to 5 are examples when the winding temperature of each part is constant. Sample No. In Nos. 2 and 3, the temperature was not specified, and an untransformed part remained partially, or it became excessively thick in the central part of the inner oxide layer width or the central part of the alternate length. Sample No. 5 is because the coiling temperature of each part is not adjusted so that the ferrite fraction of each part becomes substantially constant (if this point is outside the scope of claim 4, there is no condition underlined in Table 2) However, this is outside the scope of claim 4), and the metal structure and internal oxidation state of the hot-rolled steel strip are determined to some extent before being covered with the heat insulating material, and the variation cannot be eliminated. Sample No. No. 4 has a constant coiling temperature, but the Mn content is less than 2.3%, so the state shown in FIG. 2B (the ferrite fraction of each part is substantially constant) In other words, the coiling temperature of each part was adjusted), and good results were obtained.

  Sample No. 6 to 13 are examples when the winding temperature is changed. Sample No. When the coiling temperature is out of the specified range as in 6 to 8, the untransformed part remains or there is a part that becomes excessively high in temperature, so the variation cannot be eliminated.

  Sample No. 9 and sample no. When the cooling rate deviates from the specified range as shown in FIG. As in 6-8, the cooling is performed with the variation remaining.

  Sample No. 11 is sample No. As in the case of No. 5, the coiling temperature of each part is not adjusted so that the ferrite fraction of each part becomes substantially constant, so the effect of the present invention cannot be obtained.

  Sample No. In No. 12, since the winding temperature of each part is not adjusted so that the ferrite fraction of each part is substantially constant, it is impossible to achieve both reduction in hardness and internal oxidation variation over the entire length and width of the steel strip.

  Sample No. As shown in FIG. 13, the coiling temperature is outside the range of the present invention, and it is impossible to achieve both reduction in hardness and internal oxidation variation in the entire length and width of the steel strip.

  Furthermore, sample no. When the amount of Mn increases as in 15 and 16, the transformation is further delayed, so that it is difficult to cause the transformation to be uniform over the entire length within the scope of the present invention, and variations in hardness and TS remain.

  Sample No. When the finish rolling temperature is different as in 17 and 18, the intended transformation cannot be obtained, and it becomes difficult to suppress variations within the specified range of the present invention.

  From the above, no. 4 and no. As shown in Fig. 14, a hot-rolled steel strip with uniform hardness, strength, and surface quality over its entire length and width can only be achieved by satisfying all of the finish rolling temperature, coiling temperature, cooling rate after coiling, and heat-retaining condition. It was confirmed that

  No. No. 19 satisfies the finish rolling temperature, the coiling temperature, the cooling rate after coiling, and the heat-retaining condition, so that a hot-rolled steel strip having a uniform hardness, strength, and surface quality can be obtained over its entire length and width. . No. In No. 20, since the coiling temperature of each part is not adjusted so that the ferrite fraction of each part becomes substantially constant, it is impossible to achieve both reduction in hardness and internal oxidation variation over the entire length and width of the steel strip.

  No. Since Nos. 21 and 22 have Mn contents outside the scope of the present invention, it is impossible to achieve both reduction in hardness and internal oxidation variation in the entire length and width of the steel strip. No. From FIG. 22, it can be seen that, even if the coiling temperature is adjusted, if the Mn content is outside the range of the present invention, it is difficult to achieve both reduction in hardness and internal oxidation variation in the entire length and width of the steel strip.

  No. 23 and 24, it is understood that the Si content within the range of the present invention is important for obtaining a hot-rolled steel strip having a uniform hardness, strength, and surface quality over the entire length and width.

  No. From 25 and 26, it is understood that the C content within the range of the present invention is important for obtaining a hot-rolled steel strip having a uniform hardness, strength, and surface quality over the entire length and width.

  No. 27 is an invention example in which the coil was not covered with a heat insulating material.

Claims (7)

A hot-rolled steel strip containing, by mass%, Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, C: 0.03 to 0.3% and having the following characteristics.
(Characteristic)
Vickers hardness, tensile strength, and internal oxidation at a total of 9 measurement points at the center in the longitudinal direction and at both ends in the longitudinal direction of the hot rolled steel strip unwound from the coil. When measuring the thickness,
The difference between the maximum value and the minimum value of Vickers hardness is less than 50 HV,
The difference between the maximum value and the minimum value of tensile strength is less than 100 MPa,
The difference between the maximum value and the minimum value of the internal oxide thickness is less than 3 μm.   Furthermore, it contains at least one selected from Nb: 0.0005 to 0.15%, Ti: 0.0005 to 0.15%, and V: 0.0005 to 0.15%. Item 2. The hot-rolled steel strip according to item 1.   A cold-rolled steel strip obtained by cold rolling the hot-rolled steel strip according to claim 1 or 2. With respect to a steel material containing Mn: 2.0 to 3.0%, Si: 0.01 to 3.0%, and C: 0.03 to 0.3% in terms of mass%, finishing temperature: 850 to 1100 When the steel strip is subjected to finish rolling at ℃ to form a steel strip, the steel strip is wound in a coil shape at a temperature of 400 to 700 ° C, and the steel strip after winding is cooled to a cooling stop temperature: 20 to 400 ° C.
The transformation curve during cooling of the steel strip, the coil outer peripheral portion corresponding to the tail end portion of the steel strip, the coil central portion corresponding to the central portion of the steel strip, and the coil inner peripheral portion corresponding to the tip portion of the steel strip The coiling temperature of each part is set so that the ferrite fraction of each part becomes substantially constant and the cooling rate of each part satisfies Equation 1 based on the cooling curve of .
(Formula 1)
0.005 ≦ cooling rate (° C./s)≦0.035
However, cooling rate = ((temperature immediately after winding-temperature after 10000 seconds after winding) / 10000).   The method for manufacturing a hot-rolled steel strip according to claim 4, wherein the coil is covered with a heat insulating material from a predetermined time point immediately after coil winding up to 1000 seconds later.   6. The hot-rolled steel strip according to claim 4, wherein the coiling temperature of the coil outer peripheral part and the coil inner peripheral part is 30 to 300 ° C. higher than the coiling temperature of the coil central part. Production method.   The method for manufacturing a hot-rolled steel strip according to any one of claims 4 to 6, wherein the coiling temperature of each part is adjusted by adjusting the water injection conditions on the run-out table.