JP2008190008A - Manufacturing method of hot-rolled steel sheet having excellent aging resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet having excellent workability and excellent aging resistance, which is used for automobiles, household appliances, building materials or the like. <P>SOLUTION: A steel strip having the composition consisting of, by mass, 0.04-0.25% C, 0.001-0.5% Si, 0.05-1.5% Mn, ≤0.09% P, ≤0.015%S, 0.01-0.08% Al, 0.0005-0.015% N, and the balance Fe with inevitable impurities is subjected to the hot rolling. Thereafter, the steel strip is cooled to the temperature below 400°C at the average cooling rate of ≥60°C/s, wound and cooled, and further subjected to the cold rolling of the elongation of 0.1-1.0% by using a roll having the diameter to satisfy inequality (1), t/R≥0.0055, where t denotes the plate thickness and R denotes the diameter of the roll. Thus, the hot-rolled steel plate having the excellent aging resistance can be obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a hot-rolled steel sheet excellent in aging resistance with little material deterioration due to aging, and in particular, hot-rolling with good workability and excellent aging resistance used for automobiles, home appliances, building materials, etc. The present invention relates to a method for manufacturing a steel sheet.

  A thin steel plate used after being processed into a complicated shape is required to have high ductility and to have little variation in ductility. However, when so-called aging occurs in which solid solution C or solid solution N in steel accumulates around movable dislocations and it becomes difficult to move the movable dislocations, ductility decreases. Aging also occurs at room temperature, and when there is a large amount of solute C or solute N in the steel, the ductility decreases in just a few months. Therefore, since there is a problem that ductility deteriorates with the passage of time, it is difficult to store for a long time after manufacture, and it is difficult to maintain the necessary materials for use after several months by the user. On the other hand, hot-rolled steel sheets are manufactured by a manufacturing process in which a steel sheet manufactured by hot rolling is wound into a coil shape and allowed to cool to near room temperature, and the cooling temperature history differs between the inside of the coil and the outer periphery. For this reason, the amount of solid solution C, solid solution N, etc. in the steel sheet remaining after manufacture differs in the longitudinal direction of the steel sheet. As a result, the degree of decrease in ductility due to aging in which solute C, solute N, etc. in the steel sheet gathers in the dislocations tends to be different, which causes variations in material. In particular, the BH (Bake Harding) steel sheet, which is used for automobiles and has a relatively low strength during molding and is easy to process and has increased strength by baking after painting, has a solid solution amount of C and N in the steel sheet. Therefore, this aging phenomenon becomes remarkable, and this is a major cause of deterioration of product performance due to a decrease in ductility and a variation in ductility.

  From such a background, there is a demand for a hot-rolled steel sheet that has a small decrease in ductility due to aging and a small variation in ductility.

  Various techniques have been proposed for preventing the deterioration of ductility due to aging. Examples of techniques for improving the aging resistance depending on steel components include, for example, mass%, C = 0.0001 to 0.20%, and Si = 2. 0.0% or less, Mn = 3.0% or less, P = 0.15% or less, S = 0.015% or less, Al = 0.20% or less, N = 0.001 to 0.10%, and Al and N are contained so as to satisfy 0.52Al / N <10, and one or more of Cr, Mo, and V are respectively Cr = 2.5% or less, Mo = 1.0% Hereinafter, V = 0.1% or less, and (Cr + 3.5Mo + 39V)) ≧ 0.1, heat that is excellent in paint bake-hardening performance and room temperature aging resistance composed of the remaining Fe and inevitable impurities There is a rolled steel sheet (for example, refer to Patent Document 1). However, it is technically difficult to overcome the variation in ductility degradation in the longitudinal direction of the hot-rolled steel sheet due to aging only by adjusting the steel components, and there is a problem of cost increase due to the additive element.

  Moreover, in order to reduce the material deterioration by aging, the technique which performs temper rolling after annealing is known about the cold-rolled steel plate. For example, when the annealed steel sheet having a thickness of 1.0 mm or less is temper-rolled, the relationship that the ratio of the steel sheet thickness (t) to the temper rolling mill work roll diameter (2R) is 2R / t ≦ 350. There has been proposed a temper rolling method for cold-rolled steel sheets that satisfies the above requirements (see, for example, Patent Document 2).

  However, this method relates to a cold-rolled steel sheet, and in the case of a cold-rolled steel sheet, by performing continuous annealing, there is less variation in the amount of solute C or solute N in the longitudinal direction of the steel sheet, and Since the plate thickness is only 1.0 mm or less, a remarkable effect has only been found. Therefore, this technique cannot be applied as it is to a thick hot-rolled steel plate that is not subjected to continuous annealing.

  Moreover, as a technique which performs temper rolling to a hot-rolled steel sheet, for example, in mass%, C: 0.030 to 0.100%, Si: 0.05 to 1.0%, Mn: 0.10 to 1. 00, S: 0.015% or less, Al: 0.025 to 0.100%, N: 0.0015 to 0.0150%, the steel containing the balance Fe and inevitable impurities is heated to 1200 ° C or higher, Hot rolled steel sheet by finishing rolling at a temperature of Ar3 point + 30 ° C. or more and 950 ° C. or less, then cooling to 500 ° C. or less at a cooling rate of 30 ° C./s within 3 seconds, and winding at 400 to 500 ° C. Then, pickling the hot-rolled steel sheet, temper rolling at a rolling reduction of 0.4 to 1.0%, a method for producing a hot-rolled steel sheet excellent in bake hardenability has been proposed (for example, (See Patent Document 3).

  However, this technique is intended to improve the bake hardenability of the hot-rolled steel sheet, and is not intended to suppress the decrease in ductility due to aging, but rather the room temperature aging resistance deteriorates.

  From such a background, it is the actual situation that a hot-rolled steel sheet with a small decrease in ductility due to aging is required.

JP 2002-53933 A JP 59-73104 A Japanese Patent Laid-Open No. 1-180917

  Therefore, in view of the above circumstances, the present invention does not necessarily add a special element to the components of the hot-rolled steel sheet, and increases the number of movable dislocations in the steel sheet in the hot-rolling process and the post-process after hot rolling. Even when solid solution C or solid solution N is present and accumulates on movable dislocations due to aging, by leaving movable dislocations that can be easily moved during processing, workability with reduced deterioration in ductility due to aging is good and aging resistance An object of the present invention is to provide a method for producing a hot-rolled steel sheet having excellent properties.

  The present inventor has earnestly studied about a method for producing a hot-rolled steel sheet having excellent aging resistance without necessarily adding a special element to the components of the hot-rolled steel sheet. As long as there is solid solution C or solid solution N, even if the solid solution C or N accumulates on the movable dislocation by aging, it is necessary to leave a movable dislocation that can easily move during processing. The present inventors have found that a decrease in ductility due to aging can be suppressed. As a method of increasing the movable dislocation, we found that the movable dislocation was increased by controlling the cooling conditions after hot rolling, and that the movable dislocation was further increased by performing skin pass rolling with a small diameter roll after cooling. The present invention has been completed.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.04 to 0.25%,
Si: 0.001 to 0.5%,
Mn: 0.05 to 1.5%,
P: 0.09% or less,
S: 0.015% or less,
Al: 0.01 to 0.08%,
N: 0.0005 to 0.015%
The steel slab of the component which consists of remainder Fe and an unavoidable impurity is hot-rolled, and after cooling to less than 400 degreeC by average cooling rate 60 degreeC / s or more, it winds up and cools, and also the following formula ( A method for producing a hot-rolled steel sheet having excellent aging resistance, characterized by performing cold rolling with an elongation of 0.1 to 1.0% using a roll having a diameter satisfying 1).
t / R ≧ 0.0055 (1)
(Where t is the plate thickness and R is the roll diameter)

(2) The steel sheet is further mass%,
Cr: 2.5% or less,
Mo: 1.0% or less,
V: The method for producing a hot-rolled steel sheet having excellent aging resistance according to the above (1), wherein one or two or more of 0.1% or less is contained in a total amount of 0.1% or more.

(3) The steel sheet is further mass%,
Ca: 0.0002 to 0.01%,
Zr: 0.001 to 0.10%,
REM: 0.0005 to 0.10%
A method for producing a hot-rolled steel sheet having excellent aging resistance according to the above (1) or (2), which comprises one or more of the following.

  ADVANTAGE OF THE INVENTION According to this invention, the hot-rolled steel plate excellent in the aging resistance which reduced the fall of the ductility by aging can be obtained also by long-term storage after manufacture of a hot-rolled steel plate. Moreover, it is possible to impart aging resistance without using an expensive element as a component of the hot-rolled steel sheet, and further, it is possible to impart aging resistance to the entire hot-rolled steel sheet without variation. It plays.

  The present invention will be described in detail below.

  The forming process of the steel sheet is made possible by the presence of movable dislocations in the steel sheet. If there are few movable dislocations, the ductility (formability) will be inferior. The decrease in ductility such as elongation due to aging is due to the fact that solute C and solute N present in hot-rolled steel sheets accumulate on movable dislocations over time, and movable dislocations are reduced without moving movable dislocations. Is due to

  In view of this, the present inventor paid attention to the movable dislocations present in the hot-rolled steel sheet in order to reduce the decrease in ductility due to aging of the hot-rolled steel sheet. A special additive element is considered to be able to reduce the decrease in ductility due to aging because even if dissolved C or solute N accumulates in movable dislocations over time, many movable dislocations still remain. In order to prevent the deterioration of ductility due to aging of hot-rolled steel sheets with a thick plate thickness of about 1.2 to 6.5 mm even without the addition of steel, intensive research has been conducted on increasing movable dislocations in hot-rolled steel sheets. It was.

  As a result, it is possible to increase the number of movable dislocations in the hot-rolled steel sheet by setting the composition of the hot-rolled steel sheet, sheet thickness, cooling conditions after hot-rolling, winding conditions, and skin pass rolling conditions to a specific range. The present invention has been completed by finding that a hot-rolled steel sheet excellent in aging resistance can be produced with a significant decrease in ductility, and with little variation in the amount of reduced ductility even in the longitudinal direction of the coil.

In the present invention, the following a. ~ C. The conditions are important.
a. C, Si, Mn, P, S, Al, N, and the like as steel components are included in a specific range.
b. After hot rolling, the steel is cooled to an average cooling rate of 60 ° C./s or higher and a temperature of less than 400 ° C., preferably 300 ° C. or lower, and wound on a coil.
c. Skin pass rolling is performed under the condition of the following formula (1).

The relationship between the skin pass roll diameter and the plate thickness is
t / R ≧ 0.0055 (1)
(Where t is the thickness of the plate and R is the diameter of the skin pass roll), and cold rolling is performed at an elongation of 0.1 to 1.0%.

  First, the reason why the steel component and the component range of the present invention are limited will be described.

C: 0.04 to 0.25%
C is an inexpensive element necessary for increasing the strength of the steel, and if its content is less than 0.04%, its effect is poor. On the other hand, if it exceeds 0.25%, ductility is lowered and weldability is also reduced. Reduce. Therefore, the C content is set to 0.04 to 0.25%.

Si: 0.001 to 0.5%
Si is an element that improves the strength ductility balance of the hot-rolled steel sheet by a solid solution strengthening action, and is required to be 0.001% or more. However, if it exceeds 0.5%, the strength increases too much and the formability decreases. Therefore, the upper limit was made 0.5%.

Mn: 0.05 to 1.5%
Mn is useful as a solid solution strengthening element, forms MnS, and suppresses cracking of S during hot rolling, and has an effect of suppressing normal temperature aging caused by solid solution N, so 0.05% or more is necessary. On the other hand, if the added amount exceeds 1.5%, the strength becomes too high and the ductility is deteriorated and the adhesiveness of the hot dip galvanizing is inhibited, so the upper limit was made 1.5%.

P: 0.09% or less P is an element that is inevitably contained from the viewpoint of steelmaking, but has an effect of increasing the strength, and can be contained by 0.09% or less. If it exceeds 0.009%, the fatigue strength after spot welding becomes poor, or the strength increases too much to cause surface shape defects during press working, so the upper limit was made 0.09%.

S: 0.015% or less S is an element that is inevitably contained from the steel making as with P, and causes hot cracking or deteriorates workability, so 0.015% is made the upper limit. did.

Al: 0.01 to 0.08%
Al is added as a deoxidizer and has the effect of increasing the ductility of the resulting hot-rolled steel sheet, and it is necessary to add 0.01% or more in order to exert the effect. However, Al combines with N to form AlN and lowers the BH property, so the upper limit was made 0.08%.

N: 0.0005 to 0.015%
N has the effect of increasing the BH property by solid-dissolving in the steel sheet, thereby precipitating as a nitride by baking coating treatment, and it is necessary to contain 0.0005% or more of N in order to ensure the effect. However, if N is too much, ductility due to aging deteriorates, so 0.015% was made the upper limit.

One or more of Cr: 2.5% or less, Mo: 1.0% or less, V: 0.1% or less: 0.1% or more in total amount One or more of Cu, Ni, and Cr Is an element contained in a steel sheet by reusing scrap as a steelmaking raw material. These components have the effect of increasing the strength of the steel sheet and improving the aging property at room temperature. However, it is not necessary to add them positively in terms of cost, and if the amount is too large, the workability deteriorates. Even if it contains 5% or less, Mo: 1.0% or less, V: 0.1% or less, and one or more of them in a total amount of 0.1% or more, the effect of the present invention is impaired. It is not a thing.

One or more of Ca: 0.0002 to 0.01%, Zr: 0.001 to 0.10%, REM: 0.0005 to 0.10% Ca, Zr, REM (rare earth element) Is an element that is effective in controlling the form of MnS formed in steel and has an effect of suppressing a decrease in workability during hot rolling. For this effect, each of Ca, Zr, and REM (rare earth elements) 0.0002% or more, 0.001% or more, and 0.0005% or more are necessary. However, if the amount is excessively increased, workability is deteriorated. Therefore, 0.01%, 0.10%, and. The upper limit was 10%.

  Next, hot rolling conditions will be described.

  In hot rolling, a steel material is heated to a temperature of Ar3 or higher in a heating furnace, and the steel material extracted from the heating furnace is hot-rolled as usual to obtain a steel plate. The steel sheet after finish rolling is cooled to less than 400 ° C. at an average cooling rate of 60 ° C./s or more by ROT cooling. During this cooling, transformation from the γ phase (austenite) to the α phase (ferrite) occurs, and movable dislocations occur in the steel sheet due to the volume change. The generated movable dislocations disappear faster as the temperature is higher. Therefore, the generated movable dislocations can be left without disappearing by cooling to a low temperature region at a high cooling rate. As a fast cooling rate that does not eliminate movable dislocations, an average cooling rate of 60 ° C./s or more is required.

  In addition, since the disappearance of movable dislocations decreases when the temperature falls below 400 ° C., cooling at an average cooling rate of 60 ° C./s or more in ROT cooling is performed until a temperature range below 400 ° C. is reached. More preferably, it is cooled to 300 ° C. or lower. If the coil is cooled to a temperature of less than 400 ° C. and wound on the coil, the cooling from the temperature to room temperature has little effect on the material due to the difference in the cooling rate inside and outside the coil, and there is no material variation in the coil longitudinal direction. It is also effective for suppression.

  Therefore, in the present invention, the cooling of the steel sheet after finish rolling in the γ region is performed by rapidly cooling the average cooling rate at 60 ° C./s or more, and the cooling arrival temperature is less than 400 ° C.

  Here, the “average cooling rate” means that if there is an area where there is no cooling zone due to temperature measurement or the like in the middle of the ROT cooling equipment, the air is cooled in this part. Mean cooling rate to the side.

  It is important in the present invention to previously form movable dislocations in the entire hot-rolled steel sheet under such hot-rolled cooling conditions.

  In addition, the movable dislocation formed in the steel sheet decreases when the steel sheet is once held at a high temperature in the α region by annealing. For this reason, many movable dislocations cannot be obtained in the steel sheet when the cold-rolled steel sheet is annealed.

  Next, skin pass rolling (cold rolling) conditions after hot rolling cooling will be described.

  Skin pass rolling is usually performed in the final rolling process, meaning rolling with extremely low elongation, and is performed for the purpose of correcting the plate shape of a steel plate to a smooth surface or adjusting the surface roughness. However, in addition to this purpose, for example, when a yield point appears in a steel sheet during forming, it may be performed to prevent a stretcher strain pattern that occurs in a portion that has reached the stress.

  An object of the present invention is to prevent ductility deterioration such as elongation due to aging by skin pass rolling. However, as the elongation rate of the skin pass rolling is increased, especially when it exceeds 1%, the elongation immediately after the skin pass rolling is lowered. Therefore, even if the elongation reduction due to aging can be prevented, the overall decrease in elongation cannot be prevented. It will be.

  Therefore, the present inventor has examined in detail how the reduction in elongation due to aging can be prevented by skin pass rolling with a low elongation of 1% or less. As a result, it has been found that, by generating movable dislocations during the production of the hot-rolled sheet described above and performing skin pass rolling using a small-diameter roll, it is possible to prevent elongation (ductility) deterioration due to aging even with skin pass rolling with a small elongation rate. It was.

  Specifically, (1) by forming a movable dislocation throughout the steel sheet by cooling at a cooling rate of 60 ° C./s or higher when manufacturing a hot-rolled sheet, and (2) winding at a low temperature of less than 400 ° C. The movable dislocations can be further increased in the steel sheet by rolling with a small diameter roll at the time of skin pass rolling without eliminating the formed movable dislocations. By increasing the number of movable dislocations, even if solid solution C and solid solution N accumulate in the movable dislocations, a large number of movable dislocations that can move during processing remain, thereby reducing the reduction in ductility due to the aging of the steel sheet. It can be done.

  In skin pass rolling, the number of movable dislocations can be increased by increasing the elongation regardless of the roll diameter. However, when skin pass rolling is performed using a small-diameter roll, the number of movable dislocations near the surface of the steel sheet increases even if the elongation rate is the same as that of the large-diameter roll. To do.

  An increase in the number of movable transitions due to the influence of the skin pass roll diameter in skin pass rolling will be described with reference to FIGS. FIG. 1 is an image diagram for explaining the state of increase in the number of movable dislocations in skin pass rolling with a small diameter roll, and FIG. 2 explains the state of increase in the number of movable dislocations in skin pass rolling with a large diameter roll. It is an image figure for.

  As shown in FIG. 1, in the skin pass rolling of the steel plate 2 by the small diameter roll 1, many movable dislocations are introduced finely and uniformly in the vicinity 3 of the steel plate surface. This is because stress acts not in the compression direction of the roll but in the flow direction of the steel sheet, the shear strain in the vicinity of the steel sheet increases, and a large number of movable dislocations 5 having a small density and uniform density are introduced in the vicinity 3 of the steel sheet surface. Because. However, since the shear strain is reduced in the steel plate inside 4, it is considered that the movable dislocation 6 having a coarse density is introduced. On the other hand, as shown in FIG. 2, in the skin pass rolling of the steel plate 2 with the large diameter roll 7, the shear strain in the vicinity of the steel plate surface does not increase as in the small diameter roll. It is considered that the movable dislocation 6 having a low density is introduced inside.

  When comparing the skin pass rolling between the small diameter roll and the large diameter roll, there is no difference in the state of movable dislocation inside the steel sheet, but in the case of the small diameter roll, there are more movable dislocations near the steel sheet surface than the large diameter roll. As a whole, skin pass rolling with a small-diameter roll has more movable dislocations than a large-diameter roll.

  Therefore, a test was conducted to clarify the skin pass roll diameter capable of suppressing the decrease in elongation due to aging. 0.06% C-0.004% Si-0.21% Mn-0.006% P-0.004% S-0.045% Al-0.0035% N-balance Fe The steel piece was hot-rolled to a finishing temperature in the γ region to obtain a steel plate having a thickness (t) of 1.8 to 3.4 mm. Subsequently, as a cooling condition by ROT cooling, it was cooled at a cooling rate of 85 ° C./s and wound around a coil at 300 ° C. Next, each coil was unwound, and skin pass rolling with an elongation of 0.5% was performed using any roll having a diameter (R) of 250 mm, 300 mm, 360 mm, 400 mm, and 600 mm. The ductility investigation was conducted by processing a JIS No. 5 test piece in parallel with the rolling direction immediately after skin pass rolling and at a width of 1/4 position of each steel plate held at room temperature for 6 months, and conducting a tensile test according to the test method described in JIS Z 2241. And the elongation at break was measured. A value obtained by subtracting the breaking elongation value of the tensile test after holding for 6 months from the breaking elongation value of the tensile test immediately after skin pass rolling was calculated as the ductility reduction margin, and the relationship with the plate thickness and roll diameter was arranged.

As a result of investigating the roll diameter of skin pass rolling, which has the effect of suppressing ductility deterioration due to aging, the upper limit of the roll diameter increases as the plate thickness of the steel plate increases, and the ratio of the plate thickness (t) to the roll diameter (R) t / The relationship as shown in FIG. 3 was obtained between R and the ductility reduction allowance after 6 months (elongation reduction allowance (%)). That is, it has been found that if the following formula (1) is satisfied, an effect of suppressing a decrease in ductility due to aging can be obtained.
t / R ≧ 0.0055 (1)
(Where t is the plate thickness and R is the roll diameter)
When t / R is less than 0.0055, it is considered that a large density of uniform dislocations is not introduced near the surface of the steel sheet, so that the effect of suppressing the reduction in ductility cannot be obtained.

  When the front and back surfaces of the steel plate after skin pass rolling were ground and investigated, the effect of the roll satisfying the formula (1) is not exhibited when grinding from the surface to the inner thickness of the plate thickness. It is considered that the increase in the number of dislocations between the surface and the inner 1/4 of the plate thickness contributes.

  Since the sheet thickness of the hot-rolled steel sheet is usually about 1.2 to 6.5 mm, the upper limit of the roll diameter necessary for skin pass rolling is also determined according to the sheet thickness. However, when the roll diameter is less than 180 mm, the roll rigidity is insufficient, and therefore, variation in the elongation in the width direction of the steel sheet is likely to occur, which is not preferable. Therefore, t / R is preferably 0.036 or less.

  The reason why the lower limit of the elongation rate of skin pass rolling is set to 0.1% is that if it is less than 0.1%, there are very few movable dislocations due to skin pass rolling, and the effect is not exhibited.

  By generating a large number of movable dislocations, material variations due to differences in the amount of solute C and the amount of solute N in the coil longitudinal direction can be improved.

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

  The steels A to T having the composition shown in Table 1 were melted in a converter and made into a slab having a thickness of 250 mm by a continuous casting method. These slabs were heated in a heating furnace at a temperature of 1150 to 1235 ° C., extracted, and hot-rolled. In the hot rolling, hot finish rolling was performed in the γ region, and the steel sheet was rolled to a thickness of 2.0 to 4.5 mm. Subsequently, as shown in Table 2, it was rapidly cooled to 300 to 550 ° C. at an ROT average cooling rate of 45 to 92 ° C./s, wound around a coil, and air cooled to room temperature.

  Thereafter, skin pass rolling was performed under the conditions shown in Table 2 while unwinding the coil. And about the coil inside and coil outer peripheral part of a hot-rolled steel plate immediately after skin pass rolling, a JIS No. 5 test piece was processed in parallel to the rolling direction from the 1/4 position of each width immediately after production and after 6 months at room temperature, A tensile test was performed according to the test method described in Z2241, and the elongation at break was measured. The results were as shown in Table 3.

  In accordance with the present invention are Invention Examples 1 to 23, containing a predetermined amount of steel components, cooled to a temperature of less than 400 ° C. at a cooling rate of 60 ° C./s or more after hot rolling, and wound up at room temperature. Is a hot-rolled steel sheet subjected to skin pass rolling with t / R ≦ 0.0055 and elongation = 0.1 to 1.0%, and the ductility reduction after 6 months is less than 2.0%. The material variation in the longitudinal direction (the difference between the elongation at the coil center and the elongation at the coil outer periphery) was also small.

  In Comparative Examples 1 to 3, the steel component was removed, and since the amount of C or N was too large, the ductility deterioration due to aging was large, and the material variation in the longitudinal direction was also large. Comparative examples 4 and 5 were cases where the cooling rate of ROT cooling was too slow. Since the generation of movable dislocations was not sufficient, the ductility was greatly reduced due to aging, and the material variation in the longitudinal direction was also large. In Comparative Examples 6 and 7, the coiling temperature after ROT cooling was too high, and the movable dislocations generated by the rapid cooling disappeared, so the ductility decreased due to aging and the material variation in the longitudinal direction was large. Comparative Examples 8 and 9 were cases where t / R was too small. Since the generation of movable dislocations by skin passes was not sufficient, the ductility deterioration due to aging was large, and the material variation in the longitudinal direction was also large. Comparative Examples 10 and 11 were cases where the elongation rate of skin pass rolling was too small. Since the generation of movable dislocations by the skin pass was not sufficient, the ductility deterioration due to aging was large, and the material variation in the longitudinal direction was also large.

To explain the state of increase in the number of movable dislocations in skin pass rolling with small diameter rolls

FIG.
It is an image figure for demonstrating the state of the increase in the number of the movable dislocations by the skin pass rolling by a large diameter roll. It is the figure which showed the relationship between ratio (t / R) of a plate | board thickness and a skin pass rolling roll diameter, and the ductility fall allowance after six months.

Explanation of symbols

1 small diameter roll 2 steel plate 3 steel plate surface vicinity 4 inside 5 density is fine and uniform movable dislocation 6 density coarse movable dislocation 7 large diameter roll

Claims (3)

% By mass
C: 0.04 to 0.25%,
Si: 0.001 to 0.5%,
Mn: 0.05 to 1.5%,
P: 0.09% or less,
S: 0.015% or less,
Al: 0.01 to 0.08%,
N: 0.0005 to 0.015%
The steel slab of the component which consists of remainder Fe and an unavoidable impurity is hot-rolled, and after cooling to less than 400 degreeC by average cooling rate 60 degreeC / s or more, it winds up and cools, and also the following formula ( A method for producing a hot-rolled steel sheet having excellent aging resistance, characterized by performing cold rolling with an elongation of 0.1 to 1.0% using a roll having a diameter satisfying 1).
t / R ≧ 0.0055 (1)
(Where t is the plate thickness and R is the roll diameter) The steel sheet is further mass%,
Cr: 2.5% or less,
Mo: 1.0% or less,
The method for producing a hot-rolled steel sheet having excellent aging resistance according to claim 1, wherein V: 0.1% or less of one type or two or more types is contained in a total amount of 0.1% or more. The steel sheet is further mass%,
Ca: 0.0002 to 0.01%,
Zr: 0.001 to 0.10%,
REM: 0.0005 to 0.10%
1 or 2 types or more of these are contained, The manufacturing method of the hot-rolled steel plate excellent in the aging resistance of Claim 1 or 2 characterized by the above-mentioned.

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