JPH11181526A - Production of hot rolled steel plate excellent in workability and non-aging characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel plate excellent in workability and non- aging characteristic over the entire length of a coil by the use of ordinary hot rolling equipment without addition of specific elements. SOLUTION: A steel slab, having a composition consisting of, by weight, <=0.08% C. <=0.05% Si, <=0.30% Mn, <=0.030% P, <=0.020% S, 0.010-0.060% Al, <=0.0050% N, and the balance Fe with inevitable impurities, is heated to 1,050-1,200 deg.C and hot rolling is finished at a temp. not lower than the Ar3 point, and the resultant steel plate is coiled at a coiling temp. CTM ranging from 660 to 720 deg.C. At this time, coiling is performed while regulating coiling temp. CTTB at the tip and the back end of a coil to a value satisfying the equation, CTTB=CTM+A×log (CRTB/CRM). In the equation, the symbol A stands for 30 to 40, CRM is cooling rate ( deg.C/hr) in the central part of a coil until 500 deg.C is reached, and CRTB is the mean value of respective cooling rates ( deg.C/hr) at the tip and the back end of a coil until 500 deg.C is reached.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet having excellent workability and non-aging properties used in automobiles, home appliances, building materials, containers, etc., and having uniform mechanical properties in the longitudinal direction of the coil. It relates to a manufacturing method.

[0002]

2. Description of the Related Art In recent years, in the field of steel sheets for processing typified by steel sheets for automobiles, hot rolled steel sheets have been used in place of conventionally used cold rolled steel sheets from the viewpoint of material cost reduction. Have been. For this reason, the demand for improved workability of hot-rolled steel sheets is increasing year by year, and many attempts have been made to produce hot-rolled steel sheets having excellent workability.

For example, (1) a method using ultra-low carbon aluminum killed steel (Japanese Patent Laid-Open No. 49-89621), (2) Ti or N
b using an ultra-low carbon IF steel to which b is added
No. 97431), and (3) a method of adding special elements such as B, Nb, Ti, V, Cr, and Zr to low-carbon aluminum killed steels (Japanese Patent Application Laid-Open Nos. 52-17319 and 52-19763).
No. 23518, JP-A-62-13849, JP-A-63
216925, JP-A-02-104637, JP-A-02-209423, JP-A-02-217419, and (4) a method of reducing P (JP-A-02-20942).
No. 4), (5) Method for reducing Mn, Al, N (Japanese Patent Publication No. Sho 6)
No. 3-64491), and (6) a method of defining a cooling rate after finish rolling (Japanese Patent Publication No. 05-86451).

[0004] When a material is subjected to severe processing, deterioration of workability due to aging often poses a problem, and attempts have been made to prevent aging. For example, (7) a method of adding Cr and Zr to a low carbon aluminum killed steel (Japanese Patent Laid-Open No. 52-235)
No. 18), (8) Method for reducing N (Japanese Patent Laid-Open No. 57-131)
No. 324), (9) 250-450 at the time of cooling after winding
And a method in which the temperature is kept at 0 ° C. (JP-A-02-19424).

[0005]

However, the above (1)
In the case of (5), (7) and (8), the cost of the refining process in the steel making process and the cost of the added elements cannot be avoided. In the case of (6), although workability is improved, aging deterioration is a problem. In (9), equipment for keeping heat is required, which leads to high equipment costs.

On the other hand, the material of the hot-rolled steel sheet is greatly affected by not only the winding temperature but also the cooling rate after winding. For this reason, the workability and the non-aging property are greatly deteriorated at the coil tip and tail end where the cooling rate is high. Such deterioration is also observed in a cold-rolled steel sheet manufactured through a cold-rolling-annealing process, but is more remarkable in a hot-rolled steel sheet than in a cold-rolled steel sheet. Regarding this problem, JP-A-58-37128 and JP-A-59-37128
As disclosed in JP-A-16227 and JP-A-05-43946, studies have been made on cold-rolled steel sheets but not on hot-rolled steel sheets.

[0007] The present invention has been made in view of such a problem, so that it can be used in ordinary hot rolling equipment without adding a special element.
It is an object of the present invention to provide a method capable of manufacturing a hot-rolled steel sheet having excellent workability and non-aging properties over the entire length of a coil.

[0008]

First, a basic experiment which led to the present invention will be described. A steel having the following components was vacuum-melted in a laboratory, and the steel slab was heated to 1130 ° C., hot-rolled to a thickness of 2.3 mm at a finishing temperature of about 880 ° C. or higher, and 600 to 740 corresponding to a winding temperature. Charged into a heating furnace at a temperature within the range of ℃, the cooling rate from the same temperature to 500 ℃ 30 ℃
/ Hr, 70 ° C / hr, furnace cooling to 130 ° C / hr,
Thereafter, temper rolling of 1% was performed. Component (wt%) C: 0.05%, Si: 0.01%, Mn: 0.22
%, P: 0.008%, S: 0.008%, Al: 0.
025%, N: 0.0022%

A JIS No. 5 tensile test piece was sampled from the obtained hot-rolled steel sheet, the elongation (El) was measured, and the yield point rise (A) after a heat treatment at 100 ° C. for 60 minutes was further measured.
I, aging index) and non-aging properties were evaluated. The results are shown in FIGS. 2 (A) and 2 (B).

From FIG. 2 (A), the non-aging property of the low carbon aluminum killed steel is determined by the cooling rate (30 ° C./h) corresponding to the center of the coil.
In a place where r) is slow, the winding temperature is 660 to 710 ° C., and the aging index is remarkably improved to 20 N / mm ² or less. In a place where the cooling rate corresponding to the coil tip and tail ends is fast, this temperature range is It was found to shift to higher temperatures. Further, from FIG. 2 (B), good El was shown at 48% or more in the above-mentioned temperature range, and the same tendency was recognized as to the influence of the cooling rate. This seems to be because the precipitation form of AlN and the precipitation form of cementite are greatly affected by the winding temperature and the cooling rate.

Therefore, it can be seen that if the winding temperature is controlled for each part of the coil in accordance with the cooling rate, a hot-rolled steel sheet having excellent workability and non-aging properties and uniform in the longitudinal direction of the coil can be obtained. That is, depending on the winding temperature and the cooling rate of the coil central portion, and the cooling rate of the inner and outer peripheral portions of the coil (portions corresponding to the coil tip portion and the rear end portion), the coil tip portion,
By controlling the coiling temperature at the tail end and controlling the coiling temperature by this, it is possible to improve the workability and non-aging properties without adding special elements, and to achieve uniform heat in the coil longitudinal direction. Rolled steel sheet can be manufactured. In an actual coil, it is considered that by increasing the winding temperature at the leading end and the trailing end of the coil, a heat retaining effect is obtained, the cooling rate is reduced, and the non-aging property is further improved.

According to the method for producing a hot-rolled steel sheet of the present invention based on such knowledge, C: 0.08% or less, Si: 0.05% or less, Mn:
0.30% or less, P: 0.030% or less, S: 0.
020% or less, Al: 0.010 to 0.060%, N
: 0.0050% or less and 1% of steel slab consisting of Fe and unavoidable impurities
After heating to 0.50 to 1200 ° C., the hot rolling is terminated at _three or more points of Ar, and a winding temperature CT in the range of 660 to 720 ° C.
_When winding at _M , the winding temperature CT _TB at the coil tip and tail ends each having a length of 5% or more of the total coil length from the tip and tail ends of the coil is wound at the temperature of the following formula (1). Is what you take. CT _TB = _C T _M + A × log (CR _{T B} / _C R _M ) (1) However, A: 30 to 40 (constant), C R _M : Cooling rate of the center of the coil up to 500 ° C (° C / hr), CR _TB : 500
Average value of cooling rate (° C / hr) of coil tip / tail end up to ° C. Incidentally, CT _M equals the coiling temperature in the coil center portion, CR _M and below CR _TB are measured in advance in advance coil entire length from the coil after winding when wound in CT _M. Further, the cooling rates all mean an average cooling rate.

The reasons for limiting the components of the present invention will be described. C: 0.08% or less When C is contained in a large amount, not only deteriorates workability such as elongation, but also exists as solid solution C and degrades non-aging property. 0.0
8%.

Si: 0.05% or less When Si is contained in a large amount, Al-based inclusions increase and workability deteriorates. In addition, the scale property is also deteriorated, and the surface property of the steel sheet is impaired.

Mn: 0.30% or less Mn, as in the case of C, will cause a deterioration in workability if added in a large amount.

P: 0.030% or less P is dissolved in steel, increases the strength of the steel, and deteriorates the workability.

S: 0.020% or less Since S forms sulfide-based inclusions and degrades workability, S is preferably lower. In addition, S causes scab flaws, sliver flaws, and the like, and the surface properties of the steel sheet are impaired.
S: 0.020% or less.

Al: 0.010-0.060% Al is added as a steel deoxidizer. From the viewpoints of deoxidizing effect and economy, the content is set to 0.010% or more and 0.060% or less.

N: 0.0050% or less N, like C, not only deteriorates mechanical properties such as elongation, but also exists as solid solution N and degrades non-aging property. % Or less.

Next, the hot rolling conditions will be described. If the slab heating temperature exceeds 1200 ° C., the scale loss increases and the mechanical properties deteriorate.
° C. On the other hand, if the temperature is lower than 1050 ° C., it becomes difficult to secure the finishing temperature to Ar ₃ points or more.
050 ° C.

When the finishing temperature of the hot rolling falls below the Ar ₃ point,
Since not only the workability of the hot-rolled steel sheet is remarkably deteriorated, but also a sudden change in the rolling load occurs, and there is an inconvenience in operation such as difficulty in controlling the thickness of the steel sheet, the Ar is set to ₃ points or more.

The winding temperature after hot rolling is important in the present invention. As apparent from examples described later, the cooling rate CR _M and the coil distal portion of the coil central portion when the advance coil total length wound in CT _M, cooling speed C of the average of the rear end portion
By measuring the R _TB and controlling the cooling water amount, water pressure, etc. of the steel strip running on the table, the winding temperature of the coil front and rear ends can be set within the temperature range that satisfies the formula (1). By controlling, a hot-rolled steel sheet excellent in non-aging property having an aging index of 20 N / mm ² or less can be obtained. In this case, CR _M, to that the cooling rate of the CR _TB to 500 ° C. is, AlN precipitation 5
Since it is completed at up to 00 ° C, the effect on non-aging is 5
This is because the cooling rate is up to 00 ° C. The reason why the lengths of the coil tip and the rear end are set to 5% or more of the entire coil length is that if the length is less than 5%, the heat retaining effect is too small, and the non-aging property of the coil front and rear ends deteriorates. Because it becomes. In addition, the upper limit of the length of the coil front end portion and the rear end portion is sufficient to be 20% or less of the entire coil length.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the mechanical properties and non-aging properties of the steel type according to the present invention will be described. Table 1 vacuum melted in the laboratory
Was heated to 1150 ° C, hot rolled to a sheet thickness of 2.3 mm at a finishing temperature of about 880 ° C or higher, and charged into a heating furnace at 680 ° C corresponding to the winding temperature. The furnace was cooled so that the cooling rate from the temperature to 500 ° C. was 30 ° C./hr, and then 1% temper rolling was performed.

Using the obtained hot-rolled steel sheet, the tensile properties were determined with a JIS No. 5 tensile test piece, and the yield point increase (AI) after heat treatment at 100 ° C. × 60 mm was determined to evaluate the non-aging property. The results are shown in Table 1.

[0025]

[Table 1]

In Table 1, it can be seen that steel types A to F are examples of the invention and have good workability and non-aging properties. On the other hand, steel type G is poor in workability because it has more P than steel type B, and steel type H is inferior in workability and non-aging because it has more C than steel type B. Further, the steel type I has a large Mn as compared with the steel type E, and thus has poor workability. Also, steel type J has a higher N content than steel type A, and therefore has poor workability and non-aging properties. Steel type K has a higher Si content than steel type A, and thus has good workability and non-aging properties. The scale properties were poor, and the surface properties of the steel sheet were poor. Steel type L had a larger amount of S than steel type A, so that workability was poor, and slivers and slivers were generated, and the surface properties of the steel plate were also poor.

Next, an example of controlling the winding temperature at the coil tip and tail will be described. A steel having the following components was melted in a converter and slab was formed by continuous casting. This slab was hot-rolled to a finished plate thickness of 2.3 mm under the conditions shown in Table 2,
The leading and trailing ends of the coil were wound at a controlled winding temperature determined according to equation (1). The winding temperature was controlled by controlling the amount of cooling water on the table. Component (wt%) C: 0.05%, Si: 0.01%, Mn: 0.19
%, P: 0.010%, S: 0.008%, Al: 0.
027%, N: 0.0024%

[0028] Incidentally, when applying equation (1), the cooling rate of the coil central portion when wound in advance coil entire length at the same volume collected temperature (CT _M) (CR _M), the distal end portion, the tail end Investigate the average cooling rate (CR _TB ) and log (CR _TB / CR
_M ) was calculated. CR _M is charged with thermocouples spaced Yutakaana measured from the side surface in the central portion of the coil (depth 150 mm), was determined by measuring the temperature change. On the other hand, CR _TB inserts a thermocouple from the side of the coil at the position of the second turn from the top and back ends of the coil, inserts a thermocouple, measures the temperature change, and calculates the average value. _TB .

With respect to each of the obtained coils, test samples were taken from the tip, center, and tail ends, and were subjected to JI
The tensile properties were further measured at 100 ° C. × 60
The non-aging property was evaluated by calculating the yield point increase (AI) after the min heat treatment. The results are also shown in Table 2. Moreover, the samples No. 1 to 7 in Table _{2, log (CR TB / CR} M)
FIG. 1 shows a graph in which the effects of (CT _TB −C T _M ) on non-aging are summarized. Incidentally, reference numerals used in the graph represents a sample No., the value of (CT _TB -CT _M) was calculated for the front end and rear end portions in each sample. In the graph, “○” indicates that both the workability and the non-aging property are good, and “X” indicates that the non-aging property is poor.

[0030]

[Table 2]

From Table 2 and FIG. 1, the samples Nos. 1, 5, and 6 in which the winding temperature was controlled based on the equation (1) showed good processing over the tip, center, and tail ends of the coil. It can be seen that a steel sheet having good aging and non-aging properties was obtained. On the other hand, in Nos. 3 and 7, the winding temperatures at the leading end and the tail end are higher than the temperature determined by equation (1), and in Nos.
The non-aging property of the tip and tail ends deteriorates, and good non-aging property is not obtained over the entire length of the coil. In addition, No. 8 has a short control length of the winding temperature at the tip end and the tail end.
The non-aging property of the tail end is deteriorated. Regarding the workability, in some of the comparative examples, El at the tail end is less than 48%, but is generally good.

[0032]

According to the present invention, the leading and trailing ends of the coil are wound at a winding temperature that satisfies the formula (1) despite the use of an aluminum killed steel containing no special element. Thus, a hot-rolled steel sheet excellent in workability and non-aging property can be obtained over the entire length of the coil. In addition, since it can be carried out only by controlling the coiling temperature at the leading end and the trailing end of the coil, no special equipment is required, the operation can be carried out using ordinary equipment, and the productivity is excellent.

[Brief description of the drawings]

1 is a graph log (CR _TB / CR _M) and that (CT _TB -CT _M) has organized the effect on non-aging in the embodiment.

FIG. 2 (A) is a graph showing the relationship between the temperature corresponding to the winding temperature and the non-aging property according to the cooling speed, and FIG. 2 (B) is the graph representing the relationship between the temperature corresponding to the winding temperature and the elongation according to the cooling speed. It is a displayed graph.

Claims (1)

[Claims] 1. C .: not more than 0.08% by weight,
i: 0.05% or less, Mn: 0.30% or less, P:
0.030% or less, S: 0.020% or less, Al:
0.010% to 0.060%, N: 0.0050% or less, and the balance consisting of Fe and inevitable impurities was 1 piece.
After heating to 500 to 1200 ° C., hot rolling is terminated at _three or more points of Ar, and a coiling temperature CT in the range of 660 to 710 ° C.
_When winding at _M , the winding temperature CT _TB at the coil tip and tail ends each having a length of 5% or more of the total coil length from the tip and tail ends of the coil is wound at the temperature of the following formula (1). Manufacturing method of hot rolled steel sheet with excellent workability and non-aging property. CT _TB = CT _M + A × log (CR _TB / C R _M ) (1) where A: 30 to 40 (constant) C R _M : Cooling rate at the center of the coil up to 500 ° C (° C /
hr) CR _TB : Average value of cooling rate (° C / hr) at coil tip and tail end up to 500 ° C