JP3289591B2 - Manufacturing method of hot rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet which is soft and has excellent workability over the entire length of the coil without deterioration of the material at the front and rear ends of the coil, with good yield.

[0002]

2. Description of the Related Art Soft-rolled hot-rolled steel sheets of the SPHC and SPHD classes are relatively inexpensive and show good workability, and are therefore widely used in parts for automobiles, home appliances, parts for general machinery, and the like. When manufacturing such a soft hot-rolled steel sheet, AlN
Is hot-rolled at the time of hot rolling in order to sufficiently precipitate and coarsen the grains, grow crystal grains, and increase softness and ductility.
However, when the hot-rolled coil is wound at a high temperature, AlN precipitation coarsens in the center of the coil and the duct becomes soft and highly ductile. Even in winding, AlN does not sufficiently precipitate and coarsen, and the crystal grains become fine, hard and low ductility as compared with the central part of the coil, and the material becomes uneven in the longitudinal direction of the coil. This is not preferable from the viewpoint of manufacturing a steel sheet having required characteristics and providing a uniform product to a consumer, and causes a decrease in yield, for example, by cutting off the rear end of the coil tip.

In order to avoid such a problem, the following proposals have conventionally been made. (1) A technique for increasing the precipitation and coarseness of AlN by making the coil front and rear ends non-water-injected on the runout and winding at a higher temperature than the coil center (Japanese Patent Publication No. 56-36051). (2) As in (1) above, the winding temperature at the rear end of the coil is set to be higher than that at the center, and cooling at the rear end of the coil is 3
A technique in which the precipitation is coarsened by slow cooling at a rate of not more than ℃ / min (JP-A-5-43946). (3) After winding, a technique of attaching a heat insulating cover to the coil and gradually cooling the coil to precipitate and coarsen AlN over the entire length of the coil (Japanese Patent Publication No. Sho 60-13048, Japanese Patent Publication No. Sho 60-13049) . (4) A technique of increasing the ratio of the amount of Al to the amount of N and promoting the coarsening of AlN by combining this with high-temperature winding (Japanese Patent Publication No. 57-21011). However, no water injection is applied to the rear end of the coil (1), N
If the ratio of the amount of Al to the amount of Al is increased, only (4) does not sufficiently cause coarsening of AlN precipitation at the rear end of the coil, and the mechanical test values at the rear and center of the coil include tensile strength ( T
S) causes a difference of about 20 MPa and elongation (El) of about 4%. Technology that gradually cools the front and rear ends of the coil after high-temperature winding
(2) is not intended for hot-rolled steel sheets, but is a technique applied when manufacturing cold-rolled steel sheets by continuous annealing. However, this method promotes AlN precipitation coarsening at the rear end of the coil tip. Can also be applied to soft hot rolled steel sheets. However, in order to gradually cool the rear end portion of the coil, that is, the outer peripheral portion of the coil, it is necessary to bind the coil after winding, and a reduction in productivity cannot be avoided. At the coil tip, that is, on the inner peripheral side of the coil, it is described that the mandrel is enlarged immediately after winding and the coil is tightened.However, after the coil is pulled out from the mandrel after winding is completed, the coil is loosened, and the coil is wound again. The cooling rate at the tip is faster. For this reason, there is a difference of about 0.1 to 0.2 in the Rankford value between the coil tip and the center,
It is hard to say that AlN is sufficiently precipitated and coarse over the entire length of the coil. Regarding the technology (3) for cooling the entire coil by installing a heat insulating cover, the increase in equipment costs due to the heat insulating cover and the time required to install the heat insulating cover inevitably reduce productivity.

[0004]

As described above, according to the prior art, the coarseness of AlN precipitation at the front and rear ends of the coil is insufficient and a uniform material cannot be obtained over the entire length of the coil. Alternatively, there are many problems such as an increase in equipment costs and a decrease in productivity, which impairs economic efficiency.

[0005] The present invention has been made in view of the above circumstances, without causing a special capital investment and reduced productivity.
An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet which is soft and has excellent workability over the entire length of the coil without deterioration of the material at the front and rear ends of the coil, with good yield and economical production.

[0006]

As described above, AlN does not sufficiently precipitate and coarsen at the rear end portion of the coil at the time of winding, and the crystal grains become fine and hard and have low ductility as compared with the center portion of the coil. Therefore, in order to avoid this, it is necessary to promote the precipitation coarsening of AlN. Then, the present inventors diligently studied the precipitation coarsening behavior of AlN and obtained the following conclusions.

(1) In the case where the precipitation of AlN is suppressed as much as possible before winding, and the degree of supersaturation of the precipitation is maximized, the coarsening of AlN during the winding is remarkably promoted. (2) In order to suppress the precipitation of AlN and keep AlN in a solid solution state until winding, immediately after slab is continuously cast, it is subjected to hot rolling without heat holding or heating to a specific temperature range. It is preferable to perform "direct rolling".

(3) Fine Al ₂ O ₃ generated by reoxidation of molten steel during slab casting not only suppresses grain growth and hardens and reduces ductility of a hot-rolled sheet, but also acts as a precipitation nucleus of AlN. This makes it extremely difficult to keep AlN in a solid solution state before winding. Therefore, in order to promote the coarsening of AlN during winding, it is necessary to reduce the reoxidized Al ₂ O ₃ as much as possible. Here, since reoxidation of molten steel is mainly caused by iron oxide in slag as an oxygen source, it is important to reduce total iron (T.Fe) in slag in a ladle.

(4) According to (1), (2), and (3) above, when winding is performed not only at the center of the coil but also at the rear end of the coil, the A
1N sufficiently precipitates and coarsens. As a result, the front and rear ends and the center of the coil are equally soft and highly ductile, and the material variation in the longitudinal direction of the coil is significantly reduced.

[0010] The first invention has been made based on such knowledge, and is expressed by C: 0.02 to 0.07% by weight, Si ≦
0.1%, Mn: 0.1-0.5%, P ≦ 0.025
%, S: 0.005 to 0.025%, Sol. Al: 0.0
In the method for producing a hot-rolled steel sheet containing 3 to 0.08% and N ≦ 0.0035%, continuous casting is performed with the total iron content in the slag in the ladle being 3% or less, and hot rolling is performed immediately after continuous casting. A method for producing a hot-rolled steel sheet, wherein the finishing temperature at that time is at least the Ar ₃ transformation point and the winding temperature is 600 to 700 ° C.

Further, the present inventors have also found the following points. (5) In addition to reoxidized Al ₂ O ₃ , MnS also acts as a precipitation nucleus for AlN. By reducing this, variability in the material in the longitudinal direction of the coil is further improved.

(6) In order to reduce MnS acting as precipitation nuclei of AlN, it is effective to coarsen MnS and reduce the number of MnS serving as nuclei. It is realized by holding in the area.

The second invention has been made based on the above (5) and (6). In the hot rolling, after the rough rolling, the rough bar is heated in a temperature range of 900 ° C. to 1100 ° C. for 20 seconds to 200
This is a method for producing a hot-rolled steel sheet, which is held for seconds.

It should be noted that the fact that AlN is brought into a solid solution state until winding by direct-feed rolling is described in JP-A-59-110.
No. 722, Japanese Patent Publication No. 56-21330, and the like. However, JP-A-59-110722 is a technique relating to the production of a cold-rolled steel sheet by batch annealing. In order to prevent the precipitation of AlN by low-temperature winding, the method of the present invention is not disclosed. At the time of removal, coarsening of precipitation of AlN is not promoted. Japanese Patent Publication No. 56-21330 discloses that in addition to preventing precipitation of AlN by low-temperature winding on the premise of batch annealing, precipitation of AlN by high-temperature winding on the premise of continuous annealing is also described. . However, as mentioned above,
It is not enough to carry out direct rolling to keep AlN in a solid solution state until winding, and it is not enough to reduce reoxidized Al ₂ O ₃ by optimizing the total iron (T.Fe) in the slag.
More preferably, it is necessary to increase the size of MnS by heat retention of a coarse bar. Unless this is performed, even if high-temperature winding is performed, the precipitation and coarsening of AlN will not be promoted much, and the rear end of the coil tip will not be promoted. Material deterioration is inevitable.

Further, the steel sheet obtained by the present invention may be a soft steel sheet having excellent workability, but “excellent in workability” means that the steel sheet is creased by stretching, stretching flange, bending, drawing, or the like. A predetermined shape and size can be easily formed without generating defects such as defective dimensions. The difficulty of such processing depends not only on the material of the steel plate but also on processing conditions such as pressing conditions, and thus it is difficult to specifically define the processing. However, for example, it is one standard to secure elongation of 30% or more. Also, it is important for processing to be soft,
Considering the capacity of the press equipment and the wear of the mold, 400
MPa or less is desirable.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the reasons for addition and limitation of the additional components of the steel sheet produced by the method of the present invention will be described. C is preferably low to make the steel sheet hard and low ductility, and the upper limit is made 0.07%. However, lowering C to less than 0.02% unnecessarily increases steelmaking costs, so the lower limit is made 0.02%.

Si hardens the steel sheet, causes deterioration of surface properties such as generation of red scale, and also causes deterioration of plating adhesion, so that it is not preferable, and it is necessary to reduce it to 0.1% or less. .

Mn must be added in an amount of 0.1% or more for the purpose of fixing S as MnS and securing hot ductility.
However, if added in excess of 0.5%, the steel sheet becomes hard and low ductile, so the upper limit is made 0.5%.

Since P is an element that makes the steel sheet hard and low ductility, the upper limit must be suppressed to 0.025%. S is
Not only does workability decrease through the formation of MnS,
It is an element that acts as a precipitation nucleus of AlN and hinders keeping AlN in a solid solution state until winding, and the upper limit needs to be 0.025% in order not to make such an adverse effect appear. However, lowering S to less than 0.005% unnecessarily increases steelmaking costs.
05%.

Sol. Al is an element that promotes the precipitation and coarsening of AlN. For this purpose, it is necessary to add 0.03% or more. However, the effect is saturated by adding more than 0.08%,
The upper limit is set to 0.08%, because it only increases costs.

N increases the absolute amount of AlN itself and makes the steel sheet hard and low ductility. Al addition has the effect of promoting Al coarsening because the coarsening of AlN is rate-determined by the diffusion of Al, but it is preferable, but N addition does not contribute to the promotion of coarsening of AlN and increases the absolute amount of AlN itself. The steel sheet is made hard and low ductile through the reduction in grain growth. In order to prevent such adverse effects from manifesting, it is necessary to reduce N to 0.0035% or less.

In addition to the above-mentioned elements, Cu, Cr, Sn, etc. may be added as needed to add required characteristics. Next, the reasons for limiting the manufacturing conditions when the slab having the above component composition is melted and continuously cast are described. In the present invention, hot rolling is performed immediately after continuous casting. "Immediately subjected to hot rolling" refers to "direct rolling" in which hot rolling is immediately performed without holding or heating to a specific temperature range. In the present invention, in order to promote precipitation coarsening of AlN during winding, the precipitation of AlN is suppressed as much as possible until winding, and the supersaturation degree of precipitation is maximized. The method of subjecting the slab to hot rolling (performing direct-feed rolling), once lowering the temperature of the slab, and then reheating the slab is not used.
This is because, in the conventional method in which the slab is once cooled and then reheated, it is necessary to heat the slab at a high temperature of 1250 ° C. or more for 4 hours or more in order to completely re-dissolve the AlN precipitated during the temperature decrease. This is because, in direct rolling, AlN does not precipitate because the slab does not once cool down, and it is possible to maintain AlN in a solid solution state until the start of hot rolling without special consideration. . Further, the method of the present invention is preferable also from the viewpoint of energy consumption and scale loss.

Further, in the present invention, the total iron content (T.Fe) of the slag in the ladle at the time of casting is reduced to 3% or less in order to reduce reoxidized Al ₂ O _{3 serving} as precipitation nuclei of AlN as much as possible. FIG. 1 shows Steel Groups A to C shown in Table 1 (Sol. Al,
Except for N, it is common to Groups A to C. For Sol. Al and N,
In group A, Sol. Al: 0.045% to 0.051%, N:
0.0015% to 0.0020%, Sol.Al in group B:
0.022% to 0.026%, N: 0.0015% to
0.0020%, Sol. Al in group C: 0.047% or more
0.052%, N: 0.0039% to 0.0044%, which is almost constant among the Steel groups), and after continuous casting, the finishing temperature is 840 ° C and the winding temperature is 6 ° C.
Tensile test values (TS, El) at the center of the coil and corresponding tensile test values at the rear end of the coil when directly rolled to a thickness of 2.3 mm at 40 ° C. (no heat retention of the coarse bar after rough rolling) The difference (ΔTS, ΔEl) between T.S. This is the result of sorting by Fe. Here, ΔTS, ΔEl, the coil front end and the coil rear end are defined as follows.

ΔTS: (TS at coil front end + TS at coil rear end) / 2− (TS at coil center) ΔEl: (El at coil front end + El at coil rear end)
/ 2- (El at the center of the coil) Coil tip: 2 m from the end of coil Coil rear: 2 m from the end of coil From FIG. 1, Sol. Al and N are within the scope of the present invention. In the Steel A group, T. in the slag By suppressing Fe to 3% or less, soft ductility is increased. Moreover, even if the coiling temperature of the coil front and rear end and the coil center is the same without any special consideration such as setting the coiling temperature of the coil front and rear end higher than that of the coil center,
The material deterioration of the coil front and rear ends with respect to the coil center is Δ
It can be seen that the level can be suppressed to a very slight level of about 10 MPa in TS and about 2% in ΔEl. On the other hand, T.C. When Fe exceeds 3%, even in Steel A group in which Sol. Al and N are in the range of the present invention, hard ductility is reduced, ΔTS exceeds 30 MPa, ΔEl increases to about 5%, and the coil front and rear ends are increased. The material deterioration is remarkable.

Sol. Al and N are out of the scope of the present invention.
Steel group B and group C not only have hard low ductility as a whole, but also have T.G. Regardless of how Fe is reduced, the material degradation at the rear end of the coil is 20MP in ΔTS.
a, ΔEl does not decrease to about 4% or less. From these experimental results, in the present invention, in addition to optimizing the components such as Sol. Al and N, the T.T. F
It is understood that e needs to be reduced to 3% or less. T. F
e is reduced to 3% or less according to a conventional method, for example, by performing slag reforming by introducing Al into the slag in the slag. Fe is controlled to 3% or less. The charging of Al may be performed either after tapping the converter or in the ladle before casting.

The steelmaking and casting methods are described in Component T. There is no special provision as long as Fe is optimized. In other words, the vacuum degassing process may or may not be performed,
It is also optional to reform the slag during the ladle.

According to the present invention, T.P. Fe is set to 3% or less, and the reoxidized Al ₂ O _{3 serving} as the precipitation nucleus of AlN is reduced as much as possible. In addition to this, the influence of MnS serving as the precipitation nucleus of AlN as in the case of the reoxidized Al ₂ O ₃ is also roughly reduced. It is better to reduce the heat retention than the bar. That is, after the rough rolling, the coarse bar is moved in a specific temperature range (900 to 1100 ° C.) for a specific time (20 to 200 ° C.).
Seconds), the MnS is coarsened by holding, and the number of MnS serving as precipitation nuclei of AlN is reduced, whereby the precipitation of AlN is further suppressed.

FIG. 2 shows the steel of the present invention, which is shown in Table 1 of Steel.
D, and after continuous casting of this, after performing coarse bar heat retention, a finishing temperature of 860 ° C., a winding temperature of 620 ° C., and a sheet thickness of 3.2.
5 shows the results of arranging the material deterioration amounts (ΔTS, ΔEl) of the leading and trailing end portions of the coil with respect to the center portion of the coil at the time of direct rolling to mm in terms of the holding temperature and the holding time of the coarse bar. For comparison, a hot-rolled sheet not subjected to rough bar heat retention was also prepared, and its ΔTS and ΔEl were evaluated. From FIG. 2, the coarse bar is 900
It can be seen that when the temperature is held in the temperature range of 1 to 100 ° C. for 20 seconds to 200 seconds, the deterioration of the material at the front and rear ends of the coil is reduced as compared with the case where the rough bar heat is not maintained. That is, ΔTS: 10 MPa, ΔEl:
In the case where the above coarse bar heat retention was performed with respect to −1.7%, ΔTS ≦ 5 MPa, ΔEl ≧ −1.0% and ΔTS,
ΔEl decreases. On the other hand, when the heat retention of the rough bar is not in the above range, the deterioration of the material at the front and rear ends of the coil is not much different from the case where the heat retention of the rough bar is not performed, or conversely increases. For example, if the holding temperature of the coarse bar is too high, exceeding 1100 ° C., the driving force for the precipitation coarsening of MnS is insufficient, and the holding effect is not substantially generated. Alternatively, if the holding time is as short as less than 20 seconds, the coarsening time of MnS is not secured, and
S does not coarsen sufficiently. In such a case, ΔTS, ΔE
l is 5 to 15 MPa and -1.0 to -2.0%, which is a result that is not much different from the case where the coarse bar heat retention is not performed. When the holding temperature is lower than 900 ° C. or when the holding time is longer than 200 seconds, the precipitation of AlN is promoted in addition to the coarsening of MnS, so that ΔTS and ΔEl are 15 MPa or more and −2.0 MPa.
% Or less results in an increase rather than performing coarse bar heat retention. Based on the results of these studies, according to the present invention, when the heat retention of the rough bar after the rough rolling is performed in order to further reduce the deterioration of the material at the front and rear ends of the coil, the holding temperature is set to 9 °.
00 ° C to 1100 ° C, and the holding time is 20 seconds to 200 seconds. The holding temperature may be constant during the holding, and there is no particular problem whether the temperature is raised or lowered between 900 and 1100 ° C.

Regarding the finish rolling conditions, the finish temperature is Ar
If the temperature is lower than the ₃ transformation point, the material, particularly the elongation, deteriorates. Therefore, the finishing temperature must be equal to or higher than the Ar ₃ transformation point.
Regarding the winding temperature, the lower limit is set to 6 in order to sufficiently precipitate and coarsen AlN even at the rear end of the coil at the time of winding.
The temperature must be set to 00 ° C. However, if the winding temperature exceeds 700 ° C., problems such as deterioration of the surface properties and an increase in the scale of acid pickling will occur, so the upper limit is set to 700 ° C.

[0030]

Next, embodiments of the present invention will be described. (Example 1) Steels E to L shown in Table 1 were melted. Of these, the components of the steels E to I do not change, but at the time of casting, the T.I. Those having changed Fe were prepared (branch numbers 1 to 4). After continuous casting, these steels were rolled to a plate thickness of 3.2 mm by direct-feed rolling without keeping the heat of the rough bars. As a comparison, a slab which was once cooled to room temperature after continuous casting, reheated to 1230 ° C. for 1 hour, and then subjected to hot rolling was also prepared. For these coils, details of the manufacturing conditions and tensile test values (TS, E
l), Table 2 summarizes the material deterioration (ΔTS, ΔEl) at the front and rear ends of the coil.

Ingredients, T. in slag In Examples in which the Fe and hot rolling conditions are both within the range of the present invention, the coil is soft and highly ductile, and it is found that the deterioration of the material at the front and rear ends of the coil is particularly small (at about 10 MPa or less at ΔTS and about −2 at ΔEl). %
that's all). In contrast, T. in the components and slags. In a comparative example in which any of Fe and hot rolling conditions deviated from the present invention, the hardness and the ductility were reduced, and the material deterioration at the rear end portion of the coil tip was significantly larger than that of the example of the present invention.

(Example 2) Steels F-2 and G- described in Table 1
After continuous casting of 1, H-3 and I-2, they were directly rolled to a thickness of 1.2 mm. This was done for coarse bar heat retention, except for comparison. Table 3 summarizes details of the manufacturing conditions, tensile test values at the center of the coil, and material deterioration (ΔTS, ΔEl) at the front and rear ends of the coil for the obtained coil. In the embodiment in which both the holding temperature and the holding time of the heat retention of the coarse bar are within the scope of the present invention, the coil is soft and highly ductile. It can be seen that it has been further improved as compared to the case without the case. That is, when the coarse bar heat retention is not performed, ΔTS: 7 to 10
MPa, ΔEl: −1.2 to −1.7, but those subjected to coarse bar heat retention have ΔTS ≦ 5 MPa, ΔEl ≧ −
1.0%. On the other hand, in the case of the comparative example in which either the holding temperature of the coarse bar heat retention or the holding time deviates from the present invention, the material deterioration of the rear end portion of the coil tip is not much different from the case where the coarse bar heat retention is not performed. Or rather increase.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

As described above, according to the present invention,
In particular, the total iron content in the slag is regulated to 3% or less, and the slab is continuously cast and then directly rolled, so that there is no special equipment investment or a reduction in productivity. No, it is possible to economically produce hot-rolled steel sheets with equal softness and excellent workability over the entire length of the coil with good yield.
Its industrial value is high.

[Brief description of the drawings]

FIG. 1 shows the total iron content (TF) in the slag in a ladle with respect to the deterioration of the material at the center of the hot-rolled coil and the material at the front and rear ends of the coil.
The figure which showed the influence of e).

FIG. 2 is a diagram in which material deterioration amounts at the front and rear ends of a hot-rolled sheet coil are arranged in relation to a holding temperature and a holding time of a coarse bar.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshimasa Funakawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Takuya Terauchi 1-1-2, Marunouchi, Chiyoda-ku, Tokyo Japan (72) Inventor Masaaki Yamamoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-7-90384 (JP, A) JP-A-63-262412 ( JP, A) JP-A-2-66111 (JP, A) JP-B 64-11695 (JP, B2) JP-B 62-5214 (JP, B2) JP-B 7-42496 (JP, B2) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C21C 7/00-7/10

Claims (2)

(57) [Claims] (1) C: 0.02 to 0.07% by weight,
Si ≦ 0.1%, Mn: 0.1-0.5%, P ≦ 0.0
25%, S: 0.005 to 0.025%, Sol. Al:
In a method for producing a hot-rolled steel sheet containing 0.03 to 0.08% and N ≦ 0.0035%, continuous casting is performed with the total iron content in the slag in the ladle being 3% or less, and hot casting is performed immediately after continuous casting. A method for producing a hot-rolled steel sheet, which is subjected to rolling, wherein the finishing temperature is at least the Ar ₃ transformation point and the winding temperature is 600 to 700 ° C. 2. The method for producing a hot-rolled steel sheet according to claim 1, wherein in the hot rolling, after the rough rolling, the rough bar is held in a temperature range of 900 to 1100 ° C. for 20 to 200 seconds.