JP2001226743A - Good workability hot rolled steel sheet excellent in surface property and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a good workability hot rolled steel sheet excellent in surface properties without being subjected to a refining stage after the subsequent coiling after rolling in a hot rolled steel sheet and to provide a producing method therefor. SOLUTION: This good workability hot rolled steel sheet excellent in surface properties contains by weight, 0.015 to 0.06% C, <=0.1% Si, 0.08 to 0.5% Mn, 0.005 to 0.015% S, <=0.0035% N and 0.0005 to 0.020% Mg, in which B is added as well so as to satisfy 1<B<N<2 by the ratio of the quantity to be added, oxides containing Mg with a diameter of <=0.1 μm are distributed at intervals of <=5 μm by the average intergranular distance, also, solid solution carbon is contained by the quantity of <=0.5 ppm, and is obtained by finishing hot roiling in the temperature range of the Ar3 transformation point or more, and also at the time of being passed through the temperature range of (the Ar3 transformation point -150 deg.C) from the above temperature range, performing cooling at a prescribed cooling rate and coiling, and the method for producing the same is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet and a method of manufacturing the same, and more particularly, to preventing a striped pattern on a steel sheet surface from being bent when a coiled hot-rolled steel sheet is unwound. The present invention relates to a hot-rolled steel sheet having good workability and excellent surface properties for imparting hole expandability, and a method for producing the same.

[0002]

2. Description of the Related Art A coiled hot-rolled steel sheet, and particularly a black scale material (an acid-free washing material), does not need to go through a refining process unless it is necessary to correct the shape. However, when the coil is rewound, the buckle may occur.
At this time, measures are taken to prevent buckling by pressure rolls or skin pass rolling. Recently, the threading condition by this method has been studied in detail, and as disclosed in Japanese Patent Application Laid-Open No. H7-111209, the waist break can be performed regardless of the size of the threading material and the sheet temperature by the tension at the time of threading and the roll diameter of the pressure roll. A method of preventing this is disclosed.

However, this method describes a means for rewinding a hot rolled material for refining or the like.
Therefore, there is no need for a fine-tuning board for black scale material, and the cost increases accordingly. In addition, Japanese Unexamined Patent Application Publication No.
In the -56732 publication, the cooling conditions and the winding temperature after finish rolling with B-added steel are specified, solute nitrogen is precipitated as BN during high-temperature hot rolling, and solute carbon is converted into cementite in grains as cementite. By efficiently depositing, it is possible to prevent generation of buckling and stretcher strain.

[0004]

However, in this method, it is disclosed that air cooling time is secured to secure ductility and the transformation rate to α is sufficiently increased. The resulting structure is likely to be non-uniform. As a result, there is a concern about deterioration of hole expandability, which is one of the important factors for workability. The hole expandability is also affected by the cleanliness due to inclusions. Therefore, an object of the present invention is to provide a hot-rolled black scale material (acid-free washing material) which does not need to pass through the refining process.
The object of the present invention is to prevent the occurrence of buckling and to impart workability, particularly excellent hole expandability.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have proposed a low-carbon steel (0.048C
-0.01Si-0.26Mn-0.014P-0.0
08S-0.0025B-0.0015N), the effects of the amount of Mg added, the cooling rate after finish rolling, and the winding temperature on the occurrence of buckling were found, and the following was found. (1) It is known that the cause of occurrence of buckling in hot-rolled black scale material (acid-free washing material) is caused by solid solution carbon or solid solution nitrogen remaining after hot rolling. A detailed examination of the actual situation shows that, especially in low-carbon steel to which B is added, it is thought to be caused by differences in cooling conditions after winding, and fluctuations in the amount of residual solute carbon have a large effect. I do. Conventionally, when the winding temperature is relatively high at about 550 ° C., in order to completely fix the solid solution carbon, it is preferable that the entire area of the coil is cooled to about room temperature if possible by the next step.
However, depending on the season, it may be rewound in a temperature range of about 100 ° C. depending on the season, and it is considered that the occurrence of hip break is inevitable depending on how the pressure roll is applied.

Accordingly, the present inventors have utilized oxides as precipitation nuclei in order to completely fix solute carbon in a hot-rolled sheet, and particularly to realize precipitation of cementite in a low-temperature region,
In particular, it has been found that an oxide containing Mg has the effect. That is, it has been found that when an oxide having an appropriate size is distributed in the grains, MnS is used as a nucleus in addition to MnS, and cementite is efficiently precipitated during cooling after winding. Regarding the effect of the oxide containing Mg, the relationship between the particle size and distribution state and the occurrence of buckling is shown in FIGS.

That is, it has been found that when the oxide containing Mg is present at a certain density or higher, the occurrence of bending is completely eliminated. The cause of this phenomenon is that the oxide containing Mg suppresses austenite grain growth during hot rolling from the change in the amount of solute carbon shown in FIG. It is presumed that the same effect as the increase in the number of precipitation sites of cementite in the rolled sheet was obtained, and the precipitation was promoted. The examination of oxides containing Mg was carried out on a hot rolled sheet using a transmission electron microscope using a replica. At that time, the magnification was set to 10,000 times, and 20 visual fields were examined. As for the size, the major axis and the minor axis were measured, and the average value was defined as the size of the oxide.

(2) In the winding step, it is necessary to reduce the amount of solute carbon by precipitating cementite efficiently. FIG. 3 shows the result of an investigation on the relationship between the winding temperature and the occurrence state of hip break. Here, the cooling rate at the time of transformation from γ to α was 50 ° C./s. As a result, it was found that when the winding temperature was 200 to 450 ° C., no buckling occurred. This is probably because cementite nucleation and growth occurred efficiently in this temperature range.
In addition, from the measurement of the amount of solute carbon by the measurement of internal friction,
It is important that the content be 0.5 ppm or less.

(3) Further, as a result of investigating hole expandability,
As shown in FIG. 4, it was found that in the steel to which Mg was added, the hole expandability was also improved. Table 1 shows the results of investigation of inclusions by the method described in JIS G 0555. M
It is considered that the amount of so-called A and B-based inclusions is reduced as a form of oxide as inclusions by adding g, and as a result, factors that promote the generation and propagation of cracks are reduced. The inclusions were surveyed using a grid plate having a grid line thickness of 5 μm, a grid line number of 20 vertically and horizontally, and a grid spacing of 0.4 mm according to the method described in JIS G 0555, and a measurement magnification of 400 times. Was carried out. 60 views
The field of view was determined from the number of intersections between the inclusions and the lattice, and the cleanliness was determined from S = {n / (f × p)} × 100. Here, f: the number of visual fields, p: the total number of grid points, and n: the grid points occupied by inclusions. Based on the above findings, a workable hot-rolled steel sheet having excellent surface properties that does not cause buckling during unwinding and a method for producing the same have been established.

[0010]

[Table 1]

The gist of the present invention is as follows: (1) By weight ratio, C: 0.015 to 0.07%, Si:
0.1% or less, Mn: 0.08 to 0.5%, S: 0.0
03-0.015%, N: 0.0035% or less, B is contained so as to satisfy 1 <B / N <2 in a ratio of added amount, and 0.0005-0.02% of Mg is further added. An oxide containing the remainder of Fe and unavoidable impurity elements and containing Mg of 0.1 μm or less in diameter, distributed at intervals of 5 μm or less in average interparticle distance, and containing a solid solution carbon amount of 0.5 ppm or less. Highly workable hot-rolled steel sheet with excellent surface properties.

(2) A good workability hot-rolled steel sheet having excellent surface properties, characterized by having a plating layer on the surface of the hot-rolled steel sheet of (1). (3) After the steel described in (1) above is converted into a slab by continuous casting, rough rolling is performed immediately after reheating or casting.
r ₃ and ends the finish rolling at a temperature range of not lower than the transformation point, but starts to cool from the temperature range, Ar ₃ transformation point - when the
At the time of rewinding by cooling to a temperature range of 200 to 450 ° C. at a cooling rate of 50 ° C./s or more when passing through a temperature range of (Ar ₃ transformation point −150 ° C.) A method for producing a hot-rolled steel sheet having good workability and excellent surface properties without causing breakage.

(4) The above-mentioned (3), wherein the rough rolling is completed, and the sheet bar is once wound around a coil and subjected to finish rolling as it is, or the finish bar is connected to a preceding sheet bar and then subjected to finish rolling. 4. A method for producing a hot-rolled steel sheet having excellent surface properties according to item 1. (5) The hot-rolled steel sheet having excellent surface properties according to any one of (3) to (4), wherein rough rolling is performed immediately after casting to a slab of 100 mm or less. Manufacturing method.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the component composition in the present invention will be described. C must be 0.07% or less. If it is added in excess of this, the amount of cementite precipitated on the hot-rolled sheet increases, and not only does the ductility significantly decrease, but also solute carbon remains excessively, preventing breakage. If the amount is less than 0.015%, the driving force of cementite precipitation is greatly reduced, and the amount of dissolved carbon is not reduced in the winding step, but remains on the hot-rolled sheet.

If Si is contained in a large amount in steel, S
The scale containing i is formed unevenly and impairs the appearance.
Therefore, since the hot-rolled black scale material (acid-free washing material) as the object of the present invention is not particularly preferable, the content is set to 0.1% or less. Mn is added when increasing the strength of steel, but excessive addition greatly reduces ductility. Therefore, the upper limit is 0.5%. However, if the content is less than 0.08%, MnS is not formed, which causes hot embrittlement. Therefore, the lower limit is set.

S forms an A-based inclusion (JI
SG0555) not only deteriorates ductility but also causes excessive hot cracking, so the upper limit is 0.015%. Further, it is not preferable to make the content lower than 0.003% because it leads to an increase in cost in steel making. P is an element mainly added for the purpose of increasing the strength,
In the present invention, since it does not require active addition, it need not be specified. However, an excessive content is not preferred because ductility is reduced. For this reason, the content is inevitably included as 0.03% or less, but an excessive reduction is not preferable because it increases steelmaking costs.

Since Al is also an element added for deoxidation, it does not contribute to achieving the object of the present invention. However, if the content is less than 0.005%, the intended effect is not exhibited. On the other hand, if it is added in excess of 0.1%, it will remain in the steel as an oxide, and there is a concern that deterioration in ductility and deterioration in hole expandability will occur. Since N causes the occurrence of buckling as in C, it is necessary to completely fix N as a precipitate in the hot-rolled sheet stage. In the present invention, BN
To fix solid solution nitrogen. However, if it is contained more than necessary, the amount of added B increases accordingly. for that reason,
The upper limit is 0.0035%.

B is added for fixing N and for the purpose of improving the hardenability so as to introduce mobile dislocations due to transformation strain during cooling after finish rolling. If the purpose is to fix only N, B / N = 1.0
However, in order to secure the amount of strain introduced during the transformation by lowering the Ar ₃ transformation point, it is necessary to lower the transformation point. Therefore, such B
In order to effectively exhibit the effect of (1), it is necessary to satisfy 1 <B / N <2. The lower limit is set for the above-mentioned reason, and the upper limit is set so that excessive addition of B hardens the steel material and avoids cracking at the slab stage. That is, the range was 1 <B / N <2.

Mg is the most important element in the present invention. That is, in order for the oxide containing the same to exhibit the effect of suppressing austenite grain growth, 0.0005%
It is necessary. However, even if it is added in an amount of 0.02% or more, the effect is not only saturated, but also the cost is significantly increased. Note that the inclusion of trace amounts of Cu, Ni, Sn and Cr due to the use of scrap does not impair the effects of the present invention. The heating temperature in the hot rolling step of the present invention is not particularly limited because BN can precipitate even at a relatively high temperature. In the present invention, there is no need to specify the MnS precipitation size as disclosed in JP-A-4-56732.

The cooling condition after hot rolling is one of the most important control factors in the present invention. That is, in order to secure the driving force necessary for the precipitation of cementite, it is necessary to increase the cooling rate from the Ar ₃ transformation point to the (Ar ₃ transformation point -150 ° C.) after finish rolling. Therefore, 50 ° C
If the speed is lower than / s, the driving force for precipitation is insufficient, and it becomes difficult to efficiently precipitate cementite. Therefore, this cooling rate is set as the lower limit. By the way, it is considered that the cooling rate can be further relaxed by increasing the amount of Mn or B to be added. 50 ° C /
s or more. The present invention is also characterized in that a strain is introduced at the time of transformation from γ to α to secure a movable dislocation density.

The winding temperature must be within a temperature range in which cementite is efficiently deposited during cooling after winding into a coil. In the present invention, the temperature is set to 200 to 450 ° C. However, if the temperature is lower than 200 ° C., C is not sufficiently diffused, so that C is insufficiently fixed during the winding step. As a result, there is a concern about occurrence of hip break. On the other hand, 45
If the temperature exceeds 0 ° C., the amount of precipitation becomes insufficient, and C remaining in a solid solution state increases, so that there is also a danger that a buckling may occur. Furthermore, in this temperature range, film boiling tends to occur, so that the uniformity of the winding temperature is deteriorated. The present invention has been made mainly for the purpose of omitting the passing of the hot-rolled steel sheet, particularly the black scale material (acid-free washing material), in the refining process. It may be applied to what is planed.

[0022]

(Example 1) C: 0.045%, Si: 0.01%, Mn: 0.26
%, P: 0.014%, S: 0.008%, Al: 0.
027%, N: 0.0024%, B: 0.0025%,
A steel containing 0.0025% of Mg was output from a converter and made into a slab by continuous casting. After hot rolling at 1200 ° C., rough rolling was performed, and then finish rolling was completed in the temperature range shown in Table 2,
A 2.3 mm hot rolled sheet was used. Thereafter, cooling and winding were performed under the conditions shown in Table 2. Here, Ar ₃
The transformation point is 916-50 [C (%)] + 27 [Si
(%)] Is approximately 892 ° C. when estimated by −64 [Mn]. In addition, the size of the oxide containing Mg and the average interparticle distance were 0.07 and 3.5 μm, respectively, on average. For the evaluation of the material, the wound coil was unwound to investigate the occurrence of buckling, processed into a No. 5 test piece described in JIS Z 2201, and a tensile test was performed according to the test method described in JIS Z 2241. In addition, the hole expandability was investigated using a 60-degree conical punch with a diameter of 10 mm.
The hole of (d ₀ ) was pushed and expanded, and the hole diameter (d) at the time when the crack penetrated the plate thickness was measured and evaluated by d / d ₀ . Table 2 shows the results.

[0023]

[Table 2]

Nos. 1, 2, 3, according to the method of the present invention
There is no break in 4, 5, 6, 7 and 8, and
Since the material is soft, ductility is high and d / d ₀ exceeding 2.5 is obtained. On the other hand, when the winding temperature was out of the range of the present invention, No. In No. 11, even if transformation strain is introduced by quenching after finish rolling, not only the YP-El is large but also the breakage of the hip cannot be prevented due to the large amount of residual solid solution carbon. On the other hand, no. In 10,
Since precipitation in grains is insufficient, solute carbon remains, so that not only YP-El is large, but also bending can not be prevented. No. In No. 9, since the cooling rate after finish rolling deviated from the range of the present invention, the strain introduced during the transformation from γ to α intended in the present invention was small, and the driving force for precipitation of cementite was small. As a result, a high Y
In addition to showing P-El, hip breakage occurs and the hole expandability is poor. Further, No. ₃ in which the finishing temperature was lower than the Ar ₃ transformation point. In No. 12, since the strain introduced at the time of transformation from γ to α is small, not only the hip break cannot be prevented but also the hole expandability is not good. Also,
Low ductility due to non-uniform texture.

Example 2 Various steels shown in Table 3 were output from a converter and slabs were produced by continuous casting. Hot rolling is 1150-12
After heating at 50 ° C., rough rolling and finish rolling were performed to produce a hot-rolled sheet having a thickness as shown in Table 4. The finish rolling was completed in the temperature range not lower than the Ar ₃ transformation point. Furthermore, the amount of water in the cooling zone was adjusted so that the cooling rate after finish rolling was within the scope of the present invention. After cooling,
Winding was performed at 350 ° C., and the occurrence of buckling was investigated while unwinding the wound coil, and material evaluation and hole expandability evaluation by a tensile test were performed in the same manner as in Example 1. Table 2 shows the results.

[0026]

[Table 3]

[0027]

[Table 4]

The B, C, D, E, F and G steels according to the present invention have hole expandability of more than 2.5 without occurrence of breakage. On the other hand, steel A with a low carbon content does not reduce the amount of solute carbon during winding, causes breakage, and has a large YP-El. In the case of H steel with a high carbon content, the material becomes harder and the YP-El
Is large, causing breakage. Also, the hole expandability is low. In steel I, the amount of N greatly deviated from the range of the present invention, and as a result, B / N also deviated to a low level, so that dissolved nitrogen remained,
Not only is the ductility low, but the hip break cannot be prevented. Also,
In the J steel, the N content is within the range of the present invention, but the added amount of B is low, and the B / N deviates from within the range of the present invention.

Further, in the case of O steel in which B / N deviates from the range of the present invention to a high degree and a large amount of solid solution B remains, no breakage occurs but the strength is too high.
In the K steel in which the amount of Si is deviated from high, scale flaws caused by Si are generated. L steel and S with low Mn content
In the case of the N steel with a large amount deviated, edge cracks occurred at the edge portion by hot rolling. In addition, since bainitic ferrite cannot be obtained from L steel, hole expandability is low. Further, in the M steel in which the amount of Mg was deviated low, the precipitation amount of oxides containing Mg was insufficient, so that refinement of crystal grains could not be realized, and the precipitation site of cementite was reduced, so that YP-El was large. ,
A hip break has occurred.

(Example 3) The D and E steels of Example 2 were subjected to hot-dip rolling plating and electroplating. The hot-dip galvanizing was carried out by pickling and then Ni-plating, followed by heating to 420 ° C. by an electric heating method, and then immersing in pure zinc and a zinc bath containing 0.20 and 0.51% of Al and Mg, respectively. On the other hand, for electroplating, Zn-Ni plating was performed. In each case, good plated steel sheets were obtained.

[0031]

Industrial Applicability According to the present invention, a hot rolled steel sheet, in particular, a black scale material (an acid-free washing material) can be prevented from breaking and having high hole expandability without passing through a refining process. Thus, a hot-rolled steel sheet having good workability can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the oxide size, the state of occurrence of buckling, and the amount of solute carbon.

FIG. 2 is a graph showing the relationship between the average oxide particle distance, the occurrence of buckling, and the amount of solute carbon.

FIG. 3 is a diagram showing a relationship between a winding temperature and a state of occurrence of hip break.

FIG. 4 is a diagram showing the effect of Mg addition on hole expandability.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K032 AA00 AA01 AA02 AA04 AA16 AA21 AA26 AA27 AA29 AA31 BA01 CA02 CA03 CC04 CD03 CE01 4K037 EA00 EA01 EA02 EA05 EA15 EA18 EA22 EA23 EA25 FC04 FA02 FC03 FA04 HA05 JA06

Claims (5)

[Claims] 1. A weight ratio of C: 0.015 to 0.07%, Si: 0.1% or less, Mn: 0.08 to 0.5%, S: 0.003 to 0.015%, N: 0.0035% or less, O: 0.0035% or less B is contained so as to satisfy 1 <B / N <2 in an addition amount ratio, and Mg is added in an amount of 0.0005 to 0.02%. The balance consists of Fe and unavoidable impurity elements and has a diameter of 0.1
The oxide containing Mg of less than μm
A good-workability hot-rolled steel sheet having excellent surface properties, wherein the hot-rolled steel sheet is distributed at intervals of not more than m and contains a solid solution carbon amount of not more than 0.5 ppm. 2. A highly workable hot-rolled steel sheet having excellent surface properties, wherein the hot-rolled steel sheet according to claim 1 has a plating layer on a surface thereof. 3. After the steel according to claim 1 is cast into a slab by continuous casting, rough rolling is performed immediately after reheating or casting, and finish rolling is completed in a temperature range not lower than the Ar ₃ transformation point; Cooling is started from that temperature range. At that time, the cooling rate when passing through the temperature range from the Ar ₃ transformation point to (Ar ₃ transformation point -150 ° C.) is set to 50 ° C./s or more, and 200 to 450 ° C.
A method for producing a hot-rolled hot-rolled steel sheet having excellent surface properties which does not cause breakage during rewinding after cooling to the temperature range described above. 4. The method according to claim 3, wherein the rough rolling is completed, the sheet bar is once wound into a coil and subjected to finish rolling as it is, or finish rolling is performed after connecting to a preceding sheet bar. Of hot-rolled steel sheet with good workability and excellent surface properties. 5. A rough rolling is carried out immediately after casting to a slab of 100 mm or less.
The method for producing a hot-rolled steel sheet having excellent workability according to any one of the above items.