JP3341611B2 - Manufacturing method of yield strength controlled steel sheet for building - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having a low yield ratio for a welded structure, which is used for construction, civil engineering and the like and requires a high-precision stress design, and which has a low yield strength. The present invention relates to a method for manufacturing a yield-strength-controlled steel sheet for buildings, which has small variations.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of ensuring the safety of buildings in the event of a large earthquake, the ultimate yield strength is to prevent the collapse of columns by absorbing the seismic energy by yielding the beams prior to the yielding of the columns. Design methods have begun to be applied to structures such as buildings.

[0003] In order to sufficiently absorb seismic energy, it is necessary that the steel material can be greatly deformed when an earthquake occurs, and for that purpose, the steel material is required to have a low yield ratio. Further, in order to yield the beam prior to the yield of the column as intended by the designer, it is required that the yield strength of the steel material be small.

[0004] As a method for producing a low yield ratio high-tensile steel sheet having a tensile strength of 490 MPa or more, a method in which on-line controlled rolling and accelerated cooling are generally performed so as not to impair productivity.

[0005] In a steel sheet manufactured by hot rolling, a part of the primary scale generated during heating and a part of the secondary scale generated during hot rolling are sufficiently reduced during descaling performed during hot rolling. It remains on the surface of the steel sheet without being removed, causing unevenness in the scale layer thickness on the surface of the steel sheet.
Uneven scale may occur.

[0006] In the process of producing a steel sheet which is controlled rolling and then accelerated cooling performed on-line, unevenness of scale occurs due to the above-mentioned unevenness in scale, and when uneven cooling is stopped, variation in steel sheet strength is increased. I have. When a steel sheet with a large variation in strength is used for a building,
When an earthquake occurs, the columns and beams do not yield as intended by the designer, and as a result, there is a high risk that the building will collapse.

[0007] The primary scale generated during heating is removed by injection of high-pressure water by a hydraulic scale breaker (HSB) arranged on the inlet side of the rough rolling mill, and the secondary scale generated during rolling is coarse. It is removed by the injection of high-pressure water by a hydraulic scale breaker (HSB) arranged on the entry side of each of the rolling mill and the finishing mill.

[0008]

However, with respect to the above-mentioned conventional technique for removing secondary scales, in particular, when scale removal is not performed sufficiently due to interference of high-pressure water or uneven stagnation during descaling. As a result, the steel sheet is rolled down while the scale unevenness is formed on the surface of the steel sheet, which causes a problem that the characteristic values of the steel sheet such as the strength vary.

In order to solve the above-mentioned problem relating to scale control, Japanese Patent Application Laid-Open No. Hei 7-48622 discloses that immediately after rolling, a steel sheet is water-cooled to a temperature range of 700 to 800 ° C. to form a secondary scale. (Hereinafter, referred to as prior art) is disclosed. However, depending on the prior art, in order to perform this, in the case of controlled rolling, it is necessary to install equipment or the like that uniformly cools the steel sheet between the rough rolling mill and the finishing rolling mill, and for this reason, it is enormous. Equipment costs are required.

[0010] Accordingly, an object of the present invention is to solve the above-mentioned problems, to prevent scale unevenness from occurring on the surface of a steel sheet, to provide a tensile strength of 490 MPa or more, a yield ratio of 80% or less, and an index of the variation in yield strength within the same steel sheet. , The variation width of the yield strength 3σ (σ is a standard difference, hereinafter referred to as “3σYS”) is 30M.
It is an object of the present invention to provide a method for producing a yield strength control steel sheet for building, which is a high yield strength steel sheet having a low yield ratio of Pa or less.

[0011]

SUMMARY OF THE INVENTION From the above-mentioned viewpoints, the present inventors have found that there is no scale unevenness on the steel sheet surface.
Thereby, the tensile strength is 490 MPa or more, the yield ratio is 80% or less, and the variation 3σYS in the yield strength is 30.
In order to develop a method of manufacturing a steel sheet with a controlled yield strength, which is a high-strength steel sheet having a low yield ratio and a low yield ratio, which is equal to or less than MPa, the present inventors have conducted intensive studies.

The present inventors first observed in detail the scale unevenness occurring on the surface of a steel sheet. As a result, the unevenness of the scale generated on the surface of the steel sheet is compared with the fine powdery red scale having a thickness of 20 μm or more and the thickness of the uniform and excellent adhesion of 10 μm.
It turned out that it consisted of a thin black-gray scale of μm or less.

The formation of the red scale is caused by thick scale partially remaining due to interference or stagnation of high-pressure water during descaling during high-temperature rolling of the steel sheet. The remaining thick scale is pulverized by rolling to increase its surface area, and is oxidized to grow even thicker. When the steel sheet having the thick red scale thus grown remaining on the surface is rolled, scale unevenness occurs due to the thick red scale layer.

A steel sheet having a chemical composition shown in Table 1 was subjected to controlled rolling and accelerated cooling, and a 50 mm-thick steel sheet having a thickness 1/8 (t / 8) from the surface and from the surface. FIG. 1 shows the results of investigating the influence of the scale morphology on the yield strength at the quarter thickness (t / 4) portion.

[0015]

[Table 1]

As is apparent from FIG. 1, the yield strength of the steel sheet at the t / 8 and t / 4 portions where the thick red scale is generated is greatly reduced to 50 to 60 MPa as compared with the metal ground. On the other hand, the yield strength of the steel sheet in the portion where the thin black ash scale was generated was only slightly reduced by about 10 MPa as compared with the metal ground.

From the above, if scale unevenness including a red scale is formed on a steel sheet that has been accelerated and cooled, the influence thereof appears in the form of a variation in strength. Since it reaches the part of the thickness (t / 4) of 1 /, it was found that it is necessary to remove the red scale in order to suppress the variation of the yield strength at the time of accelerated cooling.

The present invention has been made on the basis of the above-mentioned findings, and has a C content of 0.04 to 0.18 wt.
0.05-0.8 wt.%, Mn: 0.2-1.7 wt.%, And Al: 0.07 wt.% Or less, and at least one element selected from the group consisting of: Ti: 0.
03 wt.% Or less (including 0), Nb: 0.05 wt.% Or less (including 0), V: 0.1 wt.% Or less (including 0), C
u: 0.7 wt.% or less (including 0), Ni: 0.7 wt.% or less (including 0), Cr: 1.3 wt.% or less (including 0),
And Mo: 0.8 wt.% Or less (including 0), and the rest: rough rolling and controlled rolling started from a temperature range of 950 ° C. or less on a slab consisting of Fe and unavoidable impurities. By performing hot rolling and then accelerated cooling, in the step of manufacturing a yield strength control steel sheet for building, the steel slab is passed through a rolling mill only by descaling without applying reduction, and then the descaling is performed. 1 second or more after passing, without rolling again, or passing the rolling mill to perform the rolling and descaling, immediately before the control rolling or during the control rolling early pass, The feature is that it is performed at least once or more.

[0019]

Next, in the method of the present invention,
The reason for limiting the chemical composition of the steel slab to the above range will be described. (1) C: C is 0.04w to secure the strength of the steel sheet.
It is necessary to contain t.% or more. However,
When the C content exceeds 0.18 wt.%, The weldability is significantly deteriorated. Therefore, the C content should be limited to the range of 0.04 to 0.18 wt.%. (2) Si: Si must be contained in an amount of 0.05% by weight or more as a deoxidizing agent. However, when the Si content exceeds 0.8 wt.%, Weldability, ductility, and toughness deteriorate. Therefore, the Si content should be limited to the range of 0.05 to 0.8 wt.%. (3) Mn: Mn is used to improve the strength and toughness.
It is necessary to contain 2 wt.% Or more. However, when the Mn content exceeds 1.7 wt.%, The hardenability is remarkably increased, and it is at the time of welding. The crack sensitivity increases. Therefore, the Mn content should be limited to the range of 0.2 to 1.7 wt.%. (4) Al: Al is an element added as a deoxidizing agent. However, if the Al content exceeds 0.07 wt.%, The cleanliness of the steel decreases, and the toughness of the weld deteriorates. Therefore,
The Al content should be limited to 0.07 wt.% Or less.
Note that the deoxidizing action is not particularly limited, since Si can be substituted. (5) Ti, Nb, V, Cr and Mo: Ti, Nb,
V, Cr and Mo are all elements that contribute to improving the strength of steel. Therefore, at least one of them is added as needed. However, the Ti content is less than 0.03 wt%, the Nb content is less than 0.05 wt.%, The V content is less than 0.1 wt.%, The Cr content is less than 1.3 wt.%, And The Mo content should each be limited to 0.8 wt.% Or less. When the contents of Ti, Nb, V, Cr and Mo exceed the above ranges, the weldability and the toughness are deteriorated. (6) Cu: Cu is an element effective in securing strength. However, if the Cu content exceeds 0.7 wt.%, It causes cracking during hot rolling. Therefore, Cu
The content should be limited to 0.7 wt.% Or less. (7) Ni: Ni is also an effective element for securing strength, and has an effect of preventing cracks from occurring during hot rolling by Cu. However, when the Ni content is 0.7
If it exceeds wt.%, the effect is saturated. Therefore, the Ni content should be limited to 0.7 wt.% Or less.

Hot rolling, comprising rough rolling and controlled rolling starting from a temperature range of 950 ° C. or less, under the same rolling conditions as for a conventional low yield ratio high strength steel sheet for building, is performed on a steel slab having the above chemical composition. And accelerated cooling. In the present invention, in the hot rolling, the steel slab is passed through a rolling mill only by descaling without applying rolling, and then, after a lapse of 1 second or more after the descaling, without applying rolling again. Alternatively, it is necessary to perform at least one step of immediately before the control rolling or between the passes at the initial stage of the control rolling, in which the step of applying the rolling and passing the rolling through the rolling mill to perform the descaling is performed. Hereinafter, this point will be described.

At the beginning of the controlled rolling, the steel sheet was subjected to a descaling process according to the descaling pattern shown in FIG. That is, spray headers 3 and 4 for spraying high-pressure water toward the front and back surfaces of the steel plate 1 are provided on the entrance and exit sides of the pair of finish rolling rolls 2 and 2 ', respectively. While performing a forward pass and a reverse pass of the steel sheet 1 between 2 and 2 ′, high-pressure water is sprayed on the surface of the steel sheet 1 by one of the spray headers 3 and 4 according to the traveling direction of the steel sheet 1, 1 was subjected to descaling. The descaling process described above
The following six types of descaling patterns were performed.

Descaling pattern This is a conventional descaling pattern for comparison. As shown in FIG. 2 (a), a steel sheet 1 is forward-passed between finishing rolls 2 and 2 'in the direction of the arrow. , Steel plate 1
, High-pressure water is sprayed from the spray header 3 onto the surface of the steel sheet 1 to perform the first-stage descaling, and then the steel sheet 1 is placed between the finish rolling rolls 2 and 2 ′. Make a reverse pass in the direction of the arrow to
, High-pressure water was sprayed from the spray header 4 onto the surface of the steel sheet 1 to perform a second-stage descaling while applying a reduction at a predetermined reduction rate.

FIG. 2 shows a descaling pattern for comparison.
As shown in (b), high-pressure water is sprayed from the spray header 3 onto the surface of the steel sheet 1 while the steel sheet 1 is forward-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow without reducing the steel sheet 1 to one step. The eyes are descaled, and then the steel sheet 1 is immediately reverse-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow without any gap, and the steel sheet 1 is sprayed while being reduced at a predetermined reduction rate. High-pressure water was sprayed from the header 4 onto the surface of the steel plate 1 to perform the second-stage descaling.

Descaling Pattern The descaling pattern of the method of the present invention,
As shown in (a), high-pressure water is sprayed onto the surface of the steel sheet 1 from the spray header 3 while the steel sheet 1 is forward-passed in the direction of the arrow between the finish rolling rolls 2 and 2 'without applying any reduction. The descaling of the eyes is performed, and after 1 second, the steel sheet 1 is reverse-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow, and the steel sheet 1 is subjected to a reduction by a predetermined reduction rate while being spray-headed. From 4 onward, high pressure water was sprayed onto the surface of the steel sheet 1 to perform the second-stage descaling.

Descaling Pattern The descaling pattern of the method of the present invention,
As shown in (b), high-pressure water is sprayed from the spray header 3 onto the surface of the steel sheet 1 while the steel sheet 1 is forward-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow without reducing the steel sheet 1 to one step. The descaling of the eyes is performed, and after 2 seconds, the steel sheet 1 is reverse-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow, and the steel sheet 1 is subjected to a reduction by a predetermined reduction rate while being spray-headed. From 4 onward, high pressure water was sprayed onto the surface of the steel sheet 1 to perform the second-stage descaling.

Descaling Pattern The descaling pattern of the method of the present invention,
As shown in (c), high-pressure water is sprayed onto the surface of the steel sheet 1 from the spray header 3 while the steel sheet 1 is forward-passed in the direction of the arrow between the finish rolling rolls 2 and 2 'without any reduction. The eyes are descaled, and after 1 second, the steel sheet 1 is reverse-passed in the direction of the arrow between the finish rolling rolls 2 and 2 'without applying a reduction, and the steel sheet 1 is reduced by a predetermined reduction rate. While spraying, high-pressure water is sprayed from the spray header 4 onto the surface of the steel sheet 1 to perform the second-stage descaling.
Is passed forward between the finish rolling rolls 2 and 2 ′ in the direction of the arrow, and while the steel sheet 1 is being reduced at a predetermined reduction rate, high-pressure water is sprayed from the spray header 3 onto the surface of the steel sheet 1 to perform the third step. Was descaled.

Descaling Pattern The descaling pattern of the method of the present invention,
As shown in (d), high-pressure water is sprayed from the spray header 3 onto the surface of the steel sheet 1 while the steel sheet 1 is forward-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow without rolling down the steel sheet 1 to perform one step. The descaling of the eyes is performed, and after 1 second, the steel sheet 1 is reverse-passed between the finish rolling rolls 2 and 2 'in the direction of the arrow, and the steel sheet 1 is subjected to a reduction by a predetermined reduction rate while being spray-headed. 4, high-pressure water is sprayed onto the surface of the steel sheet 1 to perform the second-stage descaling, and then after 1 second, the steel sheet 1 is reversed between the finish rolling rolls 2 and 2 ′ in the direction of the arrow without any reduction. While passing the steel sheet 1 at a predetermined reduction rate, high-pressure water is sprayed from the spray header 3 onto the surface of the steel sheet 1 to perform the third-stage descaling. 1 The steel sheet 1 is subjected to a reverse pass in the direction of the arrow between the finish rolling rolls 2 and 2 ′, and while the steel sheet 1 is subjected to reduction at a predetermined reduction rate, the steel sheet 1 is removed from the spray header 4.
The fourth stage of descaling was performed by injecting high-pressure water onto the surface of.

FIG. 4 shows the rate of occurrence of red scale on the surface of the steel sheet descaled according to the above descaling pattern. As is clear from FIG. 4, when descaling was performed according to the descaling pattern according to the method of the present invention, the rate of occurrence of red scale on the surface of the steel sheet was zero.

This is due to the following two effects. That is, the first is that immediately after the first stage of descaling, even if there is any remaining part of the scale, it is pulverized by a finish rolling roll and pressed by pressing through a rolling mill without applying any reduction. Because there is no. Second, by providing a waiting time of 1 second or more between descaling, a crack develops in the remaining scale using the temperature difference between the scale layer and the metal, and the second stage of descaling. This is because the scale can be completely removed.

FIG. 5 shows the average yield strength, the average yield ratio, and the variation in the yield strength (3σYS) of the steel sheet subjected to the descaling pattern described above.
Is shown. In addition, the tensile test value is 1
The average value was shown as zero.

As is apparent from FIG. 5, when descaling is performed by the scaling pattern of the present invention, the surface of the steel sheet is mainly covered with a uniform black ash scale, and the scale of the steel sheet surface is reduced. Since the layer thickness is almost constant, the variation in yield strength after accelerated cooling (3
σYS) could be suppressed to 30 MPa or less.

As described above, in the present invention,
The hot rolling of the billet is passed through a rolling mill without depressing only by descaling, and after elapse of 1 second or more, again, only the descaling or the rolling with the descaling is passed through the rolling mill. It is necessary to perform at least one time immediately before the control rolling and at least once during the initial pass of the control rolling, and to perform accelerated cooling after performing such hot rolling, thereby greatly reducing the variation in yield strength. Becomes possible.

[0033]

Next, the method of the present invention will be further described with reference to examples and comparative examples. A steel having a chemical composition within the scope of the present invention shown in Table 2 was smelted and prepared into a slab by continuous casting including a light reduction process.

[0034]

[Table 2]

Next, after rough rolling of the slab, hot rolling is performed under the conditions shown in Table 3 and then accelerated cooling is performed. During the initial pass of controlled rolling, the slab is subjected to the above-described method of the present invention shown in FIG. The descaling is performed according to the scaling pattern or the descaling pattern according to the comparison method shown in FIG.
1 to B4, C1, C2, D1, E1, F1, G1, G
2, H1 and I1, and comparative steel plates A5, A6, B
6, B6, C3, D2, E2, F2, G3, H2 and I2 were prepared.

[0036]

[Table 3]

With respect to each of the steel sheet of the present invention and the comparative steel sheet thus prepared, their surface properties, average yield strength, average tensile strength, average yield value, and variation in yield strength (3σYS) were examined. The results are shown in FIG. In addition, as a tensile test piece (JIS 14A), red scale,
Four pieces each were collected in the L direction from the thickness t / 4 of the ash scale and the metal basement. The tensile test value was shown as an average value of ten randomly sampled samples.

[0038]

[Table 4]

As is clear from Tables 3 and 4, at the beginning of the controlled rolling, the descaling pattern ~
After descaling according to the present invention, the steel sheets A1 to A4, B1 to B4, C1, C2,
In D1, E1, F1, G1, G2, H1, and I1, the surface properties are thin and uniform black-gray scale, the average yield ratio is 80% or less, and the variation in yield strength (3σYS) is 30 MPa. The results were as follows, with a low yield ratio and a small variation in yield strength.

On the other hand, at the beginning of the controlled rolling, the comparative steel plates A5, A6,
In the case of B6, B6, C3, D2, E2, F2, G3, H2, and I2, the surface properties include scale unevenness including a thick red scale, and the average yield ratio was 80%.
The variation in yield strength (3σYS) is 43
It was ~ 98 and it was big.

[0041]

As described above, according to the method of the present invention, there is no scale unevenness on the surface of the steel sheet, the tensile strength is 490 MPa or more, the yield ratio is 80% or less, and the yield strength varies. (3σYS) of 30 MPa or less, industrially useful, capable of economically producing a building yield strength control steel sheet as a high-strength steel sheet for welded structures requiring high-precision stress design. The effect is brought.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of scale morphology on yield strength.

FIG. 2 is a diagram showing a conventional descaling pattern.

FIG. 3 is a diagram showing a descaling pattern according to the method of the present invention.

FIG. 4 is a diagram showing a red scale occurrence area ratio for each descaling pattern.

FIG. 5 is a diagram showing variations in average yield strength, average yield ratio, and yield strength of a steel sheet for each descaling pattern.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2, 2 'roll 3 Spray header 4 Spray header

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-253811 (JP, A) JP-A-8-206723 (JP, A) JP-A-6-279923 (JP, A) JP-A-7-207 268456 (JP, A) JP-A-58-48623 (JP, A) JP-A-7-48622 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02 B21B 3 / 02 C22C 38/00 C22C 38/58

Claims (1)

(57) [Claims] 1. C: 0.04 to 0.18 wt.%, Si: 0.05 to 0.8 wt.%, Mn: 0.2 to 1.7 wt.%, And Al: 0.07 wt.% %: At least one element selected from the group consisting of: Ti: 0.03 wt.% Or less (including 0), Nb: 0.05 wt.% Or less (including 0), V: 0.1 wt.% Or less (including 0), Cu: 0.7 wt.% Or less (including 0), Ni: 0.7 wt.% Or less (including 0), Cr: 1.3 wt.% Or less (0 And Mo: 0.8 wt.% Or less (including 0), and the remainder: Starting from rough rolling and a temperature range of 950 ° C. or less for a steel slab composed of Fe and unavoidable impurities. In the process of producing a yield strength control steel sheet for building by performing hot rolling consisting of controlled rolling and then accelerated cooling, the steel slab is subjected to reduction. It is passed through a rolling mill only by descaling, and then, after one second or more has passed since the descaling, without passing down again, or passing through a rolling mill to perform rolling down and descaling. A method for producing a yield strength control steel sheet for building, wherein the step is performed at least once or more immediately before the control rolling or during the initial pass of the control rolling.