JP2745058B2 - Method for manufacturing hot-rolled steel sheet with excellent coating adhesion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a hot-rolled steel sheet for processing used in automobile parts such as underbody parts and bumpers, pipes, drums, and the like, and particularly descaling such as pickling. The present invention relates to a method for manufacturing a hot-rolled steel sheet which can be processed without performing a treatment and has excellent paint adhesion.

[Conventional technology and its problems]

The secondary scale of the steel sheet manufactured by hot rolling is inevitable in peeling during forming and has poor coating adhesion, so after performing descaling treatment such as pickling, processing is performed,
Thereafter, a method of painting was adopted. However, since this is economically disadvantageous, a hot-rolled steel sheet having excellent scale adhesion that does not require descaling has been disclosed in JP-A-61-194112,
This is proposed in JP-A-59-222533 and JP-A-62-136529.

In the method described in JP-A-61-194112, the winding temperature is set to 50.
The temperature must be below 0 ° C and above 360 ° C. When this temperature range is controlled by water cooling which is usually performed, there is a problem that temperature unevenness is likely to occur in the coil longitudinal direction or the coil width direction. The same publication also stated that the winding temperature was moderated,
The following winding method has been proposed.
, Which is economically disadvantageous.

Next, according to the methods described in JP-A-59-222533 and JP-A-62-136529, it is necessary to cool in a non-oxidizing atmosphere after winding, and an air shut-off / atmosphere adjusting device is indispensable for this. Disadvantageous.

Further, these existing bills suggest measures for improving the adhesion of the scale, but do not consider the adhesion of the paint.

[Problems to be solved by the invention]

The present invention eliminates the difficulties of the cooling control technology as described above, does not use special equipment such as an air shutoff / atmosphere adjustment device after winding, and is economical and does not require the addition of special elements. It is an object of the present invention to provide a method for producing a hot-rolled steel sheet with a scale excellent in coating adhesion as high as that of a material.

[Means for solving the problem]

The present inventors have conducted various studies to solve the above problems, and as a result, the content of Si, Al in steel greatly affects scale adhesion and coating adhesion, and adjusts the Si, Al to a specific range. As a result, they have found that a scale layer having excellent adhesion can be obtained stably without lowering the winding temperature. The present invention has been made based on this new finding, and the gist is as follows.

(1) C ≦ 0.2wt%, Si ≦ 0.04wt%, P ≦ 0.025wt%, S ≦
0.015wt%, Al ≦ 0.02wt%, and 2 × Al (wt%) + Si (wt%) ≦ 0.06 between Al and Si, with the balance being
After smelting steel consisting of Fe and unavoidable elements, insert the slab obtained by the continuous casting method into a heating furnace, or once make it a cold piece, reheat it and rescale it one or more times during rough rolling. After removing the scale and removing the scale after the rough rolling and before the finish rolling, the finish rolling was started at a temperature of 1100 ° C or less, and 900
After finishing rolling at a temperature of ℃ or less, start cooling with water injection within 5 seconds, quench at a cooling rate of at least 15 ℃ / second, 550 ℃
A method for producing a hot-rolled steel sheet with scale excellent in coating adhesion, characterized by winding at a temperature of at least 700 ° C.

(2) C ≦ 0.2wt%, Si ≦ 0.04wt%, P ≦ 0.025wt%, S ≦
0.015 wt%, Al ≦ 0.02 wt%, and 2 × Al (wt%) + Si (wt%) ≦ 0.06 between Al and Si, and
0001wt% ≦ B ≦ 0.0050wt%, 0.005wt% ≦ Nb ≦ 0.07wt%,
0.001wt% ≦ Ti ≦ 0.1wt%, 0.0005wt% ≦ Ca ≦ 0.01wt%, containing one or more kinds of steel, the balance being Fe and unavoidable elements. Insert the obtained steel slab into a heating furnace, or once chilled, reheat, perform one or more scale removals during rough rolling, and perform scale removal after rough rolling and before finish rolling. After 1100
Start the finish rolling at a temperature below 900C, finish the finish rolling at a temperature below 900C, start water injection cooling within 5 seconds,
A method for producing a hot-rolled steel sheet with excellent coating adhesion, characterized by rapidly cooling at a cooling rate of 15 ° C / sec or more and winding at a temperature of 550 ° C to 700 ° C.

(Operation)

The effects brought about by the limitation of the components and hot rolling conditions in the present invention are described below.

C is used to ensure the strength of the steel sheet, but cementite (Fe ₃ C) is formed in the steel, and its increase deteriorates the scale adhesion and weldability.
The upper limit of C is set to 0.2 wt%.

Since Si concentrates on the surface of the ground iron during heating and hot rolling and deteriorates the scale adhesion and the coating adhesion at the same time, the smaller the better, the better. Therefore, the upper limit is made 0.04 wt%.
Furthermore, since Si interacts with Al, 2 x Al (wt%) + Si
(Wt%) ≦ 0.06.

P and S also concentrate on the surface of the iron base during heating and hot rolling, and deteriorate the scale adhesion.
% Or less and S of 0.015 wt% or less can prevent scale adhesion deterioration. For the purpose of the present invention, it is preferable that both P and S are small, but the processing cost increases for both de-P and de-S. Therefore, the content should be appropriately determined according to the above-mentioned values in consideration of the economical allowable range. Is good.

Al is the most important element for obtaining the properties of the steel sheet aimed at by the present invention. That is, when the amount of Al exceeds 0.02 wt%, scale adhesion near both ends in the longitudinal direction of the coil is deteriorated due to the concentration of Al on the surface of the ground iron during heating and hot rolling.
Since Al interacts with Si, 2 x Al (wt%) + Si
(Wt%) ≦ 0.06.

 Further, the effects of Al and Si will be described in detail.

FIG. 1 shows an example of the effect of Al on scale adhesion. Here, the scale adhesion was measured by bending a test piece having a plate thickness of tmm by 180 ° at a bending radius of 1.5 t, and then rubbing the bent back portion strongly with a cloth. It was decided to display it. A rating of 1 indicates good adhesion without exposing the base steel. Score 2 is a state in which the base iron is exposed in a dot-like manner, but most of the base iron is not exposed (peeling area 10
%Less than). Score 3 is a state in which the base steel is continuously exposed and the peeled area is about 10 to 50%. In the grade 4, the base steel was completely exposed, and the adhesion was poor when the peeled area was 50% or more.
The chemical composition of the steel sheet is 0.08 to 0.10 wt% C, 0.015 to 0.020%.
wt% Si, 0.30 ~ 0.35wt% Mn, 0.015 ~ 0.017wt% P, 0.009 ~
0.005wt% S, Al is 0.015wt%, 0.032wt%, 0.040wt
%. The steel of this component is melted in a converter and then cast into a slab by continuous casting, once chilled, heated in a heating furnace at 1200 ° C, and scale-removed with high-pressure water at the time of rough rolling and immediately before finishing. , Start finish rolling at 1900-1030 ° C, 890
Finish rolling was completed at 850850 ° C., water injection cooling was started within 5 seconds, and quenched at a cooling rate of 15 ° C./s or more, and then wound at 600 to 650 ° C. As can be seen from FIG. 1, when the amount of Al is large, the scale adhesion deteriorates closer to the end in the coil longitudinal direction, whereas when the amount of Al is small, the deterioration of scale adhesion near the coil end decreases. It can be seen that the yield is improved. FIG. 2 shows the effect of Si and Al on the scale peeling score at a position 10 m from the end in the coil longitudinal direction.
The chemical composition of the steel sheet is 0.07 ~ 0.18wt% C, 0.009 ~ 0.08wt%
% Si, 0.003 to 0.04 wt% Al, and other chemical components, hot rolling conditions and other conditions were within the scope of claim 1. As is clear from FIG. 2, by limiting the amounts of Si and Al within the range of the present invention, the scale peeling score becomes 1, and the scale adhesion becomes good. FIG. 2 also shows that Si and Al have an interaction and affect the scale adhesion.

Next, FIG. 3 shows the relationship between the coating adhesion and the scale peeling score of each scale peeling score section collected from each part of the hot rolled coil used in FIG. Here, the coating adhesion is as follows: a grid-like cut at 1 mm intervals is placed on the coated steel sheet up to the base steel, immersed in warm water at 40 ° C for 3 days, and the grid-cut part is peeled off with tape and painted in 100 mm ² Visualize peeling area
We decided to express it in 10 levels of paint adhesion rating. In addition, the coating process is ED (Nippon Paint U100) after chemical conversion treatment with zinc phosphate
Paint, then paint on it (intermediate coating: Luga Bake KPX3)
6, Top coat: Neoaramic B531).
The chemical composition of the pickling material used for comparison was 0.08 wt% C-0.02 wt%
Si-0.33wt% Mn-0.016wt% P-0.010wt% S-0.019wt
% Al. FIG. 3 shows that the better the scale adhesion (the smaller the scale peeling score), the better the coating adhesion, and that a scale peeling score of 1 gives the same coating adhesion as pickling materials.

The functions of improving the scale adhesion and coating adhesion by reducing Al are still largely unknown, but during heating or hot rolling, the Al in the steel concentrates on the surface of the base iron, and the oxide film of the Al This Al is formed at the interface with the scale.
It is considered that the formation of the oxide film on the scale affects the adhesion to the scale. Also, scale formation of Al (hematite, etc.)
It is also conceivable that the influence on the scale and the adhesion of the coating are related to the adhesion. Further, FIG. 4 shows the result of investigation of the concentration of each element near the interface between the base iron and the scale. 4th
The figure shows the distribution of Fe, O, Si, and Al concentrations in the cross-section
It was investigated in. Fig. 4 (a) shows 0.02wt% Si, 0.0
Contains 40wt% Al, poor scale adhesion and coating adhesion,
(B) contains 0.02wt% Si and 0.015wt% Al, and has good scale adhesion and coating adhesion. The manufacturing conditions for the steel sheet are within the scope of the first aspect. From FIG. 4, it was confirmed that when Al increased, the concentration of Si from the surface layer of the steel sheet to the vicinity of the interface with the scale tended to be promoted. From this, it is presumed that Al itself deteriorates the scale adhesion, and also promotes the concentration of Si on the surface layer of the steel sheet to deteriorate the scale adhesion. Therefore, for the purposes of the present invention, it is necessary to simultaneously limit the amounts of Al and Si.

 The above is the reason for limiting the upper limit range of Al and Si.

B fixes N in the steel and prevents aging. In the present invention
Since it is necessary to reduce the amount of Al, N in the steel may remain in a solid solution. Therefore, when aging becomes a problem, it is necessary to prevent aging by adding B. To obtain the effect, B needs to be added in an amount of 0.0001% by weight or more. However, an increase in B promotes slab cracking, so the upper limit of addition is made 0.005 wt%.

Nb is effective as a precipitation strengthening element for improving strength and for fixing C and N. In order to exert its effect, the Nb content must be 0.005 wt% or more, and 0.07 wt%
If it exceeds the upper limit, the upper limit is set to 0.07 wt%.
And

Ti, like Nb, is effective as a precipitation strengthening element to improve the strength. To exhibit its effect, 0.02 wt% or more is required, but other Ti can be used as a deoxidizer like Al and , N is effective for non-aging by fixing N and improvement of elongation. In particular, when Ti is used as a deoxidizing material, Al can be easily reduced, which is effective for the purpose of the present invention. To obtain this deoxidizing effect and the effect of fixing C and N, Ti is 0.001.
It is necessary to add at least wt%. Therefore, the lower limit of the amount of Ti added is set to 0.001 wt%. On the other hand, the addition of a large amount of Ti promotes cracking of the slab, so the upper limit is made 0.1 wt%.

Ca is an element that combines with S to form spheroids of the A-based inclusions to improve workability. To obtain the effect, addition of 0.0005 wt% or more is required. However, if the content exceeds 0.01 wt%, the amount of inclusions increases and workability deteriorates.
0.01 wt%.

 Next, the hot rolling conditions will be described.

The reason why the scale is removed one or more times during the rough rolling is to remove the primary scale generated in the heating furnace. Since the primary scale is thick and brittle, the steel sheet rolled with the primary scale adhered has poor scale adhesion.
Furthermore, it is necessary to remove the scale before the finish rolling.
This is because during the period from rough rolling to finish rolling, the steel sheet is exposed to a high temperature for a relatively long time, so that a thick and brittle scale tends to occur, and such a scale deteriorates the scale adhesion of the manufactured steel sheet. In addition, it is to promote the generation of flaws due to the bite of the scale.

Finish rolling starts at 1100 ° C or less, finishes at 900 ° C or less, starts water cooling within 5 seconds after finish rolling, and limits the cooling rate to 15 ° C / sec or more. The reason for this is to reduce the amount of secondary scale produced by lowering the exposure time to a high temperature as much as possible, thereby preventing the deterioration of scale adhesion.

The reason why the winding temperature was limited to 550 ° C or more and 700 ° C or less was 57
This is because the 4FeO → Fe ₃ O ₄ + Fe transformation that occurs at 0 ° C. or less sufficiently proceeds, and the scale composition is mainly composed of Fe ₃ O ₄ to improve the scale adhesion. At a winding temperature of 550 ° C or higher and 700 ° C or lower, nucleation of Fe ₃ O ₄ occurs at the interface between the base iron and the scale, so that the consistency between the base iron and the scale is good, and the scale adhesion is improved. . If the winding temperature is lower than 550 ° C, the transformation of 4FeO → Fe ₃ O ₄ + Fe does not proceed sufficiently or the nucleation of the transformation occurs not only at the interface between the base iron and the scale. And the adhesion to the scale are deteriorated, and the adhesion to the scale is deteriorated. Also, the winding temperature is 700
When the temperature exceeds ℃, the amount of scale generated increases, and the adhesiveness of the scale tends to vary. Therefore, the winding temperature is set to 550 to 700 ° C in order to stably obtain good scale adhesion.
It is necessary to limit to.

〔Example〕

Continuously cast steel after smelting steel of the chemical composition shown in Table 1,
It was inserted in a heating furnace, and heated at 1050 to 1250 ° C. and then hot-rolled. The scale removal was performed during rough rolling and before finish rolling. Table 2 shows the hot rolling conditions, the scale adhesion and the coating adhesion. Finish rolling start temperature is other than No.2-4 and No.3-3
1100 ° C or less, No.2-4 is 1100 ° C, No.3-3 is 1130
° C. The scale adhesion and the coating adhesion were evaluated by the methods described above, and the coating adhesion was evaluated as acceptable for the pickling material (paint adhesion rating of 7 or more). In Table 2, No.1-1,2-
1,3-1 hot rolling conditions are within the scope of the present invention. No.1-2
Has a low cooling rate, so that scale adhesion and coating adhesion are inferior. Nos. 1-3 and 2-2 each have a too low winding temperature, and No. 2-3 has a too high winding temperature, resulting in poor scale adhesion and coating adhesion. No. 2-4 has a high finish rolling start temperature and a too high finish end temperature at the same time, and No. 3-2 has a poor finish adhesion temperature and poor scale adhesion and coating adhesion. No. 2-5 is poor in scale adhesion and coating adhesion because the water injection start time is too late. No.3-3 is especially 10m from the end in the coil longitudinal direction because the finish rolling start temperature is too high
Poor scale adhesion and coating adhesion at position.

Next, steel having the chemical composition shown in Table 3 was melted and continuously cast.
After charging in a heating furnace and heating at 1000 to 1280 ° C., hot rolling was performed. The scale removal was performed during rough rolling and before finish rolling. The finish rolling start temperature is 1100 ° C. or less in all cases. Table 4 shows the hot rolling conditions, scale adhesion and coating adhesion. Steels D to N are steels of the present invention and O to V steels are comparative steels. O, P steel has excess Si, and Q, S, T, V steel has
Al is excessive, and R and U steels are excessive in 2 × Al (wt%) + Si (wt%). Table 4 shows the hot rolling conditions, scale adhesion and coating adhesion of these steels. The evaluation method is the same as in Table 2. The hot rolling conditions of the 4th No. 4 to 22 are within the scope of the present invention. Among them, the No. 4 to 14 using the component system of the present invention have good scale adhesion and coating adhesion. On the other hand, Nos. 15 to 22 using the comparative component system were poor in both the scale adhesion and the coating adhesion at the 10 m portion. In particular, No. 16 has poor scale adhesion and coating adhesion even at the center of the coil length. Furthermore, N
o.23 to 30 are comparative examples in which the component system of the present invention was used, but the hot rolling conditions were out of the range of the present invention. No.23,25,29 has too high winding temperature, No.24,26,27,30 has too high finishing temperature, and No.28 has too low winding temperature, all have scale adhesion and painting Poor adhesion.

[Effects of the Invention] According to the production method of the present invention, a hot-rolled steel sheet with excellent scale adhesion and coating adhesion over the entire length of the coil can be produced at low cost without using special equipment such as an atmosphere adjusting device. It can be obtained stably. For this reason, the yield can be increased during the manufacture of undercarriage parts, frames, drums, and the like of automobiles, and the frequency of care for press dies due to scale peeling can be reduced. Furthermore, it is possible to paint on the scale as it is, which is extremely economical.

[Brief description of the drawings]

Fig. 1 shows the scale peeling score and Al
Fig. 2 shows the relationship with the amount.
FIG. 3 shows the relationship between the scale peeling score and the amount of Al and Si at the 10 m position, FIG. 3 shows the relationship between the coating adhesion score and the scale peeling score, and FIGS. It is a figure which shows the concentration distribution of Fe, O, Si, and Al of the cross-section board thickness direction by EPMA about the hot-rolled steel plate of two different steel types. Fig. 1 Horizontal axis: coil longitudinal position T10: coil inner winding 5m part T15: coil inner winding 15m part T30: coil inner winding 30m part T45: coil inner winding 45m part M: coil 1/2 length part B45: outside coil Rolled 45m section B30: Coil outer winding 30m section B15: Coil outer winding 15m section B10: Coil outer winding 10m section Hot-rolled steel sheet in Fig. 4 (a): (0.09wt% C-0.02wt% Si-
0.33wt% Mn-0.015wt% P-0.008wt% S-0.040wt% Al,
Finishing temperature 870 ° C, Winding temperature 590 ° C) Scale peeling score 3 Coating adhesion score 1 Hot rolled steel sheet in Fig. 4 (b): (0.09wt% C-0.02wt% Si-
0.38wt% Mn-0.017wt% P-0.009wt% S-0.015wt% Al,
Finishing temperature 880 ° C, Winding temperature 610 ° C) Scale peeling score …… 1 Paint adhesion score… 9

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tadayoshi Wada 5-10-1 Fuchinobe, Sagamihara City, Kanagawa Prefecture New Within the 2nd Technical Research Institute of Nippon Steel Corporation (72) Inventor Tomomi Honda 1st Kimitsu, Kimitsu City, Chiba Prefecture New Nippon Steel Corporation Kimitsu Works (56) References JP-A-63-241137 (JP, A) JP-A-61-195702 (JP, A) JP-A-54-116321 (JP, A) 1963-1988 (JP, A)

Claims (2)

(57) [Claims] (1) C ≦ 0.2 wt%, Si ≦ 0.04 wt%, P ≦ 0.025 wt%,
S ≦ 0.015wt%, Al ≦ 0.02wt%, and this Al and Si
In 2 × Al (wt%) + Si (wt%) ≦ 0.06, the remainder is made of steel composed of Fe and unavoidable elements, and then a steel slab obtained by continuous casting is inserted into a heating furnace, Alternatively, after once cold-flaked, reheat, perform scale removal one or more times during rough rolling, further perform scale removal after rough rolling and before finish rolling, and finish rolling at a temperature of 1100 ° C or less. Start,
After finishing rolling at a temperature of 900 ° C or less, start cooling with water injection within 5 seconds, quench at a cooling rate of 15 ° C / second or more,
A method for producing a hot-rolled steel sheet with scale excellent in coating adhesiveness, characterized in that it is wound at a temperature of 0 ° C to 700 ° C. 2. C ≦ 0.2 wt%, Si ≦ 0.04 wt%, P ≦ 0.025 wt%,
S ≦ 0.015wt%, Al ≦ 0.02wt%, and this Al and Si
2 × Al (wt%) + Si (wt%) ≦ 0.06
0.0001wt% ≦ B ≦ 0.0050wt%, 0.005wt% ≦ Nb ≦ 0.07
wt%, 0.001wt% ≦ Ti ≦ 0.1wt%, 0.0005wt% ≦ Ca ≦ 0.01wt
% Or more, and the balance is made of steel consisting of Fe and unavoidable elements, and then the steel slab obtained by the continuous casting method is inserted into a heating furnace, or once as a cold flake. After that, re-heated, performed one or more scale removal during rough rolling, and further performed scale removal after rough rolling and before finish rolling.
Finish rolling is started at a temperature of 00 ° C or less, and after finishing rolling at a temperature of 900 ° C or less, water cooling is started within 5 seconds, quenched at a cooling rate of 15 ° C / second or more, and cooled to 550 ° C or more A method for producing a hot-rolled steel sheet with excellent coating adhesion, wherein the hot-rolled steel sheet is wound at a temperature of not more than ℃.