JPH02190422A - Method for producing hot-rolled steel sheet with excellent paint adhesion as scale - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to hot-rolled steel for processing used in automobile parts such as suspension parts and bumpers, pipes, drum pipes, etc. The present invention relates to a method for producing a hot rolled steel sheet that can be processed without descaling and has excellent paint adhesion.

[Conventional technology and its problems]

The secondary scale of hot-rolled steel sheets inevitably peels off during forming processing, and paint adhesion is poor. Therefore, descaling treatment such as pickling is performed before processing, and then painting. method was adopted. However, this is economically disadvantageous, so Jikai 461-19411 is a hot rolled steel sheet with excellent scale adhesion that does not require descaling.
Publication No. 2, JP-A-59-222533, JP-A-6
This is proposed in Japanese Patent No. 2-136529.

In the method described in JP-A-61-194112, it is necessary to set the winding temperature to 500° C. or lower and 360° C. or higher.

When this temperature range is controlled by water cooling, which is commonly used, there is a problem in that temperature unevenness tends to occur in the longitudinal direction or the width direction of the coil. Further, the same publication proposes a method of relaxing the winding temperature and winding at 600° C. or lower, but this requires the addition of Cr, which is economically disadvantageous.

Next, JP-A-59-222533 and JP-A-62-
The method described in Japanese Patent No. 136529 requires cooling in a non-oxidizing atmosphere after winding, and requires an atmosphere blocking/atmosphere adjustment device for this purpose, which is economically disadvantageous.

Furthermore, although these existing bills propose measures to improve scale adhesion, they do not take paint adhesion into consideration.

[Problem to be solved by the invention]

The present invention solves the difficulties of cooling control technology as described above,
Production of hot-rolled steel sheets on a scale that is economical and has excellent paint adhesion comparable to that of pickled materials, without the use of special equipment such as atmospheric isolation or atmosphere adjustment equipment after winding, and without the need for the addition of special elements. The purpose is to provide a method.

[Means to solve the problem]

The inventors of the present invention have conducted various studies to solve the above problems, and have found that the content of Si and N in steel greatly affects scale adhesion and paint adhesion, and that it is necessary to adjust the Si and Al content within a specific range. The inventors have discovered that a scale layer with excellent adhesion can be obtained stably without lowering the winding temperature. The present invention was made based on this new knowledge,
The summary is as follows.

(1) C≦0.2wt%, Si≦0.04wt%7
P≦0.025wt%, S≦O, 'O15wt%, Al
≦0.02 wt%, and in this Al and Si, 2 The obtained steel slab is inserted into a heating furnace, or once it has been made into a cold slab, it is reheated, and scale is removed at least once during rough rolling, and then scale is removed after rough rolling and before finish rolling. After that, finish rolling was started at a temperature of 1100°C or less, and after finish rolling was finished at a temperature of 900°C or less, water injection cooling was started within 5 seconds, and the cooling rate was 15°C /
A method for producing a scale hot-rolled steel sheet with excellent paint adhesion, characterized by rapidly cooling it in seconds or more and winding it at a temperature of 550°C or more and 700'C or less.

(2) C≦0.2wt%, Si≦0.04wt%, P
≦0.025i%, S≦0.015 wt%, A! ≦
0.02wt%, and in this Al and Si, 2×Al(wt%) +Si(wtχ)≦0.06, and 0.0001wt%≦B≦0.0050wtχ
.

0.005 wt%≦Nb≦0.07wt%, 0000
1wt%≦Ti≦0.1wt%, 0.0005wt
%≦Ca≦0.01 wt%; 1. After melting steel, the balance of which is Fe and unavoidable elements, the steel slab obtained by continuous casting is inserted into a heating furnace, or once it is made into a cold slab, it is reheated and rolled more than once during rough rolling. After performing scale removal after rough rolling and before finish rolling, start finish rolling at a temperature of 1100'C or less, and finish finish rolling at a temperature of 900'C or less, 5 Start water injection cooling within seconds, cooling rate 15
A method for producing a scale hot-rolled steel sheet with excellent paint adhesion, characterized by rapidly cooling at a rate of 550'C/second or more and winding at a temperature of 1,700°C or less.

[For production]

The effects brought about by limiting the components and hot rolling conditions in the present invention will be described below.

C is used to ensure the strength of steel plates, but it forms cementite (FesC) in the steel, and its increase worsens scale adhesion and weldability, so the upper limit of C is set at 0.2 wt%. shall be.

St concentrates on the surface of the steel base during heating and hot rolling, deteriorating scale adhesion and at the same time deteriorating paint adhesion, so less is better, and therefore the upper limit is set to 0.04 wt%. Furthermore, since Si interacts with Al, 2XAl(w
t%) +Si (wt%)≦0.06.

P and S also concentrate on the surface of the steel base during heating and hot rolling, deteriorating scale adhesion, but in the present invention, P is reduced to 0.02
Deterioration of scale adhesion can be prevented by setting S to 0.015 wt% or less and S to 0.015 wt% or less. For the purpose of the present invention, it is desirable to have less P and S, but since the processing cost for both P and S increases, the content should be appropriately determined to be below the above value in consideration of economic tolerance. good.

AZ is the most important element in obtaining the properties of the steel sheet targeted by the present invention. That is, when the M amount exceeds 0.02eet%, scale adhesion near both longitudinal ends of the coil deteriorates due to M concentration on the surface of the steel base during heating and hot rolling. Also, since M interacts with Si, 2XIV (
wt%) +Si (wt%)≦0.06.

Furthermore, the effects of Al and Si will be explained in detail.

An example of the influence of M on scale adhesion is shown in FIG.

Here, scale adhesion is measured by bending a test piece with a thickness of t1 180 degrees with a bending radius of 1.5t, rubbing the back of the bend strongly with a cloth, and visually observing the scale peeling on the back of the bend.
It was decided to display the scale peeling score in stages. Rating 1
There is no bare metal exposed and the adhesion is good. Rating 2 means that the base metal is exposed in spots, but most of the base metal is not exposed (peeling area of 10% or less). Rating 3 indicates that the subway is continuously exposed,
The peeled area is about 10 to 50%. A rating of 4 means that the base metal is completely exposed and the peeled area is 50% or more, indicating poor adhesion. In addition, the chemical composition of the steel plate is 0.08 to 0.10
wt%C, 0,015-0.020wt%Si, 0.
30-0.35 wt%Mn, 0.015-0.01
7wt%P, 0.009-0.005wt%S,
M is 0, 015wt%, 0.032wt%, 0.040
Three levels were set: Steel with this composition is melted in a converter, then continuously cast into a slab, and once cooled into pieces,
Heating in a heating furnace to "C", removing scale with high pressure water during rough rolling and before inversion during finishing, starting finish rolling at 1090 to 1030 °C, finishing finish rolling at 890 to 850 °C, 5
Water injection cooling was started within seconds, and after cooling at a cooling rate of 15°C/s or more, the film was wound up at 600 to 650°C.

As can be seen in Figure 1, when there is a large amount of Ali, scale adhesion deteriorates closer to the ends in the longitudinal direction of the coil, whereas when IVfi decreases, the deterioration of scale adhesion near the ends of the coil decreases, increasing yield. can be seen to be improved. Further, FIG. 2 shows the influence of Si and M on the scale peeling score at a position 10 m from the longitudinal end of the coil. still,
The chemical composition of the steel plate is 0.07-0.18wt%C, 0.0
09~0.08wt%Si, 0.003~0.04wt
%M, and other chemical components, hot rolling conditions, and other conditions were within the range described in claim 1. As is clear from FIG. 2, by limiting the amounts of Si and M within the range of the present invention, the scale peeling score is 1 and the scale adhesion is good. Moreover, from FIG. 2, it can be seen that St and Al interact with each other and influence the scale adhesion.

Next, FIG. 3 shows the relationship between the paint adhesion and the scale peeling score of each part sampled from each part of the hot rolled coil used in FIG. 2 and having a scale peeling score. Paint adhesion is determined by making base grain-shaped cuts at 11 m 111 intervals up to the base metal on a painted steel plate, immersing it in 40℃ hot water for 3 days, and then peeling off the base grain cut portion with tape. It was decided to express the area using a 10-level paint adhesion rating based on visual inspection. In addition, the painting process is done by chemical conversion treatment with zinc phosphate and then ED (
Nippon Pain 1-Uloo) was painted, and then painted on top of that (
Intermediate coating: Rugabake KPX36, top coating: Neoaramink B531). The chemical composition of the pickling material used for comparison was 0.08wt%C0, 02wt%Si0
．． 33wt%Mn 0.016. wt%P 0.0
10wtχS 0.019wt%M. From Figure 3, the better the scale adhesion (the smaller the scale peeling score), the better the paint adhesion, and the scale peeling score is 1.
It can be seen that paint adhesion comparable to that of pickled materials can be obtained.

The mechanism of scale adhesion improvement and paint adhesion improvement due to low M range is still unclear, but M in the steel concentrates on the surface layer of the steel during heating or hot rolling, and an oxide film of M forms on the surface of the steel. Since it is formed at the interface between M and scale, it is thought that the formation of this M oxide film influences the scale adhesion.

It is also considered that the influence of M on scale formation (hematite, etc.) is involved in scale adhesion and paint adhesion. Furthermore, the results of investigating the enrichment status of each element near the base metal-scale interface are shown in FIG. Figure 4 shows Fe, O
, St, /V concentration in the thickness direction of the steel plate cross section.
This was investigated by MA. Incidentally, the manufacturing conditions of the steel plate are within the range described in claim 1.

From FIG. 4, it was confirmed that as M increases, the concentration of Si from the steel plate surface layer to the vicinity of the interface with scale is promoted. From this fact, it is presumed that M itself deteriorates scale adhesion, and also promotes the concentration of St on the surface layer of the steel sheet, thereby degrading scale adhesion. Therefore, for the purpose of the present invention, it is necessary to limit the amounts of Al and Si at the same time.

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

B fixes N in the steel and prevents aging. In the present invention/
Since it is necessary to lower Vl, there is a possibility that N in the steel remains in solid solution. Therefore, if aging is a problem, it is necessary to prevent aging by adding B. To obtain this effect, B needs to be added in an amount of 0.0001 wt% or more. However, since an increase in B promotes slab cracking, the upper limit of B addition is set at 0.05 wt%.

Nb is effective as a precipitation-strengthening element for improving strength and is also effective for fixing C and N. In order to exhibit this effect, the Nb content must be at least t% (O, OO5), and if it exceeds 0.07 wt%, the strength improvement effect disappears, so the upper limit is set to o, o'rwt%.

Like Nb, Ti is effective in improving strength as a precipitation-strengthening element, and 0.02 wt% or more is required to exhibit its effect, but Tl can also be used as a deoxidizing agent like Al. It is effective in preventing aging and improving elongation by fixing C and H. In particular, when Ti is used as a deoxidizer, M can be easily reduced, which is effective for the purpose of the present invention.

To obtain this deoxidizing effect and C and N fixation effect, Ti should be 0.
It is necessary to add 0.001 wt% or more. Therefore, the lower limit of the amount of Ti added is set to 0.001 wt%. On the other hand, the upper limit is 0.1w because adding a large amount of Ti promotes cracking of the slab.
%.

Ca is an element that combines with S to make A-based inclusions spheroidal and improves workability, and to obtain this effect, 0.0005
It is necessary to add more than wt%. However, 0.01wt
If it exceeds 0.01 wt%, the amount of inclusions will increase and the workability will deteriorate, so the upper limit is set at 0.01 wt%.

Next, the hot rolling conditions will be described.

The reason why scale removal is performed at least once during rough rolling is to remove primary scale generated in the heating furnace. Since this primary scale is thick and brittle, a steel plate that is rolled with the negative scale still attached has poor scale adhesion. Furthermore, it is necessary to remove scale before finish rolling. This is because the steel plate is exposed to high temperatures for a relatively long period of time between rough rolling and finish rolling, so thick and brittle scales are likely to occur, and such scales deteriorate the scale adhesion of the steel plate after manufacturing. This is also to encourage the occurrence of flaws due to scale encroachment.

The reason why finish rolling was started at 1100°C or lower, ended at 900°C or lower, water injection cooling was started within 5 seconds after finishing rolling, and the cooling rate was limited to 15°C/second or higher was to keep the temperature of the steel plate as low as possible. This is to reduce the amount of secondary scale produced and prevent deterioration of scale adhesion by lowering the temperature and shortening the time of exposure to high temperatures as much as possible.

The reason why the winding temperature was limited to 550°C or higher and 700°C or lower is that 4FeO-'Fe,O occurs at 570°C or lower.

+Fe transformation is allowed to proceed sufficiently, and the scale composition is reduced to Fe, O.
This is to improve scale adhesion by making it a main component. If the coiling temperature is between 550°C and 700°C, nucleation of Fe and O occurs at the interface between the steel base and scale, resulting in good consistency between the base steel and scale (and improved scale adhesion). If the zero winding temperature is less than 550℃,
4FeO-+Fe, 04+Fe transformation does not proceed sufficiently, or nucleation of transformation occurs outside the interface between the base iron and scale, resulting in poor consistency between the base iron and scale, and scale adhesion deteriorates. do. Also, the winding temperature is 7
When the temperature exceeds 00°C, the amount of scale generated increases, and scale adhesion tends to vary. Therefore, in order to obtain stable and good scale adhesion, the winding temperature was set at 550°C.
It is necessary to limit the temperature to ~700°C.

〔Example〕

Steel having the chemical composition shown in Table 1 was continuously cast after being melted, inserted into a heating furnace, heated at 1050 to 1250°C, and then hot rolled. Scale removal was carried out during rough rolling and before finish rolling. Table 2 shows the hot rolling conditions, scale adhesion, and paint adhesion. Finish rolling start temperature is No, 2-4, No
, 3-3 is below 1100℃, No. 2-
4 is 1110″C, No. 3-3 is 1130°C.

The scale adhesion and paint adhesion were evaluated using the methods described above, and the paint adhesion was comparable to that of pickled materials (paint adhesion rating of 7 or higher).
was passed. In Table 2, 1-1.2-1.3
-1 hot rolling conditions are within the scope of the present invention. No, 1-
Sample No. 2 has poor scale adhesion and paint adhesion due to slow cooling rate. No. 1-3. Since the winding temperature of both No. 2-2 is too low, and because the winding temperature of No. 2-3 is too high, the scale adhesion and paint adhesion are poor. In Nα2-4, the finishing rolling start temperature is too high and the finishing finishing temperature is too high.
No. 3-2 had poor scale adhesion and paint adhesion because the finishing temperature was too high. No. 2-5 had poor scale adhesion and paint adhesion because the water injection start time was too late.

In No. 3-3, the finish rolling start temperature was too high, so the scale adhesion and paint adhesion were particularly poor at a position 10 m from the longitudinal end of the coil.

Next, steel having the chemical composition shown in Table 3 was continuously cast after being melted, charged into a heating furnace, heated at 1000 to 1280°C, and then hot rolled. Scale removal was carried out during rough rolling and before finish rolling. The finish rolling start temperature is 1100°C or lower in all cases. Table 4 shows the hot rolling conditions, scale adhesion, and paint adhesion. D-N steel is a steel based on the composition of the present invention, and O~
V steel is a comparative composition system. O, P steel has excess Si, Q, S, T.

■ Steel has excess M, R and U steel have 2X/V (wt%) +S
i (wt%) is in excess. Table 4 shows the hot rolling conditions, scale adhesion, and paint adhesion of these steels.

The evaluation method is the same as in Table 2. Although the hot rolling conditions of Nα4 to 22 in Table 4 are within the scope of the present invention, Nos. 4 to 14 using the plain wood invention component system have good scale adhesion and paint adhesion, whereas in comparison N using component system
α15 to 22 have poor scale adhesion and paint adhesion at the 10 m section. In particular, Nα16 has poor scale adhesion and paint adhesion even at the center of the coil length. Furthermore, Nα
Nos. 23 to 30 are comparative examples in which the component system of the present invention was used, but the hot rolling conditions were outside the range of the present invention. k23,25
．． 29 has too high winding temperature, Nα24, 26, 27.3
0 has a finishing temperature that is too high, and k2B has a winding temperature that is too low, resulting in poor scale adhesion and paint adhesion.

[Effects of the Invention] According to the manufacturing method of the present invention, hot-rolled steel sheets with scales that have excellent scale adhesion and paint adhesion over the entire length of the coil can be produced at low cost without using special equipment such as an atmosphere conditioning device. can be obtained stably.

For this reason, it is possible to increase the yield when manufacturing automobile suspension parts, frames, drum pipes, etc., and to reduce the frequency of maintenance of press molds due to scale peeling. Furthermore, it is possible to paint the scale as it is, which is extremely economical.

[Brief explanation of the drawing]

Figure 1 shows the scale peeling score and / at the longitudinal position of the coil.
Figure 2 is a diagram showing the relationship between the scale peeling score and AZ and S at a position 10 m from the longitudinal end of the coil.
Figure 3 is a diagram showing the relationship between paint adhesion rating and scale peeling rating, Figure 4 (a) and (b) are for hot rolled steel sheets of two steel types with different M levels. EPMA of
FIG. 3 is a diagram showing the concentration distribution of Fe, o, st, and AZ in the thickness direction of a cross-sectional plate according to the present invention. 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 Winding 45 m section B30: Coil outer winding 30 m section B15: Coil outer winding 15 m section B10: Coil outer winding 10 m section Hot rolled steel plate in Figure 4 (A): (0,09C-0,02S
i-0,33Mn-0,015P O,008S-0
, 040M, finishing exit temperature 870℃ 9 winding temperature 590℃
) Hot-rolled steel plate in Figure 4 (b): (0,09C-0,0
2St-0,38Mn-0,017P-0,009S-
0,015M, finishing exit temperature 880℃ 1 winding temperature 610℃
°C) 00.015 wtZ, Nl mouth aθ32wtZ Δaθ elevation wt/, Al Coil longitudinal device (a) Fig. 2 i (wtZ> Fig. 3 Go J4 Scale peeled salmon evaluation wood (b)

Claims (2)

[Claims] (1) C≦0.2wt%, Si≦0.04wt%, P≦
0.025wt%, S≦0.015wt%, Al≦0.
02wt%, and 2× in this Al and Si.
After melting a steel in which Al (wt%) + Si (wt%)≦0.06 and the balance consists of Fe and unavoidable elements, insert the steel piece obtained by continuous casting into a heating furnace, or Once it is made into cold pieces, it is reheated, scale is removed at least once during rough rolling, scale is removed after rough rolling and before finish rolling, and finish rolling is started at a temperature of 1100℃ or less. After finish rolling at a temperature of 900°C or less, 5
A method for producing a hot-rolled steel sheet with excellent paint adhesion in scale, characterized by starting water injection cooling within seconds, rapidly cooling at a cooling rate of 15°C/second or more, and winding at a temperature of 550°C or more and 700°C or less. . (2) C≦0.2wt%, Si≦0.04wt%, P≦
0.025wt%, S≦0.015wt%, Al≦0.
02wt%, and 2× in this Al and Si.
Al (wt%) + Si (wt%)≦0.06, and 0.0001wt%≦B≦0.0050wt%, 0.
005wt%≦Nb≦0.07wt%, 0.001wt
%≦Ti≦0.1wt%, 0.0005wt%≦Ca≦
After melting steel containing 0.01 wt% of one or more elements, with the remainder consisting of Fe and unavoidable elements, the steel slab obtained by continuous casting is inserted into a heating furnace or cooled once. After cutting into pieces, reheating, removing scale at least once during rough rolling, further removing scale after rough rolling and before finish rolling, and then starting finish rolling at a temperature of 1100 ° C. or less, After finishing finish rolling at a temperature of 900°C or less, water injection cooling is started within 5 seconds, rapid cooling is performed at a cooling rate of 15°C/second or more, and winding is performed at a temperature of 550°C or more and 700°C or less. A method for manufacturing hot-rolled steel sheets in scale with excellent paint adhesion.