JPS5825491A - Cold rolled steel plate having excellent phosphatability - Google Patents

Abstract

PURPOSE:To improve the weldability, phosphatability and corrosion resistance after painting of a cold-rolled steel plate by depositing composite Fe-Zn materials electrolytically at specific rates on the surfaces of the cold-rolled steel plate having the surface of high cleanliness. CONSTITUTION:With a cold-rolled steel plate having surfaces of high cleanliness as the cathode, the steel plate is electrolyzed in a metallic salt soln. of Fe, Zn, thereby allowing composite deposits of Fe-Zn to deposit on the surfaces of the cathode at 5-1,000mg/m<2>. Then, the phosphatability of the cold-rolled steel plate is improved drastically. Here, the rate of deposition is limited at the above- mentioned values in order to make the composite Fe-Zn deposition layers to be deposited on the surfaces discontinuous.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cold-rolled steel sheet with excellent phosphating properties. A cold rolled steel sheet with excellent weldability, phosphate treatment properties, and post-painting corrosion resistance when used in the final application after applying phosphate treatment as a base treatment for painting on the edges. It is to provide t*.

In recent years, in order to overcome the problems of productivity and economy with the conventional patch type box annealing furnace method, manufacturers of cold rolled steel sheets have been using the continuous casting method, which has excellent workability. Various techniques have been proposed for economically producing cold-rolled steel sheets. The basis of this technology is to heat a cold-rolled copper strip above its recrystallization temperature, then cool it to a predetermined temperature, over-age it for a predetermined time within a predetermined temperature range, and then perform secondary cooling. An arm turn is used.

In this continuous annealing, copper strip surface cleaning equipment, heating equipment, soaking equipment, cold rolling equipment, overaging equipment, secondary cooling equipment, drying equipment, and temper rolling equipment are installed in series. Since this requires an extremely long line of equipment and an enormous equipment cost, various attempts have been made to shorten the furnace length by shortening the heat cycle in particular.

One development of this proposal is to use a direct-fired furnace with a high heat transfer coefficient instead of the conventional radiant tube furnace for heating, to increase the heating rate and shorten the heating time, and to effectively use the sensible heat of the direct-fired furnace exhaust gas. This not only improves thermal efficiency but also reduces cooling time by increasing the cooling rate by using water cooling or air/water cooling instead of conventional jet cooling or other gas cooling methods. It has been proposed to shorten the aging time as well.In that case, the cooling rate can be varied over a wide range for air-water cooling, so it is possible to easily follow different material requirements depending on the application, and Since cooling can be stopped at a desired copper strip temperature, reheating to the overaging temperature can be omitted during overaging treatment, and for this reason, it is becoming widely used.

Although it is possible to achieve high efficiency of cold-rolled steel sheets by adopting the continuous annealing method, unfortunately, according to the knowledge of our authors, conventional methods such as radiant tube furnace heating and jet cooling cannot be used. It has been found that even when continuously annealed in a reducing atmosphere, the phosphating property is lower than that of cold rolled steel sheets produced by box annealing.

Phosphateability problems are particularly problematic when combining rapid heating with a direct-fired furnace, as proposed, and rapid cooling, such as steam-water deprivation or water cooling.

This is because direct-fired furnace heating, air-water cooling, or water-cooling is essentially an oxidizing atmosphere, and the surface of the steel strip is oxidized during the heating and cooling processes. It is essential to remove the oxidized layer on the surface of the copper strip at some point during the rounding and continuous annealing process, but even though the nine oxide layer formed in a direct-fired furnace can be reduced in a high-temperature soaking furnace, it is regenerated during cooling. It is difficult to reduce the oxidation in a low-temperature overaging furnace, and the cycle shortening effect cannot be expected, and if the reduction is incomplete, the final performance of the paint resistance -
The oxidized layer must be removed by pickling, polishing, or grinding before the temper rolling process because it significantly deteriorates the corrosion resistance.This method removes carbon, which adversely affects corrosion resistance along with the surface oxidized layer. It is possible to remove iron oxide powder containing iron oxide particles and impurities, and a cold-rolled steel sheet with a beautiful appearance can be obtained, but unfortunately, the phosphating property deteriorates during use.

The present invention solves these difficulties and provides a steel sheet with dramatically improved phosphate treatment properties.

By the way, conventionally, as a means to improve the processability of steel sheets with poor phosphate treatment properties, it has been known to spray a sodium phosphate suspension after press forming and degreasing, but immediately before phosphate treatment. .

It is also known to add trace amounts of heavy metal salts to the phosphate treatment solution itself in order to increase its reactivity.

However, for example, the method of spraying a suspension onto the components of the processed layer requires adding one new step to the series of phosphate treatment steps, which is a heavy burden in terms of equipment and cost, and it is difficult to use the existing line. This may not be possible depending on the specifications. The method of adding heavy metal salts to the treatment liquid itself accelerates the reaction of other parts when treating parts such as automobiles, which are made up of parts with various surface textures, and may result in an excessive coating on some parts. There is a risk of precipitation. Of course, the cost of the chemical conversion treatment liquid itself also increases.

Furthermore, in order to improve the phosphatizing property of cold rolled steel sheets, the present applicant has already reported that Zn is deposited at 0.2 to 21/d on cold rolled steel sheets as a pretreatment for phosphate treatment. It is publicly known from Japanese Patent Publication No. 46-7442. However, in recent years, cationic electrodeposition coating has become the mainstream for car manufacturers, etc., and as a result, phosphate treatment has also changed from Hop*1t@(Zn3(PO4)2) to Phos.
phophyl 11 to (Zn2Fe(PO4),
) It is changing to II. However, in response to this situation, in the prior art, the above-mentioned phosphate treatment properties cannot necessarily be said to be good, and furthermore, the above-mentioned electrodeposition coating properties suffer from the generation of hydrogen during coating. It was found that this was not good because the coating film was prone to blistering.

The present inventors have completely eliminated the drawbacks of these conventional techniques, found a technique that can be easily and reliably applied in the copper strip manufacturing process, and has a large treatment effect. We succeeded in improving the processability to the same level as box-type annealed material.

Specifically, F and Zn are combined on a high-purity steel surface to form a
The objective is to provide a cold-rolled steel sheet having discontinuous and invisible fine precipitates in the range of 100OW1/j.The steel sheet of the present invention can be used as a patch-type box-shaped annealed material in a normal process as shown in Examples later. In addition to having the same or higher phosphate treatment properties, when painted, the corrosion resistance of the paint is superior to that of the patch annealed material.

The reason why paint corrosion resistance deteriorates is that graphite and other carbon-like substances and silicon, At, or compounds that occur on the surface of ordinary patch-type box-type annealed materials inhibit the formation of phosphate films and deteriorate paint corrosion resistance, resulting in surface contamination. This is believed to be because after the substance is removed by pickling or polishing/grinding, the treatment effects of the present invention, which will be described later, are exhibited and a phosphate film with extremely few film defects is formed densely.

It has not been completely elucidated why the steel sheet of the present invention has good phosphatizing properties.

The composite precipitates of F and Zn precipitated on the steel surface due to the local battery action acting between the composite precipitates of Fe and Zn precipitated on the steel surface during the phosphate film formation reaction and the steel surface. This is considered to be a benefit in that the dissolution of precipitates is promoted due to its relatively base nature, and as a result, phosphate crystal nuclei are generated quickly in the early stages of phosphate film formation.

Therefore, in the present invention, it is necessary that the composite precipitated layer of F and Zn precipitated on the steel surface is discontinuous, and if the precipitation amount is such that the reduced metal precipitated layer is continuous, phosphate treatment is difficult. However, even if the deposited layer is discontinuous, in order for the crystal nuclei of the phosphate film to occur densely, the density of the 5ite where local battery action takes place, that is, the density of the discontinuous part, must be appropriate. It is necessary to be within a certain range. When the amount of precipitation is small, the density of phosphate crystal nuclei generated is low, resulting in rounding, and no improvement in phosphate treatment properties is observed. As the amount of precipitation increases, the phosphate crystal nucleus density gradually increases, reaches a maximum, and then decreases again. However, as the phosphate crystal nucleus density increases, the amount of phosphate crystals attached decreases, so it becomes extremely If the amount of adhesion decreases, the corrosion resistance after painting will deteriorate.

In this invention, the upper and lower limits of the precipitation amount of composite precipitates of r and Zs on the steel surface are determined from the above-mentioned considerations for phosphate treatability. and finish) appearance.

OUs properties such as spot weldability and workability are set to be equivalent to cold rolled steel sheets, and the amount of adhesion on the steel surface is set to 5 to 10.
It is selected within the range of 00100O/-. Below 5 K/I, there is almost no effect of improving phosphate treatability.

If the depth of the trench is 1000 cm/- or more, the amount of phosphate crystals deposited decreases and the corrosion resistance after painting deteriorates.

One of the features of the present invention is that a composite precipitate of F and Zn is deposited on the steel surface.

As mentioned above, when only Zn is precipitated alone, Phosphopby
Although phosphate treatment of the l l 1 to (Zn2P*(PO4)2) type is required, it cannot necessarily be said to be a favorable treatment, and the paintability during cationic electrodeposition coating also deteriorates during coating. There is a problem that hydrogen is easily generated and the coating film is blistered, and furthermore, if the amount of precipitation is large, there is a problem with weldability.

The steel sheet of the present invention on which a composite precipitated layer of F. and zn is precipitated is a nine-cold rolled steel sheet that overcomes the above-mentioned problems and has excellent phosphate treatment properties.

That is, in the case where only Zn is precipitated alone, a local cell effect acts between the Zn precipitate and the steel surface during the phosphate film formation reaction, and in this case, since Zrr is more base than the steel, the precipitated Zn Dissolution occurs, and phosphate crystal nuclei are formed at the initial stage of phosphate film formation, but the resulting phosphate crystals are composed of a large number of Hopsit (Zng(PO4)z), as described above. The problem occurs during cationic electrodeposition.

On the other hand, in the case where a composite precipitate of F and Zvs of the present invention is precipitated, during the phosphoric acid film forming reaction.

In fact, a local cell effect acts between these composite precipitates and the steel surface, and since the composite precipitates of F and Zn are more base than the steel, dissolution of the precipitates occurs, and this occurs during the initial stage of phosphate film formation. Formation of phosphate crystal nuclei takes place, and in this case, the resulting phosphate crystals are of a Phosphophyllitv-rich type, resulting in a base film that is well suited for cationic electrodeposition.

In this case, the precipitated F・-Zn composite precipitate (F@/F
@＋Z! I) (2) The ratio needs to be 505i to 80III, and if the (F/F*+Zn) ratio is less than 5091, the resulting phosphate crystal will be Pho Phophyllit.
This results in a mold with less oxidation, and the paintability during cationic electrodeposition coating deteriorates. Furthermore, when the (F*/Fa+Zn) ratio exceeds 80-, the p1 local cell effect due to the slight potential difference between the Fje-Zn composite precipitate and the steel surface does not work and does not contribute to the formation of phosphate crystal nuclei.

In addition, as described above, the F@-Z♂ composite precipitate of the present invention has a lower Zn content in the precipitate than when Zn is precipitated alone. This also improves weldability.

The steel plate of the present invention can be easily manufactured by the following method. A composite precipitate of F.--Zn can be easily deposited on a steel surface by electrolyzing in these metal salts using the steel plate as a cathode using an electroplating method.

Specifically, (1) When performing normal continuous annealing in an It-based atmosphere, an electrolytic cell and a washing tank are installed on the outlet side of the final cooling zone of the continuous annealing furnace, and the electrolytic cell In the cathode electrolysis tjl of a predetermined amount of electricity
After L9, wash with water and dry.

(2) When performing cooling in an oxidizing atmosphere such as direct-fired furnace heating or water cooling, hot water cooling, or air/water cooling, electrolytic If high-temperature de-furnacing is performed in electrolytic Asahi (3) Hakoya annealing by installing tanks in series, a pickling tank, a water washing tank, an electrolytic tank, and a washing tank are installed in series in front of the temper rolling mill. Electrolytic treatment is applied to the surface after the oxide layer has been removed.

Next, as an example of the method of the present invention, a case in which continuous annealing is followed by pickling and then electrolytic treatment will be described with reference to FIG.

1 is entry side handling equipment such as uncoiler, shear, welder, etc., 2 is entry side looper, 3 is primary tropical, 4
5 is a jet type direct-fired heating furnace with 18 tons of jet burners, 6 is a soaking zone, 7 is a primary air-water cooling zone, 8 is an overaging zone, 9 is a secondary cooling zone, 1G is an oxidation zone layer removing device, 1
1 is a water washing tank, 12 is an electrolytic treatment tank, and 13 is a water washing tank. 14
15 is a dryer, 15 is an outlet rolling machine, 16 is a temper rolling mill, and 17 is an outlet rolling equipment including an oil applicator, a shear, a coiler, etc. The cold-rolled steel strip S passes through the entry side root arm 2, is guided to a second heating zone 3, a secondary preheating zone 4, and then to a direct-fired heating furnace 5.

In the direct-fired heating furnace 5, the copper band temperature is at least 600℃.
In order to ensure that the temperature increase rate of the copper strip in the high temperature range of 40° C. or higher, preferably 400° C. or higher, is maintained at 40° C./or higher regardless of the thickness of the copper strip, the following furnace operation is performed.

In other words, the high temperature combustion gas from the direct-fired heating furnace 5 is ζO
It is led from the discharge port 20 to the secondary preheating zone 4 via the collecting channel 4-21, where it is used for the secondary preheating of the copper strip, and then led to the rectifier and pelleter 24, where it is used for heat exchange with the combustion air in the direct-fired furnace 5. - is carried out to lower the temperature, and then it is guided by the primary heat exchanger 3 and collides with the surface of the copper strip as a jet to raise the temperature of the steel strip. The preheated air is supplied 25 to the nozzle 19. - The Tsujiko Tropical 3 is composed of a plurality of jet preheating zones 22 that can be turned on and off freely, and the number of ON zones and the jet flow velocity are changed according to the plate thickness, so that the copper strip at the outlet of the First Tropical Control temperature. - The combustion gas in the next tropical zone 3 first enters the upper part, and the combustion gas coming out from there is further supplied to the lower part via the path 23. -By passing through the next Tropical Zone 3, the temperature of the copper zone will rise from room temperature to 150°C.
The temperature is raised to ~300℃, but in this case, if the plate thickness is thin, the number of ON zones and the jet flow velocity are reduced to preheat to a relatively low temperature, and if the plate thickness is thick, the on-zone Preheat to a relatively high temperature by increasing the number and jet flow rate.

- The copper strip that has left the next issue tropic 3 is then led to the secondary heat exchanger 4, where it exchanges heat with the high-temperature combustion gas from the direct-fired heating furnace 5 and is heated to a copper strip temperature of 400 to 500°C. Subsequently, the temperature is raised from 400 to 500°C to the soaking temperature in the direct-fired heating furnace 5. The direct-fired heating furnace 5 is composed of a plurality of combustion zones 18, and the combustion flame stream from the axial flow type slit paddle 19 provided in the six zones 18 collides with the surface of the steel plate as a jet stream.

Examples of fuel include, but are not limited to, 1, for example, CO
G is used to burn at an air ratio of about 0.95±0.05,
The furnace can be operated at a furnace temperature of 1200c or higher. If the air ratio exceeds 1, oxidation of the surface of the copper strip in the direct-fired furnace will become extremely severe, so the furnace should be operated so that the air ratio does not exceed 1.

In the direct-fired heating furnace 5, 400℃ or higher, at most 500℃
From the above, the copper strip passed through at a heating rate of t40℃/&E or higher until reaching the soaking temperature is 700 to 80℃ depending on the target material.
After reaching the soaking temperature of 60℃, it was heated to 5-40℃ in the soaking zone 6.
Soaking annealing for seconds is performed.

The atmosphere in the soaking zone 6 is, for example, a reducing atmosphere.

In the soaking zone 6, the nine copper strip is subjected to soaking annealing at 700 to 860°C for 5 to 40 seconds, and then to the primary air-water cooling zone 7.
50 to a range of 300-500℃ by passing through
It is rapidly cooled at a cooling rate of ℃/s6c or more. The purpose of rapid cooling after recrystallization and grain growth of the rolled structure by soaking is to increase the degree of supersaturation of solid solute carbon by diffusing from cementite into ferrite grains during the heating and soaking process. , to promote the precipitation of solid solution carbon in the subsequent over-aging treatment. Moreover, if air-water cooling is adopted, it is easy to control the end point of rapid cooling (41), and cooling can be stopped at the overaging treatment temperature.

The steel strip cooled to a temperature of 300°C to 500°C in the air-water cooling zone 7 is then cooled to a temperature of 300°C to 500°C in the overaging zone 8.
It is over-aged.

The atmosphere in the overaging zone 8 is usually a non-reducing atmosphere or an oxidizing atmosphere.

The copper strip subjected to the over-aging treatment is then rapidly cooled in a secondary cooling zone 9, and then washed in hot water 11 after removing all oxides adhering to the copper strip in a cooling tank 10. The material is subjected to electrolytic treatment in a tank 12, washed with water 13, and then transported to a dryer 14.

Next, the present invention will be explained with reference to examples.

Examples 1 to 4 were conducted using the following materials.

Material 1: Continuously cast aluminum killed steel (・C: 0.0371g,
The cold-rolled material (Si: 0.03G, Mn: 0.2311G) (0.8 pieces) was passed through the nine equipment shown in Fig. A steel sheet with an oxide film thickness of 2001% ll/II' as F in the oxide film.

Material 2: Kiya, Dedo steel (C: 0.051111.81:
0.01-, Mn: 0.2311G) was treated in the same manner as Material 1 to produce a mourning steel plate with an oxide film thickness of 250 cm/ze as F in the oxide film.

Material 3 A cold-rolled material with almost the same composition as knee material 1 was electrolytically cleaned and then soaked at 700°C for 30 minutes in mα at dp -20°C.
Furnace cooling and nine continuous annealing materials.

Material 4: A cold-rolled material with the same composition as material 1 was electrolytically cleaned,
dp The steel plate was soaked at 700°C for 24 hours in HNX at -40°C, and when the plate temperature reached 400°C during furnace cooling, it was poured out into the air.

The evaluation method for samples in Examples and Comparative Examples is as follows.

In the Examples and Comparative Examples, the phosphate treatment was
Bt 13, a spray deaf phosphate treatment solution manufactured by Kerr
Process with 7 and 9. The treatment liquid is TA15-17, MuR25-3
0, accelerator was adjusted to 0.8 to 1.5/India. 120 “Scan heavy electric current all (X3
00) to observe the state of precipitation, from ◎: a level where fine crystals are precipitated almost uniformly over the entire surface, to xx:
Visual judgment was made by dividing into 9 ranks to a level where no crystal precipitation was observed.

In the evaluation after the 120" treatment, the amount of film was measured in the usual manner, and the crystal size was determined from micrographs.

The results of the 88T (salt spray test) are as follows: 2ON 21 μm of anionic electrocoating paint Nirekron 7000AK manufactured by Kansai Into ■ was electrocoated on the board that had been phosphate treated at 120". After baking at 180°C x 20', it Cross-cut to reach the base material with a sharp knife, and use 5 saline solution.
After death, salt water was sprayed for 240 hours in accordance with 8Z-2371, and the cellophane tape was removed from the four slits.

[Brief explanation of the drawing]

Figure 1 shows 1 of the present invention! It is a route map showing an example of equipment when carrying out service. S...copper belt! ...Entry side handling equipment, 2...
・Entrance looper, 3...-Tsuko tropical, 4...Secondary preheating zone, 5...Jet flow direct fire heating furnace, 6...Soaking zone,
7... - secondary air-water cooling zone, 8... over-aging zone, 9...
Secondary cooling zone, 10... Oxide layer removal device, 11... Water washing, 12... Electrolytic treatment, 13... Water washing tank, 14...
・Dryer, 15... Out side roots parable - 116...
Temper rolling mill, 17... Output side handling equipment, 18.
...Fuel zone, 19.../Kooner, 20...
Exhaust port, 21... Gathering Chan z! -122...Jet preheating zone, 23...Path, 24...Requiperator.

Claims (1)

[Claims] lI! Surface height Frame cleanliness 5~1000197m of F・eZslt composite was electrolytically deposited on the cold rolled steel plate surface1
Cold-rolled steel sheet with excellent phosphate treatment properties that reduce mold and mildew.