EP3321394B1

EP3321394B1 - Process and equipment for producing cold-rolled steel strip

Info

Publication number: EP3321394B1
Application number: EP16821503.6A
Authority: EP
Inventors: Yuta TERASAKI; Hiroyuki Akimoto; Hiroyuki Masuoka
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2015-07-08
Filing date: 2016-07-07
Publication date: 2019-12-11
Anticipated expiration: 2036-07-07
Also published as: EP3321394A4; CN107709620A; CN107709620B; US20180298503A1; EP3321394A1; WO2017007036A1

Description

TECHNICAL FIELD

This application relates to a method of producing a cold rolled steel strip and a production system for a cold rolled steel strip.

BACKGROUND

In recent years, there has been strong demand for improved automobile fuel efficiency from a viewpoint of global environmental protection. There has also been strong demand for strengthening of automotive bodies from a viewpoint of ensuring passenger safety upon collision. To respond to these demands, the simultaneous achievement of both weight-reduction and strengthening of automotive bodies is being actively promoted through strengthening and sheet metal thinning (weight-reduction) of cold rolled steel sheets used as a material for automotive components. However, since many automotive components are produced through forming of a cold rolled steel sheet, the cold rolled steel sheet serving as a material for these components is required to have excellent formability in addition to high strength.
There are various methods for increasing the strength of a cold rolled steel sheet such as solid solution strengthening through Si addition, which is an effective means for strengthening without significant loss of formability. However, the addition of a large amount of Si to a cold rolled steel sheet, and particularly the addition of 0.5 mass% or more of Si, is known to result in the formation of a large amount of Si-containing oxides such as SiO₂ and Si-Mn-based composite oxides at the steel sheet surface during slab reheating, during hot rolling, or during annealing after cold rolling. These Si-containing oxides significantly reduce chemical convertibility such that high-strength cold rolled steel sheets containing a large amount of Si have poor chemical convertibility. Moreover, high-strength cold rolled steel sheets containing a large amount of Si suffer from a problem of having poor post-coating corrosion resistance and being more susceptible to coating peeling than normal cold rolled steel sheets when exposed to a harsh corrosive environment after electrodeposition coating, such as in a warm salt water immersion test or a wet-dry combined cyclic corrosion test. Consequently, it is difficult to use high-strength cold rolled steel sheets containing a large amount of Si in body applications for which coating is essential.
Patent literature (PTL) 1 and 2 provide techniques for solving this problem. PTL 1 and 2 each describe a method of producing a cold rolled steel sheet including subjecting a steel sheet that has been cold rolled and subsequently continuously annealed to pickling by continuously feeding the steel sheet into a mixed acid (nitric acid and hydrochloric acid, nitric acid and hydrofluoric acid, or the like) to immerse the steel sheet, and subsequently subjecting the steel sheet to repickling by continuously feeding the steel sheet into a non-oxidizing acid (hydrochloric acid, sulfuric acid, or the like) to immerse the steel sheet. The described method removes Si-containing oxides at the steel sheet surface through the pickling and removes iron-based oxides that are produced in the pickling through the repickling, and thereby enables production of a cold rolled steel sheet having excellent chemical convertibility and post-coating corrosion resistance in harsh corrosive environments.
PTL 3 relates to a method and device for producing an Si-containing cold rolled steel sheet, the method comprising steps of cold rolling a steel containing 0.5 to 3.0 mass% Si, continuously annealing the cold rolled steel sheet, pickling the surface of the continuously annealed steel sheet, and repickling the surface of the pickled steel sheet with a non-oxidative acid.
PTL 4 relates to a method for producing a cold rolled steel sheet, wherein a continuously annealed steel sheet after cold rolling is pickled with a mixture of nitric acid and hydrochloric acid.
PTL 5 relates to a method of continuously annealing a cold rolled steel strip and carrying out pickling with a mixed acid of nitric acid and hydrofluoric acid.
PTL 6 relates to method of pickling stainless steel using a mixture of nitric acid and hydrofluoric acid.

CITATION LIST

Patent Literature

PTL 1: JP 2012-132092 A
PTL 2: JP 2012-188693 A
PTL 3: EP 2 692 902 A1
PTL 4: EP 2 612 956 A1
PTL 5: JP 3 021164 B2
PTL 6: JP 3 225880 B2

SUMMARY

(Technical Problem)

However, our studies have demonstrated that when a cold rolled steel strip is continuously passed along a production system capable of implementing two-stage pickling such as described above, and the cold rolled steel strip is continuously subjected to this two-stage pickling, as time passes, the surface appearance quality of the cold rolled steel strip produced at that time tends to become poorer. Specifically, we realized that as time passes, the surface of the cold rolled steel strip straight after the pickling in the first stage is discolored to a reddish-brown color due to adhered matter, and this discoloration is not removed by the repickling in the second stage. Among cold rolled steel strips having poor surface appearance quality as described above, we found that there were cold rolled steel strips that also had poor chemical convertibility and post-coating corrosion resistance in harsh corrosive environments.
In view of the problems set forth above, it would be beneficial to provide a method of producing a cold rolled steel strip and a production system for a cold rolled steel strip that enable continuous production with long-term stability of a cold rolled steel strip having excellent chemical convertibility, post-coating corrosion resistance in harsh corrosive environments, and surface appearance quality.

(Solution to Problem)

Through diligent studies, we discovered that there is a correlation between the surface appearance quality of a cold rolled steel strip and the iron ion concentration (hereinafter, also referred to simply as the "Fe concentration") in a mixed acid used when the cold rolled steel strip is subjected to pickling in the first stage. Specifically, we discovered that the surface of a cold rolled steel strip pickled with this mixed acid had a higher tendency to be discolored to a reddish-brown color when the Fe concentration in the mixed acid was higher.
We studied the cause of this and found that the pickling rate increases as Fe gradually elutes from a cold rolled steel sheet during the pickling and the Fe concentration in the mixed acid rises. As a result, the reaction heat that is generated exceeds the cooling capability of equipment for circulating the mixed acid and the temperature of the mixed acid rises. Moreover, we realized that when the cold rolled steel strip exits a pickling tank into the atmosphere, drying is promoted and discoloration occurs due to drying proceeding in a state in which mixed acid solution remains on the cold rolled steel strip. Therefore, although it is a prerequisite that a certain degree of pickling weight loss is ensured from a viewpoint of securing good chemical convertibility and post-coating corrosion resistance, it is also necessary to appropriately control the pickling rate (i.e., the mixed acid temperature) in accordance with the Fe concentration in the mixed acid in order that surface appearance quality does not deteriorate.
This disclosure is based on the findings described above and the invention is defined in the appended claims.

(Advantageous Effect)

The disclosed method of producing a cold rolled steel strip and production system for a cold rolled steel strip enable continuous production with long-term stability of a steel strip having excellent chemical convertibility, post-coating corrosion resistance in harsh corrosive environments, and surface appearance quality.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view illustrating a cold rolled steel strip production system 100 according to a disclosed embodiment;
FIG. 2A is a scanning electron microscope (SEM) image illustrating a film surface in a comparative example;
FIG. 2B illustrates glow discharge spectroscopy (GDS) analysis results for the comparative example;
FIG. 2C is an image illustrating a sample in the comparative example after testing to evaluate post-coating corrosion resistance;
FIG. 2D is an image illustrating the surface of the sample in the comparative example;
FIG. 3A is an SEM image illustrating a film surface in Example 1;
FIG. 3B illustrates GDS analysis results for Example 1;
FIG. 3C is an image illustrating a sample in Example 1 after testing to evaluate post-coating corrosion resistance;
FIG. 3D is an image illustrating the surface of the sample in Example 1 ;
FIG. 4A is an SEM image illustrating a film surface of a sample in Example 2 corresponding to an Fe concentration of 5 g/L;
FIG. 4B is an SEM image illustrating a film surface of a sample in Example 2 corresponding to an Fe concentration of 15 g/L; and
FIG. 4C is an SEM image illustrating a film surface of a sample in Example 2 corresponding to an Fe concentration of 20 g/L.

DETAILED DESCRIPTION

(Method of producing cold rolled steel strip)

A method of producing a cold rolled steel strip according to one disclosed embodiment includes: subjecting a steel strip that has been cold rolled and subsequently continuously annealed to pickling by continuously feeding the steel strip into a mixed acid solution containing a first acid that is oxidizing and is nitric acid and a second acid that is non-oxidizing to immerse the steel strip; and subsequently subjecting the steel strip to repickling by continuously feeding the steel strip into an acid solution containing a third acid that is non-oxidizing to immerse the steel strip.

(Pickling)

In annealing performed to impart desired structure, strength, and workability on a cold rolled steel strip using a continuous annealing furnace, a non-oxidizing or reducing gas is normally used as an atmosphere gas, and the dew point is strictly controlled. Consequently, in the case of a normal cold rolled steel strip in which the additive amount of alloy is small, oxidation of the surface of the steel strip is suppressed. However, in the case of a cold rolled steel strip containing 0.5 mass% or more of Si or Mn, oxidation of Si, Mn, and the like, which are easily oxidized compared to Fe, occurs even if the composition and dew point of the atmosphere gas are strictly controlled during annealing. Consequently, it is not possible to avoid the formation of Si-containing oxides such as Si oxide (SiO₂) and Si-Mn-based composite oxides at the surface of the steel strip. Si-containing oxides are formed not only at the surface of the steel strip, but also at an inner part of the steel substrate, which impairs etching properties of the steel strip surface in chemical conversion treatment (zinc phosphate treatment) carried out as foundation treatment for electrodeposition coating, and negatively affects formation of a sound chemical conversion treatment film. Moreover, in recent years, there has been progress toward the use of a lower chemical conversion treatment liquid temperature with the aim of reducing the amount of sludge produced in chemical conversion treatment and reducing running cost, and thus it is becoming the case that chemical conversion treatment is carried out under conditions in which reactivity of the chemical conversion treatment liquid with respect to the steel strip is significantly reduced compared to under conventional conditions. In such circumstances, the deterioration of chemical convertibility becomes more noticeable.
In the pickling of the present embodiment, a cold rolled steel strip is continuously fed into a mixed acid solution containing a first acid that is oxidizing and is nitric acid and a second acid that is non-oxidizing to immerse the cold rolled steel strip and remove a Si-containing oxide layer from the surface of the cold rolled steel strip. The thickness of the Si-containing oxide layer is normally approximately 1 µm from the steel strip surface, but varies depending on the chemical composition of the steel strip and the annealing conditions (temperature, time, atmosphere).
The oxidizing first acid is nitric acid. The reason that the first acid is required in the mixed acid solution is that, among Si-containing oxides, although Si-Mn-based composite oxides readily dissolve in acid, SiO₂ displays poor solubility, and thus, in order to remove this SiO₂, it is necessary to use an oxidizing acid, namely nitric acid, so as to remove steel substrate together with Si-containing oxides at the surface of the steel strip.
The concentration of nitric acid in the mixed acid solution is set within a range of higher than 110 g/L and not higher than 188 g/L. This is because a concentration of 110 g/L or lower reduces the permissible Fe concentration upper limit in the mixed acid solution and shortens the time that continuous pickling treatment can be performed using the same mixed acid solution without waste liquid treatment, whereas a concentration of higher than 188 g/L makes it difficult to dissolve iron-based oxides by the repickling in the subsequent stage. When the concentration of nitric acid is high, the Fe concentration in the mixed acid solution tends to rise more quickly, and thus the permissible Fe concentration upper limit tends to be reached more quickly. This shortens the time that continuous pickling treatment can be performed using the same mixed acid solution without waste liquid treatment. In view of the above, the concentration of nitric acid is more preferably 140 g/L or lower, and even more preferably 130 g/L or lower.
The non-oxidizing second acid is one or more selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid. In particular, the use of hydrochloric acid, sulfuric acid, and/or hydrofluoric acid is preferred. The reason for using a non-oxidizing acid such as described above is to suppress the formation of iron-based oxides that precipitate on the steel strip surface in accompaniment to pickling with the oxidizing first acid.
The concentration of the second acid in the mixed acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L. This is because a concentration of 4.5 g/L or lower makes it difficult to dissolve iron-based oxides by the repickling in the subsequent stage, whereas a concentration of higher than 12.5 g/L reduces the pickling weight loss per unit time and may result in residual SiO₂ in the steel strip surface layer. The concentration of the second acid is more preferably 6.5 g/L to 8.5 g/L.
Conditions that influence the amount of Si-containing oxides include the structure of the steel strip and the annealing conditions. A suitable pickling time for removing Si-containing oxides is determined by taking into account these conditions. The concentration of nitric acid, the sheet passing speed, and the pickling line length may be set so as to ensure this suitable pickling time.

(Repickling)

Fe that dissolves from the steel strip surface through the pickling forms iron-based oxides and these iron-based oxides precipitate on and cover the steel strip surface, leading to reduced chemical convertibility. These iron-based oxides are removed after the pickling in the present embodiment by continuously feeding the steel strip into an acid solution containing a third acid that is non-oxidizing to immerse the steel strip. The term "iron-based oxide" is used to refer to an oxide having iron as a main component in which the atomic concentration of iron among constituent elements of the oxide other than oxygen is 30% or higher. These iron-based oxides are oxides that are present with a non-uniform thickness on the steel strip surface and differ from a natural oxide layer that is present as a uniform layer of several nanometers in thickness. Note that iron-based oxides formed at the surface of the cold rolled steel strip are known to be amorphous based on observation using a transmission electron microscope (TEM) and analysis results of a diffraction pattern obtained by electron beam diffraction.
The non-oxidizing third acid is one or more selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid. In particular, the use of hydrochloric acid, sulfuric acid, and/or hydrofluoric acid is preferred. Among these acids, hydrochloric acid is suitable because residual matter tends not to remain at the steel strip surface as with sulfate ions in the case of sulfuric acid since hydrochloric acid is a volatile acid, and because the destructive effect on iron-based oxides by chloride ions is large. Alternatively, an acid obtained by mixing hydrochloric acid and sulfuric acid may be used. The second acid used in the pickling and the third acid used in the repickling may be the same type of acid or different types of acids. However, it is preferable to use the same type of acid from a viewpoint of simplification of the production system.
The concentration of the third acid in the acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L. This is because a concentration of 4.5 g/L or lower makes it difficult to dissolve iron-based oxides, whereas a concentration of higher than 12.5 g/L may lead to discoloration due to the presence of residual acid solution on the steel strip surface. The concentration of the third acid is more preferably 6.5 g/L to 8.5 g/L.
An appropriate pickling time in the repickling is determined based on the pickling weight loss required to remove iron-based oxides formed by the pickling in the first stage, pickling efficiency determined by the acid composition, and pickling length. In general, the acid temperature is approximately 30 °C to 60 °C and the pickling time is approximately 10 s.
The cold rolled steel strip that is subjected to the pickling and repickling described above after continuous annealing may then be processed to obtain a cold rolled steel sheet as a product sheet through normal processing steps such as temper rolling and a leveling process.
The total pickling weight loss in the pickling and repickling is preferably 8 g/m² or more. When the total pickling weight loss is 8 g/m² or more, Si-containing oxides and iron-based oxides tend not to remain at the steel strip surface and higher chemical convertibility is obtained.

(Control of acid concentration in mixed acid solution)

The following describes the control of acid concentration in the mixed acid, which is a feature of our techniques. As previously explained, when a cold rolled steel strip is continuously passed along a production system capable of implementing two-stage pickling such as described above, and two-stage pickling of the cold rolled steel strip is performed continuously, as time passes, the surface of the cold rolled steel strip straight after the pickling in the first stage at that time experiences a phenomenon of reddish-brown discoloration due to adhered matter. Moreover, we discovered that this phenomenon tends to occur more easily as the Fe concentration in the mixed acid rises. In other words, we discovered that as the Fe concentration in the mixed acid rises, there is an increase in the discoloration area ratio of the surface of the cold rolled steel strip straight after pickling treatment using this mixed acid.
As previously explained, this is caused by a rise in the temperature of the mixed acid solution associated with a rise in the Fe concentration in the mixed acid. Accordingly, in the present embodiment, it is necessary to appropriately control the pickling rate (i.e., the temperature of the mixed acid) in accordance with the Fe concentration in the mixed acid. Specifically, the concentration of the first acid (nitric acid) in the mixed acid solution is lowered and the concentration of the second acid (for example, hydrochloric acid) in the mixed acid solution is raised as the Fe concentration in the mixed acid solution rises.
In the present embodiment, this control of acid concentration is used to maintain the temperature of the mixed acid solution constantly within a range of 45 °C to 55 °C. This is because a temperature of lower than 45 °C reduces the pickling weight loss per unit time and may lead to residual SiO₂ in the surface layer of the steel strip, whereas a temperature of higher than 55 °C may lead to discoloration of the steel strip surface starting to occur.
The Fe concentration in fresh mixed acid that has not been used for steel strip pickling is zero. Suppose that the concentrations of the first acid and the second acid in the fresh mixed acid are taken to be roughly in the middle of the preferred ranges therefor. For example, the concentration of the first acid may be set as 132.5 g/L and the concentration of the second acid may be set as 6.5 g/L.
Thereafter, the Fe concentration in the mixed acid is measured over time. The Fe concentration may be measured continuously or may be measured intermittently at fixed intervals.
The Fe concentration is classified into a number of levels and set concentrations for the first acid and the second acid are predetermined for each level. When the Fe concentration transitions to a next level, the concentrations of the first acid and the second acid are adjusted. For example, at a stage at which the Fe concentration in the mixed acid reaches 15 g/L, the concentration of the first acid may be adjusted to 125.0 g/L and the concentration of the second acid may be adjusted to 7.5 g/L. As further time passes, at a stage at which the Fe concentration in the mixed acid reaches 20 g/L, the concentration of the first acid may be adjusted to 110.0 g/L and the concentration of the second acid may be adjusted to 8.5 g/L.
In another configuration, relationship formulae between Fe concentration and set concentrations for the first acid and the second acid is predetermined, and the concentrations of the first acid and the second acid are adjusted from moment to moment in accordance with a gradual rise in the Fe concentration in the mixed acid.
Although no specific limitations are placed on the timing of adjustment of acid concentration in the mixed acid, the values for acid concentration at each level, and so forth, they may be determined as appropriate in consideration of the composition of the steel strip, the annealing conditions, and so forth.
According to the present embodiment, the control of acid concentration enables the temperature of the mixed acid to be maintained within a preferred range without an increase in the pickling rate even when the Fe concentration in the mixed acid rises. This enables continuous production with long-term stability of a cold rolled steel strip having excellent chemical convertibility, post-coating corrosion resistance in harsh corrosive environments, and surface appearance quality.

(Cold rolled steel strip production system)

The following describes a cold rolled steel strip production system 100 according to one disclosed embodiment that can be used to implement the method of producing a cold rolled steel strip described above. The production system 100 includes, in this order, a water tank 10 that holds water, a mixed acid tank 12 that holds a mixed acid solution (nitric/hydrochloric acid) containing nitric acid as the first acid and hydrochloric acid as the second acid, a water tank 14 that holds water, an acid tank 16 that holds hydrochloric acid as the third acid, and a water tank 18 that holds water.
A sheet feeder includes rollers 11, 13, 15, 17, and 19 that are respectively immersed in the five tanks mentioned above and a plurality of rollers 21 positioned above the tanks. The sheet feeder can continuously feed a steel strip P that has been cold rolled and subsequently continuously annealed and can immerse the steel strip P in the water tank 10, the mixed acid tank 12, the water tank 14, the acid tank 16, and the water tank 18 in this order.
The production system 100 also includes a nitric acid stock solution tank 20 that holds nitric acid and serves as a first stock solution tank and a hydrochloric acid stock solution tank 22 that holds hydrochloric acid and serves as a second stock solution tank and a third stock solution tank. A first pipe 24 extends from the nitric acid stock solution tank 20, and a second pipe 26 and a third pipe 28 extend from the hydrochloric acid stock solution tank 22.
The first pipe 24 and the second pipe 26 are connected to a mixed acid solution circulation tank 30. In the mixed acid solution circulation tank 30, nitric acid fed from the nitric acid stock solution tank 20 and hydrochloric acid fed from the hydrochloric acid stock solution tank 22 are mixed and held. A first valve 32 is provided in the first pipe 24 such that the feed rate of nitric acid from the nitric acid stock solution tank 20 can be adjusted. A second valve 34 is provided in the second pipe 26 such that the feed rate of hydrochloric acid from the hydrochloric acid stock solution tank 22 can be adjusted.
The third pipe 28 is connected to an acid solution circulation tank 40. The acid solution circulation tank 40 holds hydrochloric acid fed from the hydrochloric acid stock solution tank 22. A valve is also provided in the third pipe such that the feed rate of hydrochloric acid from the hydrochloric acid stock solution tank 22 can be adjusted.
Two fourth pipes 38 that link the mixed acid solution circulation tank 30 and the mixed acid tank 12 are provided as pipes for circulating the mixed acid solution between the mixed acid solution circulation tank 30 and the mixed acid tank 12. A valve is provided in each of the fourth pipes 38 and these valves enable adjustment of the circulation rate of the mixed acid solution. The mixed acid solution circulation tank 30 is provided with a heat exchanger 36. When the temperature of the mixed acid solution rises due to reaction heat, the temperature can be lowered through the heat exchanger 36.
Two fifth pipes 42 that link the acid solution circulation tank 40 and the acid tank 16 are provided as pipes for circulating hydrochloric acid solution between the acid solution circulation tank 40 and the acid tank 16. A valve is provided in each of the fifth pipes 42 and these valves enable adjustment of the circulation rate of the hydrochloric acid solution. The acid solution circulation tank 40 is provided with a heat exchanger 44. A rise in the temperature of the hydrochloric acid solution due to reaction heat can be suppressed through the heat exchanger 44.
The production system 100 includes an Fe concentration meter 52 that measures the Fe concentration in the mixed acid solution in the mixed acid tank 12. Fe gradually elutes from the cold rolled steel strip over the course of the pickling, resulting in a gradual rise in the Fe concentration in the mixed acid. The rise in the Fe concentration in the mixed acid is detected at appropriate timing by the Fe concentration meter 52. For example, the Fe concentration meter 52 may be an analyzer that, by near infrared spectroscopy, irradiates the mixed acid solution with near infrared at intervals of 1 minute and calculates the Fe concentration in the mixed acid solution from the change in the spectrum after the irradiation. The mixed acid solution fed to the Fe concentration meter 52 may be sampled from the mixed acid tank 12 as illustrated in FIG. 1, or may be sampled from the fourth pipe 38 that leads from the mixed acid tank 12 to the mixed acid solution circulation tank 30. Note that the production system 100 has a configuration in which the mixed acid can be sampled from the circulation tank 30 and fed to the Fe concentration meter 52. This is in order to measure the Fe concentration of fresh mixed acid solution when mixed acid solution in the circulation tank 30 is replaced.
A controller 54 controls the first valve 32 and the second valve 34 based on output of the Fe concentration meter 52. Specifically, the controller 54 reduces the feed rate of nitric acid from the nitric acid stock solution tank 20 and increases the feed rate of hydrochloric acid from the hydrochloric acid stock solution tank 22 as the Fe concentration in the mixed acid solution rises so as to lower the concentration of nitric acid in the mixed acid solution and raise the concentration of hydrochloric acid in the mixed acid solution. The specific method of control is as previously described. The controller 54 may be implemented by a central processing unit (CPU) in a computer.
Although FIG. 1 illustrates an example in which acid concentration in the mixed acid is automatically controlled through the controller 54, the disclosed production method is not limited to this example and an operator may alternatively adjust the first valve 32 and the second valve 34 based on measurement results of the Fe concentration meter 52.
A waste liquid pipe 46 extends from the mixed acid solution circulation tank 30 and a waste liquid pipe 48 extends from the acid solution circulation tank 40 such as to feed waste liquid to a waste liquid pit 50 from each of these tanks. The waste liquid fed to the waste liquid pit is subjected to pH treatment and N₂ treatment in disposal. The Fe concentration in the nitric/hydrochloric acid solution gradually rises, but it is preferable to set the permissible Fe concentration upper limit as a value of 25 g/L or lower. This is because an Fe concentration of higher than 25 g/L in the nitric/hydrochloric acid solution makes it difficult to suppress a decrease in chemical convertibility even through adoption of our techniques. When the Fe concentration approaches 25 g/L, nitric/hydrochloric acid is discharged to the waste liquid pit 50 from the mixed acid solution circulation tank 30, and the mixed acid solution circulation tank 30 is replenished with fresh nitric acid and hydrochloric acid from the stock solution tanks 20 and 22. The permissible Fe concentration upper limit in the nitric/hydrochloric acid solution is more preferably set as a value of 15 g/L or lower from a viewpoint of ensuring better chemical convertibility. Moreover, the permissible Fe concentration lower limit in the nitric/hydrochloric acid solution is preferably set as 10 g/L or higher from a viewpoint of operational efficiency. Although no specific limitations are made about discharge of hydrochloric acid from the acid solution circulation tank 40, discharge may be performed at a timing other than during operation once a certain period of use has elapsed.
As one embodiment, a feed rate A of nitric acid to the mixed acid solution circulation tank 30 from the nitric acid stock solution tank 20 may be set as 0.8 m³/hr to 1.6 m³/hr and a feed rate B of hydrochloric acid to the mixed acid solution circulation tank 30 from the hydrochloric acid stock solution tank 22 may be set as 0.1 m³/hr to 0.3 m³/hr. A and B are adjusted at the timing at which the concentrations of nitric acid and hydrochloric acid are to be adjusted. Moreover, a circulation rate C by the mixed acid solution circulation tank 30 may be set as 25 m³/hr to 90 m³/hr, a waste liquid discharge rate D from the mixed acid solution circulation tank 30 may be set as 0 m³/hr to 5 m³/hr, a feed rate E of hydrochloric acid to the acid solution circulation tank 40 from the hydrochloric acid stock solution tank 22 may be set as 1.0 m³/hr to 2.0 m³/hr, a circulation rate F by the acid solution circulation tank 40 may be set as 25 m³/hr to 90 m³/hr, and a waste liquid discharge rate G from the acid solution circulation tank 40 may be set as 0 m³/hr to 5 m³/hr. There is no particular need to adjust C, D, E, F, and G during operation.
Note that by providing the water tank 14 as in the present embodiment, it is possible to prevent nitric/hydrochloric acid carried out from the mixed acid tank 12 by the steel strip P becoming mixed into hydrochloric acid in the acid tank 16. This is preferable because it enables reliable removal of iron-based oxides by repickling in the acid tank 16.

(Chemical composition of cold rolled steel strip)

Although no specific limitations are placed on the chemical composition of the cold rolled steel strip for which our techniques are adopted, a Si content of 0.5 mass% to 3.0 mass% is required. Si is an effective element for strengthening steel because it can increase the strength of steel without significantly reducing workability. However, Si is an element that has a negative impact on chemical convertibility and post-coating corrosion resistance. To achieve strengthening through Si addition, it is necessary to add 0.5 mass% or more. Moreover, when the Si content is less than 0.5 mass%, the necessity of adopting our techniques is low because the impact of poorer chemical conversion treatment conditions is small. On the other hand, a Si content of more than 3.0 mass% causes steel hardening, has a negative impact on reliability and sheet passing performance (manufacturability), and leads to reduced ductility of the steel strip itself. Therefore, Si is added within a range of 0.5 mass% to 3.0 mass%. The preferred range for Si addition is 0.8 mass% to 2.5 mass%.
No specific limitations are placed on components other than Si and any values within the compositional range of a normal cold rolled steel strip are permissible. However, in a case in which our techniques are adopted for a high-strength cold rolled steel sheet having a tensile strength TS of 590 MPa or more that is to be used in an automotive body or the like, it is preferable that the chemical composition is as follows.

C: 0.01 mass% to 0.30 mass%

C is an effective element for strengthening steel and is also an effective element for forming bainite, martensite, and retained austenite having a transformation induced plasticity (TRIP) effect. These effects are obtained through addition of 0.01 mass% or more of C. Moreover, weldability is not significantly reduced so long as the additive amount of C is 0.30 mass% or less. Accordingly, C is preferably added within a range of 0.01 mass% to 0.30 mass%. C is more preferably added within a range of 0.10 mass% to 0.20 mass%.

Mn: 1.0 mass% to 7.5 mass%

Mn is an element that has effects of strengthening steel through solid solution strengthening, raising quench hardenability, and promoting formation of retained austenite, bainite, and martensite. These effects are exhibited when 1.0 mass% or more of Mn is added. On the other hand, excessive addition of Mn leads to increased raw material cost, but addition of 7.5 mass% or less is permissible. Accordingly, Mn is preferably added within a range of 1.0 mass% to 7.5 mass%. Mn is more preferably added within a range of 2.0 mass% to 5.0 mass%.

P: 0.05 mass% or less

P is an element that has little negative impact on deep drawability relative its significant solid solution strengthening ability and is an effective element for achieving strengthening. The P content is preferably 0.005 mass% or more to achieve these effects. On the other hand, it is preferable to set an upper limit of 0.05 mass% because P impairs spot weldability. The P content is more preferably 0.02 mass% or less.

S: 0.01 mass% or less

S is unavoidably mixed into steel as an impurity, and is a harmful component that precipitates as MnS and reduces stretch flangeability of a steel sheet. The S content is preferably limited to 0.01 mass% or less and more preferably 0.005 mass% or less in order that stretch flangeability is not reduced. The S content is even more preferably 0.003 mass% or less. Industrially, a S content of 0.0001 mass% or more is obtained in view of desulfurization cost.

Al: 0.06 mass% or less

A1 is an element that is added as a deoxidizer in a steel making process and is also an effective element for separating non-metal inclusions that reduce stretch flangeability as slag. Therefore, the Al content is preferably 0.01 mass% or more. However, it is preferable to set an upper limit of 0.06 mass% because excessive Al addition leads to increased raw material cost. The Al content is more preferably within a range of 0.02 mass% to 0.06 mass%.
In the cold rolled steel strip for which our techniques are adopted, Fe and incidental impurities make up the balance exclusive of the components described above. However, the following components may optionally be contained.
For example, Ti, Nb, and V are useful elements that not only form precipitates such as carbides and nitrides and increase the strength of steel, but also suppress ferrite growth to refine structure, and improve formability and particularly stretch flangeability. These effects are obtained when 0.005 mass% or more of each of these elements is added and reach saturation when more than 0.3 mass% is added. Accordingly, it is preferable to add one of Ti, Nb, and V within a range of 0.005 mass% to 0.3 mass%, or to add two or more of Ti, Nb, and V, each within a range of 0.005 mass% to 0.3 mass%. Addition of each of these elements within a range of 0.005 mass% to 0.2 mass% is more preferable.
Mo and Cr are elements that improve quench hardenability of steel, promote formation of bainite and martensite, and contribute to strengthening. These effects are obtained when 0.005 mass% or more of each of these elements is added and reach saturation when more than 0.3 mass% is added. Accordingly, it is preferable that Mo and Cr are each added within a range of 0.005 mass% to 0.3 mass%. Mo and Cr are more preferably each added within a range of 0.005 mass% to 0.2 mass%.
B is an effective element for raising quench hardenability of steel and can be added in an amount of 0.001 mass% to 0.006 mass%. Addition of 0.002 mass% or less of B is more preferable. Ni and Cu are effective elements for strengthening steel and can each be added within a range of 0.001 mass% to 2.0 mass%.
N is an element that causes greatest deterioration of an anti-aging property of steel and deterioration of the anti-aging property is significant particularly when the N content is more than 0.008 mass%. Accordingly, the N content should be as small as possible and is preferably 0.008 mass% or less. The N content is more preferably 0.006 mass% or less. Industrially, a N content of 0.001 mass% or more is obtained.
Ca and REM have an effect of causing spheroidization of sulfides and are effective elements for enhancing stretch flangeability. These effects are obtained when 0.001 mass% or more is added, but addition of more than 0.1 mass% reduces cleanliness of steel. Accordingly, it is preferable that Ca and REM are each added within a range of 0.001 mass% to 0.1 mass%.

EXAMPLES

Operations according to the followings examples and comparative example were performed using a production system that was the same as the production system illustrated in FIG. 1 with the exception that a controller was not included. A cold rolled steel strip that had a chemical composition containing, in mass%, 0.125 % of C, 1.40 % of Si, 1.90 % of Mn, 0.02 % of P, and 0.002 % of S, the balance being Fe and incidental impurities, and that had been annealed under a reducing atmosphere in a continuous annealing furnace was passed along the production system and was subjected to pickling and repickling.

(Comparative example)

The concentration of nitric acid in the mixed acid was set as 132.5 g/L and the concentration of hydrochloric acid in the mixed acid was set as 6.5 g/L. The Fe concentration in the mixed acid at the start of operation was 0 g/L. Although the Fe concentration gradually rose over the course of operation, the nitric acid concentration and hydrochloric acid concentration in the mixed acid were not adjusted. The concentration of hydrochloric acid in repickling was set as 3 g/L. A sample was taken from the steel strip at a section that had been pickled once the Fe concentration in the mixed acid solution reached 20 g/L and had subsequently been repickled. The sample was subjected to evaluation as described below. The total pickling weight loss in the pickling and repickling was 5.9 g/m².

(Example 1)

At the start of operation, the concentration of nitric acid in the mixed acid was set as 132.5 g/L and the concentration of hydrochloric acid in the mixed acid was set as 6.5 g/L. The Fe concentration in the mixed acid at the start of operation was 0 g/L. Since the Fe concentration gradually rose over the course of operation, the concentration of nitric acid was adjusted to 125.0 g/L and the concentration of hydrochloric acid was adjusted to 7.5 g/L at a stage at which the Fe concentration in the mixed acid reached 15 g/L, and the concentration of nitric acid was adjusted to 110.0 g/L and the concentration of hydrochloric acid was adjusted to 8.5 g/L at a stage at which the Fe concentration in the mixed acid reached 20 g/L. Adjustment of the nitric acid concentration and hydrochloric acid concentration in the mixed acid was carried out by an operator. The concentration of hydrochloric acid in repickling was set as 6 g/L. A sample was taken from the steel strip at a section that had been pickled once the Fe concentration in the mixed acid solution reached 20 g/L and had subsequently been repickled. The sample was subjected to evaluation as described below. The total pickling weight loss in the pickling and repickling was 21.3 g/m².

The samples of the comparative example and Example 1 were subjected to chemical conversion treatment under the following conditions. The grain size of phosphate film chemical conversion crystals and the film mass were measured. A grain size of 5 µm or less and a film mass of 1.0 g/m³ to 3.0 g/m³, which are typical control values, were taken to be preferred ranges. The film surface was observed at × 1,000 magnification by an SEM to confirm whether there were locations at which chemical conversion crystals were not present. In addition, GDS analysis was used to measure depth direction distributions of O, Si, Mn, and Fe in a sample surface layer and confirm whether a Si peak was present at the surface layer.

Chemical conversion treatment conditions

Each sample was subjected to chemical conversion treatment under the following conditions using a degreasing agent "FC-E2011", a surface-modifying agent "PL-X", and a chemical conversion treatment agent "PALBOND PB-L3065" produced by Nihon Parkerizing Co., Ltd. such that the film coating weight was 1.7 g/m² to 3.0 g/m².
Degreasing: Treatment temperature 40 °C, treatment time 120 s
Spray degreasing and surface modification: pH 9.5, treatment temperature room temperature, treatment time 20 s
Chemical conversion treatment: Chemical conversion treatment liquid temperature 35 °C, treatment time 120 s
The mean grain size was 6 µm in the comparative example and 4 µm in Example 1. The film mass was 0.9 g/m³ in the comparative example and 2.5 g/m³ in Example 1. FIG. 2A is an SEM image illustrating the film surface in the comparative example and FIG. 3A is an SEM image illustrating the film surface in Example 1. As illustrated, locations at which chemical conversion crystals were not present were observed in the comparative example, whereas chemical conversion crystals were observed uniformly in Example 1. In the GDS analysis results, a Si peak was detected at the surface layer in the comparative example as illustrated in FIG 2B, whereas a Si peak was not detected at the surface layer in Example 1 as illustrated in FIG. 3B. These results demonstrate that the comparative example had poor chemical convertibility and Example 1 had excellent chemical convertibility.

<Post-coating corrosion resistance evaluation>

The samples of the comparative example and Example 1 were subjected to chemical conversion treatment under the conditions described above and were further subjected to electrodeposition coating on the surface of the chemical conversion treatment film using an electrodeposition coating material "V-50" produced by Nippon Paint Co. Ltd. such as to obtain a film thickness of 25 µm. A cutter was used to form a cross-cut scar of 45 mm in length in the surface of the resultant test piece. The test piece was then subjected to a corrosion test in which 90 cycles were repeated with each cycle comprising salt spraying (5 mass% NaCl aqueous solution: 35 °C, relative humidity: 98 %) for 2 hours, followed by drying (60 °C, relative humidity: 30 %) for 2 hours, followed by wetting (50 °C, relative humidity: 95 %) for 2 hours. After this test, the test piece was washed with water and dried, and then a tape peeling test was performed on the cut scar section. The maximum total peeling width both left and right of the cut scar section was measured. Post-coating corrosion resistance can be evaluated as good when this maximum total peeling width is 6.0 mm or less.
FIG. 2C is an image illustrating the test piece of the comparative example after the tape peeling test and FIG. 3C is an image illustrating the test piece of Example 1 after the tape peeling test. In the comparative example, the maximum total peeling width was 7.9 mm and post-coating corrosion resistance was poor, whereas in Example 1, the maximum total peeling width was 5.6 mm and post-coating corrosion resistance was good.

FIG. 2D is an image illustrating the surface of the sample in the comparative example and FIG. 3D is an image illustrating the surface of the sample in Example 1. As illustrated, the surface in the comparative example was discolored reddish-brown, whereas Example 1 did not experience such discoloring and had good surface appearance.

(Example 2)

The Fe concentration in the mixed acid at the start of operation was 5.0 g/L. Relationships between Fe concentration and concentrations of nitric acid and hydrochloric acid for ensuring the required pickling weight loss were set in advance by the following relationship formulae (1) and (2). The nitric acid concentration at the start of operation was set as 132.5 g/L and the hydrochloric acid concentration at the start of operation was set as 5.5 g/L. Since the Fe concentration in the mixed acid gradually rose over the course of operation, the concentration of nitric acid and the concentration of hydrochloric acid were adjusted in accordance with formulae (1) and (2) in response. $Nitric acid concentration (g / L) = 140 - 1.5 \times Fe concentration (g / L)$
$Hydrochloric acid concentration (g / L) = 4.5 + 0.2 \times Fe concentration (g / L)$
The concentration of hydrochloric acid in repickling was set as 8 g/L. Samples were taken from the steel strip at sections that had been pickled once the Fe concentration in the mixed acid solution reached 5 g/L, 15.0 g/L, and 20 g/L, and had subsequently been repickled. These samples were subjected to evaluation as described below. The total pickling weight loss for the pickling and repickling was 11.0 g/m² for the sample corresponding to the Fe concentration of 5 g/L, 12.0 g/m² for the sample corresponding to the Fe concentration of 15 g/L, and 12.0 g/m² for the sample corresponding to the Fe concentration of 20 g/L.
The samples were subjected to evaluation of chemical convertibility, post-coating corrosion resistance, and surface appearance by the same methods as for the comparative example and Example 1.

FIG. 4A is an SEM image illustrating the film surface of the sample corresponding to the Fe concentration of 5 g/L, FIG. 4B is an SEM image illustrating the film surface of the sample corresponding to the Fe concentration of 15 g/L, and FIG. 4C is an SEM image illustrating the film surface of the sample corresponding to the Fe concentration of 20 g/L. Chemical conversion crystals were observed uniformly in all the images. Moreover, a Si peak was not detected at the surface layer in GDS analysis for any of the samples. This demonstrates that Example 2 also had excellent chemical convertibility.

<Post-coating corrosion resistance evaluation results>

The maximum total peeling width was 5.2 mm for the sample corresponding to the Fe concentration of 5 g/L, 4.8 mm for the sample corresponding to the Fe concentration of 15 g/L, and 5.6 mm for the sample corresponding to the Fe concentration of 20 g/L. Therefore, Example 2 had good post-coating corrosion resistance in the same way as Example 1.

The surfaces of the samples corresponding to the Fe concentrations of 5 g/L, 15 g/L, and 20 g/L were observed. Reddish-brown discoloring was not observed at the surface of any of the samples and all the samples had good surface appearance. However, slight staining was observed on a section of the surface of the sample corresponding to the Fe concentration of 20 g/L, whereas the samples corresponding to the Fe concentrations of 5 g/L and 15 g/L did not suffer from staining and had extremely good surface appearance. This demonstrates that it is preferable to set the upper limit for the Fe concentration as 15 g/L.

INDUSTRIAL APPLICABILITY

The disclosed method of producing a cold rolled steel strip and production system for a cold rolled steel strip enable continuous production with long-term stability of a steel strip having excellent chemical convertibility, post-coating corrosion resistance in harsh corrosive environments, and surface appearance quality. Therefore, a cold rolled steel strip produced by our techniques can be suitably used as a strengthening component of an automotive body, a component for a home appliance, a building component, or the like.

REFERENCE SIGNS LIST

100: cold rolled steel strip production system
10, 14, 18: water tank
12: mixed acid tank (for nitric/hydrochloric acid)
16: acid tank (for hydrochloric acid)
11, 13, 15, 17, 19, 21: roller (sheet feeder)
20: nitric acid stock solution tank
22: hydrochloric acid stock solution tank
24: first pipe
26: second pipe
28: third pipe
30: mixed acid solution circulation tank
32: first valve
34: second valve
36: heat exchanger
38: fourth pipe
40: acid solution circulation tank
42: fifth pipe
44: heat exchanger
46, 48: waste liquid pipe
50: waste liquid pit
52: Fe concentration meter
54: controller

Claims

A method of producing a cold rolled steel strip comprising:
subjecting a steel strip containing 0.5 mass% to 3.0 mass% of Si that has been cold rolled and subsequently continuously annealed to pickling by continuously feeding the steel strip into a mixed acid solution containing a first acid that is nitric acid and a second acid that is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid to immerse the steel strip; and

subsequently subjecting the steel strip to repickling by continuously feeding the steel strip into an acid solution containing a third acid that is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid to immerse the steel strip, wherein

concentration of the first acid in the mixed acid solution is set within a range of higher than 110 g/L and not higher than 188 g/L, and concentration of the second acid in the mixed acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L,

concentration of the third acid in the acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L,

Fe concentration in the mixed acid solution is measured over time, and

the concentration of the first acid in the mixed acid solution is lowered and the concentration of the second acid in the mixed acid solution is raised so as to maintain the temperature of the mixed acid solution constantly within a range of 45 °C to 55 °C as the Fe concentration in the mixed acid solution rises in a manner such that:
(i) a number of levels of the Fe concentration is predetermined, set concentrations for the first acid and the second acid are predetermined for each level, and when the measured Fe concentration transitions to a next level, the concentrations of the first acid and the second acid are adjusted, or

(ii) a relationship formula between the Fe concentration and set concentrations for the first acid and the second acid is predetermined, and the concentrations of the first acid and the second acid are adjusted from moment to moment in accordance with the relationship formula with a gradual rise in the measured Fe concentration in the mixed acid.
The method of producing a cold rolled steel strip according to claim 1, wherein
the second acid and the third acid are hydrochloric acid.
The method of producing a cold rolled steel strip according to any one of claims 1 or 2, further comprising
immersing the steel strip in water after the pickling and before the repickling.
The method of producing a cold rolled steel strip according to any one of claims 1 to 3, wherein
the pickling and the repickling have a total pickling weight loss of 8 g/m² or more.
A production system (100) for a cold rolled steel strip comprising:
a first stock solution tank (20) holding a stock solution of a first acid that is nitric acid, a second stock solution tank (22) holding a stock solution of a second acid that is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid, and a third stock solution tank (22) holding a stock solution of a third acid that is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid;

a first pipe (24) extending from the first stock solution tank (20), a second pipe (26) extending from the second stock solution tank (22), and a third pipe (28) extending from the third stock solution tank (22);

a mixed acid solution circulation tank (30) to which the first pipe (24) and the second pipe (26) are connected, and in which the first acid fed from the first stock solution tank (20) and the second acid fed from the second stock solution tank (22) are mixed and held;

a first valve (32) disposed in the first pipe (24) for adjusting a feed rate of the first acid from the first stock solution tank (20) and a second valve (34) disposed in the second pipe (26) for adjusting a feed rate of the second acid from the second pipe (26);

an acid solution circulation tank (40) to which the third pipe (28) is connected and that holds the third acid fed from the third stock solution tank (22);

a mixed acid tank (12) holding a mixed acid solution containing the first acid and the second acid;

an acid tank (16) holding an acid solution containing the third acid;

at least two fourth pipes (38) linking the mixed acid solution circulation tank (30) and the mixed acid tank (12) for circulating the mixed acid solution between the mixed acid solution circulation tank (30) and the mixed acid tank (12);

at least two fifth pipes (42) linking the acid solution circulation tank (40) and the acid tank (16) for circulating the acid solution between the acid solution circulation tank (40) and the acid tank (16); and

a sheet feeder (11, 13, 15, 17, 19, 21) continuously feeding a steel strip (P) containing 0.5 mass% to 3.0 mass% of Si that has been cold rolled and subsequently continuously annealed, and immersing the steel strip (P) in the mixed acid tank (12) and the acid tank (16) in this order;

wherein concentration of the first acid in the mixed acid solution is set within a range of higher than 110 g/L and not higher than 188 g/L, and concentration of the second acid in the mixed acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L, and

concentration of the third acid in the acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L,

the production system (100) further comprising:
a concentration meter (52) measuring Fe concentration in the mixed acid solution in the mixed acid tank (12); and
a controller (54) controlling the first valve (32) and the second valve (34) based on output of the concentration meter (52) such as to decrease the feed rate of the first acid from the first stock solution tank (20) and increase the feed rate of the second acid from the second stock solution tank (22), and thereby to lower the concentration of the first acid in the mixed acid solution and raise the concentration of the second acid in the mixed acid solution so as to maintain the temperature of the mixed acid solution constantly within a range of 45 °C to 55 °C as the Fe concentration in the mixed acid solution rises in a manner such that:
(i) a number of levels of the Fe concentration is predetermined, set concentrations for the first acid and the second acid are predetermined for each level, and when the measured Fe concentration transitions to a next level, the concentrations of the first acid and the second acid are adjusted, or

(ii) a relationship formula between the Fe concentration and set concentrations for the first acid and the second acid is predetermined, and the concentrations of the first acid and the second acid are adjusted from moment to moment in accordance with the relationship formula with a gradual rise in the measured Fe concentration in the mixed acid.
The production system for a cold rolled steel strip according to claim 5, further comprising
a water tank (14) that holds water and is positioned between the mixed acid tank (12) and the acid tank (16), wherein
the sheet feeder (11, 13, 15, 17, 19, 21) continuously feeds the steel strip (P) into the water tank (14) after the steel strip exits the mixed acid tank (12), and subsequently continuously feeds the steel strip (P) into the acid tank (16).
The production system for a cold rolled steel strip according to claim 5 or 6, wherein
the second acid and the third acid are the same type of acid, and the second stock solution tank (22) and the third stock solution tank (22) are the same tank.