JPH04304348A - Production of hot-dip galvanized steel plate excellent in r-value - Google Patents

Abstract

PURPOSE:To produce a hot-dip galvanized hot rolled steel plate excellent in surface characteristics of plating, adhesive strength of plating, and r-value by successively subjecting a steel with specific composition to casting, finish rolling, coiling, dry descaling, plate temp. regulation, and hot-dip galvanizing under respectively specified conditions. CONSTITUTION:A steel having a composition consisting of, by weight, 0.10-0.60% Mn, 0.001-0.012% S, and the balance Fe with inevitable components is cast and solidified at 1400-1200 deg.C at >=5.0 deg.C/min cooling rate and formed into a cast slab. The finish rolling of this cast slab is started within 60min from the initiation of solidification and finished at a temp. not lower than the Ar3 point, and the resulting plate is coiled at >=350 deg.C. Subsequently, the plate is subjected to dry descaling at >=350 deg.C, to plate temp. regulation prior to plating up to 400-600 deg.C, and to hot-dip galvanizing. At this time, the atmosphere is regulated to nonoxidizing or reducing atmosphere at least, from the completion of dry descaling an thereafter. By this method, the hot rolled steel plate having superior r-value, deep drawability, and workability can be produced with high productivity and workability.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a hot-rolled steel plate coated with Zn or an alloy of Zn and Al as a main component.
The present invention relates to a method for producing a hot-rolled steel sheet for processing with an excellent ▲r▼ value.

0002

[Prior Art] Generally, after hot rolling, Zn-based hot-dip galvanized steel sheets are wound into coils and naturally cooled to less than 100°C over a long period of time, and then descaled at about 80°C in a pickling line. , and then further processed in a hot-dip plating line. In addition, in the case of a Zn-based hot-dip-plated cold-rolled steel sheet, cold rolling is performed between pickling and hot-dip plating. In addition, as a hot-dip plating line, 7
It is heated to about 00 to 800 degrees Celsius to burn off foreign substances called smut (Fe oxide, Fe + silicate, etc.), oils and other surface impurities that adhere to the surface of the steel plate during the pickling process and subsequent transportation, and then heats it with hydrogen. The so-called Sendzimer method, in which the surface cleanliness necessary for plating adhesion is ensured by reduction in an atmospheric gas, and then hot-dip plating by cooling to near the plating bath temperature in a non-oxidizing or reducing atmosphere, or a hydrogen atmosphere. It is heated to about 700-800℃ in gas to remove surface impurities such as smut and oil to ensure the surface cleanliness necessary for plating adhesion, and then heated to around the plating bath temperature in a non-oxidizing or reducing atmosphere. The so-called non-oxidation furnace method is used, which involves cooling the material to a temperature of 100°C before hot-dip plating.

In the case of Zn-based hot-dipped hot-rolled steel sheets, surface impurities such as smut and oil that adhere to the steel sheet surface during the pickling process and subsequent transportation are removed by combustion and reduction, thereby achieving the surface cleanliness necessary for plating adhesion. 700-8 to ensure
Heating to around 00°C is unavoidable.

However, in order to ensure the surface cleanliness necessary for plating adhesion, it is necessary to
Heating to a high temperature of around 00°C requires a huge amount of energy and a long heating furnace, which imposes a large operating cost. Furthermore, cooling the plating bath temperature (approximately 500° C.), which is an intermediate temperature between the two, results in a large loss of thermal energy. Furthermore, hydrogen is stored in the steel sheet in a high-temperature hydrogen atmosphere, and after plating, this hydrogen is released to the interface between the steel sheet and the plating, causing bulge-like defects on the plating surface. As a method to solve the above problems, for example, a method of coating with Ni or a Ni-based alloy as a pre-treatment for plating (Japanese Patent Laid-Open No. 61-44168)
has been disclosed, but although it is possible to reduce the heating temperature,
Since the pretreatment temperature is below 150℃, the coil must be cooled once, which not only makes it impossible to shorten the number of days required after winding, but also makes it impossible to utilize the heat retained in the coil, so energy loss is still inevitable. Furthermore, the number of pretreatment steps increases, which increases operating costs. In addition, blistering defects on the plating surface (not only deterioration of appearance but also peeling resistance and
To avoid this (leading to deterioration of corrosion resistance), the heating temperature should be set to
Method for reducing temperature to 00 to 720°C (Japanese Patent Application Laid-open No. 52-9554
Publication No. 3) is disclosed, but due to cleaning with acid,
Since the steel plate temperature once drops below 100℃, not only is it not possible to shorten the number of days required after winding, but the heat retained in the coil cannot be used, and heating temperatures of 600 to 720℃ still require a huge amount of energy and time. Since it requires a heating furnace, the effect of reducing operating costs is small.

On the other hand, regarding the improvement of the ▲r▼ value of hot-rolled steel sheets, as described in Japanese Patent Application Laid-open No. 59-226149,
A method for producing ultra-low carbon steel using Ti by oil-lubricated rolling,
Alternatively, there is a method of manufacturing by adding alloys such as Nb and Ti, as described in JP-A-62-192539.

[0006]

Problems to be Solved by the Invention Problems with the above manufacturing method include the following points.

■In the process of applying Zn plating to hot rolled steel sheets,
Because the temperature of the steel sheet must be lowered to room temperature for cleaning with acid, not only is it not possible to shorten the number of days required after winding, but the heat retained in the coil cannot be utilized, and the heating process before plating still requires a huge amount of energy and time. The operating cost is high because it requires a heating furnace.

[0008] The rolling oil used in oil lubrication has a high concentration, and rolling itself is inevitably carried out under conditions at the very edge of the slip limit, and there are significant restrictions on rolling temperature, rolling speed, rollable size, etc. The deterioration of production efficiency cannot be denied.

[0009] Furthermore, it becomes necessary to take time and effort to remove oil adhering to the rolls, which deteriorates workability and production efficiency.

■When using alloys such as Nb and Ti,
Alloy cost is high and not economical.

The object of the present invention is to provide a method for producing a Zn-based plated steel sheet that is free from these problems, is economical, has good workability, and has an excellent ▲r▼ value.

0012

[Means for Solving the Problems] In order to achieve the above-mentioned problems, the present invention provides Mn: 0.10 to 0.60% by weight,
A steel containing S: 0.001 to 0.012% and other Fe and other unavoidable components is cast and solidified in a temperature range of 1400°C to 1200°C at a cooling rate of ≥5.0°C/min to form a slab. , Start finish rolling of the slab within 60 minutes from the start of solidification, finish at Ar3 point or higher, coil at 350°C or higher, then start dry descaling at 350°C or higher, then roll at 350°C to 600°C. Dry descaling at ℃, then adjusting the plate temperature before plating to 400℃ to 600℃, and Zn-based hot-dip plating on the steel plate are performed continuously, and at least after the dry descaling is completed, it is non-oxidizing or reducing. Z that has excellent plating surface properties and plating adhesion, and has an excellent ▲r▼ value, which is characterized by atmosphere adjustment.
The method is a method for producing an n-type hot-dipped hot-rolled steel sheet.

[0013] Note that the hot-rolled steel sheet for processing in the present invention refers to
In the field of manufacturing and using hot-rolled steel sheets for processing, steels commonly used for such steel sheets, for example, C:
0.01% to 0.10%, Mn: 0.10% to 0.60
%, Si: 0.001-0.06% and P: 0.00
1% ~ 0.050%, S: 0.001% ~ 0.012%
This refers to steel that contains Fe and other unavoidable components.

[0014]

[Operation] In order to achieve the above object, the present inventors conducted various experimental studies and obtained the following findings.

[0015] Steel having the above components is cast and solidified in the temperature range of 1400°C to 1200°C at a cooling rate of 5°C/min or more to form a slab, and finish rolling of the slab is started within 60 minutes from the start of solidification. However, when the rolling is finished below the Ar3 point, Mn
The amount of S that precipitates as S is ensured in a solid solution state until the rolling, and precipitates as fine MnS during the rolling,
The precipitated fine MnS suppresses austenite recrystallization and forms a fully developed austenite rolling texture, which transforms into ferrite with a high concentration of (112) planes, resulting in a steel sheet with a high ▲r▼ value. I found out what I can get.

[0016] If the winding temperature is less than 350°C, a distorted pattern called buckling will occur during unwinding, impairing the appearance quality after plating, and making it impossible to effectively utilize the heat retained in the coil, resulting in energy loss. As a result, the equipment required to adjust the temperature to 400 to 600° C. before plating becomes long, making it impossible to enjoy the effect of reducing equipment costs.

[0017] Further, in this step, in order to improve the descaling efficiency of the subsequent process, it is also possible to adopt process conditions that reduce the scale thickness. For example, rolling in an inert atmosphere such as Ar, N2, etc., cooling between rolling stands with cooling water containing a solvent that has a scale control effect, cooling on a hot run table, N2 sealing atmosphere of the wound coil BO
It is possible to employ cooling within the X.

Next, regarding descaling conditions, telescaling is limited to dry descaling that does not use an acid solution. This avoids the generation of smut that adheres to the surface of the steel sheet due to the pickling process, making it possible to ensure the surface cleanliness necessary for plating adhesion without having to heat and burn off foreign matter. A specific dry descaling method is vacuum arc (10-1 to 10
-6 Torr), plasma, reaction (reduction), magnetic polishing (
(Using magnetic powder of tens to hundreds of microns), shot blasting, sand blasting, grid blasting, wire brushing, grinder, etc. can be used singly or in combination.

[0019] In order to maintain the surface cleanliness necessary for the plating adhesion obtained by dry descaling, at least after dry descaling is completed, an inert gas atmosphere such as argon or nitrogen, or a mixture of an inert gas and hydrogen is used. It is necessary to maintain the atmosphere in a non-oxidizing or reducing atmosphere such as a hydrogen atmosphere. It goes without saying that dry descaling may be performed in a non-oxidizing or reducing atmosphere.

In particular, it is desirable to use a reducing atmosphere for materials that are difficult to plate and contain a large amount of Si or the like.

[0021] Furthermore, dry descaling must be started at 350°C or higher and completed at 350-600°C. The reasons for this limitation are explained below.

[0022] If the dry descaling temperature is less than 350°C, a strain pattern called buckling occurs due to deformation of the steel plate due to bending and unbending during sheet passing, which impairs the appearance quality. Furthermore, since the heat retained in the coil cannot be used effectively and energy loss occurs, the equipment required to adjust the plate temperature to 400 to 600°C before plating becomes long, making it impossible to enjoy the effect of reducing equipment costs.

On the other hand, when the dry descaling temperature is 600
When the temperature exceeds 0.degree. C., hydrogen in the reducing atmosphere tends to be absorbed into the steel sheet, which tends to cause blistering defects on the plating surface. In addition, from the perspective of the energy cost and equipment cost required to adjust the plate temperature before plating to 400 to 600 °C,
If the temperature exceeds ℃, the loss becomes large.

As for the process conditions after dry descaling, in order to maintain the surface cleanliness necessary for the plating adhesion obtained by dry descaling, after dry descaling, the process is continuously carried out in a non-oxidizing or reducing atmosphere. So 4
The plate temperature before plating is adjusted to 00 to 600°C, and hot-dip plating is performed. In particular, it is desirable to use a reducing atmosphere for materials that are difficult to plate and contain a large amount of Si or the like. The reasons for limiting the plate temperature before plating are described below. If the temperature is less than 400°C, so-called "wettability" cannot be ensured, resulting in non-plating or deterioration of plating adhesion. Further, if the temperature is lower than 350° C., a strain pattern called buckling occurs due to deformation of the steel sheet due to bending and unbending during sheet passing, and there is a risk that the appearance quality may be impaired. On the other hand, when the temperature exceeds 600°C, deterioration and fluctuation of the steel sheet material occur due to tempering of the transformed structure, redissolution of precipitates, grain growth, etc., and hydrogen in the reducing atmosphere is likely to be absorbed into the steel sheet. , which tends to cause blistering defects on the plating surface. Further, the alloying reaction between Zn and Fe progresses excessively, and a brittle plating layer such as a Γ phase appears, deteriorating the plating adhesion. In addition, the energy cost and
Also from the point of view of equipment cost, if the temperature exceeds 600°C, losses will be large. Furthermore, according to this manufacturing method, since the material is built in during the hot rolling process, it goes without saying that there is no need for recrystallization annealing to adjust the material.

Furthermore, after completion of plating, the required characteristics,
Various post-treatments such as skin pass, chromate treatment, bonding treatment, and painting can be appropriately selected depending on the application.

[0026] By utilizing the above knowledge, the present invention has achieved the object.

[0027]

[Example] Steel having the chemical composition shown in Table 1 was manufactured under the hot rolling conditions shown in Table 2, and dry descaling and hot dipping were performed under the conditions shown below.

[0028]

[Table 1]

[0029]

[Table 2] (Note) ■ CCS: Casting cooling rate ■ FTO: Finish rolling start temperature ■ FTE: Finish rolling end temperature ■ MST: Time from start of solidification to start of rolling ■
CT: Winding temperature

[0030]

[Example 1] Using the coils manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating using a vacuum arc were performed under the following conditions. Plating bath temperature: 470°C, pre-plating plate temperature: 480°C Plating bath components: Zn-0.2%Al Atmosphere: 80%N2 -20%H2 Plating amount: 190g/m2 Dry descaling temperature: 350°C Then, The adhesion of the plating was evaluated using a DUPONT impact tester by visually observing blistering defects and bends on the plating surface, and the material was evaluated by measuring the ▲r▼ value (tensile test). Note that the evaluation was conducted 30 days after plating. The results are shown in Table 3.

[0031] For various component systems of steel types A to E, steel sheets can be produced that exhibit excellent plating adhesion, are free from bulge-like defects and buckles, and have a good ▲r▼ value.

Further, the bare corrosion resistance and painted corrosion resistance were evaluated by a salt spray test, the chemical conversion treatment property was evaluated by the amount of chemical conversion film deposited, and the paint adhesion was evaluated by an Erichsen test, and all properties were good. However, compared to the inventive example, steel No. 5 had a cooling rate outside the lower limit of the invention during solidification, and the finish rolling end temperature was Ar3.
Comparative examples of Steel No. 6 where the point was cut, Steel No. 7 where the time from solidification to start of finish rolling was outside the upper limit, and Steel No. 8 where S was outside the lower limit of the present invention, the ▲r▼ value was 0. It was low, below .90.

[0033]

[Table 3]

[0034]

[Example 2] Using the coils manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating were performed using a combination of magnetic polishing and reduction under the following conditions. Plating bath temperature: 470°C, plating bath component: Zn-0.10
%Al atmosphere: 85%N2 -15%H2, plating amount:
90 g/m2 Thereafter, plating adhesion was evaluated using a DUPONT impact tester, visually inspecting the plating surface for blistering defects and buckling, and material deterioration was evaluated using a tensile test. In addition, the evaluation is after plating, 30
It was carried out after a day had passed.

The results are shown in Table 4. No. In No. 1, the dry descaling temperature was too high, so a bulge-like defect occurred. Also, even if dry desks are arranged within the conditions, No. If the pre-plating plate temperature is too high as in 2, blistering defects will occur.
Furthermore, since alloying of Zn and Fe progressed excessively, plating adhesion also deteriorated. On the other hand, No. In No. 3, the plate temperature before plating was too low, resulting in poor plating adhesion and unplated areas. No. In No. 5, the dry descaling temperature was too low, so bending occurred during coil unwinding. Also, No. In No. 7, the annealing temperature was too low and the steel plate could not be recrystallized, so a high ▲r▼ value could not be obtained. No. 9 is an example where an appropriate descaling temperature could not be secured.
As a result, a broken back occurred.

[0036]No. Items 4, 6, and 8 meet the conditions of the present invention, exhibiting excellent plating adhesion, with no bulging defects or buckling, and having a high ▲r▼ value and excellent deep drawability. Obtained. (Legend is the same as Table 3)

[0037]

[Table 4]

[0038]

[Example 3] Using the coils manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating using a vacuum arc were performed under the following conditions. Steel number: 2 Descaling temperature: 600°C Atmosphere: 100% N2 Plating adhesion: 150 g/m2 After that, the plating adhesion was checked using a DUPONT impact tester, visually inspecting the plating surface for blistering defects and bends, and checking for material deterioration. Evaluated by test. In addition, the evaluation is after plating, 30
It was carried out after a day had passed. The results are shown in Table 5. Shows excellent plating adhesion to various bath components, eliminating blistering defects and
A good plated product was obtained with no bending observed. (Legend is the same as Table 3)

[0039]

[Table 5]

[0040]

Effects of the Invention The present invention uses the above-mentioned means and utilizes the above-mentioned effects, without using alloys such as Ti, Nb, etc. as in the conventional method, and without performing oil-lubricated rolling. It is possible to economically produce workable hot-rolled steel sheets with good ▲r▼ value and excellent deep drawability with good productivity and workability, and to apply galvanization to the surface more economically. Therefore, it has a great industrial effect on the field of art.

Claims (1)

[Claims] [Claim 1] Mn: 0.10 to 0.60 in weight%
%, S: 0.001 to 0.012%, and other Fe
and 1400°C to 1200°C for steel consisting of unavoidable components.
The slab is cast and solidified at a cooling rate of ≧5.0°C/min, and the slab is finished rolled for 60 minutes from the start of solidification.
Start within minutes and finish at Ar3 point or higher, wind up at 350°C or higher, then dry descaling at 350°C or higher, adjust the plate temperature before plating to 400°C to 600°C, and Zn-based hot-dip plating on the steel sheet. Zn-based hot-dip plating with excellent plating surface properties and plating adhesion, and an excellent ▲r▼ value, which is carried out continuously and at least after dry descaling is completed, the atmosphere is adjusted to be non-oxidizing or reducing. A method for producing hot rolled steel sheets.