JPH04304349A - Production of hot-dip galvanized steel plate excellent in r-value and deltar-value - Google Patents

Abstract

PURPOSE:To produce a plated hot rolled steel plate excellent in surface characteristics of plating, adhesive strength of plating, r-value, and DELTAr-value by successively subjecting a steel with specific composition to casting, holding, 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, 0.001-0.006% S, and the balance Fe with inevitable impurities is cast and solidified by cooling at <=5 deg.C/min cooling rate at 1400-1200 deg.C. Further, after holding in the temp. region of 1200-1100 deg.C at 5-30 deg.C/min average temp. fall rate, finish rolling is started within 60min from the initiation of solidification in the temp. region not higher than (Ar3 point +100 deg.C) and finished in the isothermal temp. region 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 and thereafter.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention provides 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 that has a good Δr value, which is an index showing the anisotropy of the ▲r▼ value and the r value. [0002] Generally, after hot rolling, Zn-based hot-dipped hot-rolled steel sheets are wound into coils and naturally cooled to less than 100°C over a long period of time, and then processed at about 80°C in a pickling line. It is descaled and then 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. Problems to be Solved by the Invention The above manufacturing method has the following problems. ■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. [0011] Furthermore, in the case of JP-A-62-192539, the Δr value is large and cannot be said to be good. The object of the present invention is to provide a method for producing Zn-based plated steel sheets that are free from these problems, are economical, have good workability, and have excellent ▲r▼ values and ▲r▼ values. [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 0.001 to 0.006% S 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. , the temperature range of the slab from 1200℃ to 1100℃ 5
After keeping the slab warm at an average cooling rate of ℃/min or more and 30℃/min or less, finish rolling of the slab is started within 60 minutes from the start of casting in a temperature range of Ar3 point + 100℃ or less, and the rolling is carried out at Ar3 The process is completed in an isothermal temperature range above 350°C, followed by dry descaling at 350°C or above, temperature adjustment before plating from 400°C to 600°C, and continuous Zn-based hot-dip plating on the steel sheet. Zn with excellent plating surface properties and plating adhesion, and excellent ▲r▼ and Δr values.
The method is a method for producing hot-dipped hot-rolled steel sheets. [0014] 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% - 0.006%
This refers to steel that contains Fe and other unavoidable components. [Operation] In order to achieve the above object, the present inventors conducted various experimental studies and obtained the following knowledge. [0016] The 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 the temperature range of the slab is 5°C.
The temperature is maintained at an average cooling rate of 30° C./min or more and 30° C./min or less, finish rolling of the slab is started at a temperature of Ar3 point +100° C. or less within 60 minutes from the start of solidification, and Ar3
When finishing isothermal finish rolling at or above the point, the amount of S precipitated as MnS during finish rolling was ensured in a sufficient solid solution state until finish rolling, and by this ensuring, the precipitation amount and precipitation size were controlled during finish rolling. Fine MnS precipitates, which effectively controls austenite recrystallization, fully develops the rolling texture of austenite, and ferrite transformed from this rolling texture has a high accumulation of (112) planes. ▲r▼ value improves. At this time, due to the above-mentioned finish rolling, a large shear strain acts on the surface layer of the steel sheet and randomizes the texture of the surface layer, so that the Δr value becomes small. [0017] Regarding the temperature history from 1200°C to 1100°C, it is sufficient that the average temperature drop rate is 5°C/min or more and 30°C/min or less, and depending on the thickness of the slab, simple air cooling may be sufficient, or coolant Cooling or heating may be used. Further, complete heat retention may be partially included during this time, and rolling for adjusting the width and thickness may be interposed. [0018] The lower limit of the winding temperature after rolling is 350
℃. The reasons for this limitation are explained below. If the winding temperature is less than 350°C, a distorted pattern known as 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. The equipment required to adjust the temperature to 400 to 600°C becomes long, making it impossible to enjoy the effect of reducing equipment costs. Further, in this step, in order to improve the telescaling efficiency in the subsequent step, 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, descaling is limited to dry descaling without using 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 alone or in combination. [0021] 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. [0023] 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. [0024] If the dry descaling temperature is less than 350°C, a distortion 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., distortion patterns called buckling may occur 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. [0027] Furthermore, after completion of plating, the required characteristics and
Various post-treatments such as skin pass, chromate treatment, bonding treatment, and painting can be appropriately selected depending on the application. [0028] By utilizing the above knowledge, the present invention has achieved the object. [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. [Table 1] [Table 2] (Note) CCS: Casting cooling rate
Temperature lowering conditions: ■ Temperature lowering rate 20℃/min FTO: Finish rolling start temperature
■1200℃×10 minutes constant FTE: Finish rolling end temperature
■1100℃×10 minutes constant MST: From the start of solidification
■Cooling rate 50℃/min
Time until rolling start CT: Coiling temperature [0032] [Example 1] Using a coil manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating with a vacuum arc were performed under the following conditions. went. Plating bath temperature: 470°C, pre-plating plate temperature: 480°C Plating bath components: Zn-0.2%Al Atmosphere: 80%N2-20%H2 Plating deposition amount: 190g/m2 Dry descaling temperature: 350 [0034] Thereafter, plating adhesion was evaluated using a DUPONT impact tester by visually observing blistering defects and folds on the plating surface, and by measuring the ▲r▼ value (tensile test). Note that the evaluation was conducted 30 days after plating. The results are shown in Table 3. [0035] Hot-rolled steel sheets with a high ▲r▼ value and a small ▲r▼ value, which exhibited excellent plating adhesion with various composition systems of steel types A to F, were free from bulging defects and buckles, and had a small ▲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 present invention example, Steel No. 4 did not satisfy the average temperature drop condition, Steel No. 5 did not satisfy the condition that the rolling start temperature was Ar3 + 100℃ or less, and Steel No. 6 did not satisfy the cooling rate condition during solidification. In all cases, the ▲r▼ value is low, and steel No. 7, whose heat retention conditions are outside the upper limit, has a good ▲r▼ value, but Δr
Steel No. 8, which had a large value and the rolling temperature was less than Ar3, was
Steel numbers 9, 11, and 1 with low r▼ values and S content outside the conditions
3 also has a low ▲r▼ value. In addition, Steel No. 10, which does not satisfy the S content condition and the heat retention condition, has a low ▲r▼ value, and Steel No. 12, which does not meet the S content condition and the heat retention condition, has a good ▲r▼ value, but Δ
Steel No. 14, which has a large r and a long time from the start of casting to the start of finish rolling, has a low ▲r▼ value. [Table 3] r=(rC +rL +2·rD )/4Δr=(rC
+rL )/2-rD [Example 2] Using the coils manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating were carried out using a combination of magnetic polishing and reduction under the following conditions. went. Plating bath temperature: 470°C, plating bath components: Zn-0.10
%Al atmosphere: 85%N2 -15%H2, plating amount:
90 g/m2 [0040] Thereafter, plating adhesion was evaluated using a DUPONT impact tester by visually observing blistering defects and buckles on the plating surface, and material deterioration was evaluated using a tensile test. Note that the evaluation was conducted 30 days after plating. 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. [0042]No. Samples Nos. 4, 6, and 8 met the conditions of the present invention, exhibited excellent plating adhesion, had no bulging defects, and had no buckling, and yielded plated products with good ▲r▼ values and ▲r▼ values. (Legend is the same as Table 3) 00
[Table 4] [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. [0045] Steel number: 2 Descaling temperature: 600°C Atmosphere: 100% N2 Plating adhesion: 150 g/m2 [0046] After that, the plating adhesion was checked using a DUPONT impact tester by visually checking the plating surface for blistering defects and buckling. , Material deterioration was evaluated using a tensile test. Note that the evaluation was performed 30 days after plating. The results are shown in Table 5. Excellent plating adhesion was exhibited to various bath components, and a good plated product was obtained with no blistering defects or buckling. (The legend is the same as in Table 3) [Table 5] [Effects of the Invention] The present invention uses the above-described means and utilizes the above-described effects to produce Ti, Nb, etc. as in the conventional method. This makes it possible to economically produce hot-rolled steel sheets with good ▲r▼ value and low workability ▲r with good productivity and workability without using alloys or without oil-lubricated rolling. Moreover, since the surface layer can be galvanized more economically, 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: 0.001-0.006%, other Fe
and 1400°C to 1200°C for steel consisting of unavoidable components.
The slab is solidified by casting at a cooling rate of ≥5.0°C/min in the temperature range of 1200°C to 1100°C, and the average temperature decrease is 5°C/min or more and 30°C/min or less. After keeping the slab warm at a constant speed, finish rolling of the slab is started within 60 minutes from the start of solidification in a temperature range below the Ar3 point +100°C, and the rolling is finished in an isothermal temperature range above the Ar3 point, and then finished at 350°C.
After winding in the above manner, dry descaling at 350°C or higher, temperature adjustment before plating to 400°C to 600°C, and Zn-based hot-dip plating on the steel sheet are performed continuously, and at least after completion of dry descaling, no A method for producing a Zn-based hot-dipped hot-rolled steel sheet with excellent plating surface properties and plating adhesion, and excellent ▲r▼ and ▲r values, the method comprising adjusting the atmosphere to be oxidizing or reducing.