JPH09316617A - Production of high workability plated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a plated steel sheet excellent in workability and plating adhesion at a low cost even if cold rolling and pickling are obviated. SOLUTION: A steel slab having a componental compsn. contg., by weight, <=0.02% C, <=2% Si, <=3% Mn, 0.01 to 0.2% Ti, 0.001 to 0.2% Nb, 0.0001 to 0.003% B, <=0.01% N, <=0.2% P and <=, 0.05% S, and in which [C], [Ti], [Nb] and [N], i.e., each content of C, Ti, Nb and N satisfies the inequality of [C]/12+[N]/14<=[Ti]/48+[Nb]/93 is heated to the temp. range of the Ac3 transformation point or above and is subjected to hot rough rolling. Next, it is subjected to descaling treatment by high pressure water under >=300kg/cm<2> discharge pressure, is thereafter subjected to hot finish rolling so as to requlate the draft in the temp. range of 500 deg.C to the Ar3 transformation point to >=60% and the finishing temp. to <=800 deg.C under lubrication and is coiled, subsequently, >=50% of scales in the surface of the steel sheet is reduced in an atmosphere heated at 750 deg.C to the Acs transformation point by an annealing furnace of continuous plating equipment, and after that, plating is applied thereto.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a plated steel sheet for use as a building material, a steel sheet for cooling / heating / hot water supply equipment, a steel sheet for automobiles, etc., which is required to have high corrosion resistance and drawability.

[0002]

2. Description of the Related Art The production of plated steel sheets is carried out by hot rolling the raw material for the plated steel sheets, removing the iron oxide layer covering the steel sheet surface with a pickling facility, and performing cold rolling, followed by continuous hot dipping equipment. It is common to perform plating with an electroplating device or the like. Here, if the hot-rolled material is used as it is, it cannot be recrystallized by accumulating processing strain, and the drawability suitable for the above purpose cannot be obtained. Therefore, recrystallization should be performed by cold rolling. Therefore, it is necessary to obtain a plated steel sheet having excellent workability such as elongation and drawing characteristics. The removal of the iron oxide layer is essential because the iron oxide layer hinders the plating and becomes a starting point of peeling of the plating layer, resulting in poor adhesion of the plating.

In contrast to the general production of the above-mentioned plated steel sheet, Japanese Patent Laid-Open Nos. 6-145937 and 6-279967 omit the pickling step and the cold rolling step mainly for the purpose of cost reduction. Techniques for doing so are disclosed. That is, it has been proposed to carry out a reduction treatment in a reducing atmosphere gas without removing the iron oxide layer on the surface of the hot rolled sheet, and then perform hot dip galvanizing.

However, there is no mention in the above publication of the deterioration of workability which is a concern due to the omission of cold rolling, and regarding the damage (peeling) of the plating suffered by deformation such as bending, Not resolved. Incidentally, in JP-A-6-145937, the adhesion of plating is not mentioned. On the other hand, JP-A-6-
No. 2,799,67 discloses an improvement in plating adhesion by using a hot rolled steel sheet with a relatively thin iron oxide layer of 1.1 to 4.6 μm, but there is no mention of means for obtaining the thin iron oxide layer. Has not been done. Furthermore, when using a steel type in which easily oxidizable components such as Si and Mn are present in the steel in order to increase the strength of the steel plate, these components are oxidized during annealing before plating and are concentrated on the steel plate surface. There is also a problem in that the treatment hinders the reaction between the steel sheet and the molten metal, eventually leading to non-plating defects.

[0005]

Therefore, the present invention proposes a method capable of producing a plated steel sheet excellent in workability and plating adhesion at low cost even if cold rolling and pickling are omitted. With the goal.

[0006]

Means for Solving the Problems The inventors diligently studied the temperature and working conditions of hot rolling, the descaling conditions after rough rolling, the iron oxide layer thickness of hot rolled steel sheets, and the steel sheet material after annealing, Further, the steel sheet on which the iron oxide layer was formed was reduced under various conditions and then plated, and the plating characteristics were investigated. As a result, by incorporating the work strain in the hot-rolled sheet, deterioration of workability can be prevented even if cold rolling is omitted, and by obtaining a hot-rolled steel sheet with a sufficiently thin iron oxide layer, oxidation It was found that the adhesion of plating can be secured without removing the iron layer.

That is, according to the present invention, C: 0.02 wt% or less, Si: 2 wt% or less, Mn: 3 wt% or less, Ti: 0.01 to 0.2.
wt%, Nb: 0.001 to 0.2 wt%, B: 0.0001 to 0.003 wt
%, N: 0.01 wt% or less, P: 0.2 wt% or less and S: 0.
A steel slab containing 05 wt% or less and having a composition such that each of the C, Ti, Nb, and N contents [C], [Ti], [Nb], and [N] satisfies the following formula (1): Heated to a temperature range of ₃ transformation points or higher and subjected to hot rough rolling, and then the discharge pressure was 300 kg / cm
After performing descaling treatment with high pressure water of ² or more,
In the annealing furnace of the continuous plating equipment, hot finish rolling with a rolling reduction of 60% or more and an end temperature of 800 ° C or less in the temperature range of 500 ℃ or more and Ar ₃ transformation point or less is performed under lubrication, and after winding. This is a method for producing a high workability plated steel sheet, which is characterized by reducing 50% or more of the iron oxide layer on the surface of the steel sheet at a steel sheet temperature of 750 ° C or more and Acs transformation point or less and then performing plating. Note [C] / 12 + [N] / 14 ≦ [Ti] / 48 + [Nb] / 93 ---- (1). Here, the Acs transformation point changes from α phase to γ
It is the temperature at which the transformation to the phase begins.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the procedure of the present invention will be specifically described. First, as a material for plated steel sheets, C: 0.
02 wt% or less, Si: 2 wt% or less, Mn: 3 wt% or less, Ti: 0.
01-0.2 wt%, Nb: 0.001-0.2 wt%, B: 0.0001-0.
003 wt%, N: 0.01 wt% or less, P: 0.2 wt% or less and S: 0.05 wt% or less, and each content of C, Ti, Nb and N [C], [Ti], [Nb] And [N] is the above formula
Use a steel slab with a composition that satisfies (1).

That is, C and N are interstitial solid solution elements and are effective in increasing the strength of the steel sheet, but on the other hand, they lower the workability represented by elongation and r-value, so that the present invention Then, by strengthening with components other than C and N, C and N are suppressed to 0.02 wt% or less and 0.01 wt% or less, respectively, in the steelmaking stage.

Further, in order to make the adverse effects of the remaining C and N on the workability harmless, Ti and Nb are added to cause C and N to react and precipitate, thereby ensuring excellent workability. Specifically, Ti: 0.01-0.2 wt% and Nb: 0.001-0.2 wt%
It is important to add% to the contents of C and N under the above formula (1).

This is because Ti has a higher reactivity than Nb and is selectively consumed for precipitation of N and C, and is easily oxidized and consumed by oxygen in steel. Below the range, the effect of addition does not appear. On the other hand, adding more than 0.2 wt% saturates the effect and increases the cost.

Since Nb has a low reactivity with elements other than C as compared with Ti, it is effective even if added in a small amount.
If it is less than 1 wt%, the number of atoms present is too small compared to the amounts of C and N, and the effect does not appear. On the other hand, adding more than 0.2 wt% saturates the effect and increases the cost. Further, Ti and Nb can be secured in sufficient amounts to precipitate C and N by adding them in a range satisfying the above formula (1) depending on the amounts of C and N.

Si and Mn are components that enable the strength of the steel sheet to be increased without impairing the workability, and can be added up to 2 wt% and 3 wt% respectively. If the upper limit is exceeded, cracks are likely to occur in the steel plate edge portion during hot working, and scales are abnormally formed, so that a clean steel plate surface cannot be obtained. The lower limits of Si and Mn are not particularly limited. Adjusted for required strength. However, in order to avoid an increase in steel cost, 0.001% and 0.01 wt% are preferable, respectively.

If B is less than 0.0001 wt%, improvement of brittleness due to grain boundary strengthening of steel cannot be expected. On the other hand, when it exceeds 0.003 wt%, the brittleness improving effect is saturated and the workability of steel is rapidly deteriorated.

P is relatively inexpensive and can maintain high workability to enhance strength, but in steel with reduced C and N, it precipitates at grain boundaries and causes embrittlement, so 0.2 wt. %
Is the upper limit. The lower limit is not particularly limited, but is preferably 0.001 wt% or more from the viewpoint of not increasing the steelmaking cost. Further, by coexisting B with B, it becomes possible to measure high strength while maintaining high workability without embrittlement of the steel sheet with a relatively small amount of addition and at low cost.

Next, in the hot rolling step, prior to the rough rolling, the steel slab adjusted to the above components is heated to the Ac ₃ transformation point or higher. Specifically, the slab is heated to around 1200 ° C. to reduce the deformation resistance of the steel plate in the subsequent rough rolling. It should be noted that the slab cast by continuous casting or the like may be directly subjected to the rough rolling step before cooling without heating the slab again. After that, after rough rolling, descaling with high-pressure water is performed prior to finish rolling. As a result, the scale produced up to the rough rolling is removed.

The descaling with high-pressure water is used to almost completely remove the oxide scale produced and grown up to the rough rolling. High-pressure water is used because scales can be removed efficiently without flaws on the steel plate surface. And, the discharge pressure of high-pressure water needs to be 300 kgf / cm ² or more. Because below 300 kgf / cm ² ,
Incomplete removal of scale, resulting in finish rolling,
This is because the scale on the hot-rolled steel sheet after winding becomes thick and non-uniform.

Therefore, if the oxide scale thickness is reduced by descaling with high-pressure water, the hot-rolled steel sheet surface can be made beautiful, and therefore continuous descaling by pickling is not required. By reducing in a heating furnace of a hot-dip galvanizing apparatus, it becomes possible to manufacture beautiful plated steel sheets with good plating adhesion. On the other hand, from hot-rolled steel sheets with thick oxide scale and inferior surface properties produced without using descaling by high-pressure water, beautiful plated steel sheets with good plating adhesion can be produced without descaling by pickling. Is difficult.

In the next finish rolling step, rolling with a rolling reduction of 60% or more below the Ar ₃ transformation point is performed under lubrication, and the final finishing rolling temperature is set to 800 ° C. or less. That is, by setting the rolling reduction below the Ar ₃ transformation point to 60% or more under lubrication, it is possible to incorporate a working strain with less shear stress into the hot-rolled steel sheet after cooling, and to reduce it in the subsequent reduction furnace. At the time of recrystallization, a structure advantageous for imparting high workability appears. Therefore, the workability can be secured without performing cold rolling.

Further, the reason why the final finishing temperature is set to 800 ° C. or lower is to suppress the growth of the iron oxide layer immediately after hot rolling. Specifically, the iron oxide layer thickness in the hot rolled sheet is 4
It can be suppressed to about μm or less.

The hot-rolled steel sheet is taken up, then reduced at the same time as recrystallization annealing in an annealing furnace of a continuous hot dip coating apparatus, and then plated. That is, in the continuous hot-dip galvanizing apparatus, the iron oxide layer is reduced and recrystallization occurs in the steel sheet in the annealing furnace.

Here, in order to carry out both reactions rapidly, it is essential to adjust the steel plate temperature to 750 ° C. or higher, while Ac
s If it exceeds the transformation point, the structure becomes random and a structure advantageous for workability cannot be obtained.

Further, the reduction of the iron oxide layer needs to be performed at 50% or more, preferably less than 100%. The reason is that if the reduction rate is less than 50%, the iron oxide layer often remains and peels off when subjected to impact or processing, which is not practical. On the other hand, when the content becomes 100%, hydrogen atoms start to be occluded, and if the hydrogen atom heightens, hydrogen is discharged from the steel after plating and loses its place, vaporizes at the plating interface and progresses to local plating delamination. In particular, when Si and Mn are contained in high concentrations, 100% reduction causes surface concentration due to oxidation of Si and Mn, which impedes wettability in the subsequent plating process and induces non-plating defects. Will be done.

The atmosphere is 3 which is a general reducing gas.
Although N ₂ containing at least ₂ % H ₂ can be used, the H ₂ concentration is preferably 7% or more for efficient reduction.

The steel sheet that has undergone the prescribed reduction and recrystallization annealing is cooled to about the plating bath temperature and then introduced into the plating bath for plating. For example, as a zinc-based plating bath, in addition to Zn and Fe, in order to improve various performances,
It is possible to contain Al, Mg, Mn, Ni, Co, Cr, Si, Pb, Sb, Bi, Sn and the like alone or in combination.

Finally, the steel plate plated by immersion is
After adjusting to a required basis weight between ^{20 and} 250 g / m ² by gas wiping, etc., cool by air cooling, air cooling or water cooling, and then leveler or temper rolling if necessary to obtain the product. . Further, in order to improve the corrosion resistance and the like, it is possible to perform a chromate treatment, a phosphate treatment or the like after cooling or temper rolling, and it is also effective to further paint. Similarly, a lubrication treatment can be performed as a post-treatment.

On the other hand, when steel sheets are assembled and used by spot resistance welding or the like, it is possible to perform plating by a molten Zn bath containing 0.1 to 0.2 wt% of Al, adjust the basis weight, and then heat alloy. It is valid. The basis weight here is 20g
If it is less than 80 m / m ³ , corrosion resistance will be insufficient, and if it exceeds 80 g / m ³ , the plating will peel off easily during bending and drawing,
It is preferably in the range of 20 to 80 g / m ³ . Similarly, the Fe content in the plating is 7 to 12 wt%. Because 7wt
If it is less than 10%, the unalloyed pure Zn layer remains on the plating surface and hinders spot resistance weldability, and this pure Zn layer easily elutes from flaws after coating. Is abruptly embrittled, resulting in significant peeling of the plating during processing.

Although the hot-dip galvanized steel sheet has been mainly described above, the present invention is not limited to the hot-dip galvanized steel sheet, but is similarly applicable to other hot-dip galvanized steel sheets or electroplated steel sheets. For example, 55% Al-Zn plating, Al plating, Pb
Plated steel sheets such as plating, Sn plating or Zn-Ni plating are suitable. In any case, it suffices to plate after the reduction treatment of 50% or more and less than 100%, and a steel sheet having excellent plating characteristics can be obtained without being restricted by the plating type. In a hot-dip galvanizing line, a plating tank is usually arranged continuously in an annealing furnace, and thus it is particularly suitable for the present invention.

[0029]

Example A slab having a steel composition shown in Table 1 was heated to 1200 ° C. and hot-rolled according to Table 2 to obtain a 0.8 mm-thick hot rolled sheet. In finish rolling, lubrication with mineral oil was applied.
For comparison, according to the conventional method, cold-rolled sheet was also produced by performing pickling after hot rolling and then cold rolling under the conditions shown in Table 3. Next, this hot rolled sheet was cut into a test piece of 60 × 200 mm, washed with acetone, subjected to reduction treatment and recrystallization annealing with a vertical molten metal plating simulator, and then subjected to zinc-based plating. Table 2 shows the hot rolling and annealing conditions and the iron oxide layer thickness of the hot rolled sheet, and Table 4 shows the plating conditions. With respect to the plated steel sheet thus obtained, the reduction ratio of the iron oxide layer was measured, and the mechanical properties and plating adhesion were evaluated. The reduction rate and mechanical properties of the iron oxide layer are shown in Table 2, and the evaluation results of plating adhesion are shown in Table 4, respectively. Here, the reduction ratio of the iron oxide layer was obtained by separately obtaining the amount of iron oxide dissolved and removed by pickling in advance, calculating the reduced iron oxide amount from the weight reduced by reduction annealing, and calculating the ratio.

The plating adhesion was evaluated by a ball impact test and a 180 ° outward bending test. That is, in the ball impact test, a hammer having a hemispherical convex surface having a diameter of 1/2 inch was applied to the back side of the test surface, and a hemispherical concave saucer was placed on the test surface side, and a 2 kg weight was applied to a 70 cm. The cell was dropped from the height of the test piece, the hitting core was hit, the cellophane adhesive tape was attached to the protruding test surface, and then peeled off, and the surface of the plated steel sheet was observed. In the 180-degree outer bending test, a vinyl adhesive tape was applied to the test surface in advance, a 0.9 mm steel plate was put in the spacer, and the test surface was bent 180 degrees with the hydraulic press to the outside, and then flattened again. After bending back, the vinyl tape was peeled off, and the surface of the plated steel sheet was observed.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

The alloyed hot-dip Zn plating was also evaluated in the same manner. That is, a test piece similar to the above was prepared using a slab having the steel composition shown in Table 1. The hot rolling conditions and the annealing conditions before plating are shown in Table 5, the pickling and cold rolling conditions according to the conventional method are shown in Table 6, and the galvannealing conditions are shown in Table 7. . The plated steel sheet thus obtained was evaluated for mechanical properties and plating adhesion. The evaluation results are also shown in Tables 5 to 7, respectively.

The plating adhesion was evaluated by a 90-degree inner bending test and a 180-degree outer bending test. That is, a vinyl adhesive tape is applied to the test surface in advance, and in the 90-degree internal bending test,
After bending 90 degrees with the test surface inside along a die of 1 mm radius, and then bending it back to a flat state again, in the 180 degree outer bending test, put a 0.9 mm steel plate in the spacer and use a hydraulic press. After bending by 180 degrees with the test surface outside, the plate was bent back to a flat state again, the vinyl tape was peeled off, and the surface of the plated steel sheet was observed.

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

[0040]

According to the present invention, a plated steel sheet obtained by plating without removing the iron oxide layer can be provided with both excellent workability and plating adhesion.
A plated steel sheet can be provided at low cost.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location C22C 38/00 301 C22C 38/00 301T 38/14 38/14 C23C 2/40 C23C 2/40

Claims (1)

[Claims] 1. C: 0.02 wt% or less, Si: 2 wt% or less, M
n: 3 wt% or less, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2
wt%, B: 0.0001 to 0.003 wt%, N: 0.01 wt% or less,
P: 0.2 wt% or less and S: 0.05 wt% or less, and each content of C, Ti, Nb and N [C], [Ti], [Nb]
A steel slab having a chemical composition satisfying the following formula and [N] is heated to a temperature range of Ac ₃ transformation point or higher and subjected to hot rough rolling, and then degassed with high pressure water having a discharge pressure of 300 kg / cm ² or more. After scaling, the rolling reduction is 60% or more and the end temperature is 800 ° C in the temperature range of 500 ° C or higher and Ar ₃ transformation point or lower.
The following hot finish rolling is performed under lubrication, and after winding up, 50% or more of the iron oxide layer on the steel sheet surface is reduced at a steel sheet temperature of 750 ° C or higher and Acs transformation point or lower in an annealing furnace of continuous plating equipment. A method for producing a high workability plated steel sheet, which comprises performing plating. Note [C] / 12 + [N] / 14 ≦ [Ti] / 48 + [Nb] / 93