RU2418094C2 - High strength hot-galvanised steel sheet and high strength annealed after galvanising steel sheet with excellent mouldability and ability to application of electro-deposit; procedures and devices for fabrication of such sheets - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: procedure for fabrication of said steel sheet or steel sheet annealed upon galvanising consists in continuous hot-galvanising high strength steel sheet containing Si and Mn or C, Si and Mn. During galvanising there is ensured presence of oxides of Si of at least one type chosen from FeSiO₃, FeSiO₄, MnSiO₃ and Mn₂SiO₄ on surface of steel or surface of division of steel sheet and electro-deposit. Also, oxides of SiO₂ type should be present on internal side of steel sheet surface to form a zinc coated layer or a layer of electro-deposit out of alloy of zinc on high strength steel sheet; after zinc coating steel sheet is annealed. Steel sheet contains in wt %, C from 0.05 to 0.25 %, Si from 0.3 to 2.5 %, Mn from 1.5 to 2.8 %, P 0.03 % or less, S 0.2 % or less, Al from 0.005 to 0.5 %, N 0.0060 % or less, the rest is Fe and unavoidable impurities. At fabrication of the said steel sheet in a reduction furnace there is implemented equipment including a device for introduction of generation of gas containing CO₂ at amount from 1 to 100 vol. %, the rest is N₂, H₂O, O₂, CO₂ and unavoidable impurities. The equipment controls potential of oxygen on surface of steel sheet in furnace.

EFFECT: production of high strength hot-galvanised sheet and high strength annealed after galvanising steel sheet of required appearance with excellent adhesion of electro-deposit, mouldability and resistance to corrosion.

22 cl, 6 dwg, 18 tbl, 10 ex

Description

FIELD OF THE INVENTION

The present invention relates to high-strength hot dip galvanized steel sheet and high-strength steel sheet annealed after galvanizing, and methods for manufacturing such sheets, and more particularly, relates to a galvanized steel sheet having a good appearance without gaps in electroplating, and excellent adhesion and galvanic coating, shape to corrosion, and made with the possibility of use for various applications, such as steel sheet for construction m materials or cars.

State of the art

The steel sheet most often used as a galvanized steel sheet having good corrosion resistance is hot dip galvanized steel sheet. This hot dip galvanized steel sheet is usually made by degassing a steel sheet, then heating it in a non-oxidizing atmosphere furnace, re-annealing it in a reduction furnace to clean the surface and guarantee quality, immersing it in a hot dip galvanizing bath and controlling the amount of sludge. This gives such distinguishing features as excellent formability, corrosion resistance, adhesion of electroplating, etc., which are widely used for automobiles, applications related to building materials, etc.

In particular, in order to guarantee the function of protecting passengers at the time of a collision and to reduce weight in order to increase fuel economy in recent years, it is required to make galvanized steel sheet stronger in the automotive sector.

To increase the strength of the steel sheet without reducing its machinability, the addition of elements such as Si, Mn, and P is effective. Among them, in particular, Si is easily oxidized even in comparison with Fe, and therefore it is known that if electroplating is applied to the steel sheet containing Si under normal conditions of hot dip galvanizing, during the annealing process, Si in steel will concentrate on the surface and cause defects in the gaps of the electroplating, and the adhesion of the electroplating will drop. In addition, the addition of these elements delays fusion, as a result of which — compared with mild steel — the temperature rises and the required fusion time increases. At this elevated temperature, longer fusion causes the transformation of residual austenite in the steel sheet into perlite and reduces workability, as a result of which the effects of the added elements are nullified.

As a technology for suppressing defects in electroplating gaps in a steel sheet containing Si, Japanese Patent Publication (A) No. 55-122865 describes a method for oxidizing steel, which results in a surface with an oxide film of a thickness of 400 to 10,000 Å, followed by annealing and deposition plating on it in a hydrogen-containing atmosphere. However, in this known technical solution, practical implementation of the regulation of the recovery time of the film of iron oxides is difficult. If the recovery time is too long, this causes a concentration of Si on the surface, and if it is too short, an iron oxide film remains on the surface of the steel, resulting in the problem that the measures taken do not completely exclude defects in electroplating, and the problem, consisting in the fact that if the film of iron oxides on the surface becomes too thick, then the peeling oxides will adhere to the rolls and cause visual defects.

To cope with these problems, the inventors proposed in Japanese Patent Publication (A) No. 2001-232355 and Japanese Patent Publication No. 2003-105516 a manufacturing method that prevents the concentration of Si on the surface by oxidizing the surface of the steel sheet and then restoring it in a controlled atmosphere reduction furnace .

In addition, the inventors proposed in Japanese Patent Publication (A) No. 2001-295018 a Si-containing high strength, hot dip galvanized steel sheet having excellent corrosion resistance and representing a steel sheet having a content of Si formed on its surface from 0.2 to 2, 0 wt.%, With a layer of Zn-Al-Mg-plating, deposited by hot dip galvanizing and containing from 2 to 19 wt.% Al, from 1 to 10 wt.% Mg, and the rest is Zn and inevitable impurities, as well proposed in Japanese Patent Publication (A) No. 2004-323970 high hot dip galvanized steel sheet having excellent ability to apply electroplating, representing a steel sheet having a Si content of from 2.0 to 3.0 wt.% and containing, from the inside of its surface, oxide particles of one or more of Si oxides, Mn oxides or complex oxides of Si and Mn.

In addition, Japanese Patent Publication (A) No. 56-33463 and Japanese Patent Publication (A) No. 57-79160 describe methods for suppressing defects in electroplating gaps by pre-coating the electroplating containing Cr, Ni, Fe, etc., onto a surface steel sheet. In addition, Japanese Patent Publication (A) No. 2002-161315 describes a method for forming an inner oxide layer directly below a surface of a steel sheet on a continuous annealing line, removing simultaneously formed surface oxides by etching, and then plating the steel on a continuous galvanizing line.

Disclosure of invention

At the same time, the manufacturing technology, both described above and set forth in other primary sources, is not able to completely prevent defects in the omissions of electroplated coatings and poor adhesion. In the technical solution according to Japanese Patent Publication (A) No. 55-122865, the practical implementation of regulating the recovery time of a film of iron oxides is difficult. If the recovery time is too long, this causes a concentration of Si on the surface, and if it is too short, an iron oxide film remains on the surface of the steel, so that this cannot completely exclude defects in the gaps of electroplating.

Therefore, Japanese Patent Publication (A) No. 2001-323355 and Japanese Patent Publication (A) No. 2003-105516 provide for controlling the reducing atmosphere and creating such an internal oxidation state to produce SiO ₂ , which helps to suppress the defects of electroplating gaps due to concentration Si on the surface. This method can significantly reduce the defects in the gaps of electroplating, arising due to the concentration of Si on the surface, but even it cannot completely prevent the defects in the gaps of electroplating and poor adhesion.

The reason is that even if the methods described in the aforementioned patents can prevent the concentration of Si on the surface due to external oxidation, it is still not possible to completely prevent the disclosure of SiO ₂ on the surface of the steel sheet. Therefore, to prevent defects in gaps in electroplating or poor adhesion, tight control of SiO ₂ becomes necessary.

In addition, Japanese Patent Publication (A) No. 2004-323970 proposes controlling a reducing atmosphere for introducing oxide particles of at least one type selected from Si oxides, Mn oxides and complex Si and Mn oxides into the surface of the steel sheet so that in order to improve the ability to apply electroplating, but although this method can significantly reduce the defects of galvanizing coatings arising due to the concentration of Si on the surface, it cannot completely prevent the defects of galvanizing coatings and unsatisfying full adhesion.

Improving the adhesion of hot dip galvanized steel sheet cannot be expected due to its fusion like an annealed steel sheet after galvanizing, which makes it difficult to increase the adhesion of the electroplated steel sheet containing Si, in which the adhesion at the interface between the electroplated coating and the steel sheet can easily decrease sharply. For this reason, even though the technical solutions corresponding to Japanese Patent Publication (A) No. 2001-323355, Japanese Patent Publication (A) No. 2003-105516 and Japanese Patent Publication (A) No. 2004-323970, electroplating adhesion guaranteed to the extent that the plating does not peel off during the bending test, it is not possible to sufficiently guarantee the adhesion of the plating based on a tough assessment such as a DuPont impact test.

In addition, in the case of such a method of pre-plating, as in Japanese Patent Publication (A) No. 56-33463 and Japanese Patent Publication (A) No. 57-79160, equipment for applying electroplating becomes necessary, so that this method cannot be applied there, where there is no place for this equipment. In addition, there is also the problem that the installation of equipment for pre-plating causes an increase in production costs. In addition, double annealing, as in the technical solution according to Japanese Patent Publication (A) No. 2002-161315, also causes a problem of rising production costs.

In addition, the manufacturing technology of the steel sheet containing Si, described so far, focused on guaranteeing the ability to apply electroplating and did not consider the improvement of formability and various other aspects of the operational characteristics when used as a steel sheet with electroplating.

Therefore, the present invention provides a solution to the aforementioned problems, and it proposes a high-strength hot dip galvanized steel sheet and a high-strength sheet annealed after galvanizing with excellent electroplating adhesion, formability and corrosion resistance, as well as methods for manufacturing such sheets.

The inventors conducted an advanced search in the field of electroplating on high-strength steel sheet and as a result found that by electroplating a steel sheet to which Si and Mn were added in certain or more significant quantities using continuous hot dip galvanizing equipment optimized according to the conditions heat treatment and electroplating conditions, it is possible to control the types and positions of Si oxides and obtain high-strength hot dip galvanized steel sheet and high-strength annealed after galvanizing steel sheet of proper appearance and with excellent adhesion of electroplating, formability and corrosion resistance, which leads to the embodiment of the present invention. That is, the true meaning of the present invention is as follows.

(1) Hot dip galvanized steel sheet with excellent formability and electroplating ability, consisting of a high strength steel sheet containing, in wt.%:

C: 0.05 to 0.25,

Si: 0.3 to 2.5,

Mn: from 1.5 to 2.8,

P: 0.03 or less

S: 0.02 or less

Al: 0.005 to 0.5,

N: 0.0060 or less

and the rest is Fe and inevitable impurities,

on which there is a galvanized layer containing Al in an amount of from 0.05 to 10 wt.% and Fe in an amount of from 0.05 to 3 wt.%, and the rest is Zn and inevitable impurities, wherein the hot dip galvanized steel sheet is characterized by the presence of oxides containing Si, with an average content of 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and the presence of Fe- Zn alloy with an average grain size of from 0.5 to 3 microns on the side plating.

(2) Hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, in wt.%:

C: 0.05 to 0.25,

Si: 0.3 to 2.5,

Mn: 1.5 to 2.8,

P: 0.03 or less

S: 0.02 or less

Al: 0.005 to 0.5,

N: 0.0060 or less

and the rest is Fe and inevitable impurities,

on which there is a galvanized layer containing Al in an amount of from 0.05 to 10 wt.% and Fe in an amount of from 0.05 to 3 wt.%, and the rest is Zn and inevitable impurities, wherein the hot dip galvanized steel sheet is characterized by the presence of oxides containing Si, with an average content of 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and the presence of Fe- Zn alloy with an average grain size of from 0.5 to 3 microns on the side of the plating grains in a ratio of 1 to 500 microns or more.

(3) Hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, in wt.%:

C: 0.05 to 0.25,

Si: 0.3 to 2.5,

Mn: 1.5 to 2.8,

P: 0.03 or less

S: 0.02 or less

Al: 0.005 to 0.5,

N: 0.0060 or less

and the rest is Fe and inevitable impurities,

on which there is a galvanized layer containing Al in an amount of from 0.05 to 10 wt.% and Mg in an amount of from 0.01 to 5 wt.%, and the rest is Zn and unavoidable impurities, wherein the hot dip galvanized steel sheet is characterized by the presence of oxides containing Si, with an average content of 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer.

(4) Hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, in wt.%:

C: 0.05 to 0.25,

Si: 0.3 to 2.5,

Mn: 1.5 to 2.8,

P: 0.03 or less

S: 0.02 or less

Al: 0.005 to 0.5,

N: 0.0060 or less

and the rest is Fe and inevitable impurities,

on which there is a galvanized layer containing from 4 to 20 wt.% Al, from 2 to 5 wt.% Mg and from 0 to 0.5 wt.% Si, and the rest is Zn and inevitable impurities, the above-mentioned hot dip galvanized steel sheet characterized by the presence of oxides containing Si, with their average content from 0.6 to 10 wt.%, at the grain boundaries of crystals and in crystal grains on the side of a steel sheet of 5 μm or less from the interface between the high-strength steel sheet and the plating layer.

(5) Annealed after galvanizing a steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, in wt.%:

C: 0.05 to 0.25,

Si: 0.3 to 2.5,

Mn: 1.5 to 2.8,

P: 0.03 or less

S: 0.02 or less

Al: 0.005 to 0.5,

N: 0.0060 or less

and the rest is Fe and inevitable impurities,

on which there is a zinc alloy plating layer containing Fe, and the rest is Zn and unavoidable impurities, said steel sheet being characterized by the presence of oxides containing Si, with an average content of 0.6 to 10 wt.%, at the grain boundaries of the crystals and in crystal grains on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and the presence of oxides containing Si, with an average content of from 0.5 to 1.5 wt.% in the plating layer.

(6) High-strength hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating according to any one of paragraphs. (1) to (5), characterized in that the said oxides containing Si are oxides of one or more types selected from SiO ₂ FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ .

(7) High strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating according to (5), characterized by the presence of Si oxides of at least one type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , in the plating layer on the surface of the steel sheet and the presence of SiO ₂ on the inner side of the surface of the steel sheet.

(8) High strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating according to (5), characterized by the presence of Si oxides of at least one type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , in the plating layer and the presence of SiO ₂ on the side of the steel sheet of the plating layer and in the steel sheet.

(9) High-strength hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating according to any one of paragraphs. (1) to (4), characterized by the presence of Si oxides of at least one type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ on the surface or surface side of the steel sheet and the presence of SiO ₂ on the inner side of the surface of the steel sheet.

(10) High strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating according to any one of paragraphs. (1) - (9), characterized in that the relationship between the limit F (MPa) tensile strength and elongation L (% ) satisfies the inequality

L≥51-0.035 × F.

(11) A method of manufacturing a high strength hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is performed by immersion of a high strength steel sheet containing Si and Mn, during which the presence of Si oxides of at least one of the type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ on the surface of the steel or the interface of the steel sheet and plating and provide the presence of oxides of the type SiO ₂ on the inner side of the surface of the steel sheet to form a galvanized layer or a zinc alloy plating layer on a high strength steel sheet.

(12) A method of manufacturing a high strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is carried out by immersion of a high strength steel sheet containing Si and Mn, during which the presence of Si oxides at least one type selected from FeSiO _3, FeSiO _4, MnSiO ₃ and Mn ₂ SiO _4, on the steel surface or surfaces of the steel sheet and the plating section and provide a presence type oxides SiO ₂ VNU renney surface side of the steel sheet to form the galvanized layer on the high-strength steel sheet, followed by annealing of the sheet after the galvanizing.

(13) A method of manufacturing a high-strength hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is carried out by immersion of a high-strength steel sheet containing C, Si and Mn, during which the presence of Si oxides at least one type selected from FeSiO _3, FeSiO _4, MnSiO ₃ and Mn ₂ SiO _4, on the steel surface or surfaces of the steel sheet and the plating section and provide a presence type oxides SiO ₂ and the inner side of the steel sheet surface to form a galvanized layer or to form a plating layer of zinc oxide on the high-strength steel sheet.

(14) A method of manufacturing a high-strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is performed by immersion of a high-strength steel sheet containing C, Si and Mn, during which the presence of Si oxides at least least one type selected from FeSiO _3, FeSiO _4, MnSiO ₃ and Mn ₂ SiO _4, on the steel surface or surfaces of the steel sheet and the plating section and provide a presence type oxides SiO ₂ in morning surface side of the steel sheet to form the galvanized layer, and then annealing is carried out after galvanizing the sheet.

(15) A method of manufacturing a high-strength hot dip galvanized steel sheet and a high-strength steel sheet annealed after galvanizing with excellent formability and the ability to apply electroplating, according to any one of (11) to (14), characterized in that the high-strength hot dip-galvanized steel sheet and high-strength the steel sheet annealed after galvanizing contains, in wt.%, C: from 0.05 to 0.25, Si: from 0.3 to 2.5, Mn: from 1.5 to 2.8, P: 0.03 or less, S: 0.02 or less, Al: from 0.005 to 0.5, N: 0.0060 or less, and the rest is Fe and unavoidable impurities.

(16) A method of manufacturing a high strength hot dip galvanized steel sheet and a high strength steel sheet annealed after galvanizing with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is performed by immersion of a high strength steel sheet containing C, Si and Mn, during which it is restored this sheet is in the recovery zone with an atmosphere containing H ₂ in an amount of from 1 to 60 vol.%, and the rest is one or more components such as N ₂ , H ₂ O, O ₂ , CO ₂ and inevitable impurities, and controlled by reaching the following parameter logPO _{2 the} partial pressure of oxygen in the said atmosphere:

Where

T is the maximum peak temperature (K) of the steel sheet,

[Si%] is the Si content (wt.%) In the steel sheet.

(17) A method for manufacturing a high-strength hot dip galvanized steel sheet and a high-strength hot dip galvanized steel sheet after hot dip galvanizing with excellent formability and the ability to apply electroplating according to claim 16, characterized in that the sheet is oxidized in the oxidation zone in front of the recovery zone in the atmosphere with a component from 0 , 9 to 1.2 by the ratio of air and fuel in the combustible mixture, and then restore it in the recovery zone.

(18) A method for manufacturing a high-strength hot dip galvanized steel sheet and a high-strength steel sheet annealed after galvanizing with excellent formability and the ability to apply electroplating according to (16), characterized in that the sheet is oxidized in the oxidation zone in front of the recovery zone in the atmosphere with dew point 273 To or more, and then restore it in the recovery zone.

(19) A method of manufacturing a high-strength hot dip galvanized steel sheet with excellent formability and the ability to apply electroplating, which consists in finishing the rolling of a steel slab containing C, Si and Mn, at a point temperature of Ar ₃ or higher, cold rolling of the sheet with compression by 50-85%, and then its hot dip galvanizing, during which use equipment for continuous hot dip galvanizing, having a recovery zone with the atmosphere, containing th H ₂ in an amount of from 1 to 60 vol.%, and the rest is one or more components such as N ₂ , H ₂ O, O ₂ , CO ₂ and inevitable impurities, and controlled with the achievement of the following parameter logPO ₂ partial pressure of oxygen in the atmosphere mentioned:

Where

T is the maximum peak temperature (K) of the steel sheet,

[Si%] is the Si content (wt.%) In the steel sheet,

the sheet is annealed in the temperature range of the joint existence of two phases of ferrite and austenite from 1023 K to 1153 K, this sheet is cooled from the maximum peak temperature to 923 K with an average cooling rate of 0.5 to 10 degrees per second, then it is cooled from 923 K up to 773 K with an average cooling rate of 3 degrees per second or more and further from 773 K with an average cooling rate of 0.5 degrees per second or more for hot dip galvanizing to form a hot dip galvanized layer on the surface said cold-rolled steel sheet, wherein said manufacturing method is characterized in that the cooling time is controlled from 773 K to 623 K after plating, reaching a range of 25 seconds to 240 seconds.

(20) A method of manufacturing a high-strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating, which consists in finishing the rolling of a steel slab containing C, Si and Mn, at a point temperature of Ar ₃ or higher, cold rolling of the sheet with compression by 50-85%, and then its hot dip galvanizing, during which use equipment for continuous hot dip galvanizing, having a recovery zone with an atmosphere containing H ₂ in an amount of from 1 to 60 vol.%, and the rest is one or more components such as N ₂ , N ₂ O, O ₂ , CO ₂ and inevitable impurities, and controlled with the achievement of the following parameter logPO _{2 the} partial pressure of oxygen in the said atmosphere :

Where

T is the maximum peak temperature (K) of the steel sheet,

[Si%] is the Si content (wt.%) In the steel sheet,

the sheet is annealed in the temperature range of the joint existence of two phases of ferrite and austenite from 1023 K to 1153 K, this sheet is cooled from the maximum peak temperature to 923 K with an average cooling rate of 0.5 to 10 degrees per second, then it is cooled from 923 K up to 773 K with an average cooling rate of 3 degrees per second or more and further from 773 K with an average cooling rate of 0.5 degrees per second or more to a range from 693 K to 733 K and can withstand this sheet when cooled from 773 K to a galvanic temperature baths over time from 25 seconds to 240 seconds, then hot dip galvanizing is carried out to thereby form a hot dip dip layer on the surface of said cold rolled steel sheet, then the mentioned steel sheet is fused onto which said hot dip dip layer is formed to form a zinc alloy plating layer on the surface of said steel sheet, wherein said method of manufacturing an annealed after galvanizing steel sheet is characterized in that its galvanization by immersion is carried out in a hot dip galvanizing bath, the composition of which provides for the presence of Al in the bath in an effective concentration of 0.07 to 0.105 wt.%, and the rest is Zn and inevitable impurities, and carry out the annealing of the subsequent galvanizing at a temperature T (K) satisfying the inequality

720≤T≤690 × exp (1.35 × [Al%]),

where [Al%] is the effective concentration of Al (wt.%) in the bath, characteristic of the galvanizing bath.

(21) A method for manufacturing a high strength hot dip galvanized steel sheet and a high strength hot dip galvanized steel sheet with excellent formability and plating ability according to any one of (15), (19) and (20), characterized in that the high strength hot dip galvanized steel sheet and high-strength annealed after galvanizing steel sheet contain, in wt.%, C: from 0.05 to 0.25, Si: from 0.3 to 2.5, Mn: from 1.5 to 2.8, P : 0.03 or less, S: 0.02 or less, Al: from 0.005 to 0.5, N: 0.0060 or less, and the rest is Fe and unavoidable Yes.

(22) A method of manufacturing a high-strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating according to (20), characterized in that the sheet is annealed, cooled to a range of 673 K to 723 K, and then reheated to range from 703 K to 743 K for annealing after galvanizing.

(23) A method of manufacturing a high-strength annealed after galvanizing steel sheet with excellent formability and the ability to apply electroplating according to (20) or (22), characterized in that they control the time from electroplating to cooling to 673 K or lower temperature to achieve a range from 30 seconds to 120 seconds.

(24) Equipment for the manufacture of high-strength hot dip galvanized steel sheet, having a non-oxidizing atmosphere furnace or direct-acting flame furnace and capable of continuous hot dip galvanizing of a steel sheet, characterized in that a device for introducing a gas containing CO ₂ into a reduction furnace is installed amount from 1 to 100 vol.%, and the rest is N ₂ , H ₂ O, O ₂ , CO ₂ and inevitable impurities.

(25) Equipment for the manufacture of high-strength hot dip galvanized steel sheet, having a non-oxidizing atmosphere furnace or direct-acting flame furnace and capable of continuous hot dip galvanizing of a steel sheet, characterized in that a reduction device for generating a gas containing CO ₂ in the amount of from 1 to 100 vol.%, and the rest is N ₂ , H ₂ About, O ₂ , CO ₂ and inevitable impurities.

Brief Description of the Drawings

Figure 1 presents a view illustrating the results of the introduction, polishing and etching of high-strength hot dip galvanized steel sheet with good ability to apply electroplating, as well as observing the cross section through the image obtained using a scanning electron microscope (SEM).

Figure 2 shows the results of the implementation and polishing of the cross section of a high-strength hot dip galvanized steel sheet with good ability to apply electroplating when tilted by 10 degrees, as well as observing the cross section by means of an SEM image.

Figure 3 shows the results of embedding and polishing the cross section of a high-strength hot dip galvanized steel (having a Zn-Al-Mg-Si composition electroplated) of a steel sheet with good ability to apply electroplating when inclined by 10 degrees, as well as observing the cross section through an image obtained from using EMS.

Figure 4 shows the results of the introduction and polishing of the cross section of a high-strength annealed after galvanizing steel sheet with good ability to apply electroplating when tilted by 10 degrees, as well as observing the cross section through the image obtained using SEM.

Fig. 5 is a side view showing an example of processing equipment for manufacturing a high-strength hot dip galvanized steel sheet in accordance with the present invention.

Fig. 6 is a side view showing an example of processing equipment for manufacturing high strength hot dip galvanized steel sheet in accordance with the present invention.

The implementation of the invention

Below will be a detailed explanation of the present invention. First, the reasons for the numerical limitations of the contents of C, Si, Mn, P, S, Al, and N will be explained. C is an essential element in attempts to increase the strength of a steel sheet by strengthening the structure due to martensite or residual austenite. The reason that the C content is set to 0.05% or more is because if the C content is less than 0.05%, cementite or perlite is easily formed on the hot dip galvanizing line, and it is difficult to quickly cool the sheet from temperature annealing using fog or spray water as a coolant and it is difficult to guarantee the required tensile strength. On the other hand, the reason that the C content is set to 0.25% or less is because if the C content exceeds 0.25%, it is difficult to form weld zones without defects by spot welding and at the same time, C and workability is getting worse.

Si is added in an amount of from 0.3 to 2.5% as an element that increases the machinability of the steel sheet, in particular strength, without significantly affecting the elongation. The reason that the Si content is set to 0.3% or more is because if the Si content is less than 0.3%, it is difficult to guarantee the required tensile strength. The reason that the Si content is set to 2.5% or less is because if the Si content exceeds 2.5%, then the effect of increasing the strength becomes saturated and a decrease in ductility occurs. In a preferred embodiment, making this wt.% Four times or more higher than the C content, the perlite and bainite conversion is noticeably delayed due to reheating for fusion, and this makes it possible to obtain a metal structure in which - even after cooling to room temperature - from 3 to 20% by volume are martensite and residual austenite mixed with ferrite.

Mn, along with C, reduces the free energy of austenite, so it is added in an amount of 1.5% or more in order to stabilize austenite until the steel strip is immersed in a galvanic bath. Further, adding this element in such a way that its content in wt.% Is 12 or more times higher than the C content, the conversion of perlite and bainite is significantly delayed, and even after cooling to room temperature a metal structure is formed in which from 3 to 20% by volume are martensite and residual austenite mixed with ferrite. At the same time, if the added quantity becomes excessive, the slab easily crackes, and the quality of spot welding deteriorates; as a result, the upper limit is 2.8%.

P is usually present in steel as an unavoidable impurity. If its quantity exceeds 0.3%, the quality of spot welding is sharply worsened. In addition, when the above-mentioned amount is exceeded, in the case of a high-strength steel sheet having a tensile strength exceeding 490 MPa, as in the present invention, the toughness and cold rolling ability are also sharply reduced, therefore, the content is set to 0.03% or less . S is also incorporated into steel as an unavoidable impurity. If the amount of Mn exceeds 0.02%, the presence of MnS flattened in the rolling direction becomes noticeable, which has a detrimental effect on the bending ability of the steel sheet, therefore, the Mn content is set to 0.02% or less.

Al is a deoxidizing steel element, and it acts to intensify the grinding of grains of hot-rolled material by AlN and suppresses coarsening of crystal grains in the sequence of heat treatment processes, thereby improving the quality, so that it is necessary to add Al in an amount of 0.005% or more. At the same time, if its amount exceeds 0.5%, then not only the costs are large, but the surface properties also worsen, therefore its amount is set equal to 0.5% or less. N is also commonly present in steel as an unavoidable impurity. If its amount exceeds 0.006%, the elongation and brittleness parameters are deteriorated, therefore, its content is set equal to 0.006% or less.

In addition, even if the steel having these ingredients as the main one additionally contains Nb, Ti, B, Mo, Cu, Ni, Sn, Zn, Zr, W, Co, Ca, rare earth elements (including Y), V, Ta , Hf, Pb, Mg, As, Sb and Bi, the total amount of which is 1% or less, the effects of the present invention are not reduced. Corrosion resistance and workability will increase, depending on the amount mentioned, or other preferred cases will arise.

Next, an electroplating layer will be explained.

The reason that the Al content in the hot dip galvanized layer is limited to a range of 0.5 to 10 wt.% Is because if the Al content exceeds 10 wt.%, The fusion reaction of Fe and Al will take too long and will be observed drop in adhesion of electroplating. In addition, the reason that the Al content is limited to a lower limit of 0.5 wt.% Or more is that if conventional hot dip galvanizing is carried out with an amount of Al less than 0.5 wt.%, The Zn fusion reaction and Fe will leak for too long during plating, and a brittle alloy layer will form on the interface of the base metal, iron, and the adhesion of the plating will deteriorate.

The reason that the Fe content is limited to a range of 0.01 to 3 wt.% Is because if the Fe content is less than 0.01 wt.%, Then the effect of increasing the adhesion of the plating will be unsatisfactory, and if it exceeds 3 wt. %, then on the interface of the base metal — iron — a layer of brittle alloy will form and the adhesion of the electroplating will deteriorate.

Even if the plating layer additionally contains Sb, Pb, Bi, Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, P, B, Sn, Zr, Hf, Sr, V, Se, or REM alone or in combination in an amount of 0.5 wt.% or less, the effects of the present invention are not reduced. Depending on the amount mentioned, an additional improvement in appearance will occur, or other preferred results will occur.

There are no specific restrictions on the amount of sludge as a result of hot dip galvanizing, but from the point of view of corrosion resistance it should be 10 g / m ² or more, and from the point of view of workability it should be 350 g / m ² or less.

In addition, the high-strength hot dip galvanized steel sheet according to the present invention can increase the adhesion of the plating due to the presence of oxides containing Si, with an average content of 0.6 to 10 wt.%, At the grain boundaries of the crystals and in the crystal grains on the side of the steel sheet in 5 μm or less from the interface between the high-strength steel sheet and the plating layer and in the grains of crystals, and the presence of a Fe-Zn alloy with an average grain size of from 0.5 to 3 μm on the side of the plating. It is believed that the reason that the adhesion of the plating is increased if oxides containing Si are present at the grain boundaries of the crystals and inside the grain of the crystals is because SiO ₂ no longer opens on the surface of the steel sheet, causing a drop in the adhesion of the plating due to the formation of oxides containing Si in the steel sheet during the annealing process.

In addition, it is believed that the reason that the adhesion of the plating is enhanced by the formation of a Fe-Zn alloy with an average grain size of 0.5 to 3 μm on the side of the plating from the interface between the high-strength steel sheet and the plating layer is because adhesion is enhanced by the reaction between the steel sheet and the plating bath.

Generally speaking, with respect to a steel sheet having a Si content of less than 0.3%, it is known that during the reaction of this steel sheet and a plating bath, Fe-Al-Zn-based intermetallic compounds are formed that increase adhesion. The inventors conducted various experiments, as a result of which it was found that in a steel sheet with a Si content of 0.3% or more, the formation of intermetallic compounds based on Fe-Al-Zn increases the adhesion of the plating. Therefore, it is believed that when SiO ₂ opens on the surface of the steel sheet, this slows down the reaction between the steel sheet and the plating bath, as a result of which the Fe-Zn alloy is not formed, and the adhesion of the plating decreases.

Figure 1 shows the results of the introduction, polishing and etching of high-strength hot dip galvanized steel sheet with good ability to apply electroplating, as well as observing the cross section through the image obtained using SEM. As should be clear from this drawing, Fe-Zn-based intermetallic compounds present in the plating layer can be clearly distinguished by observation under a microscope. If we analyze the content of Fe (%) in the intermetallic compounds, then it is approximately 7%, therefore, it is believed that the intermetallic compounds based on Fe-Zn represent the ζ phase.

This ζ phase has a monoclinic crystalline structure, which, when viewed from the point of view of the cross section, forms the shape of a rectangle or parallelogram, as shown in figure 1. Therefore, the average grain size of these Fe-Zn-based intermetallic compounds is the average size measured along the long axis and short axis of the shapes of a rectangle or parallelogram.

The reason that the average grain size of Fe-Zn-based intermetallic compounds is limited to a range of 0.5 to 3 μm is that if this size is less than 0.5 μm, the effect of increasing the adhesion of the plating is unsatisfactory. If it is more than 3 μm, then the fusion reaction of Zn and Fe takes too long, a layer of brittle alloy forms and the adhesion of the electroplating worsens.

The inventors investigated Fe-Zn-based intermetallic compounds in many electroplated coatings, which confirmed that in high-strength hot dip galvanized steel sheet with good plating adhesion, Fe-Zn-based intermetallic compounds were present in an amount of 1 grain per 500 μm or more in any transverse section.

In addition, oxides containing Si present at said crystal grain boundaries and in crystal grains can be clearly distinguished by observation under a microscope. As an example of oxides containing Si, at the grain boundaries of the crystals and in the crystal grains from the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, Fig. 2 shows the results of observation of cross sections. Figure 2 shows the results of the introduction and polishing of the cross section of a high-strength hot dip galvanized steel sheet with good ability to apply electroplating when tilted by 10 degrees, as well as observing this cross section through an SEM image. From this drawing, it will be understood that oxides containing Si present at crystal grain boundaries and in crystal grains of high-strength steel sheet can be clearly distinguished by observation under a microscope.

In addition, if we analyze the oxides at the crystal grain boundaries and in the crystal grains by means of an energy dispersive X-ray spectroscope (EDS), then peaks of Si, Mn, Fe, and O are observed, and therefore it is assumed that the observed oxides are SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ .

Further, the reason that the Al content in the Zn-Al-Mg-based plating layer is limited to the range of 0.05 to 10 wt.% Is because if the Al content exceeds 10 wt.%, A drop in the adhesion of the plating is noticeable. therefore, the Al content in the plating layer in which Si is not added should be reduced to 10 wt.% or less. In addition, the reason that the Al content is limited to 0.05 wt.% Or more is that if plating is carried out by conventional hot dip galvanizing with an Al amount of less than 0.05 wt.%, Then When applying electroplating, a Zn-Fe fusion reaction will occur, on the interface of the base metal - iron - a brittle alloy layer will form and the adhesion of the electroplating will deteriorate.

Therefore, in the plated steel material obtained by hot dip galvanizing, in the present invention, in particular in the case where the Al concentration is a high concentration, for example, in excess of 10 wt.%, Si must be added to the plating layer to ensure the adhesion of the plating.

On the other hand, the reason for limiting the Al content in the Zn-Al-Mg-Si-based plating layer to a range of 4 to 22 wt.% Is that it is necessary to add Al in an amount of 4 wt.% Or more in order to noticeably increase the corrosion resistance of the electroplating layer, and if this content is more than 22 wt.%, then the effect of increasing corrosion resistance becomes saturated.

The reason that the Si content is set to 0.5 wt.% Or less (but greater than 0 wt.%) Is because Si has the effect of increasing adhesion, but if this content exceeds 0.5 wt.%, Then the effect increased adhesion becomes saturated. This content is preferably from 0.00001 to 0.5 wt.%, And more preferably from 0.0001 to 0.5 wt.%.

The addition of Si is significant in the electroplating layer having an Al content of more than 10 wt.%, But even in the electroplating layer with an Al content of 10 wt.% Or less, the effect on increasing the adhesion of the electroplating is large, so when used as a material for a difficult to process element and t etc., as well as in other cases where high adhesion of electroplating is required, the addition of Si is effective. In addition, due to the addition of Si in the cured structure of the electroplating layer, the Mg ₂ Si phase is released. This Mg ₂ Si phase has the effect of increasing corrosion resistance, so it would be preferable to increase the amount of Si additive and prepare a metal structure in which the cured structure of the plating layer includes the Mg ₂ Si phase.

The reason that the Mg content is limited to a range of from 0.01 to 5 wt.% Is that if it is less than 0.01 wt.%, Then the effect of increasing corrosion resistance becomes unsatisfactory, and if it exceeds 5 wt. .%, then the electroplating layer becomes brittle, and the adhesion drops. The corrosion resistance increases the more, the greater the amount of Mg additive, so that to significantly increase the corrosion resistance of the plating layer, the Mg content is preferably made in the range from 2 to 5 wt.%. In addition, the aforementioned Mg ₂ Si phase is released more readily, the greater the amount of Mg additive, it would be more preferable to increase the amount of additive and prepare the metal structure with the Mg ₂ Si phase mixed in the cured layer of the electroplating layer.

Even if the plating layer additionally contains Fe, Sb, Pb, Bi, Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, P, B, Sn, Zr, Hf, Sr, V, Sc, or REM separately or together in an amount in the range of 0.5 wt.%, the effects of the present invention are not reduced. Depending on the amount mentioned, an additional improvement in appearance will occur, etc., or other preferred results may occur. There are no specific restrictions on the amount of sludge due to hot dip galvanizing, but from the point of view of resistance to corrosion, it should preferably be 10 g / m ² or more, and from the point of view of workability it should preferably be 350 g / m ² or less.

In addition, it is possible to eliminate defects in the gaps of electroplating from high-strength hot dip galvanized steel sheet due to the presence of oxides containing Si, with their average content from 0.6 to 10 wt.%, At the grain boundaries of crystals and in crystal grains on the side of the steel sheet of 5 μm or less from the interface between the high-strength steel sheet and the plating layer. The reason why defects in galvanic coating gaps can be eliminated if oxides containing Si are present at the grain boundaries of crystals and in the grains of crystals of high-strength steel sheet is because SiO ₂ grains no longer open on the surface of the steel due to the formation of oxides containing Si in a steel sheet during annealing.

Oxides containing Si present at said grain boundaries of crystals and in crystal grains can be clearly distinguished by observation under a microscope. As an example of oxides containing Si, at the grain boundaries of the crystals and in the crystal grains from the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, Fig. 3 shows the results of observation of cross sections. Figure 3 shows the results of the introduction and polishing of the cross section of a high strength hot dip galvanized steel sheet without gaps in electroplating when tilted by 10 degrees, as well as observing this cross section through an SEM image. From this drawing, it will be understood that oxides containing Si present at crystal grain boundaries and in crystal grains of high-strength steel sheet can be clearly distinguished by observation under a microscope.

In addition, if we analyze the oxides at the grain boundaries of crystals and in crystal grains by means of EDS, then peaks of Si, Mn, Fe, and O are observed, and therefore, it is assumed that the observed oxides are SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ .

In addition, the term "zinc alloy plating layer" in the present invention means a plating layer consisting mainly of a Fe-Zn alloy formed by diffusion of Fe present in the steel into a plating consisting of Zn by a fusion reaction. No specific restrictions are imposed on the Fe content, but at a content of Fe of less than 7% in the plating, the soft η phase remains on the plating surface and the formability is reduced by stamping, and if the Fe content exceeds 15 wt.%, Then on the interface of the base metal - iron - excessive formation of a brittle alloy layer will occur and the adhesion of the plating will deteriorate, so that a range of 7 to 15 wt.% is suitable.

In addition, in the general case, when continuous hot dip galvanizing is performed by immersion in a plating bath, Al is added to control the fusion reaction in the plating bath, so that the plating contains from 0.05 to 0.5 wt.% Al. In addition, during the alloying process, simultaneously with the diffusion of Fe, diffusion of elements added to the steel also occurs, so that the electroplating also contains these elements.

Even if the steel sheet according to the present invention contains or has one or more elements of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and rare earth elements mixed therein in a hot dip galvanizing bath or during galvanizing, the effects of the present invention are not reduced. Depending on the amount mentioned, corrosion resistance and workability will even increase, or other preferred cases may arise. There are no specific restrictions on the amount of sludge due to annealing after galvanizing, but from the point of view of resistance to corrosion, it should preferably be 20 g / m ² or more, and from the point of view of economy, 150 g / m ² or less.

From the high-strength steel sheet annealed after galvanizing, defects in the gaps of the electroplating can be eliminated due to the presence of oxides containing Si, with an average content of 0.6 to 10 wt.%, At the grain boundaries of crystals and in crystal grains on the side of a steel sheet of 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and due to the presence of oxides containing Si, with an average content of from 0.05 to 1.5 wt.%, in the plating layer. The reason why defects in plating coatings can be eliminated if oxides containing Si are present at the grain boundaries of crystals and in the grains of crystals of high-strength steel sheet is because Si, which causes defects in plating coatings, is no longer revealed on the surface of the steel due to the formation of oxides containing Si in the steel sheet during the annealing process.

In addition, the oxides in the plating layer diffuse into the plating during diffusion of the oxides containing Si and formed in the steel sheet during the annealing during the fusion process.

Oxides containing Si present at said grain boundaries of crystals and in crystal grains can be clearly distinguished by observation under a microscope. As an example of oxides containing Si, at the grain boundaries of the crystals and in the crystal grains from the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, Fig. 4 shows the results of observation of cross sections. Figure 4 shows the results of embedding and polishing the cross section of a high-strength annealed after galvanizing steel sheet without gaps in electroplating when tilted by 10 degrees, as well as observing this cross section by means of an SEM image. From this drawing, it will be understood that oxides containing Si present at crystal grain boundaries and in crystal grains of high-strength steel sheet can be clearly distinguished by observation under a microscope.

In addition, oxides containing Si and present in the plating layer can also be clearly distinguished by observation under a microscope.

In addition, if we analyze the oxides at the grain boundaries of crystals and in crystal grains by means of EDS, then peaks of Si, Mn, Fe, and O are observed, and therefore, it is assumed that the observed oxides are SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ .

In the present invention, a “steel layer containing oxides containing Si” is a layer in which oxides containing Si are visible under a microscope. In addition, the “average content of oxides containing Si” indicates the content of oxides contained in this steel layer, and the term “thickness of the layer of steel containing oxides containing Si” refers to the distance from the surface of the steel sheet to the part where these oxides are observed.

The content of oxides containing Si can be measured by any type of method to the extent that mass% of oxides can be measured, but a reliable method is to dissolve the layer containing oxides containing Si with acid, separate the oxides containing Si, and then measure masses. In addition, the method for measuring the thickness of a layer of steel containing oxides containing Si also does not impose specific restrictions, but a reliable method for measuring from a cross section by observation under a microscope.

In the present invention, the reason that the average content of oxides containing Si is limited to a range of 0.6 to 10 wt.% Is that if it is less than 0.6 wt.%, The suppression of the outer oxide film is insufficient and the effect prevention of defects in the gaps of electroplating is invisible, and if it exceeds 10 wt.%, then the effect of preventing defects in the gaps of electroplating becomes saturated.

In addition, the reason that the thickness of the layer of steel containing oxides containing Si is limited to 5 μm is because if said thickness is greater than 5 μm, then the effect of increasing the adhesion of the plating becomes saturated.

In addition, the reason why the content of oxides containing Si in the layer annealed after galvanizing is limited to an average content of 0.05 to 1.5 wt.% Is that if it is less than 0.05 wt.%, Then the suppression the outer oxide film is insufficient and the effect of preventing defects in the gaps of electroplating can be invisible, and if it exceeds 1.5 wt.%, then the effect of preventing defects in gaps in electroplating becomes saturated.

The content of oxides containing Si in the electroplating layer can be measured by any type of method to the extent that mass% of the oxides can be measured, but a reliable method is the process of dissolving only the electroplating layer with acid, separating the oxides containing Si, and then measuring the mass.

In the present invention, “high-strength electroplated steel sheet with excellent machinability" means a greasy sheet having an operating tensile strength of 490 MPa or more, and also a relationship between the tensile strength F (MPa) and elongation L (%), satisfying the inequality:

L≥51-0.035 × F.

The reason that the elongation L is limited to [51-0.035 × F]% or more is because when L is less than [51-0.035 × F], the steel sheet breaks during deep drawing or other extreme processing, and in other circumstances, machinability is unsatisfactory.

Next, the reasons why restrictions are imposed on the manufacturing conditions will be explained. In the present invention, to confidently form a layer of steel containing oxides containing Si, an effective method for providing internal oxidation of oxides containing Si during annealing on a continuous hot dip galvanizing line is effective.

Here, “internal oxidation of oxides containing Si” is a phenomenon when oxygen diffused into a steel sheet reacts with Si near the alloy surface and oxidizes the precipitates. The phenomenon of internal oxidation occurs when the rate of oxygen diffusion inward becomes even greater than the rate of diffusion of Si outward, that is, when the oxygen potential in the atmosphere is relatively high or the Si concentration is low. At the same time, Si does not move at all and oxidizes in place, so that the concentration of Si oxides on the surface of the steel sheet, which causes a drop in the adhesion of the electroplating, can be prevented.

However, even in the steel sheet adjusted by the internal oxidation method, the subsequent plating ability varies depending on the type of Si oxides and their positional relationship, so that Si oxides are characterized as a state in which Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ are present on the surface of the steel sheet or on the surface side, and SiO _{2 is} present on the inner side of the surface of the steel sheet. This is because even if SiO ₂ is in a state of internal oxidation, its presence on the surface of the steel sheet reduces the ability to apply electroplating.

FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO _{4 are} stable in the region where the oxygen potential is higher than that of SiO ₂ , and therefore, to obtain a state in which Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ are present on the surface of the steel sheet or on the surface side, and in which SiO _{2 is} present on the inner side of the surface of the steel sheet, it is necessary to make the oxygen potential greater than in the case of internal oxidation of one SiO ₂ .

The oxygen potential in steel decreases in the direction from the surface of the steel sheet inward, so that if such control is carried out according to the oxygen potential on the surface of the steel sheet, in which Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ are formed on the surface of the steel sheet or on the surface side, then Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ will be formed on the surface steel sheet or on the side surface, and SiO ₂ is formed on Cored oil side of the steel sheet surface, wherein the oxygen potential is reduced.

Through the use of types of Si oxides and from the positional relationships described above, it is possible to prevent defects in the gaps of electroplating due to SiO ₂ in the subsequent process of immersion in a hot dip galvanizing bathtub.

Furthermore, by galvanizing and fusing the steel sheet thus prepared, on which Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ are formed on the surface of the steel sheet or on the surface side, these Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ diffuse into the plating layer.

The oxidation state of Si is determined by the oxygen potential in the atmosphere, so that for the formation of the oxides described in the present invention, under the desired conditions, direct control of the PO ₂ parameter in the atmosphere is necessary.

It is believed that when a gas in the atmosphere contains H ₂ , H ₂ O, O ₂ , and the rest is N ₂ , the following equilibrium reaction occurs. The ratio of PH ₂ O / PH _{2 is} proportional to the square root of PO ₂ and the equilibrium constant 1 / K ₁

H ₂ O = H ₂ + 1 / 2O ₂ : K ₁ = P (H ₂ ) · P (O ₂ ) ^{l / 2} / P (H ₂ O).

The equilibrium constant K ₁ is a temperature dependent variable, so that when the temperature changes, PH ₂ O / PH ₂ and PO ₂ change individually. That is, even the region of the ratio of the partial pressure of water and the partial pressure of oxygen corresponding to the partial pressure of oxygen of the internal oxidation region of Si in a certain temperature range will correspond in another temperature range to the oxygen potential of the region in which iron is oxidized, or will correspond to the temperature potential of the internal oxidation region Si.

Therefore, even if you control the ratio of PH ₂ O / PH ₂ , the formation of oxides described in the present invention is impossible.

In addition, it is believed that when a gas in the atmosphere contains H ₂ , H ₂ O, O ₂ , and the rest is N ₂ , the following equilibrium reaction occurs. The ratio of PCO ₂ O / PCO is proportional to the square root of PO ₂ and the equilibrium constant 1 / K ₂

CO ₂ = CO + 1 / 2O ₂ : K ₂ = P (CO) · P (O ₂ ) ^1/2 / P (CO ₂ ).

In addition to this, at the same time, the following equilibrium reaction occurs, in which it is believed that H ₂ O is formed in the atmosphere

CO ₂ + H ₂ = CO + H ₂ O: K ₃ = P (CO) · P (H ₂ O) / P (CO ₂ ) · P (H ₂ ).

Hence, PO ₂ is indefinite as long as not to be determined PH ₂ O, pH _2, RCO _2, RSO and temperature, so that the formation of oxides, characterized in the present invention, it is necessary to determine or PO ₂ define all the mentioned values.

In particular, in order to reduce iron, while suppressing the external oxidation of Si in the reduction zone, and to form Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , on the surface of the steel sheet or on the surface side, conduct recovery in the recovery zone with an atmosphere containing H ₂ in an amount of from 1 to 60 volume. %, and the rest is one or more components such as N ₂ , H ₂ O, O ₂ , CO ₂ and inevitable impurities, and controlled with the achievement of the following parameter logPO _{2 the} partial pressure of oxygen in the said atmosphere:

Where

T is the maximum peak temperature (K) of the steel sheet,

[Si%] is the Si content (wt.%) In the steel sheet.

It should be noted here that all the logarithms mentioned in the present invention are decimal (ordinary) logarithms.

The reason that the content of H _{2 is} limited to a range of from 1 to 60 vol.%, Is that if it is less than 1%, then it will not be possible to sufficiently restore the oxide film formed on the surface of the steel sheet, and it cannot be guaranteed wettability by electroplating, and if this content exceeds 60%, the improvement in recovery is imperceptible, and costs increase.

The reason logPO _{2 is} limited to −0.000038T ² + 0.107T-90.4 is because iron oxides must be reduced in the reduction zone. If logPO ₂ exceeds the value of -0.000038T ² + 0.107T-90.4, then the iron oxidation region enters, so that a film of iron oxides is formed on the surface of the steel sheet and defects in the gaps of the electroplating occur.

The reason logPO _{2 is} limited to

-0.000034T ² + 0.105T-0.2 [Si%] ² +2.1 [Si%] ² -98.8 or more, is that if logPO ₂ is less than -0.000034T ² + 0.105T-0.2 [Si%] ² +2.1 [Si%] ² -98.8, then Si-containing oxides in the form of SiO ₂ open on the surface and cause defects in the gaps of the electroplating and a drop in the adhesion of the electroplating.

Doing parameter logPO ₂ constituting -0,000034T ² + 0,105T-0,2 [Si%] ² +2,1 [Si%] -98.8 ² or more is obtained a state where Si oxides at least, of the same type, selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ are present on the surface of the steel sheet or on the surface side, and SiO _{2 is} present on the inner side of the surface of the steel sheet.

In addition, in an atmosphere with an even smaller parameter logPO ₂ , an entrance to the external oxidation region of Si occurs, so that the adhesion of the electroplating noticeably decreases.

In the present invention, the maximum peak temperature T of the steel sheet, determining logPO _{2 of the} partial pressure of oxygen in the atmosphere, is defined as being in the range from 923 K to 1173 K.

The reason why T is limited to 923 K or more is that if T is less than 923 K, then the oxygen potential for external Si oxidation is small and the Fe oxidation region is established with the industrial potential range of oxygen potential and on the surface of the steel sheet FeO is formed, as a result of which the adhesion of the electroplating decreases. On the other hand, the reason T is limited to 1173 K or less is that annealing at temperatures above 1173 K requires enormous energy and is uneconomical. If the task is to obtain the mechanical properties of the steel sheet explained above, then a maximum peak temperature of 1153 K or less is sufficient.

In addition, the higher the temperature of the atmosphere in the furnace, the easier - and therefore more profitable - to raise this temperature to the temperature of the steel sheet, but if the temperature of the atmosphere is too high, the service life of the refractory materials in the furnace is reduced and costs are increased, so it is preferable value 1273 K or less.

In the present invention, the control of the parameter PO _{2 is} carried out by introducing one or more components of H ₂ O, O ₂ , CO ₂ and CO. If the temperature should be determined in the above equations of equilibrium reactions, then the equilibrium constant is determined, and based on this equilibrium constant, the partial pressure of oxygen is determined, i.e. oxygen potential. At an atmosphere temperature in the range from 773 K to 1273 K, the gas reaction reaches an equilibrium state in a short time, so that the parameter PO ₂ is determined if the pH ₂ O, PH ₂ , PCO ₂ , PCO and the temperature of the atmosphere in the furnace are determined.

Intentional introduction of O ₂ and CO into the furnace is not necessary, and when H ₂ O and CO ₂ are introduced into the furnace containing 1 volume. % H ₂ or more, O ₂ and CO are formed due to an equilibrium reaction between a part of said input components and H ₂ . Since it is possible to introduce the required amounts of H ₂ O and CO ₂ , specific restrictions are not imposed on the method of introduction, and as it can be mentioned, for example, the method of burning a gas consisting of a mixture of CO and H ₂ and introducing the resulting H ₂ O and CO ₂ , a method for burning CH ₄ , C ₂ H ₆ , C ₃ H ₈ or another hydrocarbon gas and liquefied natural gas (LNG) or other hydrocarbon mixture, and introducing the resulting H ₂ O and CO ₂ , a method for burning gasoline, diesel, heavy oil or another mixture of liquid hydrocarbons and introducing the resulting H ₂ O and CO ₂ , a method comprising burning CH ₃ OH, C ₂ H ₅ OH or another alcohol and mixtures thereof or organic solvents of various types and introducing the resulting H ₂ O and CO ₂ , and t .d.

You can also consider a method that involves burning only CO ₂ and introducing the resulting CO, but if you introduce CO ₂ at the temperature of the main annealing in a furnace with this atmosphere, the part will be restored by means of H ₂ , and CO and H ₂ O will be obtained, so that it’s different from there is no case of the introduction of H ₂ O and CO ₂ .

In addition, in addition to the methods of burning substances and introducing the resulting H ₂ O and CO _2, it is also possible to use the introduction of a gas containing a mixture of CO and H ₂ , CH ₄ , C ₂ H ₆ , C ₃ H ₈ or another hydrocarbon gas, LNG or another hydrocarbon mixture, gasoline, diesel fuel, heavy oil or another mixture of liquid hydrocarbons, or CH ₃ OH, C ₂ H ₅ OH or other alcohol and mixtures thereof, various types of organic solvents, etc., simultaneously with oxygen into the furnace for annealing and burning them in this furnace to obtain H ₂ O and CO ₂ .

This method is simpler and significantly better in controllability compared to the method of using N ₂ saturated with water vapor, or N ₂ rising at the dew point, for supplying water vapor. In addition, this is also not related to condensation in pipes, and therefore, work on pipe insulation, etc. can also be excluded from consideration.

In the present invention, there are no particular restrictions on the recovery time at PO ₂ and temperature described in the claims, but said time is preferably from 10 seconds to 3 minutes. If in a furnace with a reducing atmosphere, during the process of temperature increase, PO ₂ increases, then the sheet passes through the region where logPO ₂ exceeds the value -0.000038T ² + 0.107T-90.4, and then is restored in this region, characterized by the value -0 , 000038T ² + 0.107T-90.4 or less, so that the initially created film of iron oxides is restored. In order to obtain the desired steel sheet having Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ present on the surface of the steel sheet or on the surface side, and having SiO ₂ , present on the inner side of the surface of the steel sheet, this sheet is preferably held in said region for 10 seconds or more. However, if the shutter speed lasts more than 3 minutes, then not only is energy wasted uselessly, but also a drop in productivity is caused on the continuous production line, so this option is not preferred.

In addition, if PO ₂ and the temperature of the reducing atmosphere are in the range according to the present invention, a conventional hot dip galvanizing method can be used with immersion in a non-oxidizing atmosphere furnace or hot dip galvanizing in a radiation furnace without a convection section. Whatever method is used, in the process of raising the temperature, the sheet passes through the region in which the logPO ₂ parameter exceeds the value of -0.000038T ² + 0.107T-90.4 until the temperature of the sheet exceeds 923 K and is formed on the surface of the steel sheet a film of iron oxides, so that for its reduction, suppression of external oxidation of Si and the formation of Si oxides of at least one type selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , on the surface or surface side of the steel sheet, it is enough to ensure proper management of PO ₂ and tamper atmospheric atmosphere in the reduction zone, bringing these parameters into the range according to the present invention.

For example, as described in Japanese Patent Publication (A) No. 55-122865 and Japanese Patent Publication (A) No. 5-271891, a method of forming an iron oxide film on a steel sheet in advance, followed by annealing the steel sheet and restoring the iron oxide film, can be used.

As a method of forming a film of iron oxides, you can use the method of controlling the ratio of air and fuel in the combustible mixture with the introduction of this ratio in the range from 0.9 to 1.2 in the oxidation zone to form a film of iron oxides, or the method of controlling the dew point of the oxidation zone to achieve 273 K or more to form a film of iron oxides.

The reason for the correction of the ratio of air and fuel in a combustible mixture with its introduction in the range from 0.9 to 1.2 is that the formation of a film of iron oxides sufficient to suppress external oxidation of Si requires that the value of the ratio of air to fuel in a combustible the mixture was 0.9 or more. If it is less than 0.9, then it is impossible to form a satisfactory film of iron oxides. In addition, if the ratio of air to fuel in the combustible mixture exceeds 1.2, then the film of iron oxides formed in the oxidation zone is too thick, and peeling oxides will stick to the rolls and cause appearance defects.

In addition, the reason that the dew point of the oxidation zone is controlled to achieve a value of 273 K or more is because the formation of an iron oxide film sufficient to suppress the external oxidation of Si oxides requires a dew point of 273 K or higher. If the dew point is below 273 K, then it is impossible to form a satisfactory film of iron oxides. The upper limit of the dew point is not specifically set, but taking into account the effects of reducing the quality of the equipment, etc., a level of 373 K or less is desirable.

The film thickness of the oxides is affected not only by the ratio of air and fuel in the combustible mixture and the dew point, but also by the line speed, peak temperature, etc., so that in the preferred embodiment, these parameters are controlled, ensuring the passage of the sheet under conditions giving the film thickness oxides from 200 to 2000 Å.

However, in order to complete the recovery of the formed film of iron oxides, the recovery time at PO ₂ and the temperature described in the claims is preferably set to 20 seconds or more.

The aforementioned manufacturing method is possible due to the fact that in a furnace with a reducing atmosphere in the equipment for continuous electroplating by hot dip galvanizing, a device for introducing a gas containing CO ₂ in an amount of from 1 to 100 volume is installed. %, and the rest - N ₂ , H ₂ O, O ₂ , CO and unavoidable impurities, in this sequence, or due to the fact that in the furnace with a reducing atmosphere a device is installed that burns CO or hydrocarbons and generates a gas containing CO ₂ in amount from 1 to 100 volume. %, and the rest is N ₂ , H ₂ O, O ₂ , CO and inevitable impurities. Specific examples of processing equipment are shown in FIG. 5 and FIG. 6. Installation in a furnace with a reducing atmosphere of a device for introducing a gas containing CO ₂ in an amount of from 1 to 100 volume. %, and the rest is N ₂ , H ₂ O, O ₂ , CO and unavoidable impurities, in this sequence, or the installation in a furnace with a reducing atmosphere of a device burning CO or hydrocarbons and generating a gas containing CO ₂ in an amount from 1 to 100 volume. %, and the rest - N ₂ , H ₂ O, O ₂ , CO and inevitable impurities, can guarantee the control of the furnace with a reducing atmosphere, allowing to obtain an atmosphere giving the desired oxide layer.

Next, reasons for imposing restrictions on other operating conditions will be explained. The task is to obtain a metal structure containing martensite in an amount of 3 to 20%, as well as to achieve both high strength and good workability during stamping. If the percentage of martensite and residual austenite expressed in volume percent is less than 3%, then high strength will not be obtained. On the other hand, if the percentage of martensite and residual austenite expressed in volume percent exceeds 20% and the strength is high, the workability of the steel sheet will deteriorate and the purpose of the present invention will not be achieved.

On the slab used for hot rolling, specific restrictions are not imposed. This can be a continuous cast slab or a slab made using a thin slab casting apparatus, etc. In addition, it is possible to use a process similar to rolling combined with continuous casting (PSNL), during which the sheet is cast, and then immediately subjected to hot rolling.

From the point of view of guaranteeing the formability of the steel sheet by stamping, the final temperature of the hot rolling should be at the level of point Ar ₃ or higher. Specific restrictions are not imposed on the cooling conditions after hot rolling and the cooling temperature, but the temperature is preferably set to 1023 K or less, in order to avoid intense fluctuation of the properties at both ends of the roll and to avoid deterioration of the etching ability due to an increase in the thickness of the scale, and it is also preferably set equal to 823 K or more, since if, in particular, bainite or martensite are formed, then during the cold rolling edge cracks will easily arise, and in extreme cases the sheet will even torn. Cold rolling can be carried out under normal conditions. From the point of view of preventing unhindered working hardening of ferrite by finely dispersing martensite and residual austenite and maximizing machinability, the rolling speed makes a reduction of 50% or more. On the other hand, cold rolling with a rolling speed corresponding to a reduction of more than 85% requires an enormous load of cold rolling, so this is impractical.

During annealing in continuous equipment for continuous hot dip galvanizing by dipping followed by annealing, the annealing temperature is set in the range from 1023 K to 1153 K, which corresponds to the region of coexistence of two phases - ferrite and austenite. If the annealing temperature is less than 1023 K, then recrystallization is insufficient, and the steel sheet cannot be given sufficient workability during stamping. Annealing at temperatures above 1153 K causes an increase in production costs and accelerates the deterioration of equipment performance, so this is impractical. In addition, if gradual cooling to 923 K is carried out during immersion in a galvanic bath and cooling, then a sufficient increase in the percentage of ferrite expressed in volume percent may not occur, which will make it difficult to achieve both high strength and good workability during stamping.

The steel sheet is cooled after annealing during immersion in a plating bath. The cooling rate in this case causes cooling from the maximum peak temperature to 923 K at an average speed of 0.5 to 10 degrees per second, subsequent cooling from 923 K to 773 K with an average cooling rate of 3 degrees per second or more, further cooling from 773 K with an average cooling rate of 0.5 degrees per second or more by hot dip galvanizing by immersion up to 627 K and holding for a period of time from 25 seconds to 240 seconds when the temperature changes from 773 K to 623 K after plating.

For a cooling rate of up to 923 K, an average value of 0.5 to 10 degrees per second is set in order to improve workability by increasing the percentage of ferrite expressed in volume percent and at the same time increasing the concentration of C in austenite to reduce the generated free energy and lower the temperature at which austenite transformation begins to a temperature plating bath or lower temperature. If you set the average cooling rate to 923 K less than 0.5 degrees per second, then you may need to increase the length of the processing line in equipment for continuous hot dip galvanizing and may increase costs, and therefore set the average cooling rate to 923 K at 0.5 degrees per second.

To set the average temperature cooling rate to 923 K at a level of less than 0.5 degrees per second, this can be considered as lowering the maximum peak temperature and annealing the sheet at a temperature corresponding to a small expressed volume percent fraction of austenite, but in this case, a suitable temperature volume may turn out to be narrowed compared to the temperature range allowed in real operations. Even if the annealing temperature is only slightly lower, then austenite may not form, and then the goal will not be achieved.

On the other hand, if the average temperature cooling rate is set to 923 K at a level of more than 0.5 degrees per second, then not only can the increase in the percentage of ferrite expressed in volume percent occur, but the increase in the concentration of C in austenite may turn out to be small, which will make it difficult achievement of both high strength and good machinability.

The average cooling rate from 923 K to 773 K is set equal to 3 degrees per second or more in order to avoid the conversion of austenite to perlite in the middle of cooling. If the cooling rate is less than 3 degrees per second, then even if annealing is carried out at the temperature described in the present invention or taking place upon cooling to 923 K, the formation of perlite cannot be avoided. The upper limit of the average cooling rate is not specifically specified, but cooling a steel strip with an average cooling rate of over 20 degrees per second in a dry atmosphere would be difficult.

An average cooling rate of 773 K is set equal to 0.5 degrees per second or more in order to avoid the conversion of austenite to perlite in the middle of cooling. If the cooling rate is less than 0.5 degrees per second, then even if annealing is carried out at the temperature described in the present invention or taking place upon cooling to 773 K, the formation of perlite cannot be avoided. The upper limit of the average cooling rate is not specifically specified, but cooling a steel strip with an average cooling rate of over 20 degrees per second in a dry atmosphere would be difficult.

The reason for holding for a period of time from 25 seconds to 240 seconds when the temperature changes from 773 K to 623 K after electroplating is to help achieve the desired concentration of C in austenite and to obtain hot dip galvanizing, providing excellent machinability and high strength. If the cooling time from 773 K to 623 K after plating is less than 25 seconds, then the concentration of C in austenite becomes unsatisfactory, and this concentration of C in austenite does not reach a level guaranteeing residual austenite at room temperature, and if it is more than 240 seconds, then the transformation bainite will last too long, the amount of austenite will be small, and the formation of residual austenite in sufficient quantities will be impossible.

In hot dip galvanizing without fusion, the sheet passes through a hot dip galvanizing bath, cooling from 773 K to 623 K. The average value of the cooling speed from 773 K to 723 K and the time taken to do this are not critical if they are set in the range corresponding to the range caused by the present invention. The bath temperature characteristic of a hot dip galvanizing bath may vary depending on the composition in the bath, but with a bath composition range according to the present invention, a temperature range from 673 K to 753 K is satisfactory.

The composition in the bath, characteristic of the electrolytic bath, can be any composition to the extent that it is possible to obtain the desired steel sheet with electroplating, but in the General case, use a galvanic bath with a composition close to the composition of the required electroplating.

On the other hand, if during galvanizing, heating is carried out for alloying, and the concentration of C in austenite is unsatisfactory, the amount of austenite will become small and it will be difficult to achieve both high strength and good workability during stamping, therefore, the final cooling temperature before plating is set equal to 693 K to 733 K, and the cooling time from 773 K to the bath temperature is set equal to from 25 seconds to 240 seconds.

The final cooling temperature was set in the range from 693 K to 733 K in order to promote the concentration of C in austenite and to obtain annealing after galvanizing, providing excellent machinability and high strength.

The reason that the cooling time is set from 773 K to the temperature of the plating bath in the range from 25 seconds to 240 seconds is that if it is less than 25 seconds, then the concentration of C in austenite becomes unsatisfactory, and therefore the concentration of C in austenite cannot reach the level guaranteeing residual austenite at room temperature, and if it is more than 240 seconds, then the transformation of bainite lasts too long, the amount of austenite becomes less and the formation of residual austenite in a sufficient amount o It is ordered to be impossible.

In addition, with a holding period from 773 K to the temperature of the plating bath, if cooling and maintaining the temperature is carried out immediately with a range from 673 K to 723 K, this contributes to the concentration of C in austenite and to obtain galvanizing after annealing, providing high strength and excellent machinability . At the same time, if continuous immersion of the sheet is carried out in a plating bath with a temperature of 703 K or lower, then the sheet will be cooled by a plating bath and harden, so it is necessary to reheat it from a temperature of 703 K to a temperature of 743 K, and only then carry out hot dip galvanizing .

In the manufacture of the steel sheet annealed after galvanizing according to the present invention, the Al concentration in the hot dip galvanizing bath used is adjusted to create an effective Al concentration in the bath of 0.07 to 0.105% by weight. In this case, this effective concentration of Al in the plating bath means the concentration of Al in the bath minus the concentration of Fe in the bath.

The reason that the effective Al concentration is limited to a range of 0.07 to 0.105% by weight is because if the effective Al concentration is lower than 0.07%, then the Fe — Al — Zn phase, which forms the fusion barrier at the beginning of the plating, will be formed unsatisfactorily, and a thick G-phase will form on the interface of the plated steel sheet, as a result of which during processing it will be possible to obtain only a steel sheet annealed after galvanizing with poor adhesion of the plating. On the other hand, if the effective concentration of Al exceeds 0.105%, then a high temperature and a long fusion time will be required, and the austenite remaining in the steel will turn into perlite, which will make it difficult to achieve high strength and good machinability. Said concentration is preferably 0.099 mass% or less.

In addition, in the present invention, the fusion is preferably carried out at a temperature T (K) of annealing after galvanizing, satisfying the inequality

720≤T≤690 × exp (1.35 × [Al%]),

where [Al%] is the effective concentration of Al (wt.%) in the plating bath.

The reason the fusion temperature T is limited to a range of 720 K to 690 × exp (1.35 × [Al%]) K is because if the annealing temperature T after galvanizing is below 720 K, then either fusion will not occur, or fusion will occur unsatisfactorily, leading to the appearance of unmelted parts and an electroplated surface coated with an η phase, which worsens the formability. In addition, if T is greater than 690 × exp (1.35 × [Al%]) K, the fusion process is too long, and a thick G-phase forms on the interface of the galvanized steel sheet, as a result of which the adhesion of the electroplated coating decreases during processing.

The reason that the time after plating by hot dip galvanizing by immersion before cooling to a temperature of 673 K or less is limited to a range of 30 seconds to 120 seconds, because if it is less than 30 seconds, the fusion is unsatisfactory, leading to the appearance of fused parts and the surface of the plating coated with the η phase, worsening the formability, and if it exceeds 120 seconds, the transformation of bainite takes too long, the amount of austenite becomes less and formation in a sufficient amount of retained austenite it is impossible.

There are no particular restrictions on the method of heating in the annealing furnace after galvanizing in the present invention. Since the temperature of the present invention can be guaranteed, it is also possible to use radiation heating using a conventional gas furnace or high frequency induction heating. In addition, it is not critical to the method of cooling from the maximum peak temperature after heating for fusion. Due to the fact that the hermetic seal, etc., blocks the heating after fusion, even cooling is sufficient by leaving it in a static state. It is not a problem to provide gas cooling in order to achieve faster cooling.

Examples

Example 1

The slab of each of the compositions according to table 1 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet with a thickness of 4.5 mm, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t blitz 2, for manufacturing an annealed after galvanizing the steel sheet. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ .

The sheet was annealed by adjusting to obtain the values shown in table 2, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate Nia 1.7 degrees per second or more to 723 K, holds up as long as the plating bath is not reached 723 K, is adjusted from 773 K to the plating bath temperature for 30 seconds, and then spent the hot dip galvanizing and alloying at 773 K.

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement. The percentage of Fe in the electroplating was determined by dissolving the electroplating in hydrochloric acid containing the inhibitor, and measuring by inductively coupled plasma (ICP).

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using an X-ray diffractometer (RD). The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated by the following criteria by observing the oxides containing Si from the cross section of the steel sheet with an embedded and polished plating layer by means of an image obtained using a cylindrical mirror analyzer (CHA):

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

Oxides containing Si present in the plating layer were evaluated by observing a steel sheet with an embedded and polished plating layer by means of an SEM image of the cross section of the sheet. The state of the oxides was observed by means of an SEM image. Sheets in which oxides containing Si were observed in the plating layer were rated as “good”, and sheets in which this was not observed were rated as “bad”.

The content of oxides containing Si in the plating layer was measured by dissolving the plating in hydrochloric acid containing an inhibitor, then separating the oxides containing Si, and then measuring their mass.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of the steel sheet with an embedded and polished electroplated layer by means of an image obtained by using a DZA. Sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet, were rated as “good”, and sheets with oxides, where Fe or Mn and Si, O, observed in the same and the same provisions, not noticeable, were rated as "bad."

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

The evaluation results are shown in table 2. Samples numbered 5, 9, 12, 15, 17, 20, 23, 26, 30, 32, 35, 38, 42, and 45 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, in Si oxides were concentrated on the surface of the steel sheet and areas without electroplating appeared, as a result of which a conclusion was made on rejection in appearance. Samples numbered 6, 8, 11, 14, 18, 21, 24, 27, 29, 33, 36, 39, 41 and 44 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, they could not reduce Fe oxides, present on the surface of the steel sheet, and areas without electroplating appeared, as a result of which a conclusion was drawn on culling in appearance. Other steel sheets prepared by the method according to the present invention were high-strength annealed after galvanizing steel sheets of excellent appearance.

Example 2

The slab of each of the compositions according to table 1 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet with a thickness of 4.5 mm, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t Blitz 3 for manufacturing an annealed after galvanizing the steel sheet. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace and the zone in which the temperature was raised due to the heat lost by the furnace was adjusted until the dew point reached 283 K for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ .

The sheet was annealed by adjusting to obtain the values shown in table 3, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate Nia 1.7 degrees per second or more to 723 K, holds up as long as the plating bath is not reached 723 K, is adjusted from 773 K to the plating bath temperature for 30 seconds, and then spent the hot dip galvanizing and alloying at 773 K.

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the CO concentration, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement. The percentage of Fe in the plating was determined by dissolving the plating in hydrochloric acid containing the inhibitor, and making the measurement by ICP.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

Oxides containing Si present in the plating layer were evaluated by observing a steel sheet with an embedded and polished plating layer by means of an SEM image of the cross section of the sheet. The state of the oxides was observed by means of an SEM image. Sheets in which oxides containing Si were observed in the plating layer were rated as “good”, and sheets in which this was not observed were rated as “bad”.

The content of oxides containing Si in the plating layer was measured by dissolving the plating in hydrochloric acid containing an inhibitor, then separating the oxides containing Si, and then measuring their mass.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of the steel sheet with an embedded and polished electroplated layer by means of an image obtained by using a DZA. Sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet, were rated as “good”, and sheets with oxides, where Fe or Mn and Si, O, observed in the same and the same provisions, not noticeable, were rated as "bad."

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

The evaluation results are shown in table 3. Samples numbered 5, 9, 12, 15, 17, 20, 23, 26, 30, 32, 35, 38, 42, and 45 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, in Si oxides were concentrated on the surface of the steel sheet and areas without electroplating appeared, as a result of which a conclusion was made on rejection in appearance. Samples numbered 6, 8, 11, 14, 18, 21, 24, 27, 29, 33, 36, 39, 41 and 44 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, they could not reduce Fe oxides, present on the surface of the steel sheet, and areas without electroplating appeared, as a result of which a conclusion was drawn on culling in appearance. Other steel sheets prepared by the method according to the present invention were high-strength annealed after galvanizing steel sheets of excellent appearance.

Example 3

The slab of each of the compositions shown in table 1 was heated to 1423 K and hot rolled at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet 4.5 mm thick, which was then cooled at a temperature of 853 to 953 K. It was etched then cold-rolled to obtain a 1.6 mm thick cold-rolled steel strip, then plated using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given x in table 4, for the manufacture of annealed after galvanizing steel sheet. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace and the zone in which the temperature was raised due to the heat lost by the furnace was regulated until the dew point was reached at 283 K for use as an oxidation zone, and the non-oxidizing atmosphere furnace was regulated until the air ratio of 1.0 was reached and fuel in a combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ . The logPO ₂ parameter of the oxygen potential in the furnace was adjusted to obtain the values shown in table 4.

The sheet was annealed by adjusting to obtain the values shown in table 4, kept at a holding temperature (which was in the range from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds.

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement. The percentage of Fe in the plating was determined by dissolving the plating in hydrochloric acid containing the inhibitor, and making the measurement by ICP.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

Oxides containing Si present in the plating layer were evaluated by observing a steel sheet with an embedded and polished plating layer by means of an SEM image of the cross section of the sheet. The state of the oxides was observed by means of an SEM image. Sheets in which oxides containing Si were observed in the plating layer were rated as “good”, and sheets in which this was not observed were rated as “bad”.

The content of oxides containing Si in the plating layer was measured by dissolving the plating in hydrochloric acid containing an inhibitor, then separating the oxides containing Si, and then measuring their mass.

The appearance of the plating was judged by calculating the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

The evaluation results are shown in table 4. Samples numbered 1, 2, 3, 4, 6, 7, 9, 10, 11, 12, 15, 16, 17, 18, 19, 20, 24, 25, 26, 28, 29 and 30 proved to be suitable for the cooling rate in the annealing furnace, the effective concentration of Al in the hot dip galvanizing bath and the fusion temperature, which makes it possible to produce high-strength annealed after galvanizing steel sheet with excellent machinability.

Example 4

The slab having the composition indicated by the symbol E in table 1 was heated to 1423 K and hot rolled at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet 4.5 mm thick, which was then cooled at a temperature of 853 to 953 K. It was poisoned, then cold-rolled to obtain a 1.6 mm thick cold-rolled steel strip, then plated using in-line continuous hot dip galvanizing equipment under the conditions given in Table 5, annealed for manufacturing after galvanizing a steel sheet. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ .

The sheet was annealed by adjusting to obtain the values shown in table 5, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate Nia 1.7 degrees per second or more to 723 K, holds up as long as the plating bath is not reached 723 K, is adjusted from 773 K to the plating bath temperature for 30 seconds, and then spent the hot dip galvanizing and alloying at 773 K.

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for the equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement. The percentage of Fe in the plating was determined by dissolving the plating in hydrochloric acid containing the inhibitor, and making the measurement by ICP.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

Oxides containing Si present in the plating layer were evaluated by observing a steel sheet with an embedded and polished plating layer by means of an SEM image of the cross section of the sheet. The state of the oxides was observed by means of an SEM image. Sheets in which oxides containing Si were observed in the plating layer were rated as “good”, and sheets in which this was not observed were rated as “bad”.

The content of oxides containing Si in the plating layer was measured by dissolving the plating in hydrochloric acid containing an inhibitor, then separating the oxides containing Si, and then measuring their mass.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of the steel sheet with an embedded and polished electroplated layer by means of an image obtained by using a DZA. Sheets with oxides, where Fe or Mn and Si, O are visible in the same positions observed on the surface of the steel sheet, were rated as “good”, and sheets with oxides where Fe or Mn and Si, O are not visible in the same and the same observed positions were rated as "bad."

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

The evaluation results are shown in Table 5. Sample No. 5 had logPO ₂ in an out of range furnace according to the present invention, therefore, Si oxides were concentrated on the surface of the steel sheet and areas without electroplating appeared, thereby making a conclusion about culling in appearance. Sample No. 6 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, it was not possible to recover Fe oxides present on the surface of the steel sheet and areas without electroplating appeared, thereby making a conclusion about culling in appearance. Other steel sheets prepared by the method according to the present invention were high-strength annealed after galvanizing steel sheets of excellent appearance.

Example 5

The slab of each of the compositions according to table 6 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet with a thickness of 4.5 mm, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t blitz 7, for the manufacture of hot-dip galvanized steel sheet by dipping. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ .

The sheet was annealed by adjusting to obtain the values shown in table 7, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate 1.7 degrees per second or more up to 723 K, was delayed until the plating bath reached 723 K, brought from 773 K to the temperature of the plating bath in 30 seconds, and then plating was applied by hot dip galvanizing by immersion in a plating bath, intended for applying a Zn-Mg-Al-Si-based plating at a bath temperature of 723 K for 3 seconds, was blown with N ₂ to control the amount of plating deposited, and then cooled to 623 K for 20 seconds. The composition of the plating layer of each obtained plated steel sheet was as follows: Mg: 3%; Al: 11% and Si: 0.15%.

P02 in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, the measured value of the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature. The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible in the steel sheet deeper and inside from the oxides, where Fe or Mn and Si, O are visible in the same positions;

“Satisfactory”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

Adhesion was evaluated by attaching an adhesive tape to a galvanized steel sheet by hot dip galvanizing after a DuPont impact test and subsequent peeling. Sheets whose electroplating did not peel were rated as "good," and sheets whose electroplating were peeling were rated as "bad." The DuPont impact test was carried out using a shock head with a rounded 12.7 mm (0.5 in) at its front end and dropping a 1 kg load from a height of 1 m.

The evaluation results are shown in table 7 and table 8 (continuation of table 7). Samples numbered 3, 6, 9, 12, 17, 20, 23, 26, 29, 33, 35, 38, 41, 45, 48 had logPO ₂ in an out of range furnace according to the present invention, therefore, Si oxides were concentrated in them surface of the steel sheet and there were areas without plating, as a result of which a conclusion was made on rejection. Samples numbered 2, 5, 8, 11, 18, 21, 24, 27, 30, 32, 36, 39, 42, 44 and 47 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, oxides could not be reduced in them Fe, which were present on the surface of the steel sheet, and there were areas without electroplating, as a result of which a conclusion was made on rejection. Other steel sheets prepared by the method according to the present invention were high-strength annealed after galvanizing steel sheets of excellent appearance.

Example 6

The slab of each of the compositions according to table 6 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet with a thickness of 4.5 mm, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t blitz 9, for the manufacture of hot-dip galvanized steel sheet by dipping. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ . The logPO ₂ parameter of the oxygen potential in the furnace was adjusted to obtain the values shown in table 9.

The sheet was annealed by adjusting to obtain the values shown in table 9, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate 1.7 degrees per second or more up to 723 K, was delayed until the plating bath reached 723 K, brought from 773 K to the temperature of the plating bath in 30 seconds, and then plating was applied by hot dip galvanizing by immersion in a plating bath, intended for plating based on Zn-Mg-Al, or in a plating bath for applying plating on the basis of Zn-Mg-Al-Si, for 3 seconds, blown through N ₂ for controlling the amount of plating precipitate, then cooled to 623 K for 20 seconds. The composition of the plating layer of each obtained plated steel sheet was as shown in table 10 (continuation of table 9).

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the CO concentration, the measured value of the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for the equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

The amount of plating coating was measured by dissolving the plating in hydrochloric acid containing an inhibitor, and the weight was measured by weighing. The composition of the plating layer was determined by dissolving the plating in hydrochloric acid containing the inhibitor, and measuring it by chemical analysis.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible in the steel sheet deeper and inside from the oxides, where Fe or Mn and Si, O are visible in the same positions;

“Satisfactory”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

Adhesion was evaluated by attaching an adhesive tape to a galvanized steel sheet by hot dip galvanizing after a DuPont impact test and subsequent peeling. Sheets whose electroplating did not peel were rated as "good," and sheets whose electroplating were peeling were rated as "bad." The DuPont impact test was carried out using a shock head with a rounded 12.7 mm (0.5 in) at its front end and dropping a 1 kg load from a height of 1 m.

The evaluation results are shown in table 9 and table 10 (continuation of table 9). Sample No. 1 had an Al concentration in the electroplating layer outside the range according to the present invention, therefore, a fusion reaction of Zn and Fe occurred, and the adhesion of the coating fell, resulting in a conclusion on rejection. Sample No. 16 had an Al concentration and a Si concentration in the electroplating layer outside the range of the present invention, whereby it was concluded that the sample was rejected. Defects numbered 2, 5, 8, 11, 18, 21, 24, 27, 30, 32, 36, 39, 42, 44, and 47 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, oxides could not be reduced in them Fe, which were present on the surface of the steel sheet, and there were areas without electroplating, as a result of which a conclusion was drawn on culling in appearance. Other steel sheets prepared by the method of the present invention were high strength hot dip galvanized steel sheets with excellent plating and adhesion properties.

Example 7

The slab of each of the compositions according to table 6 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet 4.5 mm thick, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip with a thickness of 1.6 mm, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions specified in t blitz 11 for manufacturing hot-dip galvanized steel sheet by dipping. The equipment used for continuous hot dip galvanizing was equipment that heats the sheet using a non-oxidizing atmosphere furnace and then restores and anneals it in the recovery zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ . The logPO ₂ parameter of the oxygen potential in the furnace was adjusted to obtain the values shown in table 11.

The sheet was annealed by adjusting to obtain the values shown in table 11, kept at a holding temperature (ranging from a maximum peak temperature of minus 20 degrees to a maximum peak temperature) for a holding time of 60 seconds.

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

Hot dip galvanizing was applied to the sheet by immersion in a galvanic bath designed to apply Zn-Mg-Al-Si electroplating for 3 seconds and was blown with N ₂ to control the amount of electroplating deposit reaching 100 g / m ² per side . The composition of the plated layer of the obtained plated steel sheet was as follows: Mg: 3%; Al: 11% and Si: 0.15%.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible in the steel sheet deeper and inside from the oxides, where Fe or Mn and Si, O are visible in the same positions;

“Satisfactory”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

Adhesion was evaluated by attaching an adhesive tape to a galvanized steel sheet by hot dip galvanizing after a DuPont impact test and subsequent peeling. Sheets whose electroplating did not peel were rated as "good," and sheets whose electroplating were peeling were rated as "bad." The DuPont impact test was carried out using a shock head with a rounded 12.7 mm (0.5 in) at its front end and dropping a 1 kg load from a height of 1 m.

The evaluation results are shown in table 11. The method according to the present invention provided the manufacture of a high-strength hot dip galvanized steel sheet with excellent electroplating ability and adhesion.

Example 8

The slab of each of the compositions according to table 12 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet 4.5 mm thick, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t table 13, for the manufacture of hot dip galvanized steel sheet. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ .

The sheet was annealed by adjusting to obtain the values shown in table 13, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate 1.7 degrees per second or more, up to 723 K, was delayed until the galvanic bath reached 723 K, brought from 773 K to the temperature of the galvanic bath in 30 seconds, and then hot-dip galvanized by immersion in a galvanic bath designed for applying Zn-Al-based plating for 3 seconds and blowing with N ₂ to control the amount of plating coating, and then cooling to 623 K for 20 seconds. The composition of the plating layer of each obtained plated steel sheet was as shown in table 14 (continuation of table 13).

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for the equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible in the steel sheet deeper and inside from the oxides, where Fe or Mn and Si, O are visible in the same positions;

“Satisfactory”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

Fe-Zn-based intermetallic compounds present in the plating layer were evaluated by introducing into the cross-section of the coated steel sheet in the direction perpendicular to rolling to a depth of 2 cm, polishing this section and then observing the cross-section by means of an SEM image. The grain size of the Fe-Zn-based intermetallic compounds was the average value according to the measurement results along the long axis and the short axis of the observed crystal. The average grain size was the average value for 4-10 crystals selected from the observed crystals with large grain sizes. As for the observed sheets according to the invention, this time in each case four or more crystals were checked in each case.

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

The chipping property was evaluated by attaching an adhesive tape to a steel sheet, subsequent bending by 180 degrees, reverse bending, removing the tape, and considering the width of the plating adhered to the tape as the peeling width. Sheets with a peeling width of 3 mm or less were rated as “good”, and sheets with a peeling width of more than 3 mm were rated as “bad”.

Electroplating adhesion was evaluated by attaching an adhesive tape to a galvanized steel sheet by hot dip galvanizing after a DuPont impact test and subsequent peeling. Sheets whose electroplating did not peel were rated as "good," and sheets whose electroplating were peeling were rated as "bad." The DuPont impact test was carried out using an impact head with a rounded 12.7 mm (0.5 in) at its front end and dropping a 3 kg load from a height of 1 m.

The evaluation results are shown in table 13 and table 14 (continuation of table 13). Samples numbered 3, 6, 9, 12, 17, 20, 23, 26, 29, 33, 35, 38, 41, 45 and 48 had logPO ₂ in an out-of-range furnace according to the present invention, therefore, Si oxides were concentrated in them the surface of the steel sheet, which led to the appearance of areas without electroplating and a drop in adhesion of the electroplating, as a result of which a conclusion was drawn on culling the samples. Samples numbered 2, 5, 8, 11, 18, 21, 24, 27, 30, 32, 36, 39, 42, 44 and had logPO ₂ in the furnace out of range according to the present invention, therefore, they could not recover Fe oxides that were present on the surface of the steel sheet, which led to the appearance of areas without electroplating and a drop in the adhesion of the electroplating, as a result of which a conclusion was drawn on culling the samples. Other steel sheets prepared by the method of the present invention were high strength hot dip galvanized steel sheets with excellent plating and adhesion properties.

Example 9

The slab of each of the compositions according to table 12 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet 4.5 mm thick, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t table 15, for the manufacture of hot dip galvanized steel sheet. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ . The logPO ₂ parameter of the oxygen potential in the furnace was adjusted to obtain the values shown in table 15.

The sheet was annealed by adjusting to obtain the values shown in table 15, kept at a holding temperature (ranging from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds, then cooled from the maximum peak temperature to 923 K with an average cooling rate of 1 degree per second, then from 923 K to 773 K with an average cooling rate of 4 degrees per second, and additionally cooled from 773 K with an average cooling rate 1.7 degrees per second or more, up to 723 K, was delayed until the galvanic bath reached 723 K, brought from 773 K to the temperature of the galvanic bath in 30 seconds, and then hot-dip galvanized by immersion in a galvanic bath designed for applying Zn-Al-based plating for 3 seconds and blowing with N ₂ to control the amount of plating coating, and then cooling to 623 K for 20 seconds. The composition of the plating layer of each obtained plated steel sheet was as shown in table 16 (continuation of table 15).

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature. The amount of plating deposited was measured by dissolving the plating in hydrochloric acid containing an inhibitor, followed by the use of a weighing method for measurement. The composition of the plating layer was determined by dissolving the plating in hydrochloric acid containing the inhibitor, and measuring it by chemical analysis.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible in the steel sheet deeper and inside from the oxides, where Fe or Mn and Si, O are visible in the same positions;

“Satisfactory”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

Fe-Zn-based intermetallic compounds present in the plating layer were evaluated by introducing into the cross-section of the coated steel sheet in the direction perpendicular to rolling to a depth of 2 cm, polishing this section and then observing the cross-section by means of an SEM image. The grain size of the Fe-Zn-based intermetallic compounds was the average value according to the measurement results along the long axis and the short axis of the observed crystal. The average grain size was the average value for 4-10 crystals selected from the observed crystals with large grain sizes. As for the observed sheets according to the invention, this time in each case four or more crystals were checked in each case. In addition, in the comparative example corresponding to sample No. 11, Fe-Zn-based intermetallic compounds were not observed, and thick Fe-Al-based intermetallic compounds were observed.

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

Electroplating adhesion was evaluated by attaching an adhesive tape to a galvanized steel sheet by hot dip galvanizing after a DuPont impact test and subsequent peeling. Sheets whose electroplating did not peel were rated as "good," and sheets whose electroplating were peeling were rated as "bad." The DuPont impact test was carried out using an impact head with a rounded 12.7 mm (0.5 in) at its front end and dropping a 3 kg load from a height of 1 m.

The evaluation results are shown in table 15 and table 16 (continuation of table 15). Sample No. 1 had an Al concentration in the electroplating layer outside the range according to the present invention, therefore, the Zn and Fe alloying reaction took too long, a brittle alloy layer formed on the surface of the base metal — iron, and the electroplating adhesion fell, and therefore a conclusion was made on rejecting the sample. Sample No. 11 had an Al concentration in the electroplating layer outside the range according to the present invention, therefore, the fusion reaction of Zn and Fe took too long, and the adhesion of the electroplating fell, resulting in the conclusion about rejection of the sample. Other steel sheets prepared by the method of the present invention were high strength hot dip galvanized steel sheets with excellent plating and adhesion properties.

Example 10

The slab of each of the compositions according to table 12 was heated to 1423 K and subjected to hot rolling at a final temperature of 1183 to 1203 K to obtain a hot-rolled steel sheet 4.5 mm thick, which was then cooled at a temperature of 853 to 953 K. It was etched, then subjected cold rolling to obtain a cold-rolled steel strip 1.6 mm thick, then plating was applied using in-line equipment for continuous hot dip galvanizing followed by annealing in the production line under the conditions given in t table 17, for the manufacture of hot dip galvanized steel sheet. The equipment used for continuous hot dip galvanizing was equipment heating the sheet with a non-oxidizing atmosphere furnace, and then restoring and annealing it in the reduction zone. The non-oxidizing atmosphere furnace was adjusted to achieve a 1.0 ratio of air to fuel in the combustible mixture for use as an oxidation zone. The recovery zone was provided using a device for burning a mixed gas containing CO and H ₂ , and introducing the resulting H ₂ O and CO ₂ into gaseous N ₂ containing 10 volume. % H ₂ . The logPO ₂ parameter of the oxygen potential in the furnace was adjusted to obtain the values shown in table 17.

The sheet was annealed by adjusting to obtain the values shown in table 17, kept at a holding temperature (which was in the range from the maximum peak temperature minus 20 degrees to the maximum peak temperature) for a holding time of 60 seconds.

PO ₂ in a furnace with a reducing atmosphere was found using the concentration of hydrogen in the furnace, the concentration of water vapor, the concentration of CO ₂ and the concentration of CO, measured by the temperature of the atmosphere, and the equilibrium constants K ₁ and K ₂ for equilibrium reactions:

H ₂ O = H ₂ + 1 / 2O ₂ ,

CO ₂ = CO + 1 / 2O ₂ .

The tensile strength (PPR) and elongation (OA) were determined by cutting the test piece in accordance with Japanese industrial standard JIS No. 5 from a steel sheet and conducting a tensile test at ordinary temperature.

Hot dip galvanizing was applied to the sheet by immersion in an electroplating bath designed for applying Zn-Al based electroplating for 3 seconds, and was blown with N ₂ to control the amount of electroplating deposit reaching 100 g / m ² per side. The composition of the plating layer of each obtained plated steel sheet was as follows: Al: from 0.4 to 0.5% and Fe: from 0.4 to 0.6%.

Oxides containing Si present at the crystal grain boundaries and in the crystal grains of the steel sheet were evaluated by observing the steel sheet with an embedded and polished plating layer by means of an SEM image from the cross section of the sheet. The state of the inner oxide layer was observed by means of an SEM image. Sheets with Si-containing oxides observed at crystal grain boundaries and in crystal grains were rated as “good”, and sheets in which this was not observed were rated as “bad”. The thickness of the inner oxide layer was likewise observed by means of an SEM image. The thickness of the inner oxide layer was measured from the interface between the steel sheet and the plating to the part where oxides were observed at the grain boundaries of crystals and in crystal grains. The composition of the inner oxide layer was analyzed using EDS connected to SEM. Sheets with observed peaks of Si and O were rated as “good”, and those in which this was not observed were rated as “bad”.

The content of Si-containing oxides in the steel sheet was measured by dissolving the electroplating in hydrochloric acid containing the inhibitor, then dissolving the layer containing Si-containing oxides in the steel sheet with acid to separate the oxides containing Si, and then measuring their mass.

The presence of FeO was determined by measuring from the surface of the steel sheet using XRD. The sheets on which the FeO diffraction peak was not observed were rated as “good”, and those on which the FeO diffraction peak was observed were rated as “bad”.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ and SiO ₂ were evaluated according to the following criteria by observing the oxides containing Si from the cross section of a steel sheet with an embedded and polished electroplated layer by means of an image obtained with the use of DZA:

The provisions of (Fe, Mn) SiO ₃ and (Fe, Mn) ₂ SiO ₄ :

“Good”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are visible on the surface of the steel sheet;

“Bad”: sheets with oxides, where Fe or Mn and Si, O, observed in the same positions, are not noticeable.

The position of SiO ₂ :

“Good”: sheets with oxides, where Si and O, observed in the same positions, are visible in the steel sheet deeper and inside from the oxides, where Fe or Mn and Si, O are visible in the same positions;

“Satisfactory”: sheets with oxides, where Si and O, observed in the same positions, are visible inside the steel sheet;

“Bad”: sheets with oxides, where Si and O, observed in the same positions, are not visible inside the steel sheet.

Fe-Zn-based intermetallic compounds present in the plating layer were evaluated by introducing into the cross-section of the coated steel sheet in the direction perpendicular to rolling to a depth of 2 cm, polishing this section and then observing the cross-section by means of an SEM image. The grain size of the Fe-Zn-based intermetallic compounds of each obtained plated steel sheet was from 0.5 to 3 μm. As for the observed sheets according to the invention, this time in each case four or more crystals were checked in each case.

The appearance of the plating was judged by visual observation along the entire length of the roll and the percentage of the area without plating as follows. Counting results, rated 3 points or more, were considered as passing.

4: the percentage of non-plated area is less than 1%.

3: Percentage of non-plated area from 1% to 5%.

2: the percentage of area without plating is from 5% to less than 10%.

1: percentage of non-plated area 10% or more.

Electroplating adhesion was evaluated by attaching an adhesive tape to a galvanized steel sheet by hot dip galvanizing after a DuPont impact test and subsequent peeling. Sheets whose electroplating did not peel were rated as "good," and sheets whose electroplating were peeling were rated as "bad." The DuPont impact test was carried out using an impact head with a rounded 12.7 mm (0.5 in) at its front end and dropping a 3 kg load from a height of 1 m.

The evaluation results are shown in table 17 and table 18 (continuation of table 17). The method according to the present invention provided the manufacture of high strength hot dip galvanized steel sheet with excellent plating and adhesion properties.

Industrial applicability

The present invention can provide high strength hot dip galvanized steel sheet and high strength galvanized steel sheet annealed after galvanizing with good plating, formability and adhesion, as well as excellent corrosion resistance, and methods for manufacturing such sheets.

Claims (22)

1. Hot-galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a high-strength steel sheet containing, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1, 5 to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and unavoidable impurities, on which there is a galvanized layer containing Al in an amount of from 0.05 to 10 wt.% And Fe in an amount of from 0.05 to 3 wt.%, And the rest is Zn and inevitable impurities, characterized in that said hot-dip galvanized steel sheet contains oxides, Si, with an average content of 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and contains Fe-Zn an alloy with an average grain size of 0.5 to 3 μm on the plating side and contains at least one type of Si oxides selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , on the surface or surface side of the steel sheet and SiO ₂ on the inner side of the surface of the steel sheet. 2. Hot-dip galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1.5 up to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and unavoidable impurities, on which there is a galvanized layer containing Al in an amount of from 0.05 to 10 wt.% and Fe in an amount of from 0.05 to 3 wt.%, and the rest is Zn and unavoidable impurities, characterized in that said hot-dip galvanized steel sheet contains oxides containing Si, when medium its content from 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and contains a Fe-Zn alloy with an average a grain size of from 0.5 to 3 μm on the plating side in a ratio of 1 grain per 500 μm or more, and contains Si oxides of at least one type selected from FeSiO ₂ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , on the surface or surface side of the steel sheet; and SiO ₂ on the inner side of the surface of the steel sheet. 3. Hot-galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1.5 up to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and unavoidable impurities, on which there is a galvanized layer containing Al in an amount of from 0.05 to 10 wt.% and Mg in an amount of from 0.01 to 5 wt.%, and the rest is Zn and unavoidable impurities, characterized in that the said hot-dip galvanized steel sheet contains oxides containing Si, when medium its content from 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet is 5 μm or less from the interface between the high-strength steel sheet and the electroplating layer and contains at least one Si oxides types selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ on the surface or surface side of the steel sheet and SiO ₂ on the inner side of the surface of the steel sheet. 4. Hot-galvanized steel sheet with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1.5 up to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and unavoidable impurities, on which there is a galvanized layer containing from 4 to 20 wt.% Al, from 2 to 5 wt.% Mg and from 0 to 0.5 wt.% Si, and the rest is Zn and inevitable impurities, characterized in that said hot-dip galvanized steel sheet contains oxides containing Si, with an average of their content from 0.6 to 10 wt.%, at the grain boundaries of the crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and contains at least one type of Si oxides selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ on the surface or surface side of the steel sheet and SiO ₂ on the inner side of the surface of the steel sheet. 5. Hot-galvanized steel sheet annealed after galvanizing, with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1.5 to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and unavoidable impurities, on which a zinc alloy plating layer containing Fe, and the rest is Zn and unavoidable impurities, characterized in that said steel sheet contains oxides containing Si, with an average content of 0.6 to 10 m as.%, at the grain boundaries of crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and contains oxides containing Si, with an average content of from 0.5 to 1, 5 wt.% In the plating layer, and contains Si oxides of at least one type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , in the plating layer and on the surface of the steel sheet and SiO ₂ on the inside steel sheet surface. 6. Hot-galvanized steel sheet annealed after galvanizing, with excellent formability and the ability to apply electroplating, consisting of a steel sheet containing, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1.5 to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and unavoidable impurities, on which a zinc alloy plating layer containing Fe, and the rest is Zn and unavoidable impurities, characterized in that said steel sheet contains oxides containing Si, with an average content of 0.6 to 10 m as.%, at the grain boundaries of crystals and in the grains of crystals on the side of the steel sheet 5 μm or less from the interface between the high-strength steel sheet and the plating layer, and contains oxides containing Si, with an average content of from 0.5 to 1, 5 wt.% In the plating layer, and contains at least one type of Si oxides selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ , in the plating layer and SiO ₂ on the steel sheet side of the plating layer and in steel sheet. 7. A steel sheet according to claim 5 or 6, characterized in that the relationship between the tensile strength F (MPa) and the elongation L (%) satisfies the inequality
L≥51-0.035 × F. 8. A method of manufacturing a high-strength hot-dip galvanized steel sheet with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is carried out by immersion of a high-strength steel sheet containing Si and Mn, during which the presence of Si oxides of at least one type is ensured, selected from FeSiO _3, FeSiO _4, MnSiO ₃ and Mn ₂ SiO _4, on the steel surface or surfaces of the steel sheet and the plating section and provide a presence type oxide SiO ₂ on the inside with Oron steel sheet surface to form a galvanized layer or a plating layer of zinc alloy on the high-strength steel sheet. 9. A method of manufacturing a high-strength hot-dip galvanized steel sheet annealed after galvanizing, with excellent formability and the ability to apply electroplating, characterized in that continuous hot-dip galvanizing is performed by immersion of a high-strength steel sheet containing Si and Mn, during which the presence of Si oxides at least at least one type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ on the surface of the steel or the interface of the steel sheet and the plating and provide the presence of ksidov type SiO ₂ on the inner side of the steel sheet surface to form a galvanized layer on a high-strength steel sheet, followed by annealing of the sheet after the galvanizing. 10. A method of manufacturing a high-strength hot-dip galvanized steel sheet with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is performed by immersion of a high-strength steel sheet containing C, Si and Mn, during which the presence of Si oxides of at least one type selected from FeSiO _3, FeSiO _4, MnSiO ₃ and Mn ₂ SiO _4, on the steel surface or surfaces of the steel sheet and the plating section and provide a presence type oxide SiO ₂ on the internal side of the steel sheet surface to form a galvanized layer or to form a plating layer of zinc oxide on the high-strength steel sheet. 11. A method of manufacturing a high-strength hot-dip galvanized steel sheet annealed after galvanizing, with excellent formability and the ability to apply electroplating, characterized in that continuous hot-dip galvanizing is performed by immersion of a high-strength steel sheet containing C, Si and Mn, during which the presence of Si oxides is ensured, at least one type selected from FeSiO ₃ , FeSiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ on the surface of the steel or the interface of the steel sheet and electroplating and provide the presence e oxides of the type SiO ₂ on the inner side of the surface of the steel sheet to form a galvanized layer, and then annealed this sheet after galvanizing. 12. The method according to any one of claims 8 to 11, characterized in that the high-strength hot-dip galvanized steel sheet and the high-strength hot-dip galvanized steel sheet annealed after galvanizing contain, wt.%: C from 0.05 to 0.25, Si from 0, 3 to 2.5, Mn from 1.5 to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest is Fe and inevitable impurities. 13. A method of manufacturing a high-strength hot-dip galvanized steel sheet and a high-strength hot-dip galvanized steel sheet annealed after galvanizing, with excellent formability and the ability to apply electroplating, characterized in that continuous hot dip galvanizing is performed by immersion of a high-strength steel sheet containing C, Si and Mn, during which restore this sheet in the recovery zone with an atmosphere containing H ₂ in an amount of from 1 to 60 vol.%, and the rest is one or more components such as N ₂ , H ₂ O, O ₂ , CO ₂ and inevitable impurities, and controlled with the achievement of the following parameter logPO ₂ partial pressure of oxygen in the said atmosphere:

where T is the maximum peak temperature (K) of the steel sheet,
[Si%] is the Si content (wt.%) In the steel sheet. 14. The method according to item 13, wherein the sheet is oxidized in the oxidation zone in front of the reduction zone in the atmosphere with a ratio of air and fuel in the combustible mixture from 0.9 to 1.2, and then reduced in the reduction zone. 15. The method according to p. 13, characterized in that the sheet is oxidized in the oxidation zone in front of the recovery zone in the atmosphere with a dew point temperature of 273 K or more, and then restore it in the recovery zone. 16. A method of manufacturing a high-strength hot-dip galvanized steel sheet with excellent formability and the ability to apply electroplating, in which the finish rolling of a steel slab containing C, Si and Mn, at a point temperature of Ar ₃ or higher, carry out cold rolling of the sheet with compression of 50 - 85 %, and then - hot dip galvanizing it, during which the equipment is used for continuous hot-dip galvanizing, having a reduction zone with an atmosphere containing H ₂ in an amount of from 1 to 60 b%, and the rest - one or more components such as N _2, H ₂ O, O _2, CO _2, and unavoidable impurities, and controlled with the achievement of the following parameter logPO ₂ partial pressure of oxygen in said atmosphere.:

where T is the maximum peak temperature (K) of the steel sheet,
[Si%] is the Si content (wt.%) In the steel sheet, the sheet is annealed in the temperature range of the joint existence of two phases of ferrite and austenite from 1023 to 1153 K, this sheet is cooled from the maximum peak temperature to 923 K with an average cooling rate of 0.5 to 10 degrees per second, then cool it from 923 to 773 K with an average cooling rate of 3 degrees per second or more and then from 773 K with an average cooling rate of 0.5 degrees per second or more for hot dip galvanizing, so that thus form hot galvanized nny dipping layer on the surface of said cold rolled steel sheet, characterized in that the controlled cooling time from 773 to 623 K after plating with the achievement of a range of from 25 to 240 seconds. 17. A method of manufacturing a high-strength hot-dip galvanized steel sheet annealed after galvanizing, with excellent formability and the ability to apply electroplating, in which the finish rolling of a steel slab containing C, Si and Mn, at a point temperature of Ar ₃ or more, cold rolling of the sheet with compression by 50 - 85%, and then its hot dip galvanizing, during which use equipment for continuous hot dip galvanizing, having a recovery zone with the atmosphere, zhaschey H ₂ in an amount of from 1 to 60 vol%, and the rest -. one or more components such as N _2, H ₂ O, O _2, CO _2, and unavoidable impurities, and controlled with the achievement of the following logPO parameter ₂ partial pressure of oxygen in the atmosphere mentioned:

where T is the maximum peak temperature (K) of the steel sheet,
[Si%] is the Si content (wt.%) In the steel sheet, the sheet is annealed in the temperature range of the joint existence of two phases of ferrite and austenite from 1023 to 1153 K, this sheet is cooled from the maximum peak temperature to 923 K with an average cooling rate of 0.5 to 10 degrees per second, then cool it from 923 to 773 K with an average cooling rate of 3 degrees per second or more and then from 773 K with an average cooling rate of 0.5 degrees per second or more to a range from 693 to 733 K and withstand this sheet when cooled from 773 K to a temperature of an alvanic bath for a time of 25 to 240 s, then hot dip galvanizing is carried out to thereby form a hot dip galvanized layer on the surface of said cold rolled steel sheet, then said steel sheet is fused onto which said hot dip galvanized layer is formed to form a plating layer zinc alloy on the surface of said steel sheet, characterized in that the hot dip galvanizing is carried out in a hot qi bath dipping, the composition of which involves the presence of Al in the bath in an effective concentration of 0.07 to 0.105 wt.%, and the rest is Zn and inevitable impurities, and carry out the annealing of the subsequent galvanizing at a temperature T (K), satisfying the inequality
720≤T≤690 × exp (1.35 × [Al%]),
where [Al%] is the effective concentration of Al (wt.%) in the bath, characteristic of the galvanizing bath. 18. The method according to any one of paragraphs.13, 16 and 17, characterized in that the high-strength hot-dip galvanized steel sheet and high-strength hot-dip galvanized steel sheet annealed after galvanizing contain, wt.%: C from 0.05 to 0.25, Si from 0.3 to 2.5, Mn from 1.5 to 2.8, P 0.03 or less, S 0.02 or less, Al from 0.005 to 0.5, N 0.0060 or less, and the rest Fe and inevitable impurities. 19. The method according to 17, characterized in that the sheet is annealed, cooled to a range from 673 to 723 K, and then reheated to a range from 703 to 743 K for annealing after galvanizing. 20. The method according to p. 17 or 19, characterized in that they control the time from electroplating to cooling to 673 K or lower temperature to achieve a range from 30 to 120 s. 21. Equipment for the manufacture of high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 7, having a furnace with a non-oxidizing atmosphere or a direct-burning flame furnace and capable of continuous hot dip galvanizing by immersion of the steel sheet, while a gas injection device is installed in the reduction furnace, containing CO ₂ in an amount of from 1 to 100%, and the rest -. N _2, H ₂ O, O _2, CO _2, and unavoidable impurities, to manage the oxygen potential on the surface of the steel sheet in press . 22. Equipment for the manufacture of high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 7, having a furnace with a non-oxidizing atmosphere or a direct-acting flame furnace and capable of continuous hot-dip galvanizing by immersion of the steel sheet, while a gas generating device is installed in the reduction furnace, containing CO ₂ in an amount of from 1 to 100%, and the rest -. N _2, H ₂ O, O _2, CO _2, and unavoidable impurities, to manage the oxygen potential on the surface of the steel sheet in echi.

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