阅读说明：本技术 对钢板进行退火的方法 (Method for annealing steel sheet ) 是由 约翰·罗托尔 约纳什·施陶特 让-米歇尔·马泰格纳 于 2013-12-10 设计创作，主要内容包括：本发明涉及一种对钢板进行退火的方法,该方法包括：第一步骤,该第一步骤包括使该钢板的表面完全氧化,从而形成完全氧化的表面层；第二步骤,该第二步骤包括使该钢的在所述完全氧化的层下方延伸的区域中的除了铁之外的元素选择性氧化,从而形成选择性氧化的内部层；以及第三步骤,该第三步骤包括使所述完全氧化的表面层完全还原。(The invention relates to a method for annealing a steel sheet, comprising: a first step of completely oxidizing the surface of the steel sheet to form a completely oxidized surface layer; a second step comprising selectively oxidizing elements other than iron in a region of the steel extending below the fully oxidized layer, thereby forming a selectively oxidized inner layer; and a third step comprising completely reducing the completely oxidized surface layer.)

1. A method of annealing a steel sheet, the method comprising:

-a first step comprising fully oxidizing the surface of the steel sheet, thereby forming a fully oxidized surface layer;

-a second step comprising selective oxidation of elements other than iron in a region of the steel sheet extending below the fully oxidized layer, thereby forming a selectively oxidized inner layer, wherein the second step is performed by ensuring oxygen flow into a majority of the steel sheet, wherein the second step is performed at least in a radiant tube heating zone; and

-a third step comprising a complete reduction of the fully oxidized surface layer, wherein the third step is performed at least in a radiant tube soaking zone.

2. A method of annealing of steel sheets according to claim 1, wherein said method is carried out in an apparatus comprising a direct flame heating zone, said radiant tubes heating zone and said radiant tubes soaking zone, said first step being carried out in said direct flame heating zone.

3. A method of annealing of steel sheets according to claim 2, wherein said first step is performed by adjusting the atmosphere of said direct flame heating zone to an air/gas ratio greater than 1.

4. The method of annealing of steel sheets according to claim 1, wherein said method is carried out in an apparatus comprising a radiant tubes preheating zone, said radiant tubes heating zone and said radiant tubes soaking zone, said first step being carried out in said radiant tubes preheating zone.

5. The method of annealing of steel sheets according to claim 4, wherein said first step consists in containing O in an amount of 0.1 to 10% by volume₂Is performed in the oxidation chamber.

6. Method of annealing of steel sheets according to any of claims 2 to 5, wherein said second step is performed by setting the dew point of the radiant tubes heating zone to be greater than a critical value depending on the H2 content in the atmosphere of the radiant tubes heating zone.

7. The method of annealing of steel sheets according to claim 6, wherein said dew point is adjusted by spraying of water vapour.

8. The method of annealing a steel sheet according to any one of claims 1 to 7,the reduction of the third step is carried out by using a catalyst containing at least 2% of H₂And the balance is N₂Is carried out in the atmosphere of (1).

9. The method of annealing of steel sheets according to any of claims 1 to 8, wherein the steel of said steel sheet comprises up to 4% by weight manganese, up to 3% by weight silicon, up to 3% by weight aluminium and up to 1% by weight chromium.

10. Method for producing a galvanized steel sheet, wherein an annealed steel sheet obtained according to any one of claims 1 to 9 is hot dip coated by immersion in a zinc bath.

11. A method for producing a galvannealed steel sheet, wherein the galvanized steel sheet obtained according to claim 10 is further heat-treated at a temperature of 450 ℃ to 580 ℃ for 10 seconds to 30 seconds.

12. The method for producing a galvannealed steel sheet according to claim 11, wherein the heat treatment is performed at less than 490 ℃.

Technical Field

The present invention relates to a method of annealing a steel sheet. More particularly, the invention relates to a method of annealing steel sheets prior to hot dip coating and possibly prior to galvannealing treatment.

Background

The demand for increasingly lighter vehicles by increasing mechanical resistance and even by decreasing density requires more sophisticated alloying concepts for high strength steels. Alloying elements such as aluminum, manganese, silicon and chromium are preferred, but cause serious problems in coatability due to the presence of alloying element oxides on the surface after annealing.

During heating, the steel surface is exposed to the following atmosphere: the atmosphere is non-oxidizing to iron, but oxidizing to alloying elements with higher affinity for oxygen, such as manganese, aluminum, silicon, chromium, carbon, or boron, which will produce oxides of these elements at the surface. When steel contains such oxidizable elements, these elements tend to selectively oxidize at the surface of the steel, impairing the wettability of subsequent coatings.

Furthermore, when the coating layer is a hot-dip coated steel sheet which is further heat-treated to be galvannealed, the presence of such an oxide may impair diffusion of iron in the coating layer, and thus cannot be sufficiently alloyed at the conventional line speed of an industrial line.

Disclosure of Invention

The invention provides a method for annealing a steel plate, which comprises the following steps:

-a first step comprising the complete oxidation of the surface of the steel sheet, thereby forming a completely oxidized surface layer;

-a second step comprising selective oxidation of elements other than iron in a region of the steel extending below the fully oxidized layer, thereby forming a selectively oxidized inner layer; and

-a third step comprising the complete reduction of said fully oxidized surface layer.

In a first embodiment, the method may be carried out in an apparatus comprising a direct flame heating zone, a radiant tube heating zone, and a radiant tube soaking zone, the first step being carried out in the direct heating zone, the second step being carried out at least in the radiant tube heating zone, and the third step being carried out at least in the radiant tube soaking zone. The first step may be carried out by adjusting the atmosphere of the direct flame heating zone to an air/gas ratio greater than 1.

In another embodiment, the method may be performed in an apparatus comprising a radiant tube preheating zone, a radiant tube heating zone, and a radiant tube soaking zone, the first step being performed in the radiant tube preheating zone, the second step being performed in at least the radiant tube heating zone, and the third step being performed in at least the radiant tube soaking zone. The first step may be carried out in an oxidation chamber containing O2 in an amount of 0.1 to 10% by volume, preferably O2 in an amount of 0.5 to 3% by volume. Alternatively or in combination, the oxidation chamber may be subjected to water jets to oxidise the iron.

In another embodiment, the second step is performed by setting the dew point in the radiant tube heating zone to be greater than a critical value according to the H2 content in the atmosphere of the radiant tube heating zone. The dew point may be adjusted by the injection of water vapor.

In another embodiment, the reduction of the third step is performed by using an atmosphere comprising at least 2% by volume of H2 and the remainder being N2. A preferred maximum amount of H2 is 15% by volume.

The annealed steel sheet obtained according to the invention can be hot dip coated by immersion in a zinc bath and possibly heat treated at a temperature of 450 ℃ to 580 ℃, and preferably at 490 ℃ for 10 seconds to 30 seconds to produce a so-called galvannealed steel sheet.

There is no practical limit to the type of steel that can be treated according to the invention. However, it is preferred that the steel contains a maximum of 4% by weight of manganese, a maximum of 3% by weight of silicon, a maximum of 3% by weight of aluminium and a maximum of 1% by weight of chromium to ensure that the steel can be optimally coated.

During heating, the steel surface is first exposed to an oxidizing atmosphere, which forms iron oxide at the surface (so-called complete oxidation). The iron oxide prevents the alloying elements from being oxidized at the steel surface.

This first step may be carried out in a Direct Flame Furnace (DFF) used as a preheater. The oxidation capacity of the plant is adjusted by setting the air/gas ratio to be greater than 1.

This first step may alternatively be carried out in a Radiant Tube Furnace (RTF) preheating zone. In particular, such an RTF preheating zone may comprise an oxidation chamber comprising an oxidizing atmosphere. Another alternative is to set the entire preheating zone in an oxidizing atmosphere with O2 and/or H2O as oxygen donors.

After the surface oxide layer is generated, a second step of selectively oxidizing an element other than iron is performed. These elements are the elements contained in the steel that can be oxidized most easily, such as manganese, silicon, aluminum, boron, or chromium. This second step is performed by ensuring that oxygen flows into most of the steel sheet, thereby causing internal selective oxidation of the alloying elements.

In the framework of the invention, this oxidation can be carried out by controlling the dew point of the RTF heating zone to be greater than a minimum value, depending on the H2 content in the atmosphere of the RTF heating zone. Spraying moisture is one method that may be used to control the dew point to a desired value. It is noted that reducing the H2 content of the atmosphere will allow less moisture to be injected, since the dew point will also be reduced, but selective oxidation is still obtained.

In the third step, this fully oxidized layer must be reduced in order to ensure further coating properties by any kind of coating, such as phosphates, electrodeposition coatings, vacuum coatings including jet vapor deposition coating, hot dip galvanizing coatings, etc. This reduction can take place at the end of the RTF heating zone and/or during soaking and/or during cooling of the steel sheet. The reduction may be carried out using conventional reducing atmospheres and methods known to those skilled in the art.

Detailed Description

The invention will be better understood from the detailed disclosure of some non-limiting examples.

Examples of the invention

Steel sheets made of steels with different compositions, as summarized in table 1, were produced in a conventional way before being cold rolled. The steel sheet is then annealed in a plant comprising a DFF furnace followed by an RTF furnace comprising two different zones, namely an RTF heating zone and an RTF soaking zone. The dew point of the RTF heating zone was adjusted by setting different DFF heating zone outlet temperatures and injecting steam at different rates. The annealing parameters are summarized in table 2. After soaking, the annealed steel sheet was cooled by a conventional spray cooler until reaching a temperature of 480 ℃.

The steel sheet was then immersed in a zinc pot containing 0.130% by weight of aluminum and subjected to galvannealing treatment by induction heating at a temperature of 580 ℃ for 10 seconds.

The coated steel sheet was then examined and the corresponding iron content of the coating was estimated. The results of the evaluation are also summarized in table 2.

TABLE 1 Steel compositions

Grade	Carbon (C)	Manganese oxide	Silicon	Aluminium	Chromium (III)	Molybdenum (Mo)	Titanium (IV)	Niobium (Nb)	Boron
										A	0.13	2.5	0.7	--	0.3	--	0.02	0.01	0.002
B	0.2	1.8	2.0	0.65	--	--	--	--	--
										C	0.2	2.2	2.0	0.5	--	0.15	--	0.015	--

TABLE 2 annealing parameters coating estimation

ne: not estimating

Experiment 1 presented a highly reflective, Gl-type unalloyed surface. Run 2 with insufficient dew point produced alloys that were significantly somewhat random and differentiated across the width over the entire coil length. The dew value was further increased during trial 3. This results in a fully alloyed strip surface along the entire coil length.

Another advantage of the method according to the invention is that it seems that the decarburization kinetics of the steel sheet is favourably influenced by increasing the dew point of the RTF heating zone to allow a corresponding conversion of the selective oxidation from the external mode to the internal mode. This advantage is demonstrated by monitoring the reduced carbon monoxide (CO) content in the atmosphere of the RTF heating zone.