JP2020190017A - Dew point control method for reduction atmospheric furnace, reduction atmospheric furnace, method for producing cold rolled steel sheet, and method for producing hot dip galvanized steel sheet - Google Patents

Abstract

To provide a dew point control method capable of increasing and controlling the dew point in a reduction atmospheric furnace at high precision by economically excellent means.SOLUTION: In a reduction atmospheric furnace using a nitrogen gas and a hydrogen gas as reduction gases to be fed, the reduction gases are mixed with an oxygen-containing gas, and the oxygen concentration in the reduction gases to be fed is controlled, thus the dew point in the furnace is regulated to -20 to 0°C.SELECTED DRAWING: Figure 2

Description

The present invention relates to a dew point control method and a reducing atmosphere furnace of a reducing atmosphere furnace containing hydrogen gas and nitrogen gas for heat-treating a steel sheet, a method for producing a cold-rolled steel sheet, and a method for producing a hot-dip galvanized steel sheet.

In recent years, in the fields of automobiles, home appliances, building materials, etc., there is an increasing demand for high-strength steel sheets (high-tensile steel materials) that can be used for reducing the weight of structures. As the high-tensile steel material, for example, a steel sheet having good hole expanding property by containing Si in the steel, and a steel sheet having good ductility by containing Si and Al can be obtained. I know.

However, for example, when a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet containing a large amount of Si as a base material is manufactured, there are the following problems. The hot-dip galvanized steel sheet is subjected to hot-dip galvanizing after being heat-annealed at a temperature of about 600 to 900 ° C. in a non-oxidizing atmosphere or a reducing atmosphere. However, Si in steel is an easily oxidizing element and is selectively oxidized in a generally used non-oxidizing atmosphere or reducing atmosphere to concentrate on the surface and form an oxide. Since this oxide lowers the wettability with hot-dip zinc during the plating process and causes non-plating, the wettability drops sharply as the Si concentration in the steel increases, resulting in frequent non-plating and plating adhesion. There is a problem that the sex deteriorates. Furthermore, when Si in steel is selectively oxidized and concentrated on the surface, a significant delay in alloying occurs in the alloying process after hot dip galvanizing. As a result, productivity is significantly impaired. If an alloying process is attempted at an excessively high temperature in order to secure productivity, there is also a problem that the powdering resistance is deteriorated, and it is difficult to achieve both high productivity and good powdering resistance at the same time.

In response to such problems, for example, in Patent Document 1 and Patent Document 2, the surface of the steel sheet is once oxidized using a direct-fired heating furnace (DFF) or a non-oxidizing furnace (NOF), and then in a reduction zone. A method of internally oxidizing Si by reducing it, suppressing the concentration of the Si surface, and improving the wettability and adhesion of hot dip galvanizing is disclosed.

However, in the methods described in Patent Documents 1 and 2, although the plating adhesion after reduction is good, the amount of internal oxidation tends to be insufficient, and the alloying temperature is 30 to 30 to higher than usual due to the influence of Si contained in the steel. There is a problem that the tensile strength and ductility of the steel sheet are lowered due to the high temperature of 50 ° C. If the amount of oxidation is increased in order to secure a sufficient amount of internal oxidation, the oxide scale adheres to the roll in the furnace and the steel sheet is scratched, which is a so-called pickup phenomenon. Therefore, the means for simply increasing the amount of oxidation is I can't get it.

Further, in Patent Document 3, the supplied gas is humidified by a method of passing the gas through warm water, the inside of the furnace is divided and controlled by a sealing device, and the hydrogen concentration and the dew point in the annealing furnace are controlled within a predetermined range to obtain Si. Disclosed is a method of internally oxidizing to improve hot-dip galvanizing wettability and adhesion.

However, in the method described in Patent Document 3, when the amount of water brought into the furnace changes due to fluctuations in the outside temperature or the type of steel plate, the dew point of the humidifying gas tends to fluctuate due to this change, and the dew point is stably controlled to the optimum dew point range. It was difficult to do.

Patent Document 4 discloses a method of directly injecting steam into a heating furnace to raise and control the dew point.

However, in the method described in Patent Document 4, when steam is directly supplied into the furnace, a region having a high dew point of 10 ° C. or higher is locally generated, and when the steel plate passes through the region, even the ground iron is oxidized and picked up. It is known that the phenomenon will occur. In addition, a steam generating device, a device for supplying air into the furnace, and a draining device for a piping system through which steam passes are required, which increases maintenance and installation costs and is inferior in economic efficiency.

Patent Document 5 discloses a method of humidifying the gas to be charged into the furnace and adjusting the dew point in the furnace by using a humidifying unit as a water vapor permeable membrane.

Further, Patent Document 6 discloses that in order to produce a high-strength steel sheet having excellent chemical conversion treatment property, a humidifying gas is introduced into an annealing furnace to promote internal oxidation of the steel sheet.

JP-A-2010-202959 Japanese Unexamined Patent Publication No. 2011-117069 WO2007 / 043273 Japanese Unexamined Patent Publication No. 2005-264305 Japanese Patent No. 6052464 JP 2015-151604

In the method described in Patent Document 5, the problem of Patent Document 4 is solved by controlling the dew point using the steam permeable membrane module, but water leakage to the supply gas system due to the breakage of the steam permeable membrane module It is difficult to detect, and water as a liquid flows into the gas supply pipe, and there is a risk of equipment stop trouble. In addition, a chiller unit that sends water to the module is also required, which causes a problem that maintenance and installation costs are high and the economy is inferior. Further, Patent Document 6 also requires a humidifying facility like Patent Document 5, and no particular study has been made on maintainability and installation cost.

The present invention has been made in view of such a problem, and an object of the present invention is a dew point that can raise and control the dew point in the reducing atmosphere furnace with high accuracy by an economical method. The purpose is to provide a control method.

It is considered that the problems of Patent Documents 1 to 4 have been solved in Patent Document 5, but Patent Documents 5 and 6 have problems in terms of equipment and maintenance costs and equipment costs as described above. The inventor has conducted various studies on a dew point control method for solving the above problems. As a result, the following was found and the present invention was completed.

In Patent Document 5, in order to raise the dew point of the reducing atmosphere furnace, steam is directly supplied to the reducing furnace, so that various problems caused by the steam have occurred. Therefore, we came up with the idea that the above problems could be solved by supplying water to the furnace without directly supplying steam. As a method, we focused on hydrogen, which is a mixed gas of nitrogen and hydrogen, which is usually used as a supply gas to a reducing atmosphere furnace. If oxygen gas is supplied together with a mixed gas of nitrogen and hydrogen at the time of supply to the reducing atmosphere furnace, hydrogen and oxygen react in the reducing atmosphere furnace to become water, and the atmosphere is as if water vapor was supplied. Found to be formed. It is clear that the state before the supply of the gas to be water vapor to the reducing atmosphere furnace is oxygen, and the problem caused by the water vapor before supply does not occur.

Further, it is conceivable to directly supply oxygen gas to the reducing atmosphere furnace by itself, but in this case, as in the case of Patent Document 4, the furnace is rapidly supplied at the site where oxygen is supplied in the reducing atmosphere furnace. Oxygen in the inner atmosphere is oxidized to water, and an atmosphere that locally oxidizes the steel plate is formed. As a result, it is easily assumed that the surface of the steel sheet is oxidized and then comes into contact with the in-core roll before the surface of the steel sheet is reduced, and the steel plate oxide adheres to the surface of the roll in the furnace to cause scratches. .. Therefore, there is a problem in supplying oxygen gas directly to the reducing atmosphere furnace by itself.

Here, the reducing atmosphere furnace is a furnace usually used for heat treatment of steel products, and means a furnace in an atmosphere in which iron oxide is reduced without oxidizing iron.

Further, Patent Document 5 has a problem in controlling the dew point of the reducing atmosphere furnace during the production of a hot-dip galvanized steel sheet, and an object of the present invention is to solve this problem, but the present invention is not limited to the hot-dip galvanized steel sheet and is subjected to chemical conversion treatment. Even during the production of cold-rolled steel sheets, which require good surface characteristics typified by properties, it is an issue to raise and control the dew point in the reducing atmosphere furnace with high accuracy. The present invention is a technique that can be commonly applied when annealing a hot-dip galvanized steel sheet and a cold-rolled steel sheet.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] When the cold-rolled steel sheet is annealed, nitrogen gas and hydrogen gas are used as the reducing gas to be supplied to the reducing atmosphere furnace, and the reducing gas is mixed with a gas containing oxygen and supplied. A method for controlling the dew point of a reducing atmosphere furnace, which comprises adjusting the dew point in the reducing atmosphere furnace by controlling the oxygen concentration in the reducing atmosphere.
[2] The dew point control method for a reducing atmosphere furnace according to the above [1], wherein the gas containing oxygen is air.
[3] The dew point control method for a reducing atmosphere furnace according to the above [1] or [2], wherein the oxygen concentration of the reducing gas is controlled to exceed 0 vol% and 2 vol% or less.
[4] The dew point control method for a reducing atmosphere furnace according to any one of [1] to [3], wherein the dew point in the reducing atmosphere furnace is adjusted to −20 ° C. to 0 ° C.
[5] A method for producing a cold-rolled steel sheet, which comprises manufacturing the cold-rolled steel sheet by using the dew point control method of the reducing atmosphere furnace according to any one of the above [1] to [4].
[6] The dew point of the reducing atmosphere furnace is controlled by the method for controlling the dew point of the reducing atmosphere furnace according to any one of the above [1] to [4], and after the cold-rolled steel sheet is annealed, it is continuously continued. A method for producing a hot-dip galvanized steel sheet, which comprises performing hot-dip galvanizing.
[7] A reducing atmosphere furnace for a cold-rolled steel plate anneaizer, which consists of a heating device using a radiant tube, a pipe for supplying nitrogen gas and hydrogen gas, and a device A for adjusting the supply flow rate thereof, and a gas containing oxygen. A device B including a supply pipe and a supply amount adjusting device, a device C for mixing a gas containing nitrogen gas and hydrogen gas supplied from the A, and an oxygen supplied from the B to adjust the flow rate, and the mixing. A reducing atmosphere provided with an oxygen concentration measuring device in the gas, a pipe for supplying the mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace. Furnace.
[8] Continuous hot-dip zinc plating equipment A reducing atmosphere furnace for a steel plate anneaizer, a heating device using a radiant tube, a device A consisting of a pipe for supplying nitrogen gas and hydrogen gas, and a supply flow rate adjusting device, and oxygen. A device B consisting of a pipe for supplying a gas containing the above gas and a supply amount adjusting device thereof, and a device C for adjusting the flow rate by mixing the nitrogen gas and hydrogen gas supplied from the A and the gas containing oxygen supplied from the B. A device for measuring the oxygen concentration in the mixed gas, a pipe for supplying the mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace. Reducing atmosphere furnace.

Since the present invention does not require a humidifying device or a drain draining device in the pipe, the hot-dip galvanized steel sheet can be manufactured by inexpensive equipment modification.

In addition, the supply gas is mixed with a gas containing oxygen instead of water, and the oxygen concentration of the mixed supply gas and the flow rate of the supply gas can be controlled to control the dew point in the furnace, eliminating the need for a humidification unit. Dew point control, which is more maintainable and economical than before, has become possible.

It is a figure which shows one Embodiment of the continuous hot dip galvanizing facility of this invention. It is a figure which shows one Embodiment in the reducing atmosphere furnace of this invention. It is one embodiment in the reducing atmosphere furnace of the prior art (Patent Document 5). The dew point transitions in the examples of the present invention and the comparative examples are shown.

Hereinafter, embodiments of the present invention will be specifically described.

First, an example of the equipment used in this embodiment is shown with reference to the drawings. As an annealing furnace type of continuous hot-dip zinc plating equipment used when annealing and hot-dip zinc-plated steel sheets are applied to manufacture hot-dip zinc-plated steel sheets, the heating furnace that heats the steel sheet by raising the temperature is DFF (direct flame type) or NOF. (Non-oxidation type), the soaking furnace for soaking the heated steel sheet is the radiant tube furnace (RTF) type, and the all-radiant tube furnace type, in which the heating furnace to the soaking furnace are all radiant tubes, etc. It can be exemplified.

In the present invention, the furnace portion provided with the radiant tube is referred to as a reducing atmosphere furnace. That is, if the heating furnace is a DFF (direct flame type) or NOF (non-oxidizing type) and the soaking furnace is a radiant tube furnace (RTF) type, the soaking furnace is a reducing atmosphere furnace. In the all-radiant tube furnace type in which everything from the heating furnace to the soaking furnace is a radiant tube, the reducing atmosphere furnace is from the heating furnace to the soaking furnace.

Then, by using the dew point control method of the reducing atmosphere furnace of the present invention, the heating furnace is DFF (direct flame type) or NOF (non-oxidation type) and the soaking furnace is a radiant tube furnace (RTF) type, all radiant. In any of the tube type, the dew point in the reducing atmosphere furnace can be controlled with high accuracy, and the plating property is ensured even in the case of a steel plate containing a large amount of easily oxidizing elements such as Si.

FIG. 1 is a diagram showing a configuration example of a continuous hot-dip galvanizing facility equipped with an annealing furnace and a plating apparatus. In FIG. 1, 1 is a steel plate, 2 is a steel plate transport roll, 3 is a direct flame type heating zone (DFF), 4 is a reducing atmosphere furnace (radiant tube furnace type), 5 is a quenching zone, 6 is a slow cooling zone, and 7 Is a plating device.

The steel plate 1 is sent to the direct-fired heating furnace 3 by the steel plate transport roll 2, heated (oxidation treatment step), then reduced in the reducing atmosphere furnace 4 (reduction annealing step), and then the quenching zone 5 , It is cooled by the slow cooling zone 6 (cooling step), and is plated by the plating apparatus 7.

The annealing furnace in the present invention is not limited to that used as a pretreatment for plating equipment. It may be applied to the pretreatment of the chemical conversion treatment for imparting a phosphate film or the like to the annealed steel sheet. It can also be used in an annealing furnace for cold-rolled steel sheets.

FIG. 2 is a diagram showing the configuration of the reducing atmosphere furnace 4 shown in FIG. 1 and showing an embodiment of the reducing atmosphere furnace of the present invention. FIG. 2 shows the supply route of the gas supplied into the furnace in the reducing atmosphere furnace (radiant tube type) 4. Here, the gas containing oxygen may be abbreviated as the oxygen-containing gas. Further, the gas containing oxygen shall include a gas composed of oxygen and unavoidable impurities.

In FIG. 2, 8 is a nitrogen gas and hydrogen gas supply flow rate adjusting device, 9 is an oxygen-containing gas supply flow rate adjusting device, 10 is a nitrogen gas, hydrogen gas and oxygen-containing gas mixed flow rate adjusting device, and 11 is a supply mixed gas oxygen concentration meter. Reference numeral 12 is an oxygen-containing gas drying device, 14 is an in-furnace dew point measurement point, and 15 is a supply mixed gas pipe.

According to FIG. 2, the flow rate of the mixed gas of nitrogen gas and hydrogen gas is adjusted by the nitrogen gas and hydrogen gas supply flow rate adjusting device 8, and the flow rate of the oxygen-containing gas is adjusted by the oxygen-containing gas supply flow rate adjusting device 9. As a result, the mixing ratio of the gas supplied to the nitrogen gas, hydrogen gas, and oxygen-containing gas mixing flow rate adjusting device 10 is adjusted. Then, the gas flow rate to be supplied to the reducing atmosphere furnace 4 is adjusted by the nitrogen gas / hydrogen gas / oxygen-containing gas mixing flow rate adjusting device 10, and the gas is supplied into the reducing atmosphere furnace 4 through the supply gas pipe 15. .. Further, since the oxygen-containing gas before mixing may contain water, it is preferable to remove the water using the oxygen-containing gas air drying device 12. Further, it is preferable that the oxygen concentration of the supply mixed gas passing through the supply gas pipe 15 is measured by the supply mixed gas oxygen concentration meter 11, and the flow rate of the oxygen concentration is feedback-controlled as necessary. Immediately after being charged into the reducing atmosphere furnace, the supplied mixed gas reacts with oxygen in the supplied mixed gas and hydrogen in the reducing atmosphere furnace to become water, and the dew point temperature in the reducing atmosphere furnace rises.

One or a plurality of dew point sampling points 14 in the furnace are installed in the reducing atmosphere furnace, and the dew point in the reducing atmosphere furnace is measured. Then, in response to the measurement result, the flow rate of the gas and the oxygen-containing gas is controlled within an appropriate range while monitoring the mixed supply gas oxygen concentration meter 11, and the dew point in the reducing atmosphere furnace is adjusted to a desired range. The oxygen concentration of the gas supplied into the reducing atmosphere furnace is preferably controlled to be more than 0 vol% and 2 vol% or less.

When the oxygen concentration is 0 vol%, it is difficult to set the dew point in the reducing atmosphere furnace to −20 ° C. or higher. It was confirmed that the dew point in the reducing atmosphere furnace can be set to -20 ° C or higher at an oxygen concentration of 0.0001 vol%. On the other hand, if it exceeds 2 vol%, the dew point becomes partially too high in the mixed gas supply section of the reduction furnace, and there is a risk of causing oxidation of the steel sheet.

By controlling the flow rate of the mixed supply gas based on the dew point, the amount of water generated in the reducing atmosphere furnace can be adjusted, and the dew point in the reducing atmosphere furnace can be adjusted. Instead of the oxygen-containing gas, oxygen may be mixed with the reducing gas and supplied to the reducing atmosphere furnace. Considering the minimum control resolution of the nitrogen gas / hydrogen gas / oxygen-containing gas mixed flow rate regulator 10, in order to accurately control the dew point in the reducing atmosphere furnace, oxygen-containing gas or oxygen-containing gas diluted with oxygen should be used. It is preferable to absorb the control error of the oxygen-containing gas flow rate adjusting valve 11 by supplying the gas, and it is preferable to supply air as the oxygen-containing gas from the viewpoint of cost.

Although not shown by omission in FIG. 2, normally, a gas in which dry nitrogen and hydrogen having a dew point of −60 to −40 ° C. are mixed is constantly supplied to the reducing atmosphere furnace 4 as a reducing gas. More specifically, the hydrogen volume fraction in the atmosphere is preferably 1 to 50%, and the balance is preferably nitrogen and unavoidable impurities. The composition of the atmosphere is not limited to this, and gas components such as CO and CO ₂ gas may be further contained within a range that does not affect the control of the dew point, and the undried gas is used in the reducing atmosphere furnace. It may be supplied to 4. Further, the hydrogen volume fraction does not have to be limited to the above range, and the hydrogen volume fraction in the reducing gas may be set to 100% as in a batch type reduction furnace, for example.

As described above, according to the present invention, the nitrogen gas, hydrogen gas, and dry oxygen-containing gas whose flow rates are adjusted by the flow rate adjusting valves are mixed, and the gas and oxygen are contained depending on the dew point state at the dew point sampling point in the furnace. Since the flow rate of each gas can be feedback-controlled, the dew point in the reducing atmosphere furnace can be controlled.

On the other hand, FIG. 3 is a reducing atmosphere furnace in which the dew point is controlled by using the steam permeation membrane module described in Patent Document 5. In FIG. 3, 1 is a steel plate, 2 is a steel plate transfer roll, 4 is a reducing atmosphere furnace, 14 is a dew point measurement point in the furnace, 15 is a supply mixing gas pipe, 16 is a gas distributor, 17 is a gas mixing device, and 18 is. A dew point meter for supply gas, 19 is a humidifying device, and 20 is a circulating high temperature water tank.

According to the present invention, since it is not necessary to add water by the humidifying device for raising the dew point in the reducing atmosphere furnace as shown in FIG. 3, the humidifying device and the drain removing device and the pipe heat insulating construction for dew condensation measures in the supply gas pipe are performed. It becomes unnecessary and the dew point in the reducing atmosphere furnace can be controlled at low cost. As described above, according to FIGS. 1 and 2, highly accurate dew point control is possible in the reducing atmosphere furnace (reduction annealing step).

The composition of the metal band to be annealed in the present invention is not particularly limited. If it is necessary to raise the dew point in the reducing atmosphere furnace to a desired range in order to achieve a desired surface texture, it can be applied to a metal band having an arbitrary composition.

As an example of the metal strip, a steel strip, particularly a steel strip containing an easily oxidizing element such as Si or Mn, can be mentioned as an example. As the steel strip, for example, the following composition can be exemplified.

In the present invention, the composition of the steel sheet is preferably in the following range.

In addition,% notation about component means mass% unless otherwise specified.

C: 0.03 to 0.35%
C has the effect of increasing the strength of the steel strip. For that purpose, 0.03% or more is required. On the other hand, if it exceeds 0.35%, the weldability required for use as a material for automobiles and home appliances deteriorates. Therefore, the amount of C may be 0.03% or more and 0.35% or less.

Si: 0.01 to 3.00%
Si is an element effective for strengthening steel and improving ductility, and 0.01% or more is required for that purpose. On the other hand, if it exceeds 3.00%, Si forms an oxide on the surface and the appearance of plating deteriorates. Therefore, the amount of Si may be 0.01% or more and 3.00% or less.

Mn: 0.5 to 3.0%
Mn is an element useful for enhancing hardenability and increasing the strength of steel sheets. The effect cannot be obtained below 0.5%. On the other hand, if the content exceeds 3.0%, segregation of Mn occurs and the workability is lowered. Therefore, the amount of Mn is preferably 0.5 to 3.0%.

Al: 0.001 to 1.000%
Al is added for the purpose of deoxidizing molten steel, but if the content is less than 0.001%, the purpose is not achieved. On the other hand, if it exceeds 1.000%, Al forms an oxide on the surface, and the plating appearance (surface appearance) deteriorates. Therefore, the amount of Al may be 0.001% or more and 1.000% or less.

P: 0.10% or less P is one of the elements inevitably contained, and in order to make it less than 0.005%, there is a concern that the cost will increase, so 0.005% or more is desirable. On the other hand, the slab manufacturability deteriorates as P increases. Further, the content of P suppresses the alloying reaction and causes uneven plating. In order to suppress these, it is necessary to reduce the content to 0.10% or less. Therefore, the amount of P may be 0.10% or less. It is preferably 0.05% or less.

S: 0.01% or less S is an element inevitably contained in the steelmaking process. However, if it is contained in a large amount, the weldability deteriorates. Therefore, S may be 0.01% or less.
The rest is Fe and unavoidable impurities.

For the following purposes, B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, Cr: 0.001 to 1.000%. , Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Sb: 0.001 to 0.200% 1 More than a species of element may be contained as needed.

The appropriate content when these elements are added and the reasons for their limitation are as follows.

B: 0.001 to 0.005%
When B is 0.001% or more, the quenching promoting effect can be obtained. On the other hand, if it exceeds 0.005%, the plating adhesion deteriorates. Therefore, when it is contained, the amount of B may be 0.001% or more and 0.005% or less.

Nb: 0.005 to 0.050%
When Nb is 0.005% or more, the effect of strength adjustment (strength improvement) can be obtained. On the other hand, if it exceeds 0.050%, the cost will increase. Therefore, when it is contained, the amount of Nb may be 0.005% or more and 0.050% or less.

Ti: 0.005 to 0.080%
When Ti is 0.005% or more, the effect of strength adjustment (strength improvement) can be obtained. On the other hand, if it exceeds 0.080%, the plating adhesion deteriorates. Therefore, when it is contained, the amount of Ti may be 0.005% or more and 0.080% or less.

Cr: 0.001 to 1.000%
A quenching effect can be obtained when Cr is 0.001% or more. On the other hand, if it exceeds 1.000%, the surface of Cr becomes thickened, so that the weldability deteriorates. Therefore, when it is contained, the amount of Cr may be 0.001% or more and 1.000% or less.

Mo: 0.05 to 1.00%
When Mo is 0.05% or more, the effect of strength adjustment (strength improvement) can be obtained. On the other hand, if it exceeds 1.00%, the cost will increase. Therefore, when it is contained, the amount of Mo may be 0.05% or more and 1.00% or less.

Cu: 0.05 to 1.00%
When Cu is 0.05% or more, the effect of promoting the formation of the residual γ phase can be obtained. On the other hand, if it exceeds 1.00%, the cost will increase. Therefore, when it is contained, the amount of Cu may be 0.05% or more and 1.00% or less.

Ni: 0.05 to 1.00%
When Ni is 0.05% or more, the effect of promoting the formation of the residual γ phase can be obtained. On the other hand, if it exceeds 1.00%, the cost will increase. Therefore, when it is contained, the amount of Ni may be 0.05% or more and 1.00% or less.

Sb: 0.001 to 0.200%
Sb can be contained from the viewpoint of suppressing nitriding, oxidation, or decarburization of the steel sheet surface in a region of several tens of microns caused by oxidation. By suppressing nitriding and oxidation, it is possible to prevent the amount of martensite produced on the surface of the steel sheet from decreasing, and improve fatigue characteristics and surface quality. Such an effect can be obtained at 0.001% or more. On the other hand, if it exceeds 0.200%, the toughness deteriorates. Therefore, when it is contained, the amount of Sb may be 0.001% or more and 0.200% or less.

Such steel strips can be produced by known or arbitrary methods. For example, a slab having the above composition is heated and hot-rolled to obtain a hot-rolled steel sheet, and after pickling the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled as necessary to obtain a cold-rolled steel sheet. can do.

When the Si or Mn-containing steel as described above is applied to the reduction annealing step of the present embodiment, the iron oxide formed on the surface of the steel plate in the oxidation treatment step is reduced, and the oxygen supplied from the iron oxide reduces the iron oxide. The alloying elements of Si and Mn are formed as internal oxides inside the steel plate. As a result, a reduced iron layer reduced from the iron oxide is formed on the outermost surface of the steel sheet, and Si and Mn remain inside the steel sheet as internal oxides, so that oxidation of Si and Mn on the surface of the steel sheet is suppressed and the steel sheet is steel. It is possible to prevent deterioration of the wettability of hot-dip galvanizing and to obtain good plating adhesion without non-plating.

However, although good plating adhesion can be obtained, the alloying temperature of Si-containing steel is high, so that the retained austenite phase is decomposed into the pearlite phase and the martensite phase is tempered and softened, which is a desired machine. The characteristics may not be obtained. Therefore, as a result of conducting a technical study for reducing the alloying temperature, a technology for reducing the amount of solid solution Si on the surface layer of the steel sheet and accelerating the alloying reaction by forming the internal oxidation of Si more positively was developed. Invented. In order to form the internal oxidation of Si more positively, it is effective to control the atmospheric dew point in the reducing atmosphere furnace to −20 ° C. or higher.

When the dew point in the reducing atmosphere furnace is controlled to -20 ° C or higher, oxygen is supplied from the iron oxide, and even after the internal oxide of Si is formed, the internal oxidation of Si is performed by the oxygen supplied from the moisture in the atmosphere. Due to the continued occurrence of, more internal oxidation of Si is formed. Then, the solid solution Si decreases in the inner region of the surface layer of the steel sheet in which the internal oxidation is formed. When the amount of solid solution Si decreases, the surface layer of the steel sheet behaves as if it were low Si steel, the subsequent alloying reaction is promoted, and the alloying reaction proceeds at a low temperature. As a result of lowering the alloying temperature, the desired strength can be obtained without improving ductility by maintaining the retained austenite phase at a high fraction and tempering softening of the martensite phase. In the reducing atmosphere furnace 4, when the dew point reaches + 10 ° C or higher, the steel plate base iron begins to oxidize, so the upper limit is 0 ° C for the reason of minimizing the dew point distribution uniformity and the dew point fluctuation range in the reducing atmosphere furnace. It is preferable to manage it.

If it is necessary to raise the dew point in the furnace to a desired range, the conditions of the reduction annealing step are not limited. Examples of the conditions for treating the Si or Mn-containing steel include, for example, a maximum steel sheet temperature of 700 ° C. to 900 ° C. and a heating time of 30 seconds or more.

The present invention will be described in more detail by way of examples.

Annealing and hot-dip galvanizing were performed in a continuous hot-dip galvanizing facility where the heating furnace was a DFF (direct flame type) and the soaking furnace was a radiant tube (RTF) type. The composition of the steel sheet is mass%, C: 0.13, Si: 1.4, Mn: 1.9, P: 0.01, S: 0.001, balance Fe and unavoidable impurities.

Next, an alloying treatment was performed to produce an alloyed hot-dip galvanized steel sheet. In the heating furnace, a DFF obtained by dividing the burner for the heating furnace into four groups (# 1 to # 4) is used, and the three groups (# 1 to # 3) (previous stage) on the upstream side in the steel sheet moving direction are the oxidation zone and the final. The zone (# 4) (second stage) was a reduction zone, and the air ratio of the oxidation zone was controlled to 1.10 to 1.15 and the air ratio of the reduction zone was controlled to 0.85. The temperature of the steel sheet on the exit side of the heating furnace was 718 to 722 ° C. The length of each zone is 4 m.

As the heat equalizing furnace, the reducing atmosphere furnace shown in FIG. 2 was used. As shown in FIG. 2, dry nitrogen gas, hydrogen gas, and dry air were mixed and then supplied to the reducing atmosphere furnace. As shown in FIG. 2, the supply gas supply ports are three places in the lower part of the furnace, three places in the middle stage of the furnace, and three places in the upper stage of the furnace.

The maximum flow rate of the supplied mixed gas was 350 Nm ³ / Hr, and the flow rate of the mixed air was adjusted between 0.2 and 1.0 m ³ / Hr so that the oxygen concentration was 0.001 to 0.1 vol%.

The hydrogen concentration in the reducing atmosphere furnace was set to 15 vol%. The temperature inside the reducing atmosphere furnace was 800 to 830 ° C. The plating bath temperature was adjusted to 460 ° C., the Al concentration in the plating bath was 0.13% by mass, and the adhesion amount was adjusted to 45 g / m ² per side by gas wiping. The alloying temperature was adjusted to 510 to 530 ° C., and the alloying treatment was performed in an induction heating type alloying furnace so that the degree of film alloying (Fe content) was within 10 to 13% by mass.

For comparison, a conventional humidifier having a hollow fiber membrane (FIG. 3) was used as the soaking furnace. In the hollow fiber membrane humidification method, the total amount of gas charged into the reducing atmosphere furnace is 350 Nm ³ / Hr at the maximum, and it consists of 12 membrane modules, and each membrane module has a maximum of 0.5 Nm ³ / Hr of nitrogen gas and hydrogen. The mixed gas of the above and the circulating water of a maximum of 0.6 m ³ / Hr were allowed to flow. The circulating constant temperature water tank can supply a total of 6.0 m ³ / hr of pure water. The same applies to the above embodiment except for the humidifying device.

The plated appearance of the alloyed hot-dip galvanized steel sheet obtained as described above was evaluated.

The appearance of the plating was evaluated by inspection with an optical surface defect meter (detection of non-plating defects and peroxidizing defects having a diameter of 0.5 mm or more) and visual determination of uneven alloying. The plating appearance of the invention example was the same as that of the comparative example, and the yield was also the same.

The oxygen concentration in the furnace in the invention example was in the range of 50 to 400 ppm, and the oxygen concentration in the furnace in the comparative example was in the range of 20 to 200 ppm, and there was a significant difference in the oxygen concentration in the furnace between the invention example and the comparative example. Was not seen.

The obtained results are shown in FIG. FIG. 4 shows the transition of the average value of the production time and the dew point measured at the upper, interrupted, and lower three points of the reduction zone. From FIG. 4, in the example of the present invention, the dew point can be controlled by converging in the range of −20 ° C. to 0 ° C. in the same time as the dew point control by the conventional humidifier having a hollow fiber membrane shown as a comparative example. I understand.

In addition, the surface appearance was good, and the material strength also satisfied the strength level required from the steel sheet component.

In the method for controlling the dew point of the reducing atmosphere furnace and the reducing atmosphere furnace of the present invention, since the dew point can be controlled with high accuracy, it can be stably used for annealing steel containing 0.1% by mass or more of Si, and is melted. It can be widely used in the field of manufacturing galvanized steel sheets and cold-rolled steel sheets.

1 Steel plate 2 Roll 3 Direct flame type heating zone (DFF)
4 Reducing atmosphere furnace (radiant tube type)
5 Quenching zone 6 Slow cooling zone 7 Plating device 8 Nitrogen gas and hydrogen gas supply flow rate adjusting device 9 Oxygen-containing gas supply flow rate adjusting device 10 Nitrogen gas, hydrogen gas and oxygen-containing gas mixed flow rate adjusting device 11 Supply mixed gas oxygen concentration meter 12 Oxygen-containing gas drying device 14 Dew point measurement point in the furnace 15 Supply gas piping 16 Gas distribution device 17 Gas mixing device 18 Dew point meter for supply gas 19 Humidifying device 20 Circulating constant temperature water tank

The present invention has been made based on the following findings, and the gist thereof is as follows.
[1] At least hydrogen gas is used as the reducing gas to be supplied to the reducing atmosphere furnace when the cold-rolled steel sheet is annealed, and the reducing gas is mixed with a gas containing oxygen and supplied in the reducing gas. A method for controlling the dew point of a reducing atmosphere furnace, which comprises adjusting the dew point in the reducing atmosphere furnace by controlling the oxygen concentration of the reducing atmosphere furnace.
[2] When the cold-rolled steel sheet is annealed, nitrogen gas and hydrogen gas are used as the reducing gas to be supplied to the reducing atmosphere furnace, and the reducing gas is mixed with a gas containing oxygen and supplied. A method for controlling the dew point of a reducing atmosphere furnace, which comprises adjusting the dew point in the reducing atmosphere furnace by controlling the oxygen concentration in the reducing atmosphere.
[3] The method for controlling the dew point of a reducing atmosphere furnace according to the above [1] or [2] , wherein the gas containing oxygen is air.
[4] The dew point control method for a reducing atmosphere furnace according to any one of [1] to [3], wherein the oxygen concentration of the reducing gas is controlled to exceed 0 vol% and 2 vol% or less.
[5] The dew point control method for a reducing atmosphere furnace according to any one of [1] to [ 4 ], wherein the dew point in the reducing atmosphere furnace is adjusted to −20 ° C. to 0 ° C.
[6] A method for producing a cold-rolled steel sheet, which comprises producing a steel sheet by using the dew point control method of the reducing atmosphere furnace according to any one of the above [1] to [ 5 ]. By the dew point control method of the reducing atmosphere furnace according to any one of 1] to [ 5 ], the dew point of the reducing atmosphere furnace is controlled, the cold-rolled steel sheet is annealed, and then hot-dip galvanizing is continuously performed. A method for manufacturing a hot-dip galvanized steel sheet.
[8] A reducing atmosphere furnace for a cold-rolled steel plate anneaizer, which supplies a heating device using a radiant tube, a device A consisting of at least a pipe for supplying hydrogen gas and a supply flow rate adjusting device thereof, and a gas containing oxygen. A device B composed of a pipe and a supply amount adjusting device thereof, a device C for adjusting the flow rate by mixing the hydrogen gas supplied from the device A and a gas containing oxygen supplied from the device B, and the mixed gas. A reducing atmosphere furnace including an oxygen concentration measuring device inside, a pipe for supplying a mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace.
[9] A reducing atmosphere furnace for a cold-rolled steel plate anneaizer, which consists of a heating device using a radiant tube, a pipe for supplying nitrogen gas and hydrogen gas, and a device A for adjusting the supply flow rate thereof, and a gas containing oxygen. A device B including a supply pipe and a supply amount adjusting device, a device C that mixes nitrogen gas and hydrogen gas supplied from the device A, and a gas containing oxygen supplied from the device B to adjust the flow rate. Reduction provided with an oxygen concentration measuring device in the mixed gas, a pipe for supplying the mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace. Sexual atmosphere furnace.
[10] Continuous hot-dip galvanizing equipment A reducing atmosphere furnace for a steel plate anneaizer, which includes a heating device using a radiant tube, a device A including at least a pipe for supplying hydrogen gas and a supply flow rate adjusting device thereof, and oxygen. A device B including a pipe for supplying gas and a supply amount adjusting device thereof, a device C for mixing the hydrogen gas supplied from the device A and a gas containing oxygen supplied from the device B, and adjusting the flow rate. Reduction provided with an oxygen concentration measuring device in the mixed gas, a pipe for supplying the mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace. Sexual atmosphere furnace.
[11] Continuous hot-dip zinc plating equipment A reducing atmosphere furnace for a steel plate anneaizer, which includes a heating device using a radiant tube, a device A including at least a pipe for supplying hydrogen gas and a supply flow rate adjusting device thereof, and oxygen. A device B consisting of a pipe for supplying gas and a supply amount adjusting device thereof, and a device C for mixing a gas containing nitrogen gas and hydrogen gas supplied from the device A and an oxygen supplied from the device B to adjust the flow rate. A device for measuring the oxygen concentration in the mixed gas, a pipe for supplying the mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace. Reducing atmosphere furnace.

Claims (8)

When quenching a cold-rolled steel sheet, nitrogen gas and hydrogen gas are used as the reducing gas to be supplied to the reducing atmosphere furnace, and the reducing gas is mixed with a gas containing oxygen, and the oxygen in the reducing gas supplied is supplied. A method for controlling the dew point of a reducing atmosphere furnace, which comprises adjusting the dew point in the reducing atmosphere furnace by controlling the concentration. The dew point control method for a reducing atmosphere furnace according to claim 1, wherein the gas containing oxygen is air. The dew point control method for a reducing atmosphere furnace according to claim 1 or 2, wherein the oxygen concentration of the reducing gas is controlled to exceed 0 vol% and 2 vol% or less. The dew point control method for a reducing atmosphere furnace according to any one of claims 1 to 3, wherein the dew point in the reducing atmosphere furnace is adjusted to −20 ° C. to 0 ° C. A method for producing a cold-rolled steel sheet, which comprises producing a steel sheet by using the dew point control method of the reducing atmosphere furnace according to any one of claims 1 to 4. By the method for controlling the dew point of the reducing atmosphere furnace according to any one of claims 1 to 4, the dew point of the reducing atmosphere furnace is controlled, the cold-rolled steel sheet is annealed, and then hot-dip galvanizing is continuously performed. A method for manufacturing a hot-dip galvanized steel sheet. It is a reducing atmosphere furnace of a cold-rolled steel plate incubator, and consists of a heating device using a radiant tube, a pipe for supplying nitrogen gas and hydrogen gas, and a device A for adjusting the supply flow rate, and a pipe for supplying gas containing oxygen. And the device B composed of the supply amount adjusting device, the device C for mixing the nitrogen gas and hydrogen gas supplied from the A, and the gas containing oxygen supplied from the B to adjust the flow rate, and the mixed gas. An oxygen concentration measuring device, a pipe for supplying a mixed gas supplied from the device C to the reducing atmosphere furnace, and a reducing atmosphere furnace provided with a dew point measuring device installed in the reducing atmosphere furnace. Continuous hot-dip zinc plating equipment It is a reducing atmosphere furnace of a steel plate anneaizer, a heating device using a radiant tube, a device A consisting of a pipe for supplying nitrogen gas and hydrogen gas and a supply flow rate adjusting device, and a gas containing oxygen. A device B consisting of a pipe for supplying the gas and a supply amount adjusting device thereof, a device C for mixing a gas containing nitrogen gas and hydrogen gas supplied from the A and an oxygen supplied from the B, and adjusting the flow rate, and the above. Reducibility including an oxygen concentration measuring device in the mixed gas, a pipe for supplying the mixed gas supplied from the device C to the reducing atmosphere furnace, and a dew point measuring device installed in the reducing atmosphere furnace. Atmosphere furnace.