JP5990892B2 - Method for producing high-Si cold-rolled steel sheet with excellent chemical conversion properties - Google Patents

Description

  The present invention relates to a method for producing a high-Si cold-rolled steel sheet that is used after being subjected to a chemical conversion treatment such as a phosphate treatment, and in particular, a chemical conversion treatment at a tensile strength of 590 MPa or more utilizing the strengthening ability of Si. The present invention relates to a method for producing a high-Si cold-rolled steel sheet having excellent properties.

  In recent years, from the viewpoint of reducing the weight of automobiles, there is an increasing demand for cold-rolled steel sheets having a high tensile strength of 590 MPa or more. In addition, cold-rolled steel sheets for automobiles are used after being coated, and as a pretreatment for the coating, chemical conversion treatment such as phosphate treatment is performed. The chemical conversion treatment of the cold-rolled steel sheet is one of important treatments for ensuring the corrosion resistance after painting.

In order to increase the strength of the cold rolled steel sheet, addition of Si is effective. However, in a steel sheet to which Si is added (high-strength cold-rolled steel sheet), Si is oxidized even in a reducing N ₂ + H ₂ gas atmosphere in which no oxidation of Fe occurs (reducing Fe oxide) during continuous annealing. Then, a thin film of Si oxide (SiO ₂ ) is formed on the outermost surface of the steel plate. Since this Si oxide (SiO ₂ ) thin film inhibits the formation reaction of the chemical conversion film during the chemical conversion treatment, a micro region (scaling) in which the chemical conversion film is not formed is formed, and the chemical conversion treatment performance is lowered.

  In contrast to the above, as a conventional technique for improving the chemical conversion processability of a high-Si cold-rolled steel sheet, Patent Document 1 discloses that an oxide film is formed on the steel sheet surface by causing the steel sheet temperature to reach 350 to 650 ° C. in an oxidizing atmosphere. And then heated to a recrystallization temperature in a reducing atmosphere and cooled.

  Patent Document 2 discloses a cold-rolled steel sheet containing, by mass%, Si of 0.1% or more and / or Mn of 1.0% or more in an iron oxidizing atmosphere at a steel plate temperature of 400 ° C. or more. A method is described in which an oxide film is formed on the surface of the steel sheet, and then the oxide film on the surface of the steel sheet is reduced in an iron reducing atmosphere.

  Further, Patent Document 3 discloses an oxidation effective for improving chemical conversion treatment properties, etc. in the crystal grain boundaries and / or crystal grains of the surface layer of a high strength cold-rolled steel sheet containing 0.1 wt% or more and 3.0 wt% or less of Si. A high-strength cold-rolled steel sheet characterized by having an article is described.

  In Patent Document 4, when a cross section in a direction orthogonal to the steel plate surface is observed with an electron microscope at a magnification of 50000 times or more, the ratio of the Si-containing oxide in the steel plate surface length of 10 μm is arbitrarily selected. A steel sheet excellent in phosphatability is described so as to be 80% or less on the average of the places.

  Patent Document 5 includes mass%, C: more than 0.1%, Si: 0.4% or more, and Si content (mass%) / Mn content (mass%) is 0.4 or more. The surface coverage of the Si-based oxide containing Si as a main component on the steel sheet surface is 20 area% or less, and the Si-based oxide is covered within the region. A high-strength cold-rolled steel sheet excellent in chemical conversion treatment with a maximum circle diameter of 5 μm or less in contact is described.

  In Patent Document 6, in mass%, C: 0.01 to 0.3%, Si: 0.2 to 3.0%, Mn: 0.1 to 3.0%, Al: 0.01 to 2 In a high-strength steel plate containing 0.0% and a tensile strength of 500 MPa or more, an observation region having an average grain size of 0.5 μm or less on the surface of the steel plate and a width of 10 μm or more on the surface of the steel plate is taken as a cross-sectional TEM. When a thin piece is processed for observation, and the thin piece sample is measured by TEM observation under the condition that an oxide of 10 nm or less can be observed, one or two kinds of silicon oxide and manganese silicate are 70% by mass or more in total of these. The oxide species to be contained is 30% or less with respect to the grain boundary region surface viewed from the cross section, and the particle size of the oxide species present within a range of 0.1 to 1.0 μm in depth from the steel plate surface. For chemical conversion treatment, characterized by being 0.1 μm or less High-tensile steel plate is described which.

JP 55-145122 A JP 2006-45615 A JP 9-310148 A JP 2003-113441 A JP 2004-323969 A JP 2008-69445 A JP 2010-53446 A

  However, in the manufacturing method of Patent Document 1, there is a difference in the thickness of the oxide film formed on the surface of the steel sheet by the oxidation method, and sufficient oxidation does not occur, or the oxide film becomes too thick, and the subsequent reducing atmosphere. During the annealing, the oxide film may remain or peel off, and the surface properties may deteriorate. In the examples, a technique for oxidizing in the air is described. However, in the oxidation in the air, a thick oxide is formed and subsequent reduction is difficult, or a reducing atmosphere with a high hydrogen concentration is required. There is a problem.

The manufacturing method of Patent Document 2 is an N ₂ + H ₂ gas atmosphere in which Fe on the steel sheet surface is oxidized using a direct fire burner having an air ratio of 0.93 or more and 1.10 or less at 400 ° C. or higher, and then Fe oxide is reduced. Is a method of suppressing oxidation at the outermost surface of SiO ₂ that lowers the chemical conversion processability by annealing and forming a reduced layer of Fe on the outermost surface. Patent Document 2 does not specifically describe the heating temperature in an open flame burner, but when it contains a large amount of Si (0.6% or more), the amount of oxidation of Si that is easier to oxidize than Fe is large. Therefore, oxidation of Fe is suppressed, or oxidation of Fe itself is too little. As a result, formation of the surface Fe reduction layer after reduction was insufficient, and SiO ₂ was present on the steel plate surface after reduction, and there was a case where the conversion film was scaled.

  The steel plate of patent document 3 is a steel plate which improves chemical conversion property by forming Si oxide in the inside of a steel plate, and eliminating Si oxide on the surface. The manufacturing method involves winding at a high temperature (in the embodiment, good at 620 ° C. or higher) at the time of hot rolling before the cold rolling of the steel sheet, and using that heat to form Si oxide inside the steel sheet. However, the wound coil has a fast outer cooling rate and a slow inner cooling rate, which causes large temperature unevenness in the longitudinal direction of the steel sheet, and it is difficult to obtain uniform surface quality over the entire length of the coil. .

Patent Documents 4, 5, and 6 are steel sheets that define the upper limit of the amount of Si oxide covering the surface, although the way of defining is different. As a manufacturing method, the dew point (or steam hydrogen partial pressure ratio) of the reducing N ₂ + H ₂ gas atmosphere is controlled within a certain range during temperature rise or soaking of continuous annealing, and Si is oxidized inside the steel sheet. is there. The dew point range is described in Patent Document 4 as -25 ° C or higher, and in Patent Document 5 as -20 ° C to 0 ° C. Patent Document 6 adopts a method of regulating the range of the steam hydrogen partial pressure ratio in each step of preheating, temperature elevation, and recrystallization. In these methods, it is generally necessary to control the N ₂ + H ₂ gas atmosphere with a dew point of −25 ° C. or lower by introducing water vapor or air, which is problematic from the viewpoint of operational controllability. As a result, good chemical conversion properties could not be stably obtained. In addition, increasing the dew point (or increasing the steam hydrogen hydrogen partial pressure ratio) increases the oxidization of the atmosphere, so it accelerates the deterioration of the furnace walls and rolls in the furnace, and generates scale soot called pick-up soot on the steel sheet surface. There was a case of letting.

In such a background, the inventors do not control the dew point of the reducing atmosphere of the soaking furnace or the steam hydrogen partial pressure ratio, and even if Si is contained in an amount of 0.6% or more, a good chemical conversion treatment is performed. Applied for a method of manufacturing a high-Si cold-rolled steel sheet having the properties (Patent Document 7). That is, in Patent Document 7, when a cold-rolled steel sheet is continuously annealed, the steel sheet is heated using a direct fire burner having a steel sheet temperature of at least 550 ° C. or higher and an air ratio of 0.95 or higher when the temperature is raised, and then the air ratio is 0. After heating the steel plate using an open flame burner of 89 or less until the steel plate temperature reaches 700 ° C. or higher, the dew point is −25 ° C. or lower, 1 to 10 vol% H ₂ + balance N ₂ gas atmosphere This is a method for producing a high-Si cold-rolled steel sheet having excellent chemical conversion properties, characterized by soaking in a furnace.

  According to Patent Document 7, it is possible to obtain a steel sheet with good chemical conversion property even when Si is contained in an amount of 0.6 wt% or more, but depending on the state of the steel sheet before continuous annealing, that is, the steel sheet after cold rolling. It became clear that the proper steel plate temperature range for heating at an air ratio of 0.95 or higher may be different. As a result of further detailed analysis, when the coil winding temperature at the end of hot rolling is high, the appropriate temperature range for heating with an air ratio of 0.95 or higher is on the low temperature side, and hot rolling When the coil winding temperature at the end is low, the appropriate temperature range for heating at an air ratio of 0.95 or higher is on the high temperature side, and thus it has become clear that management during manufacture may be difficult. It has been clarified that the internal oxide layer formed at the time of hot rolling is greatly affected, and that the internal oxide layer formed at the time of hot rolling becomes thick when the coil winding temperature after the hot rolling is high. Even when the coil winding temperature is the same, the subsequent storage environment affects the internal oxide layer formed during hot rolling, so the storage environment, for example, the ambient temperature, the storage location, etc. are always constant. It became clear that there was a need. However, it is often difficult to keep the hot-rolled coil storage environment constant, and a technique for obtaining a cold-rolled steel sheet having good chemical conversion properties regardless of the storage environment is required. As a method for stably obtaining the internal oxide layer during hot rolling regardless of such a storage environment, a method for sufficiently increasing the coil winding temperature at the end of hot rolling can be mentioned. However, it has been found that if the temperature is too high, the corrosion resistance after painting may deteriorate. On the other hand, when the coiling temperature for hot rolling is lowered, the internal oxide layer is hardly formed regardless of the storage environment, so that this is an effective method for obtaining a hot-rolled steel sheet having a constant surface state. However, problems such as gauge hunting during cold rolling and excessive cold rolling load occur even at low coil temperatures. As described above, it is desired to obtain a cold-rolled steel sheet having good chemical conversion properties without impairing manufacturability even in a state where there is no internal oxide layer formed during hot rolling.

  In view of such circumstances, the present invention has a high chemical conversion treatment property without controlling the dew point of the reducing atmosphere of the soaking furnace or the steam hydrogen partial pressure ratio, and even if Si is contained in an amount of 0.6% or more. It aims at providing the manufacturing method of Si cold-rolled steel plate.

  As a result of intensive studies by the present inventors for solving the problems, the following findings were obtained.

By coating Ni on the surface of the steel plate before continuous annealing and performing a temperature increase treatment in an oxidizing atmosphere, even a steel plate without an internal oxide layer formed during hot rolling can have excellent chemical conversion treatment properties. I found out that
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.05 to 0.30%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1.00%, N: 0.01% or less, the inner oxide layer formed by hot rolling steel comprising the balance Fe and inevitable impurities After the thickness of the steel sheet is set to 2 μm or less, it is cold-rolled and then coated with Ni at 40 to 2000 mg / m ² , and then at least a steel plate using a direct flame burner with an air ratio of 0.95 or more. The temperature is raised from 550 ° C. to the steel plate temperature: 650 ° C., and the annealing is performed by soaking at a dew point of −25 ° C. or less, atmosphere: 1 to 10% by volume H ₂ + the balance N _2. A method for producing a high-Si cold-rolled steel sheet having excellent processability.
[2] The annealing is performed by heating the steel plate at least from a steel plate temperature: 550 ° C. to a steel plate temperature: 650 ° C. using an open flame burner having an air ratio of 0.95 or more, and then an air ratio of 0.89 or less. The temperature of the steel sheet is further raised by 30 ° C. or higher using an open flame burner, and the soaking is performed with dew point: −25 ° C. or lower, atmosphere: 1 to 10% by volume H ₂ + remaining N ₂ [1 ] The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion processability of description.
[3] The steel is further in mass%, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.00. The method for producing a high-Si cold-rolled steel sheet having excellent chemical conversion treatment properties according to the above [1] or [2], comprising one or more of 01 to 1.0%.
[4] The steel is further in mass%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, 1 type or 2 The method for producing a high-Si cold-rolled steel sheet having excellent chemical conversion properties according to any one of the above [1] to [3], which contains seeds or more.
[5] The steel is further excellent in chemical conversion treatment according to any one of the above [1] to [4], wherein the steel further contains B: 0.0003 to 0.005% by mass%. Manufacturing method of high Si cold-rolled steel sheet.
[6] In any one of the above [1] to [5], in the continuous annealing, heating using an open flame burner having an air ratio of 0.95 or more is performed up to a steel plate temperature: 700 ° C. The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion property of description.
[7] In any one of the above [1] to [6], in the continuous annealing, heating using an open flame burner having an air ratio of 0.95 or more is performed up to a steel plate temperature: 800 ° C. The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion property of description.
[8] In the continuous annealing, the heating time using a direct fire burner having an air ratio of 0.95 or more is longer than the heating time using a direct fire burner having an air ratio of 0.89 or less. The manufacturing method of the high Si cold-rolled steel plate excellent in chemical conversion property in any one of said [2]-[7].
[9] In any of the above [2] to [6], [8], in the continuous annealing, the heating using a direct fire burner having an air ratio of 0.89 or less is performed up to a steel plate temperature: 800 ° C. The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion property of the crab.
In addition, in this specification,% which shows the component of steel is mass% altogether. In the present invention, the “high-strength cold-rolled steel sheet” is a cold-rolled steel sheet having a tensile strength TS of 590 MPa or more.

  According to the present invention, oxidation of Ni and Fe on the surface of a steel sheet using an open flame burner and subsequent reduction are used to oxidize a part of Si into the steel sheet, and a part of Si is made of Ni-Si. By making it a complex oxide, the high Si cold-rolled steel sheet containing 0.6% or more of Si improves the chemical conversion processability, has a tensile strength of 590 MPa or more, and TS × El is 18000 MPa ·% or more and is workable. An excellent high Si cold-rolled steel sheet can be produced. In addition, control of the annealing atmosphere (especially controlling the dew point to a high level) is unnecessary, which is advantageous in terms of operational controllability, speeds up deterioration of the furnace wall and rolls in the furnace, and scales called pickups. It is also possible to improve the problem of generating slag on the steel sheet surface.

Sectional drawing which shows the result of having observed the cross section sheared in the arbitrary parts of a hot-rolled board with SEM.

  The reason for limiting the chemical components of the steel sheet to which the present invention is applied will be described. In addition, unless otherwise indicated, the "%" display regarding a component means the mass%.

  Si is an element that increases the strength without reducing the workability of the steel sheet, and if it is less than 0.6%, the workability, that is, TS × El is deteriorated. Therefore, it contains 0.6% or more. Preferably it contains more than 1.10%. However, if it exceeds 3.0%, the steel sheet becomes extremely brittle, the workability deteriorates, and the chemical conversion property deteriorates, so the upper limit is made 3.0%.

  In addition to Si, the chemical composition of the steel sheet is controlled to have a metal structure of ferrite-martensite, TRIP, etc., and in order to obtain a desired material, C and Mn having solid solution strengthening ability and martensite forming ability are added. Is 0.05% or more, preferably 0.10% or more, and Mn is 1.0% or more. On the other hand, when C and Mn are added excessively, the workability of the steel sheet is remarkably lowered, so C is 0.30% or less and Mn is 3.0% or less.

  Al is added as a deoxidizing material. If it is less than 0.01%, the effect is insufficient. On the other hand, if it exceeds 1.00%, the effect is saturated and uneconomical. Therefore, the Al amount is set to 0.01 to 1.00%.

  In addition, P, S, and N are contained as inevitable impurities. P is 0.1% or less, preferably 0.015% or less. S is 0.05% or less, preferably 0.003% or less. N is 0.01% or less.

  The balance is Fe and inevitable impurities.

  Moreover, in order to control a material and a metal structure, you may contain 1 type, or 2 or more types of Cr, Mo, Ni, Cu in 0.01 to 1.0% of range, respectively. In order to increase the strength of the steel sheet, one or more of Ti, Nb, and V may be contained in a range of 0.001 to 0.1%, respectively. In order to raise the intensity | strength of a raw material and the intensity | strength after coating baking, you may contain B in 0.0003 to 0.005% of range.

Next, the manufacturing method of the high Si cold-rolled steel plate excellent in chemical conversion treatment property of the present invention will be described.
The steel having the above component composition is hot-rolled, pickled as necessary, cold-rolled, then coated with Ni on the steel sheet, and then continuously annealed in a continuous annealing line. However, the thickness of the internal oxide layer of the steel sheet before cold rolling is 2 μm or less. Moreover, continuous annealing uses a direct-fired burner with an air ratio of 0.95 or more to raise the temperature of the steel plate from at least the steel plate temperature: 550 ° C. to the steel plate temperature: 650 ° C., dew point: −25 ° C. or lower, atmosphere: 1 Soaking is performed with 10% by volume H ₂ + the balance N ₂ .

  The manufacturing method of the cold rolled steel plate before cold rolling is not particularly limited, and a known method can be used.

However, in the present invention, the thickness of the internal oxide layer formed during hot rolling is 2 μm or less. It has been found that when an internal oxide layer formed during hot rolling is present, an oxidation treatment in a continuous annealing furnace can be easily achieved, so that good chemical conversion properties can be obtained by oxidation treatment and reduction treatment. However, on the other hand, the corrosion resistance after coating is deteriorated in the environment used by the user. The cause of this is not clear, but is considered as follows. The internal oxide layer formed during hot rolling is an oxide layer formed along the grain boundaries as shown in FIG. And it can be considered that salt flows in along the grain boundary, reduces the adhesion between the coating (coating film) and the steel sheet, and deteriorates the corrosion resistance.
On the other hand, since the internal oxide layer formed during the continuous annealing exists in the grains, the above-mentioned corrosion resistance after coating does not deteriorate. In the present invention, the internal oxide layer formed during hot rolling has a thickness of 2 μm or less, that is, the steel plate before cold rolling is a steel plate having no internal oxide layer, and the internal oxide layer is formed during continuous annealing. As a result, the internal oxide layer formed according to the present invention is considered to work well for the corrosion resistance after coating.
In the present invention, the steel plate having no internal oxide layer is a steel plate having an internal oxide layer thickness of 2 μm or less formed during hot rolling. In addition, as a method of obtaining a steel sheet in which the thickness of the internal oxide layer formed at the time of hot rolling is 2 μm or less, a method for lowering the hot rolling coiling temperature, a cold rolling, before charging in a continuous annealing line. In addition, there is a method of removing by a method such as pickling or mechanical grinding. Further, the thickness of the internal oxide layer formed during hot rolling can be observed by cross-sectional SEM observation sheared at an arbitrary portion, and the cracked internal oxide layer streaks as shown in FIG. It is assumed that it is 2 μm or less in the depth direction.

After cold rolling, Ni is coated to 40 to 2000 mg / m ² .
The Ni previous surface of the steel sheet of the continuous annealing is necessary to 40mg ^{/ m 2} ~2000mg ^/ m 2 coating. When the coating amount is less than 40 mg / m ² , there is no effect on oxidation and good chemical conversion treatment properties cannot be obtained. Even if the coating is more than 2000 mg / m ² , there is no problem in performance because the influence on the subsequent chemical conversion treatment is small, but this is not preferable because the line speed, the amount of electricity and the equipment cost at the time of production increase.

  The method of coating Ni on the steel sheet surface is not limited. Although an electrolytic method, a vapor deposition method, etc. are mentioned, an electrolytic method is preferable from a viewpoint of manufacturing cost. In the case of the electrolytic method, there is no limitation on the plating bath composition, and it is common to use a sulfate solution, a chloride solution, or a composite solution thereof. Moreover, there is no limitation also about the additive to an electroplating bath, You may add the high molecular compound etc. for improving electrolysis efficiency.

  The effect of coating Ni on the steel sheet surface (effect by Ni plating) is not clear, but can be considered as follows. When the steel sheet is heated in an oxidizing atmosphere, the oxidation of iron occupying most of the surface of the steel sheet and the oxidizable elements of the steel sheet components are oxidized. When a large amount of Si is contained in the steel plate, Si oxide is formed on the steel plate surface. Here, the Si oxide changes depending on the steel plate component, and forms Si—Mn composite oxide when Mn is contained in a large amount, and forms silica (SiO 2) when the amount of Mn is small. When silica is formed, since silica has low oxygen permeability, a sufficient amount of oxidation of the steel sheet cannot be obtained in an oxidizing atmosphere, and Si does not oxidize internally and remains on the outer surface. As a result, chemical conversion processability is inhibited. On the other hand, by applying (coating) Ni to the steel sheet surface layer, Ni forms a complex oxide with Si, so that a sufficient amount of oxidation is obtained without forming silica, and Si is contained inside during subsequent reduction annealing. It can be oxidized. As a result, there is no silica on the surface layer, and chemical conversion treatment properties are improved. Even when the Ni—Si composite oxide remains on the surface layer, the Ni—Si composite oxide is easily soluble and dissolves during pickling in the subsequent manufacturing process, so that the chemical conversion treatment property is improved. Even in the absence of the pickling step, the chemical conversion treatment liquid generally used in an automobile production line is acidic, so that it dissolves in the chemical conversion treatment liquid and the chemical conversion treatment performance is improved. Moreover, since Ni is contained as a chemical conversion treatment liquid component used in the current automobile production line, the chemical conversion treatment liquid can be used without being contaminated.

Next, continuous annealing is performed.
In the continuous annealing line, three steps of continuous temperature rise, soaking, and cooling are performed. A general continuous annealing line includes a heating furnace for raising the temperature of a steel sheet, a soaking furnace for soaking, and a cooling furnace. Alternatively, a preheating furnace is further provided before the heating furnace.

  At the time of temperature increase, the temperature of the steel plate is raised from at least a steel plate temperature of 550 ° C. to a steel plate temperature of 650 ° C. in a heating furnace using a direct fire burner with an air ratio adjusted to 0.95 or higher. Thereby, Fe oxide is formed on the steel plate surface. There is no problem even if the temperature is raised with a direct flame burner in which the air ratio is adjusted to 0.95 or more from a steel plate temperature of less than 550 ° C. However, since a sufficient amount of Fe oxidation cannot be obtained in a temperature range of less than 550 ° C., a direct fire burner in which the air ratio is adjusted to 0.95 or more is used for raising the temperature of the steel plate from the steel plate temperature of 550 ° C. From the viewpoint of Fe oxide formation, it is better to reach the highest possible temperature, and the temperature is preferably raised to a steel plate temperature of 700 ° C., more preferably to a steel plate temperature of 750 ° C. However, excessive oxidation causes the Fe oxide to peel off in the next reducing atmosphere furnace and cause pickup, so the temperature rise using a direct fire burner with the air ratio adjusted to 0.95 or higher is the steel plate temperature. The temperature is preferably up to 800 ° C.

  Here, the direct-fired burner heats the steel sheet by directly applying the burner flame, which is burned by mixing fuel and air, such as coke oven gas (COG), which is a by-product gas of the steelworks, to the surface of the steel sheet. is there. The direct-fired burner has an advantage that the length of the heating furnace can be shortened and the line speed can be increased because the heating rate of the steel sheet is faster than that of the radiation method. Further, when the direct fire burner has an air ratio of 0.95 or higher and the ratio of air to fuel is increased, unburned oxygen remains in the flame, and the oxygen can promote oxidation of the steel sheet. The higher the air ratio, the stronger the oxidizability. From the viewpoint of Fe oxide formation, the air ratio should be as high as possible, and the air ratio is preferably 1.10 or more. However, if oxidized excessively, the Fe oxide is peeled off in the following soaking furnace in a reducing atmosphere, causing pickup, so the air ratio is preferably 1.30 or less.

  COG, liquefied natural gas (LNG), etc. can be used for the fuel of the direct fire burner.

  When the preheating furnace is provided before the heating furnace, the atmosphere of the preheating furnace is not particularly limited. The usual conditions may be used. For example, you may introduce the high temperature combustion gas of the heating furnace provided with the direct-fired burner into the preheating furnace. In addition, a direct fire burner having an air ratio of 0.7 to 1.3 can be used for the preheating furnace.

  In a heating furnace equipped with a direct-fired burner, from the viewpoint of preventing excessive oxidation of Fe, a direct-fired burner having an air ratio of 0.95 or more is used for the front stage of the heating furnace, and a direct-fired burner having an air ratio of 0.89 or less is used for the latter stage of the heating furnace. A fire burner may be used. In this case, a direct fire burner having an air ratio of 0.95 or more is used at least from a steel plate temperature of 550 ° C. to 650 ° C. in the pre-stage of the heating furnace, and then a steel plate is further separated by using a direct fire burner having an air ratio of 0.89 or less. If the temperature is raised at or above ° C, a sufficient amount of Fe oxidation can be obtained. From the viewpoint of Fe oxide formation, it is better to reach as high a temperature as possible, and it is preferable to raise the temperature to a steel plate temperature of 750 ° C. From the standpoint of the effect of obtaining a sufficient amount of Fe oxidation, the steel plate heating time with an open flame burner with an air ratio of 0.95 or more is equal to or longer than the steel plate heating time with a direct fire burner with an air ratio of 0.89 or less It is preferable to do. The air ratio of the direct-fired burner at the latter stage of the heating furnace is preferably 0.7 or more from the viewpoint of combustion efficiency. Furthermore, the temperature rising atmosphere using an open flame burner with an air ratio of 0.89 or less is Fe reducing, and if the temperature is raised to an excessively high temperature, the Fe oxide is reduced before entering the next soaking furnace. The generation of internal oxidation of Si due to the reduction of Fe oxide in a soaking furnace is reduced. Therefore, it is preferable to raise the temperature using an open flame burner with an air ratio of 0.89 or less up to a steel plate temperature of 800 ° C.

After heating the steel sheet as described above using a direct fire burner, for example, soaking is performed in a soaking furnace equipped with a radiant tube burner. The atmospheric gas introduced into the soaking furnace is 1 to 10% by volume H ₂ + the remaining N ₂ . If it is less than 1% by volume, H ₂ is insufficient to reduce the Fe oxide on the surface of the continuously passing steel sheet. On the other hand, even if it exceeds 10% by volume, the reduction of Fe oxide is saturated, so excess H ₂ is wasted. When the dew point exceeds -25 ° C, the oxidation of H ₂ O in the furnace by oxygen becomes significant and excessive internal oxidation of Si occurs, so the dew point is limited to -25 ° C or less. Thereby, the inside of the soaking furnace becomes a reducing atmosphere of Fe and Ni, and reduction of Fe oxide and Ni oxide generated in the heating furnace occurs. At this time, oxygen separated from Fe and Ni by reduction partially diffuses inside the steel plate and reacts with Si, thereby causing internal oxidation of SiO ₂ . Since Si is oxidized inside the steel sheet and the Si oxide on the outermost surface of the steel sheet where the chemical conversion reaction occurs is reduced, the chemical conversion property of the outermost surface of the steel sheet is improved.

  It is preferable that the soaking is performed in the range of 750 ° C. to 900 ° C. from the viewpoint of material adjustment. The soaking time is preferably 20 seconds to 180 seconds.

  The process after soaking is varied depending on the type, and the process is not particularly limited in the present invention. For example, after soaking, it is cooled with gas, air, water, etc., and tempered at 150 ° C. to 400 ° C. as necessary. In order to adjust the surface properties after cooling or tempering, pickling using hydrochloric acid or sulfuric acid may be performed.

Steel A having a chemical composition shown in Table 1 was hot-rolled, pickled, and cold-rolled by a known method to produce a steel plate having a thickness of 1.5 mm. Here, the coiling temperature after hot rolling was set to 540 ° C. to 620 ° C., and the thickness of the internal oxide layer formed during hot rolling was changed. The thickness of the internal oxide layer is measured by cross-sectional SEM observation after cold rolling, and the maximum internal oxide layer is defined as the internal oxide layer thickness in the central portion of 500 μm of the 10 mm cross-section SEM observation test piece. It shows in Table 2.
The steel plate obtained as described above was subjected to electro Ni plating with a plating facility. The plating bath is as follows.
(1) Nickel sulfate hexahydrate is 240 g / L, boric acid is 30 g / L, and the pH is adjusted to 3.0 using sulfuric acid. (2) Nickel chloride heptahydrate is 260 g / L, boric acid is 30 g. / L adjusted to pH 3.0 using hydrochloric acid (3) Nickel sulfate hexahydrate 200 g / L, nickel chloride heptahydrate 60 g / L, boric acid 30 g / L and sulfuric acid Then, the pH was adjusted to 3.0, and the electroplating conditions were such that Ni plating was performed by changing the energization time at a current density of 1 A / dm ² .
Next, the temperature is raised using a heating furnace equipped with a direct-fired burner, and after cooling, a radiant tube type soaking furnace and annealing equipment are used to simulate a cooling furnace to obtain a high-strength cold-rolled steel sheet. It was. The direct fire burner used COG as the fuel and changed the air ratio in various ways. Cooling after soaking was performed with nitrogen gas, and was held at the holding temperatures shown in Table 2. Furthermore, it pickled with the acid of Table 2, and evaluated mechanical characteristics, chemical conversion treatment property, and corrosion resistance after coating.

The pickling conditions are as follows.
Hydrochloric acid pickling: acid concentration 1-20%, liquid temperature 30-90 ° C, pickling time 5-30 sec
Acid pickling: acid concentration 1-20%, liquid temperature 30-90 ° C, pickling time 5-30 sec
For mechanical properties, a JIS No. 5 test piece (JISZ2201) was taken from the direction perpendicular to the rolling direction and tested according to JISZ2241. As strength after paint baking treatment, after 5% pre-strain, after holding at 170 ° C for 20 minutes, the tensile strength in re-tension (TS _BH ) was investigated and compared with the initial tensile strength (TS ₀ ) The difference was defined as ΔTS (TS _BH −TS ₀ ). Workability was evaluated by the value of tensile strength (TS) × elongation (El).

The evaluation method of chemical conversion property is as follows.
The chemical conversion treatment solution was subjected to chemical conversion treatment by the following method using a chemical conversion treatment solution (Palbond L3080 (registered trademark)) manufactured by Nippon Parkerizing Co., Ltd.
After degreasing with a degreasing liquid Fine Cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washed with water, and then surface-adjusted with a surface conditioning liquid preparen Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. for 30 seconds, 43 ° C. After being immersed in a chemical conversion treatment solution (Palbond L3080) for 120 seconds, it was washed with water and dried with warm air.
The chemical conversion film was randomly observed with a scanning electron microscope (SEM) at a magnification of 500 times, and the scale area ratio of the chemical conversion film was measured by image processing, and the following evaluation was made based on the scale area ratio. ○ and ◎ are acceptable levels.
A: 0% (no scale)
○: 5% or less Δ: Over 5% and 10% or less ×: Over 10% The corrosion resistance evaluation method after coating is as follows.
In the same manner as described above, after a chemical conversion treatment film was formed with a chemical conversion treatment liquid (Palbond L3080) manufactured by Nihon Parkerizing Co., Ltd., electrodeposition coating was performed with an electrodeposition coating liquid (GT-10) manufactured by Kansai Paint Co., Ltd. Then, the film was left in a furnace at 170 ° C. for 25 minutes to form a 20 μm electrodeposition film. Thereafter, the cut until reaching the steel plate with a cutter knife was put in an X shape, left in a 5% NaCl solution at 50 ° C. for 10 days, and the maximum value of the coating peeling length on one side from the cut portion was measured. Evaluation was carried out as follows from the coating peeling length.
○: Less than 2.5 mm x: 2.5 mm or more Table 2 shows the evaluation results obtained as described above together with the production conditions.

From Table 2, in the example of the present invention, good workability of tensile strength (TS) of 590 MPa or more, TS × El: 18000 MPa ·% or more, good chemical conversion property, and corrosion resistance after coating are obtained.
On the other hand, the chemical conversion processability is inferior in Comparative Examples 1-4. In Comparative Examples 5 to 11, the corrosion resistance after coating is inferior.
Furthermore, when the example which changed the thickness of the internal oxide layer formed at the time of hot rolling by changing the coiling temperature of hot rolling was evaluated, the comparative examples 5-7 which have an internal oxide layer of 1 micrometer or more In the case of, the improvement tendency of chemical conversion treatment was recognized by increasing the temperature of the open flame burner. Furthermore, in the case of Comparative Examples 8 and 9 having an internal oxide layer of about 3 μm, it can be seen that good chemical conversion properties are obtained even when the furnace exit side temperature is low.

Steels A to F having chemical components shown in Table 1 were hot-rolled, pickled, and cold-rolled by a known method to produce a steel plate having a thickness of 1.5 mm. The steel plate was subjected to Ni plating in a plating line to obtain 200 mg / m ² of Ni plating. The plating bath was 240 g / L nickel sulfate hexahydrate, 30 g / L boric acid, and the pH was adjusted to 3.0 using sulfuric acid. Energization conditions were 1A / dm ^2. Subsequently, the steel sheet was passed through the steel sheet through a continuous annealing line equipped with a preheating furnace, a heating furnace equipped with a direct-fired burner, a radiant tube type soaking furnace, and a cooling furnace to obtain a high-strength cold-rolled steel sheet. A heating furnace equipped with an open flame burner is divided into four zones, and each zone has the same length. The direct fire burner used COG as the fuel, and variously changed the air ratio between the front stage (3 zones) and the rear stage (1 zone) of the heating furnace. Cooling after soaking was performed with water, air or gas as shown in Table 3. At that time, in the case of water cooling, after cooling to the water temperature, it was reheated to the holding temperature shown in Table 3 and held. Moreover, in the case of air-water and gas cooling, it cooled to the holding temperature of Table 3, and hold | maintained as it was. Furthermore, it pickled with the acid of Table 3, or was made into the product as it was.

  Mechanical properties and chemical conversion properties were evaluated for the obtained high-strength cold-rolled steel sheets. The evaluation method of mechanical characteristics and chemical conversion treatment is the same as that in Example 1.

  The evaluation results obtained as described above are shown in Table 3 together with the manufacturing conditions.

In the example of the present invention, good workability and tensile conversion (TS) of 590 MPa or more, TS × El: 18000 MPa ·% or more are obtained.
On the other hand, the chemical conversion processability is inferior in the comparative example.

  Since the high Si cold-rolled steel sheet of the present invention has high strength and excellent chemical conversion properties, it can be used as a cold-rolled steel sheet for reducing the weight and strength of the automobile body itself. In addition to automobiles, the present invention can be applied in a wide range of fields such as home appliances and building materials.

Claims (9)

In mass%, C: 0.05 to 0.30%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05 % Of steel, Al: 0.01 to 1.00%, N: 0.01% or less of steel, the balance being Fe and inevitable impurities are hot-rolled and wound at a winding temperature of 540 to 580 ° C. Then, after the thickness of the formed internal oxide layer is made 2 μm or less, it is cold-rolled and then coated with Ni of 50 to 100 mg / m ² , and then the air ratio: 0.95 or more Using a fire burner, the temperature of the steel sheet is raised from at least a steel plate temperature: 550 ° C. to a steel plate temperature: 650 ° C., and annealing is performed with a dew point: −25 ° C. or less, atmosphere: 1 to 10% by volume H ₂ + the balance N ₂ The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion processability characterized by performing. The annealing is performed by heating the steel plate at least from a steel plate temperature: 550 ° C. to a steel plate temperature: 650 ° C. using a direct fire burner having an air ratio of 0.95 or more, and then an open flame burner having an air ratio of 0.89 or less. The steel sheet is further heated at 30 ° C. or higher using a heat treatment, and is soaked with dew point: −25 ° C. or lower, atmosphere: 1 to 10% by volume H ₂ + remaining N ₂ , A method for producing a high-Si cold-rolled steel sheet having excellent chemical conversion properties.   The steel is further in mass%, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1 The manufacturing method of the high Si cold-rolled steel sheet excellent in chemical conversion treatment property of Claim 1 or 2 characterized by containing 0.0% of 1 type (s) or 2 or more types.   The steel further includes one or more of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, and V: 0.001 to 0.1% in mass%. It contains, The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion treatment property of any one of Claims 1-3 characterized by the above-mentioned.   The steel further contains, in mass%, B: 0.0003 to 0.005%, and the high Si cooling excellent in chemical conversion property according to any one of claims 1 to 4. A method for producing rolled steel sheets. Prior Symbol sintered blunt, air ratio: heating 0.95 or more using a direct flame burner, temperature of the steel strip: Kasei according to any one of claims 1 to 5, characterized in that up to 700 ° C. A method for producing a high-Si cold-rolled steel sheet having excellent processability. Prior Symbol sintered blunt, air ratio: heating 0.95 or more using a direct flame burner, temperature of the steel strip: Kasei according to any one of claims 1 to 6, characterized in that up to 800 ° C. A method for producing a high-Si cold-rolled steel sheet having excellent processability. Claim before Symbol sintered blunt, heating time using an air ratio 0.95 or more direct flame burners, characterized in that at least the heating time using the following direct flame burner air ratio 0.89 The manufacturing method of the high Si cold-rolled steel plate excellent in the chemical conversion property as described in any one of 2-7. Prior Symbol sintered blunt, heating using the following direct flame burner air ratio 0.89 steel plate temperature: Kasei according to any one of claims 2～6,8, which comprises carrying out up to 800 ° C. A method for producing a high-Si cold-rolled steel sheet having excellent processability.