JP2012036483A - METHOD OF MANUFACTURING Si-CONTAINING HOT-ROLLED STEEL SHEET EXCELLING IN PICKLING PROPERTY, AND PICKLING METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method useful for manufacturing a Si-containing hot-rolled steel sheet excelling in a pickling property, and a pickling method useful for obtaining a Si-containing hot-rolled steel sheet small in scale.SOLUTION: In this method of manufacturing a Si-containing hot-rolled steel sheet, a hot-rolled Si-containing steel sheet respectively containing 0.04-0.2% of C, 0.1-3.0% of Si, 0.1-3.0% of Mn, P ≤0.02% (without including 0%) and S ≤0.004% (without including 0%), and having a residual part of iron and unavoidable impurities is heat-treated for 5-60 min to 700°C or above in a nitrogen atmosphere with Ocontrolled to less than 1 vol.%.

Description

  The present invention relates to a pickling technique for a Si-containing hot-rolled steel sheet.

  In order to manufacture a thin steel plate by hot rolling, a slab is heated in a heating furnace, then rolled to a predetermined thickness by rough rolling and finish rolling, and further on a hot run table in which a water cooling zone (water cooling zone) is arranged. It is water-cooled to a predetermined temperature and wound in a coil shape.

  In recent years, in a high-strength steel plate that is widely used mainly for automobile applications, it is common that a relatively large amount of Si is added to ensure strength. When normal hot rolling is applied to a steel sheet containing a large amount of Si, a scale (oxidation scale), a layer enriched with Si or Mn (Si, Mn enriched layer) is formed on the surface layer, and grain boundary oxidation It is known that the portion (see FIGS. 1 and 2 described later) occurs at a depth of several μm to several tens of μm.

  If these scale, Si, Mn enriched layer and grain boundary oxidation part remain after pickling, the grain boundary oxidation part of the steel sheet peels off in the cold rolling after pickling, and the peeled steel slab generates pruning. Deteriorates the surface properties of the steel sheet. In particular, the Si, Mn concentrated layer and the grain boundary oxidation part cannot be removed by ordinary pickling.

  For these reasons, various techniques have been proposed so far in order to enhance pickling properties. As such a technique, for example, Patent Document 1 discloses pickling of a steel sheet using a mixed acid obtained by adding mercaptoacetic acid as a pickling accelerator to sulfuric acid. Patent Document 2 proposes a technique for improving the pickling property using a pickling solution having a hydrochloric acid concentration of 80 to 200 g / L (liter) and a liquid temperature of 80 to 95 ° C.

  On the other hand, in Patent Document 3, when a hot-rolled steel strip is subjected to hydrochloric acid pickling with a continuous pickling facility in which a plurality of pickling tanks are arranged in series, at least in the final pickling tank, the hydrochloric acid concentration: 3 to 3 10% by mass, oxidizing agent concentration: 0.05 to less than 5% by mass of aqueous solution, and the remaining pickling bath including the first pickling bath, hydrochloric acid concentration: 3 to 10% by mass, oxidizing agent concentration: 0 The technique of pickling using the aqueous solution below 0.05-5 mass% is proposed. Examples of the oxidizing agent include hydrogen peroxide, potassium permanganate, dichromate, nitric acid, nitrate, nitrite, perchloric acid, perchlorate, and the like.

  In various techniques proposed so far, contrivances such as addition of an oxidation accelerator and an oxidant and raising the temperature of the pickling aqueous solution have been made. However, it is not always sufficient as a technique for removing the Si, Mn enriched layer and the grain boundary oxidized portion.

JP 2005-298911 A JP 2003-073867 A Japanese Patent No. 3506127

  This invention was made paying attention to the above situations, and the 1st objective is to provide the useful method for manufacturing the Si containing hot-rolled steel plate excellent in the pickling property. is there. The second object of the present invention is to provide a useful pickling method for obtaining a Si-containing hot-rolled steel sheet with a small scale.

The production method of the Si-containing hot-rolled steel sheet of the present invention that has achieved the first object is C: 0.04 to 0.2% (meaning mass%, the same applies in the chemical composition of steel). , Si: 0.1 to 3.0%, Mn: 0.1 to 3.0%, P: 0.02% or less (excluding 0%) and S: 0.004% or less (including 0%) No.), the balance being iron and inevitable impurities, and the hot-rolled Si-containing steel sheet is heated to 700 ° C. or more for 5 to 60 minutes in a nitrogen atmosphere in which O ₂ is controlled to be less than 1% by volume. It has a gist in terms of heat treatment.

  In order to effectively exhibit the effect by the production method of the present invention, it is preferable to perform the above heat treatment on the Si-containing hot-rolled steel sheet which has been rolled after hot rolling.

  In the method of the present invention, the basic components of the target steel sheet are as described above. However, if necessary, (a) Ni: 2% or less (not including 0%), Cu: 2% or less (including 0%) 1) or more selected from the group consisting of Mo: 2% or less (not including 0%) and B: 0.01% or less (not including 0%), (b) Cr: 2% or less (0 %: Nb: 1% or less (not including 0%), V: 1% or less (not including 0%), and W: 0.3% or less (not including 0%) One or more selected, (c) Al: 0.06% or less (not including 0%), Ti: 0.1% or less (not including 0%), Ca: 0.03% or less (0% 1) or more selected from the group consisting of 0.03% or less (excluding 0%), etc., and Mg: (d) N It is also effective to suppress the content to 0.01% or less (not including 0%), properties of the steel sheet are further improved depending on the components to be or suppressed contained.

  On the other hand, the pickling method that can achieve the second object is a hydrochloric acid concentration of 5% by mass or more and 20% by mass or less with respect to the hot-rolled steel sheet obtained by the method of the present invention as described above. Hydrogen concentration: 5% by mass or more and less than 8% by mass, and has a gist in that pickling is performed using an aqueous solution having a temperature of 50 to 75 ° C.

According to the present invention, after hot rolling, an oxide scale layer formed on the surface of a Si-containing hot-rolled steel sheet by heat treatment under a predetermined condition in a nitrogen atmosphere in which the oxygen (O ₂ ) concentration is appropriately suppressed. Thus, the steel sheet is excellent in pickling performance. Moreover, the outstanding pickling property is realizable by pickling with respect to the steel plate obtained by the said method using the aqueous solution which controlled hydrochloric acid concentration, hydrogen peroxide concentration, and temperature appropriately.

It is sectional drawing which showed typically the structure of the surface of the conventional hot-rolled steel plate after coil winding. It is sectional drawing which showed typically the structure of the hot-rolled steel plate surface after coil winding when this invention is applied.

  The inventors of the present invention have studied the structure in which the surface of the Si-containing hot-rolled steel sheet is easily pickled and removed from various angles. As a result, the present inventors have found that the surface of the Si-containing hot-rolled steel sheet can be easily pickled and removed by heating at a high temperature in a nitrogen atmosphere in which the oxygen concentration is suppressed after coil winding.

  The surface of a general hot-rolled (hot rolled) steel sheet after coil winding has a structure as shown in FIG. 1 (schematic diagram). That is, a surface layer scale (non-porous structure) 1 is formed on the outermost surface, a Si, Mn concentrated layer (non-porous structure) 2 is formed on the inner side, and Si, Mn is further formed near the surface layer of the steel plate 3. The grain boundary oxidation part 4 which is slightly concentrated is formed. These surface scale 1, Si, Mn enriched layer 2 and grain boundary oxidation part 4 are all difficult to remove by ordinary pickling.

On the other hand, if the coil is wound and heated at a high temperature in a nitrogen atmosphere in which the oxygen concentration is suppressed, the following phenomena (1) to (3) occur, and the pickling property is considered to be improved.
(1) Oxygen constituting the surface layer scale 1 diffuses toward the base material (steel plate 3), and the surface layer scale 1 has a porous (oxygen-deficient) structure and is easily removed by pickling.
(2) Oxygen constituting the Si, Mn enriched layer 2 diffuses to the base material (steel plate 3) side, and the Si, Mn enriched layer 2 has a porous (oxygen deficient) structure and is easily removed by pickling.
(3) While Si and Mn in the grain boundary oxidation part 4 are further concentrated locally, a structure (Si, Mn deficient layer) having a composition close to pure iron deficient in Si and Mn is also formed partially. Si and Mn-deficient layers are easily pickled and removed.

  FIG. 2 schematically shows the structure of the surface of the hot-rolled steel sheet after coil winding when the present invention is applied. That is, a surface scale 1a having a porous structure is formed, and Si and Mn enriched layers (non-porous structure) 2 (FIG. 1) existing inside the porous scale 1a are almost eliminated. Further, near the surface layer of the steel plate 3. A portion 5 where Si and Mn are concentrated is formed from the Si and Mn enriched layer 2 and a Si and Mn deficient layer 6 is formed. The surface layer scale 1a, the portion 5 where Si and Mn are concentrated, and the Si and Mn deficient layer 6 are all easily removed by ordinary pickling.

  In order to manufacture the Si-containing hot-rolled steel sheet of the present invention, it is necessary to strictly define the production conditions. The reasons for defining these requirements are as follows.

[Nitrogen atmosphere with O ₂ controlled to less than 1% by volume]
When the oxygen (O ₂ ) concentration in the atmosphere is 1% by volume or more, oxygen is easily supplied from the surface of the steel plate during the heat treatment, and it becomes difficult to form a Si, Mn-deficient layer in the grain boundary oxidation portion. In the present invention, the heat treatment atmosphere is a nitrogen atmosphere because an oxide layer serving as an oxygen supply source is not excessively attached.

[Heat treatment temperature: 700 ° C or higher]
When the heat treatment temperature is less than 700 ° C., oxygen diffusion becomes insufficient, and it becomes difficult to form a Si, Mn deficient structure (Si, Mn deficient layer 6 shown in FIG. 2) in the grain boundary oxidized portion. The heat treatment temperature is preferably 750 ° C. or higher. However, if the temperature is too high, the scale layer (surface layer scale 1a shown in FIG. 2) becomes too thick even though it is porous, and it is difficult to supply oxygen to the lower layer. Thus, the portion 5 where Si and Mn are concentrated in the grain boundary oxidation portion, and further the Si and Mn deficient layer 6 are hardly formed. From such a viewpoint, the heating temperature is preferably 1000 ° C. or lower (more preferably 900 ° C. or lower).

[Heat treatment time: 5 to 60 minutes]
If the heat treatment time is less than 5 minutes, the diffusion of oxygen becomes insufficient, and the Si and Mn deficient structure (Si and Mn deficient layer 6 shown in FIG. 2) is hardly formed in the grain boundary oxidized portion. The heat treatment time is preferably 10 minutes or longer, but if the heat treatment is carried out for a long time, the productivity is lowered, so it is necessary to set it to 60 minutes or less (preferably 30 minutes or less).

  The method of the present invention defines the heat treatment conditions after hot rolling, but the hot rolling treatment may be performed according to a conventional method. For example, it is preferable that the heating temperature when heating a steel slab (slab) is 1000-1300 degreeC from a viewpoint of ensuring finishing temperature. Moreover, the finishing temperature of hot rolling is set to a temperature range of 800 to 950 ° C. from the viewpoint of not forming a texture that impairs workability, and the cooling rate after finishing rolling is 30 to 120 ° C. in order to suppress the formation of pearlite. / Second is preferable (more preferably about 50 to 100 ° C./second).

  The present invention assumes that the structure of the steel sheet surface is appropriately controlled after winding, but the temperature (winding temperature) when winding is about 550 to 750 ° C. Preferably there is. When the coiling temperature is lower than 550 ° C., hematite is generated and oxygen supply is excessive. On the other hand, when the coiling temperature exceeds 750 ° C., the scale is excessively attached and the oxygen supply becomes excessive. More preferably, it is 600 degreeC or more and 700 degrees C or less.

  In the present invention, after hot rolling (and after winding), by appropriately controlling the heat treatment conditions, a steel plate having excellent pickling properties is obtained. The chemical composition of this steel plate is a high strength steel plate. Any material satisfying the above characteristics may be used. From such a viewpoint, as basic components, C: 0.04 to 0.2%, Si: 0.1 to 3.0%, Mn: 0.1 to 3.0%, P: 0.02% or less (0 %) And S: 0.004% or less (not including 0%), respectively. The reason for adding each element is as follows.

[C: 0.04 to 0.2%]
C is an element effective for increasing the strength of a steel material (that is, a steel plate). If the C content is less than 0.04%, it will not be possible to meet the needs of high strength for automobiles, while if the C content exceeds 0.2%, the cold workability will deteriorate. A preferable C content is 0.06% or more and 0.15% or less.

[Si: 0.1 to 3.0%]
Si is an important element that can ensure ductility and workability while expressing the strength of steel. In the steel plate which is the subject of the present invention, the content was set to 0.1% as the minimum amount of Si necessary for securing the strength. However, if the Si content is excessive, ductility is impaired, so the content was made 3.0% or less. A preferable Si content is 0.3% or more and 2.8% or less.

[Mn: 0.1 to 3.0%]
Mn is an element useful for securing the strength and toughness of the steel material. In order to exert such effects, it is necessary to contain at least 0.1% or more. However, if the Mn content is excessive, the ductility deteriorates, so it is necessary to make it 3.0% or less. A preferable Mn content is 0.3% or more and 2.8% or less.

[P: 0.02% or less (excluding 0%)]
P is an element inevitably contained, but the presence of a trace amount of P effectively acts to delay the precipitation of cementite and suppress the transformation. However, if the P content exceeds 0.02% and becomes excessive, the ductility is lowered and the plating adhesion (for example, hot dip galvanizing) is deteriorated, so the upper limit must be limited to 0.02%. . The P content is preferably 0.01% or less. In addition, it is difficult to make P content in steel materials 0% on industrial production.

[S: 0.004% or less (excluding 0%)]
S is an element that is inevitably contained, but forms MnS of sulfide inclusions, which segregates during hot rolling of the steel material, and thus embrittles the steel material, so the upper limit is made 0.004% It is necessary to. The S content is preferably 0.003% or less. In addition, it is difficult to make S content in steel materials 0% on industrial production.

  Components other than the chemical component composition described above are substantially iron. In the case where the balance is substantially iron, it is naturally allowed that inevitable impurities (for example, impurities (O, N, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc.) are included in the steel sheet.

  The steel materials targeted by the present invention include (a) Ni: 2% or less (not including 0%), Cu: 2% or less (not including 0%), Mo: 2% or less (0) as necessary. %) And B: one or more selected from the group consisting of 0.01% or less (not including 0%), (b) Cr: 2% or less (not including 0%), Nb: 1% 1 or more selected from the group consisting of: (not including 0%), V: 1% or less (not including 0%) and W: 0.3% or less (not including 0%), (c) Al : 0.06% or less (not including 0%), Ti: 0.1% or less (not including 0%), Ca: 0.03% or less (not including 0%), and Mg: 0.03% Including one or more selected from the group consisting of the following (not including 0%), or (d) N content is 0.01% or less (not including 0%) It is also effective to win, characteristics of the steel sheet are further improved depending on the components to be or suppressed contained. The reasons for limiting the range when containing or suppressing these elements are as follows.

[Ni: 2% or less (not including 0%), Cu: 2% or less (not including 0%), Mo: 2% or less (not including 0%), and B: 0.01% or less (0% 1 or more selected from the group consisting of
Ni, Cu, Mo and B are all effective elements for improving the hardenability. Of these, Ni is added in an appropriate amount to provide a continuous annealing line (CAL), an increase in the martensite ratio at the time of cooling, and the action of refining the martensite lath structure, and the subsequent continuous galvanizing annealing line (CGL). ) In the two-phase region reheating-cooling treatment, the final composite structure after cooling is made good, and various moldability can be improved. Although the effect of such Ni can be exhibited even in a small amount, it is preferably 0.1% or more (more preferably 0.2% or more). However, since Ni is an expensive element, it is preferable to make it 2% or less from the viewpoint of manufacturing cost. More preferably, it is 1.5% or less, More preferably, it is 1.0% or less.

  Cu is an element that improves the hardenability like Ni, and effectively acts to improve various moldability by the same action as Ni. In order to exhibit such an effect, Cu is preferably contained in an amount of 0.1% or more, more preferably 0.2% or more. However, since Cu is an expensive element, it is preferably made 2% or less from the viewpoint of manufacturing cost. More preferably, it is 1.5% or less, More preferably, it is 1.0% or less.

  Mo has the effect of improving the hardenability like Ni and Cu, and effectively acts to improve various moldability by the same action as Ni. Further, Mo is an element that effectively acts in strengthening the solid solution without impairing the plating property. In order to exhibit such an effect, Mo is preferably contained in an amount of 0.1% or more, more preferably 0.2% or more. However, since Mo is an expensive element, it is preferable to make it 2% or less from the viewpoint of manufacturing cost. More preferably, it is 1.5% or less, More preferably, it is 1.0% or less.

  B has the effect of improving hardenability like Ni and Cu, and is contained as necessary. In order to exert such an effect, B is preferably contained in an amount of 0.0001% or more, more preferably 0.0002% or more. However, if B is contained excessively, the plating property is deteriorated, so 0.01% or less is preferable. More preferably, it is 0.005% or less, and still more preferably 0.001% or less.

[Cr: 2% or less (not including 0%), Nb: 1% or less (not including 0%), V: 1% or less (not including 0%), and W: 0.3% or less (0% 1 or more selected from the group consisting of
Cr, Nb, V and W are all effective elements for improving the strength of the steel sheet. Among these, Cr can be contained as necessary in order to impart strength to the steel (steel plate) and the cold forged product. In order to exhibit such an effect, it is preferable to contain Cr 0.01% or more. However, if Cr is excessively contained, the ductility is impaired, so 2% or less is preferable. More preferably, it is 1.0% or less.

  Nb is an element that can obtain a fine structure with a small amount of addition and can increase strength without impairing toughness. In order to exert such effects, Nb is preferably contained in an amount of 0.001% or more, more preferably 0.005% or more. However, when Nb is contained excessively, carbides are generated, and the balance between strength and workability is deteriorated due to a decrease in martensite volume or precipitation strengthening. For these reasons, when Nb is contained, its content is preferably 1% or less, more preferably 0.5% or less, and still more preferably 0.1% or less.

  V, like Nb, is an element that generates carbides, and contributes to improving the strength of the steel sheet. In order to exert such an effect, V is preferably contained in an amount of 0.001% or more, more preferably 0.005% or more. However, if V is contained excessively, not only will the cost be increased, but the yield point (yield ratio) of the steel sheet will be raised and workability will be lowered. Therefore, when V is contained, its content is 1 % Or less, more preferably 0.5% or less, and still more preferably 0.1% or less.

  W is an element that contributes to improving the strength of the steel sheet by precipitation strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. In order to exhibit such an effect, W is preferably contained in an amount of 0.001% or more, more preferably 0.005% or more. However, if W is contained excessively, the carbonitride precipitates excessively and the formability is deteriorated. Therefore, when W is contained, the content is preferably 0.3% or less, more preferably. Is 0.2% or less, more preferably 0.1% or less.

[Al: 0.06% or less (not including 0%), Ti: 0.1% or less (not including 0%), Ca: 0.03% or less (not including 0%), and Mg: 0.0. One or more selected from the group consisting of 03% or less (excluding 0%)]
Al, Ti, Ca and Mg all act effectively as a deoxidizer. Among these, Al acts as a deoxidizing agent and also exhibits an effect of preventing coarsening of austenite crystal grains during normalizing heating. In order to exhibit such an effect, Al is preferably contained in an amount of 0.01% or more, and more preferably 0.02% or more. However, since the toughness deteriorates when Al is contained excessively, when Al is contained, the content is preferably 0.06% or less, more preferably 0.05% or less, and still more preferably 0. 0.04% or less.

  Ti also acts effectively as a deoxidizer. In order to exhibit such an effect, Ti is preferably contained in an amount of 0.01% or more, more preferably 0.02% or more. However, if Ti is contained excessively, the effect becomes saturated, and the crystal grains become unstable. Therefore, when Ti is contained, the content is preferably 0.1% or less. Preferably, it is 0.08% or less, more preferably 0.05% or less.

  Ca and Mg are also elements used for deoxidation, and are contained if necessary. In order to exhibit such an effect, it is preferable to contain 0.002% or more, more preferably 0.003% or more. However, if Ca and Mg are contained excessively, the moldability deteriorates. Therefore, the content is preferably 0.03% or less, more preferably 0.02% or less, and still more preferably 0. .01% or less.

[N: 0.01% or less (excluding 0%)]
N is preferably suppressed as much as possible because it forms coarse nitrides to deteriorate bendability and hole expansibility and causes blowholes during welding. From such a viewpoint, it is preferable to suppress N to 0.01% or less. Although it is difficult to make the N content 0%, the lower the value, the better the characteristics.

  The steel sheet produced as described above exhibits good acidity under normal pickling conditions, in which the pickling property itself becomes good. The present inventors also examined conditions (composition and temperature of the pickling aqueous solution) under which better pickling performance is exhibited when pickling such a steel sheet. As a result, in an aqueous solution containing a predetermined amount of hydrochloric acid, particularly when hydrogen peroxide is contained in an amount of 5% by mass or more and pickling in a state in which the temperature is appropriately controlled, the pickling speed can be further increased. It has also been found that extremely excellent pickling properties can be exhibited. The effects of the component composition and temperature control of this pickling aqueous solution are as follows.

[Hydrochloric acid concentration: 5% by mass or more and 20% by mass or less]
If the hydrochloric acid concentration is less than 5% by mass, the pickling rate is too slow, and the grain boundary oxidation part cannot be sufficiently removed. However, when the hydrochloric acid concentration exceeds 20% by mass, the pickling becomes peracid washing, so that the steel sheet is dissolved more than necessary, and the pickling yield decreases.

[Hydrogen peroxide concentration: 5% by mass or more and less than 8% by mass]
When the hydrogen peroxide concentration is less than 5% by mass, the pickling rate is too slow, and the grain boundary oxidation part cannot be sufficiently removed. However, when the hydrogen peroxide concentration is 8% by mass or more, it becomes difficult to control the temperature within an appropriate range (below) due to a rapid temperature rise of the pickling aqueous solution. Furthermore, it becomes per pickling, dissolves a steel plate more than necessary, and the pickling yield decreases.

[Pickling aqueous solution temperature: 50 to 75 ° C.]
If the temperature of the aqueous solution when pickling is less than 50 ° C., the pickling speed is too slow, and the grain boundary oxidized portion cannot be sufficiently removed. However, if this temperature exceeds 75 ° C., a mist of hydrochloric acid is generated, causing problems such as corrosion of peripheral equipment. Furthermore, it becomes per pickling, dissolves a steel plate more than necessary, and the pickling yield decreases.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
The steel (steel types A to V) having the chemical composition shown in Table 1 below is melted and the slab obtained by casting the molten steel is heated to 1250 ° C. (Temperature control is performed by measuring a slab with a thermocouple embedded therein. Heating is performed in the same manner as general slabs), finishing temperature: 870-900 ° C., thickness: hot-rolling to 2.6 mm, and then cooling at an average cooling rate: 40 ° C./second, The coil was wound up (winding temperature: 650 ° C.) and then allowed to cool in the air.

  A steel sheet piece cut into 2 cm × 2 cm from the coil is heat-treated under the conditions shown in Table 2 below, and after standing to cool, in a 15 mass% hydrochloric acid aqueous solution containing 5 mass% hydrogen peroxide at 60 ° C. for 1 minute. After pickling, the state of formation and removal of the surface scale and the grain boundary oxidation part is observed with a cross-sectional optical microscope, and if it can be removed, the pickling property is good (pickling evaluation: “◯”) and can be removed. The case where it was not pickled was regarded as poor pickling performance (pickling evaluation: “×”). The results are shown in Table 2 below together with the heating conditions (atmosphere, temperature of washing aqueous solution, time).

  From these results, we could consider as follows. First, test no. In the case of No. 1, a thick surface layer scale of 30 μm was formed after heat treatment, and a Si, Mn-deficient layer was not formed, and these layers could not be removed even by pickling. Test No. In No. 2, the surface scale was as thin as 15 μm after the heat treatment, but the Si, Mn-deficient layer was not formed, and only the surface scale was removed after pickling, and the grain boundary oxidized portion was not removed.

  Test No. 3 to 5, a surface layer scale of about 20 μm and a grain boundary oxidation portion of about 15 μm were formed after the heat treatment. In the grain boundary oxide, a Si, Mn-deficient layer was formed, and after the pickling, both the surface scale and the grain boundary oxidation part were removed.

  Test No. 6 and 7, a surface layer scale of about 20 μm and a grain boundary oxidation portion of about 15 μm were formed after the heat treatment. In the grain boundary oxidation part, the Si and Mn deficient layers were only partially formed, and the surface layer scale was removed after pickling, but the grain boundary oxidation part remained without being removed.

[Example 2]
A slab obtained by melting steel (steel types A to V) having the chemical composition shown in Table 1 and casting the molten steel is heated to 1250 ° C. (temperature control is a measurement slab in which a thermocouple is embedded. After heating to a thickness of 2.6 mm at a finishing temperature of 870 to 900 ° C. and then cooling at an average cooling rate of 40 ° C./sec. The coil was wound up (winding temperature: 650 ° C.) and then allowed to cool in the air.

A steel sheet piece cut into 2 cm × 2 cm from the coil was heat-treated at 700 ° C. for 60 minutes in a 0.5% O ₂ —N ₂ atmosphere and allowed to cool, and then the conditions shown in Table 3 below (the concentration of hydrochloric acid in the pickling aqueous solution, Pickling with hydrogen peroxide concentration and temperature). The surface scale after pickling and the formation and removal of the grain boundary oxidation part were observed with a cross-sectional optical microscope, and when it was removed, pickling was good (pickling evaluation: “◯”) and removed. The case where there was no pickling was regarded as poor pickling performance (pickling evaluation: “×”). The results are shown in Table 3 below together with the pickling conditions (hydrochloric acid concentration, hydrogen peroxide concentration, temperature in the pickling aqueous solution).

  From these results, we could consider as follows. First, test no. In No. 8, the pickling aqueous solution in which 1% of hydrogen peroxide was added to 15% hydrochloric acid was used for pickling at 50 ° C., but the surface scale could not be removed. Test No. In No. 9, the pickling aqueous solution temperature was raised to 75 ° C., but a part of the surface scale could be removed, but the grain boundary oxidation part could not be removed.

  Test No. No. 10 was obtained by pickling at 50 ° C. using a pickling aqueous solution in which 3% hydrogen peroxide was added to 15% hydrochloric acid, but only a part of the surface scale could be removed, and the grain boundary oxidation part was completely Could not be removed. Test No. In No. 11, even if the pickling aqueous solution temperature was raised to 75 ° C., a part of the grain boundary oxidation part could not be removed.

  Test No. In Nos. 12 and 13, pickling was performed at 50 ° C and 75 ° C using a pickling solution containing 5% hydrogen peroxide in 15% hydrochloric acid, but the surface scale and grain boundary oxidation part were completely removed. We were able to. Moreover, the temperature of the pickling aqueous solution did not rise rapidly during pickling.

  Test No. In No. 14, pickling was performed at 65 ° C. using a pickling aqueous solution in which 8% hydrogen peroxide was added to 15% hydrochloric acid, but both the surface layer scale and the grain boundary oxidation part could be completely removed. . Moreover, the temperature of the pickling aqueous solution did not rise rapidly during pickling.

  Test No. In No. 15, pickling aqueous solution in which 10% hydrogen peroxide was added to 15% hydrochloric acid was used for pickling at 50 ° C., but both the surface layer scale and the grain boundary oxidation part could be completely removed. . However, the temperature of the pickling aqueous solution suddenly increased during pickling (temperature exceeding 75 ° C.) and became uncontrollable at an early stage.

  Test No. In No. 16, pickling was performed at 75 ° C. using a pickling aqueous solution in which 10% hydrogen peroxide was added to 3% hydrochloric acid, but the surface scale could not be removed.

  Test No. In No. 17, pickling aqueous solution in which 5% hydrogen peroxide was added to 5% hydrochloric acid was used, and pickling was performed at 50 ° C., but both the surface layer scale and the grain boundary oxidation part could be completely removed. .

  Test No. In No. 18, pickling aqueous solution in which 8% hydrogen peroxide was added to 5% hydrochloric acid was used, and pickling was performed at 75 ° C., but both the surface layer scale and the grain boundary oxidation part could be completely removed. . Moreover, the temperature of the pickling aqueous solution did not rise rapidly during pickling.

  Test No. In No. 19, pickling aqueous solution in which 5% hydrogen peroxide was added to 20% hydrochloric acid was used, and pickling was performed at 50 ° C., but both the surface layer scale and the grain boundary oxidation part could be completely removed. . Moreover, the temperature of the pickling aqueous solution did not rise rapidly during pickling.

  Test No. In No. 20, pickling aqueous solution in which 8% hydrogen peroxide was added to 20% hydrochloric acid was used for pickling at 75 ° C., but both the surface layer scale and the grain boundary oxidation part could be completely removed. . Moreover, the temperature of the pickling aqueous solution did not rise rapidly during pickling.

  Test No. In No. 21, pickling aqueous solution in which 5% hydrogen peroxide was added to 25% hydrochloric acid was used, and pickling was performed at 50 ° C., but both the surface layer scale and the grain boundary oxidation part could be completely removed. . However, the temperature of the pickling aqueous solution suddenly increased during pickling (temperature exceeding 75 ° C.) and became uncontrollable at an early stage.

1, 1a Surface scale 2 Si, Mn enriched layer 3 Steel plate 4 Grain boundary oxidation part 5 Si, Mn enriched part 6 Si, Mn deficient layer

Claims (7)

C: 0.04 to 0.2% (meaning mass%, the same applies to the chemical components of steel), Si: 0.1 to 3.0%, Mn: 0.1 to 3.0%, P: Hot-rolled Si-containing steel sheet containing 0.02% or less (excluding 0%) and S: 0.004% or less (not including 0%), respectively, the balance being iron and inevitable impurities In a nitrogen atmosphere in which O ₂ is controlled to be less than 1% by volume, a method for producing a Si-containing hot-rolled steel sheet having excellent pickling properties, wherein the heat treatment is performed at 700 ° C. or more for 5 to 60 minutes.   The method for producing a Si-containing hot-rolled steel sheet according to claim 1, wherein the heat treatment is performed on the Si-containing hot-rolled steel sheet that has been wound after hot rolling.   The steel sheet is further Ni: 2% or less (not including 0%), Cu: 2% or less (not including 0%), Mo: 2% or less (not including 0%), and B: 0.01% The method for producing a Si-containing hot-rolled steel sheet according to claim 1 or 2, comprising one or more selected from the group consisting of the following (excluding 0%).   The steel sheet is further Cr: 2% or less (not including 0%), Nb: 1% or less (not including 0%), V: 1% or less (not including 0%), and W: 0.3% The method for producing a Si-containing hot-rolled steel sheet according to any one of claims 1 to 3, which contains one or more selected from the group consisting of the following (excluding 0%).   The steel sheet is further Al: 0.06% or less (excluding 0%), Ti: 0.1% or less (not including 0%), Ca: 0.03% or less (not including 0%) and The method for producing a Si-containing hot-rolled steel sheet according to any one of claims 1 to 4, which contains at least one selected from the group consisting of Mg: 0.03% or less (excluding 0%).   The method for producing a Si-containing hot-rolled steel sheet according to any one of claims 1 to 5, wherein the steel sheet has a N content suppressed to 0.01% or less (not including 0%).   The hydrochloric acid concentration: 5% by mass or more and 20% by mass or less, the hydrogen peroxide concentration: 5% by mass or more, 8 with respect to the Si-containing hot-rolled steel sheet obtained by the production method according to claim 1. A pickling method for a Si-containing hot-rolled steel sheet, characterized by pickling using an aqueous solution of less than mass% and having a temperature of 50 to 75 ° C.

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