JPWO2017043660A1 - Steel plate and enamel products - Google Patents

Abstract

This steel sheet is in mass%, C: 0.0060% or less, Si: 0.0010 to 0.050%, Mn: 0.05 to 0.50%, P: 0.005 to 0.100%, S : 0.0500% or less, Al: 0.0010 to 0.010%, Cu: 0.010 to 0.045%, O: 0.0250 to 0.0700%, N: 0.0010 to 0.0045% The balance is made of Fe and impurities, the structure contains ferrite, and the average crystal grain size of the ferrite at a position 1/4 of the plate thickness in the plate thickness direction from the surface is 20.0 μm or less, and Fe and Mn are And the number density of the oxide having a diameter greater than 1.0 μm and not greater than 10 μm is 1.0 × 10 3 pieces / mm 2 or more and 5.0 × 10 4 pieces / mm 2 or less. And the oxidation is 0.1 to 1.0 μm in diameter. Number density of is 5.0 × 103 cells / mm @ 2 or more.

Description

The present invention relates to steel plates and enamel products.
This application claims priority on September 11, 2015 based on Japanese Patent Application No. 2015-179722 for which it applied to Japan, and uses the content here.

  The enamel product is obtained by baking glass on the surface of a steel plate for enamel. Enamel products have functions of heat resistance, weather resistance, chemical resistance, and water resistance, so that they have been widely used as materials for kitchen utensils such as pots and sinks, and building materials. Such enamel products are generally manufactured by processing a steel plate into a predetermined shape, assembling the product into a product shape by welding or the like, and then performing enamel treatment (firing treatment).

  The enamel steel sheet used as a material for enamel products is required to have firing strain resistance, nail skip resistance, adhesion, bubble resistance, sunspot defect, and the like. In addition, in the production of an enamel product, since it is usually pressed to obtain a product shape, good moldability is required.

  In addition, the enamel treatment improves the corrosion resistance in a severe corrosive environment containing sulfuric acid or the like, so that the enamel product is also applicable to the energy field such as power generation equipment. In such a field, there is a need for reliability with respect to fatigue or the like in aged use, and further, for the purpose of reducing the weight of parts, it is required to increase the strength of steel plates used. It is known that the reliability of the above-mentioned fatigue, etc., is affected by changes in the structural form of the steel sheet in the manufacturing process of enamel products-enamel processing, that is, changes in strength due to differences in the structural form within the steel sheet. ing.

  So far, for example, Patent Document 1 discloses a technique for preventing the deterioration of the resistance to clawing due to the coarsening of the crystal grain size with respect to the change in the structure morphology associated with the enamel treatment. In Patent Document 1, based on a known high-oxygen steel, the composition, size, shape, ratio, and number of inclusions are optimized, and at the same time, a small amount of Ni, Cr, V, and Mo are added. Accordingly, it is described that, by adding Nb, B, and Ti and optimizing the manufacturing conditions of the steel sheet, it is possible to reduce the decrease in resistance to claw even when repeated enamel treatment is performed. ing.

  Further, in Patent Document 2, the structural form of a steel plate for enamel, that is, ferrite, is a problem in which deflection during firing occurs due to strength reduction accompanying crystal grain growth in enamel processing of high-oxygen steel, that is, dimensional accuracy deteriorates. It is described that it is effective to make the particle size uniform by making the particle size uniform. In Patent Document 2, Ni and Cr are added in order to refine the structure of a hot-rolled steel sheet and make grain growth uniform during annealing in the steel sheet manufacturing process.

  However, in both Patent Documents 1 and 2, it is thought that certain characteristics can be secured in the enamel product subjected to the enamel treatment accompanied by the structural change, but in order to solve the problems related to the crystal grain growth in the enamel process, Addition is essential. That is, to solve the problem, it is necessary to add an expensive alloy element. Regarding Patent Document 2, the addition of Cr coarsens the oxide to make it difficult to hinder ferrite grain growth, thereby improving the uniformity of ferrite grain size and suppressing abnormal grain growth, resulting in mixed grains. Is suppressed. However, this method, which does not use the suppression of grain growth by pinning precipitates and inclusions, results in non-uniform grain size when the temperature in the member fluctuates during enamel processing, and the desired effect cannot be obtained. There is a possibility. In this case, the strength after enamel treatment cannot be obtained stably. Further, in Patent Document 2, since the problem is to suppress the deflection of the member after the enamel treatment and only the yield stress before and after the enamel treatment is examined, the change in the tensile strength that affects the fatigue characteristics is unknown.

  As described above, there is no high-strength steel sheet that sufficiently satisfies the manufacturing process in consideration of the strength characteristics that are important characteristics of enamel steel sheets, which are guidelines for nail skip resistance and steel sheet reliability. However, there are still problems to further improve the characteristics.

Japanese Unexamined Patent Publication No. 2001-316760 Japanese Unexamined Patent Publication No. 2000-063985

  The present invention develops the technology of the steel sheet for enamel described above, and after the enamel treatment, excellent enamel characteristics (claw resistance, adhesion, appearance) and strength characteristics (by enamel treatment). It is an object of the present invention to provide a steel sheet that does not cause a decrease in tensile strength or has a property that can stably suppress a decrease in tensile strength. Moreover, this invention makes it a subject to provide the enamel product provided with the said steel plate and excellent in the enamel characteristic.

The present invention was obtained by various studies in order to overcome the problems of conventional steel plates for enamels, and in particular, the chemical composition, Based on knowledge obtained as a result of examining the influence of manufacturing conditions.
That is, the present invention is based on the following findings 1) to 4).
1) Claw jump resistance can be improved by controlling precipitates in steel by optimizing steel components and trapping hydrogen in steel which causes claw jump. In particular, it is possible to ensure nail resistance by allowing an oxide of more than 1.0 μm to 10 μm to be present in the steel and optimizing the diameter and number of the oxides.
2) Nb is a rare metal, and it is environmentally advantageous not to use it. However, when Nb is not contained, the strength reduction after enamel treatment becomes large. This is because when Nb is contained, Nb suppresses grain growth during heating and heat retention in the enamel treatment, but when Nb is not contained, this effect cannot be obtained.
3) Even if Nb is not contained, the strength after enamel treatment is achieved by optimizing the steel plate component before the enamel treatment, that is, the steel plate component, crystal grain size, and diameter and number of oxides in the steel. It becomes possible to ensure stably (that is, the strength reduction by enamel processing can be suppressed). In particular, it is effective to optimize the number density of the oxide of 0.1 to 1.0 μm in order to suppress grain growth during the enamel treatment, which is a major factor of strength reduction due to the enamel treatment.
4) By controlling the steelmaking conditions, the oxide size is controlled, and the hot rolling conditions, cold rolling conditions, annealing conditions, and temper rolling conditions are controlled appropriately, so that the form of precipitates in the final product can be controlled. It is possible to control.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
(1) The steel plate which concerns on 1 aspect of this invention is the mass%, C: 0.0060% or less, Si: 0.0010-0.050%, Mn: 0.05-0.50%, P: 0 0.005 to 0.100%, S: 0.0030 to 0.0500%, Al: 0.0010 to 0.010%, Cu: 0.010 to 0.045%, O: 0.0250 to 0.0700 %, N: 0.0010 to 0.0045%, balance: Fe and impurities, the structure contains ferrite, and the average crystal grain size of the ferrite at a position 1/4 of the thickness in the thickness direction from the surface Is 20.0 μm or less, contains an oxide containing Fe and Mn, and among the oxides, the number density of the oxides having a diameter larger than 1.0 μm and not larger than 10 μm is 1.0 × 10 ³ / mm ² or more and 5.0 × 10 ⁴ cells / mm ² or less, or Diameter is the number density of the oxides of 0.1~1.0μm is 5.0 × 10 ³ cells / mm ² or more.
(2) The steel sheet according to (1) is B, Cr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, Mg in the impurities in mass%. The total of one or more of: may be limited to 0.100% or less.
(3) The steel sheet according to (1) or (2) may be limited to Nb: 0.010% or less in terms of mass% in the impurities.
(4) The steel sheet according to any one of (1) to (3) may be a cold-rolled steel sheet.
(5) The steel plate according to any one of (1) to (4) may be an enamel steel plate.
(6) The enamel product which concerns on another aspect of this invention is equipped with the steel plate as described in any one of said (1)-(5).

The steel sheet according to the above aspect of the present invention is excellent in formability, strength after enamel treatment, and claw resistance. It also has excellent aging resistance, enamel adhesion, and appearance after enamel treatment. Therefore, it is suitable as a steel plate for enamel, which is a base material for enamel products applied to kitchen utensils, building materials, energy fields and the like.
The enamel product according to the above aspect of the present invention is excellent in enamel characteristics. Therefore, it is suitable for uses such as kitchen utensils, building materials, and energy fields.

It is a photograph which shows an example of an oxide with a diameter of 0.1-1.0 micrometer. It is a photograph which shows an example of an oxide with a diameter larger than 1.0 micrometer and 10 micrometers or less.

  A steel plate according to an embodiment of the present invention (hereinafter referred to as a steel plate according to the present embodiment) will be described in detail. The steel plate according to this embodiment is suitably used as a base material (enamel steel plate) for enamel products.

<Chemical component>
First, the reason for limiting the chemical components (chemical composition) of the steel sheet according to this embodiment will be described. “%” For ingredients means mass% unless otherwise specified.

<C: 0.0060% or less>
The smaller the content of C, the better the ductility. On the other hand, if the C content exceeds 0.0060%, bubble defects are likely to occur. Therefore, the C content is 0.0060% or less. In order to improve ductility, a lower C content is desirable. However, lowering the C content increases steelmaking costs, so the C content is preferably 0.0015% or more.

<Si: 0.0010 to 0.050%>
Si is an element having an effect of controlling the composition of the oxide. In order to obtain this effect, the Si content is set to 0.0010% or more. On the other hand, excessive Si content may inhibit the enamel characteristics, and at the same time, may cause a large amount of Si oxide to be formed by hot rolling, thereby reducing the resistance to claw jumping. Therefore, the Si content is set to 0.050% or less. From the standpoint of improving foam resistance, sunspot resistance, etc. and obtaining better surface properties after enamel treatment, it is preferably 0.0080% or less.

<Mn: 0.05 to 0.50%>
In relation to the O content, Mn is an important component that affects the composition of oxides that exert an effect on the claw resistance of a steel plate for enamel, and at the same time contributes to increasing the strength of the steel plate. Mn is an element that prevents hot brittleness caused by S during hot rolling. In order to obtain these effects, the Mn content is set to 0.05% or more. Usually, when the Mn content is high, the enamel adhesion is deteriorated and bubbles and black spots are easily generated. However, when the oxide is present in the steel, the deterioration of these characteristics is small. However, when the Mn content is excessive, ductility deteriorates. Therefore, the upper limit of the Mn content is 0.50%.

<P: 0.005-0.100%>
P is an element effective for increasing the strength of a steel sheet. P also has the effect of suppressing strength reduction due to enamel processing. In order to obtain these effects, the P content is set to 0.005% or more. P is an element effective for increasing the recrystallization temperature and suppressing the growth of crystal grains during enamel treatment. When obtaining this effect, the P content is preferably 0.015% or more. On the other hand, if the P content is excessive, P may segregate at a high concentration at the grain boundaries of the steel sheet during enamel processing, which may cause bubbles, black spots, and the like. Therefore, the P content is 0.100% or less. Preferably it is 0.075% or less.

<S: 0.0500% or less>
S is an element that forms Mn sulfide. In some cases, the sulfide is compositely precipitated in the oxide, and in the case of the composite precipitation, the resistance to claw jumping can be further improved. In order to obtain this effect, S may be contained. When acquiring the said effect, it is desirable to make S content into 0.0030% or more. More preferably, it is 0.0100% or more, More preferably, it is 0.0150% or more. However, when the S content is excessive, the effect of Mn necessary for controlling the oxide in the steel may be reduced. Therefore, the upper limit of the S content is 0.0500%. Preferably it is 0.0300% or less.

<Al: 0.0010 to 0.010%>
Al is a strong deoxidizing element. Therefore, careful control is required. If the Al content exceeds 0.010%, it will be difficult to keep the required amount of O in the steel, and it will be difficult to control oxides that are effective in resistance to claw jumping. Therefore, the Al content is set to 0.010% or less. On the other hand, if the Al content is less than 0.0010%, bubble defects are likely to occur in the slab, and the slab is more refined than usual in the steel making stage, so that a great load is imposed on the steel making process. Therefore, the lower limit of the Al content is set to 0.0010%.

<Cu: 0.010 to 0.045%>
Cu is an element that controls the reaction between vitreous and steel during enamel treatment and improves the adhesion of the enamel. In order to obtain the effect, the Cu content is set to 0.010% or more. On the other hand, when the content of Cu is excessive, not only the reaction between glass and steel is inhibited, but ductility may be deteriorated. In order to avoid such adverse effects, the Cu content is set to 0.045% or less. Preferably it is 0.029% or less, More preferably, it is 0.019% or less.

<O: 0.0250 to 0.0700%>
O is an element that forms an oxide at the same time as directly affecting the nail resistance and ductility, and affects the nail resistance in relation to the Mn content. In order to obtain excellent ductility and nail resistance, the O content is set to 0.0250% or more. Preferably, it is 0.0400% or more. On the other hand, when the O content is excessively high, ductility deteriorates, the Mn content for forming a necessary amount of oxide increases, and the alloy cost increases. Therefore, the O content is set to 0.0700% or less.
In this embodiment, the O content is determined by reacting oxygen in a steel sample of about 0.5 g with a graphite crucible according to JIS G1239, and measuring the generated CO by an infrared absorption method to quantify the concentration. taking measurement.

<N: 0.0010 to 0.0045%>
N is an interstitial solid solution element, and when contained in a large amount, ductility deteriorates. Moreover, when there is much N content, anti-aging property will deteriorate. For this reason, the upper limit of N content is made 0.0045%. The lower limit is not particularly limited. However, in the current technology, it is extremely expensive to melt to less than 0.0010%, so the lower limit of the N content is set to 0.0010%.

The steel sheet according to the present embodiment basically includes the above elements, with the balance being Fe and impurities. Impurities are components that are mixed from raw materials such as ore or scrap when manufacturing steel materials industrially or due to various factors in the manufacturing process, and do not adversely affect the steel plate according to the present embodiment. It means what is allowed in the range.
In the steel sheet according to the present embodiment, it is preferable to limit the content of elements contained as impurities to a range described later.

Cr, Ni, B, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, Mg: 0.100% or less in total Cr, Ni, B, As, Ti, Se, Ta , W, Mo, Sn, Sb, La, Ce, Ca, and Mg are elements that do not need to be positively contained, but are inevitably mixed impurities. In general, these elements are rarely mixed alone, and are often mixed with two or more elements such as Cr and Ni.
If these elements are contained excessively, the reaction with the oxide-forming elements cannot be ignored, and it becomes difficult to control the desired oxide, so the total content is preferably limited to 0.100% or less. More preferably, it is 0.050% or less, More preferably, it is 0.010% or less.
In addition, when these elements act as deoxidizing elements, it may be difficult to adjust the free oxygen by affecting the value of free oxygen. For this reason, the upper limit of each element is preferably set in a range that does not affect the value of free oxygen in the casting stage.

Nb: 0.010% or less Nb is a rare metal, and it is environmentally advantageous not to use it. Therefore, Nb is not added in the steel plate according to this embodiment. Nb may be mixed as an impurity, but is an element that affects the number of inclusions, and the Nb content is preferably limited to 0.010% or less.

<Oxide>
Next, the limitation reason regarding the oxide containing Fe and Mn present in the steel sheet according to the present embodiment will be described.
The steel structure of the steel sheet according to the present embodiment includes an oxide containing Fe and Mn as elements of the deoxidation product. In the steel plate according to the present embodiment, Nb is not added, so Nb is not included in the oxide as an element of the deoxidation product. The oxide preferably does not contain Al, Cr, Si, or the like. This is achieved by adding the above elements so as to limit the content or not to affect the oxide composition. However, even when Al, Cr, Si or the like is not added as a deoxidizer when adjusting the molten steel components, about 6% or less of Al, Cr, Si, or the like may be detected from the oxide when the oxide analysis is performed. Since it is considered that Al, Cr, Si, etc. contained as impurity elements are incorporated in the oxide, the component in the oxide containing about 15% or less, preferably about 6% or less is deoxidized. It is not counted as a product element.
That is, the oxide contained in the steel sheet according to the present embodiment is substantially composed only of Fe, Mn, and O (even if Al, Cr, and Si are inevitably contained, the total content thereof is 15% or less. It is preferable that However, it may be combined with sulfides such as MnS. When the oxide does not contain Nb, Al, Cr, Si or the like as an element of the deoxidation product, the oxide can be finely dispersed by adjusting free oxygen during casting. On the other hand, when Al, Nb, Cr, Si, etc. are included as deoxidation product elements, the number and size of oxides should be controlled within a desired range by adjusting free oxygen during the casting process of the present application. Becomes difficult.
In the steel plate according to the present embodiment, the number density of the oxide having a diameter greater than 1.0 μm and not greater than 10 μm is 1.0 × 10 ³ pieces / mm ² or more and 5.0 × 10 ⁴ among the oxides. It is necessary that the number density of the oxide having a diameter of 0.1 piece / mm ² or less and a diameter of 0.1 to 1.0 μm is 5.0 × 10 ³ pieces / mm ² or more.

An oxide having a diameter of more than 1.0 μm improves nail resistance. If the oxide is smaller than this range, the effect of improving the resistance to claw jumping is reduced. From the viewpoint of the effect of improving the nail skipping resistance, it is not necessary to specifically limit the upper limit of the diameter. However, although depending on the oxygen content, if the number of coarse oxides increases, the number density of the oxides decreases and the effect of inhibiting hydrogen permeation decreases. Moreover, a coarse oxide tends to become a starting point of cracking during processing, and reduces ductility. For this reason, it is preferable that the diameter of the oxide utilized for improving the nail resistance is 10 μm or less, preferably 5 μm or less. That is, an oxide having a diameter of more than 1.0 to 10 μm is controlled in order to improve the nail resistance.
In order to improve the nail resistance, it is necessary to contain 1.0 × 10 ³ pieces / mm ² or more of an oxide containing Fe and Mn having a diameter of more than 1.0 to 10 μm. If the number density is smaller than this, it is impossible to ensure excellent nail skipping resistance. On the other hand, if the oxide is present in excess of 5.0 × 10 ⁴ pieces / mm ² , voids are generated more than necessary at the interface between the oxide and the steel plate base material during processing, and the strength after enamel treatment is reduced. . Therefore, the upper limit of the number density is 5.0 × 10 ⁴ pieces / mm ² . Preferably, it is 1.0 × 10 ⁴ pieces / mm ² or less. An oxide having a diameter exceeding 1.0 μm often has a round shape as shown in FIG.

  On the other hand, an oxide having a diameter of 1.0 μm or less has an effect of suppressing grain growth in a heat treatment (enamel treatment) process in the production of enamel products. If the diameter exceeds 1.0 μm, the effect of suppressing grain growth due to heat input during heat treatment is lost, so the upper limit of the diameter of the oxide utilized for suppressing grain growth is 1.0 μm or less. For this effect, the diameter of the oxide in the steel is desirably small, preferably 0.8 μm or less, and more preferably 0.5 μm or less. The diameter of the oxide present in the steel is preferably as small as possible, but if it is too small, the analysis of the oxide, that is, the identification of the oxide containing Fe and Mn becomes difficult. Therefore, in the steel plate according to this embodiment, the lower limit of the oxide diameter for which the number density is controlled is set to 0.1 μm or more. That is, in order to suppress grain growth in the heat treatment step, an oxide having a diameter of 0.1 to 1.0 μm is controlled. An oxide having a diameter of 0.1 to 1.0 μm often has an angular shape as shown in FIG.

In order to suppress grain growth in the heat treatment step, it is necessary to contain 5.0 × 10 ³ pieces / mm ² or more of an oxide containing Fe and Mn having a diameter of 0.1 μm to 1.0 μm. This is because when the number density is smaller than this, the effect of suppressing grain growth in the heat treatment step cannot be obtained sufficiently. Although there is no upper limit of the oxide density in terms of suppressing grain growth, when the oxide density exceeds 1.0 × 10 ⁵ pieces / mm ² , the number density of oxides exceeding 1.0 μm is small as a result. As a result, resistance to nail flaking is reduced. Therefore, the density of the oxide containing Fe and Mn with a diameter of 0.1 μm to 1.0 μm is set to 1.0 × 10 ⁵ pieces / mm ² or less.
In the steel plate according to the present embodiment, by controlling the oxide within the above range, it is possible to suppress the grain growth and prevent the strength from being reduced during the enamel treatment without containing Nb.

  Furthermore, the oxide containing Fe and Mn with a diameter of 0.1 μm to 1.0 μm also has the effect of refining the crystal grain size after cold rolling recrystallization, so when using a member processed from bending workability or steel material It also contributes to suppression of fracture and fatigue failure.

The method for identifying the oxide described above is not particularly limited. In this embodiment, since the target oxide is one in which Fe, Mn, and O are detected at the same time, for example, a scanning electron microscope (FE-SEM) and an energy dispersive X-ray dispersive analyzer are used for the identification. (EDAX) may be used. The measurement method may be a normal method. However, since it is necessary to determine the concentration of a minute region in particular, it is necessary to pay attention to making the beam diameter of the electron beam sufficiently small (for example, 0.1 to 0.5 μm). is there.
The diameter and number density of the oxide can be measured by the following method. That is, in FE-SEM, the magnification is 5000 times or more, the number of fields of view is 10 or more, the size and number of the oxides in the field of view are measured, and the major axis of the oxide is the oxide diameter. For the number density, the number of oxides whose major axis is 0.1 μm to 1.0 μm or more than 1.0 μm to 10.0 μm among the oxides in the field of view is calculated, and the number is expressed in unit area (mm ^2). ) Divided by the total area of the field of view to obtain the number per unit area.
In the steel plate according to the present embodiment, slag and refractory mixed in the manufacturing process may exist as inclusions, but the composition does not include Mn and Fe, and this has no effect of suppressing strength reduction. Such inclusions are not counted.
As the oxide to be measured in this embodiment, Fe, Mn, and O may be detected at the same time. For example, MnS may be complexly precipitated.

<Metallic structure>
Next, the structure (metal structure) of the steel sheet according to the present embodiment will be described.
The structure of the steel sheet according to this embodiment is mainly composed of ferrite. Therefore, it is effective to reduce the crystal grain size in order to improve the strength.
When the steel sheet according to this embodiment is processed into an enamel product, ferrite grain growth occurs due to heat treatment (enamel treatment), the crystal grain size changes, and as a result, the strength (tensile strength) decreases. Further, the fatigue characteristics are also reduced due to the decrease in strength. Reducing the crystal grain size after heat treatment is effective for ensuring the strength of the steel plate after heat treatment. In order to reduce the crystal grain size after the heat treatment, it is important to reduce the grain size before the heat treatment and to suppress the grain growth accompanying the heat treatment.

The average crystal grain size of the ferrite in the steel sheet structure before heat treatment needs to be 20.0 μm or less at a position of 1/4 of the sheet thickness in the sheet thickness direction from the surface (1 / 4t: t is the sheet thickness). It is. When the average crystal grain size exceeds 20.0 μm, it is difficult to increase the strength of the steel sheet. In order to increase the strength, it is desirable that the average crystal grain size is small. However, as the average crystal grain size decreases, the ductility deteriorates. Therefore, it is necessary to determine an optimum crystal grain size for a desired product shape. Preferably, it is 15.0 μm or less, more preferably 13.0 μm or less, and further preferably 11.0 μm or less. The average crystal grain size of ferrite may be measured according to the cutting method described in JIS G0552.
In order to obtain good ductility, the area ratio of ferrite is 90% or more, more preferably 95% or more, and still more preferably 99% or more. The balance is, for example, an oxide or iron carbide.

  Usually, breakage and fatigue breakage tend to occur from the surface of a steel sheet when bending or using a member obtained by processing a steel material. Therefore, in order to improve these characteristics, it is desirable that the crystal grain size of the steel sheet surface layer is particularly small. The crystal grain size of the steel sheet is greatly influenced by the concentration of elements in the steel, particularly P, and the crystal grain size tends to decrease as the P concentration increases.

Next, the enamel product according to this embodiment will be described.
The enamel product according to the present embodiment includes the steel plate according to the present embodiment. For example, it is a product obtained by performing processing, welding, and enamel processing on the steel sheet according to the present embodiment.

<Manufacturing method>
The preferable manufacturing method of the steel plate which concerns on this embodiment is demonstrated.
If the steel plate according to the present embodiment has the above-described configuration, the effect can be obtained, and the manufacturing method does not need to be limited. However, as will be described later, a production method including steps of steelmaking, casting, hot rolling, cold rolling, continuous annealing, and temper rolling is preferable because it can be stably produced.
In each step, preferable conditions will be described.
The manufacturing point is the control of oxides having the effect of improving the resistance to claw resistance by the oxide containing Fe and Mn and suppressing the abnormal grain growth during the enamel treatment. A relatively large oxide diameter is desirable for improving nail-foil resistance, and a small oxide diameter is desirable for suppressing abnormal grain growth. When the oxygen concentration in the steel is high, an oxide having a large diameter is generated. On the other hand, when the oxygen concentration is low, the oxide diameter is reduced.
The oxide of 0.1 to 1.0 μm is angular as shown in FIG. 1, and it is considered that free oxygen often reacts with the steel components after solidification. Therefore, by adjusting the free oxygen at the steelmaking stage and stirring the solidification interface by electromagnetic stirring to adjust the concentration of components such as oxygen at the solidification interface, the crystallization of the oxide of 0.1 to 1.0 μm is achieved. You can control the number.
Further, it is considered that inclusions of more than 1.0 to 10 μm have a round shape as shown in FIG. 2 and are often produced in a liquid state at the molten steel stage. Therefore, the number of inclusions of more than 1.0 and 10 μm or less can be controlled by controlling the aggregation and floating of inclusions by controlling the casting speed, the stirring of the molten steel, the degree of superheating of the molten steel, and the like.

<Steel making process, casting process>
It is desirable that ΔT (the degree of superheat of molten steel) in the mold is in the range of 20 to 35 ° C., and the casting speed is in the range of 1 to 1.5 m / min. By setting it as the above-mentioned conditions, inclusions having a large diameter can be aggregated and floated in the mold, and the number thereof can be controlled. In order to promote the aggregation of inclusions by giving a flow in the mold, the inside of the mold may be magnetically stirred.
Further, in order to precipitate a fine oxide during or after solidification, free oxygen in the mold is degassed by secondary refining or by adding a small amount of deoxidizing element that does not affect the oxide composition. It is preferable to cast at a temperature in the range of 1200 to 1500 ° C. at a rate of 1.0 to 5.0 ° C./sec after controlling to about 700 ppm. By setting it as the above-mentioned conditions, dissolved oxygen can remain at high temperature, and inclusions with a small diameter can be formed at low temperature.
That is, by controlling the steelmaking conditions and the casting conditions, it is possible to control the presence state of both the large diameter oxide and the small diameter oxide.
The amount of dissolved oxygen (free oxygen) can be measured in a tundish using an oxygen concentration cell. If the secondary refining is stable, it is not necessary to measure each time.

<Hot rolling process>
When heating the slab prior to hot rolling, the heating temperature is preferably 1150 to 1250 ° C. When the heating temperature is higher than 1250 ° C., the amount of primary scale generated is large and the yield is lowered. On the other hand, if it is less than 1150 degreeC, the rolling load will become high because of the temperature fall during rolling. Moreover, in hot rolling, it is preferable that a rolling rate is 30 to 90% and a finishing temperature is Ar3 to 950 ° C. The coiling temperature after hot rolling is preferably 550 to 750 ° C. The Ar3 temperature can be obtained by measuring thermal expansion after adding a thermal history and processing simulating hot rolling with a small test piece.
The oxide containing Fe and Mn produced in the steel making process and the casting process is stretched by hot rolling. By setting the hot rolling rate (cumulative rolling reduction in hot rolling) to 30% or more, it becomes possible to sufficiently stretch the oxide containing Fe and Mn in the steel. If the hot rolling rate exceeds 90%, the oxide in the steel may be stretched too much to obtain good nail resistance.
If the finishing temperature in the hot rolling is less than Ar3, the rolling is performed at or below the transformation point, and mechanical properties such as ductility as a product deteriorate, and at the same time, the strength change of the steel plate increases, so that the rolling tends to become unstable. In addition, when the finishing temperature is less than Ar3, the microstructure of the hot-rolled steel sheet becomes a mixed grain including coarse grains, and there is a concern that ridging may occur after processing in a cold-rolled annealed sheet using this hot-rolled steel sheet. . Therefore, the finishing temperature needs to be Ar3 or higher, more preferably 900 ° C or higher. On the other hand, when the finishing temperature exceeds 950 ° C., the crystal grain size becomes coarse, and it becomes difficult to secure a desired strength.
The coiling temperature after hot rolling is preferably 550 ° C. or higher. If it is less than 550 degreeC, the structure form after cold rolling and continuous annealing will become difficult to ensure the ductility and r value which are required for a process. Further, when the coiling temperature exceeds 750 ° C., the crystal grain size becomes large, so that it is difficult to ensure the desired steel plate strength.

<Cold rolling process>
The hot-rolled steel sheet is pickled as necessary and then cold-rolled. The cold rolling rate in cold rolling is important for determining the characteristics of the product, and is preferably 65 to 85%. The oxide containing Fe and Mn formed in the steel making process and the casting process is stretched in accordance with the rolling rate in the hot rolling process. Thereafter, the film is further drawn in a cold rolling process, but cold rolling is processing at about 150 ° C. at the maximum, and the above oxide is hard and thus is not easily drawn. Therefore, it is preferable to perform cold rolling at a cold rolling rate of 65% or more in order to stretch the film appropriately.
At this time, voids are generated at both ends in the rolling direction of the oxide. The presence of the voids effectively works against the nail skip resistance but works against the ductility. For this reason, the presence of more voids than necessary causes a decrease in ductility, which in turn impairs the workability and the strength characteristics of the product after enamel treatment. For this reason, the upper limit of the cold rolling rate is set to 85%. When cold rolling is performed at a cold rolling rate higher than this, the voids formed at the initial stage of rolling can be seen to be crushed and disappear due to an increase in the cold rolling rate. However, since they are not systematically connected, it is assumed that the introduction of strain due to processing becomes a starting point of fracture and deteriorates ductility.

<Continuous annealing process>
Continuous annealing is performed on the cold-rolled steel sheet. As for the annealing temperature of a continuous annealing process, it is desirable to set it as 700-850 degreeC. For the purpose of characterizing mechanical properties such as strength, the annealing temperature may be less than 700 ° C. On the other hand, when the annealing temperature exceeds 850 ° C., it is preferable for mechanical properties because ductility and the like are improved. However, voids generated in the cold rolling process tend to disappear due to diffusion, and the nail skip resistance deteriorates. To do. For this reason, it is preferable that the annealing temperature of a continuous annealing process sets an upper limit to 850 degreeC.

  After annealing, temper rolling may be performed mainly for shape control. In temper rolling, the amount of strain introduced into the steel sheet changes depending on the temper rolling rate simultaneously with shape control. At this time, when the temper rolling ratio is increased, that is, when the amount of strain introduced into the steel sheet is increased, abnormal grain growth during enamel processing is promoted. For this reason, it is not desirable for the temper rolling rate to give more strain than necessary with the rolling rate at which shape control is possible as the upper limit. From the viewpoint of shape control, the rolling rate of temper rolling is preferably 1.5% or less.

  As described above, a steel plate having desired characteristics, specifically a cold rolled steel plate for enamel can be obtained.

  The enamel product according to the present embodiment is obtained by processing the steel plate according to the present embodiment into a predetermined shape, then assembling the steel plate into a product shape by welding or the like, and performing enamel processing. The enamel treatment may be performed under known conditions. For example, by heating a steel plate coated with glaze to, for example, 800 to 850 ° C. and holding it for 1 to 10 minutes, the vitreous vitreous and the steel plate can be brought into close contact with each other. That's fine.

  Steels having the composition shown in Tables 1 and 2 were melted in a converter, and slabs were formed by continuous casting. ΔT in casting mold during casting, casting speed is as shown in Tables 3 and 4, electromagnetic stirring is used, cooling rate in the range of 1200 to 1500 ° C, and dissolved oxygen amount is controlled in the range of Table 3 and Table 4. The number and density of oxides and the amount of oxygen were controlled. The dissolved oxygen amount (free oxygen) was confirmed by the method described above. These slabs were heated at a temperature of 1150 ° C. to 1250 ° C. in a heating furnace, hot-rolled at a finishing temperature of 900 ° C. or higher, and wound at 700 to 750 ° C. to obtain hot rolled steel sheets. And after pickling, the rolling rate of cold rolling was changed in the range of Table 3 and Table 4, and it was set as the cold-rolled steel plate, and also after performing continuous annealing at 780 degreeC, temper rolling was given, and plate thickness 0. The steel plate was 8 mm. In order to make the sheet thickness after temper rolling constant, the sheet thickness of the hot-rolled steel sheet was changed with respect to the rolling rate of cold rolling.

Various evaluation was implemented using said steel plate.
<Mechanical properties>
For mechanical properties, tensile strength (TS) and elongation at break (EL) were measured according to JIS Z2241, using a JIS No. 5 test piece as a tensile test. Those having an elongation at break of 30% or more were judged to be excellent in moldability.
<Observation of structure and precipitate>
For the precipitates in the steel, the cross section parallel to the cold rolling direction was observed with an SEM, and the diameter and number density of the oxide were measured by the method described above. The average crystal grain size of ferrite was measured using a cutting method described in JIS G0552.
<Strength characteristics after enamel treatment>
In addition, in order to evaluate the strength reduction due to grain growth after enamel treatment, the steel sheet was subjected to heat treatment simulating enamel at a furnace temperature of 830 ° C. for 5 minutes, and the tensile strength was obtained by a tensile test in the same manner as described above. The ratio of the strength after heat treatment to the previous strength was determined.
In consideration of the stability of the strength after the heat treatment, the Vickers hardness of the steel material was measured before and after the heat treatment, and the ratio before and after the heat treatment was determined for the minimum value of the measurement results.
Specifically, in each of the steel materials before and after the heat treatment, the 5-point Vickers hardness was measured at a load of 0.98 N at a position of 1/4 of the plate thickness, and the average value was taken as the hardness at the measurement position. Moreover, said measurement was performed in the 10 or more position which opened the space | interval of 20 mm or more, and the minimum value of the measurement result (hardness) was obtained in each before and after heat processing. And the ratio was calculated | required about the minimum value of the measurement result before and behind heat processing.
The tensile strength after enamel treatment is 0.85 (85%) or more of the tensile strength before enamel treatment, and the minimum value of hardness after enamel treatment is 0.85 or more of the minimum value of hardness before enamel treatment. In this case, it was determined that the strength reduction due to the enamel treatment could be stably suppressed.
<Aging resistance>
Aging resistance was evaluated by an aging index. The aging index is a yield stress difference between before and after JIS No. 5 tensile test piece is applied with 10% pre-strain and subjected to aging at 100 ° C. for 60 minutes. When the yield stress difference was 30 MPa or less, it was judged that the aging resistance was excellent (OK).

The enamel characteristics were examined as follows.
<Nail jump resistance>
The nail jump resistance was evaluated by a dry type powder electrostatic coating method in which a glaze of 100 μm was applied and baked in the atmosphere at a furnace temperature of 830 ° C. for 5 minutes. The steel plate after the enamel treatment is put into a constant temperature bath at 160 ° C. for 10 hours, and a nail skipping acceleration test is performed. A: Excellent, B: Slightly excellent, C: Normal, D: There is a problem The case of D was determined to be rejected.
<Enamel adhesion>
For enamel adhesion, the steel plate treated with the enamel was dropped from a 1-meter height with a 2 kg ball head weight, and the enamel peeling state of the deformed part was measured with 169 palpation needles. It was evaluated with. If the area ratio of the unpeeled portion was 40% or more, there was no problem, and if it was less than 40%, it was judged that the adhesion was inferior.
<Appearance>
The appearance after enamel treatment is the same as above, and visually observe the enameled steel sheet, and observe the condition of bubbles and sunspots. “Excellent”, “Excellent”, “Normal”, “Slightly inferior”, “Remarkably” Evaluated in 5 grades, “Inferior”, “Excellent”, “Excellent”, “Normal”, “Slightly inferior”, no problem, and “Remarkably inferior”, it was judged that bubbles and black spots occurred. .

The evaluation results are summarized in Tables 5 and 6. In the example of the present invention, no precipitate having a diameter of more than 10 μm was observed in the oxide containing Fe and Mn in the steel. In addition, among oxides containing Fe and Mn, the number of oxides having a diameter of more than 1.0 μm and 10 μm or less per unit area within the range of the present invention may satisfy the nail resistance. confirmed. Further, among oxides containing Fe and Mn per unit area, the number of oxides having a diameter of 1.0 μm or less per unit area within the range of the present invention is a decrease in strength due to grain growth after enamel treatment. Was confirmed to be small. In the description regarding the oxide density in Tables 5 and 6, E represents an index, for example, 1.0E + 03 indicates 1.0 × 10 ³ .
In all of the inventive examples, 90% or more of the ferrite structure was present.

  From the results of Tables 5 and 6, within the scope of the present invention, the conventional enamel steel plate is superior in nail skipping resistance without deteriorating aging resistance, enamel adhesion, and appearance, and tensile by enamel treatment. It was confirmed that it is possible to provide an enameled steel sheet that can stably suppress a decrease in strength.

  The steel sheet according to the above aspect of the present invention is excellent in formability, claw resistance and strength properties after enamel treatment when applied to kitchen utensils, building materials, energy fields, etc. after enamel treatment. Therefore, it is suitable as a steel plate for enamel and has high industrial applicability.

Claims (6)

% By mass
C: 0.0060% or less,
Si: 0.0010 to 0.050%,
Mn: 0.05 to 0.50%,
P: 0.005 to 0.100%,
S: 0.0500% or less,
Al: 0.0010 to 0.010%,
Cu: 0.010 to 0.045%,
O: 0.0250-0.0700%,
N: 0.0010 to 0.0045%,
The balance: Fe and impurities,
The structure contains ferrite, the average crystal grain size of the ferrite at a position of 1/4 of the plate thickness from the surface in the plate thickness direction is 20.0 μm or less,
An oxide containing Fe and Mn is included, and among the oxides, the number density of the oxide having a diameter larger than 1.0 μm and not larger than 10 μm is 1.0 × 10 ³ pieces / mm ² or more, 5.0 steel sheet, wherein × 10 is ^four or / mm ² or less, and is diameter number density of the oxides of 0.1~1.0μm is 5.0 × 10 ³ cells / mm ² or more. In the impurities, in mass%,
A total of one or more of B, Cr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg: limited to 0.100% or less. Item 2. A steel sheet according to item 1. In the impurities, in mass%,
Nb: 0.010% or less,
The steel plate according to claim 1, wherein   The steel sheet according to claim 1 or 2, wherein the steel sheet is a cold-rolled steel sheet.   It is a steel plate for enamels, The steel plate of Claim 1 or 2 characterized by the above-mentioned. An enamel product comprising the steel plate according to any one of claims 1 to 3.