JP2006219717A - Steel sheet for vessel having superior deformation resistance, surface characteristic and weldability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for a vessel, which has superior deformation resistance, surface characteristics and weldability, and keeps them from being deteriorated, and to provide a manufacturing method therefor. <P>SOLUTION: The steel sheet comprises, by mass%, 0.0800% or less C, 0.0600% or less N, 2.0% or less Si, 2.0% or less Mn, 0.10% or less P, 0.05% or less S, 2.0% or less Al, and the balance iron with unavoidable impurities. The manufacturing method includes the step of controlling the concentration of carbon and/or nitrogen so that the total amount of C and N can be 6,000 ppm or less in the surface layer of 1/8 sheet thickness deep from the surface of the steel sheet, and the total amount of C and N in the center layer of 1/4 sheet thickness thick of the steel sheet can be less than the total amount of C and N of the surface layer by 30 ppm or more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a steel plate for containers that is remarkably excellent in deformation resistance, surface characteristics, and weldability suitable for use in metal containers such as beverage cans, and a method for producing the same.

In recent years, container steel sheets have been made extremely thin to reduce the cost of containers, and one of the problems that has become apparent when containers are manufactured using such ultrathin container steel sheets is the deformation of containers. is there. This deformation is not only due to the action of external force that occurs during container handling in the general market after filling the container and the contents, but also the increase and decrease of the internal pressure of the container, that is, the pressure increase and contents during the heat treatment of the contents This is caused by a decompression process for holding an object, a pressure increase which is essential depending on contents such as carbonated beverages, and a change in internal pressure accompanying a temperature change during distribution or holding.
In order to improve the deformation resistance of the container, it is necessary to use not only the container design but also a harder material. However, generally hard materials have low ductility and cause problems such as material breakage during can molding.

On the other hand, steel plates for containers are made of materials with various hardness depending on the application. To improve productivity, prepare the same material in the upper process, and make the material by changing the conditions in the lower process as much as possible. Is efficient. As a method for this, there is a method based on the rolling reduction of re-rolling (2CR) after annealing.
However, in this method, in order to produce a steel sheet with the same thickness by changing the rolling reduction of re-cold rolling (2CR), it is necessary to create the cold-rolled sheet thickness in the previous process separately. I cannot expect a marked improvement in sex. In addition, since dislocation strengthening by processing is used as a curing means, defects such as deterioration in ductility and inferior flange formability during can molding are likely to occur compared to normal solid solution strengthening.

Therefore, as a method for adjusting the strength without significantly impairing ductility in the steps after annealing, a technique by nitriding during annealing has been proposed (see Patent Documents 1 to 3).
Further, the present inventor has applied a technique for efficiently improving the deformation resistance of a member formed of a steel plate, particularly when applying the technology for nitriding a steel plate to a thin steel plate having a thickness of 0.4 mm or less. It has been proposed (see Patent Document 4).
Japanese Patent Laid-Open No. 08-170122 Japanese Patent Laid-Open No. 08-176788 JP 2001-107148 A JP 2004-218061 A

By the way, with the conventional nitriding technology during annealing, the surface hardness can be separated by nitriding and the productivity can be improved. However, by controlling the hardness of the inner layer, the resistance to ultra-thin materials can be improved. There was a problem that improvement in deformability was not sufficient. In addition, the manufacturing method has been proposed to be maintained in an ammonia-containing atmosphere, but the nitriding method for controlling the material of the inner surface layer is hardly mentioned, and these techniques are used to describe this method. It is difficult to improve the deformation resistance of a container formed of an ultrathin material intended by the invention.
Further, although the technique proposed by the present inventors for nitriding a steel plate can efficiently improve the deformation resistance of a member formed by a steel plate having a thin plate thickness of 0.4 mm or less, depending on the conditions, There is a possibility that not only the deformability but also the surface characteristics and weldability may cause problems.

For example, with regard to deformation resistance, the effect of improving deformability is small for the amount of nitriding by nitriding treatment, and for surface characteristics, not only surface treatment properties such as adhesion of plating, coating, film, etc. are deteriorated. Further, the appearance such as surface gloss may be deteriorated.
In addition, with regard to weldability, the appropriate welding current range becomes remarkably small so as to hinder production, and in particular, a decrease in surface properties can cause deterioration in corrosion resistance and weldability, It can be a big problem.

  The present invention has been made in view of the above-mentioned circumstances. By applying nitriding to the material of the surface layer and the inner layer of the material, the deformation which becomes a problem in a container manufactured using an extremely thin steel plate is applied. When controlling and improving, deformation resistance, surface characteristics, weldability that can prevent deterioration of weldability, especially when only nitriding is applied, are extremely good. It aims at providing a steel plate and its manufacturing method.

  The present inventor has improved the intended deformation resistance even when nitriding is applied while examining the relationship between the composition of the steel sheet having a thickness of 0.4 mm or less manufactured through the nitriding process and the nitriding conditions and the material. It turns out that there are cases where the effect is not clear, and at the same time the surface properties and weldability deteriorate, and as a result of further investigation, by limiting the components to a specific range and further using heat treatment in a carburizing atmosphere, As well as controlling the material of the surface layer and inner layer of the steel, it is possible to satisfactorily control the surface condition of the steel sheet that affects the surface characteristics and weldability, and the deformation resistance desired for containers made of ultra-thin steel sheets The present invention has been completed by finding that it is possible to significantly improve the properties, surface characteristics and weldability, and the gist of the present invention is as follows.

(1) A steel plate manufactured through a carbon and / or nitrogen concentration control step,
In mass%, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% Hereinafter, Al: 2.0% or less, the balance consists of iron and inevitable impurities,
The total amount of C and N in the surface layer whose depth from the surface of the steel sheet is 1/8 of the plate thickness is 6000 ppm or less,
The difference between the total amount of C and N in the surface layer and the total amount of C and N in the central layer having a layer thickness of 1/4 of the plate thickness of the steel sheet is 30 ppm or more. Steel sheet with extremely good surface properties and weldability.

(2) The area ratio of nitrides and / or carbides on the surface of the steel sheet is that of nitrides and / or carbides in a cross section parallel to the surface of the central layer, which is ¼ of the thickness of the steel sheet. The steel sheet having an extremely good deformation resistance, surface characteristics, and weldability according to (1), wherein the steel sheet has an area ratio of 1.5 times or more.

(3) The surface roughness (Ra) of the steel sheet is 0.90 μm or less, and the number of concavo-convex vertices per inch of the surface length of the steel sheet is 250 or more ( A steel sheet having remarkably good deformation resistance, surface characteristics and weldability as described in 1) or (2).

(4) Further, by mass%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.015% or less, Cr: 20% or less, Ni: 10% or less, Cu: 5% or less (1), (2) or (3), wherein the steel sheet has excellent deformation resistance, surface characteristics, and weldability, characterized by containing at least one selected from the group of .

(5) Further, it is characterized by containing, in mass%, one or more selected from the group of Sn, Sb, Mo, Ta, V and W in a total of 0.1% or less ( 1) A steel plate having remarkably good deformation resistance, surface characteristics, and weldability according to any one of (4).

(6) The steel sheet having excellent deformation resistance, surface characteristics, and weldability according to any one of (1) to (5), wherein the steel sheet has a thickness of 0.400 mm or less.

(7) By mass%, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0 0.05% or less, Al: 2.0% or less, a steel sheet manufacturing method in which hot rolling, cold rolling, and recrystallization heat treatment are sequentially performed on a steel material in which the balance is iron and inevitable impurities,
At the same time as or after the recrystallization heat treatment, the concentration of carbon and / or nitrogen is controlled,
The total amount of C and N in the surface layer whose depth from the surface of the steel material is 1/8 of the plate thickness is 6000 ppm or less, and the total amount of C and N in the surface layer is 1/4 of the plate thickness of the steel plate. A method for producing a steel sheet having significantly good deformation resistance, surface characteristics, and weldability, characterized in that a difference between the total amount of C and N in the central layer of the layer thickness is 30 ppm or more.

(8) The carbon concentration control is carburizing treatment, and the nitrogen concentration control is nitriding treatment. Deformation resistance, surface characteristics, and weldability according to (7) are remarkably good. A method of manufacturing a steel sheet.

(9) The carburizing treatment is characterized in that the steel material is maintained in an atmosphere containing carburizing gas at 0.02% or more in a state of 550 to 800 ° C. for 1 second or more and 360 seconds or less (8 The method for producing a steel sheet having excellent deformation resistance, surface characteristics and weldability as described in (1).

(10) The nitriding treatment is characterized in that the steel material is held in an atmosphere containing ammonia gas at 0.02% or more in a state of 550 to 800 ° C. for 1 second or more and 360 seconds or less (8). A method for producing a steel sheet having excellent deformation resistance, surface characteristics and weldability as described in 1.

(11) When performing the nitriding treatment, 0.10 <Carburizing gas concentration (volume%) / Ammonia gas concentration (volume%) <10. Deformation resistance and surface according to (10) A method of manufacturing a steel sheet with extremely good characteristics and weldability.

(12) The product of the temperature and time in the thermal history of the temperature range of 550 ° C. or higher for either one or both of the carburizing treatment and the nitriding treatment is 48000 or less, and then the average from 550 ° C. to 300 ° C. The method for producing a steel sheet having excellent deformation resistance, surface characteristics, and weldability according to (9), (10) or (11), wherein the cooling rate is 10 ° C./second or more.

(13) The deformation resistance according to any one of (7) to (12), wherein cold rolling is performed again at a rolling rate of 0.5% to 70% after the recrystallization heat treatment. , A method for producing a steel sheet with remarkably good surface properties and weldability.

  According to the steel sheet having extremely good deformation resistance, surface characteristics and weldability of the present invention, the total amount of C + N in the surface layer whose depth from the surface of the steel sheet is 1/8 of the plate thickness is 6000 ppm or less, and Since the difference between the total amount of C + N of the surface layer and the total amount of C + N in the central layer having a layer thickness of 1/4 of the plate thickness of the steel plate is 30 ppm or more, a metal container or the like manufactured using this steel plate Deformation resistance, surface characteristics and weldability during production of various members having a three-dimensional structure can be improved.

  According to the method for producing a steel sheet having excellent deformation resistance, surface characteristics, and weldability according to the present invention, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% by mass%. Below, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less, hot-rolled to a steel material whose balance is iron and inevitable impurities When performing the cold rolling and the recrystallization heat treatment in sequence, since the carbon and / or nitrogen concentration control treatment is performed simultaneously with or after the recrystallization heat treatment, the total amount of C and N in the surface layer and the center layer of the steel material Can be controlled below a predetermined amount. Therefore, it is possible to easily manufacture a steel sheet having extremely good deformation resistance, surface characteristics, and weldability.

An embodiment of a steel plate having excellent deformation resistance, surface characteristics, and weldability and a method for producing the same according to the present invention will be described.
Note that this embodiment is described in detail for better understanding of the gist of the invention, and thus does not limit the present invention unless otherwise specified.

The steel sheet of the present invention is a steel sheet manufactured through a carbon and / or nitrogen concentration control step,
In mass%, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% Hereinafter, Al: 2.0% or less, the balance consists of iron and inevitable impurities,
The total amount of C and N in the surface layer whose depth from the surface of the steel sheet is 1/8 of the plate thickness is 6000 ppm or less,
And the difference of the total amount of C and N of the said surface layer and the total amount of C and N in the center layer of the layer thickness of 1/4 of the plate | board thickness of the said steel plate is 30 ppm or more.

Here, the reason why the composition of the steel sheet is limited as described above will be described.
C is most effective for the characteristics of the steel sheet, and is particularly necessary for avoiding deterioration of workability, and its content is preferably 0.0800% or less, more preferably 0.0600% or less. More preferably, it is 0.040% or less.
In the steel sheet of the present invention, N having the same properties as C can be increased after annealing by heat treatment, so the C content required from the standpoint of securing the strength may be low.

The content of C set from such a viewpoint can ensure the necessary strength even if it is 0.0050% or less, and may be sufficient if the necessary strength can be ensured even if it is 0.0020% or less. In addition, when the C content is 0.0015% or less, there is a balance with the N content, but in an ordinary container, it is possible to manufacture a non-standard extremely soft material.
Also, if the surface hardening characteristic of the present invention is performed by heat treatment in a carburizing atmosphere, the effects on workability, deformation resistance and weldability, and the stability of these effects related to fluctuations in manufacturing conditions are improved. The content of C when considered is preferably 0.0050 to 0.035%, more preferably 0.010 to 0.030%, and still more preferably 0.015 to 0.025%.

Similar to the C content, the N content is added to avoid deterioration of workability and is necessary to avoid deterioration of surface characteristics such as plating.
A preferred range for the N content is 0.600% or less, and a more preferred range is 0.300% or less.
Further, from the viewpoint of workability, it is needless to say that the N content is preferably low, preferably 0.0050% or less, and more preferably 0.0030% or less.
In addition, if the surface hardening characteristic of the present invention is performed by heat treatment in a nitriding atmosphere, the effects on workability, deformation resistance, weldability, and the stability of these effects related to fluctuations in manufacturing conditions are improved. The content of N when considered is preferably 0.0050 to 0.100%, more preferably 0.010 to 0.080%, and still more preferably 0.020 to 0.060%.

  It should be noted here that as described later, the C and N contents in the final product are present in order to impart a deformation resistance effect, good surface characteristics, and weldability of the can. The effect is different from C and N existing before heat treatment in an atmosphere or heat treatment in a nitriding atmosphere.

Si is added to adjust the strength, but if it is too much, the workability deteriorates, so the content is made 2.0% or less.
In the steel sheet of the present invention, Si forms C, N and carbides and nitrides that enter the steel by nitriding at the grain boundaries, and not only causes brittle cracks, but also impairs the effects of the present invention. In some cases, depending on the steel composition, the content may need to be limited to 1.5% or less, and further to 1.0% or less.

Since Mn is added for adjusting the strength, it is preferable to add it. However, if it is too much, the workability deteriorates, so the content is made 2.0% or less.
P is added for adjusting the strength, and if it is too much, the workability deteriorates, so the content is made 0.10% or less.
S deteriorates the hot ductility and becomes an impediment to casting and hot rolling, so the smaller the content, the more preferable it is 0.05% or less.
Al is an element added for deoxidation, but if the content is large, casting becomes difficult, and there is a risk of causing problems such as an increase in surface wrinkles, so the content is made 2.0% or less.

In addition to the above composition, the steel sheet of the present invention has
Further, in terms of mass%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.015% or less, Cr: 20% or less, Ni: 10% or less, Cu: 5% or less It is preferable to contain one or more selected ones.

Here, the reason for limiting the content of each element as described above will be described.
Since Ti raises the recrystallization temperature of a steel plate and significantly deteriorates the annealing passability of the ultrathin steel plate targeted by the present invention, the smaller the content, the better. The content is made 0.050% or less. In normal applications where a particularly high r value is not required, it is not necessary to add Ti, and therefore the content thereof is 0.03% or less, more preferably 0.02% or less.
On the other hand, Ti is left in a solid solution state before the carburizing heat treatment, and Ti carbide is formed in the steel sheet surface layer portion in particular by C entering the steel sheet from the surface with carburizing, and only the steel sheet surface layer is preferentially cured. Further, the effect aimed by the present invention can be further enhanced.

In addition to the above composition, the steel plate of the present invention is provided with the characteristics not specified in the present invention,
Furthermore, it is preferable to contain 0.1% or less in total of one or more selected from the group consisting of Sn, Sb, Mo, Ta, V, and W by mass%.

Here, the division | segmentation of the site | part of the thickness direction of a steel plate is demonstrated based on FIG.
Here, the surface layer portion whose depth from the surface of the steel plate is 1/8 of the plate thickness is referred to as “surface layer 1/8 thickness”, and the central layer portion of the steel plate thickness of 1/4 is referred to as “center”. Referred to as "layer 1/4 thickness". Corresponding portions of these “surface layer 1/8 thickness” and “center layer 1/4 thickness” are shown in FIG.
In addition, although the site | part corresponding to "surface layer 1/8 thickness" exists in both surfaces of a steel plate, in this invention, what corresponds to the limited range of this invention is made into any one surface of a steel plate. It is relatively easy to change the front and back carbon distribution and the carbide distribution on the surface by carburizing method, surface treatment before carburizing, and some treatment after carburizing. This also applies to steel sheets made of different materials. This is because the deformation resistance intended by the present invention can be obtained on only one side, and at least the carburized surface can improve surface characteristics and weldability.

Here, “the amount of C + N of the surface layer 1/8 thickness” means the total amount of C and N in the surface layer 1/8 thickness, and after polishing the steel plate and leaving only a corresponding portion, Measure by performing analysis.
Similarly, the “C + N amount of the center layer 1/4 thickness” is the total amount of C and N in the center layer 1/4 thickness, and after polishing and removing both surfaces, Use analysis values.
This analysis value is determined by an analysis result of an appropriate number of times so that an abnormal value can be excluded, although a slight variation usually occurs due to local segregation of component elements or non-uniformity of the structure.

In the steel sheet of the present invention, the amount of C + N having a surface layer thickness of 1/8 is 6000 ppm or less.
The average thickness of the surface layer 1/8 is 6000 ppm. When the steel material of the present invention is produced under normal process conditions, the C + N amount on the outermost surface is 10,000 ppm or more, and surface problems such as defective plating occur. This is because it becomes easier.
Therefore, the upper limit of the C + N amount of the surface layer 1/8 thickness is preferably 3000 ppm or less, more preferably 2000 ppm or less.

In the present invention, it is necessary to make a difference in the amount of C + N between the surface layer portion and the center layer portion of the steel sheet.
This difference is defined by [C + N amount of 1/8 thickness of surface layer]-[C + N amount of 1/4 thickness of center layer], and this value is preferably 30 ppm or more. The reason is that if this value is less than 30 ppm, there is a risk that the intended deformation resistance of the present invention is reduced.
This value is preferably 50 ppm or more, more preferably 70 ppm or more, still more preferably 100 ppm or more, still more preferably 150 ppm or more, still more preferably 200 ppm or more, and further preferably 300 ppm or more.

Furthermore, in this invention, the size and number density of the carbide | carbonized_material or nitride which exist in the specific position of the plate | board thickness direction of a steel plate are prescribed | regulated. Hereinafter, the carbide or nitride to be defined in the present invention is defined as “specific precipitate”.
This specific precipitate can be identified by a diffraction pattern such as an electron microscope, an X-ray analyzer attached to the electron microscope, or chemical analysis. In order to control the size, area ratio, and number density of the specific precipitate, the temperature and time in the temperature range of 550 to 800 ° C., which will be described later, and the cooling rate in the cooling process from 550 to 300 ° C. are appropriately controlled. This effect is effective under normal conditions, as is the case with general precipitate formation, the cooling rate is fast, the lower the temperature, the finer and denser the nitride size, while the slower the cooling rate, The longer the time required for the predetermined cooling temperature, the easier the nitride grows and the nitride size becomes coarser.

The specific precipitate of the present invention includes not only a precipitate of carbide or nitride alone, but also a composite precipitate of carbide and nitride, and further a composite precipitate of oxide, sulfide and the like.
When a composite precipitate is formed, it is difficult to specify the type of one precipitate and the size of each compound, but it is clearly divided into a portion where one precipitate is carbide or nitride and the other Except in some cases, it is a single specific precipitate.
The method for observing the specific precipitate is not particularly limited, but is directly observed using an energy dispersive X-ray microanalyzer (EDX) attached to a scanning electron microscope (SEM) or other surface observation apparatus. It is also possible to observe an extracted replica or the like obtained by the SPEED method. However, in the case of an extracted replica or the like, it is necessary to pay attention when creating the replica so that the replica consists only of information on the observation surface on the steel plate. This is because, in the case of the SPEED method or the like, if the amount of electrolytic extraction is too large, information in the plate thickness direction is superimposed, and more precipitates are observed than when the steel plate is directly observed.

Therefore, the amount of electrolytic extraction should be kept within 2 μm in terms of steel plate thickness. In addition, when the specific precipitate is very fine and extraction is not good, the thin film may be observed with a transmission electron microscope (TEM).
The determination of the composition is analyzed by EDX, and when the observed non-metallic element is C, carbide is used, and when N is nitride, nitride is used. Further, because the size is small, the characteristic spectrum of C or N is not clear, but Fe, Ti, Nb, B, Cr, etc. are detected, and clear spectra of O, S, etc. are not observed, and carbides Alternatively, a precipitate that can be almost determined to be a specific precipitate from a form comparison between the nitride and another precipitate that can be specified is also a specific precipitate in the present invention.

Moreover, you may use an electron beam diffraction pattern etc. for the qualification of a precipitate. The identification of the specific precipitate is not based on a technique such as EDX or electron diffraction pattern, and any analytical instrument that is currently significantly improved in performance may be used. In short, it is only necessary that the type, size, and number density of the precipitates can be determined by an appropriate method.
The lower limit value of the specific precipitate is set to 0.02 μm because the resolution of a commonly used analytical instrument at the time of filing of the present invention is approximately 0.02 μm.

Moreover, although the nitride which the shape extended | stretched may be seen, about the thing whose shape is not isotropic, let the average of a major axis and a minor axis be the diameter of the precipitate.
Needless to say, it is necessary to polish the steel plate in the cross-sectional observation at the 1/4 thickness position of the steel plate. Alternatively, etching can be performed.
When the surface of the steel plate is polished or etched, the observation surface is not strictly the surface of the steel plate, and it is needless to say that it is preferable not to perform processing, and any method that does not require polishing or the like should be selected. Even when the surface is processed, the reduction in plate thickness due to the processing should be limited to about 2 μm or less.
In addition, the area ratio and number are not based on the observation from the surface direction of the plate (two-dimensional observation) but from the cross-sectional direction (observation on the surface and 1/4 thickness position is not two-dimensional but one-dimensional). You may calculate a density, a diameter, etc. Note that the number and diameter of nitrides can also be obtained using an image analysis device or the like.

One of the characteristics of the present invention is to make a difference in the state of specific precipitates on the steel sheet surface.
Hereinafter, the surface state of the steel sheet, which is an important requirement of the present invention, will be described.
The present invention mainly controls the form of specific precipitates on the surface of a steel sheet, and the steel sheet is subjected to carbon and / or nitrogen concentration control, that is, heat treatment in a carburizing atmosphere or heat treatment in a nitriding atmosphere. By covering the surface with a material different from that of the steel plate, the surface non-uniformity is increased, and the characteristics related to the target surface state are improved.
In addition, since the properties related to the surface state depend on the form of specific precipitates formed on the surface, the concentration of carbon and / or nitrogen is controlled, that is, the heat history after heat treatment in a carburizing atmosphere or heat treatment in a nitriding atmosphere. The specific precipitates are also controlled depending on the cooling conditions.

As a method for limiting the state of the specific precipitate on the surface of the steel sheet, in the present invention, the area ratio of the specific precipitate is used, and the area ratio of nitride and / or carbide on the surface of the steel sheet and 1 / th of the plate thickness of the steel sheet. The ratio of the area ratio of nitride and / or carbide in the cross section parallel to the surface of the central layer having a layer thickness of 4, ie, [area ratio of nitride and / or carbide on the surface] / [plate thickness 1 / 4 area area ratio of nitride and / or carbide], and this ratio is 1.5 or more, preferably 3 or more, more preferably 6 or more, more preferably 10 or more, and still more preferably 30 or more, more preferably 100 or more.
When this ratio is less than 1.5, the effect of the present invention is reduced and the intended steel sheet cannot be obtained. On the other hand, it goes without saying that the effect of the present invention basically increases as the ratio increases.

The area ratio of the specific precipitate can be similarly defined by comparing the steel plate of the final product with the steel plate before being subjected to heat treatment in a carburizing atmosphere or heat treatment in a nitriding atmosphere. In this case, the surface nitride and / or carbide area ratio in the final product steel sheet, and the surface nitride and / or carbide area ratio in the steel sheet before heat treatment in a carburizing atmosphere or heat treatment in the nitriding atmosphere Ratio, that is, [area ratio of nitride and / or carbide on the surface of the final product plate] / area ratio of nitride and / or carbide on the surface of the steel plate before nitriding or carburizing treatment] In this case as well, the ratio is set to 1.5 or more, preferably 3 or more, more preferably 6 or more, still more preferably 10 or more, still more preferably 30 or more, and still more preferably 100 or more, as described above.
Needless to say, the larger the ratio, the greater the effect of the present invention.

There is no problem even if this specific precipitate covers the entire surface of the steel sheet. However, the specific deposit formed in a film shape is easy to break, and it is not a little broken in the sheet passing in the manufacturing process. Therefore, it is necessary to pay attention to the case where the film of the specific precipitate on the surface is very thick, which may be the starting point of the fracture of the steel plate or may be recognized as some surface defect. Therefore, high concentration of C and N on the surface of the steel sheet caused by excessive addition of C and N to the steel material should be avoided.
Further, as the form of the specific precipitate on the surface, it is preferable that the fine precipitates are uniformly dispersed rather than the coarse ones being dispersed sparsely.
However, too fine ones may have a small contribution to the target surface modification effect of the present invention, so those having a diameter of 0.10 μm or more are targeted.

Another feature of the present invention is to control the surface roughness of the steel sheet.
Hereinafter, control of the surface roughness of the steel sheet of the present invention will be described.
Although various things can be considered about the surface roughness of a steel plate, in this invention, suppose that surface roughness (Ra) and the number of the peak of an unevenness | corrugation per inch (PPI) are employ | adopted.
The method for measuring the surface roughness (Ra) and the PPI is not particularly limited, and a commonly used method such as a stylus method or a laser method, or two-dimensional or three-dimensional measurement is used.

In the present invention, the surface roughness (Ra) of the steel sheet is 0.90 μm or less and the PPI is 250 or more.
Here, when the surface roughness (Ra) exceeds 0.90 μm or the PPI is less than 250, the characteristics caused by the surface irregularities such as color tone, adhesion, weldability and the like intended by the present invention are obtained. to degrade.
The surface roughness (Ra) is preferably 0.80 μm or less, more preferably 0.70 μm or less, further preferably 0.60 μm or less, and further preferably 0.50 μm or less.
The PPI is preferably 300 or more, more preferably 350 or more, still more preferably 400 or more, still more preferably 450 or more, and further preferably 500 or more.

Qualitatively, it is preferable that unevenness with uniform height exists at a high density.
The lower limit of the surface roughness (Ra) is not particularly limited, and is controlled to a value according to the purpose by nitriding / carburizing conditions, temper rolling conditions, and the like. This lower limit value is practically 0.02 μm or more.
Further, the upper limit of PPI is not limited, and is controlled by nitriding / carburizing conditions, temper rolling conditions, and the like according to the purpose.

Basically, the surface roughness (Ra) is lower and the PPI is higher when the above-mentioned independent specific precipitates or the independently exposed steel surface are finely dispersed.
Specifically, short-time nitridation is performed in a high-concentration ammonia atmosphere and short-term carburization is performed in a high-concentration carburizing atmosphere, and then cooling is performed at a high speed. Of course, the surface condition is the previous steel composition, crystal grain size, annealing temperature and cold rolling conditions, as well as rolling reduction and number of passes during temper rolling after nitriding and carburizing, roll roughness, metal plating Since it is also affected by plating conditions and the like, it is difficult to limit to a specific range, but the basic control is the same as that normally performed.

In the conventional technology, in order to control the surface roughness of the steel sheet, the unevenness of the rolling roll is transferred in the temper rolling after annealing, or the form of the metal coated on the surface by special electrolytic treatment or plating is controlled. Furthermore, since the roughness strongly depends on the state of adhesion to the surface of the steel plate such as metal plating, the form of the coating by plating has been precisely performed.
On the other hand, in the present invention, since these conditions are hardly received, a great production advantage can be obtained.

For example, with regard to the unevenness of the rolling roll, conventionally, the unevenness of the rolling roll is worn out by rolling, and therefore it is necessary to frequently perform replacement of the rolling roll and uneven processing in order to define the unevenness on the surface of the steel sheet within a preferable range. Not only that, production was interrupted for its management, which caused an excessive load in terms of productivity and labor.
On the other hand, in the present invention, the surface state of the steel sheet is hardly affected by the temper rolling method, and a large amount of processing can be performed without almost managing the wear of the unevenness of the rolling roll.

In addition, regarding the form of metal plating, it is possible to uniformly disperse a very fine and uniform metal plating coating without particularly controlling the plating conditions and the like. Thereby, the productivity of the steel sheet can be greatly improved.
In general, temper rolling is usually performed after annealing, but in the present invention, fine cracks are also formed on the surface of the steel sheet by bending of a hearth roll when passing through a normal continuous annealing line. Since the irregularities are formed, temper rolling is not essential.

As described above, it is not clear why the surface roughness of the steel sheet is hardly affected by the method and conditions for generating the roughness, but the cause of the roughness is the steel sheet itself, that is, the specific precipitate on the steel surface. This is thought to be due to the distributed state. Hereinafter, this mechanism will be described.
The steel sheet of the present invention is basically in a state different from a steel sheet, which is basically homogeneous Fe, by covering a very large portion of the steel sheet surface with a specific precipitate as compared with a normal one. Is a point. When the surface of the steel plate is partially covered with the specific precipitate, there is a difference in surface characteristics between the specific precipitate portion and the portion where the steel plate is exposed.

Since this specific precipitate is greatly different in deformation characteristics from the components that make up steel, deformation due to bending during the skin pass and through-sheeting during steel plate production is limited to the specific precipitate portion and the exposed steel plate in the micro area. Different in part.
Thereby, in the manufacturing process of the steel sheet of the present invention, the influence of the processing conditions such as the skin pass is alleviated on the surface roughness, and strongly depends only on the state of the specific precipitate on the steel sheet surface. Further, even when almost the entire surface is covered with the specific precipitate, the specific precipitate is refined by a slight deformation during the production of the steel sheet, and forms a uniform surface roughness.

  Further, such specific precipitates greatly change in wettability and reactivity with a coating substance in a surface treatment such as plating, for example, from a normal steel plate. Therefore, the coating substance in the surface treatment shows non-uniformity depending on the presence state of specific precipitates on the steel sheet surface. Therefore, if the specific precipitate formed on the surface of the steel sheet or the exposed steel surface is refined during the production of the steel sheet, the coating material by the surface treatment can be refined regardless of the surface treatment conditions.

The steel plate of the present invention is
In mass%, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% In the following, when a steel material containing Al: 2.0% or less and the balance being iron and inevitable impurities is subjected to hot rolling, cold rolling, and recrystallization heat treatment in sequence, carbon is simultaneously with or after the recrystallization heat treatment. And / or nitrogen concentration control, the total amount of C and N in the surface layer whose depth from the surface of the steel material is 1/8 of the plate thickness is 6000 ppm or less, and the total amount of C and N in the surface layer It can be obtained by setting the difference from the total amount of C and N in the central layer having a layer thickness of 1/4 of the thickness of the steel sheet to 30 ppm or more.

Here, the carbon concentration control is preferably carburizing treatment, and the carburizing treatment is preferably heat treatment in a carburizing atmosphere. The nitrogen concentration is preferably controlled by nitriding treatment, and the nitriding treatment is preferably heat treatment in a nitrogen atmosphere.
Hereinafter, the conditions for carburizing treatment, which are important requirements of the present invention, and the conditions for nitriding treatment applied as necessary will be described.
The carburizing process and the nitriding process may be performed independently or separately. Further, the order of processing is not particularly limited. However, it is advantageous that the effect of the present invention is easily manifested by supplementing the effect of decarburization of the surface layer C due to nitriding by carburizing, and shortening the heat treatment time to cure only the surface layer of the steel plate. Therefore, it is preferable to perform carburization after nitriding, and it is more preferable to perform processing in a so-called carburizing / nitriding atmosphere that is processed simultaneously.

It should be noted that it is not necessary to increase both the C content and the N content of the steel sheet before and after these heat treatments.
For example, when applying carburizing treatment, in special cases, significant decarburization has occurred in some process before carburizing treatment specified by the present invention, and considering the entire process from the slab to the final product, This is because although the C content in the steel sheet average is reduced, a significant C increase occurs in the steel sheet surface layer, and as a result, the steel sheet of the present invention can be obtained. The same applies to the nitriding treatment and the N content. As a result, with respect to deformation resistance, surface characteristics, and weldability, which are the main purposes of the present invention, the respective amounts of C and N in the steel sheet average are determined in the present invention. Will not have an important meaning.

  For the above reasons, in the present invention, even when the expression “carburizing process” is simply performed, for example, when it is performed simultaneously with the nitriding process, as a result, a reduction in the C amount that is considered to be caused by nitriding. The amount is large, and the amount of C may decrease before and after processing. Considering such cases, the present invention uses expressions such as “heat treatment in a carburizing atmosphere” instead of simply “carburizing treatment” in parts where more strict expressions are considered necessary. That is, depending on the heat treatment atmosphere, decarburization may occur despite the heat treatment in an atmosphere containing a carburizing gas. In this case, the heat treatment was performed in an atmosphere containing no carburizing gas. This is because the amount of decarburization is kept small compared to the case.

The important point in the present invention is to obtain a highly preferable concentration distribution for the purpose and a specific precipitate distribution in the surface layer including the steel sheet surface by performing the heat treatment in a carburizing atmosphere.
For example, when the heat treatment in the nitriding atmosphere is not performed or when the material change due to nitriding is small, the heat treatment in the carburizing atmosphere is a main means for controlling the above distribution. In addition, when the above distribution is controlled mainly by nitriding, the heat treatment in the carburizing atmosphere is mainly for suppressing and improving defects in deformation resistance, surface characteristics, and weldability caused by decarburization accompanying nitriding. It becomes the main means.

  In order to achieve the target strength, particularly with respect to deformation resistance, in the present invention, basically, nitriding treatment is preferable to carburizing treatment. The reason for this is that N has a larger solid solution limit in steel than C and can utilize the effect of strengthening a large solid solution, and it is easy to obtain a large amount of fine precipitates by rapid cooling heat treatment from a high temperature. is there. Therefore, as the most preferable form of the invention, the necessary deformation resistance is imparted by nitriding, and the adverse effect on the surface properties and weldability due to the change in the surface C state that may inevitably occur at that time, By avoiding the carburizing treatment and realizing a preferable carbide form, the surface characteristics and weldability are improved more than those without carburizing treatment.

Hereinafter, in the present invention, “specific heat treatment” is used as a term encompassing “heat treatment in a carburizing atmosphere” and “heat treatment in a nitriding atmosphere”.
It is advantageous from the viewpoint of productivity that this specific heat treatment is performed simultaneously with or after the recrystallization annealing after the cold rolling and continuously with the recrystallization annealing. The annealing method can be applied regardless of batch type or continuous annealing. However, the continuous annealing method is far more advantageous from the viewpoint of the productivity of the specific heat treatment and the uniformity of the material in the coil.
Further, in order to obtain a large effect by controlling the material of the inner layer as specified by the present invention, it is disadvantageous that the specific heat treatment time and the subsequent heat history are prolonged. It is preferably performed in equipment. Moreover, when there is no special reason, it shall apply to a continuous annealing material.
In particular, in the continuous annealing process, the atmosphere in the furnace is partially controlled, the process of recrystallization in the first half and the specific heat treatment in the second half are many such as productivity, uniformity of material, easy control of carburizing or nitriding state, etc. There are benefits.

  In addition, if a specific heat treatment is performed before recrystallization is completed, recrystallization is remarkably suppressed, an unrecrystallized structure remains, and remarkable deterioration in workability may occur. This limit is complicatedly determined by steel components, specific heat treatment conditions, recrystallization annealing conditions, and the like, but it is easy to find a condition in which no unrecrystallized structure remains after an appropriate trial. The specific heat treatment conditions include not only changes in the C and N contents of the steel sheet due to the specific heat treatment, but also steel components and recrystallization annealing conditions, as well as the heat history after the specific heat treatment, etc. It is necessary to determine the diffusion from the inside to the inside and the change in the shape of the specific precipitate on the plate surface. If only the material determined by the Rockwell hardness is used as an index, the preferred deformation resistance, surface characteristics and weldability intended by the present invention cannot be obtained.

The specific heat treatment conditions need to be determined in detail with reference to the results of trials of an appropriate number of times, but the basic idea is as follows, and the present invention is defined based on the following.
That is, the specific heat treatment is preferably performed at a plate temperature of 550 to 800 ° C. It is also possible to set the temperature of the specified heat treatment to this temperature by passing the steel plate through this atmosphere as in the normal annealing, and the temperature of the specified heat treatment itself is more than that. The specific heat treatment may be allowed to proceed by keeping the temperature low and allowing a steel plate heated to a temperature in this range to enter.

  When the temperature in the specific heat treatment is raised to this temperature, the efficiency of the specific heat treatment of the steel sheet may be reduced due to alteration and decomposition of the atmosphere unrelated to the specific heat treatment of the steel sheet, so that the temperature is set to 550 to 750 ° C. Preferably it is 600-700 degreeC, More preferably, it is 630-680 degreeC. As for the atmosphere of the specific heat treatment, for example, in the case of nitriding, it is preferable to contain 0.02% or more of a nitriding gas such as ammonia gas by volume ratio as the nitriding gas. In the case of treatment, it is preferable that the carburizing gas contains 0.02% or more of a hydrocarbon gas such as acetylene, ethylene, methane, butane and ethane by volume ratio.

  The type of these gases and atmospheres is not particularly limited as long as it has an effect equivalent to that of nitriding or carburizing, and the remainder is various gases such as hydrogen gas, nitrogen gas, carbon dioxide gas, and argon. A relatively inert gas is possible. In particular, ammonia gas is highly effective for increasing the nitriding efficiency, and a predetermined amount of nitriding can be obtained in a short time. Therefore, N diffusion to the center of the steel sheet can be suppressed, and a favorable effect can be obtained for the present invention. it can. The concentration of the nitriding gas for the specific heat treatment may be 0.02% or less, but is preferably 0.1% or more, more preferably 0.2% or more, more preferably 1.0% or more, more preferably Is 5% or more, and if it is further 10% or more, a sufficient effect can be obtained even by heat treatment in 5 seconds or less.

  Similarly, the concentration of the carburizing gas is preferably 0.1% or more, more preferably 0.2% or more, further preferably 1.0% or more, more preferably 5% or more, and further 10% or more. As a result, a sufficient effect can be obtained even by heat treatment in 5 seconds or less. In normal annealing, annealing is performed in an atmosphere mainly composed of nitrogen gas and hydrogen gas. However, dew point changes, oxygen, water vapor, etc. are mixed to the same extent as in normal heat treatment, and carburization and nitriding occur. In consideration of the influence, it is possible to control within a preferable range after an appropriate trial.

  Although the time for the specific heat treatment is not particularly limited, assuming a steel sheet thickness of about 0.400 mm at the maximum in relation to the temperature condition of the present invention of 550 ° C. or higher, C or N in the steel during the specific heat treatment In consideration of the diffusion of, the specific precipitate distribution and the like intended by the present invention can no longer be obtained, and the upper limit is 360 seconds. Moreover, even if the specific heat treatment efficiency is improved, one second is required to obtain the concentration distribution in the thickness direction of the steel sheet, the specific precipitate distribution on the surface, the surface roughness, and the like required by the present invention. The time is preferably 2 to 60 seconds, more preferably 3 to 20 seconds.

  In order to control the concentration distribution in the thickness direction of the steel sheet, the distribution of specific precipitates on the surface, the surface roughness, etc., the thermal history of the steel sheet after the specific heat treatment is also important. Considering the thickness of the target steel sheet and the diffusion of C or N in the steel, holding at a high temperature for a long time is not preferable. However, by appropriately smoothing the C or N distribution by this heat treatment, Since the effect can be made more prominent, a history in a temperature range of 550 ° C. or higher is important for this purpose, and the product of temperature and time in this temperature range is preferably 48000 or less.

  This value of 48000 corresponds to 80 seconds at 600 ° C. and 60 seconds at 800 ° C., but when the temperature changes continuously, it is in a time region of about 5 seconds so that the effect is appropriately evaluated. It is also possible to divide and record temperature changes and obtain the sum of products of temperature and time for each region. Of course, the sum of the product of temperature and time may be obtained by dividing into temperature regions having a certain temperature range. The value of the product of temperature and time is preferably 24000 or less, more preferably 12000 or less, and the specific heat treatment conditions are usually set so that the distribution of C or N in the steel is almost determined at the end of the specific heat treatment. Is preferred.

  The cooling rate after the specific heat treatment greatly affects the effect of the invention in connection with the thermal history. That is, even in a low temperature and short time period in which the C or N distribution hardly changes, the formation state of the specific precipitates during the cooling process changes greatly, and thus there may be a significant difference in the distribution of the specific precipitates. By setting the average cooling rate from 550 ° C. to 300 ° C. to 10 ° C./second or more, the state of the specific precipitate formed in the holding process at a high temperature is maintained, and more solid solution C or N is left to remain. It is possible to improve the deformation resistance by making the portion relatively hard compared to the center layer. Preferably it is 20 degreeC / second or more, More preferably, it is 50 degreeC / second or more. However, care should be taken because aging may be a problem depending on the use if the solid solution C or N is excessively left.

The meanings of the above heat treatment conditions are as follows.
The dispersion state of the specific precipitates greatly depends on the initial state in which the specific precipitates are formed, in other words, the state at the time of nucleation of the specific precipitates and the subsequent growth behavior of the specific precipitates. Nitride nucleation is considered to be a phenomenon in which nucleation occurs finely and finely in a state where precipitation elements are dissolved in a supersaturated state at a low temperature, like precipitation in ordinary steel. That is, supersaturated solid solution elements try to form some precipitates, but if the temperature at that time is low, sufficient diffusion does not occur and the diffusion distance becomes short, resulting in a fine precipitation form.

  In the present invention, in order to control N to be in a supersaturated state by nitriding, it is preferable that the ammonia gas concentration is high. However, if the temperature is too low, sufficient nitriding does not occur, and even if the ammonia gas concentration is high, it may not be possible to control to a sufficiently supersaturated state. Thereafter, the growth of specific precipitates occurs during the specific heat treatment. In general, the precipitate grows as the temperature and the time increase, but during the specific heat treatment in the steel of the present invention, C and N are supplied from the surface, and at the same time, the diffusion of C and N into the steel plate occurs. Since it is not always in an equilibrium state, it must be noted that the growth behavior of precipitates in a steel sheet having a homogeneous component does not apply as it is.

  Although it is difficult at this time to completely formulate such a situation and accurately present the optimum conditions, the manufacturing conditions are basically as in the present invention in accordance with the above-described idea. As an optimal situation, in a temperature range where nitriding and carburizing to some extent sufficiently occur and diffusion does not occur excessively, in the case of heat treatment in an atmosphere assumed in the present invention, it is about 550 to 800 ° C. It is preferable to perform nitrification and growth of specific precipitates on the steel sheet surface by nitriding and carburizing at a relatively high gas concentration in the temperature range.

In particular, considering time factors for growth, it is possible to carry out miniaturization even at a low temperature at which diffusion is further suppressed even if it is held for a long time.
Since the idea for this control is not limited to nitriding / carburizing with a specific gas, it goes without saying that the atmospheric gas is not limited to a specific gas. Controls in the direction of refining the precipitation nuclei using the generally known nucleation / growth phenomenon metallurgy, and it is easy to set favorable conditions according to the specific heat treatment method It is.

Although the present invention can be achieved only by carburizing treatment, it is preferable from the viewpoint of the effect that nitriding treatment is involved as described above, and the effect of carburizing treatment is nitriding which is more effective for property control. Since it works very favorably when carburizing compensates for characteristic deterioration due to decarburization when the treatment is performed, it is preferable that both nitriding treatment and carburizing treatment are performed.
In such a case, the remarkable effect of the invention can be obtained by controlling the ratio of the carburizing gas concentration and the ammonia gas concentration in the specific heat treatment within a specific range.
That is, when the effect of carburizing is too small for nitriding, it is difficult to avoid the adverse effects due to decarburization. On the other hand, when the effect of carburizing is too great, the combined effect with nitriding cannot be obtained.

Therefore, in the present invention, if the carburizing gas concentration (volume%) / ammonia gas concentration (volume%) is RE as a condition for performing the nitriding treatment, it is preferable that 0.10 <RE <10. More preferably, 0.20 <RE <8, still more preferably 0.30 <RE <5, still more preferably 0.40 <RE <3, and still more preferably 0.40 <RE <2.
Here, the carburizing gas concentration and the ammonia gas concentration can be determined by collecting the atmosphere in the heat treatment furnace, but the concentration of the atmosphere in the furnace changes due to the decomposition on the furnace wall and the steel plate. Therefore, in the present invention, it is determined from the flow rates of the carburizing gas and ammonia gas introduced into the furnace.

  As described above, the mechanism by which the adverse effect on the deformation resistance is suppressed by the specific heat treatment can be considered to simply compensate for the deterioration of the surface hardening due to the decarburization, but quantitatively, There are also points that cannot be explained simply by the amount of decarburization and the amount of carburization, and it is possible to consider the effects of changes in the hardening mechanism and precipitates due to the interaction with nitriding, but the characteristics of the present invention are rather surface characteristics and weldability. Is a positive impact.

  The reason why remarkable surface characteristics and weldability improvement effect can be obtained by the present invention is due to the surface roughness due to the hardened surface, the difference in unevenness, or specific precipitates formed on the surface in large quantities It is. These change, for example, the contact resistance at the time of weldability, and change the interfacial reaction with the coating material at the time of surface treatment, and not only have a favorable effect directly, but also, for example, the plating form on the surface changes. This also shows an interactive effect such that the weldability changes in a preferable direction.

In particular, in the manufacture of thin steel plates for containers, re-rolling may be performed after recrystallization annealing in order to adjust hardness or plate thickness. The rolling reduction ranges from about 1%, which is close to the skin pass used for shape adjustment, to 50% or more, which is the same as that in cold rolling. In the present invention, recold rolling similar to conventional steel can be applied. is there.
For example, when aiming at shape correction or the like, rolling is performed in a range of about 0.5% to 1.5%, and the steel of the present invention is usually rolled at this level.

  It is necessary to pay particular attention to the steel according to the present invention when a high re-cold rolling rate exceeding 1.5% is applied in order to increase the strength or reduce the thickness. Considering simply, when re-cold rolling is applied to the steel of the present invention where the surface layer is hard and the inner layer is soft, only the soft inner layer is preferentially work-hardened, and only the surface layer given in the present invention is given priority in order to improve deformation resistance. However, in the steel of the present invention, if the re-rolling rate is a normal level, the hard surface layer with a high C and N content is rather high by re-rolling. The part hardens preferentially, and the hardness difference of the inner layer formed of the steel of the present invention becomes clearer.

  This is because the surface layer is easy to work harden due to a large amount of solid solution C, N and specific precipitates, while the inner layer is restrained by the surface layer and cannot be preferentially deformed, It is not selectively cured so as to greatly exceed the surface layer. Nonetheless, if the re-cold rolling rate is remarkably high, the steel plate itself becomes sufficiently hard, and it becomes possible to obtain sufficient can strength without controlling the material distribution in the plate thickness direction as in the present technology, At the same time, since the effect of improving surface characteristics and weldability through the control of surface roughness, which is a feature of the present invention, tends to be small, the significance of increasing the re-cold rolling rate far beyond the normal application range is small.

In addition, if the re-rolling ratio increases, the workability decreases, so careless application under high pressure should be avoided. From the above, when recold rolling is applied to the steel of the present invention, it is preferable to be up to about 70%. The re-rolling time is after the specific heat treatment in the step of continuously performing the recrystallization annealing and the specific heat treatment, which is a preferable process from the viewpoint of productivity, but the recrystallization annealing and the specific heat treatment are performed in separate steps. In some cases, it may be performed before the specific heat treatment.
In addition, when considering a welded portion, in a normal material, the material may be locally softened by the heat of welding, and processing distortion may be concentrated in flange forming or the like, which may deteriorate formability. Alternatively, in the steel of the present invention containing a large amount of N, the softening due to this welding heat is also suppressed, so that the formability of the welded portion can be improved.

The preferable range of the thickness of the steel sheet of the present invention is 0.400 mm or less.
This is because when the thickness of the steel plate is thicker than this, the deformation of the formed member hardly causes a problem. Moreover, when the thickness of the steel plate is thick, the thickness of the surface layer hardening by the specific heat treatment becomes relatively small, and the effect of the invention is hardly exhibited.
A more preferable range of the thickness of the steel sheet is 0.300 mm or less, more preferably 0.240 mm or less, and further preferably 0.200 mm or less. Particularly, a steel sheet having a thickness of 0.180 mm or less has a very remarkable effect. Can be obtained.

The effect of the present invention does not depend on the heat history and the manufacturing history before annealing after adjusting the components of the steel material. The slab for hot rolling is not limited to manufacturing methods such as the ingot method and continuous casting method, and it does not depend on the heat history until hot rolling, so the slab reheating method and the cast slab are reheated. The effects of the present invention can also be obtained by the CC-DR method in which hot rolling is directly performed without thinning, and also by thin slab casting in which rough rolling is omitted.
The effect of the present invention can also be obtained by two-phase rolling in which the finishing temperature is α + γ two-phase region or continuous hot rolling in which a rough bar is joined and rolled regardless of the hot rolling conditions. .

In addition, when the steel of the present invention is used as a container material having a welded portion, the softening of the heat-affected zone is suppressed, and in particular, the effect of improving the strength of the welded portion because the surface layer portion having a high C and N concentration is quenched and hardened. Also have. This becomes even more pronounced when elements such as B, Nb, etc. that normally suppress the softening of the heat affected zone are added. On the other hand, in a so-called two-piece can manufactured through drawing or ironing, the plate surface is hardened so that the coefficient of friction with the molding die is lowered and the moldability is improved.
Furthermore, since the surface layer is hardened and resistance to bending deformation is increased, bending buckling of the steel sheet during forming becomes difficult to occur, that is, an effect of suppressing generation of wrinkles also appears.

  The present invention is intended for a steel sheet having a relatively small thickness classified as a steel sheet for containers, but the application is not limited to containers, and any member having any three-dimensional structure may be used. These components are used for applications such as improved surface treatment properties such as plating, suppression of deformation due to external force after molding, improved appearance such as color tone, improved adhesion of coatings such as coating, and improved weldability. The effect of the invention is exhibited.

Usually, although the steel plate of this invention is used as a negative | original plate for surface-treated steel plates, the effect of this invention is not impaired at all by surface treatment. As the surface treatment for cans, tin, chromium (tin-free) nickel, etc. are usually applied. Moreover, it can be used without impairing the effect of this invention also as a negative | original plate for laminated steel plates which coat | covered the organic membrane which has come to be used in recent years.
In addition, it is possible to obtain a sufficient effect regardless of the crystal grain size, texture, or the like.
The means for obtaining the steel sheet of the present invention is not limited to nitriding / carburizing treatment, and can be performed by liquid, plasma, ion implantation or the like.

  Since the present invention is based on the condition that a certain area of the surface is covered with a specific precipitate, it can be applied as long as the C and N concentration is increased on the surface by some method. In particular, without increasing the total content of C and N in the steel sheet, the C and N distribution in the thickness direction of the steel sheet is changed to increase the concentration of C and N only on the surface, and the specific precipitate If the process is such that the change in the workability of the steel sheet is reduced, it is advantageous. In addition, the present invention can be realized not only in the process of nitriding and carburizing a steel sheet but also in the process of modifying the steel sheet surface by adhering some C and N-containing substances to the steel sheet surface. In particular, if any specific precipitate having low reactivity with the steel plate is adhered to the surface, it is advantageous because the influence on the properties such as workability of the steel plate base material itself is reduced.

Next, the steel of this invention is demonstrated in an Example and a comparative example.
Here, about the steel of each component of Examples and Comparative Examples shown in Table 1, after hot rolling, cold rolling, annealing with specific heat treatment, skin pass or re-cold rolling is performed, and then steel plates are manufactured. The surface of the steel plate was Sn-plated to produce a Sn-plated steel plate which is one of the most common steel plates for containers.
Next, the color tone, adhesion of the surface coating, and weldability were evaluated for each of the Sn-plated steel sheets of Examples and Comparative Examples.
Adhesion, the two steel plates to the epoxy phenolic paint was baked dry 25 mg / m ² coated on both sides, and thermocompression bonding a nylon-based adhesive a test piece, T The test piece in a state wetted with tap water A mold peel test was performed to measure peel strength.

Here, an adhesive having a high peel strength was regarded as having good adhesion, and superiority or inferiority was determined.
The color tone was evaluated based on the L value obtained using a spectrocolorimeter after applying 10 μm of a transparent polyester resin and drying. The L value indicates that the higher the value, the better the color tone, and the superiority or inferiority was determined with this value.
Welding is performed by changing the welding current in seam welding, which is normally applied to 3-piece cans. Splash generation during welding (chilli generation), weld strength by peel test (hain test), steel plate during welding The weldable current range was determined from the surface damage of the weld due to the arc current between the surface and the polar ring, and the superiority or inferiority of the weldability was determined by the range and the lower limit. Here, particularly good ones were designated as “◎”, good ones as “◯”, ordinary ones as “Δ”, and poor ones as “x”. A wider range is preferable because the manufacturing stability is higher, and a lower limit is preferable because plating peeling or material change due to a rise in the temperature of the weld is less likely to occur.

Deformation resistance was evaluated by two methods. The evaluation equivalent to a three-piece can is based on the amount of indentation of the die when the barrel of a three-piece can manufactured with the prototype material of each of the examples and comparative examples is pushed in with a cylindrical die having a diameter of 10 mm and a length of 40 mm. In the correlation between the indentation load and the indentation load, the load (rheometer strength) serving as an inflection point corresponding to yield was used as an index of deformation resistance.
In addition, the evaluation equivalent to a two-piece can is made by increasing the internal pressure of a two-piece can manufactured using the above-mentioned prototype material and having a concave bottom, and the pressure when the can bottom buckles into a convex shape. It was used as an index of deformability. The higher the pressure value, the smaller the deformation due to external force and the better the deformation resistance. The roughness was measured using a laser type three-dimensional roughness meter.

Regarding the component change in the plate thickness direction, since each sample is manufactured by a normal method before the specific heat treatment, the change in the element in the plate thickness direction is negligible and can be ignored for the effect of the present invention. It was a thing.
Table 1 shows hot rolling, cold rolling, annealing, specific heat treatment conditions and the like of each of the examples and comparative examples. All the specific heat treatments are performed after the middle of annealing, and it is considered that recrystallization was completed before carburization or nitridation by the specific heat treatment occurred.
In addition, the C and N amounts in Table 1 are values of the plate thickness average of the steel plate as a product.
Table 2 shows the characteristics of the steels of the examples and comparative examples.

In Table 1, “A” is a product of temperature and time at 550 to 800 ° C., “B” is a cooling rate at 550 to 300 ° C., and “C” is simultaneous treatment of nitriding and carburizing. , “D” is carburizing after nitriding, and “E” is carburizing gas concentration / ammonia gas concentration.
In Table 2, “F” is [C + N amount of ¼ thickness of surface layer] − [C + N amount of ¼ thickness of center layer], and “G” is [carbide or nitride on the surface] Area ratio of the object] / [area ratio of carbide or nitride at the 1/4 position of the plate thickness].
In addition, “H” is a can body buckling load (kgf) when the can shape is 3P, and a can bottom buckling pressure (kgf / cm ² ) when the can shape is 2P. The 2P can is indicated by “*”.

  It was confirmed that good deformation resistance and weldability were obtained by controlling the thickness direction state within the range of the present invention by the production method of the present invention. Some of those subjected only to nitriding had a large amount of decarburization, and some of the characteristics were deteriorated.

  The present invention limits the components of the steel material to a specific range and further uses heat treatment in a carburizing atmosphere to control the material of the surface layer portion and the inner layer portion of the steel material, and also affects the surface characteristics and weldability. Since the surface state of the steel plate is well controlled, it can be widely applied not only to steel plates for containers but also to steel plates for members having any three-dimensional structure, and its industrial utility value is extremely large.

It is a schematic diagram which shows the division | segmentation of the site | part of the plate | board thickness direction of a steel plate.

Claims (13)

A steel plate manufactured through a carbon and / or nitrogen concentration control step,
In mass%, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% Hereinafter, Al: 2.0% or less, the balance consists of iron and inevitable impurities,
The total amount of C and N in the surface layer whose depth from the surface of the steel sheet is 1/8 of the plate thickness is 6000 ppm or less,
The difference between the total amount of C and N in the surface layer and the total amount of C and N in the central layer having a layer thickness that is ¼ of the thickness of the ultrathin steel plate is 30 ppm or more. Steel sheet with remarkably good deformability, surface properties and weldability.   The area ratio of nitrides and / or carbides on the surface of the steel sheet is the area ratio of nitrides and / or carbides in a cross section parallel to the surface of the central layer, which is a quarter of the thickness of the steel sheet. The steel sheet having a remarkably good deformation resistance, surface characteristics, and weldability according to claim 1, wherein the steel sheet is 1.5 times or more.   The surface roughness (Ra) of the steel sheet is 0.90 μm or less, and the number of concavo-convex vertices per inch of the surface length of the steel sheet is 250 or more. 2. A steel sheet having remarkably good deformation resistance, surface characteristics and weldability.   Further, in terms of mass%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.015% or less, Cr: 20% or less, Ni: 10% or less, Cu: 5% or less 4. The steel sheet according to claim 1, 2 or 3, wherein the steel sheet contains one or more selected ones or more, and has excellent deformation resistance, surface characteristics and weldability.   Furthermore, it contains 1% or more selected from the group of Sn, Sb, Mo, Ta, V, and W in a mass% of 0.1% or less in total. 4. A steel plate having excellent deformation resistance, surface characteristics, and weldability according to any one of 4 above.   6. The steel sheet according to claim 1, wherein the steel sheet has a thickness of 0.400 mm or less, and has excellent deformation resistance, surface characteristics, and weldability. In mass%, C: 0.0800% or less, N: 0.0600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% Hereinafter, Al: 2.0% or less, a steel sheet manufacturing method in which a steel material comprising the balance iron and inevitable impurities is subjected to hot rolling, cold rolling, and recrystallization heat treatment in order,
At the same time as or after the recrystallization heat treatment, the concentration of carbon and / or nitrogen is controlled,
The total amount of C and N in the surface layer whose depth from the surface of the steel material is 1/8 of the plate thickness is 6000 ppm or less, and the total amount of C and N in the surface layer is 1/4 of the plate thickness of the steel plate. A method for producing a steel sheet having significantly good deformation resistance, surface characteristics, and weldability, characterized in that a difference between the total amount of C and N in the central layer of the layer thickness is 30 ppm or more.   8. The method of manufacturing a steel sheet with excellent deformation resistance, surface characteristics, and weldability according to claim 7, wherein the carbon concentration control is carburizing treatment, and the nitrogen concentration control is nitriding treatment. .   The said carburizing process hold | maintains 1 second or more and 360 second or less in the atmosphere which contains carburizing gas 0.02% or more in the state whose temperature of the said steel materials is 550-800 degreeC. A method for producing a steel sheet with extremely good deformation resistance, surface characteristics and weldability.   9. The anti-nitriding treatment according to claim 8, wherein the nitriding treatment is held for 1 second to 360 seconds in an atmosphere containing 0.02% or more of ammonia gas in a state where the temperature of the steel material is 550 to 800 ° C. 9. A method for producing a steel sheet with remarkably good deformability, surface characteristics and weldability.   The deformation resistance, surface characteristics, and weldability according to claim 10, wherein, in performing the nitriding treatment, 0.10 <Carburizing gas concentration (volume%) / Ammonia gas concentration (volume%) <10. Is a method for producing a steel sheet with extremely good quality.   The product of the temperature and time in the thermal history of the temperature range of 550 ° C. or higher for either one or both of the carburizing treatment and the nitriding treatment is 48000 or less, and then the average cooling rate from 550 ° C. to 300 ° C. is set. The method for producing a steel sheet having excellent deformation resistance, surface characteristics, and weldability according to claim 9, wherein cooling is performed at 10 ° C./second or more.   The cold-rolling is performed again at a rolling rate of 0.5% or more and 70% or less after the recrystallization heat treatment, deformation resistance, surface characteristics according to any one of claims 7 to 12, A method for producing a steel sheet with extremely good weldability.

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