JPH07216497A - Steel sheet or steel sheet parts with high fatigue strength and their production - Google Patents

Abstract

PURPOSE:To attain superior formability at the time of plastic working and the property of improving the strength of a base material at the time of nitriding treatment by specifying a composition and controlling an internal structure. CONSTITUTION:A steel sheet, as a stock, has a composition consisting of, by weight ratio, 0.10-0.70% C, 0.05-1.0% Si, 0.05-0.50% Mn, <=2.00% Ni, 0.01-1.00% acid-soluble Al, 0.002-0.010% N, 3-50ppm B, <=0.010% Ca, <=0.010% S, <=0.020% P, and the balance essentially Fe. This steel sheet is box-annealed and spheroidal cementite is dispersed in graphite, followed by plastic working to form into required parts shape. Then, the parts are further soaked at 460-500'C in a gaseous mixture of N2 and H2 or in a gaseous mixture, prepared by adding CO and/or CO2 to the former gaseous mixture, for 1 to 30hr, by which a surface layer nitrided structure of <=50mum thickness and an internal structure consisting of ferrite, graphite, and cementite in which grain size is regulated to two levels of >=0.2mum and <=0.05mum can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel plate or a thin steel plate component having high fatigue strength, which is suitable as a high strength component for automobiles and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Generally, steel parts requiring a complicated shape and high fatigue strength are subjected to gas nitriding or molten hydrocyanate after being subjected to plastic working on a steel plate or a bar steel manufactured by a steel manufacturer. Those whose surface is hardened by a treatment such as nitriding (tuftride) are used. In general, the steel type used for such treatment requires a high elongation at the stage of plastic working, and therefore a steel having a low C content is used. However, in the case of such a steel type, since the C content is low, even if the tensile strength increases due to work hardening during plastic working, it softens due to the temperature increase during the nitriding treatment. Therefore, although the effect of increasing the fatigue strength can be obtained by the formation of the surface nitrided layer, there is a limit to the increase in the strength. For these reasons, it is desired to develop a material steel sheet and a method for increasing the strength thereof, which can simultaneously increase the strength of the material itself when performing the nitriding treatment for the purpose of increasing the hardness of the surface layer.

JP-A-64-25946 and JP-A-2-10
Japanese Patent No. 7742 discloses a steel sheet having a low carbon to medium carbon material steel sheet having a structure of ferrite and graphite or ferrite, graphite and cementite, and having improved formability and hardenability. However, these inventions do not show a method capable of improving the strength of the base material during the nitriding treatment.

The nitriding treatment in the production of gears for automobile transmissions is intended for medium carbon steel or low carbon steel, as disclosed in JP-A-2-107742. As a method for increasing the base metal strength during the nitriding treatment for such steel types, Cu is added to precipitate ε-Cu in the nitriding temperature range, or Nb or the like is added to NbN in the nitriding temperature range. , NbC
Means for forming precipitates such as However, the addition of these alloying elements raises the material strength before plastic working, resulting in problems such as insufficient formability.

A solution to this problem is shown in, for example, Iron and Steel, 78, 8 (1992), p.1383 for the purpose of manufacturing gears for automobile transmissions. The method presented in the above paper is Cu,
This is a steel for which Nb is not added and is subjected to hot forging and cold forging to form gear teeth, and if necessary, this product is carburized or nitrocarburized to increase the hardness of the teeth. However, the effect of increasing the strength of the base material itself cannot be expected in the soft nitriding treatment.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems. The purpose of the present invention is to
It is an object of the present invention to provide a thin steel plate or a thin steel plate component having a formability equivalent to that of a general low carbon steel at the time of plastic working of a steel plate and having an increased base metal strength at the time of nitriding treatment after the plastic working, and a manufacturing method thereof.

[0007]

The summary of the present invention is as follows.
The thin steel plate or the thin steel plate component of (1) and the manufacturing method of (2).

(1) C: 0.10 to 0.70% by weight, Si:
0.05 to 1.00%, Mn: 0.05 to 0.50%, Ni: 2.00% or less (without addition), sol.Al: 0.01 to 1.00%, N: 0.002 to
0.010%, B: 3 to 50 ppm and Ca: 0.010% or less (there may be no addition), and the balance substantially consists of Fe and unavoidable impurities, P in the impurities is 0.020% or less, S
Is 0.010% or less and the surface layer has a thickness of 50 μm.
A thin steel plate or thin steel plate part having the following nitriding layer and having an internal structure made of ferrite, graphite and cementite divided into two levels having a diameter of 0.2 μm or more and 0.05 μm or less.

(2) C: 0.10 to 0.70% by weight, Si:
0.05 to 1.00%, Mn: 0.05 to 0.50%, Ni: 2.00% or less (without addition), sol.Al: 0.01 to 1.00%, N: 0.002 to
0.010%, B: 3 to 50 ppm and Ca: 0.010% or less (there may be no addition), and the balance substantially consists of Fe and unavoidable impurities, P in the impurities is 0.020% or less, S
The material steel sheets 0.010% or less, after the spheroidized cementite and graphite were dispersed in the steel by box annealing at six hundred and fifty to seven hundred eighty ° C., subjected to plastic working, further 400 to 650 ° C. with N ₂ and H a mixed gas of ₂ CO or the mixed gas,, CO ₂
In a mixed gas atmosphere containing at least one of the above, soaked for 1 hour or more and 30 hours or less, and a surface layer structure composed of a nitride layer having a thickness of 50 μm or less, ferrite, graphite and a grain size of 0.2 μm or more and 0.
A method for producing a thin steel plate or a thin steel plate part, which comprises forming an internal structure made of cementite divided into two levels of 05 μm or less.

Ni: 2.00% or less, Ca: 0.010% or less means
Any of these may be 0%, which means that the upper limits are 2.00% and 0.010%, respectively, if necessary.

The above-mentioned thin steel plate component means a component formed from a thin steel plate as a material by deep drawing, rolling, spinning or the like, for example, a gear, a pulley or a hub.

As a solution to the above-mentioned problems, the present inventor has proceeded with the study of the following concrete means from the viewpoint of both material and manufacturing method.

In terms of materials, steel grade of 0.10% C or more, and at the stage of the steel sheet after annealing, C is dispersed in the steel as graphite to reduce the strength and maintain the same elongation as conventional mild steel sheets. .

C in steel during heating for nitriding
Is re-precipitated inside the steel sheet (base material portion) as ultrafine cementite that acts as a substitute for precipitates containing Cu, V, etc.

By this reprecipitation, the tensile strength is increased according to the amount of C and the strength is increased up to 200 MPa.

In terms of the manufacturing method: 400-65 to enable precipitation of ultrafine cementite
Use a manufacturing process that allows nitriding at 0 ° C.

As a result, the present inventor has accomplished the present invention by obtaining the following findings.

In order to promote graphitization in order to secure softening and high elongation after annealing and to form ultrafine cementite after nitriding treatment, the lower limit of C content is 0.
Must be 10%.

In order to graphitize cementite in the annealing step, it is effective to add B, which has a smaller action than Si and sol.Al, which have a large solid solution hardening action. Similarly, Ni and Ca, which have a small solid solution hardening effect, should be added if necessary.

Mn has an upper limit of 0 to secure a graphite structure.
The lower limit should be 0.05% in order to secure toughness by precipitating as 50% and MnS.

It is necessary to reduce the amounts of P and S, but especially C
In the medium carbon region with the upper limit of 0.2 to 0.4%, it is required to reduce the amount of P as much as possible in order to promote graphitization and obtain high elongation, and the upper limit of P must be 0.020%. .

A material steel plate having such a chemical composition
Cementite is graphitized by box annealing in the range of 650-780 ℃, and spheroidized cementite and graphite are dispersed.

In order to efficiently achieve both nitriding treatment after plastic working and increasing the material strength by precipitating ultrafine cementite, the nitriding condition is NH ₃ + H ₂ atmosphere and 400 to 650 ° C. It is necessary to soak for 1 hour to 30 hours in the temperature range.

[0024]

The chemical composition and structure of the thin steel sheet or the material of the thin steel sheet component of the present invention, and the method of manufacturing the thin steel sheet and the structure thereof will be described below.

I. Chemical composition of raw steel sheet C: 0.10 to 0.70% In order to obtain a graphitized structure after annealing and to form ultrafine cementite after nitriding, a certain amount of C content is required. The lower limit was 0.10%. On the other hand, if the content is excessively increased to exceed 0.70%, the formation of graphite is promoted during box annealing of the raw steel sheet,
The upper limit was set to 0.70% because graphite in steel adversely affects as inclusions and deteriorates formability.

Si: 0.05 to 1.00% Si is a component necessary for graphitization of cementite, and it is necessary to add a certain amount or more as a deoxidizing agent, so the lower limit was made 0.05%. On the other hand, since it is necessary to reduce the tensile strength at the time of forming the steel sheet after annealing as much as possible, Si having a high solid solution hardening ability avoids its high content as much as possible, and its upper limit value is
It was 1.00%.

Mn: 0.05 to 0.50% Mn stabilizes cementite and suppresses the decomposition of cementite during annealing and soaking.

Therefore, if the Mn content exceeds 0.50%, it becomes a significant inhibiting factor for the precipitation of graphite. However, Mn has the effect of improving the hardenability of steel or improving the toughness by combining with S in the steel to form MnS, and it is necessary to contain a certain amount or more, so the lower limit was made 0.05%. .

Sol.Al: 0.01 to 1.00% The higher the content of sol.Al, the easier the precipitation of graphite. This effect cannot be obtained at less than 0.01%. On the other hand, if the content exceeds 1.00% and is excessive, it causes adverse effects such as solid solution hardening of ferrite and increase of oxide precipitation in steel. As a result, the toughness of the product after the nitriding treatment may be significantly deteriorated. Therefore, the upper limit is set to 1.00%.

N: 0.002 to 0.010% N is an element which is unavoidably contained in steel, but it is combined with B during annealing to form BN and promotes the formation of graphite with BN as a nucleus, so a certain amount. It is necessary to control the above content, and the lower limit was made 0.002%. On the other hand, if the B content exceeds 0.010%, the N content in the steel excessively increases, leading to deterioration of elongation, etc., so the upper limit is 0.010%.
And

B: 3 to 50 ppm B improves the toughness after nitriding and promotes graphite formation after annealing, so it was decided to add B in a certain amount or more. The minimum effective B amount for obtaining these effects is 3 pp
On the other hand, when it exceeds 50 ppm, FeB is formed during hot rolling, which adversely affects toughness. Therefore, the range of B content is 3 to 50 ppm.

P: 0.020% or less P is said to segregate at the interface between cementite and ferrite, and is an element that suppresses the migration of C element and significantly inhibits the precipitation of graphite. Therefore, the P content should be as low as possible. Is good. In particular, in order to shorten the box annealing time for graphitization (maximum soaking time within 36 hours), the allowable upper limit of P is 0.020
%.

S: 0.010% or less S, like P, is an element that inhibits graphitization, and if its content increases, the box annealing time required for graphitization increases. Further, since the toughness is remarkably reduced, S in steel must be reduced as much as possible. Therefore, the upper limit of S is 0.010.
%.

Ni: 2.00% or less Ni is an element that promotes graphitization together with Si, but does not have a solid solution hardening ability for ferrite as much as Si, and is an element effective in drawing out softening of a steel sheet after annealing. . For this reason,
It was added as needed for the purpose of promoting graphitization. However, if the content exceeds 2.00% in excess, it causes solid solution hardening of ferrite and increases cost, so the upper limit was made 2.00%.

Ca: 0.010% or less Addition of Ca to steel has an effect of reducing solid solution oxygen in the steel and an effect of reducing Al-based oxides. In order to promote graphitization during annealing, it is necessary to maintain the sol.Al content within the above range, which may cause an increase in Al-based oxide in steel. Further, since the Mn content also needs to be suppressed as described above, the effect of fixing and reducing the solid solution S in the steel as MnS may not be sufficiently obtained in some cases.

Therefore, the amount of Al-based oxide is reduced and the amount of Ca is reduced.
When it is necessary to fix S as a system sulfide, it is preferable to add Ca as needed. When including,
If 0.010% is exceeded, these effects saturate, leading to an increase in cost, and Ca-based oxides and sulfides in the steel increase, so the upper limit was made 0.010%.

II. Structure of Steel Plate Steel Plate after Annealing: The steel structure of the base material before plastic working is assumed to be a dispersion of spheroidized cementite and graphite by box annealing under predetermined conditions. Although graphite precipitation is effective for improving formability, not all of cementite needs to be graphitized, while graphitization of cementite requires very large thermal energy. For this reason, in order to secure high formability at low cost, spheroidized cementite and graphite
Seeds were dispersed.

At this time, the preferable size of the spheroidized cementite is coarse particles having a diameter of 0.2 μm or more.

Thin steel plate or thin steel plate part of the product: The thin steel plate or thin steel plate part of the present invention has a nitride layer with a thickness of 50 μm or less on the surface layer, and the internal structure of the base material is ferrite, graphite and grain size. Consists of three types of cementite divided into two levels of 0.2 μm or more and 0.05 μm or less.

[Nitride Layer on Surface Layer] The nitride layer on the surface layer is a layer mainly composed of Fe _X N, and is a surface hardened layer necessary for imparting high fatigue strength and wear resistance. If the thickness exceeds 50 μm, the toughness deteriorates and the cost for nitriding also increases. A desirable lower limit for the thickness is 15 μm. This thickness corresponds to one layer (one piece) or more of ferrite grain size.

[Internal Microstructure after Nitriding Treatment and Grain Size of Cementite] The internal base metal structure is obtained by soaking under predetermined conditions during the nitriding treatment, and is composed of the above-mentioned three types. By this, the tensile strength and fatigue strength are improved.

High fatigue strength is due to the formation of the above-mentioned nitride layer,
The particle size precipitated from C diffused from graphite into ferrite is
It is obtained by using ultrafine cementite of 0.05 μm or less.

When a certain amount of cementite is graphitized during the annealing of the material steel sheet, the cementite that cannot be graphitized and remains as a spheroidized structure grows to a grain size of 0.2 μm or more and remains (hereinafter, this coarse grain). Cementite is referred to as Fe ₃ CI). However, since such coarse-grained cementite has a small amount of precipitation and has little influence on the tensile strength, the fatigue strength does not increase. On the other hand, ultrafine cementite with a grain size of 0.05 μm or less (hereinafter referred to as Fe ₃ C III) does not remain in the ferrite after annealing of the steel sheet, but is precipitated by C diffused from graphite during the nitriding treatment. When Fe ₃ C III precipitates in ferrite, it increases its tensile strength and simultaneously increases fatigue strength.

However, if cementite having a grain size of more than 0.05 μm and less than 0.2 μm (hereinafter referred to as Fe ₃ C II) is present in the steel after annealing the raw steel sheet and before the nitriding treatment, it is precipitated during the nitriding treatment. Since the incoming Fe ₃ C III is adsorbed by Fe ₃ C II and coarsens, the strength increasing effect cannot be obtained.

Therefore, excellent formability before nitriding treatment and
In order to achieve both high fatigue strength after nitriding treatment, Fe ₃ CI of 0.2 μm or more is dispersed during annealing of the steel sheet, and after nitriding treatment, Fe ₃ CI of 0.05 μm or less
It is necessary to have a structure in which Fe ₃ C III is dispersed.

III. Manufacturing Method and Conditions The thin steel plate or thin steel plate part of the present invention is manufactured by the following method. That is, a steel slab having the chemical composition described above is hot-rolled by a conventional method, or if necessary, cold-rolled by a conventional method to obtain a material thin steel sheet, which is spheroidized cementite and graphite by box annealing. After being dispersed into a steel sheet having good formability and strength and elongation characteristics similar to mild steel, it is subjected to cold working such as deep drawing and cold forging, or plastic working into a component shape or plate shape by a method such as cold rolling. Then, the surface is nitrided, and at the same time, the internal structure is appropriately controlled.

Annealing conditions for precipitating graphite and spheroidized cementite in the raw steel sheet: In order to improve the formability of the steel sheet before the nitriding treatment, the raw thin steel sheet is annealed in a box to graphitize the cementite and then the spheroidized cementite is formed. And graphite need to be dispersed. However, in order to graphitize all the cementite, long-time box annealing is required, which causes an increase in cost. Further, when the whole amount of this cementite is graphitized, the ferrite which is the matrix becomes coarse, and the strength and the elongation are also lowered. Therefore, it is necessary to form a structure in which cementite is spheroidized to an appropriate size and remains in a dispersed state, and the remaining cementite is graphitized and dispersed.

Desirable box annealing conditions are 650 to 780 ° C., 12
~ 24 hours range. It takes a very long time to graphitize cementite in steel, and box annealing is used. At this time, as a temperature condition necessary for graphitization, a soaking temperature of 650 ° C. or higher is required, and at a temperature lower than this, it takes a very long time to graphitize cementite, which is not established as an industrial method. . On the other hand, if the temperature exceeds 780 ° C., the matrix becomes austenite and cementite is decomposed and solid-solved in the matrix, so that a desired structure exhibiting high fatigue strength cannot be obtained.

Nitriding condition after plastic working: In order to form a nitride layer on the surface layer, it is necessary to heat in an atmosphere containing nitrogen, but the atmosphere is N for the purpose of shortening the treatment time for forming the nitride layer. NH ₃ using a mixed gas of ₂ and H ₂ or a mixed gas of one or more of CO and CO ₂ added to this mixed gas.
Or NH ₃ + H _2, or better to a mixed atmosphere of _{NH 3 -N 2 -CO-H 2} -CO 2.

The treatment temperature is soaked in the range of 400 to 650 ° C. As shown in Example 2 below, below 400 ° C or
At temperatures above 650 ° C, it is difficult to form the desired nitride layer thickness and Fe ₃ C III, which leads to strength increase, and only Fe ₃ CI and Fe ₃ CII can be obtained as cementite.

Soaking time for nitriding treatment is 1 hour or more 30
Not more than time. As shown in Example 2 to be described later, if less than 1 hour, the desirable lower limit of the nitrided layer thickness of 15 μm cannot be ensured, and it is difficult to form Fe ₃ C III that causes an increase in strength. On the other hand, when it exceeds 30 hours, the thickness exceeds the upper limit of 50 μm, Fe ₃ C III is coarsened, and only Fe ₃ CI and Fe ₃ C II are obtained as cementite, and the strength increasing effect is impaired.

The thin steel plate or thin steel plate part manufactured by the above process and conditions has a desired nitriding layer on the surface layer and
Ferrite, graphite and grain size 0.2
Cementite having two levels of μm or more and 0.05 μm or less is formed, and high fatigue strength is obtained.

[0053]

【Example】

[Example 1] A steel ingot of the steel having the chemical composition shown in Table 1 melted in a laboratory vacuum melting furnace was forged to a plate thickness of 30 mm, and subsequently hot-rolled into a thin steel plate having a plate thickness of 4 mm, and then further rolled. Cold rolled to a thickness of 2.0 mm to obtain a steel plate. These were subjected to box annealing under the annealing conditions shown in Table 2 and examined for mechanical properties.

After 5% plastic strain was further applied to the annealed steel sheet, 50% NH ₃ -20% N ₂ -10% CO -10% H ₂ -10% CO
_A nitriding layer was formed on the surface by soaking for 12 hours in 480 ° C atmosphere gas consisting of ₂ and the nitriding depth after nitriding, grain size of cementite in the base material, tensile strength, and surface hardness were investigated. . All the tensile tests at this time were performed using JIS 13B test pieces.

Further, the fatigue strength after nitriding treatment was measured by the following method.

After the above-mentioned annealing, a notch having a depth of 2 mm, a tip angle of 30 °, and a radius of 0.5 mm was formed on both sides of the center portion and the width direction of a test piece having the same plate thickness and a parallel portion width of 20 mm and a length of 50 mm. After cutting and nitriding this test piece under the same conditions as above, 4Hz
The stress amplitude of the cycle was measured in the range of 200 to 500 N / mm ^{2 up} to 2 × 10 ⁶ times. The results are shown in Table 2. ○ in the column of the grain size of cementite must be present in the steel sheet,
X also means not present.

As is clear from Table 2, in the steel sheets manufactured from the steels 2 to 4 having the chemical composition defined in the present invention, all have good elongation after annealing and increase in strength after nitriding treatment. It is recognized and has a high fatigue strength. On the other hand, in the steel sheet manufactured from Steel 1 having a chemical composition outside the scope of the present invention, there is almost no increase in strength due to the nitriding treatment and the fatigue strength is low. Further, the steel sheet produced from the steel 5 also has a low elongation after annealing and is not suitable for plastic working.

[0058]

[Table 1]

[0059]

[Table 2]

Example 2 A steel ingot of steel having the chemical composition shown in Table 3 melted in a laboratory vacuum melting furnace was forged to a plate thickness of 30 mm,
Subsequently, it was hot-rolled into a thin steel plate having a thickness of 4 mm to obtain a raw steel plate. After cold-rolling these to a plate thickness of 1.5 mm, they were box-annealed under the annealing conditions shown in Table 4 and examined for mechanical properties.

After 5% of plastic strain was further applied to the annealed steel sheet, it was nitrided under the conditions shown in Table 4 to form a nitride layer on the surface layer. The particle size, tensile strength and surface hardness of cementite were investigated. In addition, all the tensile tests at this time were performed using JIS13B test pieces. RX shown in the column of nitriding atmosphere in Table 4 is RX gas (butane decomposition gas: for example, 30% CH ₄ -60% H ₂ -4% N ₂
-5% CO-1% CO ₂ ).

Furthermore, the fatigue strength after the nitriding treatment was determined in Example 1.
It measured by the method similar to. The results are shown in Table 4.

As is clear from Table 4, the steel sheets manufactured by the processes B, C, E, F, H, I and J satisfying the conditions defined in the present invention all have good formability after annealing, Moreover, a high surface hardness and an increase in tensile strength are observed after the nitriding treatment.

In Process A, the nitriding temperature is low, and the desired thickness of the nitrided layer and the Fe ₃ C III structure are not obtained, so that the surface layer is not hardened and the strength is not increased. Further, in the process D, the nitriding temperature is too high, and the desired Fe ₃ C III structure is not obtained, so that the surface hardness is decreased and the tensile strength is not increased so much.

On the other hand, in the process G, the nitriding time is short, and the desired thickness of the nitrided layer and the Fe ₃ C III structure are not obtained, so that the surface layer hardening and the increase in tensile strength are small. In Process K, the nitriding treatment time was excessively long and the desired Fe ₃ C III structure was not obtained, so that the tensile strength increase was small.

In the processes A, D, G and K in which the increase in tensile strength after the nitriding treatment is small, the fatigue strength is also low.

[0067]

[Table 3]

[0068]

[Table 4]

Example 3 An ingot of steel having the chemical composition shown in Table 5 melted in a laboratory vacuum melting furnace was forged to a plate thickness of 30 mm,
Then, it was hot-rolled into a thin steel plate with a plate thickness of 1.8 mm to obtain a raw steel plate. These hot-rolled steel sheets were box-annealed under the annealing conditions shown in Table 6, and the same mechanical properties as in Example 1 were investigated.

After 5% plastic strain was applied to the annealed steel sheet, a nitride layer was provided on the surface under the same conditions as in Example 1, the nitride layer depth after the nitriding treatment, the grain size of cementite in the base metal part, The tensile strength, surface hardness and fatigue strength after nitriding treatment were investigated. The results are shown in Table 6.

As can be seen from Table 6, in the steel sheet produced from steel 24 having a Si content exceeding the range of the present invention, formability after annealing cannot be obtained even if the annealing temperature is within the range defined by the present invention. . In the steel sheet made of steel 25 having an Mn content exceeding the range of the present invention, graphite precipitation is suppressed in the annealing stage and the elongation is insufficient. In the case of steel plate made of steel 26, the annealing temperature is too low, so the graphitization of cementite is insufficient and Fe ₃ CII
Is generated, and the fatigue strength is low. Since the annealing temperature of the steel sheet produced from steel 27 is too high, the nitriding treatment also produces Fe ₃ CII, and the fatigue strength is low. A steel sheet manufactured by satisfying all the conditions defined in the present invention has a desired structure and good strength.

[0072]

[Table 5]

[0073]

[Table 6]

[0074]

According to the present invention, by applying a nitriding treatment to control the surface layer thickness and internal structure, it is possible to increase the strength of the base material and obtain a thin steel plate or a thin steel plate part having improved fatigue strength. it can. If these are applied to high-strength parts for automobiles, etc., long life can be achieved.

Claims (2)

[Claims] 1. A weight ratio of C: 0.10 to 0.70%, Si: 0.05.
~ 1.00%, Mn: 0.05 ~ 0.50%, Ni: 2.00% or less (may be no addition), sol.Al: 0.01 ~ 1.00%, N: 0.002 ~ 0.01
0%, B: 3 to 50 ppm and Ca: 0.010% or less (there may be no addition), and the balance substantially consists of Fe and unavoidable impurities, P in the impurities is 0.020% or less, S is 0.
010% or less, has a nitrided layer with a thickness of 50 μm or less as its surface structure, and the internal structure is composed of ferrite, graphite, and cementite divided into two levels with a diameter of 0.2 μm or more and 0.05 μm or less. Thin steel plate or thin steel plate part. 2. A weight ratio of C: 0.10 to 0.70%, Si: 0.05
~ 1.00%, Mn: 0.05 ~ 0.50%, Ni: 2.00% or less (may be no addition), sol.Al: 0.01 ~ 1.00%, N: 0.002 ~ 0.01
0%, B: 3 to 50 ppm and Ca: 0.010% or less (there may be no addition), and the balance substantially consists of Fe and unavoidable impurities, P in the impurities is 0.020% or less, S is 0.
After spheroidizing cementite and graphite are dispersed in the steel by box annealing at 650 to 780 ℃, the material thin steel sheet containing 010% or less is subjected to plastic working, and further N ₂ and H at 400 to 650 ℃.
₂ or a mixed gas atmosphere in which at least one of CO and CO ₂ is added to this mixed gas, soaked for 1 hour to 30 hours, and a surface layer structure composed of a nitride layer having a thickness of 50 μm or less,
Ferrite, graphite and grain size of 0.2μm or more and 0.05μm
A method for producing a thin steel plate or a thin steel plate part, comprising forming an internal structure made of cementite divided into the following two levels.