WO2016148037A1

WO2016148037A1 - Steel sheet for carburization having excellent cold workability and toughness after carburizing heat treatment

Info

Publication number: WO2016148037A1
Application number: PCT/JP2016/057684
Authority: WO
Inventors: 武広土田; 桂梶原
Original assignee: 株式会社神戸製鋼所
Priority date: 2015-03-13
Filing date: 2016-03-10
Publication date: 2016-09-22
Also published as: JP2016169433A

Abstract

This steel sheet for carburization having excellent cold workability and toughness after carburizing heat treatment has a sheet thickness of 2-10 mm, a component composition containing specific amounts of C, Mn, Al and N with the balance made up of iron and unavoidable impurities, and a steel structure composed of ferrite and carbides. With respect to the ferrite, the number of ferrite crystal grains having an aspect ratio of 3 or less is 60% or more of the number of all ferrite crystal grains, and the average crystal grain size of all crystal grains is within the range of 3-50 μm. With respect to the carbides, the number of carbides having an aspect ratio of 2 or less is 80% or more of the number of all carbides, and the average circle-equivalent diameter of all carbides is 0.6 μm or less.

Description

Carburizing steel plate with excellent cold workability and toughness after carburizing heat treatment

The present invention relates to a carburized steel sheet that exhibits good cold workability during processing before heat treatment and exhibits good toughness after carburizing heat treatment, and more particularly, presses from steel sheets in automobiles, trains, industrial machinery, etc. Machine structural parts that are hardened by carburizing or carbonitriding to improve wear resistance and fatigue resistance after the molding process, especially gears, clutch plates, dampers, brake plates, reclining sheets The present invention relates to a carburized steel plate useful as a material for manufacturing door lock parts and the like.

In recent years, the needs for improving the fuel efficiency of automobiles and the need for higher strength (strength, toughness, durability) and cost reduction in industrial machinery have become stronger, and machine structural parts are more durable and can be manufactured at low cost. It is demanded. In response to the need for higher strength for such mechanical structural parts, steel materials that are generally used and their manufacturing methods are steel materials that are hot-forged steel bars (hot forging materials), and cutting and carburizing and quenching are used. (For example, refer to Patent Document 1). Further, in order to ensure strength and toughness after carburizing and quenching, JIS-SCM420 steel to which expensive Mo is added as a steel material component is generally employed. On the other hand, in order to reduce CO ₂ emissions in the component manufacturing process, there is an increasing demand for cold forging of components such as gears that have been processed by hot forging. Further, as a cost reduction method, press forming such as punching, bending, drawing, etc., using a steel plate as a raw material, instead of cutting with a steel bar as a raw material, is also being studied. In addition, there is a demand to reduce the steel price itself as much as possible.

On the other hand, hole expansibility may be used as an evaluation of press formability, but as a method of improving the hole expansibility of a steel sheet, carbide is spheroidized by spheroidizing annealing and the size is controlled within an appropriate range. There are many known techniques for improving hole expansibility (see, for example, Patent Document 2). In Patent Document 2, as a method for improving the punchability of a steel sheet, the punchability can be improved by defining the spheroidization rate and average particle size of the spherical carbide, and further, the crystal grains are prevented from coarsening during carburizing. For this purpose, it is disclosed that Ti or B may be added. However, in the specified range of the spherical carbide shown in the same document, the punchability is not sufficiently improved, and it is difficult to secure the internal strength of the component, particularly when the C content is 0.3% by mass or less. It cannot be applied to high-strength parts as the subject of the invention.

In Patent Document 3, the stretch flangeability of a steel sheet is improved by setting the average ferrite grain size to 1 to 10 μm, the standard deviation of ferrite grain size to 3.0 μm or less, and the inclusion shape ratio to 2.0 or less. It is disclosed. However, no steel sheet having a C content of 0.3% or less, which is an object of the present invention, is found in the examples of this document, and is not intended for carburizing use. Also, since the P content is large, carburizing temporarily. However, sufficient toughness cannot be obtained.

As mentioned above, the carburizing steel sheet that combines high hole expandability, high strength, and low cost at a high level has hardly been studied so far.

Japanese Patent No. 3094856 Japanese Patent No. 4465057 Japanese Patent No. 4276504

Then, this invention aims at providing the steel plate for carburization which can implement | achieve the cost reduction of the steel materials used for manufacture of a machine structural component, combining cold workability and the toughness after carburizing heat processing. .

The steel plate for carburizing according to the first invention of the present invention,
The plate thickness is 2-10mm,
Ingredient composition is mass%,
C: 0.05 to 0.30%,
Mn: 0.3 to 3.0%,
Al: 0.015 to 0.1%,
N: each containing 0.003 to 0.030%,
The balance consists of iron and inevitable impurities,
Steel structure
Made of ferrite and carbide,
For the ferrite, the number of ferrite crystal grains having an aspect ratio defined by the major axis / minor axis of 3 or less is 60% or more of the number of all ferrite crystal grains, and the average crystal grain size of all the ferrite crystal grains is In the range of 3-50 μm, and
For the carbide, the number of carbides having an aspect ratio defined by the major axis / minor axis of 2 or less is 80% or more of the number of all carbides, and the average equivalent circle diameter of all the carbides is 0.6 μm or less. It is characterized by being.

The steel plate for carburizing according to the second invention of the present invention,
In the first invention,
Ingredient composition is mass%, and
Cr: more than 0% and 3.0% or less,
Mo: more than 0% and 1.0% or less,
Ni: At least one selected from the group consisting of more than 0% and not more than 3.0%
Is included.

The carburized steel sheet according to the third aspect of the present invention is
In the first or second invention,
Among the inevitable impurities, Si is 0.5% or less, P is 0.030% or less, and S is 0.035% or less.

The carburized steel sheet according to the fourth aspect of the present invention is
In any one of the first to third inventions,
Ingredient composition is mass%, and
Cu: more than 0% and 2.0% or less,
Co: at least one selected from the group consisting of more than 0% and not more than 5%
Is included.

The carburized steel sheet according to the fifth aspect of the present invention is
In any one of the first to fourth inventions,
Ingredient composition is mass%, and
V: more than 0% and 0.5% or less,
Ti: more than 0% and 0.1% or less,
Nb: at least one selected from the group consisting of more than 0% and 0.1% or less
Is included.

The carburized steel sheet according to the sixth aspect of the present invention is
In any one of the first to fifth inventions,
Ingredient composition is mass%, and
Ca: more than 0% and 0.08% or less,
Zr: at least one selected from the group consisting of more than 0% and not more than 0.08%
Is included.

The carburized steel sheet according to the seventh aspect of the present invention is
In any one of the first to sixth inventions,
Ingredient composition is mass%, and
Sb: more than 0% and 0.02% or less
Is included.

The carburized steel sheet according to the eighth aspect of the present invention is
In any one of the first to seventh inventions,
Ingredient composition is mass%, and
REM: more than 0% and 0.05% or less,
Mg: more than 0% and 0.02% or less,
Li: more than 0% and 0.02% or less,
Pb: more than 0% and 0.5% or less,
Bi: At least one selected from the group consisting of more than 0% and 0.5% or less
Is included.

According to the present invention, in the steel structure in which carbides are dispersed in ferrite, the ferrite crystal grains are equiaxed and refined, and the carbides are spheroidized and refined to ensure cold workability, It has become possible to provide a carburized steel sheet that can obtain a predetermined toughness after carburizing heat treatment. In addition, this has made it possible to reduce the cost of steel materials used in the manufacture of machine structural parts.

The external appearance shape and dimension of the Charpy impact test piece used in the Example are shown, (a) is a side view, and (b) is a front view.

Hereinafter, the carburized steel sheet according to the present invention (hereinafter, also referred to as “the steel sheet of the present invention” or simply “the steel sheet”) will be described in more detail. The steel sheet of the present invention overlaps with the hot forging material (high-strength, high-toughness case-hardening steel) described in Patent Document 1 above, but the steel structure is a structure in which carbides are dispersed in ferrite. The difference is that the ferrite crystal grains are equiaxed and refined, and the carbides are spheroidized and refined.

[Thickness of the steel sheet of the present invention: 2 to 10 mm]
First, the steel sheet of the present invention has a thickness of 2 to 10 mm. If the plate thickness is less than 2 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 10 mm, it is difficult to achieve the tissue form defined in the present invention, and the desired effect cannot be obtained. A preferred plate thickness is 3 to 9 mm, and a more preferred plate thickness is 4 to 7 mm.

Next, the component composition constituting the steel sheet of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
<C: 0.05 to 0.30%>
C is an element indispensable for securing the core strength of the carburized (or carbonitrided) quenching part finally obtained. If it is less than 0.05%, sufficient strength cannot be obtained. However, if it is contained excessively, the toughness is deteriorated, and the machinability and cold forgeability are deteriorated and the workability is impaired, so 0.30% is made the upper limit. A preferable content of C is in the range of 0.08 to 0.25%.

<Mn: 0.3 to 3.0%>
Mn is an element effective for deoxidation of molten steel, and in order to exert its effect effectively, it must be contained in an amount of 0.3% or more. However, if it is excessively contained, cold workability and machinability are reduced. In addition to having an adverse effect and increasing the amount of segregation to the crystal grain boundary, it lowers the grain boundary strength and thus adversely affects the impact characteristics, so it must be suppressed to 3.0% or less. A preferable content of Mn is in the range of 0.5 to 2.0%.

<Al: 0.015 to 0.1%>
Al is an element contained in steel as a deoxidizing material for steel, and has an action of binding to N in steel to produce AlN and preventing coarsening of crystal grains. In order to exert such an effect effectively, it must be contained at 0.015% or more, but the effect is saturated at about 0.1%, and beyond that, it combines with oxygen to form a non-metallic inclusion, Since it adversely affects the characteristics, etc., the upper limit was set to 0.1%. Preferably it is 0.08% or less, More preferably, it is 0.06% or less, Most preferably, it is 0.04% or less.

<N: 0.003-0.030%>
N combines with Al, V, Ti, Nb, etc. in steel to produce nitrides, and has the effect of suppressing the coarsening of crystal grains. The effect is by containing 0.003% or more. Effectively demonstrated. Preferably, it is 0.005% or more. However, these effects saturate at about 0.030%, and if it is contained more than that, nitrides become inclusions and adversely affect the physical properties, so addition beyond this must be avoided. Preferably it is 0.02% or less, More preferably, it is 0.015% or less.

The steel sheet of the present invention basically contains the above components, and the balance is iron and inevitable impurities, but it is desirable to suppress Si, P and S which are inevitably mixed in as little as possible for the following reasons.

<Si: 0.5% or less>
Si effectively acts as a strengthening element or a deacidifying element, but promotes grain boundary oxidation to deteriorate bending fatigue properties and adversely affects cold forgeability. Therefore, in order to eliminate such obstacles, the content must be suppressed to 0.5% or less, and particularly when a high level of bending fatigue characteristics is required, it is desirable to suppress the content to 0.1% or less. It is. From such a viewpoint, the more preferable content of Si is in the range of 0.02 to 0.1%.

<P: 0.030% or less>
P segregates at the grain boundaries and lowers the toughness, so the upper limit was set to 0.030%. The more preferable content of P is 0.020% or less, and further preferably 0.010% or less.

<S: 0.035% or less>
S generates MnS and contributes to the improvement of machinability. However, when the present invention is applied to gears and the like, not only the impact characteristics of the vertical eyes but also the impact characteristics of the horizontal eyes are important, and the impact characteristics of the horizontal eyes are improved. In order to reduce the anisotropy, the S content must be suppressed to 0.035% or less. The more preferable content of S is 0.025% or less, and further preferably 0.020% or less.

In addition to the above basic components, the steel sheet of the present invention can contain the following permissible components within the range not impairing the action of the present invention.

<Cr: more than 0% and 3.0% or less,
Mo: more than 0% and 1.0% or less,
Ni: at least one selected from the group consisting of more than 0% and not more than 3.0%>
These elements are useful elements in that they have a function of enhancing hardenability or refining the hardened structure, particularly Cr has an excellent effect of improving hardenability, and Mo is an incompletely hardened structure. This effectively acts to improve the hardenability and the grain boundary strength, and Ni contributes to the improvement of impact resistance by refining the structure after quenching. Such an effect is preferably exhibited by including at least one of Cr: 0.2% or more, Mo: 0.08% or more, and Ni: 0.2% or more. If it exceeds 0.0%, Cr produces carbides and segregates at the grain boundaries, lowers the grain boundary strength and adversely affects the toughness, the above effect of Mo is saturated at about 1.0%, Since the effect is saturated at 3.0%, addition beyond that is economically useless.

<Cu: more than 0% and 2.0% or less,
Co: at least one selected from the group consisting of more than 0% and 5% or less>
Cu is an element that effectively acts to improve corrosion resistance, and the effect is preferably exerted by inclusion of 0.3% or more, but the effect is saturated at 2.0%, so it is contained more than that. Is useless. If Cu is contained alone, the hot workability of the steel material tends to deteriorate. Therefore, in order to avoid such adverse effects, Ni having the effect of improving the hot workability should be used in the above content range. Is desirable.

Also, Cu and Co are elements that have an effect of strain aging and hardening the steel material, and are effective in improving the strength after processing. In order to effectively exhibit such an action, these elements are preferably contained in an amount of 0.1% or more, and more preferably 0.3% or more. However, if the Co content is excessive, the effects of strain aging and hardening of the steel material, and the effect of improving the strength after processing are saturated, and there is a possibility of promoting cracking, so the Co content is It is recommended to set it to 5% or less, further 4% or less, especially 3% or less.

<V: more than 0% and 0.5% or less,
Ti: more than 0% and 0.1% or less,
Nb: at least one selected from the group consisting of more than 0% and 0.1% or less>
These elements combine with C and N to form carbides and nitrides, refine the crystal grains and contribute to the improvement of toughness (impact resistance), but the effect is saturated near the upper limit, respectively. Since there is a risk of adversely affecting the machinability and cold workability, each must be suppressed to the upper limit value or less. Preferable lower limit values for effectively exhibiting the effect of addition of these elements are V: about 0.03%, Ti: about 0.005% and Nb: about 0.005%.

<Ca: more than 0% and 0.08% or less,
Zr: at least one selected from the group consisting of more than 0% and 0.08% or less>
Ca wraps hard inclusions with flexible inclusions, and Zr spheroidizes MnS, both of which contribute to improving machinability, and both elements are reduced by anisotropy due to spheroidization of MnS. Although it has the effect | action which improves the impact characteristic of a horizontal eye, since those effects are saturated at 0.08%, respectively, it is 0.08% or less, Furthermore, 0.05% or less, Especially 0.01% or less It is recommended to do. In addition, the preferable lower limit for making the said effect of these elements exhibit effectively is Ca: 0.0005% (further 0.001%), Zr: 0.002%.

<Sb: more than 0% and 0.02% or less>
Sb is an element effective for suppressing the grain boundary oxidation and increasing the bending fatigue strength, but since the effect is saturated at 0.02%, the addition of more is economically useless. A preferable lower limit value for effectively exhibiting the effect of addition of Sb is 0.001%.

<REM: more than 0% and 0.05% or less,
Mg: more than 0% and 0.02% or less,
Li: more than 0% and 0.02% or less,
Pb: more than 0% and 0.5% or less,
Bi: at least one selected from the group consisting of more than 0% and 0.5% or less>
REM is an element that, like Ca, spheroidizes compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, REM is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated, and an effect commensurate with the content cannot be expected, so 0.05% or less, further 0.03% or less, particularly 0.01% by mass or less is recommended. .
In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.

Mg, like Ca, is an element that spheroidizes sulfide compound inclusions such as MnS to enhance the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, Mg is preferably contained in an amount of 0.0002% or more, more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Li can spheroidize sulfide compound inclusions such as MnS and increase the deformability of steel like Ca, and lower the melting point of Al-based oxides to make them harmless and improve machinability. It is a contributing element. In order to effectively exhibit such an action, Li is preferably contained in an amount of 0.0002% or more, and more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Pb is an effective element for improving machinability. In order to effectively exhibit such an action, Pb is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, if it is contained excessively, production problems such as the occurrence of rolling defects occur, so 0.5% or less, further 0.4% or less, and particularly 0.3% by mass or less are recommended.

Bi is an element effective for improving the machinability like Pb. In order to effectively exhibit such an action, Bi is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, since the effect of improving the machinability is saturated even if contained excessively, 0.5% by mass or less, further 0.4% or less, particularly 0.3% or less is recommended.

Next, the structure characterizing the steel sheet of the present invention will be described.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention has a steel structure in which carbides are dispersed in ferrite. In particular, with respect to ferrite crystal grains, the degree and size of equiaxing are controlled within a specific range, and The degree and size of spheroidization are controlled within a specific range.

<The steel structure consists of ferrite and carbide>
Since the steel sheet of the present invention is produced by further spheroidizing and annealing a hot rolled sheet of ferrite + pearlite main structure, for example, as described later in “Preferred production method”, the steel structure is made of ferrite. The carbide (mainly cementite) is dispersed (precipitated) therein.

<About the ferrite, the number of crystal grains having an aspect ratio defined by the major axis / minor axis of 3 or less is 60% or more of the total number of crystal grains>
The shape of ferrite crystal grains is equiaxed grains to improve stretch flangeability (hole expandability) and ensure the hardness (internal hardness) of the plate thickness center that is deep from the surface after heat treatment Necessary for compatibility. For this reason, the number of ferrite crystal grains that are equiaxed grains and have an aspect ratio (major axis / minor axis) of 3 or less is 60% or more, preferably 70% or more, more preferably 80% of the total number of ferrite crystal grains. % Or more.

<Average crystal grain size of all ferrite crystal grains is in the range of 3-50 μm>
If the ferrite crystal grains become too large, the surface properties deteriorate, causing surface cracks, and the hole-expandability deteriorates. For this reason, the average crystal grain size of all ferrite crystal grains is 50 μm or less, preferably 40 μm or less, and more preferably 30 μm or less. On the other hand, as the lower limit value, the finer the ferrite crystal grains, the better the characteristics, but it is necessary to increase the rolling ability and the cooling ability, and the productivity is lowered. For this reason, the average crystal grain size of all ferrite crystal grains is 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more.

<About the carbides, the number of carbides having an aspect ratio defined by the major axis / minor axis of 2 or less is 80% or more of the total number of carbides, and the average equivalent circle diameter of all the carbides is 0.6 μm or less>
If the carbide is flattened or coarsened, cracks will occur in the punched section during punching, leading to quality deterioration when the part is made into parts, and it will also be the starting point of fracture when drilling further. Therefore, it is necessary to spheroidize and refine the carbide. For this reason, the number of carbides that are spherical carbides with an aspect ratio (major axis / minor axis) of 2 or less is 80% or more, preferably 83% or more, more preferably 85% or more of the total number of carbides. The average equivalent circle diameter of all carbides is 0.6 μm or less, preferably 0.55 μm or less, and more preferably 0.5 μm or less.
Here, “carbide” is mainly cementite having a structure of Fe ₃ C. However, when Cr, V, Ti, Nb or the like is contained, cementite, VC, TiC, NbC is also included. However, fine VC, TiC, NbC, etc. (equivalent circle diameter less than 0.1 μm) that cannot be observed in the carbide size measurement by SEM observation described later are not included. Such fine carbide having an equivalent circle diameter of less than 0.1 μm does not affect the punching workability and hole expansibility described above.

[Method for measuring the aspect ratio of ferrite crystal grains]
With respect to the ferrite crystal grains, the maximum ferret diameter and the minimum ferret diameter were measured, and the ratio (major axis / minor axis) was defined as the aspect ratio.

[Method for measuring average ferrite crystal grain size]
By analyzing the image taken by the scanning electron microscope, the individual centroid diameters of all ferrite crystal grains are obtained, and the average grain size of all ferrite crystal grains is obtained by arithmetically averaging the centroid diameters by the number of all crystal grains. It was.

[Measurement method of carbide aspect ratio and average equivalent circle diameter]
A SEM (Scanning Electron Microscope) specimen was prepared by polishing and etching the 1/4 depth surface of the plate thickness, and four fields of view of 8000 times images were taken, and image analysis software (“ Image-Pro Plus ”(Media Cybernetics) measured the aspect ratio (major axis / minor axis) and equivalent circle diameter of all particles having an equivalent circle diameter of 0.1 μm or more, and the aspect ratio was 2.0 or less. The number of particles and the total number of particles were counted to calculate the ratio, and the average value of equivalent circle diameters was calculated.

Next, a preferred manufacturing method for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
The steel sheet of the present invention is obtained, for example, by melting and casting raw material steel having the above-mentioned composition to form a slab, and by subjecting the slab as it is or surface chamfered to each step of heating, hot rough rolling, and finish rolling. It is possible to produce a spheroidizing material obtained by further subjecting the hot-rolled plate as the intermediate material to further spheroidizing annealing. Thereafter, pickling and skin pass may be further performed according to necessary conditions such as surface condition and plate thickness accuracy.

Here, the steel structure before spheroidizing annealing (that is, the steel structure as a hot-rolled sheet) is composed of ferrite: 10 to 50%, pearlite: 15 to 50%, and balance: bainite. Regarding the crystal grains of all phases including pearlite (hereinafter referred to as “total crystal grains”), the number of crystal grains having an aspect ratio (major axis / short axis) of 3 or less is 60% or more of the total crystal grains. In addition, it is important that the average crystal grain size of all the crystal grains is in the range of 3 to 50 μm.

Because it is necessary to finally control the form of carbide (mainly cementite) to a predetermined range through the subsequent spheroidizing annealing, as the structure in the previous stage, ferrite and pearlite are appropriately added as described above. It is necessary to include. If there is too much ferrite, not only will the ferrite be prone to coarsening, but the carbides will also be prone to coarsening. Moreover, when there is too much pearlite, in other words, the case where the cooling rate after hot rolling is insufficient or the case where the alloying elements are insufficient, the entire structure becomes coarse, so after spheroidizing annealing Of ferrite and cementite are likely to be coarsened. Conversely, if there is too much bainite, the hardness of the steel sheet becomes too high, making it difficult to handle in operation.

[Preparation of molten steel]
First, a desired oxide can be generated by adding a predetermined alloy element in a predetermined order to molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted. Particularly in the present invention, it is extremely important to adjust the total oxygen amount after adjusting the dissolved oxygen amount so that coarse oxides are not formed.

“Dissolved oxygen” means oxygen in a free state that does not form oxides and exists in molten steel. Total oxygen means the sum of all oxygen contained in molten steel, that is, free oxygen and oxygen forming oxides.

First, the dissolved oxygen content of the molten steel is adjusted to a range of 0.0010 to 0.0060%. When the amount of dissolved oxygen in the molten steel is less than 0.0010%, the amount of dissolved oxygen in the molten steel is insufficient, so that a predetermined amount of Al—O-based oxide cannot be secured, and a desired size distribution cannot be obtained. In addition, when the amount of dissolved oxygen is insufficient, when REM is added, REM forms sulfides, so that inclusions become coarse and deteriorate characteristics. Therefore, the amount of dissolved oxygen is set to 0.0010% or more. The dissolved oxygen is preferably 0.0013% or more, more preferably 0.0020 or more.

On the other hand, if the amount of dissolved oxygen exceeds 0.0060%, the amount of oxygen in the molten steel becomes too large, and the reaction between the oxygen in the molten steel and the above elements becomes violent, which is not preferable for melting work, and is coarse. Oxide is produced and the characteristics are deteriorated. Therefore, the amount of dissolved oxygen should be suppressed to 0.0060% or less. The amount of dissolved oxygen is preferably 0.0055% or less, more preferably 0.0053% or less.

By the way, the amount of dissolved oxygen in molten steel primarily refined in a converter or electric furnace usually exceeds 0.010%. Therefore, in the production method of the present invention, it is necessary to adjust the amount of dissolved oxygen in the molten steel to the above range by some method.

Examples of the method for adjusting the amount of dissolved oxygen in the molten steel include a method of vacuum C deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Mn, and Al. The amount of dissolved oxygen may be adjusted by appropriately combining these methods. Moreover, you may adjust the amount of dissolved oxygen using a ladle heating type refining apparatus, a simple molten steel processing facility, etc. instead of the RH type degassing refining apparatus. In this case, since the amount of dissolved oxygen cannot be adjusted by vacuum C deoxidation, a method of adding a deacidifying element such as Si may be adopted to adjust the amount of dissolved oxygen. When employing a method of adding a deoxidizing element such as Si, the deoxidizing element may be added when steel is removed from the converter to the ladle.

After the dissolved oxygen content of the molten steel is adjusted to the range of 0.0010 to 0.0060%, the molten steel is stirred, and the oxides in the molten steel are floated and separated so that the total oxygen content in the molten steel is 0.0010 to 0.00. Adjust to 0070%. As described above, in the present invention, the molten steel in which the amount of dissolved oxygen is appropriately controlled is stirred to remove unnecessary oxides, and then generation of coarse oxides, that is, coarse inclusions can be prevented.

If the total oxygen amount is less than 0.0010%, the desired amount of oxide is insufficient, and the amount of oxide that contributes to the fine size distribution of inclusions cannot be ensured. Therefore, the total oxygen amount is set to 0.0010% or more. The total oxygen amount is preferably 0.0015% or more, more preferably 0.0018% or more.

On the other hand, if the total oxygen amount exceeds 0.0070%, the amount of oxide in the molten steel becomes excessive, and coarse oxides, that is, coarse inclusions are generated, and the characteristics deteriorate. Therefore, the total oxygen amount should be suppressed to 0.0070% or less. The total oxygen amount is preferably 0.0060% or less, more preferably 0.0050% or less.

Since the total amount of oxygen in the molten steel changes generally in correlation with the stirring time of the molten steel, it can be controlled by adjusting the stirring time. Specifically, the total amount of oxygen in the molten steel is appropriately controlled while appropriately measuring the total amount of oxygen in the molten steel after stirring the molten steel and removing the floating oxide.

When adding REM to a steel material, after adjusting the total oxygen amount in molten steel to the said range, it casts, after adding REM. The desired oxide can be obtained by adding the above elements to the molten steel with the total oxygen content adjusted.

The form of REM added to the molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, etc., pure Ca, Fe—Si—La alloy, Fe—Si—Ce alloy, Fe— A Si—Ca alloy, a Fe—Si—La—Ce alloy, a Fe—Ca alloy, a Ni—Ca alloy, or the like may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, it is necessary to satisfy the preferred range defined in the present invention.

When REM is added in the present invention, it is preferable to stir the molten steel within a range not exceeding 40 minutes after the addition of REM for the purpose of promoting the removal of coarse oxides. When the stirring time exceeds 40 minutes, fine oxides aggregate and coalesce in the molten steel, so that the oxides become coarse and the characteristics deteriorate. Therefore, the stirring time is preferably within 40 minutes. The stirring time is more preferably within 35 minutes, and further preferably within 30 minutes. The lower limit of the stirring time of the molten steel is not particularly limited, but if the stirring time is too short, the concentration of the additive element becomes non-uniform, and the desired effect cannot be obtained as a whole steel material. Accordingly, a desired stirring time corresponding to the container size is required.

As described above, molten steel with an adjusted composition can be obtained. It casts using the obtained molten steel, and obtains a steel piece.

Then, hot rolling including heating and finish rolling, rapid cooling after hot rolling, slow cooling after quenching stop, rapid cooling after slow cooling, and winding are performed to produce a hot rolled sheet as an intermediate material.

[heating]
Heating before hot rolling is performed at 1150 to 1300 ° C. An austenite single phase is obtained by this heating. Thereby, solid solution elements (including additive elements such as V and Nb) are dissolved in austenite. If the heating temperature is less than 1150 ° C., it cannot be dissolved in austenite, and coarse carbides are formed, so that the effect of improving fatigue characteristics cannot be obtained. On the other hand, heating temperatures exceeding 1300 ° C. are difficult to operate. Moreover, when Ti is contained as an additive element, the TiC solution solution temperature or higher and 1300 ° C. or lower are necessary also in terms of solid solution of Ti having the highest solution temperature among carbides. A more preferable lower limit of the heating temperature is 1200 ° C.

[Hot rough rolling]
In rough rolling, the microstructure control of recrystallized austenite is performed in order to ensure the proportion of equiaxed grains having a predetermined shape defined in the present invention. The rough rolling temperature is set to 900 to 1200 ° C. in consideration of securing the temperature of the subsequent finish rolling, and the austenite grains in the rough rolling are refined and repeatedly recrystallized, so that the proportion of equiaxed grains having a predetermined shape is increased. Can be controlled. The rough rolling temperature is more preferably 900 to 1100 ° C, still more preferably 900 to 1000 ° C.

[Hot finish rolling]
Hot rolling is performed so that the finish rolling temperature is 800 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation occurs at a high temperature and the precipitated carbides in the ferrite are coarsened, so that a certain finish rolling temperature is required. The finish rolling temperature is more preferably 850 ° C. or higher in order to coarsen austenite grains and increase the grain size of bainite.

[Difference between entry temperature and exit temperature of hot finish rolling]
The difference between the entry side temperature and the exit side temperature of hot finish rolling is set to 150 ° C. or less. When this temperature difference exceeds 150 ° C., the temperature before finish rolling is high, and the crystal grains (austenite grains) at this high temperature stage become coarse and the recrystallized grains generated during finish rolling also become large. Cheap. Further, when the temperature difference between the entry side and the exit side is large, the recrystallized structure generated during finish rolling tends to be non-uniform, and as a result, ferrite crystal grains having a large aspect ratio tend to remain. For these reasons, the number of ferrite crystal grains having an aspect ratio of 3 or less is less than 60% of the total number of crystal grains. This temperature difference is more preferably 100 ° C. or less.

[Rapid cooling after hot rolling]
After finishing the finish rolling, quenching is performed at a cooling rate (quenching rate) of 20 ° C./s or more within 5 s, and the quenching is stopped at a temperature of 580 ° C. or more and less than 670 ° C. (quenching stop temperature). This is because the precipitation carbide formed in the ferrite is refined by lowering the starting temperature of the ferrite transformation. When the cooling rate (quenching rate) is less than 20 ° C./s, pearlite transformation is promoted, or when the quenching stop temperature is less than 580 ° C., pearlite transformation or bainite transformation is promoted, and cold workability is lowered. On the other hand, when the quenching stop temperature is 670 ° C. or higher, the precipitated carbides in the ferrite are coarsened, and fatigue resistance characteristics cannot be ensured. The quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° C.

[Slow cooling after rapid cooling stop]
After the rapid cooling stop, it is slowly cooled at a cooling rate (slow cooling rate) of 5 ° C./s or more and less than 20 ° C./s. By setting the slow cooling rate to 5 ° C./s or more, the formation of pro-eutectoid ferrite during hot rolling is suppressed, the precipitated carbides in ferrite are appropriately refined, and the grain structure of the hot rolled sheet is controlled. This is to control the texture form in the final steel sheet. When the slow cooling rate is less than 5 ° C./s, the amount of pro-eutectoid ferrite is increased and coarse grains are produced, and coarse grains are produced in the final steel plate, resulting in a non-uniform state of carbides, resulting in cold workability. Deteriorate.

[Rapid cooling after slow cooling, winding]
After the above-described slow cooling, the film is wound at over 550 ° C and below 650 ° C. When the coiling temperature is higher than 650 ° C., many surface oxide scales are formed and the surface properties are deteriorated. On the other hand, when it is less than 550 ° C., many martensites are formed and cold workability is lowered.

A hot-rolled sheet is manufactured as described above. The steel sheet of the present invention is obtained by further subjecting the manufactured hot-rolled sheet to spheroidizing annealing.

[Spheroidizing annealing]
The spheroidizing annealing conditions need to be lower and shorter than usual. That is, the heating temperature is preferably 705 to 740 ° C., the holding time is 2 to 6 hours depending on the heating temperature, and the average cooling rate up to 680 ° C. thereafter is preferably 0.001 to 0.01 ° C./s. If the heating temperature is too high, the carbide (mainly cementite) tends to coarsen, whereas if the heating temperature is too low, pearlite remains and the aspect ratio of the carbide tends to increase. You may hold | maintain isothermal on the way so that an average cooling rate may become in the said prescription | regulation range. When the average cooling rate exceeds 0.01 ° C./s, regenerated pearlite is generated and the aspect ratio of the spherical carbide is increased. On the other hand, when the average cooling rate is less than 0.001 ° C./s, not only industrially it takes too much time, but also the spherical carbide is easily coarsened.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

After the steel having the component composition shown in Table 1 below was melted by a vacuum melting method and cast into a 120 mm thick ingot, this was hot rolled under the conditions shown in Table 2 below to produce a hot-rolled sheet, Further, spheroidizing annealing was performed to produce a spheroidizing material. In all the tests, the cooling after the quenching stop in the hot rolling was performed under the condition of slow cooling for 5 to 20 s at a cooling rate of 10 ° C./s or less.

Using a vacuum melting furnace (capacity 150 kg), a test steel containing chemical components shown in Table 1 was melted, cast into a 150 kg ingot, and cooled. In melting the test steel in a vacuum melting furnace, the components are adjusted for elements other than Al, REM, and Ca, and deoxidized using at least one element selected from C, Si, and Mn. The amount of dissolved oxygen in the molten steel was adjusted. The total amount of oxygen in the molten steel was adjusted by stirring the molten steel in which the amount of dissolved oxygen was adjusted for about 1 to 10 minutes to float and separate oxides in the molten steel.

In addition, when adding REM and Ca, the molten steel which adjusted the component was obtained by adding to the molten steel which adjusted the total amount of added oxygen. REM was added in the form of a misch metal containing about 25% La and about 50% Ce, and Ca was added in the form of a Ni—Ca alloy, a Ca—Si alloy, or a Fe—Ca compact. .

Then, the obtained ingot was hot-rolled under the conditions shown in Table 2 to produce a hot rolled plate having a predetermined thickness, and further subjected to spheroidizing annealing to produce a spheroidizing material.

For the spheroidized material thus obtained, the aspect ratio, the crystal grain size, and the number of ferrite crystal grains, and the carbides were measured by the measurement method described in the above [Mode for Carrying Out the Invention]. The aspect ratio, equivalent circle diameter, and number of them were investigated.

Moreover, in order to evaluate the cold workability of each of the above spheroidized materials, the hole expansion rate was measured, and those having a hole expansion rate of 48% or more were accepted.

Further, from each of the spheroidized materials, a sub-size Charpy test piece (see FIG. 1) having a width of 5 mm is cut out so that the rolling direction is in the longitudinal direction of the test piece and the plate thickness direction is in the width direction of the test piece. The surface other than the notched surface was plated with copper so as not to be carburized (that is, only the surface having the R notch was carburized), and then carburized heat treatment was performed under the following conditions.

[Carburizing heat treatment conditions]
In a gas atmosphere of carbon potential (CP value) = 0.8%, after holding at 900 ° C. × 2.5 h and further carburizing by holding at 850 ° C. × 0.5 h, oil quenching was performed at 100 ° C. Then, after holding at 160 ° C. for 2 hours and performing a tempering treatment, it was air-cooled.

<Toughness after carburizing heat treatment>
And in order to evaluate the toughness after carburizing heat processing, the Charpy impact test was implemented about each said test piece, and the impact value of 50 J / cm < ² > or more was set as the pass.

<Internal hardness after carburizing heat treatment>
Further, in order to evaluate the material strength after the carburizing heat treatment, as the internal hardness, using a Vickers hardness tester, the measurement position: the central portion of the cross section at a position of 15 mm from the end surface in the longitudinal direction of each Charpy test piece, the load: Vickers hardness (Hv) was measured under the conditions of 10 kg, number of measurements: 5 times, and those with 300 Hv or more were regarded as acceptable.

These measurement results are shown in Table 3 below.

As shown in Table 3 above, Steel No. 1, 2, and 6 to 20 are all manufactured using recommended steel rolling conditions and spheroidizing annealing conditions using steel grades that satisfy the requirements of the compositional composition of the present invention. It is a satisfactory invention steel, and the hole expansion ratio, impact value and internal hardness after carburizing heat treatment all satisfy the acceptance criteria, and after securing the good toughness and internal toughness while ensuring good cold workability It was confirmed that a carburized steel sheet showing hardness was obtained.

In contrast, Steel No. Nos. 3 to 5 and 21 to 29 are comparative steels that do not satisfy at least one of the component composition and the structure requirements defined in the present invention, and are at least one of the hole expansion ratio, toughness and internal hardness after carburizing heat treatment. Does not meet the acceptance criteria.

For example, steel No. No. 3, although the requirements of the component composition are satisfied, the heating temperature before hot rolling is too low outside the recommended range, so excessive pearlite is formed at the stage of hot rolling, and as a result, the stage of spheroidizing material The ferrite crystal grains are flattened and the hole expandability is inferior.

Steel No. No. 4, although the requirements of the component composition are satisfied, the plate thickness after hot rolling is too large outside the specified range, so that ferrite is excessively formed at the stage of hot rolling, and as a result, the stage of spheroidizing material The ferrite crystal grains are coarsened and the hole expandability is inferior.

Steel No. No. 5 satisfies the requirements for the component composition, but the difference between the entry side temperature and the exit side temperature in the finish rolling is too large outside the recommended range, the ferrite crystal grains are flattened, and the hole expandability is inferior.

Steel No. Although 21 (steel type q) has a hot rolling condition in the recommended range, since the C content is too low, ferrite is excessively formed at the stage of hot rolling and the internal hardness after carburizing heat treatment is inferior.

On the other hand, steel No. 22 (steel grade r), although the hot rolling conditions are in the recommended range, the C content is too high, so excessive pearlite is formed at the hot-rolled sheet stage, and the ferrite crystal grains are flattened at the spheroidizing stage. The hole expandability and toughness after carburizing heat treatment are inferior.

Steel No. Although No. 23 (steel type s) has a hot rolling condition in the recommended range, the Mn content is too low, the ferrite crystal grains are flattened, and the internal hardness after carburizing heat treatment is inferior.

On the other hand, steel No. 24 (steel grade t), although the hot rolling conditions are in the recommended range, the Mn content is too high, and ferrite formation is insufficient at the hot-rolled sheet stage, while pearlite is excessively formed, and the spheroidizing material stage. The hole expandability is inferior.

Steel No. No. 25 (steel grade u), although the hot rolling conditions are in the recommended range, the Al content is too low, the hole expandability is inferior, and it is estimated that oxide inclusions increase due to insufficient deoxidation at the melting stage. Therefore, toughness is also inferior.

On the other hand, steel No. Although No. 26 (steel type v) has a hot rolling condition in the recommended range, the Al content is too high, the hole expandability is inferior, and the solid solution Al becomes a cause of embrittlement and the toughness is also deteriorated.

Steel No. In No. 27 (steel type w), although the hot rolling conditions are in the recommended range, the N content is too high, the hole expandability is inferior, and the solid solution N becomes a brittle factor and the toughness is also deteriorated.

Steel No. In No. 28, since the heating and holding time of the spheroidizing annealing is too long, the carbide is coarsened and the hole expandability is inferior.

Steel No. In No. 29, since the heating temperature of spheroidizing annealing is too high and the cooling rate after heating and holding is too high, regenerated pearlite is generated, the carbide is flattened, and the hole expandability is poor.

From the above, the applicability of the present invention was confirmed.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on March 13, 2015 (Japanese Patent Application No. 2015-051444), the contents of which are incorporated herein by reference.

The carburized steel sheet of the present invention exhibits good cold workability during processing before heat treatment, but exhibits good toughness after carburizing heat treatment, and gears, clutch plates, dampers, brake plates, reclining seats, door lock parts It is suitable for such as.

Claims

The plate thickness is 2-10mm,
Ingredient composition is mass%,
C: 0.05 to 0.30%,
Mn: 0.3 to 3.0%,
Al: 0.015 to 0.1%,
N: each containing 0.003 to 0.030%,
The balance consists of iron and inevitable impurities,
Steel structure
Made of ferrite and carbide,
For the ferrite, the number of ferrite crystal grains having an aspect ratio defined by the major axis / minor axis of 3 or less is 60% or more of the number of all ferrite crystal grains, and the average crystal grain size of all the ferrite crystal grains is In the range of 3-50 μm, and
For the carbide, the number of carbides having an aspect ratio defined by the major axis / minor axis of 2 or less is 80% or more of the number of all carbides, and the average equivalent circle diameter of all the carbides is 0.6 μm or less. A carburizing steel sheet having excellent cold workability and toughness after carburizing heat treatment, characterized by being.
The carburized steel sheet according to claim 1, wherein the component composition further includes at least one of the following (a) to (f).
(A) at least one selected from the group consisting of Cr: more than 0% and 3.0% or less; Mo: more than 0% and 1.0% or less; Ni: more than 0% and 3.0% or less
(B) By mass%, at least one selected from the group consisting of Cu: more than 0% and 2.0% or less, Co: more than 0% and 5% or less
(C) at least one selected from the group consisting of V: more than 0% and 0.5% or less, Ti: more than 0% and 0.1% or less, and Nb: more than 0% and 0.1% or less.
(D) at least one selected from the group consisting of Ca: more than 0% and not more than 0.08% and Zr: more than 0% and not more than 0.08% in mass%.
(E) Mass%, Sb: more than 0% and 0.02% or less (f) Mass%, REM: more than 0% and 0.05% or less, Mg: more than 0% and 0.02% or less, Li: 0% At least one selected from the group consisting of more than 0.02% or less, Pb: more than 0% to 0.5% or less, Bi: more than 0% to 0.5% or less
The steel plate for carburizing according to claim 1 or 2, wherein among the inevitable impurities, Si is 0.5% or less, P: 0.030% or less, and S: 0.035% or less.