JP2003013137A - Method of manufacturing steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing steel, which can provide a finer ferrite than that by a conventionally controlled rolling/cooling method or a hot forging method, and which can be easily industrially performed because of machining with a low deformation resistance, without needing Cu as an essential element. SOLUTION: The method of manufacturing steel includes a step for controlling segregation in grain boundaries so as to make a ratio of an average segregation concentration (mass%) of particular elements in austenitic grain boundaries against an average concentration (mass%) in the grains to be 1.2 or more, by heating and holding the steel containing the particular elements at a specific heating temperature in an austenitic region and for a specific holding time, and a step for cooling it to a supercooling austenitic region, and hot working it to cause dynamic transformation to ferrite and dynamic recrystallization of ferrite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing steel, and more particularly to a method for producing steel which has a fine structure obtained by hot working and has a low deformation resistance during working. Steel obtained by the manufacturing method of the present invention, as a structural material, a steel wire rod, a steel bar,
It is preferably used in various fields of shaped steel, thin steel plate, thick steel plate, steel pipe and forged parts formed by hot forging.

[0002]

2. Description of the Related Art As a strengthening method for steel, a solid solution strengthening method, a precipitation strengthening method, a transformation strengthening method, a crystal grain refining method and the like are known. In particular, the crystal grain refining method can improve the strength without lowering the toughness, and is therefore the most effective method for obtaining a steel excellent in strength-toughness balance,
In addition, since it is not necessary to add a large amount of expensive element to steel, it is said that high strength steel can be manufactured at low cost.

As a method for industrially applying the above effect of the grain refinement method, there is known a controlled rolling-controlled cooling method in which hot rolling conditions and subsequent cooling conditions are controlled. This controlled rolling-controlled cooling method includes processing in the austenite recrystallization region (increase of ferrite nucleation sites due to austenite grain refinement), processing in austenite unrecrystallized region (ferrite nucleation site due to introduction of austenite intragranular transformation). Increase) and accelerated cooling after processing (increase in nucleation rate due to increase in driving force) to refine the crystal grains. However, in the conventional controlled rolling-controlled cooling method, for example, in the case of Si-Mn steel, the limit is that the obtained ferrite grain size is about 15 μm.

Various crystal grain refining methods have been studied in order to overcome such a limitation of crystal grain refinement. Above all, on the assumption that the current facilities are used, a large reduction working method for performing a strong reduction of 50% or more in a low temperature austenite region is effective. For example, in JP-A-11-92855 and JP-A-11-323481, by performing a large reduction of 50% or more in the austenite unrecrystallized region, finer than in the controlled rolling-controlled cooling method,
It is described that fine particles having a particle size of about 2 to 4 μm can be obtained. However, in the low temperature austenite region (austenite unrecrystallized region), the deformation resistance at the time of processing is significantly increased. Therefore, considering the current equipment capacity, it is not possible to industrially carry out the method described in the above publication. It is difficult.

Further, in Japanese Unexamined Patent Publication No. 2000-192139, steel having a composition containing Cu: 0.5% by mass or more is heated to a temperature in the austenite region and then held for a specific time, and then 800 ° C. The following describes a thermomechanical treatment method for steel that is hot-worked in a supercooled austenite region to dynamically cause ferrite transformation. According to this method, finer ferrite can be obtained than in the conventional controlled rolling-controlled cooling method, and processing can be performed with low deformation resistance. However, when Cu is added, depending on the content of other elemental elements in the steel, hot cracking is likely to occur in continuous casting, which may make industrial production difficult. However, there is a problem that the manufacturing cost increases due to the necessity of adding.

Further, in order to obtain desired mechanical properties, many parts for mechanical structures such as automobile parts are manufactured by forming steel into parts by hot forging and then performing tempering treatment such as quenching and tempering. ing. In recent years, from the viewpoint of cost reduction by omitting the process, application of non-heat-treated hot forging without tempering treatment of quenching and tempering has been attempted. The required properties of such hot forged parts include fatigue strength as well as workability during forging. Since the fatigue strength is related to the tensile strength, strengthening the steel by refining the crystal grains is also advantageous for improving the fatigue strength.

On the other hand, since improvement in tensile strength lowers machinability, it is important to improve the durability ratio, which is the ratio of fatigue strength to tensile strength. For example, JP-A No. 11-152542 discloses a hot forged non-heat treated steel that achieves a high durability ratio and a method for manufacturing the same. In this technique, steel containing a predetermined amount of V is hot forged at 1050 to 1300 ° C., and then the austenite temperature region is rapidly cooled as much as possible to suppress the precipitation of coarse carbides and carbonitrides, and V in the ferrite. It aims to obtain a high durability ratio by increasing the ferrite hardness / pearlite hardness ratio by strengthening ferrite by finely precipitating carbides and carbonitrides. However, in the technique of the above-mentioned publication, since hot forging is performed in a high temperature range, there is a limit in refining crystal grains, and there is also a limit in improving fatigue strength and durability ratio.

[0008]

DISCLOSURE OF THE INVENTION The present invention obtains a finer ferrite than that of the conventional method of producing steel, that is, the conventional controlled rolling-controlled cooling method and the conventional hot forging method, overcoming the problems of the prior art. And a method of manufacturing a steel excellent in fatigue strength, which can be carried out industrially easily because it is processed with low deformation resistance, and which does not require the addition of Cu. To aim.

[0009]

As a result of intensive studies to solve the above problems, the present inventors have found that a steel containing a specific element is austenite at a specific heating temperature and a specific holding time. By heating and holding at, by controlling the segregation of the above elements to the austenite grain boundaries, it becomes easier to produce supercooled austenite by subsequent cooling, and the ferrite grain size is significantly refined by processing in this supercooled austenite region. It was found that the above-mentioned processing and the above-mentioned processing have low deformation resistance, and have completed the present invention. The details will be described below.

First, the present inventors have found that C, S, P, N
The steel to which at least one element selected from the group consisting of i, Cr, B, Sn, and As is added is heated and held in the austenite region, so that in the subsequent cooling process, the transformation point from austenite to ferrite (transformation It was found that the starting temperature and T _s ) are significantly reduced. For example, when 3.0 mass% Ni-added steel is heated and held in the austenite region for a specific time or longer, the transformation point from austenite to ferrite is 80
It significantly drops from 0 ° C to 690 ° C.

The present inventor has found that such a decrease in transformation point
It is considered that the segregation of additional elements to the austenite grain boundaries is the cause, and after the steel is heated and held in the austenite region, it is quenched in water to freeze the austenite grains as martensite, and the FE- Concentration analysis by TEM was performed. As a result, it was found that the additive element segregated at the grain boundaries so that the ratio of the average segregation concentration (mass%) in the austenite grain boundaries to the average concentration (mass%) in the grains was 1.2 or more. It was It was also found that the transformation point may not be sufficiently lowered when the heating temperature is lowered or the holding time is shortened to adjust the ratio to be less than 1.2. The present inventors, from these results, while the additive element in the steel is heated and held in the austenite region,
When sufficiently segregated to the austenite grain boundaries, the interface energy of the austenite grain boundaries is reduced, ferrite generation from the grain boundaries, which is the most effective nucleation site of ferrite, is suppressed, and the transformation point is lowered. Thought.

Further, when supercooled austenite produced by such a decrease in the transformation point of steel is processed, a so-called dynamic transformation in which transformation of austenite to ferrite is induced occurs. It was also found that dynamic recrystallization of ferrite occurs and the grain size can be significantly reduced to the extent that the crystal grain size is 10 μm or less.

Further, it has been found that the deformation resistance becomes much lower than that in the ordinary working in the low temperature austenite region due to the dynamic transformation into ferrite during the working and the dynamic recrystallization of the ferrite. This is because during processing, the strain is released due to the formation of a large amount of soft ferrite due to dynamic transformation before the hard austenite is significantly work-hardened. It is considered that by causing the crystal, the deformation stress does not increase due to work hardening of the ferrite, and the deformation resistance is kept low. Such knowledge is extremely important industrially, and it means that it is possible to greatly relax the restrictions on the equipment for grain refinement necessary for strengthening steel and improving fatigue properties.

The present inventor has completed the present invention based on the above findings. Hereinafter, unless otherwise specified, the component composition is expressed by mass ratio. That is, the present invention is C: 0.01 to 0.65%,
S: 0.01 to 0.1%, P: 0.01 to 0.1%, N
i: 0.5 to 3.0%, Cr: 0.5 to 3.0%, B:
0.0005-0.02%, Sn: 0.01-0.1
%, And As: at least one element selected from the group consisting of 0.01 to 0.1%, with the balance being Fe and unavoidable impurities, the balance being at least one of these elements. In the austenite region, the average segregation concentration (mass%) at the austenite grain boundary of the elements satisfying the following formula (1) is maintained by heating at a heating temperature T and a holding time t that satisfy the following formula (1), and Of grain boundary segregation so that the ratio of the average concentration (mass%) at 1.2 is 1.2 or more, then the step of cooling to the supercooled austenite region, and then hot working into ferrite And a step of inducing dynamic recrystallization of ferrite.

[0015]

[Equation 2]

However, in the formula (1), P: heat treatment parameter (= average diffusion distance (m) during heat treatment), x: one element selected from the group of the above elements, D _x : diffusion of element x Coefficient (m ² / s), Q _x : activation energy of element x (J / mol), T: heating temperature (° C.), t: retention time (s), R: gas constant (= 8.3314 J / ( K ・ mo
l)).

It is one of the preferred embodiments of the present invention that the steel further contains at least one element selected from the following groups A to F. A group: Si: 2.0% or less and V: 0.1% or less B group: Mn: 2.0% or less and Mo: 3% or less C group: Ti: 0.1% or less and Nb: 0.1 % Or less D group: Al: 0.10% or less E group: Ca: 0.01% or less F group: Rare earth metal element (REM): 0.02% or less

The working ratio in the hot working is 30.
% Or more is preferable.

Further, it is particularly preferable that the hot working is hot forging for producing a hot forged part.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The steel used in the present invention is C: 0.01 to 0.6.
5%, S: 0.01 to 0.1%, P: 0.01 to 0.1
%, Ni: 0.5 to 3.0%, Cr: 0.5 to 3.0
%, B: 0.0005 to 0.02%, Sn: 0.01 to
It contains at least one element selected from the group consisting of 0.1% and As: 0.01 to 0.1% (hereinafter also referred to as "essential segregation element"). These essential segregation elements lower the transformation point due to segregation to austenite grain boundaries, and the steel used in the present invention is required to contain at least one of them in the above content. The reasons for limiting the content of the essential segregation element will be described below.

If C is contained in an amount of 0.01% or more, the transformation point is lowered due to segregation to austenite grain boundaries. Further, when C exceeds 0.65%, the toughness of steel may be significantly reduced. Therefore, the content of C is 0.01 to 0.65%.

When S is contained in an amount of 0.01% or more, the transformation point is lowered due to segregation to austenite grain boundaries. Further, if S is contained in excess of 0.1%, the toughness of steel may be significantly reduced. Therefore, the content of S is 0.01 to 0.1%.

When P is contained by 0.01% or more, the transformation point is lowered due to segregation to the austenite grain boundaries. Further, if P is contained in excess of 0.1%, it may cause embrittlement at the grain boundary. Therefore, the content of P is 0.
It is from 01 to 0.1%.

When Ni is contained in an amount of 0.5% or more, the transformation point is lowered due to segregation to austenite grain boundaries. Further, when Ni exceeds 3.0%, the balance between strength and ductility may be deteriorated. Therefore, N
The content of i is 0.5 to 3.0%.

When Cr is contained in an amount of 0.5% or more, the transformation point is lowered due to segregation to austenite grain boundaries. Further, if Cr is contained in excess of 3.0%, martensitic transformation is likely to occur. Therefore, the content of Cr is 0.5 to 3.0%.

When B is contained in an amount of 0.0005% or more, the transformation point is lowered due to segregation to austenite grain boundaries.
Further, when B is contained in an amount of more than 0.02%, a boron compound may be formed to reduce the toughness. Therefore, the content of B is 0.0005 to 0.02%.

If Sn is contained in an amount of 0.01% or more, the transformation point is lowered due to segregation to the austenite grain boundaries. Further, if Sn is contained in excess of 0.1%, it may cause grain boundary embrittlement. Therefore, the Sn content is
It is 0.01 to 0.1%.

When As is contained by 0.01% or more, the transformation point is lowered due to segregation to the austenite grain boundaries. If As is contained in excess of 0.1%, it may cause embrittlement at the grain boundaries. Therefore, the content of As is
It is 0.01 to 0.1%.

When the steel used contains two or more kinds of essential segregation elements, at least one kind of essential segregation element may satisfy the above content, but other essential segregation elements may be contained in the above content. If it is contained in a larger amount, the above-mentioned problems may occur. Therefore, it is preferable that the contents of all the essential segregation elements are equal to or less than the respective upper limits described above.

The steel used in the present invention is not particularly limited with respect to other elements as long as it contains the above-mentioned essential segregation element in the above content and the balance is Fe and inevitable impurities.

The steel used in the present invention further includes the following A to
It is one of the preferable embodiments of the present invention to contain at least one element selected from the group F. A group: Si: 2.0% or less and V: 0.1% or less B group: Mn: 2.0% or less and Mo: 3% or less C group: Ti: 0.1% or less and Nb: 0.1 % Or less D group: Al: 0.10% or less E group: Ca: 0.01% or less F group: Rare earth metal element (REM): 0.02% or less

Since Si improves the strength of the ferrite phase by solid solution strengthening, it may be contained in the steel.
If the content is more than 2.0%, the weldability may be deteriorated. Therefore, when the steel obtained by the present invention is used for the purpose of welding, the Si content is
It is preferably 2.0% or less.

V contributes to the improvement of strength by precipitating V (C, N). However, if V is contained in excess of 0.1%, the cleanliness of steel may be deteriorated. Therefore, the V content is preferably 0.1% or less.
From the viewpoint of improving strength, the V content is 0.00
It is preferably at least 5%.

Mn improves the hardenability and lowers the transformation point of steel, so it may be contained, but if it is contained in excess of 2.0%, martensitic transformation is likely to occur. . Therefore, the Mn content is preferably 2.0% or less.

Mo has the effect of lowering the transformation point. To obtain this effect, the content is preferably 0.01% or more. When Mo is contained in excess of 3%,
Martensite transformation is likely to occur. Therefore,
The Mo content is preferably 3% or less.

Ti forms Ti (C, N) to suppress coarsening of austenite grains during austenite heating,
Improves toughness, but if it is contained in excess of 0.1%,
On the contrary, it may reduce toughness. Therefore, T
The content of i is preferably 0.1% or less. From the viewpoint of improving toughness, the Ti content is 0.00
It is preferably at least 5%.

Nb forms Nb (C, N) to suppress austenite grain growth and improve toughness.
If it is contained in excess of 100%, the effect is saturated and the cost is increased. Therefore, the Nb content is preferably 0.1% or less. When Nb is contained in order to exert the above effects, the Nb content is 0.00
It is preferably at least 5%.

Al may be contained in order to exert a strong deoxidizing action and improve the cleanliness of the steel, but it may be contained in an amount of 0.
If the content exceeds 10%, the cleanliness of steel may be rather deteriorated. Therefore, the Al content is 0.
It is preferably 10% or less.

By incorporating Ca, the precipitation of sulfide can be controlled. However, Ca is 0.01%
If it is contained in excess of 10%, nonmetallic inclusions may be formed in the steel and the properties of the steel may be deteriorated. Therefore, C
The content of a is preferably 0.01% or less. Note that the content of Ca is 0.
It is preferably 001% or more.

REM has the effect of suppressing grain growth of austenite grains and refining the austenite grains. However, if REM is contained in excess of 0.02%, the cleanliness of steel may be deteriorated. Therefore, the total content of REM is preferably 0.02% or less.
In order to refine the austenite grains, the total REM content is preferably 0.001% or more.

In the first step of the method for producing steel of the present invention, the steel having the above composition is added to at least one element of the essential segregation elements (when Cu is added in combination, at least the essential segregation elements and Cu are added). For one element, preferably both of at least one element of the essential segregation element and Cu), a heating temperature T and a holding time t satisfying the following expression (1) are applied to the following formula by heating and holding in the austenite region. In the process of controlling the grain boundary segregation so that the ratio of the average segregation concentration (mass%) in the austenite grain boundaries of the element satisfying (1) to the average concentration (mass%) in the grain is 1.2 or more. is there.

[0042]

[Equation 3]

However, in the formula (1), P: heat treatment parameter
(= Average diffusion distance (m) during heat treatment), x: the above
Group of elements (When Cu is added in combination,
And an element selected from Cu), D_x: Expansion of element x
Dispersion coefficient (m²/ S), Q_x: Activation energy of element x
(J / mol), T: heating temperature (° C.), t: holding time
(S), R: gas constant (= 8.314J / (K · mo
l)). Here, the heating temperature T is austenite.
The temperature of the region. Also, D_xAnd Q _xPart of Table 1
Shown in.

[0044]

[Table 1]

The present inventor experimentally investigated the relationship between the heating temperature and the holding time when segregation of the essential segregation element to the austenite grain boundaries was promoted and the transformation point was lowered, and at least one of the essential segregation elements was investigated. It was found that when the heating temperature T and the holding time t satisfying the above formula (1) are held in the austenite region by heating and holding, the transformation point is remarkably lowered. Then, the ferrite grain size is remarkably refined by working in the supercooled austenite region caused by the reduction of the transformation point. On the other hand, if the heating temperature is too low or the holding time is too short, the transformation point does not decrease remarkably even when heated and held in the austenite region.

The above formula (1) is obtained experimentally by measuring the transformation point of the steel when the heating temperature and the holding time are variously changed to change the heat treatment parameter (P value). Is. FIG. 1 is a graph showing an example of the relationship between the P value and the transformation point (T _s ) when the essential segregation element and Cu are added individually. As is clear from FIG.
When the P value for the essential segregation element satisfies the above formula (1), the transformation point remarkably decreases, as in the case of Cu.

The average segregation concentration (mass%) at the austenite grain boundaries of the elements satisfying the above formula (1) and the average concentration within the grains ( Controlling the grain boundary segregation so that the ratio of (mass%) is 1.2 or more is one of the features of the present invention. FIG. 2 shows the average segregation concentration (mass%) at the austenite grain boundaries and the average concentration (mass%) within the grain of the element satisfying the above formula (1) measured by FE-TEM (beam diameter 2 nm). the ratio is a graph showing an example of the relationship between the transformation point (T _s). As is clear from FIG. 2, when the ratio is 1.2 or more, the transformation point is significantly lowered. In addition,
In controlling the grain boundary segregation of the elements satisfying the above expression (1) to a predetermined amount, the effect of the heating temperature and the holding time during heating and holding on the concentration ratio of the grain boundary segregation is shown in a table for each element. It is convenient to keep it. If the holding time during heating and holding is too long, the once segregated element may form a solid solution and diffuse, and the transformation point may not be sufficiently lowered. Therefore, in the present invention, it is necessary to control the grain boundary segregation within the range where the ratio is 1.2 or more so that the holding time does not become too long. Preferably, the ratio is 1.5 or more.

The second step of the method for producing steel of the present invention is a step of cooling to a supercooled austenite region. In the present invention, the “supercooled austenite region” means a temperature T ₀ below which the chemical free energy of the austenite phase and the chemical free energy of the ferrite phase, which are thermodynamically calculated from the chemical composition of the steel, become equal. It means a region existing as an austenite state. The cooling rate is not particularly limited, but it is preferably 0.1 to 20 ° C./s.

The reason why the material is cooled to the supercooled austenite region after the heating and holding is that it is necessary to process the supercooled austenite in the hot working step described later. In hot working in the austenite region that is not undercooled, the dynamic transformation from austenite to ferrite (dynamic γ → α transformation) described below does not occur, and since the austenite is work hardened, the deformation resistance is extremely high. Get higher In hot working when ferrite appears before working, dynamic γ →
The α transformation does not occur, and the strain accumulated in the ferrite causes an extremely low toughness.

The third step of the steel manufacturing method of the present invention is a step of hot working to induce dynamic transformation into ferrite and dynamic recrystallization of ferrite. After being cooled to the supercooled austenite region, hot working causes a mechanical force to be applied, whereby a dynamic transformation from austenite to ferrite occurs. After that, if you continue processing,
Since mechanical force is continuously applied, dynamic recrystallization of the generated ferrite occurs and the crystal grains are refined.

The average ferrite grain size of the obtained crystal grains is
It can be adjusted by the working rate in hot working.
Even if the processing rate is small, the dynamic transformation to ferrite and the dynamic recrystallization of ferrite occur, but if the processing rate is increased, the dynamic recrystallization of ferrite is more likely to occur and the crystal grains are finer. Becomes For example, in order to obtain a fine-grained steel having an average ferrite grain size of 10 μm or less, it is necessary to actively use dynamic recrystallization of ferrite. Therefore, the working rate in hot working is preferably 30% or more, and 50 % Or more is more preferable. In the present invention, the "working rate" refers to the rate of reduction of the cross-sectional area of the work piece due to working. Specifically, in the case of plate rolling, the working rate means a reduction rate, and in the case of working a round bar or the like, it means a surface reduction rate.

Further, in the present invention, if the total working ratio in hot working is the same, the average ferrite grain sizes obtained are almost the same. For example, when the hot working is hot rolling, the obtained average ferrite grain size is almost the same whether the rolling is performed in one pass or in multiple passes.

The hot working has an advantage of low deformation resistance. This is because the strain is released by the large amount of soft ferrite generated by dynamic transformation before the hard austenite is significantly work hardened, and the dynamically transformed ferrite undergoes processing to undergo dynamic recrystallization. It is considered that the deformation resistance does not increase due to the work hardening of the ferrite, and the deformation resistance is kept low.

The steel obtained by the manufacturing method of the present invention is preferably composed mainly of ferrite with a ferrite fraction of 50% or more. The ferrite-based steel is not limited to a steel having a ferrite structure in its entirety, and may have a second phase such as pearlite or martensite as long as the ferrite fraction (ferrite area ratio) is 50% or more. .

When the present invention is applied to the production of a hot forged part, that is, at a heating temperature T and a holding time t which satisfy the above formula (1) in the austenite region for at least one essential segregation element, By heating and holding, the grain boundary segregation is controlled so that the ratio of the average segregation concentration in the austenite grain boundary of the element satisfying the above formula (1) to the average concentration in the grain is 1.2 or more, and thereafter, By cooling to the supercooled austenite region and performing hot forging in this supercooled austenite region to produce hot forged parts, it is possible to perform hot forging with low deformation resistance, and it is also possible to obtain a fine grain structure. Become. In addition, in order to obtain a fine grain structure having an average ferrite grain size of 10 μm or less, it is preferable that the processing rate is 30% or more. The hot forged part thus obtained has an advantage that it has excellent fatigue strength characteristics because it has a small ferrite grain size, and that it can be used as a part with no heat treatment.

The ferrite-based steel obtained by the production method of the present invention has an advantage that it is excellent in strength and toughness because the ferrite structure is fine. The use of the ferritic steel is not particularly limited, but from the above advantages,
As a structural material, it is preferably used in various fields of steel wire rods, steel bars, shaped steels, thin steel plates, thick steel plates, steel pipes, and mechanical structural parts formed into parts by hot forging.

[0057]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto. (Example 1) Steels 1 to 20 having the compositions shown in Table 2 were melted,
Rolled to a width of 30 mm x length of 60 mm x thickness of 100 mm,
It was used as a test material. Steel 1 is the steel used in the comparative example, steels 2 to 10 and steels 12 to 20 are the steels used in the invention examples, and steel 11 is the steel used in the reference example.

[0058]

[Table 2]

[0059]

[Table 3]

This test material was heated and held in the austenite region at the heating temperature and holding time shown in Table 3 and then cooled to the supercooled austenite region at a cooling rate of 2 ° C./s. Hot rolling was performed at the processing temperature and processing rate shown.

In Table 3, T _{0 of} steels 1 to 20, P value during heating and holding, segregated elements, average segregation concentration (mass%) at austenite grain boundaries of elements segregated at austenite grain boundaries, and intragrain Ratio of average concentration (mass%) at, transformation point in the absence of processing, average ferrite grain size of the obtained steel,
Deformation resistance in hot rolling is shown.

T _{0 of} was calculated from the composition.
For the P value of, using the heating temperature T and the holding time t shown in Table 3, and the D _x and Q _x shown in Table 1,
It was calculated from the above formula (1). The ratio of the segregated elements and and their average grain boundary segregation concentration to the intragranular average concentration was calculated from the concentration at five points measured at the grain boundaries and within the grains by FE-TEM with a beam diameter of 2 nm. The average ferrite grain size of the obtained steel was obtained by multiplying the average intercept length measured by the intercept method by 1.128. The deformation resistance in the hot rolling was calculated from the rolling conditions and the rolling load during the hot rolling.

In Table 3, Invention Examples 1 to 18
Is a steel containing an essential segregation element, the P value satisfies the above formula (1), and the ratio of the grain boundary average segregation concentration to the intragranular average concentration of the segregated element is 1.2 or more. That the undercooling transformation point is lowered due to the occurrence of supercooled austenite, and that the deformation resistance is small, and the dynamic transformation into ferrite and the dynamic recrystallization of ferrite occur obviously. Therefore, it corresponds to the steel manufacturing method of the present invention. On the other hand, Comparative Example 1 uses steel that does not substantially contain the essential segregation element. Further, Reference Example 1 uses a steel which does not substantially contain the essential segregation element and which contains Cu alone.

Further, for steels 1 to 11 in Table 3, a graph showing the relationship between the workability of the obtained steel and the average ferrite grain size, the workability of the obtained steel and the deformation resistance in hot rolling. And a graph showing the relationship with are prepared and shown in FIGS. 3 and 4.

From Table 3 and FIG. 3, it is understood that according to the steel manufacturing method of the present invention (Invention Examples 1 to 9), ferrite-based steel having a small ferrite grain size can be obtained. Further, when the working rate in hot working is 30% or more, the structure is made finer and the ferrite grain size can be 10 μm or less. Further, when the working rate is high, the ferrite grain size is close to 1 μm. You can On the other hand, when the steel containing no essential segregation element is used (Comparative Example 1), the ferrite grain size is as coarse as about 60 μm. The same is true even if the processing rate is increased.

Further, it can be seen from Table 3 and FIG. 4 that the deformation resistance in hot working is low in the steel manufacturing method of the present invention (Invention Examples 1 to 9). On the other hand, when the steel containing no essential segregation element is used (Comparative Example 1), the deformation resistance in hot working is high. Inventive Examples 10 to 2
Regarding 0, the illustration of the relationship between the processing rate of steel and the average ferrite grain size and the relationship between the processing rate of steel and the deformation resistance in hot rolling was omitted, but from Table 3, invention examples 1 to It can be seen that the same average ferrite grain size and deformation resistance as those of No. 9 are obtained. Also, comparing each invention example with reference example 1, it can be seen that each invention example has the same deformation resistance and average ferrite grain size as reference example 1.

[0067]

[Table 4]

[0068]

[Table 5]

(Example 2) Steels 21 to 21 having the compositions shown in Table 4
Welded 29, width 150mm x length 400mm x thickness 1
After rolling to 50 mm, homogenization annealing was performed at 1250 ° C. Then, a test piece for hot forging having a diameter of 80 mm and a height of 40 mm was cut out and hot forged. Hot forging is No. 4
At the heating temperature and holding time shown in the table, the material is heated and held in the austenite region and then up to the temperature just above the transformation point of each steel.
Cooling was carried out at a cooling rate of ° C / s, and hot forging was carried out at a processing temperature and a processing rate shown in Table 5 using a 300t press machine.
The average ferrite grain size, tensile strength TS and fatigue strength of the steel material after hot forging were measured.

[0070]

[Table 6]

In Table 5, T _{0 of} steels 21 to 29, P value at the time of heating and holding, average segregation concentration (mass%) at the austenite grain boundary of the segregated element, and the element segregated at the austenite grain boundary and the intragranular content The ratio of average concentration (mass%), the transformation point in the absence of processing, the average ferrite grain size of the obtained steel, the deformation resistance during hot forging, the tensile strength TS, the fatigue strength, and the durability ratio are shown.

The T ₀ , P value, segregated element, ratio of average segregation concentration at grain boundaries and average concentration within grains, and average ferrite grain size are
It was determined by the same method as that described in Example 1. The deformation resistance during hot forging was calculated by measuring the load of the load cell during processing. The tensile strength TS was measured by taking a JIS No. 14A tensile test piece so as to include the center part of the hot forged material and performing a tensile test. Regarding the fatigue strength, a JIS No. 1 rotary bending test piece was sampled from the center of the hot forged material, and a fatigue test was conducted using an Ono type rotary bending tester to obtain a 10 ⁷ fatigue limit, which was taken as the fatigue strength. Durability ratio is the ratio of fatigue strength to tensile strength.

The deformation resistance remains high in Comparative Example 3 using the steel containing substantially no essential segregation element and Comparative Example 4 in which the P value does not satisfy the above expression (1). In contrast,
Inventive Examples 19 to 26 have achieved low deformation resistance.
Further, in Comparative Examples 3 and 4, the average ferrite grain size decreases due to the increase in the processing rate, but the average ferrite grain size remains coarse even if the processing rate is 70%. On the other hand, in Invention Examples 19 to 26,
By increasing the working rate from 20% to 30%, the average ferrite grain size is significantly reduced, and the grain size is reduced to 10 μm or less. When the processing rate is further increased, the average ferrite grain size can be reduced to about 1 μm. Further, the durability ratio is about 0.5 in Comparative Examples 3 and 4, but in Invention Examples 19 to 26, even if the processing rate is less than 30%, it is up to about 0.55, and if the processing rate is 30% or more, it is about 0.60. Can be significantly improved.

[0074]

[Table 7]

[0075]

[Table 8]

[0076]

As described above, according to the method for producing steel of the present invention, fine ferrite can be obtained as compared with the conventional controlled rolling-controlled cooling method, and the steel is processed with a low deformation resistance. It is extremely useful because it can be performed easily. Further, in the steel manufacturing method of the present invention, since steel containing no Cu can be used, it is used for manufacturing steel used for a wide range of applications. Further, since the ferrite-based steel obtained by the present invention has a fine ferrite structure, it has an excellent strength-toughness balance and is extremely useful. Furthermore, if the method for producing steel of the present invention is used when producing a hot forged part, the deformation resistance of the steel during forging is small, so the forging load can be suppressed small, and further, the forged part with excellent fatigue strength characteristics. Can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing an example of a relationship between a P value and a transformation point when an essential segregation element and Cu are added individually.

FIG. 2 shows an average segregation concentration (mass%) at an austenite grain boundary and an average concentration (mass%) within a grain of an element satisfying the above formula (1) measured by FE-TEM (beam diameter 2 nm). It is a graph showing an example of a relation between a ratio and a transformation point.

FIG. 3 is a graph showing the relationship between the workability and the average ferrite grain size of the steels obtained in the examples.

FIG. 4 is a graph showing the relationship between the workability of the steels obtained in the examples and the deformation resistance in hot rolling.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Akihiro Matsuzaki             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Toshiyuki Hoshino             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Shinichi Amano             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works F-term (reference) 4K032 AA01 AA02 AA04 AA05 AA06                       AA08 AA11 AA12 AA16 AA19                       AA20 AA22 AA23 AA24 AA27                       AA29 AA31 AA32 AA35 AA36                       AA40 BA01 BA02 BA03 CA01                       CA02 CB01 CB02

Claims (4)

[Claims] 1. A mass ratio of C: 0.01 to 0.65%,
S: 0.01 to 0.1%, P: 0.01 to 0.1%, N
i: 0.5 to 3.0%, Cr: 0.5 to 3.0%, B:
0.0005-0.02%, Sn: 0.01-0.1
%, And As: at least one element selected from the group consisting of 0.01 to 0.1%, and the balance consisting of Fe and inevitable impurities. In the austenite region, the average segregation concentration (mass%) at the austenite grain boundary of the elements satisfying the following formula (1) is maintained by heating at a heating temperature T and a holding time t that satisfy the following formula (1), and Of controlling the grain boundary segregation so that the ratio of the average concentration (mass%) at 1.2 is 1.2 or more, then cooling to the supercooled austenite region, and then hot working into ferrite. And a step of inducing dynamic recrystallization of ferrite. [Equation 1] However, in the formula (1), P: heat treatment parameter (= average diffusion distance (m) during heat treatment), x: one element selected from the group of the above elements, D _x : diffusion coefficient of element x (m ² /
s), Q _x : activation energy of element x (J / mo
l), T: heating temperature (° C.), t: holding time (s), R:
It is a gas constant (= 8.314 J / (K · mol)). 2. The method for producing steel according to claim 1, wherein the steel further contains at least one element selected from the following groups A to F. A group: Si: 2.0% or less and V: 0.1% or less B group: Mn: 2.0% or less and Mo: 3% or less C group: Ti: 0.1% or less and Nb: 0.1 % Or less D group: Al: 0.10% or less E group: Ca: 0.01% or less F group: Rare earth metal element (REM): 0.02% or less 3. The method for producing steel according to claim 1, wherein the working rate in the hot working is 30% or more. 4. The method for producing steel according to claim 1, wherein the hot working is hot forging for producing a hot forged part.