JP2001123224A - Method for producing high strength and high toughness non-heattreated parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing high strength and high toughness non-heattreated parts having proof stress of >=550 MPa, yiel ratio of >=0.85 and impact value of >=100 J/cm2 in a Charpy impact test at room temperature using a JIS No. 3 Charpy impact test piece. SOLUTION: Steel having a chemical composition containing 0.02 to 0.10% C, <=0.5% Si, 1.0 to 3.0% Mn, <=0.04% P, <=0.40% S, <=2.0% Cu, <=2.0% Ni, <=2.0% Cr, <=1.0% Mo, <=0.1% Nb, <=0.05% Ti, <=0.1% Nd, <=0.005% B, <=0.1% Al and <=0.01% N, and the balance Fe with impurities, in which 80 to 97% of the structure are composed of ferrite and bainitic ferrite, and also, 50% or more of the structure other than the ferrite and bainitic ferrite are composed of pseudo martensite is subjected to cold working.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a high-strength, high-toughness non-refined part. Specifically, it has a 0.2% proof stress (or yield strength) of 550 MPa or more, a yield ratio of 0.85 or more (“0.2% proof strength / tensile strength” or “yield strength / tensile strength”), and JIS No. 3 The present invention relates to a method for producing a high-strength, high-toughness non-heat treated component having an impact value of 100 J / cm ² or more in a Charpy impact test at room temperature using a Charpy impact test specimen.

[0002]

2. Description of the Related Art Machine structural parts such as automobiles and various industrial machines, especially power transmission parts and important safety parts, require high strength and excellent toughness. For this reason, the above components have conventionally been generally manufactured by quenching and tempering steel made of steel for machine structural use. However, recently, with emphasis on energy saving and global environmental issues, there has been an increasing movement to omit or simplify the quenching and tempering steps.

[0003] In response to the above-mentioned movement, alloying elements such as V and Ti are added to steel, and quenching and tempering are performed by utilizing the precipitation hardening effect of the steel, even after being left to cool after hot forging (hot working). Non-heat treated steel for hot forging that can secure tensile strength close to the so-called "heat treated steel" has been developed.However, the toughness of this non-heat treated steel is low, and the toughness required for the above-mentioned various components is reduced. It was not enough to secure.

[0004] Furthermore, the non-heat treated steel has a higher yield ratio ("0.2% proof stress / tensile strength" or "yield strength / tensile strength") than the heat treated steel. Therefore, in the case of the non-heat treated steel for hot forging, there is also a problem that it is difficult to cut into a high-strength final part shape and the productivity is low.

That is, generally, it is determined whether or not a mechanical component is permanently deformed in response to an external load. Therefore, when high-strength parts are required, it is necessary to increase the proof stress, and when trying to increase the proof stress of the non-heat treated steel, the tensile strength becomes too high because the yield ratio is low. . Since hot forging is a process for performing rough forming of parts, it is necessary to perform cutting to obtain the final product shape, and the cutting resistance during this cutting is increased due to the increase in tensile strength. Therefore, the non-heat treated steel for hot forging has a large deterioration in machinability and a drop in productivity.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to be suitable as a machine structural component such as an automobile or various industrial machines, especially a power transmission system component or an important security component. , A proof stress of 550 MPa or more, a yield ratio of 0.85 or more, and 100 J / c in a Charpy impact test at room temperature using a JIS No. 3 Charpy impact test piece (hereinafter, also simply referred to as a Charpy impact test).
An object of the present invention is to provide a method for producing a high-strength, high-toughness non-heat-treated part having an impact value of m ² or more.

[0007]

The gist of the present invention resides in a method for producing a high-strength, high-toughness non-heat-treated part as shown in the following (1) and (2).

(1) In mass%, C: 0.02 to 0.10
%, Si: 0.5% or less, Mn: 1.0 to 3.0%,
P: 0.04% or less, S: 0.40% or less, Cu: 2.
0% or less, Ni: 2.0% or less, Cr: 2.0% or less,
Mo: 1.0% or less, Nb: 0.1% or less, Ti: 0.
It contains 0.05% or less, Nd: 0.1% or less, B: 0.005% or less, Al: 0.1% or less, N: 0.01% or less, and the balance consists of a chemical composition of Fe and impurities. 80 to 97% of the structure is a mixed structure of ferrite and bainitic ferrite, and 50% or more of the structure other than ferrite and bainitic ferrite is subjected to cold working to a steel material which is pseudo martensite. Of high strength, high toughness non-heat treated parts.

(2) A steel material having the chemical composition and structure described in (1) above is subjected to cold working, and then is aged at a temperature of 600 ° C. or less. Quality parts manufacturing method.

Here, "bainitic ferrite" refers to a so-called "BI type bainite".

Incidentally, "bainite" is originally a mixed structure of "ferrite" and "cementite particles", and is usually divided into "upper bainite" and "lower bainite". The bainite morphology classification method includes a method of classifying according to the morphology of ferrite and a method of classifying carbide (cementite).
There are two methods of classifying according to the form of precipitation of ferrite. In the case of classifying in the form of ferrite, lath-like ferrite is bundled from austenite grain boundaries to form “upper bainite”, which is plate-like ferrite and its inner part. What consists of fine plate-like cementite is "lower bainite". On the other hand, when bainite is classified according to the precipitation form of carbide (cementite), the case where cementite is formed at the interface of lath-like ferrite is “upper bainite”, and the case where cementite is finely precipitated in ferrite grains is “lower bainite”. ".

The above-mentioned "upper bainite" is based on the form of ferrite and further takes into account the form of precipitation of cementite, as shown in FIG.
"B-II type bainite" and "B-III type bainite"
May be finely classified. In other words, when lath-like ferrite 3 is formed in a relatively high temperature range without precipitation of cementite 2 from austenite grain boundary 1, "B
A typical upper bainite, referred to as "-I-type bainite" (FIG. 1 (a)) and having a slightly lowered transformation temperature, with precipitation of cementite 2 at the interface of lath-like ferrite 3, is called "B-II".
Type bainite "(FIG. 1 (b)). The B-III type bainite (FIG. 1 (c)) is formed in a region where the temperature is further lowered, and in which extremely fine lath-like ferrite 3 in which particles of cementite 2 aligned in one direction are dispersed.
It is called. As described above, the “bainitic ferrite” in the present invention refers to this “BI type bainite”.
Point to.

[0013] Pseudo martensite means that when austenite is transformed into ferrite and bainitic ferrite, C is concentrated into untransformed austenite, and in the subsequent cooling process, the untransformed C in which the above-mentioned C is concentrated. Austenite refers to martensite and untransformed austenite. The pseudo martensite includes not only the above-described mixed structure of martensite and untransformed austenite, but also the structure of a martensite single phase or an untransformed austenite single phase.

The ratio of the structure means the ratio of the structure when observed under a microscope, that is, the area ratio. The area ratio of the mixed structure of ferrite and bainitic ferrite means the area ratio of the ferrite and bainitic ferrite in the structure. Refers to the sum of the rates. The “mixed structure of ferrite and bainitic ferrite” in the present invention refers to a structure in which ferrite and bainitic ferrite coexist. The structure other than ferrite, bainitic ferrite and pseudo martensite may be any structure such as pearlite and bainite containing C in supersaturation.

[0015] The non-heat treated means that no so-called "heat treatment" such as quenching and tempering is performed. Therefore, the non-heat treated part refers to a part which is not subjected to both the quenching and the tempering.

As described above, 0.2% proof stress and yield strength are collectively referred to as "proof stress" in this specification.

Hereinafter, those described in the above (1) and (2) are referred to as the invention of (1) and the invention of (2), respectively.

In order to solve the above-mentioned problems, the inventors of the present invention have proof stress of 550 MPa or more, yield ratio of 0.85 or more, and 100 J / c in Charpy impact test.
As a result of conducting various studies in order to provide a method for producing a high-strength, high-toughness non-heat treated component having an impact value of m ² or more, the following findings were obtained.

(A) Cold drawing to a steel material whose chemical composition is a specific low carbon-Mn system and in which the proportion of ferrite, bainitic ferrite and pseudo martensite in the structure is a specific value. By performing cold working such as cold drawing or cold extrusion, the yield strength and the yield ratio can be increased. In this case, if the area reduction rate of the cold working is set to 10% or more, the yield strength and the yield ratio can be stably increased.

The “area reduction rate” is defined as {(cross-sectional area of steel before cold working) − (cross-sectional area of steel after cold working)} /
(Cross-sectional area of steel material before cold working).

(B) After aging at 300 to 600 ° C. after cold working, the yield strength is increased, and as a result, the yield ratio can be further increased.

(C) The steel has an appropriate amount of Cu, Ni, Mo, N
When b, Ti and B are contained, the above-mentioned effect is further enhanced.

The present invention has been completed based on the above findings.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by mass".

(A) Chemical composition C: 0.02 to 0.10% C has the effect of increasing the strength. However, if the content is less than 0.02%, it is difficult to secure a desired yield strength of 550 MPa or more. On the other hand, when it exceeds 0.10%, toughness is deteriorated. Therefore, the content of C is set to 0.0
2 to 0.10%. Note that a desirable range of the C content is 0.04 to 0.08%.

Si: 0.5% or less Si need not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, the content of Si is preferably set to 0.1% or more. But,
If it is contained in a large amount, the toughness of the ferrite ground is deteriorated, so that the cold workability is reduced. In particular, the content is 0.1.
If it exceeds 5%, the cold workability is significantly reduced. Therefore, the content of Si is set to 0.5% or less. In addition, in order to obtain sufficient cold workability, the content of Si is set to 0.3%.
It is preferable to set the following.

Mn: 1.0 to 3.0% Mn is an inexpensive hardenability improving element, which suppresses the formation of pearlite during hot working and has the effect of forming pseudo martensite and bainitic ferrite. Having. To obtain this effect, the Mn content needs to be 1.0% or more. However, when the content exceeds 3.0%, segregation becomes remarkable, and workability, especially cold workability, is deteriorated. Therefore, the content of Mn is set to 1.0 to 3.0%. In addition, the desirable range of Mn content is 1.5-2.
5%.

P: 0.04% or less Since P deteriorates toughness, its content is preferably made as low as possible. However, in order to reduce the content of P, secondary refining is required, and economic In some cases, this may be a problem from the viewpoint of performance, so the amount may be determined according to the performance and cost required for the component. However, if the content of P exceeds 0.04%, the toughness is significantly reduced.
Therefore, the content of P is set to 0.04% or less. In order to obtain good toughness (Charpy impact properties), the content of P is preferably set to 0.025% or less.

S: 0.40% or less S may not be added. If added, it has the effect of forming MnS in the steel to enhance machinability. To ensure this effect, the content of S is preferably set to 0.01% or more. However, when the S content exceeds 0.40%, MnS elongates during cold working such as cold drawing, cold drawing or cold extrusion to cause longitudinal cracks. Therefore, the content of S is set to 0.40% or less. In order to obtain good cold workability, the content of S is preferably set to 0.10% or less.

Cu: 2.0% or less Cu need not be added. If added, it has the effect of improving the quenchability and suppressing the formation of pearlite during hot working, and the effect of increasing the strength after aging by precipitation by aging treatment. In order to surely obtain such an effect, the content of Cu is preferably set to 0.5% or more. However, if Cu
Even if the content exceeds 0%, the above effect is saturated and the cost is increased. Therefore, the content of Cu is set to 2.
0% or less. In addition, when adding Cu, it is preferable to add together with Ni described below.

Ni: 2.0% or less Ni may not be added. If added, it has the effect of improving the hardenability and suppressing the formation of pearlite during hot working. To ensure this effect, the content of Ni is preferably 0.5% or more. However, Ni was added to 2.
Even if the content exceeds 0%, the above effect is saturated and the cost is increased. Therefore, the content of Ni is set to 2.
0% or less. In addition, when adding the above-mentioned Cu, it is preferred to add Ni simultaneously.

Cr: 2.0% or less Cr need not be added. If added, there is an effect of suppressing the generation of pearlite during hot working. To ensure this effect, the content of Cr is preferably set to 0.2% or more. However, even if Cr is contained in an amount exceeding 2.0%, the above effect is saturated, and the cost is increased.
Therefore, the content of Cr is set to 2.0% or less. In order to suppress the product cost, it is desirable that the Cr content be 1.0% or less.

Mo: 1.0% or less Mo may not be added. If added, it has the effect of improving the pearlite hardenability and suppressing the formation of pearlite during hot working, and the effect of forming carbides by aging to increase the strength after aging. To ensure this effect, M
It is preferable that the content of o is 0.2% or more. However, even if Mo is contained in an amount exceeding 1.0%, the above-described effect is saturated and the cost is increased. Therefore, Mo
Was made 1.0% or less. In order to suppress the product cost, the content of Mo is desirably 0.6% or less.

Nb: 0.1% or less Nb may not be added. If added, the austenite grain size is made finer, the structure after hot working is made finer, and there is an effect of increasing the toughness. Further, it has an effect of forming carbides by aging treatment to increase the strength after aging. In order to reliably obtain these effects, it is preferable that the content of Nb is 0.01% or more. However, even if Nb is contained in an amount exceeding 0.1%, the above effect is saturated and the cost is increased. Therefore, the content of Nb is set to 0.1% or less.
In order to reduce product cost, the content of Nb should be 0.06
% Is desirable.

Ti: 0.05% or less Ti may not be added. If added, the austenite grain size is made finer, the structure after hot working is made finer, and there is an effect of increasing the toughness. Further, it has an effect of forming carbides by aging treatment to increase the strength after aging. In order to ensure these effects, it is preferable that the content of Ti is 0.01% or more. However, even if the content of Ti exceeds 0.05%, the above effect is saturated, and the cost is increased. Therefore, the content of Ti is set to 0.05% or less. In order to suppress the product cost, the content of Ti should be reduced to 0.
It is desirable to set it to 03% or less.

Nd: 0.1% or less Nd may not be added. If added, it has the effect of making MnS spherical and increasing cold workability such as cold drawability and machinability. To ensure this effect, Nd is 0.01
% Is preferable. However, even if Nd is contained in an amount exceeding 0.1%, the above effect is saturated and the cost is increased. Therefore, the content of Nd is set to 0.1% or less. In order to reduce product cost, N
It is desirable that the content of d be 0.05% or less.

B: 0.005% or less B may not be added. If added, there is an effect of suppressing the generation of pearlite during hot working. In order to surely obtain this effect, the content of B is preferably 0.0005% or more. However, even if B is contained in an amount exceeding 0.005%, the above-mentioned effect is saturated and the cost is increased. Therefore, the content of B is set to 0.005% or less. In order to reduce product cost, the content of B should be 0.0
It is desirably 025% or less.

Al: 0.1% or less Al may not be added. If added, it has the effect of deoxidizing the steel. To ensure this effect, Al should be 0.0
The content is preferably 1% or more. However, Al
If the content exceeds 0.1%, cold workability such as cold drawability decreases. Therefore, the content of Al is 0.1
% Or less. In order to stably secure good cold workability, the Al content is preferably set to 0.05% or less.

N: 0.01% or less N degrades cold workability, so its content is desirably kept as low as possible. In particular, when the content exceeds 0.01%, the cold workability significantly decreases.
Therefore, the content of N is set to 0.01% or less. In order to ensure good cold workability stably, N
Is preferably 0.006% or less.

(B) Structure To ensure a proof stress of 550 MPa or more, a yield ratio of 0.85 or more, and an impact value of 100 J / cm ² or more in a Charpy impact test, the chemical composition described in the above item (A) must be used. 80-97% of the structure of the non-heat treated part has a mixed structure of ferrite and bainitic ferrite, and
5 of structure other than ferrite and bainitic ferrite
It is necessary to make 0% or more pseudo martensite.

If the area ratio of the mixed structure of ferrite and bainitic ferrite in the structure is less than 80%, the toughness is reduced, and a desired impact value cannot be obtained. On the other hand, the area ratio of the mixed structure of ferrite and bainitic ferrite was 9%.
If it exceeds 7%, it is difficult to secure strength by cold working.

When pseudo martensite is less than 50% of the structure other than ferrite and bainitic ferrite, the ratio of pearlite and bainite containing supersaturated C to the structure is large, and the toughness is reduced. Therefore, a desired impact value cannot be obtained.

Therefore, the microstructure of the ferrite and bainitic ferrite was 80-97%, and 50% of the microstructure other than ferrite and bainitic ferrite.
The above was defined as pseudo martensite.

The proportion of pseudo martensite in the structure other than ferrite and bainitic ferrite was 5%.
What is necessary is just 0% or more, and 100% may be pseudo martensite.

As described above, the ratio of the tissue refers to the ratio of the tissue when observed with a microscope, that is, the area ratio.
The area ratio of the mixed structure of ferrite and bainitic ferrite indicates the sum of the area ratios of ferrite and bainitic ferrite in the structure. "Mixed structure of ferrite and bainitic ferrite" refers to a structure in which ferrite and bainitic ferrite coexist, and structures other than ferrite, bainitic ferrite and pseudo martensite include pearlite and bainite containing C in supersaturation. As described above, anything may be used.

The tissue ratio can be measured, for example, by cutting out a test piece for microscopic examination, polishing it to a mirror surface, collecting an extraction replica using 5% picral as a corrosive solution, and observing it with a transmission electron microscope at a magnification of about 10,000. I just need.

As a method for producing the above specified structure, for example, in order to reduce the austenite grain size and the fine precipitation of bainitic ferrite, a steel ingot or a steel slab is subjected to 1000 to 100%. After heating to a temperature of 1250 ° C., hot rolling is performed so that the cumulative draft at 900 ° C. or less is 50% or more, and the rolling is finished at 750 ° C. or more, and then 0.5 to 10 ° C./sec. At least 500 at cooling rate
There is a process of performing so-called "controlled rolling-controlled cooling" in which the temperature is cooled down to ° C. The chemical composition of the above item (A) is designed so that a desired structure is formed by rolling and cooling a steel ingot or a steel slab under the above conditions.

(C) Cold working In order to secure a proof stress of 550 MPa or more, a yield ratio of 0.85 or more, and an impact value of 100 J / cm ² or more in a Charpy impact test, the above items (A) and (B) The steel having the chemical composition and structure described in the section needs to be subjected to cold working by cold drawing, cold drawing, cold extrusion, or the like. If the area reduction rate of this cold working is 10% or more,
Since the yield strength and the yield ratio can be easily increased, the yield strength of 550 MPa or more and the yield ratio of 0.85 or more can be stably imparted to the non-heat treated component. If the area reduction rate is further increased to 20% or more, the proof stress and the yield ratio can be increased more stably. The area reduction rate can be changed according to the strength required for the component and the specifications of the cold working equipment, and the upper limit does not need to be particularly defined. But,
When cutting is performed after cold working to finish the final component shape, the tensile strength is preferably kept low, so the upper limit of the area reduction is preferably about 80%.

The “area reduction ratio” is expressed by {(cross-sectional area of steel before cold working) − (cross-sectional area of steel after cold working)} / (cross-sectional area of steel before cold working). The point is as described above.

By satisfying the conditions of the above items (A) and (B) and the condition of this item (C),
The method for producing a high-strength, high-toughness non-refined part according to the invention of (1) is obtained.

(D) Aging If the aging treatment is performed at an appropriate temperature after the cold working, the proof stress can be increased without substantially changing the tensile strength increased by the cold working, and as a result, the yield ratio can be increased. Can be. Therefore, in order to further increase the proof stress and the yield ratio of the non-heat treated part, the steel having the chemical composition described in the above item (A) and the structure described in the above item (B) must
It is preferable to perform the cold working described in the section and then further perform the aging treatment.

However, if the temperature of the aging treatment exceeds 600 ° C., overaging occurs and the strength is reduced, so that desired tensile properties cannot be secured. For this reason, in the invention of (2),
After subjecting the steel material having the chemical composition and structure described in (A) and (B) to the cold working described in (C),
It is specified that aging is performed at a temperature of 600 ° C. or less.

If the temperature of the aging treatment performed after the cold working is lower than 300 ° C., the above-mentioned effect may not be obtained at all. Therefore, the aging treatment after the cold working is performed at 300 ° C. It is better to carry out at the above temperature.
When the temperature is 50 to 450 ° C., the yield strength and the yield ratio can be stably increased.

The aging time need not be particularly specified, but is preferably about 30 minutes to 2 hours in order to increase productivity.

It should be noted that, depending on the shape of the target high-strength, high-toughness non-heat treated part, the steel having the chemical composition and structure described in (A) and (B) may be added to the cold steel described in (C). In addition, after the working, a cutting process may be performed to obtain a desired shape, or after the cutting process, aging may be performed at a temperature of 600 ° C. or less as described in (D). Good.

Hereinafter, the present invention will be described in detail with reference to examples.

[0057]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were melted in a test vacuum furnace by a conventional method. Steels A1 to A22 in Tables 1 and 2 are examples of the present invention in which the chemical composition is within the range specified by the present invention, and steels B1, steel B2, and steel C1 have a content range in which any of the components is specified by the present invention. This is a comparative example deviating from the above. Among the steels of the comparative example, steel C1 is JIS SC
It is steel corresponding to M435.

[0058]

[Table 1]

[0059]

[Table 2]

(Example 1) Of the steel smelted as described above, a steel ingot of steel A6 shown in Table 1 was heated to 1250 ° C, and then the cumulative draft at 1050 to 800 ° C was 75%. Hot rolling is performed at 800 ° C., and then cooled to room temperature at a cooling rate of 3 ° C./sec.
m round bars were produced.

A JIS No. 14A tensile test piece and a JIS No. 3 Charpy impact test piece were cut out from the center of the thus obtained round bar having a diameter of 30 mm, and tensile properties (proof stress, tensile strength) and impact properties (impact) at room temperature were obtained. Values) were investigated.

Further, a test piece for examining the structure was cut out, mirror-polished, and observed by a transmission electron microscope by an extraction replica method to examine the structure. In this case, 5% picral was used for the corrosion.

Next, the above-mentioned round bar having a diameter of 30 mm was subjected to a peeling process to a diameter of 20 mm by a usual method, and then subjected to an acid pickling treatment and a lubrication treatment by a usual method, followed by cold drawing. Was. The area reduction rate by wire drawing was set to three conditions of 5%, 10%, and 40%. Then, J is drawn from the center of each drawn round bar.
Cut out IS14A tensile test specimen and JIS No. 3 Charpy impact test specimen, and tensile properties at room temperature (proof stress, tensile strength)
And impact characteristics (impact value) were investigated.

Tables 3 and 4 show the results of the above tests.

[0065]

[Table 3]

[0066]

[Table 4]

As can be seen from Tables 3 and 4, when a round bar having the chemical composition and structure specified in the present invention is subjected to cold drawing (cold working), the yield strength and the yield ratio can be increased. When cold working of 10% or more is performed, the proof stress and the yield ratio can be stably increased, the proof stress is 550 MPa or more, the yield ratio is 0.85 or more, and the JIS No. 3 Charpy impact test piece is used. 100 in the Charpy impact test at room temperature used
It is clear that the desired property of an impact value of J / cm ² or more can be secured. It is also clear that when the area reduction rate of the cold working is 40%, the proof stress and the yield ratio can be increased more stably.

(Example 2) Each round bar produced by wire drawing in Example 1 was subjected to aging treatment at 300 to 700 ° C for 1 hour. After this, JI from the center of each aged round bar
S14A tensile test pieces and JIS No. 3 Charpy impact test pieces were cut out, and the tensile properties (proof stress, tensile strength) and impact properties (impact value) at room temperature were investigated.

Table 5 summarizes the test results.

[0070]

[Table 5]

As shown in Table 5, a round bar having the chemical composition and structure specified in the present invention was subjected to cold drawing (cold working) at a reduction in area of 10% or more, and then at a temperature of 600 ° C. or less. It is clear that aging can further increase the yield strength and the yield ratio while securing desired toughness (impact properties).

(Example 3) Of the steels listed in Tables 1 and 2 which were produced by the test vacuum furnace, steels A1 to A5 and steels A7 to A2
2. After heating the steel ingots of steel B1 and steel B2 to 1250 ° C., hot rolling is performed so that the cumulative rolling reduction at 1,050 to 800 ° C. becomes 75%, and the rolling is finished at 800 ° C.
The mixture was cooled to room temperature at a cooling rate of ° C./sec to produce a round bar having a diameter of 30 mm.

A test piece for examining the structure was cut out from the thus obtained round bar having a diameter of 30 mm, mirror-polished, and observed by a transmission electron microscope by an extraction replica method to examine the structure. In this case, 5% picral was used for the corrosion.

Table 6 shows the results of the above organizational survey.

[0075]

[Table 6]

Next, the above-mentioned round bar having a diameter of 30 mm was subjected to a pillaring process to a diameter of 20 mm by an ordinary method, and then subjected to pickling and lubrication by an ordinary method.
% Cold drawing was performed. Thereafter, a JIS 14A tensile test piece and a JIS No. 3 Charpy impact test piece were cut out from the center of each drawn round bar, and the tensile properties (proof stress, tensile strength) and impact properties (impact value) at room temperature were investigated. did.

On the other hand, for steel C1, 1250
° C, then hot-rolled at 850 ° C with a diameter of 30mm
And then allowed to cool in the air. The round bar having a diameter of 30 mm thus obtained was subjected to 860 ° C. heating / oil cooling quenching and tempering. The tempering is 600
JIS 14A tensile test specimen and JIS 3 Charpy specimen from the center of the round bar after tempering.
The impact test piece was cut out, and the tensile properties (proof stress, tensile strength) and impact properties (impact value) at room temperature were investigated. In addition, the structure | tissue examination of the tempered material which used this steel C1 as a raw material steel was not performed.

Table 7 shows the tensile properties and the impact properties when the steels A1 to A5, the steels A7 to A23, the steels B1 and the steels B2 were subjected to cold drawing with a reduction in area of 40%, and the steels C1 as the raw materials. The following summarizes the tensile properties and the impact properties when this was subjected to a tempering treatment.

[0079]

[Table 7]

As can be seen from Table 7, when a round bar having the chemical composition and structure specified in the present invention is subjected to cold drawing (cold working) with a reduction in area of 40%, the yield strength and the yield ratio can be increased, and the yield strength can be increased. 550M
It is clear that the desired characteristics of an impact value of 100 J / cm ² or more can be ensured in a Charpy impact test at room temperature using a JIS No. 3 Charpy impact test specimen with a yield ratio of 0.85 or more and a JIS No. 3 Charpy impact test piece.

On the other hand, since the chemical compositions of the steels B1 and B2 deviate from the specified conditions of the present invention, desired toughness (impact properties) cannot be obtained when using these steels as raw materials. If the steel C1 is subjected to a tempering treatment, the yield strength and the toughness satisfy the targets, but the yield ratio is low and 0.8%.
5 has not been reached.

(Embodiment 4) Each round bar produced by wire drawing in the above-mentioned embodiment 3 was aged at 400 ° C for 1 hour. Then, from the center of each aged round bar, JIS14A
No. 10 tensile test piece and JIS No. 3 Charpy impact test piece were cut out, and the tensile properties (proof stress, tensile strength) and impact properties (impact value) at room temperature were investigated.

Table 8 summarizes the test results.

[0084]

[Table 8]

As shown in Table 8, a round bar having the chemical composition and structure specified in the present invention was subjected to cold drawing and then further heated to 400 ° C.
If aging is performed, while securing the desired toughness (impact properties),
It is clear that the yield strength and the yield ratio can be further increased.

On the other hand, since the chemical compositions of the steels B1 and B2 deviate from the specified conditions of the present invention, desired toughness (impact properties) cannot be obtained when using these steels as raw materials.

[0087]

The high-strength, high-toughness non-refined part produced by the method of the present invention has a yield strength of 550 MPa or more, a yield ratio of 0.85 or more, and 100 J / cm ² in a Charpy impact test.
Since it has the above-mentioned impact value, it can be used as a machine structural component such as an automobile or various industrial machines, especially a power transmission system component and an important security component.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram schematically showing classification according to the form of upper bainite, where (a) is “BI type bainite”, (b) is “B-II type bainite”, and (c) is “B-III”. Type bainite ".

[Explanation of symbols]

 1: austenite grain boundary, 2: cementite, 3: lath ferrite

Claims (2)

[Claims] C .: 0.02 to 0.10% by mass, S
i: 0.5% or less, Mn: 1.0 to 3.0%, P: 0.
04% or less, S: 0.40% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo:
1.0% or less, Nb: 0.1% or less, Ti: 0.05%
Hereinafter, Nd: 0.1% or less, B: 0.005% or less, A
l: 0.1% or less, N: 0.01% or less, and the balance is composed of Fe and the chemical composition of impurities.
7% is a mixed structure of ferrite and bainitic ferrite, and 50% or more of the structure other than ferrite and bainitic ferrite is cold worked to a steel material which is pseudo martensite. Manufacturing method for tough non-tempered parts. 2. A high-strength, high-toughness, non-heat treated component, which is obtained by subjecting a steel material having the chemical composition and structure according to claim 1 to cold working and then aging at a temperature of 600 ° C. or less. Manufacturing method.