WO2013084265A1

WO2013084265A1 - Steel for mechanical structures and manufacturing method therefor

Info

Publication number: WO2013084265A1
Application number: PCT/JP2011/006857
Authority: WO
Inventors: 稔本庄; 長谷　和邦
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2011-12-07
Filing date: 2011-12-07
Publication date: 2013-06-13
Also published as: IN2014KN01143A

Abstract

According to the present invention, a high-strength and high-toughness steel for mechanical structures is obtained without tempering and with little alloying, said steel comprising a ferrite and pearlite composition and having a component composition that includes 0.35-0.60 mass% C, 0.1-1.0 mass% Si, 0.1-1.5 mass% Mn, not more than 0.025 mass% P, not more than 0.025 mass% S, 0.01-0.10 mass% Al, and not more than 0.0015 mass% O, with the remainder comprising unavoidable impurities and Fe, and including 10-50% processed ferrite.

Description

Steel for machine structure and manufacturing method thereof

The present invention relates to a machine structural steel suitable for automobile parts, machine structural parts, etc., in particular, a machine structural steel that does not require tempering treatment by quenching and tempering, and a method for producing the same.

Conventionally, in the fields of construction machinery, industrial machinery and ships, parts that require high strength and high toughness include carbon steel materials for machine structures represented by S45C, and low mechanical structures containing Cr and Mo. Alloy steel quenching and tempering treatment materials have been used. However, in recent years, from the viewpoint of cost reduction or CO ₂ emission reduction, development of non-tempered steel that can omit the quenching and tempering treatment has been actively promoted.

As a typical non-heat treated steel, there is a ferrite / pearlite type non-heat treated steel (for example, see Non-Patent Document 1). This is because, in the cooling process after rolling, the ferrite and pearlite transformation precipitates almost at the same time, strengthening the ferrite with V carbide and obtaining the same strength as the quenched and tempered material, but the toughness is lower than the quenched and tempered material There was a problem.

Therefore, an attempt was made to improve the fertility of ferrite-pearlite type non-heat treated steel. For example, as a method of improving toughness, Patent Document 1 discloses rolling (first rolling) in which coarse austenite crystal grains before rolling are recrystallized in the austenite recrystallization region, and austenite in the austenite non-recrystallized region. It is described that strain is imparted (second rolling) and the generation of pro-eutectoid ferrite in the crystal grains is promoted by the strain, thereby obtaining a fine ferrite-pearlite structure to improve low-temperature toughness.

Furthermore, Patent Document 2 states that the surface reduction rate of rough rolling in the temperature range below T1 ° C. is 25% or less, and then the surface reduction is performed in the temperature range from (T2-200) to T2 ° C. according to the following T2. After finishing rolling with a rate of 25% or more, toughness is improved by cooling to 650 ° C. at a cooling rate of 5 ° C./s or more.
T1 (° C.) = − 5440 / (log [V] [C] −3.314) −173
T2 (° C.) = 910−203 [C] +44.7 [Si] −30 [Mn] −20 [Cu] −15.2 [Ni]
-11 [Cr] +31.5 [Mo] +104 [V] +400 [Ti] +460 [Al] +700 [P]

Japanese Patent No. 3214731 JP 2009-215576 A

However, in the technique described in Patent Document 1, although the low-temperature toughness is improved, the strength is reduced by rolling performed to improve the toughness, so it is necessary to add more V than the expected amount. However, it did not necessarily meet the demands of the industry for cost reduction by using low alloys.

Further, in the technique described in Patent Document 2, since accelerated cooling is performed after rolling, a bainite structure or the like is generated, the strength of the steel is increased, and the toughness may be decreased.

Therefore, an object of the present invention is to provide a high-strength and high-toughness steel for machine structures with a non-tempered and low alloy composition.

In order to solve the above problems, the inventors manufactured steels in which the addition amount of C, Si, Mn, P, S, Al, and O was changed and the fraction of processed ferrite was changed, and the strength and toughness. Investigated earnestly. As a result, it has been found that the strength and toughness of the material are improved by optimizing the addition amount of C, Si, Mn, P, S, Al and O, and controlling the fraction of processed ferrite within an appropriate range. The present invention has been completed.

That is, the gist configuration of the present invention is as follows.
(1) C: 0.35 to 0.60 mass%,
Si: 0.1 to 1.0 mass%,
Mn: 0.1-1.5% by mass,
P: 0.025 mass% or less,
S: 0.025 mass% or less,
Al: 0.01 to 0.10% by mass and O: 0.0015% by mass or less, having a component composition consisting of the balance inevitable impurities and Fe, and comprising a ferrite and pearlite structure containing 10 to 50% processed ferrite Machine structural steel.

(2) In addition to the above component composition, Cr: 1.0% by mass or less,
Cu: 1.0 mass% or less,
Mo: 1.0% by mass or less,
W: 1.0 mass% or less and Ni: 1.0 mass% or less, 1 type or 2 types or more selected from the above, Steel for machine structure as described in said (1) characterized by the above-mentioned.

(3) In addition to the above component composition, Nb: 0.1% by mass or less,
The steel for machine structural use as described in (1) or (2) above, containing one or more selected from Ti: 0.2% by mass or less and V: 0.15% by mass or less.

(4) In addition to the above component composition, B: 0.0002 to 0.005 mass%
The steel for machine structure according to any one of (1) to (3), characterized in that

(5) In addition to the above component composition, Pb: 0.01 to 0.40 mass%,
Bi: 0.01 to 0.40 mass% and Ca: 0.0005 to 0.0100 mass%
The structural structural steel according to any one of (1) to (4) above, which contains one or more selected from among the above.

(6) C: 0.35 to 0.60 mass%,
Si: 0.1 to 1.0 mass%,
Mn: 0.1-1.5% by mass,
P: 0.025 mass% or less,
S: 0.025 mass% or less,
Al: 0.01% to 0.10% by mass and O: 0.0015% by mass or less, and the material consisting of the remainder inevitable impurities and Fe is heated to 900 to 1250 ° C, and the cumulative area reduction at a temperature of ₃ points or less is 80%. Rolling with a reduction rate of 10% or more per pass in a temperature range of ₃ points or less of Ar is performed at least in one pass, and the finishing temperature is (Ar ₃ −10 ° C.) to (Ar ₃ −150 ° C.) A method for producing steel for machine structural use, characterized in that hot rolling is performed and then left to cool.

(7) In addition to the above materials, Cr: 1.0% by mass or less,
Cu: 1.0 mass% or less,
Mo: 1.0% by mass or less,
One or two or more types selected from W: 1.0% by mass or less and Ni: 1.0% by mass or less are contained.

(8) In addition to the above materials, Nb: 0.1% by mass or less,
The process for producing steel for machine structure as described in (6) or (7) above, comprising one or more selected from Ti: 0.2% by mass or less and V: 0.15% by mass or less .

(9) In addition to the above materials, B: 0.0002 to 0.005 mass%
The method for producing steel for machine structure according to any one of (6) to (8), characterized in that

(10) In addition to the above materials, Pb: 0.01-0.40 mass%,
Bi: 0.01 to 0.40 mass% and Ca: 0.0005 to 0.0100 mass%
1 or 2 types or more chosen from among these, The manufacturing method of the steel for machine structure in any one of said (6) to (9) characterized by the above-mentioned.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to stably manufacture the non-heat-treated steel for machine structures having high strength and toughness without increasing the alloy component of the steel material. Furthermore, the quenching and tempering process can be omitted, and contributes to the reduction of CO ₂ emissions, thereby providing an industrially beneficial effect.

It is a figure which shows the relationship between a processed ferrite fraction and yield strength. It is a figure which shows the relationship between a processed ferrite fraction and tensile strength. It is a figure which shows the relationship between a processed ferrite fraction and an impact value.

Next, the steel for machine structure of the present invention will be described first from the reasons for limiting each component in the component composition.
C: 0.35-0.60 mass%
C is an essential element for ensuring the necessary strength. If it is less than 0.35% by mass, the strength of the steel decreases, so 0.35% by mass or more is added. On the other hand, if it exceeds 0.60% by mass, the toughness of the steel is reduced. From the above, the C content is set to 0.35 to 0.60 mass%.

Si: 0.1-1.0 mass%
Si is an effective element for adjusting the deoxidizer and strength in the steelmaking process. To obtain these effects, 0.1% by mass or more is necessary. On the other hand, if it exceeds 1.0% by mass, the toughness is impaired, so the range is 0.1 to 1.0% by mass.

Mn: 0.1-1.5% by mass
Mn is an important element for adjusting the strength, but in order to obtain the effect, 0.1% by mass or more is necessary. On the other hand, if it exceeds 1.5% by mass, the toughness is impaired, so 0.1 to 1.5% by mass The range.

P, S: 0.025% by mass or less P and S are elements that deteriorate toughness, and it is preferable to reduce them as much as possible, but 0.025% by mass is allowed.

Al: 0.01-0.10 mass%
Al is an element added as a deoxidizer, and its effect is poor when it is less than 0.01% by mass. On the other hand, if it exceeds 0.10% by mass, the toughness is adversely affected, so Al is in the range of 0.01 to 0.10% by mass. To do.

O: 0.0015% by mass or less O is preferably as low as possible because it combines with Si and Al to form hard oxide-based nonmetallic inclusions, resulting in a decrease in toughness. In the present invention, the O content is 0.0015% by mass or less.

In the present invention, it is possible to add one or more of the following elements.
Cr: 1.0 mass% or less, Cu: 1.0 mass% or less, Mo: 1.0 mass% or less, W: 1.0 mass% or less, and Ni: 1.0 mass% or less Cr, Cu, Mo, W and Ni are solid solution strengthening elements. It is an effective element for strength adjustment. Therefore, as needed, it is possible to add any one or two or more of the above five types, preferably 0.05% by mass or more. On the other hand, if any element exceeds 1.0% by mass, the toughness decreases, and therefore, it is preferably 1.0% by mass or less.

Similarly, one or more of the following elements can be further added.
Nb: 0.1% by mass or less, Ti: 0.2% by mass or less, and V: 0.15% by mass or less Nb has an effect of preventing coarsening of crystal grains by forming carbonitrides and forms precipitates with C. Although it is an element useful for obtaining strength, if added over 0.1 mass%, the toughness decreases, so it was made 0.1 mass% or less.

Ti has an effect of fixing N in steel as TiN and preventing coarsening of crystal grains, and also forms a precipitate with C in the same manner as Nb, and thus is an element useful for obtaining strength. When added in excess of 0.2% by mass, the toughness decreases, so the content was made 0.2% by mass or less.

V is an element that contributes to the improvement of strength by forming precipitates with C like Nb and Ti. The effect is saturated even if added over 0.15% by mass. Further, since precipitates increase and toughness decreases on the contrary, the content is preferably 0.15% by mass or less.

Furthermore, it is possible to add the following elements.
B: 0.0002 to 0.005 mass%
B is an element that increases the strength of the steel after tempering by increasing the hardenability, and can be contained as necessary. In order to acquire the said effect, adding at 0.0002 mass% or more is preferable. However, if added over 0.005 mass%, the toughness deteriorates. Therefore, B is preferably 0.0002 to 0.005 mass%.

In the present invention, in order to improve machinability, it is possible to add one or more of the following elements.
Pb: 0.01 to 0.40 mass%, Bi: 0.01 to 0.40 mass%, and Ca: 0.0005 to 0.0100 mass%.
Pb: 0.01-0.40 mass%
Pb is an element that improves machinability. In order to obtain the effect, Pb is preferably added in an amount of 0.01% by mass or more. On the other hand, if added over 0.40 mass%, the toughness is lowered, so 0.010 to 0.40 mass% is preferable.

Bi: 0.01-0.40 mass%
Bi is an element that improves machinability. In order to acquire the effect, it is preferable to add at 0.01 mass% or more. On the other hand, if added over 0.40% by mass, the toughness is remarkably lowered, so 0.01 to 0.40% by mass is preferable.

Ca: 0.0005 to 0.0100 mass%
Ca is an element that improves machinability. In order to acquire the effect, adding at 0.0005 mass% or more is preferable. On the other hand, even if added over 0.0100% by mass, the effect is saturated, so 0.0005 to 0.0100% by mass is preferable.

Next, the structure of the steel for machine structure of the present invention will be described.
By applying a low-temperature transformation structure such as martensite, bainite or a mixed structure other than ferrite and pearlite as a base structure to steel for machine structure, represented by a large diameter steel bar having a diameter of 100 mmφ or more, For example, it becomes difficult to make the structure in the cross section of the steel bar uniform, and internal cracking is likely to occur due to the influence of transformation stress generated by thermal stress and transformation that occurs during cooling. For this reason, in the present invention, the base structure is not a low-temperature transformation structure but a ferrite and pearlite structure.

Means for achieving high strength in the ferrite and pearlite structures include a method of increasing the pearlite fraction of the second phase, a method of further reducing the ferrite structure, and a method of hardening the ferrite by solid solution strengthening and precipitation strengthening. Alternatively, a method of rolling in a two-phase region (austenite + ferrite) to increase a part of ferrite to a high dislocation density, etc. can be considered.

Among the above methods, the method of refining ferrite is advantageous for increasing the yield stress (hereinafter referred to as YP), but the increase in tensile strength (hereinafter referred to as TS) is small. However, sufficient strength cannot be achieved by this method alone. The method for increasing the pearlite fraction requires addition of a large amount of C. However, excessive addition of C is not preferable because it causes a decrease in toughness as described above. The method of strengthening ferrite by adding a solid solution strengthening element or a precipitation strengthening element requires a large amount of alloy elements to be added, resulting in an increase in alloy costs and a decrease in toughness.

On the other hand, the use of processed ferrite can suppress the addition of C and alloy elements to the minimum, and can increase YP and TS. In other words, the method using the processed ferrite can increase the strength without controlled cooling (accelerated cooling) after hot rolling, so the thermal stress generated during cooling and the transformation stress generated along with the transformation It is possible to increase the strength while suppressing the occurrence of internal cracks due to the effect of.

Therefore, in the present invention, as a means for increasing the strength of the steel for machine structural use, a method is adopted in which the microstructure of the steel is a ferrite and pearlite structure containing processed ferrite in an area ratio of 10 to 50%. is there. Here, the reason why the processed ferrite fraction is in the range of 10 to 50% of the entire steel structure in terms of the area ratio is as follows.
That is, if the fraction of the processed ferrite is less than 10%, the steel cannot be sufficiently strengthened. On the other hand, if it exceeds 50%, the strength increases but the toughness decreases and the two phases (austenite + ferrite). This is because the risk of roll breakage accompanying an increase in load during zone rolling increases.
The processed ferrite is a ferrite introduced with work strain formed by hot rolling in a two-phase region (austenite + ferrite) below the Ar ₃ transformation point. Usually, the ferrite is traced, If the length of the minor axis and the major axis is determined, and the ratio of the major axis to the minor axis (aspect ratio) is defined as ferrite having a ratio of 2 or more, the area occupied in the microstructure can be quantified. The rate can be measured.

Next, production conditions for obtaining the steel of the present invention will be described.
In the production of the steel for machine structural use of the present invention, first, the steel having the above-described component composition is melted by a usual method using a converter, an electric furnace or the like, and a usual method such as a continuous casting method or an ingot casting method is used. Use a steel material such as slab, billet or bloom. In addition, after melting, treatment such as ladle refining or vacuum degassing may be added.

Thereafter, the steel material is charged into a heating furnace, reheated, and then hot-rolled to obtain, for example, a non-tempered steel bar having desired dimensions, structure, and characteristics. At this time, the reheating temperature of the steel material needs to be in the range of 900 to 1250 ° C. When the heating temperature is less than 900 ° C., the deformation resistance during rolling becomes large, so that hot rolling becomes difficult. On the other hand, heating above 1250 ° C may cause surface marks, increase scale loss and fuel consumption. Preferably, it is in the range of 900 to 1200 ° C.

In the subsequent hot rolling, it is first necessary to perform rolling at a temperature of ₃ points or less of Ar. If rolling is not performed at this temperature, the microstructure of the steel does not contain processed ferrite, and the necessary strength and toughness cannot be ensured. For rolling at a temperature of ₃ points or less of Ar, it is necessary to carry out at least one pass of rolling having a reduction in area of 10% or more per pass. This is because when the area reduction ratio is less than 10%, the amount of processed ferrite produced is reduced, and the effect of enhancing the strength and toughness by the two-phase region rolling cannot be obtained sufficiently.
When the cumulative area reduction rate at a temperature of ₃ points or less of Ar exceeds 80%, the rolling load increases and rolling becomes difficult, or the number of rolling passes increases, leading to a decrease in productivity. Furthermore, the amount of processed ferrite exceeds 50%, and the strength of the steel increases too much, leading to a decrease in toughness. Therefore, the cumulative area reduction rate at ₃ points or less of Ar is 80% or less.

Furthermore, it is necessary to perform the finishing temperature in the hot rolling under the conditions of (Ar ₃ −10 ° C.) to (Ar ₃ −180 ° C.). When the rolling finishing temperature exceeds (Ar ₃ −10 ° C.), the effect of enhancing the toughness by the two-phase rolling cannot be obtained sufficiently, whereas when the rolling finishing temperature is less than (Ar ₃ −180 ° C.), the rolling load increases due to the increase in deformation resistance. As a result, it becomes difficult to roll, the amount of processed ferrite exceeds 50%, the strength of the steel increases too much, and the toughness decreases.

It is preferable to cool the cooling following the hot rolling. This is because when accelerated cooling is performed after hot rolling, the structure becomes a low-temperature transformation structure such as martensite, bainite, or a mixed structure other than ferrite + pearlite, and it is difficult to make the structure in the cross section uniform. In addition, internal cracking is likely to occur due to the thermal stress generated during cooling and the transformation stress generated with the transformation. Therefore, it is preferable to cool the cooling following hot rolling. Specifically, the cooling may be performed at 0.5 ° C./s or less.

Hereinafter, a steel bar will be specifically described as an example of steel for machine structural use.
Steel having the component composition shown in Table 1 is melted in a vacuum melting furnace or converter to form a bloom, and this bloom is charged into a heating furnace and heated, and then hot rolled in accordance with the conditions shown in Table 2. Hot rolled into a round bar. A JIS No. 4 tensile test piece and a JIS No. 3 Charpy impact test piece were cut out from a 1/4 depth portion from the surface of the obtained rolled steel bar, and the mechanical properties were evaluated.

Three Charpy impact tests were conducted at a test temperature of 20 ° C., and the average impact value was evaluated. In addition, a sample for tissue observation is collected from the same position as the above-mentioned specimen collection position, and observed with an optical microscope at a magnification of 400 times in five fields, and the generated ferrite is traced. “Image-Pro” manufactured by NIPPON ROPER (Trade name) to determine the lengths of the short axis and long axis of the ferrite, quantify the area of the ferrite (processed ferrite) with an aspect ratio of 2 or more in the microstructure, The rate was determined.

The rolling temperature was measured by installing a radiation thermometer on the entry side and the exit side of the mill. Further, the finish rolling temperature in Table 2 refers to the temperature on the exit side during final rolling.
Yield strength, tensile strength, and toughness (impact value) were judged to have improved characteristics when improved by 10% or more compared to conventional S45C (reference steel).

Table 3 shows the processed ferrite fraction, yield strength, tensile strength, impact value, and structure evaluation results. The results of Table 3 are summarized in FIGS. 1 to 3. Steels B-4 to 8 satisfying the component composition, structure and production conditions of the present invention show yield values, tensile strengths and impact values that are 10% or more better than standard steels, It can be seen that while having high strength, it has high toughness. On the other hand, even if the component composition is within the range of the present invention, the steels B-2, 3, 9 and 10 which do not have the microstructure of the present invention have yield strength, tensile strength and impact value. It can be seen that the toughness is reduced due to the standard steel level or higher strength.

Steel having the composition shown in Table 4 is melted in a vacuum melting furnace or converter to form a bloom, and this bloom is charged into a heating furnace and heated, and then hot-rolled under the conditions shown in Table 5. , Hot rolled into a round bar. The above-mentioned test was implemented and evaluated with respect to the obtained rolled steel bar.

Table 6 shows the processed ferrite fraction, yield strength, tensile strength, impact value, and structure evaluation results. Steels C-1 to C-11 satisfying the component composition, structure and production condition of the present invention have yield values, tensile strengths, and impact values that are 10% or more better than those of standard steels. It can be seen that it is strong but has high toughness. On the other hand, even if the component composition is within the scope of the present invention, Steel C-12 to 16 which does not have the microstructure of the present invention, the production conditions are outside the scope of the present invention, and the microstructure of the present invention It can be seen that the steels C-17 to 19 that do not have any of the above have lower toughness due to the yield strength, tensile strength, and impact value being either the standard steel level or higher strength. Internal cracks did not occur in the internal observation of the inventive steel, but internal cracks occurred in the comparative steel C-18 (water-cooled material).

Claims

C: 0.35-0.60 mass%,
Si: 0.1 to 1.0 mass%,
Mn: 0.1-1.5% by mass,
P: 0.025 mass% or less,
S: 0.025 mass% or less,
Al: 0.01 to 0.10% by mass and O: 0.0015% by mass or less, having a component composition consisting of the balance inevitable impurities and Fe, and comprising a ferrite and pearlite structure containing 10 to 50% processed ferrite Machine structural steel.
In addition to the above component composition, Cr: 1.0% by mass or less,
Cu: 1.0 mass% or less,
Mo: 1.0% by mass or less,
The machine structural steel according to claim 1, comprising one or more selected from W: 1.0 mass% or less and Ni: 1.0 mass% or less.
In addition to the above component composition, Nb: 0.1% by mass or less,
The steel for machine structural use according to claim 1 or 2, comprising one or more selected from Ti: 0.2 mass% or less and V: 0.15 mass% or less.
In addition to the above component composition, B: 0.0002 to 0.005 mass%
The mechanical structural steel according to claim 1, comprising:
In addition to the above component composition, Pb: 0.01-0.40 mass%,
Bi: 0.01 to 0.40 mass% and Ca: 0.0005 to 0.0100 mass%
It contains 1 type, or 2 or more types chosen from among these, Steel for machine structure in any one of Claim 1 to 4 characterized by the above-mentioned.
C: 0.35-0.60 mass%,
Si: 0.1 to 1.0 mass%,
Mn: 0.1-1.5% by mass,
P: 0.025 mass% or less,
S: 0.025 mass% or less,
Al: 0.01% to 0.10% by mass and O: 0.0015% by mass or less, and the material consisting of the remainder inevitable impurities and Fe is heated to 900 to 1250 ° C, and the cumulative area reduction at a temperature of 3 points or less is 80%. Rolling with a reduction rate of 10% or more per pass in a temperature range of 3 points or less of Ar is performed at least in one pass, and the finishing temperature is (Ar 3 −10 ° C.) to (Ar 3 −150 ° C.) A method for producing steel for machine structural use, characterized in that hot rolling is performed and then left to cool.
In addition to the above materials, Cr: 1.0 mass% or less,
Cu: 1.0 mass% or less,
Mo: 1.0% by mass or less,
The method for producing steel for machine structural use according to claim 6, comprising one or more selected from W: 1.0 mass% or less and Ni: 1.0 mass% or less.
In addition to the above materials, Nb: 0.1% by mass or less,
The method for producing steel for machine structural use according to claim 6 or 7, comprising one or more selected from Ti: 0.2% by mass or less and V: 0.15% by mass or less.
In addition to the above materials, B: 0.0002 to 0.005 mass%
The manufacturing method of the steel for machine structures in any one of Claim 6 to 8 characterized by the above-mentioned.
In addition to the above materials, Pb: 0.01-0.40 mass%,
Bi: 0.01 to 0.40 mass% and Ca: 0.0005 to 0.0100 mass%
The method for producing steel for machine structural use according to any one of claims 6 to 9, comprising one or more selected from among the above.