JP6791179B2 - Non-microalloyed steel and its manufacturing method - Google Patents

Description

INDUSTRIAL APPLICABILITY According to the present invention, non-tempered steel having a tensile strength-toughness balance equal to or higher than that of the tempered material even if it is a non-heat-rolled material, particularly mechanical structural parts used in automobiles and various industrial machines It relates to a non-tamed steel suitable for the above.

Structural parts used in transportation machinery and construction machinery, including automobiles, include not only tempered steel obtained by quenching and tempering carbon steel for machine structure and alloy steel for machine structure, but also the chemical composition of steel regardless of quenching and tempering. Non-quenched steel whose strength is ensured by adjusting the structure is used.

Non-tempered steels used for such applications generally have a ferrite-pearlite two-layer structure with V and Nb added, and yield when the tensile strength is similar to that of tempered steel. It has been pointed out that when the strength, drawing value and impact value are low, while the yield strength is set to the same level, the tensile strength, that is, the hardness, increases excessively and the machinability decreases.

Against the above background, Patent Documents 1 and 2 include structures having ferrite, bainitic ferrite, and pseudo-martensite in order to obtain high-strength, high yield ratio, and high toughness non-tempered steel. A technique is disclosed in which a prepared steel material is cold-worked and then aged at 600 ° C. or lower to precipitate Cu and Ti-Nb-based carbides.

However, in actual manufacturing, it is extremely difficult to strictly control the ratio of a plurality of tissues as described above. Further, when the precipitation strengthening by adding a large amount of Cu is used, it is necessary to add a large amount of expensive Ni to prevent high temperature cracking, so that there is a problem that it is not suitable as a structural part to be consumed in large quantities. ..

Further, in Patent Document 3, in order to obtain bainite-type non-tempered steel having excellent machinability in addition to strength and toughness, Ti carbon sulfide having a larger lubricating effect during cutting than MnS is appropriate. The technology to control the size and cleanliness is disclosed. However, according to the investigation by the present inventors, the pinning force of the dispersed particles decreases and the austenite particles become coarse depending on the balance of the addition amounts of Ti and C and the heating and cooling conditions during hot working. As a result, it was found that the toughness may decrease.

Japanese Unexamined Patent Publication No. 2001-123224 Japanese Unexamined Patent Publication No. 2001-131680 Japanese Patent No. 3489655

The present invention provides a non-microalloyed steel and a method for producing the same, which solves the above-mentioned problems of the prior art and is suitable as a material for stably producing mechanical structural parts having an excellent strength-toughness balance. The purpose.

In order to solve the above problems, the present inventors have diligently studied the effects of components and structures on the strength and toughness after hot working. As a result, we have found the following items a) to c).
a) An excellent strength-toughness balance can be obtained by making the steel structure after hot working into bainite with an area ratio of 70% or more.
b) In order to improve toughness, it is effective to reduce the amount of solid solution C in the tissue in addition to the bainite organization described in a) above. Therefore, it is necessary to add Ti, which reduces the amount of solid solution C by forming carbides, in an amount of a predetermined amount or more according to the C content in the steel material.
c) By increasing the amount of Ti carbides that become unsolidified when heated in hot working, the pinning effect of the precipitates is exhibited, and a fine bainite structure can be obtained without tempering. On the other hand, if Ti and C are added in excess, the austenite grains become significantly finer and the hardenability is lowered, so that it becomes difficult to obtain the desired bainite structure described in a) above. Further, when the amount of undissolved Ti carbide is small, the austenite grains become coarse, so that the toughness is inferior to that of the fine bainite structure. Therefore, in order to obtain a stable and excellent strength-toughness balance, it is necessary to strictly control the contents of C and Ti.

The present invention has been made based on the above findings, and its gist structure is as follows.
1. 1. By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The balance is a non-tamed steel having a composition of Fe and unavoidable impurities, and a structure in which the area ratio of the bainite phase is 70% or more.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.

2. 2. The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The non-microalloyed steel according to 1 above, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less.

3. 3. The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The non-tampered steel according to 1 or 2 above, which contains at least one selected from 1.0% or less.

4. By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The rest is a steel material with a composition of Fe and unavoidable impurities, which is hot-worked with a heating temperature of 1300 ° C or less and a finishing temperature of 850 ° C or more and 1300 ° C or less, and then in the temperature range of 700 to 550 ° C at 0.5 ° C / s. A method for producing non-tempered steel to be cooled.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.

5. The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The method for producing a non-microalloyed steel according to 4 above, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less.

6. The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The method for producing a non-microalloyed steel according to 4 or 5 above, which comprises one or more selected from 1.0% or less.

According to the present invention, it is possible to provide non-microalloyed steel and a method for producing the same, which are suitable as a material for stably producing mechanical structural parts having an excellent strength-toughness balance. That is, when various structural parts used in transportation machines and construction machines are manufactured using the steel of the present invention, it becomes possible to mass-produce parts having high strength and excellent toughness without being tempered.

It is a figure which shows the shape of the rotary bending fatigue test piece. It is a graph for comparing the strength-toughness balance between the invention example and the comparative example.

First, in the present invention, the reason why the composition of steel is limited to the above range will be described. In addition, unless otherwise specified, the "%" indication regarding the component shall mean mass%.
C: 0.10 to 0.25%
C is an element effective for microstructure refinement because it forms Ti carbides that are stably present at the heating temperature during hot working together with Ti. In addition, since it contributes to the formation of the bainite phase and the improvement of strength, it is necessary to add 0.10% or more. On the other hand, if the C content exceeds 0.25%, the toughness of the bainite phase decreases due to the increase in the solid solution C content, so the C content is set to 0.25% or less. It is preferably in the range of 0.14% or more and 0.22% or less.

Si: 0.10 to 1.00%
Si is added at 0.10% or more because it is effective not only for deoxidation but also for the formation of bainite phase. On the other hand, if the amount of Si exceeds 1.00%, it dissolves in the ferrite phase and the bainite phase, and the solid solution hardening deteriorates the toughness and machinability. Therefore, the amount of Si is set to 1.00% or less. It is preferably in the range of 0.10% or more and 0.50% or less.

Mn: 0.30 to 3.00%
Mn is added because it is effective in forming the bainite phase and improving the strength. However, if the amount of Mn is less than 0.30%, the amount of bainite phase formed is small, and it becomes difficult to secure toughness. Therefore, the amount of Mn should be 0.30% or more. On the other hand, if it exceeds 3.00%, hardenability is remarkably improved and a hard layer such as island-shaped martensite that adversely affects toughness is formed, so the amount of Mn should be 3.00% or less. It is preferably in the range of 0.80% or more and 2.00% or less.

P: 0.050% or less P is preferably as low as it segregates at the grain boundaries and reduces toughness and fatigue strength. Specifically, if it exceeds 0.050%, the above-mentioned adverse effects appear, so the amount of P was set to 0.050% or less. On the other hand, since a large manufacturing cost is required to reduce the content to less than 0.003%, it is preferable to set 0.003% as the lower limit from the viewpoint of cost.

S: 0.005% to 0.150%
S has an action of forming a sulfide with Mn and improving machinability. For that purpose, the content should be 0.005% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was set to 0.150%. It is preferably in the range of 0.010% or more and 0.060% or less.

Cr: 0.10 to 2.50%
Cr is added because it is effective in forming the bainite phase and improving the strength. However, when the amount of Cr is less than 0.10%, the amount of bainite phase formed is small, and it becomes difficult to secure toughness. Therefore, the amount of Cr is set to 0.10% or more. On the other hand, if it exceeds 2.50%, hardenability is remarkably improved and a hard layer such as island-shaped martensite that adversely affects toughness is formed, so the amount of Cr is set to 2.50% or less. It is preferably in the range of 0.50% or more and 1.50% or less.

Al: 0.010 to 0.080%
Al is an element necessary for deoxidation, but its effect cannot be sufficiently obtained if it is less than 0.010%. On the other hand, if it exceeds 0.080%, the effect is saturated, nozzle clogging occurs during continuous casting, and alumina cluster inclusions appear, resulting in a decrease in toughness and fatigue strength. Therefore, the amount of Al was limited to the range of 0.010 to 0.080%. It is preferably in the range of 0.020 to 0.050%.

Ti: 0.25 to 0.60%
Ti, together with C, forms Ti carbides that exist stably at the heating temperature during hot working, and is therefore an effective element for microstructure miniaturization. In order to exert such an effect, it is necessary to add 0.25% or more. On the other hand, the effect is saturated at 0.60%, and addition exceeding 0.60% leads to cost increase. Therefore, the amount of Ti was limited to the range of 0.25 to 0.60%. It is preferably in the range of 0.30 to 0.50%.

N: 0.0070% or less N is an element that combines with Ti to form TiN and contributes to the refinement of austenite crystal grains. However, the formation of TiN not only reduces the amount of Ti carbides that contribute to the miniaturization of austenite grains at the hot working temperature, but also causes bubbles to be generated in the steel ingot during solidification when added in excess, and forging property. The upper limit is set to 0.0070% because it causes deterioration of. It is preferably in the range of 0.0020 to 0.0070%.

In the present invention, it is not sufficient for each element to simply satisfy the above range, and T1 defined by the following formula (1) is 3.0 or less, and T2 defined by the following formula (2) is 1620 to It is important to satisfy the range of 1750.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.

That is, since the toughness of the non-tamed steel having a bainite structure decreases as the amount of solid solution C increases, the amount of Ti that reduces the amount of solid solution C through the formation of carbides is determined according to the amount of C added to the steel material. It is necessary to add more than a certain amount. Here, T1 described above is a parameter representing the amount of solid solution C in the bainite structure, and when T1 exceeds 3.0, the amount of solid solution C increases, so that the effect of improving toughness cannot be obtained. Therefore, T1 should be 3.0 or less.

Next, if the amount of Ti carbides that become unsolidified during heating in hot working is increased, the pinning effect of the precipitates is exhibited, and a fine bainite structure can be obtained without being tempered. Here, T2 described above is a parameter representing the required amount of Ti carbides that contribute to miniaturization. When T2 is 1620 or less, the amount of Ti carbides is insufficient and the pinning force is reduced, so that the austenite grains become coarse. , The toughness is reduced. On the other hand, when T2 exceeds 1750, the austenite grains become significantly finer and the hardenability is lowered, so that it becomes difficult to obtain a bainite structure having an area ratio of 70% or more. Therefore, T2 is set to 1620 to 1750.

The components in steel in the present invention contain the above components, and the balance is Fe and unavoidable impurities. Further, the following selective components can be added for the purpose of imparting other characteristics, etc., as long as the effects of the present invention are not impaired.

Mo: 1.0% or less
Mo can be added to improve hardenability and toughness. When added, it is preferably 0.05% or more. On the other hand, the effect is saturated when it exceeds 1.0%, and the cost increases when it is added in excess of 1.0%. Therefore, it is preferable to set the upper limit to 1.0%. More preferably, it is in the range of 0.10 to 0.50%.

Nb: 0.3% or less
Nb refines the crystal grains and strengthens the grain boundaries, contributing to the improvement of toughness and fatigue strength. For that purpose, it is preferable to add at 0.05% or more. On the other hand, if the effect exceeds 0.3%, it is saturated and causes an increase in cost. Therefore, it is preferable to set the upper limit to 0.3%. More preferably, it is in the range of 0.10 to 0.20%.

V: 0.3% or less V is a carbonitride-forming element like Nb, which contributes to the improvement of toughness and fatigue strength by refining crystal grains and strengthening grain boundaries. For that purpose, it is preferable to add at 0.05% or more. On the other hand, if the effect exceeds 0.3%, it is saturated and causes an increase in cost. Therefore, it is preferable to set the upper limit to 0.3%. More preferably, it is in the range of 0.10 to 0.20%.

W: 0.3% or less W forms a precipitate together with Ti and contributes to the improvement of toughness and fatigue strength. For that purpose, it is preferable to add at 0.05% or more. On the other hand, if the effect exceeds 0.3%, it is saturated and causes an increase in cost. Therefore, it is preferable to set the upper limit to 0.3%. More preferably, it is in the range of 0.10 to 0.20%.

B: 0.0100% or less B is an element effective for ensuring hardenability by adding a small amount, and for that purpose, it is preferable to add 0.0005% or more. On the other hand, even if it is added in excess of 0.0100%, the above effect is saturated. Therefore, the upper limit is preferably 0.0100%. More preferably, it is in the range of 0.0010 to 0.0040%.

Cu: 1.0% or less
Cu is an element that contributes to the improvement of hardenability and is useful for the improvement of toughness. In order to obtain these effects, Cu is preferably 0.01% or more. On the other hand, if the Cu content exceeds 1.0%, the surface surface of the rolled material becomes rough, and there is a concern that it may remain as a flaw. Therefore, the upper limit is preferably 1.0%. More preferably, it is in the range of 0.10 to 0.50%.

Ni: 1.0% or less
Ni is an element that contributes to the improvement of hardenability and is useful for improving toughness. In order to obtain these effects, Ni is preferably 0.01% or more. On the other hand, even if it is contained in excess of 1.0%, the above effect is saturated. Therefore, the upper limit is preferably 1.0%. More preferably, it is in the range of 0.10 to 0.50%.

Next, the reason why the steel structure of the non-tempered steel in the present invention is limited to the above range will be described.
Bainite phase: 70% or more in area ratio to the entire structure In the present invention, it is important that the bainite phase has an area ratio of 70% or more to the entire structure. In the present invention, Ti carbides that are undissolved in the hot working temperature range are used for pinning the austenite grains. Since the precipitates exist in a non-matching manner, the elastic strain energy (matching strain) is small, and the risk of toughness reduction due to the precipitates can be minimized. On the other hand, if fine Ti carbides are precipitated in the cooling process after hot working, it is disadvantageous from the viewpoint of improving toughness. In this respect, the bainite transformation process is less likely to generate Ti carbides in the matrix than the ferrite-pearlite transformation process. Therefore, the steel structure of the non-treated steel of the present invention is mainly composed of the bainite phase. Specifically, the bainite phase is set to 70% or more in area ratio with respect to the entire structure. It is preferably 80% or more, more preferably 90% or more. It may also be 100%. As the structure other than the bainite phase, a ferrite phase, a pearlite phase, a martensite phase and the like can be considered, but it goes without saying that the smaller the number of these structures, the more preferable.

Here, the area ratio of each phase can be obtained as follows. That is, a test piece was taken from the obtained non-tamed steel, and the vertical cross section (L cross section) parallel to the rolling direction was corroded with Nital after polishing, and L was used using an optical microscope and a scanning electron microscope (SEM). The type of phase is identified by cross-sectional structure observation (400x optical microscope structure observation), and the area ratio of each phase is determined. Since it is difficult to determine island-shaped martensite with an optical microscope, observe it using an SEM. Since the island-shaped martensite is observed by SEM as a white and raised part, it can be calculated from the average value of the area ratios of microstructure photographs of at least 5 fields of view by image processing.

Next, the production conditions of the present invention will be described.
That is, after heating the steel material having the above-mentioned composition, hot working is performed at a finishing temperature of 850 ° C or higher and 1300 ° C or lower, and then the temperature range of 700 to 550 ° C is cooled at 0.5 ° C / s or higher. Manufacture tempered steel. Hereinafter, the requirements for each manufacturing process will be described.
[Steel heating temperature: 1300 ° C or less]
If the heating of the steel material prior to hot working exceeds 1300 ° C, it becomes difficult to secure the amount of Ti carbides that contribute to the miniaturization of the structure, so the heating temperature is set to 1300 ° C or less. On the other hand, the lower limit is preferably 900 ° C. or higher in order to secure the finishing temperature in the hot working of the next process. The steel material to be heated may be a piece of steel, or a piece of steel or steel bar that has undergone hot rolling or hot forging.

[Finishing temperature in hot working: 850 ° C or higher and 1300 ° C or lower]
If the finishing temperature is less than 850 ° C., a ferrite structure is formed, which is disadvantageous for forming the matrix phase into a bainite structure having an area ratio of 70% or more. In addition, the rolling load increases, and the roundness of the rolled material also deteriorates. Therefore, the finishing temperature is set to 850 ° C or higher. If the finishing temperature exceeds 1300 ° C, it becomes difficult to secure the amount of Ti carbides that contribute to microstructure miniaturization. Therefore, the upper limit of the finishing temperature is set to 1300 ° C.

As the hot working, either one or both of hot forging and hot rolling can be applied, and it may be appropriately selected and used according to the shape of the member using the non-tamed steel of the present invention. When both hot rolling and hot forging are performed as hot working, the above-mentioned heating temperature and finishing temperature conditions may be applied to the final hot working.

[Cooling: 0.5 ° C / s or more in the temperature range of 700 to 550 ° C]
In order to obtain the above-mentioned bainite structure so that fine precipitates do not precipitate after hot working and the toughness is not impaired, it is necessary to specify the cooling rate after hot working. That is, it is necessary to cool the temperature range of 700 to 550 ° C., which is the precipitation temperature range of fine precipitates, at a cooling rate (0.5 ° C./s) or higher at which fine precipitates can be obtained.

Hereinafter, the constitution and the action and effect of the present invention will be described more specifically according to Examples. The present invention is not limited by the following examples, and can be appropriately modified within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. Is done.
Steels (No. 1 to 43) having the composition shown in Table 1 are melted in a 150 kg vacuum melting furnace, and the steel material obtained by continuous casting is heated to 1250 ° C. and then hot-rolled at a finishing temperature of 1050 ° C. After that, it was cooled to room temperature at 1 ° C./s to obtain a round steel bar having a diameter of 50 mm. The obtained round bar steel is further heated at various temperatures and then hot forged at 1150 ° C. with a finishing reduction rate of 40% to finish it into a steel bar of 30 mmφ, and then 1.2 ° C / s or 0.1 ° C / s (Table). 2) was cooled to room temperature.
Here, steels Nos. 1 to 23 shown in Table 1 are invention steels whose component compositions satisfy the present invention, and steels Nos. 24 to 43 are comparative steels whose component compositions do not satisfy the present invention, and Table 2 Nos. 44 to 46 in the No. 44 to 46 are comparative examples in which any of the cooling rate after forging, the heating temperature before hot forging, and the finishing temperature of hot forging deviates from the specified values of the present invention.

The obtained steel bars were subjected to microstructure observation, tensile test, Charpy impact test and Ono-type rotary bending fatigue test. The contents of each survey will be described in detail below.

Structure observation For structure observation, a test piece is taken from a steel bar with a diameter of 30 mmφ obtained by hot forging, and the vertical cross section (L cross section) parallel to the forging direction is corroded with nital after polishing, and an optical microscope and SEM are used. The type of phase was identified by observing the cross-sectional structure, and the area ratio of each phase was determined.

Tensile test In the tensile test, JIS No. 4 tensile test pieces were sampled from the center of a steel bar of 30 mmφ and the tensile strength was investigated.

Charpy impact test In the Charpy impact test, a JIS No. 3 test piece was sampled from the center of a 30 mmφ steel bar and the impact value at room temperature was evaluated.

Rotational bending fatigue characteristics A test piece with a parallel portion diameter of 6 mm shown in FIG. 1 was collected from a steel bar having a diameter of 30 mmφ. The obtained test piece, using a Ono-type rotary bending fatigue testing machine, the rotational speed: conducted at 3000 rpm, 10 ⁷ times as fatigue limit was measured rotating bending fatigue strength.

Table 2 and FIG. 2 show the results of the above survey. It was found that the examples of the present invention (No. 1 to 23) had an excellent strength-toughness balance as compared with the comparative examples (No. 24 to 46).

Claims (6)

By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The balance is a non-tamed steel having a composition of Fe and unavoidable impurities, and a structure in which the area ratio of the bainite phase is 70% or more.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element. The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The non-microalloyed steel according to claim 1, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less. The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The non-tampered steel according to claim 1 or 2, which contains at least one selected from 1.0% or less. By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The rest is a steel material with a composition of Fe and unavoidable impurities, which is hot-worked with a heating temperature of 1300 ° C or less and a finishing temperature of 850 ° C or more and 1300 ° C or less, and then in the temperature range of 700 to 550 ° C at 0.5 ° C / s. A method for producing a non-tempered steel having a structure having a bainite phase area ratio of 70% or more, which is cooled as described above.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element. The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The method for producing non-microalloyed steel according to claim 4, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less. The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The method for producing non-microalloyed steel according to claim 4 or 5, which comprises at least one selected from 1.0% or less.

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