JP2019151866A - Non-heat-treated steel and method for producing the same - Google Patents

Abstract

To provide a non-heat-treated steel suitable as a material for stably making a mechanical structural component with a good strength-toughness balance.SOLUTION: The non-heat-treated steel contains C: 0.10-0.25%, Si: 0.10-1.00%, Mn: 0.30-3.00%, P: 0.050% or less, S: 0.005-0.150%, Cr: 0.10-2.50%, Al: 0.010-0.080%, Ti: 0.25-0.60% and N: 0.0070% or less, in such a range that T1 defined by the following formula (1) is 3.0 or less, and T2 defined by the following formula (2) is 1620-1750, with the balance being Fe and unavoidable impurities, in its component composition. In the structure of the non-heat-treated steel, the area ratio of a bainite phase is 70% or more. T1=(C/12)/(Ti/48) (1), T2=-7430/[Log(C×Ti)-3.23] (2), where each element symbol denotes a content (mass%) of the element.SELECTED DRAWING: None

Description

  The present invention relates to a non-heat treated steel having a tensile strength-toughness balance equal to or higher than that of a tempered material even if it is a non-heat treated material as hot-rolled. As a suitable non-heat treated steel.

  Structural parts used in transportation equipment and construction machinery, including automobiles, include not only tempered steel that has been tempered and tempered carbon steel for mechanical structures and alloy steels for mechanical structures, Non-tempered steel whose strength is secured by adjusting the structure is used.

  Non-tempered steel used for such applications generally has a ferrite-pearlite double-layered structure with V and Nb added. When tensile strength is the same as that of tempered steel, it yields. It has been pointed out that when the strength, the drawing value and the impact value are low, while the yield strength is set to the same level, the tensile strength, that is, the hardness is excessively increased and the machinability is lowered.

  Under the background described above, Patent Document 1 and Patent Document 2 describe a structure having ferrite, bainitic ferrite, and pseudo martensite in order to obtain a high-strength, high yield ratio, and high toughness non-heat treated steel. A technique is disclosed in which the prepared steel material is subjected to aging treatment at 600 ° C. or lower after cold working 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. Furthermore, when precipitation strengthening due to the addition of a large amount of Cu is used, it is necessary to add a large amount of expensive Ni to prevent hot cracking. .

  Further, in Patent Document 3, in order to obtain a bainite-type non-heat treated steel that is excellent in machinability in addition to strength and toughness, Ti carbon sulfide having a greater lubricating effect during cutting than MnS is appropriately used. A technique for controlling the size and cleanliness is disclosed. However, according to the investigation by the present inventors, depending on the balance of the addition amount of Ti and C and the heating and cooling conditions during hot working, the pinning force due to the dispersed particles is reduced, and the austenite grains become coarse. As a result, it has been found that toughness may decrease.

JP 2001-123224 JP 2001-131680 A Japanese Patent No. 3348965

  The present invention solves the above-mentioned problems of the prior art, and provides a non-heat treated steel suitable for a material for stably producing a mechanical structural component having an excellent strength-toughness balance and a method for producing the same. Objective.

In order to solve the above-mentioned problems, the present inventors diligently studied the influence of components and structure on strength and toughness after hot working. As a result, the following items a) to c) were found.
a) An excellent strength-toughness balance can be obtained by making the steel structure after hot working a bainite having an area ratio of 70% or more.
b) In order to improve toughness, it is effective to reduce the amount of solute C in the structure in addition to the bainite structure of a). Therefore, it is necessary to add Ti which reduces the amount of dissolved C by carbide formation in a predetermined amount or more according to the C content in the steel material.
c) Increasing the amount of Ti carbide that becomes insoluble during heating in hot working can exhibit the pinning effect due to the precipitate, and a fine bainite structure can be obtained while remaining unheated. On the other hand, when Ti and C are added excessively, the austenite grains are remarkably refined and the hardenability is lowered, so that it becomes difficult to obtain the desired bainite structure described in the above a). Further, when the amount of undissolved Ti carbide is small, the austenite grains are coarsened, so that the toughness is inferior to that of the fine bainite structure. Therefore, in order to stably obtain an excellent strength-toughness balance, it is necessary to strictly control the contents of C and Ti.

This invention is made | formed based on said knowledge, The summary structure is as follows.
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: 0.005-0.150%,
Cr: 0.10-2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less in a 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-tempered steel having a composition of Fe and unavoidable impurities and a structure with an area ratio of bainite phase of 70% or more.
T1 = (C / 12) / (Ti / 48) (1)
T2 = -7430 / [Log (CxTi) -3.23] (2)
Here, each element symbol indicates the content (% by mass) of the element.

2. The component composition is further in mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
2. The non-heat treated steel according to 1 above, comprising one or more selected from W: 0.3% or less and B: 0.0100% or less.

3. The component composition is further in mass%.
Cu: 1.0% or less and
Ni: The non-tempered steel according to 1 or 2 above, comprising one or more 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: 0.005-0.150%,
Cr: 0.10-2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less in a 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 remainder is hot-worked at a heating temperature of 1300 ° C or lower and a finishing temperature of 850 ° C or higher and 1300 ° C or lower on a steel material having a component composition of Fe and inevitable impurities, and then the temperature range of 700-550 ° C is 0.5 ° C / s. The manufacturing method of the non-heat-treated steel cooled above.
T1 = (C / 12) / (Ti / 48) (1)
T2 = -7430 / [Log (CxTi) -3.23] (2)
Here, each element symbol indicates the content (% by mass) of the element.

5. The component composition is further in mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
5. The method for producing non-heat treated steel as described in 4 above, comprising one or more selected from W: 0.3% or less and B: 0.0100% or less.

6). The component composition is further in mass%.
Cu: 1.0% or less and
Ni: The method for producing a non-tempered steel as described in 4 or 5 above, comprising at least one selected from 1.0% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the non-heat-treated steel suitable for the raw material for producing stably the mechanical structural component excellent in the strength-toughness balance, and its manufacturing method can be provided. That is, when various structural parts used for transportation machines and construction machines are produced using the steel of the present invention, it is possible to mass-produce parts that are not tempered and that have high strength and toughness.

It is a figure which shows the shape of a rotation bending fatigue test piece. It is a graph for comparing an intensity | strength-toughness balance with an invention example and a comparative example.

First, the reason why the component composition of steel is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding a component shall mean the mass%.
C: 0.10 to 0.25%
C is an element effective for refining the structure because it forms Ti carbide that stably exists together with Ti at the heating temperature during hot working. Moreover, since it contributes also to the production | generation of a bainite phase and strength improvement, addition of 0.10% or more is required. On the other hand, if the C content exceeds 0.25%, the toughness of the bainite phase decreases due to an increase in the amount of solute C, so the C content is 0.25% or less. Preferably it is 0.14% or more and 0.22% or less of range.

Si: 0.10 to 1.00%
Since Si is effective not only for deoxidation but also for the formation of a bainite phase, it is added at 0.10% or more. On the other hand, if the Si content exceeds 1.00%, it dissolves in the ferrite phase and bainite phase, and its solid solution hardening deteriorates toughness and machinability, so the Si content is 1.00% or less. Preferably it is 0.10% or more and 0.50% or less of range.

Mn: 0.30 to 3.00%
Mn is added because it is effective in generating a bainite phase and improving the strength. However, if the amount of Mn is less than 0.30%, the amount of bainite phase produced becomes small and it becomes difficult to ensure toughness. Therefore, Mn is added at 0.30% or more. On the other hand, if it exceeds 3.00%, the hardenability is remarkably improved, and a hard layer such as island martensite that adversely affects toughness is formed. Therefore, the Mn content is 3.00% or less. Preferably it is 0.80% or more and 2.00% or less of range.

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

S: 0.005% to 0.150%
S forms sulfides with Mn and has an effect of improving machinability. For that purpose, it is made to contain 0.005% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was made 0.150%. Preferably it is 0.010% or more and 0.060% or less of range.

Cr: 0.10-2.50%
Cr is added because it is effective for generating a bainite phase and improving the strength. However, if the Cr content is less than 0.10%, the amount of bainite phase produced is reduced, and it becomes difficult to ensure toughness. Therefore, the Cr content is 0.10% or more. On the other hand, if it exceeds 2.50%, the hardenability is remarkably improved, and a hard layer such as island martensite that adversely affects toughness is formed. Therefore, the Cr content is 2.50% or less. Preferably, it is 0.50% or more and 1.50% or less of range.

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

Ti: 0.25 to 0.60%
Ti is an element that is effective in refining the structure because it forms Ti carbide that stably exists together with C at the heating temperature during hot working. In order to exert such effects, addition of 0.25% or more is necessary. On the other hand, the effect is saturated at 0.60%, and addition exceeding 0.60% leads to an increase in cost. Therefore, the Ti content is limited to the range of 0.25 to 0.60%. Preferably it is 0.30 to 0.50% of range.

N: 0.0070% or less N is an element which forms TiN by combining with Ti and contributes to the refinement of austenite crystal grains. However, the formation of TiN not only reduces the amount of Ti carbide that contributes to the refinement of austenite grains at the hot working temperature, but if excessively added, bubbles are generated in the steel ingot during solidification, and forgeability Therefore, the upper limit is made 0.0070%. Preferably it is 0.0020 to 0.0070% of range.

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

  That is, because the toughness of non-tempered steel having a bainite structure decreases with an increase in the amount of solute C, Ti that decreases the amount of solute C through carbide formation depends on the amount of C added in the steel. It is necessary to add more than a fixed amount. Here, T1 described above is a parameter representing the amount of solute C in the bainite structure. If T1 exceeds 3.0, the amount of solute C increases, and thus the effect of improving toughness cannot be obtained. Therefore, T1 is set to 3.0 or less.

  Next, if the amount of Ti carbide that becomes insoluble at the time of heating during hot working is increased, the pinning effect due to the precipitate is exhibited, and a fine bainite structure can be obtained while remaining unheated. Here, the above-mentioned T2 is a parameter representing the necessary amount of Ti carbide that contributes to the refinement, and when T2 is 1620 or less, the amount of Ti carbide is insufficient and the pinning force is reduced, so the austenite grains are coarsened. The toughness will decrease. On the other hand, if T2 exceeds 1750, the austenite grains are remarkably refined and the hardenability is lowered, so that it is difficult to obtain a bainite structure having an area ratio of 70% or more. Therefore, T2 is set to 1620-1750.

  The components in steel in the present invention include the above components, and the balance is Fe and inevitable impurities. Furthermore, the following selection components can be added for the purpose of imparting other characteristics and the like within a range not impairing the effects of the present invention.

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 added over 1.0%, so the upper limit is preferably made 1.0%. More preferably, it is 0.10 to 0.50% of range.

Nb: 0.3% or less
Nb contributes to improvement of toughness and fatigue strength by refining crystal grains and strengthening grain boundaries. Therefore, it is preferable to add 0.05% or more. On the other hand, if the effect exceeds 0.3%, the effect becomes saturated and the cost increases, so the upper limit is preferably made 0.3%. More preferably, it is 0.10 to 0.20% of range.

V: 0.3% or less V, like Nb, is a carbonitride-forming element that refines crystal grains and strengthens grain boundaries to contribute to improvements in toughness and fatigue strength. Therefore, it is preferable to add 0.05% or more. On the other hand, if the effect exceeds 0.3%, the effect becomes saturated and the cost increases, so the upper limit is preferably made 0.3%. More preferably, it is 0.10 to 0.20% of range.

W: 0.3% or less W forms precipitates together with Ti and contributes to improvement of toughness and fatigue strength. Therefore, it is preferable to add 0.05% or more. On the other hand, if the effect exceeds 0.3%, the effect becomes saturated and the cost increases, so the upper limit is preferably made 0.3%. More preferably, it is 0.10 to 0.20% of range.

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

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

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

Next, the reason why the steel structure of the non-heat treated steel in the present invention is limited to the above range will be described.
Bainitic phase: 70% or more in area ratio with respect to the entire structure In the present invention, it is important that the bainite phase is 70% or more in area ratio with respect to the entire structure. In the present invention, Ti carbide which is insoluble in the temperature range is utilized for pinning austenite grains in the hot working temperature range. Since the precipitate exists in a non-matching manner, the elastic strain energy (matching strain) is small, and the risk of toughness reduction due to the precipitate can be minimized. On the other hand, if fine Ti carbides precipitate in the cooling process after hot working, it is disadvantageous from the viewpoint of improving toughness. In this respect, in the bainite transformation process, Ti carbide is less likely to be generated in the matrix phase than in the ferrite-pearlite transformation process. Therefore, the steel structure of the non-heat treated steel of the present invention is mainly composed of a bainite phase. Specifically, the bainite phase is 70% or more in terms of the area ratio with respect to the entire structure. Preferably it is 80% or more, more preferably 90% or more. It may be 100%. In addition, as a structure other than the bainite phase, a ferrite phase, a pearlite phase, a martensite phase, or the like can be considered, but it goes without saying that the smaller the structure, the better.

  Here, the area ratio of each phase can be determined as follows. That is, a test piece was taken from the obtained non-heat treated steel, and a vertical cross section (L cross section) parallel to the rolling direction was corroded with nital after polishing, and using an optical microscope and a scanning electron microscope (SEM), L The type of phase is identified by cross-sectional structure observation (400-times optical microscope structure observation), and the area ratio of each phase is obtained. Note that island martensite is difficult to determine with an optical microscope and is therefore observed using an SEM. Since the island-like martensite is observed as a white and floating portion by SEM, it can be calculated from the average value of the area ratios by image processing of a microstructure photograph of at least 5 views.

Next, the manufacturing conditions of the present invention will be described.
That is, after heating the steel material having the above-described component composition, it is subjected to hot working at a finishing temperature of 850 ° C. to 1300 ° C., and then cooled to a temperature range of 700 to 550 ° C. at 0.5 ° C./s or more. Produces tempered steel. Hereinafter, requirements for each manufacturing process will be described.
[Steel heating temperature: 1300 ℃ 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 carbide that contributes to the refinement of the structure, so the heating temperature is set to 1300 ° C or less. On the other hand, the lower limit is preferably set to 900 ° C. or higher in order to ensure the finishing temperature in the next hot working. The steel material to be heated may be a steel slab, or a steel slab or steel bar that has been subjected to hot rolling or hot forging.

[Finish temperature in hot working: 850 ℃ to 1300 ℃]
When this finishing temperature is less than 850 ° C., a ferrite structure is formed, which is disadvantageous for making the parent phase 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. For this reason, finishing temperature shall be 850 degreeC or more. Further, when the finishing temperature exceeds 1300 ° C., it becomes difficult to secure the amount of Ti carbide that contributes to the refinement of the structure, so the upper limit of the finishing temperature is 1300 ° C.

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

[Cooling: Temperature range of 700-550 ° C over 0.5 ° C / s]
In order to obtain the above-described bainite structure so that fine precipitates are not deposited after hot working and the toughness is not impaired, it is necessary to regulate the cooling rate after hot working. That is, it is necessary to cool a temperature range of 700 to 550 ° C., which is a precipitation temperature range of the fine precipitate, at a critical cooling rate (0.5 ° C./s) or more at which the fine precipitate is obtained.

Hereinafter, according to an Example, the structure and effect of this invention are demonstrated more concretely. It should be noted that the present invention is not limited by the following examples, and can be appropriately changed within a range that can be adapted to the gist of the present invention, and these are all included in the technical scope of the present invention. It is.
Steel with the composition shown in Table 1 (No. 1-43) was melted in a 150kg vacuum melting furnace and the steel material obtained by continuous casting was 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 50 mmφ round bar steel. The obtained round bar is further heated at various temperatures, hot forged at 1150 ° C with a finish reduction of 40% and finished to 30 mmφ bar, and then 1.2 ° C / s or 0.1 ° C / s (Table 2).
Here, Steel Nos. 1 to 23 shown in Table 1 are invention steels whose component compositions satisfy the present invention, and Steel Nos. 24 to 43 are comparative steels whose component compositions do not satisfy the present invention. Table 2 Nos. 44 to 46 in the comparative examples are comparative examples in which any one of the cooling rate after forging, the heating temperature before hot forging, and the finishing temperature of hot forging deviates from the specified value of the present invention.

  The obtained steel bar was subjected to a structure observation, a tensile test, a Charpy impact test, and an Ono-type rotary bending fatigue test. The details of each survey are described below.

Microstructure observation In microstructural observation, specimens were 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 was corroded with nital after polishing, using an optical microscope and SEM. The type of phase was identified by cross-sectional structure observation, and the area ratio of each phase was determined.

Tensile test In the tensile test, a JIS No. 4 tensile test piece was taken from the center of a 30 mmφ steel bar and examined for tensile strength.

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

Rotating bending fatigue characteristics A test piece having a parallel part diameter of 6 mm shown in FIG. 1 was taken from a steel bar having a diameter of 30 mmφ. With respect to the obtained test piece, the rotational bending fatigue strength was measured using an Ono-type rotary bending fatigue tester at a rotation speed of 3000 rpm and 10 ⁷ times as the fatigue limit.

  Table 2 and FIG. 2 show the results of the above investigation. The inventive examples (Nos. 1 to 23) were found to have an excellent strength-toughness balance with respect to the comparative examples (Nos. 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: 0.005-0.150%,
Cr: 0.10-2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less in a 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-tempered steel having a composition of Fe and unavoidable impurities and a structure with an area ratio of bainite phase of 70% or more.
T1 = (C / 12) / (Ti / 48) (1)
T2 = -7430 / [Log (CxTi) -3.23] (2)
Here, each element symbol indicates the content (% by mass) of the element. The component composition is further in mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The non-tempered steel according to claim 1, comprising one or more selected from W: 0.3% or less and B: 0.0100% or less. The component composition is further in mass%.
Cu: 1.0% or less and
Non-tempered steel according to claim 1 or 2, comprising one or more selected from Ni: 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: 0.005-0.150%,
Cr: 0.10-2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less in a 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 remainder is hot-worked at a heating temperature of 1300 ° C or lower and a finishing temperature of 850 ° C or higher and 1300 ° C or lower on a steel material having a component composition of Fe and inevitable impurities, and then the temperature range of 700-550 ° C is 0.5 ° C / s. The manufacturing method of the non-heat-treated steel cooled above.
T1 = (C / 12) / (Ti / 48) (1)
T2 = -7430 / [Log (CxTi) -3.23] (2)
Here, each element symbol indicates the content (% by mass) of the element. The component composition is further in mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The method for producing a non-heat treated steel according to claim 4, comprising one or more selected from W: 0.3% or less and B: 0.0100% or less. The component composition is further in mass%.
Cu: 1.0% or less and
Ni: The manufacturing method of the non-heat-treated steel of Claim 4 or 5 containing 1 or more types selected from 1.0% or less.

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