JP2021143416A - Method for producing steel component having locally softened part - Google Patents

Abstract

To provide a method for producing a locally softened high strength steel component without performing local temperature control.SOLUTION: A method for producing a steel component comprises: a step of preparing a steel sheet having a chemical composition consisting of, by mass, 0.05 to 0.40% C, 0 to 2.0% Si, 1.0 to 3.0% Mn, 0.010 to 1.0% Al, 0.0005 to 0.010% B, and the balance iron with inevitable impurities; a step of heating the steel sheet to a temperature of an Ac1 point (°C) to an Ac3 point (°C)+below 10°C; a working step of applying stain by 0.5% or higher at a temperature of 675°C to an AC3 point+below 10°C after the step of heating: a step of performing holding or gradual cooling for 1 to 120 sec at an average cooling velocity of 0 to 15°C/sec after the working step; and a step of performing cooling to an Ms point (°C)-50°C after the step of performing holding or gradual cooling. The average cooling velocity from the temperature of the step of heating to an Ms point (°C)-50°C is controlled to 10°C/sec or higher.SELECTED DRAWING: Figure 3

Description

The present disclosure relates to a method of manufacturing a steel part having a locally softened portion.

In recent years, in order to protect occupants in the event of a vehicle collision, there is a need for a technique for preferentially deforming a specific part in the event of a collision while maintaining high strength of the entire automobile frame component. Therefore, there is a demand for high-strength steel parts in which a specific portion is locally softened and / or a method for manufacturing the same, which are used in the art.

Patent Document 1 discloses a method of covering a portion to be softened when a steel sheet is heated to an austenite single-phase temperature range. As a result, the portion covered with the heat shield is less than the austenite single-phase temperature range even during heating, and the martensitic transformation of the portion after quenching is suppressed, and the portion is compared with the portion not covered with the heat shield. Softens.

Patent Document 2 discloses a method of providing a portion where the contact between the steel sheet and the mold is poor when the steel plate is brought into contact with the mold from the austenite single-phase temperature range and rapidly cooled. As a result, a soft structure (ferrite and / or pearlite) is precipitated in the portion, and the portion is softened.

JP-A-2017-78189 Japanese Unexamined Patent Publication No. 2011-179028

In Patent Documents 1 and 2, it is not possible to soften only the portion to be softened due to the influence of heat transfer or the like in the steel sheet. For example, in Patent Document 1, only the portion covered with the heat shield cover should be softened to be less than the austenite single-phase temperature range, but at the end of the portion covered with the heat shield cover, from the adjacent portion not covered with the heat shield cover. Since heat is transferred, as a result, it cannot be sufficiently softened at the end of the portion covered with the heat shield cover. In Patent Document 2, only the portion having poor contact with the mold should be softened without quenching, but heat is transferred from the portion to the adjacent portion having good contact with the mold, resulting in the mold and the mold. A softening effect can also be exerted on the adjacent portion of the. Therefore, it is difficult to locally soften only the portion to be softened by the method of softening by local temperature control as in Patent Documents 1 and 2.

The embodiment of the present invention has been made in view of such a situation, and one of the objects thereof is to produce a locally softened high-strength steel part without local temperature control. To provide a method.

Aspect 1 of the present invention is
C: 0.05 to 0.40% by mass,
Si: 0 to 2.0% by mass,
Mn: 1.0 to 3.0% by mass,
Al: 0.010 to 1.0% by mass,
P: More than 0% by mass and less than 0.100% by mass,
S: More than 0% by mass and 0.010% by mass or less,
N: More than 0% by mass and 0.010% by mass or less,
B: 0.0005 to 0.010% by mass, and the balance: a step of preparing a steel sheet having a chemical composition consisting of iron and unavoidable impurities, and
A step of heating the steel sheet to a temperature of 1 point (° C.) or more and 3 points (° C.) of Ac and less than 10 ° C.
After the heating step, a machining step of adding 0.5% or more of strain at a machining temperature of 675 ° C. or higher and Ac 3 points (° C.) + 10 ° C. or lower,
After the processing step, a step of holding at the processing temperature for 1 second or more and 120 seconds or less, or a step of slowly cooling for 1 second or more and 120 seconds or less at an average cooling rate of more than 0 ° C./sec and 15 ° C./sec or less.
After the step of holding or slowly cooling, the step of cooling to the Ms point (° C.) -50 ° C. is included.
This is a method for manufacturing steel parts, in which the average cooling rate from the temperature of the heating step to the Ms point (° C.) -50 ° C. is controlled to 10 ° C./sec or more.

Aspect 2 of the present invention
C: 0.05 to 0.40% by mass,
Si: 0 to 2.0% by mass,
Mn: 1.0 to 3.0% by mass,
Al: 0.010 to 1.0% by mass,
P: More than 0% by mass and less than 0.100% by mass,
S: More than 0% by mass and 0.010% by mass or less,
N: More than 0% by mass and 0.010% by mass or less,
B: 0.0005 to 0.010% by mass, and the balance: a step of preparing a steel sheet having a chemical composition consisting of iron and unavoidable impurities, and
A step of heating the steel sheet to a temperature of Acc 3 points (° C.) + 10 ° C. or higher and 1100 ° C. or lower, and
After the heating step, a machining step of applying a strain of 10% or more at a machining temperature of Ms point (° C.) + 50 ° C. or higher and Ac 3 point (° C.) + 10 ° C.
After the processing step, a step of holding at the processing temperature for 1 second or more and 120 seconds or less, or a step of slowly cooling for 1 second or more and 120 seconds or less at an average cooling rate of more than 0 ° C./sec and 15 ° C./sec or less.
After the step of holding or slowly cooling, the step of cooling to the Ms point (° C.) -50 ° C. is included.
This is a method for manufacturing steel parts, in which the average cooling rate from the temperature in the heating step to the Ms point (° C.) -50 ° C. is controlled to 10 ° C./sec or more.

In aspect 3 of the present invention, the steel plate is
The production method according to Aspect 1 or 2, further comprising one or more selected from the group consisting of Cu: more than 0% by mass and 0.50% by mass or less, and Ni: more than 0% by mass and 0.50% by mass or less. ..

In the fourth aspect of the present invention, the steel plate is
Ti: More than 0% by mass and less than 0.10% by mass,
10. Described in any one of aspects 1 to 3 further containing one or more selected from the group consisting of Cr: more than 0% by mass and 3.0% by mass or less, and Nb: more than 0% by mass and 0.10% by mass or less. It is a manufacturing method of.

Aspect 5 of the present invention is the manufacturing method according to any one of aspects 1 to 4, which comprises applying the strain by overhang molding.

Aspect 6 of the present invention is the manufacturing method according to any one of aspects 1 to 4, which comprises applying the strain by forging.

Aspect 7 of the present invention is the manufacturing method according to any one of aspects 1 to 4, which comprises applying the strain by bending back at the time of draw molding.

Aspect 8 of the present invention is the manufacturing method according to any one of aspects 1 to 4, which comprises applying the strain by shearing.

Aspect 9 of the present invention is the manufacturing method according to any one of aspects 1 to 8, which comprises applying the strain by a plurality of times of processing.

Aspect 10 of the present invention is the manufacturing method according to aspect 9, wherein the plurality of times of processing includes a process of applying a deformation and a process of performing the process so as to return the deformation.

According to an embodiment of the present invention, it is possible to provide a method for producing locally softened high-strength steel parts without local temperature control.

FIG. 1 is a graph showing the relationship between temperature and displacement when a steel sheet is heated from a low temperature in a four-master test. FIG. 2 is a graph showing the relationship between temperature and displacement when the steel sheet is cooled from a high temperature in the Formaster test in addition to the relationship shown in FIG. FIG. 3 is a schematic view showing a sampling position of an evaluation sample of an example. FIG. 4 is a schematic cross-sectional view taken along line XX shown in FIG.

The inventors of the present application have studied from various angles in order to realize a method for manufacturing locally softened high-strength steel parts without local temperature control.

As a result, a steel plate having a predetermined chemical composition is heated to a state in which austenite is relatively unstable, such as a two-phase region of austenite and ferrite, and a slight strain is applied to the portion to be softened, thereby only the portion to be softened. We have found a production method for growing soft structure from the portion by promoting nucleation of soft structure (ferrite and / or pearlite) and holding or slowly cooling for a certain period of time (hereinafter, the first embodiment of the present invention). ).

Further, in the above heating, even when the austenite such as the austenite single-phase region is heated to a relatively stable state, by applying a relatively large strain to the portion to be softened, the same as in the first embodiment of the present invention. At the same time, it was also found that nucleation of soft tissue can be promoted only in the portion to be softened (hereinafter, referred to as Embodiment 2 of the present invention).

The details of each requirement defined in Embodiments 1 and 2 of the present invention are shown below.
In the present specification, the “steel part” refers to a steel plate processed into a predetermined shape by the processing steps of the first and second embodiments of the present invention.

<Embodiment 1 of the present invention>
The production method according to the first embodiment of the present invention is
(A) The process of preparing the steel plate and
(B) After the step (a), the step of heating and
(C) After the step (b), the step of processing and
(D) After the step (c), the step of holding or slowly cooling, and
(E) After the step (d), the step of cooling and
including.
Hereinafter, each step will be described.

(A) Step of preparing a steel plate The chemical composition of the steel plate according to the first embodiment of the present invention is C: 0.05 to 0.40% by mass, Si: 0 to 2.0% by mass, Mn: 1.0 to 1. 3.0% by mass, Al: 0.010 to 1.0% by mass, P: more than 0% by mass and 0.100% by mass or less, S: more than 0% by mass and 0.010% by mass or less, N: more than 0% by mass It consists of 0.010% by mass or less, B: 0.0005 to 0.010% by mass, and the balance: iron and unavoidable impurities.
Hereinafter, each element will be described in detail.

(C: 0.05 to 0.40% by mass)
The C content determines the strength of the steel part. In order to obtain sufficient strength of the steel part, the C content is 0.05% by mass or more, preferably 0.10% by mass or more, and more preferably 0.20% by mass or more.

On the other hand, when the C content becomes excessive, the toughness of the steel part is remarkably lowered, and the delayed fracture of the steel part is likely to occur. Therefore, the C content is 0.40% by mass or less, preferably 0.38% by mass or less, and more preferably 0.36% by mass or less.

(Si: 0 to 2.0% by mass)
Si is an element optionally contained in the steel sheet. Si contributes to the hardness stability of the steel sheet by increasing the temper softening resistance. Therefore, it is preferable that Si is contained in the steel sheet in an amount of more than 0% by mass.

On the other hand, Si facilitates the formation of retained austenite (γ) and promotes a decrease in yield strength (YS) and segregation of Mn. Therefore, the Si content is 2.0% by mass or less, preferably 1.8% by mass or less.

(Mn: 1.0 to 3.0% by mass)
Mn contributes to increasing the strength of steel parts by enhancing the hardenability of the steel sheet. In order to exert this effect, the Mn content is 1.0% by mass or more, preferably 1.2% by mass or more, and more preferably 1.4% by mass or more.

On the other hand, if the Mn content is excessive, coarse carbides may be deposited in the steel parts. Therefore, the Mn content is 3.0% by mass or less, preferably 2.8% by mass or less, and more preferably 2.6% by mass or less.

(Al: 0.010 to 1.0% by mass)
Al is an element that acts as an antacid. In order to exert such an effect, the amount of Al is 0.010% by mass or more. The amount of Al is preferably 0.020% by mass or more, more preferably 0.025% by mass or more. However, the excessive content of Al leads to an increase in manufacturing cost, significantly increases the Ac3 point, and causes deterioration of surface quality (decarburization and thinning) due to a high temperature of the material heating temperature. Therefore, the amount of Al is set to 1.0% by mass or less. The amount of Al is preferably 0.80% by mass or less, and more preferably 0.70% by mass or less.

(P: More than 0% by mass and 0.100% by mass or less)
P is an element that is inevitably contained and deteriorates the weldability of the steel sheet, but is also an element that has an effect of contributing to the solid solution strengthening of the ferrite phase. In order to exert such an effect and not deteriorate the weldability of the steel sheet, the amount of P is set to 0.100% by mass or less. The amount of P is preferably 0.050% by mass or less, and more preferably 0.020% by mass or less. Note that P is an impurity that is inevitably mixed in steel, and it is impossible for industrial production to reduce the amount to 0% by mass, and usually exceeds 0% by mass, and further 0.00050% by mass or more. Can be contained in.

(S: More than 0% by mass and 0.010% by mass or less)
S is an element that is inevitably contained and deteriorates the weldability of the steel sheet. Therefore, the amount of S is set to 0.010% by mass or less. The amount of S is preferably 0.0080% by mass or less, and more preferably 0.0050% by mass or less. Since the amount of S should be as small as possible, the lower limit is not particularly limited, but it is impossible for industrial production to set the amount to 0% by mass, and usually exceeds 0% by mass, further 0.00010% by mass or more. Can be contained in.

(N: More than 0% by mass and 0.010% by mass or less)
N is an element that is inevitably contained, and when it is contained in an excessive amount, AlN is generated and the deoxidizing effect of Al is reduced. Therefore, the amount of N is set to 0.010% by mass or less. The amount of N is preferably 0.0080% by mass or less, and more preferably 0.0050% by mass or less. Since the amount of N should be as small as possible, the lower limit is not particularly limited, but it is impossible for industrial production to set the amount to 0% by mass, and usually exceeds 0% by mass, further 0.00010% by mass or more. Can be contained in.

(B: 0.0005 to 0.010% by mass)
B contributes to increasing the strength of steel parts by enhancing the hardenability of the steel sheet. In order to exert this effect, the B content is 0.0005% by mass or more, preferably 0.0010% by mass or more, and more preferably 0.0015% by mass or more.

On the other hand, when the B content becomes excessive, a coarse iron boron compound is precipitated, and the toughness of the steel part is lowered. Therefore, the B content is 0.010% by mass or less, preferably 0.0080% by mass or less, and more preferably 0.0060% by mass or less.

(Remaining: iron and unavoidable impurities)
In one preferred embodiment, the balance is iron and unavoidable impurities. Inevitable impurities are elements that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like.
It should be noted that, for example, there are elements such as P, S and N, which are usually preferable as the content is smaller, and therefore are unavoidable impurities, but the composition range thereof is separately specified as described above. Therefore, in the present specification, the term "unavoidable impurities" constituting the balance is a concept excluding elements whose composition range is separately defined.

Further, the steel sheet according to the first embodiment of the present invention may selectively contain the following optional elements as needed, and the characteristics of the steel parts are further improved according to the contained components.

(Cu: 1 or more selected from the group consisting of more than 0% by mass and 0.50% by mass or less, and Ni: more than 0% by mass and 0.50% by mass or less)
By containing Cu, the corrosion resistance of the steel sheet itself is improved, so that hydrogen generation due to corrosion of the steel sheet can be suppressed and the delayed fracture resistance can be improved. Cu also has the effect of promoting the formation of iron oxide: α-FeOOH, which is said to be thermodynamically stable and protective even in the rust generated in the atmosphere. By promoting the formation of the rust, it is possible to suppress the invasion of the generated hydrogen into the steel sheet, and it is possible to suppress the promoted cracking due to hydrogen in a harsh corrosive environment. Therefore, the Cu content is preferably more than 0% by mass, more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. On the other hand, if the Cu content is excessive, the plating property in the plating process at the time of manufacturing the steel sheet and the chemical conversion treatment property after hot stamping deteriorate. Therefore, the Cu content is preferably 0.50% by mass or less.
It is known that Ni has the same effect as Cu. Therefore, the Ni content is preferably more than 0% by mass, more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. On the other hand, the Ni content is preferably 0.50% by mass or less.

(Ti: 1 or more selected from the group consisting of more than 0% by mass and 0.10% by mass or less, Cr: more than 0% by mass and 3.0% by mass or less, and Nb: more than 0% by mass and 0.10% by mass or less)
Ti reduces the amount of BN produced in the steel sheet by producing TiN. As a result, the amount of the solid solution B in the steel sheet is increased, and the effect of improving the hardenability by B can be enhanced. In order to exert this effect, the Ti content is preferably more than 0% by mass, more preferably 0.0050% by mass or more, still more preferably 0.0250% by mass or more and 0.050% by mass. That is all.
On the other hand, if Ti is excessively contained in the steel sheet, carbides are precipitated at the grain boundaries, and the hardenability of the steel sheet is deteriorated. Therefore, the Ti content is preferably 0.10% by mass or less, more preferably 0.080% by mass or less, and further preferably 0.070% by mass or less.

Cr contributes to ensuring hardness and suppressing precipitation of coarse carbides during cooling. In order to exert these effects, the Cr content is preferably more than 0% by mass.
On the other hand, if Cr is excessively contained in the steel sheet, cracking of the steel sheet may occur, and the Cr content is preferably 3.0% by mass or less, more preferably 2.5% by mass or less. Yes, more preferably 2.0% by mass or less.

It is a carbide-forming element and is an element that contributes to the microstructure of steel sheets. Therefore, the Nb content is preferably more than 0% by mass, more preferably 0.0050% by mass or more.
On the other hand, as the structure of the steel sheet becomes finer, reverse transformation during heat treatment is promoted, but ferrite formation is promoted during cooling, which may lead to a decrease in strength of steel parts. Such an effect increases as its content increases. In addition, there is an inconvenience that the cold rollability is deteriorated. From this point of view, it is preferable that Nb is contained in an amount of 0.10% by mass or less. It is preferably 0.070% by mass or less, and more preferably 0.050% by mass or less.

(B) Heating Step In the first embodiment of the present invention, the steel sheet is heated to Ac 1 point (° C.) or more and Ac 3 points (° C.) + 10 ° C.
If it is less than one point of Ac, austenite transformation does not occur, and it becomes difficult to obtain a high-strength steel part after the step (e) cooling described later. On the other hand, when the temperature is set to less than 3 points of Ac + 10 ° C., nucleation of ferrite and / or pearlite, which are soft structures, can be easily promoted in the step (c) processing described later.

The Ac1 point and the Ac3 point can be obtained by investigating the temperature during heating and the displacement history due to the expansion and contraction of the steel due to the heating in the Formaster test. FIG. 1 is a graph showing the relationship between temperature and displacement when a steel sheet is heated from a low temperature in a four-master test. In the low temperature region, the steel can expand linearly with an increase in temperature at an expansion coefficient corresponding to the crystal structure (bcc) of ferrite. Further temperature increases can produce austenite with a denser crystal structure (fcc) and begin to shrink. The temperature at which the temperature begins to deviate from the straight line can be set as the Ac1 point. In the high temperature region where the temperature is further increased, all the ferrites are transformed into austenite, and can expand linearly again at an expansion rate according to the crystal structure of austenite. The temperature at which expansion begins along the straight line can be set to the Ac3 point.

(C) Processing step After the above-mentioned (b) heating step, processing is performed in which a strain of 0.5% or more is applied at a temperature of 675 ° C. or higher and Ac 3 points + 10 ° C. or lower.
At the above temperatures, a large number of grain boundaries, which are nucleation sites of ferrite and / or pearlite, which are soft structures, may be present in the steel sheet. By applying a slight strain (that is, 0.5% or more) in such an unstable state, nucleation of ferrite and / or pearlite, which is a soft structure, is remarkably promoted in the strained portion. can do. More preferably, a strain of 5.0% or more is applied, and even more preferably, a strain of 9.0% or more is applied.
The strain can be calculated by the following equation (1).

Strain (%) = | (d ₀ − d ₁ ) / d ₀ × 100 | ・ ・ ・ (1)

d ₀ is the plate thickness of the steel plate before processing or the plate thickness of the unprocessed portion of _{the steel plate after processing, and d 1} is the plate thickness of the processed portion of the steel plate after processing. In each case, the unit is mm.
The strain may be, for example, the equivalent plastic strain obtained by FEM analysis. That is, if the equivalent plastic strain obtained by FEM analysis is 0.5% or more, it can be softened in the same manner.

The Ms point can be obtained by investigating the temperature during cooling and the displacement history due to expansion and contraction of the steel due to the cooling in the Formaster test. FIG. 2 is a graph showing the relationship between temperature and displacement when the steel sheet is cooled at a relatively high cooling rate after the heating, in addition to the temperature-displacement relationship during heating described in FIG. In the medium and high temperature range, steel can shrink linearly with a temperature drop at a shrinkage rate according to the crystal structure of austenite. When the temperature is further lowered, it can transform into martensite and begin to expand. The temperature at which the temperature begins to deviate from the straight line can be set as the Ms point.

If the heating temperature in the step (b) heating is set to Ac 1 point (° C.) or more and Ac 3 points (° C.) + 10 ° C., and the processing temperature is set to less than 675 ° C., the transformation into a soft structure becomes active. The softening of the non-processed portion is also remarkable, and it becomes difficult to manufacture the locally softened steel part only in the processed portion.
When the heating temperature in the above (b) heating step is set to Ac 1 point (° C.) or higher and Ac 3 points (° C.) + 10 ° C., and the processing temperature is set to Ac 3 points + 10 ° C. or higher, crystals that are nucleation sites of soft tissue. Grain boundaries are reduced, and even a slight strain cannot promote nucleation of soft tissue.

The processing temperature may be the same as or different from the heating temperature of the step (b) heating. If different, an additional heating and / or cooling step may be included between the steps (b) and (c) above. Further, a step of holding the temperature at a constant temperature may be included after the step (b) and before the step (c).

The above processing may be any processing, and for example, press processing, overhang forming, forging, bending back at the time of draw forming, shearing and the like are preferably used.

(D) Step of holding or slowly cooling After the step of (c) processing described above, holding or slowly cooling is performed at an average cooling rate of 0 to 15 ° C./sec for 1 second or more and 120 seconds or less. That is, it is held at the above processing temperature for 1 second or more and 120 seconds or less, or slowly cooled for 1 second or more and 120 seconds or less at an average cooling rate of more than 0 ° C./sec and 15 ° C./sec or less. As a result, ferrite and / or pearlite, which are soft structures nucleated in the process (c) described above, can be grown.

If the average cooling rate is greater than 15 ° C./sec, or if the retention or slow cooling time is less than 1 second, the soft structure ferrite and / or pearlite cannot be sufficiently precipitated and grown. The holding time or slow cooling time is preferably more than 1 second, more preferably 3 seconds or more, and further preferably 6 seconds or more.
On the other hand, if the holding or slow cooling time exceeds 120 seconds, ferrite and / or pearlite, which are soft structures, precipitate and grow even in the unprocessed portion, and high-strength steel parts cannot be obtained. It is preferably 12 seconds or less.

(E) Cooling step After the above-mentioned (d) holding or slow cooling step, cooling is performed to the Ms point (° C.) -50 ° C. At this time, the average cooling rate from the heating temperature of the step (b) heating (that is, Ac1 point (° C.) or more and Ac3 point (° C.) + 10 ° C. or less) to Ms point (° C.) -50 ° C. is 10 ° C./. Control over seconds. As a result, martensitic transformation can occur at least in the unprocessed portion, and the strength of the unprocessed portion can be sufficiently ensured. If cooling at an average cooling rate of 10 ° C./sec or higher is completed at the Ms point (° C.) -50 ° C. or higher, martensitic transformation cannot be sufficiently caused in the unprocessed portion. Further, even if the average cooling rate is less than 10 ° C./sec, martensitic transformation cannot be sufficiently caused in the unprocessed portion.

After the step (e) of cooling, it can be cooled to room temperature, for example. The cooling rate from the Ms point (° C.) -50 ° C. to room temperature is not particularly limited.

<Embodiment 2 of the present invention>
The production method according to the second embodiment of the present invention is different from the production method according to the first embodiment of the present invention in terms of (b) heating step and (c) processing step. Hereinafter, those steps different from the first embodiment of the present invention will be described below as (b') heating steps and (c') processing steps.

(B') Heating Step In the second embodiment of the present invention, the steel sheet is heated to Ac 3 points (° C.) + 10 ° C. or higher and 1100 ° C. or lower. Unlike the first embodiment of the present invention, even if the heating is performed at Ac 3 points + 10 ° C. or higher in the heating step, if a relatively large strain is applied in the step of processing (c') described later, the same as the first embodiment of the present invention. Similarly, nucleation of the soft structures ferrite and / or pearlite can be significantly promoted. On the other hand, if the temperature exceeds 1100 ° C., decarburization of the steel surface becomes remarkable and the target strength cannot be secured. In addition, there is a possibility that the thickness will be reduced due to the progress of oxidation. In the case of a plating material, oxidation and alloying proceed, and the hardness of the plating becomes too high, causing problems such as peeling in a subsequent processing process (oxidation of steel sheet, pressing scratches).

(C') Processing step After the above (b') heating step, processing is performed in which strain is applied by 10% or more at a temperature of Ms point (° C.) + 50 ° C. or higher and Ac 3 point (° C.) + 10 ° C. or lower. At Ms point (° C.) + 50 ° C. or higher and Ac 3 point (° C.) + 10 ° C., austenite becomes relatively unstable. Therefore, the strain was applied by applying a relatively large strain (10% or more). Nucleation of the soft structures ferrite and / or pearlite can be significantly promoted in the portions. More preferably, a strain of 15% or more is applied, and even more preferably, a strain of 40% or more is applied. The strain can be calculated by the above equation (1). Further, the strain may be, for example, an equivalent plastic strain obtained by FEM analysis. That is, if the equivalent plastic strain obtained by FEM analysis is 10% or more, it can be softened in the same manner.

At a temperature of Acc3 point + 10 ° C. or higher, austenite becomes a relatively stable state, and even if a relatively large strain is applied, it becomes difficult to promote nucleation of ferrite and / or pearlite, which are soft tissues. On the other hand, if it is less than the Ms point (° C.) + 50 ° C., martensitic transformation may occur, and it becomes difficult to promote nucleation of ferrite and / or pearlite, which are soft tissues.

From (b') the temperature after the heating step (Ac3 point (° C.) + 10 ° C. or higher and 1100 ° C. or lower) to (c') the temperature of the processing step (Ms point (° C.) + 50 ° C. or higher and Ac3 point (° C.) + 10 ° C.) Cooling up to (less than) is not particularly limited, and any average cooling rate may be used. Further, a step of holding the temperature at a constant temperature may be included after the step (b') and before the step (c').

The processing of the above-mentioned (c') processing step may be any processing, and for example, press processing, overhang forming, forging, bending back at the time of draw forming, shearing and the like are preferably used.

In the first and second embodiments of the present invention, the strains in the processing steps (c) and (c') described above may be applied by a plurality of processings.
When the strains in the processing steps (c) and (c') are added by a plurality of times of processing, the strain due to the multiple times of processing can be calculated by the following formula (2).

ｄ_ｎはｎ回目の加工後の鋼板のうち加工部分の板厚であり、単位はｍｍである。
なお、上記式（２）のひずみは、例えば各加工後におけるＦＥＭ解析により求めた相当塑性ひずみの総和としてもよい。

d _n is the thickness of the processed portion of the steel sheet after the nth processing, and the unit is mm.
The strain of the above formula (2) may be, for example, the sum of the equivalent plastic strains obtained by FEM analysis after each processing.

For example, when the processing steps (c) and (c') are single steps, it is difficult to apply a predetermined strain (0.5% or more in the first embodiment and 10% or more in the second embodiment). In some cases. In such a case, it is advantageous to carry out the steps (c) and (c') above by a plurality of times to accumulate the strain, so that the strain can be easily increased to a predetermined value or more.

Further, when the processing steps (c) and (c') are single steps, the transport time from the steps (c) and (c') to the cooling step (e) is less than 1 second. (D) It may be difficult to secure the time (1 second or more) for the step of holding or slowly cooling. In such a case, by performing the above steps (c) and (c') by a plurality of processes, the transfer time between the plurality of processing steps is allocated to the time for holding or slowly cooling the above steps (d). It is advantageous because it can be done.

Further, the plurality of times of processing may include a processing of adding deformation and a processing of returning the deformation. This makes it possible to apply the above strain to the initial steel plate shape without changing the final steel part shape.

When the above-mentioned steps (c) and (c') include a plurality of processes, the above-mentioned (d) holding or slow cooling step may be performed after each process. For example, when two processes are included, a step of performing the first process, then performing the first holding or slow cooling step, and then performing the second process, and then performing the second holding or slow cooling step. May be done. In this case, the total of the time of the first holding or slow cooling step and the time of the second holding or slow cooling step is the time specified in step (d) of Embodiments 1 and 2 of the present invention, that is, It may be 1 second or more and 120 seconds or less.

The temperatures of the steps (a) to (e), (b') and (c') are the surface temperatures of the steel plate (or steel parts) and may be measured using a thermocouple or a radiation thermometer. In addition, the correspondence between the atmospheric temperature of the heating line, etc. and the surface temperature of the steel sheet (or steel part) measured by thermoelectric pair, etc. is investigated in advance, and the atmospheric temperature of the heating line, etc. is used to determine the correspondence between the steel sheet (or steel part). The surface temperature may be read.

According to the first and second embodiments of the present invention, there is provided a method for producing a high-strength steel part in which only a portion to which a predetermined strain or more is strained by processing is locally softened without local temperature control. It is possible to do.

Hereinafter, embodiments of the present invention will be described in more detail with reference to examples. The embodiments of the present invention are not limited by the following examples, and can be carried out with appropriate modifications to the extent that they can meet the above-mentioned and later-described intent, and all of them can be carried out with the present invention. It is included in the technical scope of the embodiment.

Steel type No. in Table 1 Using the steel having the chemical composition shown in A (Ac1 point: 778 ° C., Ac3 point: 875 ° C., Ms point: 385 ° C.), prepare a steel sheet having a thickness of 1.6 mm and an area of 100 mm × 100 mm, and prepare the steel sheet. It was heated to 880 ° C. Then, it was allowed to cool to 750 ° C. at about 12 ° C./sec, and overhang molding was performed at 750 ° C. The overhang molding was performed by pressing a hemispherical punch of φ10 mm from the back surface against the central portion of the steel plate of 100 mm × 100 mm. The overhang height was 3.0 mm. After overhanging, the mixture was slowly cooled for 6 seconds at an average cooling rate of 10.8 ° C./sec. Then, it was water-cooled to the Ms point (° C.) -50 ° C. (that is, 335 ° C.) so that the average cooling rate from 880 ° C. to 335 ° C. was 39.5 ° C./sec. Then it was allowed to cool to room temperature. The above is referred to as Production Example 1-2.
The Ac1 point, Ac3 point, and Ms point were obtained by the Formaster test. The four master test was conducted under the following conditions.
Formaster testing machine: FTM-10 manufactured by Fuji Denpa Koki
Specimen size: Plate thickness 2.0 mm x width 3.0 mm x length 10 mm (However, there are two holes of Φ0.7 mm x depth 2.0 mm for inserting a thermocouple)
Number of tests: 7 times (only the cooling rate is changed, the others are under certain conditions)
Heating rate: 10 ° C / s (room temperature to heating temperature)
Heating temperature: 950 ° C
Holding time at heating temperature: 180 seconds Cooling rate: 2, 5, 10, 15, 20, 30, and 40 ° C./s (heating temperature to room temperature)
Further, in Table 1, the steel type No. Since the Cu content of A was at the level of unavoidable impurities (less than 0.01% by mass), it was described as "-".

An evaluation sample was taken to evaluate the strain and hardness of the steel parts obtained in Production Example 1-2. The sampling position of the evaluation sample is shown in FIG. As shown in FIG. 3, the overhanging molded portion A (length 25 mm × width 5 mm) in the center of the steel part and the unprocessed portion B (length 10 mm × width 5 mm) located at a position vertically separated from the overhang molding portion A are collected. bottom.

In order to evaluate the strain of the sample, the plate thickness was determined by observing the cross section with an optical microscope.
The plate thickness of the overhanging molded portion A is the central portion of the steel part, a position 3.75 mm vertically away from the central portion (referred to as an intermediate portion), and a position 7.5 mm vertically away from the central portion (hem portion). ), Each of which was obtained. Then, using the above equation (1), the central portion of the steel part, the intermediate part and the thickness of the skirt portion and the plate thickness d ₁ of the working portion, the thickness of the steel sheet before working the thickness of the non-working part B As d ₀ , the strains at the center, middle and hem of the steel part were determined.

The Vickers hardness was measured at three points (center part, middle part and hem part) of the overhanging molded part A and at the unprocessed part B. The measurement was carried out using a Vickers hardness tester under the conditions of a load of 1 kg and a holding time of 10 seconds. As for the measurement position, when the plate thickness was d, three points of d / 4 from the surface of the steel part were measured in the plate thickness direction. FIG. 4 is a schematic cross-sectional view taken along line XX shown in FIG. 3, showing the hardness measurement position of the overhang molding portion A.
Although the hardness measurement position of the non-processed portion B is not shown, three points of d / 4 from the surface of the steel part in the vertical and horizontal directions and the plate thickness direction of the non-processed portion B were measured.

The average Vickers hardness values of the three points (center part, middle part and hem part) of the overhanging molded part A and the three points of the non-processed part B were adopted as the respective Vickers hardnesses.

The temperature (° C.) (referred to as molding temperature) at which overhang molding was performed from Production Example 1-2, the overhang height (mm), the cooling rate during slow cooling (° C / sec), the slow cooling time (seconds), and Steel parts were manufactured by changing the average cooling rate (° C./sec) from the heating temperature to the Ms point −50 ° C. (referred to as Production Examples 1-1 and 1-3 to 1-8). Then, the strain and Vickers hardness of each steel part were evaluated in the same manner as in the steel parts obtained in Production Example 1-2. The results are shown in Table 2.
In Table 2, the underlined values indicate that the values are outside the scope of the first embodiment of the present invention.

In Production Examples 1-1 to 1-8, at least one of the central portion, the middle portion and the hem portion has a Vickers hardness of 20 HV or more lower than that of the non-processed portion, and the hardness of the non-processed portion is reduced. It was judged that the product having a value of 310 HV or more was a production example satisfying the standard as "locally softened high-strength steel parts".
In a more preferable production example of the "locally softened" steel part, at least one of the central portion, the intermediate portion and the hem portion has a Vickers hardness of 40 HV or more lower than that of the unprocessed portion. A more preferable production example is one in which the Vickers hardness is reduced by 100 HV or more.
Further, as a "high-strength steel part", a more preferable production example has a Vickers hardness of a non-processed portion of 400 HV or more, and a more preferable production example has a Vickers hardness of 500 HV or more.
The same judgment is made in Examples 2 and 3 described later.

From the results in Table 2, it can be considered as follows. Production Examples 1-1 to 1-4 in Table 2 are examples that satisfy all the requirements specified in the first embodiment of the present invention, and are more than a predetermined value by processing without local temperature control. It was possible to produce a high-strength steel part in which only the portion to which strain (0.5% or more in the first embodiment of the present invention) was applied was locally softened.
On the other hand, Production Examples 1-5 to 1-8 in Table 2 are examples in which the requirements specified in the first embodiment of the present invention are not satisfied, and strains equal to or more than a predetermined value due to processing (in the first embodiment of the present invention, 0. It was not possible to produce locally softened high-strength steel parts in the portion to which 5% or more) was added.

In Production Examples 1-5 to 1-8, the forming temperature was 650 ° C. or 550 ° C., which was less than 675 ° C., so that the entire steel part including the non-processed portion was softened, and the locally softened high strength. The steel parts could not be manufactured.

Steel type No. in Table 1 Using the steel having the chemical composition shown in A, a steel sheet having a thickness of 1.6 mm and an area of 100 mm × 100 mm was prepared, and the steel sheet was heated to 880 ° C. Then, it was allowed to cool to 750 ° C. at about 12 ° C./sec, and the first overhang molding was performed at 750 ° C. The first overhang molding was performed by pressing a φ10 mm hemispherical punch against the central portion of a 100 mm × 100 mm steel sheet from the back surface. The overhang height was 3.0 mm. After the first overhang molding, the mixture was slowly cooled for 6 seconds at an average cooling rate of 10.8 ° C./sec. After the first slow cooling step, a second overhang molding was performed. The second overhang molding was performed by pressing a φ10 mm hemispherical punch from the direction opposite to the first overhang molding (that is, from the surface) against the portion where the first overhang molding was performed. .. After the second overhang molding, the mixture was slowly cooled for 6 seconds at an average cooling rate of 6.7 ° C./sec. After the second slow cooling step, water cooling was performed to the Ms point (° C.) -50 ° C. (that is, 335 ° C.) so that the average cooling rate from 880 ° C. to 335 ° C. was 26.2 ° C./sec. Then it was allowed to cool to room temperature. The above is referred to as Production Example 2-1.

The strain and Vickers hardness of the steel parts obtained in Production Example 2-1 were evaluated in the same manner as in Example 1. The strain was calculated using the above equation (2). Since the first overhang molding is performed in the same manner as in Production Example 1-2, it is assumed that the plate thickness after the first overhang molding is the same as that in Production Example 1-2, and the strain is strained. Is being calculated. The results are shown in Table 3. Since the second overhang molding was performed in the opposite direction to the first overhang molding, the second overhang height was set to a negative value.

From the results in Table 3, it can be considered as follows. Production Example 2-1 in Table 3 is an example that satisfies all of the requirements specified in the first embodiment of the present invention, and is a strain of a predetermined value or more due to processing without local temperature control (implementation of the present invention). In the first form, it was possible to produce a high-strength steel part in which only the portion to which 0.5% or more) was added was locally softened.

Steel type No. in Table 1 Using the steel having the chemical composition shown in A, a steel sheet having a thickness of 1.6 mm and an area of 100 mm × 100 mm was prepared, and the steel sheet was heated to 950 ° C. and held for 60 seconds. Then, it was allowed to cool to 550 ° C. at about 12 ° C./sec, and overhang molding was performed at 550 ° C. The overhang molding was performed by pressing a hemispherical punch of φ10 mm from the back surface against the central portion of the steel plate of 100 mm × 100 mm. The overhang height was 0.1 mm. After overhanging, the mixture was slowly cooled for 6 seconds at an average cooling rate of 4.7 ° C./sec. Then, it was water-cooled to the Ms point (° C.) -50 ° C. (that is, 335 ° C.) so that the average cooling rate from 950 ° C. to 335 ° C. was 12.5 ° C./sec. Then it was allowed to cool to room temperature. The above is referred to as Production Example 3-1.

The strain and Vickers hardness of the steel parts obtained in Production Example 3-1 were evaluated in the same manner as in Example 1.

Steel type from Production Example 3-1, temperature at which overhang molding was performed (referred to as molding temperature), overhang height (mm), cooling rate during slow cooling (° C / sec), slow cooling time (second) ) And the average cooling rate (° C./sec) from the heating temperature to the Ms point −50 ° C. were changed to produce steel parts (referred to as Production Examples 3-2-3-19). Then, the strain and the Vickers hardness were evaluated for each steel part in the same manner as in Production Example 3-1. The results are shown in Tables 4 and 5. The steel type No. in Table 1 The Ac1 point of the steel having the chemical composition shown in B was 778 ° C, the Ac3 point was 875 ° C, and the Ms point was 385 ° C.
In Tables 4 and 5, the underlined values indicate that they are outside the scope of the second embodiment of the present invention.

From the results in Tables 4 and 5, it can be considered as follows. Production Examples 3-4 to 3-6, 3-9, 3-11 and 3-14 to 3-16 in Table 4 and Production Examples 3-20 to 3-27, 3-30 to 3-32 and Table 5 in Table 5. All of 3-34 to 3-38 are examples that satisfy all the requirements specified in the second embodiment of the present invention, and strains equal to or more than a predetermined value (in the present invention) due to processing without local temperature control. In the second embodiment, it was possible to produce a high-strength steel part in which only the portion to which 10% or more) was added was locally softened.

On the other hand, Production Example No. in Table 4 3-13-3, 3-7-3-8, 3-10, 3-12-3-13, 3-17 and 3-19, and Production Examples 3-28, 3-29 and Table 5 in Table 5. Reference numeral 3-33 is an example in which the requirements specified in the second embodiment of the present invention are not satisfied, and locally in a portion where a strain of a predetermined value or more (10% or more in the second embodiment of the present invention) is applied by processing. It was not possible to produce softened high-strength steel parts.

Production Examples 3-1 to 3-3, 3-8, 3-10, 3-13 and 3-19 in Table 4 and Production Examples 3-33 in Table 5 are all in the central portion, the middle portion and the hem portion. Since the strain was less than 10%, it was not possible to produce locally softened high-strength steel parts.

In Production Example 3-7 of Table 4, in (d) the step of holding or slowly cooling, the slow cooling rate is more than 15 ° C./sec (that is, the slow cooling time is less than 1 second), and the central portion, the intermediate portion, and the hem portion are shown. Since the strain was less than 10% in all of the above, it was not possible to produce locally softened high-strength steel parts.

Production Examples 3-12 and 3-17 in Table 4 and Production Examples 3-28 and 3-29 in Table 5 have a slow cooling rate of more than 15 ° C./sec (that is, slow cooling) in the step of (d) holding or slow cooling. Since the time was less than 1 second), it was not possible to produce locally softened high-strength steel parts.

In Production Example 3-18 of Table 4, the strain applied by processing is 8% in the central portion, which does not satisfy the strain of 10% or more specified in the second embodiment of the present invention, but is referred to as the non-processed portion. The difference in hardness was 20 HV or more. This is the part No. It is possible that manufacturing conditions other than strain (heating temperature, cooling rate, slow cooling time, etc.) were preferable in the central portion of 3-18, but the details are unknown.

Steel type No. in Table 1 Using the steel having the chemical composition shown in A, a steel sheet having a thickness of 1.6 mm and an area of 100 mm × 100 mm was prepared, and the steel sheet was heated to 950 ° C. Then, it was allowed to cool to 750 ° C. at about 12 ° C./sec, and the first overhang molding was performed at 750 ° C. The first overhang molding was performed by pressing a φ10 mm hemispherical punch against the central portion of a 100 mm × 100 mm steel sheet from the back surface. The overhang height was 4.0 mm. After the first overhang molding, the mixture was slowly cooled for 6 seconds at an average cooling rate of 9.7 ° C./sec. After the first slow cooling step, a second overhang molding was performed. The second overhang molding was performed by pressing a φ10 mm hemispherical punch from the direction opposite to that of the first overhang molding (that is, from the surface) against the portion where the first overhang molding was performed. .. After the second overhang molding, the mixture was slowly cooled for 6 seconds at an average cooling rate of 5.3 ° C./sec. After the second slow cooling step, water cooling was performed to the Ms point (° C.) -50 ° C. (that is, 335 ° C.) so that the average cooling rate from 950 ° C. to 335 ° C. was 16.6 ° C./sec. Then it was allowed to cool to room temperature. The above is referred to as Production Example 4-1.

The strain and Vickers hardness of the steel parts obtained in Production Example 4-1 were evaluated in the same manner as in Example 1. The strain was calculated using the above equation (2). In Production Example 4-1 when the second overhang molding was not performed, the plate thickness of the central portion was 1.39 mm, the plate thickness of the intermediate portion was 1.22 mm, and the plate thickness of the hem portion was 1.58 mm. Since it was confirmed separately, the strain is calculated by using these plate thicknesses as the plate thickness after the first overhang molding in Production Example 4-1. The results are shown in Table 6. Since the second overhang molding was performed in the opposite direction to the first overhang molding, the second overhang height was set to a negative value.

From the results in Table 6, it can be considered as follows. Production Example 4-1 in Table 6 is an example that satisfies all of the requirements specified in the second embodiment of the present invention, and is a strain equal to or more than a predetermined value due to processing without local temperature control (implementation of the present invention). In the second form, it was possible to produce a high-strength steel part in which only the portion to which 10% or more) was added was locally softened.

In an embodiment of the present invention, it is possible to provide a method for producing locally softened high-strength steel parts without local temperature control. Such high-strength steel parts are suitable, for example, as materials for automobile skeletons.

1 Steel parts 2 Hardness measurement in the central part 1st place 3 Hardness measurement in the central part 2nd place 4 Hardness measurement in the central part 3rd place 5 Hardness measurement in the middle part 1st place 6 Hardness measurement in the middle part 2nd place 7 Middle Hardness measurement at the part 3rd place 8 Hardness measurement at the hem 1st place 9 Hardness measurement at the hem 2nd place 10 Hardness measurement at the hem 3rd place A Overhang molding part B Non-processed part

Claims (10)

C: 0.05 to 0.40% by mass,
Si: 0 to 2.0% by mass,
Mn: 1.0 to 3.0% by mass,
Al: 0.010 to 1.0% by mass,
P: More than 0% by mass and less than 0.100% by mass,
S: More than 0% by mass and 0.010% by mass or less,
N: More than 0% by mass and 0.010% by mass or less,
B: 0.0005 to 0.010% by mass, and the balance: a step of preparing a steel sheet having a chemical composition consisting of iron and unavoidable impurities, and
A step of heating the steel sheet to a temperature of 1 point (° C.) or more and 3 points (° C.) of Ac and less than 10 ° C.
After the heating step, a machining step of adding 0.5% or more of strain at a machining temperature of 675 ° C. or higher and Ac 3 points (° C.) + 10 ° C. or lower,
After the processing step, a step of holding at the processing temperature for 1 second or more and 120 seconds or less, or a step of slowly cooling for 1 second or more and 120 seconds or less at an average cooling rate of more than 0 ° C./sec and 15 ° C./sec or less.
After the step of holding or slowly cooling, the step of cooling to the Ms point (° C.) -50 ° C. is included.
A method for manufacturing steel parts, in which the average cooling rate from the temperature of the heating step to the Ms point (° C.) -50 ° C. is controlled to 10 ° C./sec or more. C: 0.05 to 0.40% by mass,
Si: 0 to 2.0% by mass,
Mn: 1.0 to 3.0% by mass,
Al: 0.010 to 1.0% by mass,
P: More than 0% by mass and 0.100% by mass or less,
S: More than 0% by mass and 0.010% by mass or less,
N: More than 0% by mass and 0.010% by mass or less,
B: 0.0005 to 0.010% by mass, and the balance: a step of preparing a steel sheet having a chemical composition consisting of iron and unavoidable impurities, and
A step of heating the steel sheet to a temperature of Acc 3 points (° C.) + 10 ° C. or higher and 1100 ° C. or lower, and
After the heating step, a machining step of applying a strain of 10% or more at a machining temperature of Ms point (° C.) + 50 ° C. or higher and Ac 3 point (° C.) + 10 ° C.
After the processing step, a step of holding at the processing temperature for 1 second or more and 120 seconds or less, or a step of slowly cooling for 1 second or more and 120 seconds or less at an average cooling rate of more than 0 ° C./sec and 15 ° C./sec or less.
After the step of holding or slowly cooling, the step of cooling to the Ms point (° C.) -50 ° C. is included.
A method for manufacturing steel parts, in which the average cooling rate from the temperature of the heating step to the Ms point (° C.) -50 ° C. is controlled to 10 ° C./sec or more. The steel plate is
The production method according to claim 1 or 2, further comprising one or more selected from the group consisting of Cu: more than 0% by mass and 0.50% by mass or less, and Ni: more than 0% by mass and 0.50% by mass or less. The steel plate is
Ti: More than 0% by mass and less than 0.10% by mass,
In any one of claims 1 to 3, further containing one or more selected from the group consisting of Cr: more than 0% by mass and 3.0% by mass or less, and Nb: more than 0% by mass and 0.10% by mass or less. The manufacturing method described. The production method according to any one of claims 1 to 4, which comprises applying the strain by overhang molding. The production method according to any one of claims 1 to 4, which comprises applying the strain by forging. The manufacturing method according to any one of claims 1 to 4, which comprises applying the strain by bending back at the time of draw molding. The production method according to any one of claims 1 to 4, which comprises applying the strain by shearing. The production method according to any one of claims 1 to 8, which comprises applying the strain by a plurality of times of processing. The manufacturing method according to claim 9, wherein the plurality of times of processing includes a processing of adding deformation and a processing of returning the deformation.

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