JP3497654B2 - Fe-Cu alloy steel having good strength, ductility, and toughness and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a body-centered cubic (bc)
c) Fe-Cu alloy steel that achieves both strength-ductility and strength-toughness better than conventional steel, characterized by distributing a Cu precipitation phase having a structure in the steel Yield strength 10kgf / mm
^It can be applied to materials with a wide range of strengths, from mild steel of about ² to high tensile steel of about 120 kgf / mm ² mainly composed of martensite structure.

[0002]

2. Description of the Related Art In recent years, weight reduction has been demanded for outer panels such as automobiles and ships and structures such as buildings, and it has been essential to increase the strength of steel sheets as a means for solving the problem. Conventionally,
The means for increasing the strength of a steel sheet is roughly divided into (1)
Add a large amount of solid solution strengthening elements such as Si, Ni and Mo, or add a large amount of precipitation strengthening elements such as Nb, Ti and V. (2) Miniaturize crystal grains or control to a composite structure. There are two methods, and the optimum strengthening method has been selected in consideration of material requirements and economic efficiency according to the application used.

The strengthening by the above-mentioned method (1) has been used for a long time regardless of whether the structure is ferrite, bainite, or martensite, but it is generally inversely proportional to high tensile strength when trying to obtain high tensile strength. Then, ductility and toughness deteriorate. Furthermore, when a large amount of elements such as Mo and Ni are added, the carbon equivalent increases and the weldability deteriorates.

[0004] On the other hand, by that strength of the method (2), two-phase structure steel of ferrite-martensite (for example Japanese Patent Publication 61
Strength and ductility, as typified by JP-A-15128).
Alternatively, it is a method capable of achieving both strength and toughness as in lower bainite steel. However, this method cannot be applied to improve strength-ductility and strength-toughness characteristics at all strength levels from low-strength steel to high-strength steel because the strength that can be achieved is determined to some extent when the structure is selected. . Because of this, steel of all strength levels
Even better strength than steel material to existing on wood - ductility, strength - it has been desired to develop a steel material having a toughness properties.

Therefore, as a result of earnestly conducting experiments and studies to achieve the above object, the inventors of the present invention added a proper amount of Cu and appropriately controlled the structure of the precipitated Cu, thereby It has been found that, regardless of the structure, it is possible to achieve both superior strength-toughness and strength-ductility than a conventional steel sheet that has been strengthened by precipitation strengthening.

[0006] as a steel plate that the addition of Cu, JP-A-61-
288015, JP-A-5-331591,
Japanese Unexamined Patent Publication No. 6-108200 is disclosed. These use ε-Cu having a face-centered cubic (fcc) structure that precipitates by subjecting to an aging treatment at around 550 ° C. after solution treatment at high temperature, and has high strength-high toughness or high strength-high strength. It says that ductility can be achieved.

However, as the present invention shows, fcc
With ε-Cu having a structure, it is not possible to achieve both strength-toughness and strength-ductility that exceed the categories of conventional precipitation strengthening methods. See also “Journal of Nuclear M
materials "(vol. 148, 1987, p.
107) is a body-centered cubic (hereinafter bc) in ferrite.
It is described that high hardness can be obtained by depositing Cu having a structure c). However, there is no description about toughness and ductility, or strength-ductility, strength-toughness characteristics, and about steels having other structures such as martensite and bainite.

[0008]

DISCLOSURE OF THE INVENTION The present invention was developed in view of the above-mentioned present situation, and ductility due to high strength, which has been a problem in the conventional strengthening method, for all steel types from low strength steel to high strength steel. It is an object of the present invention to provide a steel material capable of suppressing deterioration of toughness and achieving high strength, and a manufacturing method thereof.

[0009]

The present invention has taken the following means in order to solve the above problems. That is,
The first feature is that C: 0.2 wt% or less, Si: 3.0 wt
% Or less, Mn: 3.0 wt% or less and Cu: 0.5 to
Contains 5.0 wt%, the balance Fe and unavoidable impurities
Bcc with a particle size (diameter) of 1 to 15 nm
A structure in which Cu having a structure is dispersed at a precipitation fraction of 20 to 80%.
Rutensite structure or bainite structure or Fe
Wright tissue or a mixture of two or three of these
Have good strength, ductility, and toughness with a composite structure
Fe-Cu alloy steel.

A second aspect of the present invention, said Fe-Cu
Alloy steel with Ni: 0.5-5.0 wt%, Cr: 3.0 w
t% or less, Ti: 0.10 wt% or less, Nb: 0.15
It is characterized by containing one or more of V wt.% or less and V: 0.15 wt% or less.

The third feature of the present invention is that C: 0.2 wt%.
Hereinafter, Si: 3.0 wt% or less, Mn: 3.0 wt% or less
Below, and containing Cu: 0.5 to 5.0 wt%, the balance
Fe-Cu alloy consisting of Fe and unavoidable impurities
Reheat the steel to a temperature above 750 ° C or 75
Rolling is completed at a temperature of 0 ° C or higher, and the alloy steel is heated at 10 ° C / se.
After cooling at a cooling rate of c or higher, 250 ° C or higher and 45
The one that has been aged for 20 minutes or more at 0 ° C or less is further
Aging for 10 to 120 minutes above 480 ℃ and below 600 ℃
Good strength, ductility and toughness, characterized by being treated
Is a method for producing an Fe-Cu alloy steel having

The fourth feature of the present invention is C: 0.2 wt%
Hereinafter, Si: 3.0 wt% or less, Mn: 3.0 wt% or less
Below, and containing Cu: 0.5 to 5.0 wt%, the balance
Fe-Cu alloy consisting of Fe and unavoidable impurities
Heating the steel to a temperature above 750 ° C or 750
After finishing rolling at a temperature of ℃ or more, the alloy steel is 0.5 to 8
Characterized by cooling to room temperature at a cooling rate of ° C / sec
Fe-Cu alloy with good strength, ductility and toughness
It is a method of manufacturing steel.

The fifth feature of the present invention is that C: 0.2 wt.
% Or less, Si: 3.0 wt% or less, Mn: 3.0 wt%
The following, and containing Cu: 0.5 to 5.0 wt%,
The balance is Fe-Cu alloy steel consisting of Fe and inevitable impurities, heated to a temperature of 750 ° C or higher, or 75
After finishing rolling at a temperature of 0 ° C or higher, the alloy steel is cooled to 10 ° C /
After cooling at a cooling rate of sec or more, 480 ° C to 600
And facilities aging treatment 10 to 120 minutes at ° C., the particle size (diameter)
Cu having a bcc structure of 1 to 15 nm has a precipitation fraction of 20.
Martensite structure or bainer dispersed at ~ 80%
Ferrite structure or ferrite structure or of these
Characterized in that it is a mixed tissue of two or three kinds of
This is a method for producing an Fe-Cu alloy steel having good strength, ductility, and toughness.

The sixth feature of the present invention is C: 0.2 wt.
% Or less, Si: 3.0 wt% or less, Mn: 3.0 wt%
The following, and containing Cu: 0.5 to 5.0 wt%,
The balance is Fe-Cu alloy steel consisting of Fe and inevitable impurities, heated to a temperature of 750 ° C or higher, or 75
Rolling is completed at a temperature of 0 ° C or higher, and the alloy steel is heated at 10 ° C / se.
cooled in c above cooling rate, after the addition of 1% or more prestrain at room temperature, and facilities the aging treatment 10-120 minutes at 480 ° C. to 600 ° C., the particle size (diameter) of 1-15 nm bcc Construction
With Cu having a dispersion fraction of 20 to 80%
Site structure or bainite structure or ferai
Tissue or a mixed set of two or three of these
It is a method for producing an Fe-Cu alloy steel having good strength, ductility, and toughness, which is characterized by being woven .

A seventh feature of the present invention is that the alloy steel is
In addition, Ni: 0.5 to 5.0 wt%, Cr: 3.0 wt
% Or less, Ti: 0.10 wt% or less, Nb: 0.15w
One or two of t% or less and V: 0.15 wt% or less
Third to sixth inventions characterized by containing at least one species
Fe-Cu having good strength, ductility and toughness
It is a method for manufacturing alloy steel.

[0016] It should be noted, refers to the tensile strength is here in strength,
Good strength-ductility, strength-toughness means that there is less decrease in ductility and toughness when the strength is increased, as compared with precipitation-strengthened steel by carbide and conventional steel in which the structure of the Cu precipitation phase is not properly controlled. means. Specifically, compared with precipitation strengthening methods such as carbides such as TiC, NbC, and VC, nitrides such as TiN, NbN, VN, and AlN, oxides and ε-Cu, uniform elongation when compared at the same tensile strength. 2% or more, and the impact absorption energy vE ₀ in the V-notch Charpy test is 30
It indicates that the number has increased by J or more. The bcc structure means a body-centered cubic structure.

[0017] Here, precipitation fraction of Cu having a bcc structure shall be obtained by the process of the following (1) (4). (1) Atom probe field ion microscope (AP-FI
In M), it is confirmed that Cu is precipitated in the matrix. (2) The precipitate in the matrix is ε-Cu or 9R
-Check with a transmission electron microscope that it is not Cu. (3) The composition of the matrix is analyzed by an X-ray analysis method (energy dispersive X-ray spectroscopy) using a focused electron beam probe. (4) Substitute the analysis value in the following formula. Precipitation fraction of bcc-Cu = (analysis value of Cu in matrix / amount of added Cu) ×
100 (%) In addition to this, as a method for discriminating between bcc-Cu and ε-Cu, a wide area X-ray absorption edge fine structure (EXAF) is used.
The method using S) is also practicable.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION In the steel of the present invention, ductility due to high strength, for all steel types from low strength steel to high strength steel,
It is possible to suppress deterioration of toughness and achieve high strength.
The method is as follows: Cu particles having a body centered cubic (bcc) structure having a diameter of 1 to 15 nm in an Fe matrix (hereinafter referred to as bcc-C).
The point is to finely disperse u) as matching precipitates.

The inventors conducted mechanical tests and detailed observations on steel sheets in which various precipitate phase structures were suppressed, and as a result, F
It was found that when bcc-Cu is dispersed in e, the decrease in elongation value and the decrease in absorbed energy upon impact, which are characteristic of the conventional precipitation strengthening method, are reduced. As a result of analyzing this cause, when strain or impact is applied to the steel sheet, bcc-Cu undergoes martensite transformation into Cu particles having a 9R structure, and at the same time, some dislocations are released from the vicinity of the precipitate and at the same time. Absorption of energy occurs, and as a result,
It was found that the deterioration of ductility and toughness was less than that of other precipitation strengthened steels.

[0020] By the way, Nb (C, N), Ti (C, N)
Such carbonitrides and ε-Cu having the fcc structure cannot expect the above effects because the martensitic transformation of the precipitation phase does not occur due to the addition of strain or impact. Further bcc
It was also clarified that the precipitation strengthening method using -Cu gives good strength-ductility and strength-toughness characteristics regardless of the structure of the parent phase. That is, regardless of whether it is ferrite, bainite, martensite, or a mixed structure thereof, b
Compared with precipitation strengthening other than cc-Cu, when the same level of strength is obtained, ductility and toughness are less likely to deteriorate.

The present invention will be described in detail below.
First, the reasons for limiting the components will be described. C: C is an essential element for controlling the structure to ferrite, bainite, martensite, or a mixed structure thereof. However, if it exceeds 0.2 wt%, a large amount of cementite precipitates and the expected effect on the ductility and toughness due to Cu precipitates almost disappears, so the upper limit is 0.2 wt.
Limited to%.

[0022] Si: Si is an essential element to control tissue ferrite, bainite, martensite or a mixture tissue, and is also necessary as a deoxidizing element. However, if it exceeds 3.0 wt%, the descaling property during hot rolling deteriorates and the cost increases. Therefore, the Si content is limited to the range of 3.0 wt% or less. Mn: Mn, like Si, lowers the Ar ₃ transformation point to form a structure of ferrite, bainite, martensite,
Alternatively, it is an essential element for controlling the mixed texture. However, if it exceeds 3.0 wt %, the cost becomes high, so the Mn content is limited to the range of 3.0 wt % or less.

[0023] Cu: Cu is the most important element in the present invention. However, if it is less than 0.5 wt%, b
The effect as cc-Cu does not appear, and 5.0 wt%
If the content exceeds 1.0, surface cracking of the steel sheet due to hot brittleness of Cu becomes significant, so the Cu content range is 0.5 to 5.0 wt.
Limited to the range of%. However, if Ni is added in an amount equal to Cu, the hot embrittlement of Cu is reduced, so Cu and Ni
In the case of adding in a complex manner, Cu exceeding 5.0 wt% can be added. Cu is also an element that does not increase the carbon equivalent and is therefore effective in improving weldability.

[0024] Cr: Cr is an alternative element of Mn, Ar
_{(3) An} element used to control the structure to ferrite, bainite, martensite, or a mixed structure thereof by lowering the transformation point. However, 3.0 wt
%, The cost increases, so the Cr content should be 3.0.
The range was limited to wt% or less. Ni: Ni has an effect of suppressing hot brittleness caused by addition of Cu. However, if it is less than 0.5 wt%, the effect is not exhibited, and if it exceeds 5.0 wt%, the cost becomes high. Therefore, the appropriate addition range is 0.5 to 5.0.
Limited to wt%.

[0025] Ti: Ti is required as an element as a deoxidizing element and also to control the austenite grain size at the time of re-heating as carbonitrides. However, the addition amount of Ti is 0.10 wt.
%, The effect of addition of Cu is lost. Therefore, in both cases of single addition and composite addition, the range of Ti content is set to 0.
It was set to 10 wt% or less. Nb: Nb is necessary as an element that controls the austenite grain size during reheating as a carbonitride. However, when the amount of Nb added exceeds 0.15 wt%, the effect of adding Cu is lost, so the range of Nb content was set to 0.15 wt% or less in both cases of single addition and composite addition. V: V is necessary as an element for controlling the austenite grain size during reheating as a carbonitride. However, if the addition amount of V exceeds 0.15 wt%, the effect of addition of Cu is lost, so the V content range was set to 0.15 wt% or less in both cases of single addition and composite addition.

[0026] Next bcc-Cu of the reasons for limitation of particle size and deposition fraction is described. If the particle size of bcc-Cu is less than 1 nm, strength increase cannot be expected. On the other hand, if it exceeds 15 nm, it undergoes a phase transformation into ε-Cu or Cu having a 9R structure, and ductility and toughness sharply decrease. Therefore, the particle size of bcc-Cu is limited to the range of 1 nm to 15 nm. However, it is ideally preferable to control the size to about 1 to 8 nm from the viewpoint of improving ductility. The precipitation fraction of bcc-Cu is 80.
%, The probability that .epsilon.-Cu or Cu having a 9R structure exists will be high, causing deterioration in toughness and ductility. If it is less than 20%, the effect of Cu is not so great. Therefore, the precipitation fraction of bcc-Cu is limited to the range of 20 to 80%.

The following method is effective for controlling the bcc-Cu precipitates. (1) Reheat to 750 ° C or higher or 75
Rolling is completed at a temperature of 0 ° C or higher, and after cooling at a cooling rate of 10 ° C / sec or higher, 250 ° C or higher and 450 ° C or higher.
48 more after aging treatment for 20 minutes or more
Aging treatment is performed at 0 ° C. or higher and 600 ° C. or lower for 10 to 120 minutes.

[0028] (2) After completing the rolling at 750 ° C. is heated at above or 750 ° C. or higher, 0.5 to 8 ° C. / se
Cool to room temperature at the cooling rate of c. (3) Heat above 750 ° C or above 750 ° C
At a cooling rate of 10 ° C / sec or more after finishing rolling at
After cooling, at 480 ° C to 600 ° C for 10 to 120 minutes
Aging treatment is applied.

[0029] (4) After completing the rolling at 750 ° C. or higher in either heating or 750 ° C. or higher, cooling at 10 ° C. / sec or more cooling rate, after the addition of prestrain of 1% at room temperature,
Aging treatment is performed at 480 ° C to 600 ° C for 10 to 120 minutes. All of these heat treatments are ε-Cu having an fcc structure.
The point is not to grow or transform into Cu of 9R structure.

In the methods (1) to (4), bc is added to Fe.
As a method for finely dispersing c-Cu, the two-step aging method (1) is preferable. That is, aging is performed in a temperature range in which bcc-Cu nucleation occurs, the bcc-Cu nuclei are finely dispersed, and then aging is performed at a higher temperature to grow bcc-Cu.

[0031]

EXAMPLES Next, the present invention will be described in more detail by way of examples. The steel materials A to J adjusted to the components shown in Table 1 were treated under various conditions shown in Table 2. From the thus obtained steel sheet, a test piece for tensile test, a test piece for Charpy impact test, a test piece for transmission electron microscope observation, and a test piece for atom probe field ion microscope (AP-FIM) are cut out. It was Table 3 shows the obtained test piece No. 1 to 14
Tensile strength (TS), total elongation (T-El), Charpy impact absorption energy value at 0 ° C. (vE ₀ ), main precipitation form of Cu in Fe matrix, and precipitation of bcc-Cu with respect to total Cu addition amount The result of investigating the fraction is shown.

As is clear from Table 3, when the main precipitation form of Cu in the Fe matrix is bcc-Cu and the precipitation fraction thereof is in the range of 20 to 80%, ε-Cu or N is used.
Compared with precipitation-strengthened steels such as bC, TiC, and VC, the T-El value and vE ₀ value are higher when compared at the same strength.
For example, sample No. 1 and No. Comparing the two, the tensile strength was 720 MPa and the bcc-C
No. in which u is properly distributed. 1 is T-El 3.5
%, Which is 38 J greatly in vE _0. That is, No.
No. 1 is superior in both strength-toughness and strength-ductility.

[0033] No. 5 and No. 6 changes the cooling rate, C
This is a modification of the precipitation morphology of u. No. No. 5 is comparative steel No. 6 , T-El is 3.9% larger, and vE ₀ is 31 J larger. No. 7 and No. No. 8 is a martensitic and ferrite dual-phase steel, No. 8 9 and No. No. 10 is the one in which the precipitation morphology of Cu is changed in the ferritic steel. N in which bcc-Cu is properly dispersed as in the above.
o. 7 and No. No. 9 is superior in both strength-toughness or strength-ductility.

[0034] No. 11 precipitation hardening steel der by VC
It No. 14 is precipitation strengthened steel made of NbC and TiC . a b cc-Cu-dispersion-strengthened steel No. Compared with No. 12, the steel of the present invention No. No. 12 has the same tensile strength, and both T-El and vE ₀ are larger. No. 13 is 2
This is an example in which both the strength-toughness and the strength-ductility were decreased because the temperature of the second aging treatment exceeded 600 ° C and ε-Cu was precipitated.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3A]

[0038]

[Table 3B]

[0039]

Industrial Applicability The alloy steel of the present invention has both good strength-ductility and strength-toughness as compared with the conventional precipitation-strengthened steels, and the use of Cu further improves weldability and fatigue. Excellent in characteristics. INDUSTRIAL APPLICABILITY The present invention can be applied to an alloy steel having a ferrite structure, a martensite structure, a bainite structure, and a mixed structure thereof, and therefore a structural member such as an automobile outer plate or an underbody member as a lightweight member, shipbuilding, It can be applied to structural members such as architecture, marine structures, steel pipes, and strength members. The low cost for manufacturing a steel material effect by utilizing the low alloy cost per Cu also has.

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (7)

(57) [Claims] 1. Fe-having good strength, ductility and toughness
In Cu alloy steel, C: 0.2 wt% or less, Si:
3.0 wt% or less, Mn: 3.0 wt% or less and C
u: 0.5 to 5.0 wt% is contained, the balance is Fe and inevitable impurities, and the particle size (diameter) is 1 to 15
Cu having a bcc structure of 20 nm has a precipitation fraction of 20 to 80%.
Fe-Cu alloy steel having good strength, ductility, and toughness, which comprises a martensite structure, a bainite structure, a ferrite structure, or a mixed structure of two or three of these, dispersed in 1. 2. Ni: 0.5 to 5.0 wt%, Cr:
3.0 wt% or less, Ti: 0.10 wt% or less, Nb:
0.15 wt% or less, V: 1 of 0.15 wt% or less
1. The composition according to claim 1, which contains one or more kinds.
The described Fe-Cu alloy steel. 3. C: 0.2 wt% or less, Si: 3.0 w
t% or less, Mn: 3.0 wt% or less, and Cu:
Contains 0.5 to 5.0 wt%, the balance Fe and impenetrable
Fe-Cu alloy steel consisting of evasive impurities, 750 ℃ or more
Reheat to above temperature or press at a temperature above 750 ℃.
After completion of rolling, the alloy steel is cooled at a cooling rate of 10 ° C / sec or more.
After cooling, keep at 250 ℃ or more and 450 ℃ or less for 20 minutes
480 ℃ or more and 6 with the above aging treatment
Special aging treatment for 10 to 120 minutes below 00 ° C
Fe-Cu with good strength, ductility and toughness
Method for manufacturing alloy steel. 4. C: 0.2 wt% or less, Si: 3.0 w
t% or less, Mn: 3.0 wt% or less, and Cu:
Contains 0.5 to 5.0 wt%, the balance Fe and impenetrable
Fe-Cu alloy steel consisting of evasive impurities, 750 ℃ or more
Or heated to 750 ℃ or higher
After the completion, the alloy steel was cooled at a cooling rate of 0.5 to 8 ° C / sec.
Good strength, characterized by cooling to room temperature in degrees,
A method for producing an Fe-Cu alloy steel having ductility and toughness. 5. C: 0.2 wt% or less, Si: 3.0 w
t% or less, Mn: 3.0 wt% or less, and Cu:
Fe-Cu alloy steel containing 0.5 to 5.0 wt% and the balance Fe and unavoidable impurities is heated to a temperature of 750 ° C. or higher or after rolling at a temperature of 750 ° C. or higher, after the alloy steel is cooled at 10 ° C. / sec or more cooling rate, and facilities the aging treatment 10-120 minutes at 480 ° C. to 600 ° C., the particle size (diameter) of 1-15 nm bcc
A structure in which Cu having a structure is dispersed at a precipitation fraction of 20 to 80%.
Rutensite structure or bainite structure or Fe
Wright tissue or a mixture of two or three of these
A method for producing an Fe-Cu alloy steel having good strength, ductility, and toughness, which is characterized by having a composite structure . 6. C: 0.2 wt% or less, Si: 3.0 w
t% or less, Mn: 3.0 wt% or less, and Cu:
Fe-Cu alloy steel containing 0.5 to 5.0 wt% and the balance Fe and unavoidable impurities is heated to a temperature of 750 ° C. or higher, or rolled at a temperature of 750 ° C. or higher to finish the alloy. After cooling the steel at a cooling rate of 10 ° C / sec or more and applying a prestrain of 1% or more at room temperature, 480 ° C to
600 facilities the aging treatment 10-120 minutes at ° C., particle size (straight
Cu having a bcc structure with a diameter of 1 to 15 nm is precipitated
Martensite structure dispersed at a rate of 20-80% or
Bainite structure or ferrite structure or these
2. A method for producing an Fe—Cu alloy steel having good strength, ductility, and toughness, characterized by having a mixed structure of two or three of the above . 7. The alloy steel further comprises Ni: 0.5-.
5.0 wt%, Cr: 3.0 wt% or less, Ti: 0.1
0 wt% or less, Nb: 0.15 wt% or less, V: 0.1
Contain one or more of 5 wt% or less
Good strength and ductility according to claims 3 to 6,
A method for producing an Fe-Cu alloy steel having toughness.