JP3460659B2 - Soft high carbon steel strip with small heat treatment distortion and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to precision punching, etc.
The present invention relates to a high carbon steel strip which is soft so that it can be easily formed into a complicated shape, has a small distortion due to subsequent heat treatment such as quenching and tempering, and can be produced at a lower cost than before, and a method for producing the same. . More specifically, the present invention is capable of effectively suppressing the heat treatment distortion associated with the heat treatment such as quenching and tempering of a press part represented by a fine blanked high carbon steel sheet product, and the heat treatment. Is related to a high carbon steel strip which is soft and has excellent workability during pressing, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, S35C stipulated in JIS G3311 has been used for automobile parts that are used by hardening and tempering after molding or heat treatment such as austempering to increase strength.
.About.S70C, SCM415 to SCM440, and the like, high carbon steels having a high C content and containing a plurality of alloy components as necessary have been used. Such a high carbon steel is forged hot or cold, further cut into a desired part shape by using a cutting process, and then, necessary strength is achieved by heat treatment.

However, in recent years, with the development of press working techniques such as fine blanking, cold working into a complicated shape is possible even with a hard steel strip containing high carbon, and hot forging or cutting work is omitted. Efficient manufacturing methods are becoming popular.

In order to be subjected to these workings, it is necessary to soften steel containing high carbon as much as possible, and even in the conventional case, it is generally dealt with by softening by adjusting chemical composition, rolling and annealing conditions. ing. Specifically, a method of graphitizing cementite in steel by adjusting the chemical composition or manufacturing method (see, for example, Japanese Patent Laid-Open No. 8-120405), or coarsening cementite itself by adjusting the chemical composition and manufacturing method. , A method of softening a steel material (Example: JP-A-11-25
6268 and Japanese Patent Laid-Open No. 11-256272) have been proposed so far.

[0005]

Although these prior art techniques are effective in improving cold workability, they have many unsuitable points for stabilizing the quenching structure, and the stability of strength after heat treatment is high. ,
There has been a problem that the heat treatment strain is not sufficiently suppressed, resulting in non-uniform heat treatment strain.

As a result of experiments conducted by the present inventor, for example, in the method disclosed in Japanese Patent Application Laid-Open No. 8-120405, diffusion of carbon is restricted, carbon solid solution amount in steel is insufficient, and hardenability is low. It was found that the occurrence of deformation due to non-uniformity of the structure becomes a problem. Furthermore, JP-A-11-256268 and JP-A-11-25
In the method disclosed in Japanese Patent No. 6272, due to restrictions on alloying components that enhance hardenability, depending on quenching heating conditions and cooling conditions, deterioration of hardness due to residual pearlite structure after quenching, and occurrence of deformation become a problem. There was found.

An object of the present invention is not only excellent in cold workability, but also high carbon steel strip capable of effectively suppressing thermal strain at the time of strengthening by heat treatment such as quenching and tempering. And an inexpensive and simple manufacturing method thereof.

[0008]

Therefore, as a result of various studies on the above-mentioned problems of the prior art, the following points have been noted.

Since the crystal lattice lengths of pearlite and martensite are different, if pearlite is partially mixed in martensite, unspecified strain will occur in the thickness direction in the unit length of the product and in any direction on the surface. It was confirmed that a warp occurred.

Further, even when uniform martensite is obtained, heat treatment distortion may occur due to quenching,
It is speculated that this is because the residual stress generated during cold working changed during heating.

Furthermore, when the elongation of the material to be cold worked has a large anisotropy, the shape becomes anisotropic even at the cold working stage, and the non-uniform shape remains until after the heat treatment.
It is also preferable to limit the anisotropy of elongation of the material within a certain range in advance.

When suppressing the deformation during quenching, the means for eliminating the causes of each quenching deformation inferred as described above are summarized as follows.

(1) Metallurgical suppression of partial formation of pearlite in quenching and cooling process The suppression of pearlite formation is, in other words, the homogenization of martensite, which requires improvement of hardenability. . Factors that control the hardenability include an increase in alloying elements and an increase in carbon content, as well as coarsening of the austenite crystal grain size during quenching heating.

On the other hand, in order to improve the cold workability, it is inevitable to coarsen the carbides in the steel or reduce the alloy components for improving the hardenability. However, since both actions reduce the hardenability, coarsening of the austenite grain size at the time of quenching heating is indispensable for achieving both cold workability and hardenability. Therefore, a proper coarse austenite grain structure is generated during quenching and heating.

(2) Metallurgical Suppression of Deformation Due to Residual Stress in Quenching and Heating Deformation during quenching and heating is caused by release of residual stress generated during press molding. This residual stress depends on the deformation resistance, and the deformation resistance is governed by the yield strength of the steel material. For this reason, it is necessary to provide a structure that can reduce the yield stress to the ferrite structure that is cold-worked. In particular, since coarsening of ferrite grains is extremely effective in reducing the yield stress, a chemical composition and manufacturing conditions that enable coarsening of ferrite grains are required.

(3) Further, in order to suppress the unevenness of deformation due to the anisotropy of elongation due to cold working of the steel strip, the anisotropy of elongation peculiar to the high carbon steel strip is limited to a certain level or less. To do.

As common measures against the problems of (1) and (2), coarsening of ferrite grains before quenching heating is effective for coarsening of austenite grains of (1), and residual stress of (2) is effective. Since coarsening of ferrite grains is also effective for suppressing the yield strength to suppress the ferrite grain size, ferrite grain coarsening that satisfies the conditions (1) and (2) has been realized, and the coarse grain ferrite structure It was decided to establish the chemical composition and manufacturing conditions that would allow stable formation of

With regard to the problem (3), the spheroidization rate and grain size of cementite should be specified under specific conditions for the purpose of stabilizing elongation, and the elongation of ferrite grains should be specified to a certain level or higher. Can suppress anisotropy.

Therefore, the inventors of the present invention first obtained a cold-worked steel strip having an austenitic structure in which pearlite is not generated even under the condition of a low cooling rate during cooling during quenching under specific conditions. Microstructure observation after heat treatment was carried out using the test pieces, and the relationship between the spheroidizing condition of cementite and the grain size and ferrite grain size at the cold working stage was obtained. At the same time, the average elongation that can be exhibited with various C contents and the alloy components and compositions to suppress the anisotropy of elongation are specified, and the relationship between the anisotropy of the state of expansion of ferrite in the rolling direction is related. I asked.

Further, in order to secure the formation conditions of the coarse ferrite particles obtained as described above, the chemical composition
The range of the content ratio of each element was specified in order to limit each nitride morphology to a specific range with respect to Al, N, and Ti. That is, on the premise of suppressing the precipitation of fine AlN 3, in addition to nitride formation by Ti, the component composition range in which AlN 3 can be coarsened under specific hot rolling, cold rolling and annealing conditions was determined.

Further, manufacturing conditions, hot rolling, cold rolling, and annealing conditions for achieving desired mechanical properties and metallographic structures even with steels having various C contents were determined.

Therefore, based on these findings, in order to secure the necessary hardness after heat treatment of the high carbon steel strip and the formability capable of withstanding cold working, (i) alloy composition, (ii) metal structure, Furthermore, the present invention was completed by defining (iii) mechanical properties and (iv) manufacturing method as follows.

(I) Alloy composition: According to the present invention, the alloy composition is defined as follows. That is, in weight ratio C:
0.10 to 0.80%, Si: 0.005% to 0.30%, Mn: 0.20 to 1.60
%, And sol.Al: 0.005 to 0.100%, N: 0.001 to
0.010%, Ti: 0.001 to 0.050% satisfy the formulas (1) and (2), and X = 0.15 × sol.Al + N−0.29 × Ti−0.0030 ≧ 0 (1) Y = 0.15 × sol. Al + N-0.29 × Ti-0.0135 ≦ 0 (2) It has a steel composition containing other inevitable impurities.

If desired, Cr: 0.05-1.20%, M
o: 0.05 to 0.40%, Nb: 0.010 to 0.050%, and B:
It may contain one or more of 0.0002 to 0.0030%.

(Ii) Metallographic structure: The average ferrite grain size d and shape e in the steel defined by the following formulas (3) and (4) are as follows (3),
The condition of Eq. (4) is satisfied. 35 <d (μm) <100 ・ ・ ・ (3) e = n ₁ / n ₂ > 1.25 ・ ・ ・ (4) Here, the average ferrite grain size d specified in the equation (3) is calculated from the steel strip surface. 1.0mm x 1.0mm in the area of 1/4 depth of thickness
This is the average value of the ferrite grain size observed from the structure photograph taken at 100 times the field of view.

Further, the average ferrite grain size d may be calculated as 1.156 times the average value of the divided lengths of the crystal grains in the extending direction and the direction perpendicular to the extending direction.

The ferrite shape e (n ₁
/ N ₂ ) is the number of grains per unit line segment in the direction perpendicular to the direction of expansion, based on the rules for expanded grain specified in JIS G0552.
n ₁ is a numerical value converted from the number of particles n ₂ per unit line segment in the stretching direction.

More preferably, the average grain size of the carbide in the steel is 0.3 to 2.0 μm, and the spheroidization rate ≧ 80% is satisfied. Also,
The carbides in the steel are mainly cementite, and the spheroidization rate% of the carbides indicates the occupancy ratio of the spherical structure where the major axis / minor axis <5.

(Iii) Mechanical properties: Yield strength / tensile strength × 100 (%) (YR) is 40 to 80%, and average elongation value Elm
[= (El ₀ + 2 × El ₄₅ + El ₉₀ ) / 4)] is given by equation (5), and the elongation anisotropy index ΔEl [= (El ₀ -2 × El ₄₅ + El ₉₀ ) / 2)] is (6). Satisfy the formula. 50-40 × C (%) −2 × Mn <Elm (%) ・ ・ ・ (5) −5 <ΔEl (%) <5 ・ ・ ・ (6) However, Elm = (El ₀ + 2 × El ₄₅ + El ₉₀ ) / 4) ΔEl ＝ (El ₀ -2 × El ₄₅ ＋ El ₉₀ ) / 2) El ₀ : Elongation in rolling direction El ₄₅ : Elongation in 45 ° direction in rolling direction El ₉₀ : Elongation in 90 ° direction in rolling direction (iv) production method: finish after hot rolling at a temperature Ac ₁ + 80 ° C. range ~Ac ₁ +160 ℃, 550 winding in a range of to 700 ° C., after pickling, Ac at ₁ -80 ℃ _{~Ac 1} + 30 ℃ Box annealing is performed under the condition that the hydrogen concentration in the atmosphere is 90% or more by mass.

Alternatively, the finishing temperature Ac ₁ + 80 ° C.
~ Ac ₁ +160 ℃ After hot rolling, 550 ~ 700 ℃ in the range, after pickling, cold rolling at a rolling reduction of 5 ~ 30%, then further Ac ₁ -50 ℃ ~ Ac ₁ +30 Box annealing is performed at ℃ at a hydrogen concentration of 90% or more in the atmosphere.

[0031] As yet another method, prior to performing cold rolling at _{Ac 1 -80 ℃ ~Ac 1 + 30} ℃, hydrogen concentration in the atmosphere may be carried out box annealing at 90% or more conditions in a weight ratio .

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons why the steel composition, metal structure, mechanical properties, and manufacturing conditions are defined as described above in the present invention will be explained, but the composition ratio is not particularly mentioned in this specification. As far as it is based on mass.

A. Component content ratio of raw steel (a) C: C content is the wear resistance of the steel product after heat treatment,
A predetermined amount is blended for the purpose of improving fatigue strength. In the present invention, quenching / tempering or austempering,
Furthermore, if necessary, the tensile strength after heat treatment such as carburizing is 980 N / mm ² or more (Vickers hardness Hv: 300 or more).
In order to ensure the toughness after heat treatment, in order to set the tensile strength upper limit to 1960 N / mm ² (580 Vickers hardness) or less, and to facilitate cold working after spheroidizing annealing, The range of the amount of C added was 0.10 to 0.80%. Preferably
0.20 to 0.60%

(B) Si: In the present invention, Si is added with an upper limit of 0.30% in order to avoid deterioration of fatigue strength due to Si oxide after heat treatment. It is preferably 0.20% or less. Or Si
Even if it is extremely suppressed, there is no advantage and the cost increases, so the lower limit is made 0.005%.

(C) Mn: Mn is added in an amount of 0.20% or more for the purpose of ensuring hardenability during heat treatment or increasing tempering temperature and austempering temperature for improving toughness. However, addition of more than 1.60% causes hardening of the steel sheet in hot rolling, making it difficult to carry out pickling or cold rolling. Therefore, the range of Mn addition is 0.20 to 1.60%
And Preferably, it is 0.30 to 0.80%.

(D) sol.Al: In the present invention, the ferrite crystal grain size is defined as a specific coarse grain size. In order to satisfy this condition, it is necessary to suppress Al-based nitrides and AlN in steel as much as possible, or to coarsen and precipitate them.

Although Al is inevitably contained in the deoxidizing step in the steelmaking stage, it is known to the present inventors that inclusion of Al in a specific amount makes AlN coarse and coarse ferrite. To do.

Based on this, the content range of Al is 0.005%.
Specified as 0.100% or less. If the content is less than this,
Oxygen in the steel increases, and the fatigue strength after heat treatment decreases.On the other hand, if it exceeds this range, the excessive Al amount itself causes embrittlement after heat treatment, or coarse Al oxide decreases the fatigue strength. Since it causes adverse effects such as causing it, the content should be 0.005% or more.
It was defined as 0.100% or less. Preferably, it is 0.02 to 0.05%.

(E) N: N is an impurity element inevitably contained in the steel, but in order to control the ferrite grain size of the high carbon steel strip as a specific coarse grain, which is the object of the present invention, , It is necessary to limit the content range, and it must be contained in a specific ratio with sol.Al. Results of various evaluations so
It has been found that it is necessary to limit N within a specific range by the ratio of l.Al and Ti, and the range is 0.0010 to 0.0100%. If it is out of this range, the ferrite grain size becomes excessively coarse, and it becomes difficult to control the grain size targeted by the present invention. Therefore, in the present invention, the range of N content is 0.0010
Stipulated to ~ 0.0100%.

(F) Ti: Ti is contained in the range of 0.001 to 0.050%. Even if it falls below this range, the effect of suppressing the coarse-grained form of ferrite grains is not significantly affected, but reduction to below this range as an impurity is unnecessary in consideration of economic efficiency. On the other hand, if the content exceeds this range, coarse nitrides and oxides are formed, the fatigue strength after heat treatment is reduced, and carbides are formed in the annealed state, which leads to cold workability and residual stress suppressing effect. Inhibit. From the above viewpoint,
The Ti content range is 0.001 to 0.050%.

(G) Cr: The high carbon steel strip is formed into a steel part by cold working, and then the strength thereof can be increased by quenching / tempering accompanied by carburization as required, or austempering. At this time, in order to increase the strength or the toughness, an alloying element is appropriately contained.

Among these alloy elements, Cr has a great effect of improving strength and toughness. From this, when it is necessary to improve both strength and toughness, 0.05 to 1.20 depending on the required strength.
It may be added in the range of%.

Below this range, the balance between strength and toughness does not exhibit the desired effect. On the other hand, in a range exceeding this range, even if box annealing is performed, it is hard and cold workability and the effect of suppressing residual stress during heat treatment cannot be obtained.

(H) Mo: Mo is also effective in increasing strength and toughness, so if desired, if higher toughness is required, it is added in the range of 0.05 to 0.40%. Preferably, the upper limit is 0.30%.

Below this range, the strength / toughness balance does not exhibit the desired effect. On the other hand, if this is exceeded,
Even if box annealing is performed, it is hard and cold workability and residual stress suppressing effect during heat treatment cannot be obtained.

(I) Nb: Nb is also effective in increasing strength and toughness, so if desired, if higher toughness is required, it is added within the range of 0.010 to 0.050%.

(J) B: B has the effect of improving the hardenability and the toughness in the heat treatment, and is added in an appropriate amount in the present invention if desired. At this time, the effect of B is
In addition to being hindered by N in steel, addition of B itself also makes it difficult to control the ferrite grain size, which is the object of the present invention. Therefore, when B is added, the range is 0.0002 to 0.0030%.

At this time, it is preferable that Ti is contained at the same time, and Ti is contained in a ratio of about 10 times the B content. In particular, Ti is 3 to 10 times the N content, preferably 3 times the N content.
It is effective to control the toughness and coarse ferrite grain size by controlling the content ratio to 5 times.

(K) Relational expression of sol.Al, N and Ti: When accelerating ferrite grain growth by annealing, it is necessary to suppress coarsening or precipitation of AlN. At this time, if the ratio of sol.Al and N is controlled within a range defined by a specific relational expression, it is possible to obtain the ferrite structure defined in the present invention after annealing, and when Ti is contained. According to the findings of the present invention, when a specific coefficient is added to Ti and added to the relational expression, the ferrite grains can be controlled within the grain size range specified in the present invention.

At this time, the relational expression defining sol.Al, N and Ti is, according to the knowledge of the present inventors, "0.15 × sol.Al + N
-0.29 x Ti "as the basic formula, 0.15 x sol.Al + N-0.29 x
When the Ti value is in the range of 0.0030 to 0.0135, the ferrite grain size structure that is the object of the present invention is obtained.

Therefore, regarding sol.Al, N and Ti, the following (1)
, It is preferable that X and Y in the expression (2) are satisfied. X = 0.15 × sol.Al + N−0.29 × Ti−0.0030 ≧ 0 (1) Y = 0.15 × sol.Al + N−0.29 × Ti−0.0135 ≦ 0 (2) B. Object of the present invention Metallographic structure of steel (1) Ferrite grain size and shape: The ferrite grain size has the largest effect on YS and strongly controls its residual stress morphology. Residual stress due to cold working can be reduced due to the decrease in YS accompanying the coarsening of the ferrite grain size.

As a result of examining this YS proper region in the high carbon steel strip of the present invention, it was found that the size d of the ferrite grains is a condition exceeding 35 μm. When d is 100 μm or more,
The upper limit of d is 100 μm, since wrinkles due to stretcher strain become visible during cold working and the formability deteriorates.
Less than

The average ferrite grain size d defined here is
1.0 mm x 1.0 m in the area of the plate thickness 1/4 depth from the steel strip surface
This is the average value of the ferrite grain size observed from the microstructure photograph taken at 100 times the m field of view.

For convenience, the unit line length is 1.0 mm,
Further, the average ferrite grain size d may be calculated as 1.156 times the average value of the dividing lengths of the crystal grains in the extending direction and the direction perpendicular to the extending direction.

Regarding the ferrite grain shape, in the present invention, in order to suppress the in-plane anisotropy of mechanical properties during cold working and to make the two-dimensional residual stress and elongation during cold working uniform. It is advantageous that the ferrite grains are expanded in the rolling direction.

The degree of expansion of the ferrite structure at this time is defined by the shape e of ferrite particles based on the specification of expanded grains specified in JIS G0552, which is a unit in the direction perpendicular to the expanded direction. The number of particles per line segment n ₁ and the number of particles per unit line segment in the drawing direction n _{2 are} converted to a value, e =
It is defined by n ₁ / n ₂ . According to the invention, this ferrite shape is limited to e = n ₁ / n ₂ > 1.25.

(M) Grain size and spheroidization rate of carbides in steel: In the present invention, the carbides in steel are mainly composed of cementite, and the precipitation form of this cementite is the residual stress during cold working and during heating during heat treatment. Has a great influence on the solid solution state of the carbide and the hardenability at the time of quenching.

According to the knowledge of the inventor of the present invention, the grain size and spheroidization rate of the spheroidized carbide influence the heat treatment deformation amount in the present invention for suppressing the heat treatment deformation of the high carbon steel strip.

The form of the spheroidized carbide in the steel was determined by polishing the cross section of the steel strip, corroding it with a Nital solution, and then obtaining a plate thickness of 1/4.
The area of 100 μm width × 100 μm depth of the part is observed with a scanning electron microscope, and is specified by the data obtained at that time.

In a preferred embodiment of the invention, the spheroidized carbide, preferably spheroidized cementite, has an average particle size of 0.5.
Spheroidization rate ≧ 80% is to be satisfied at ˜2.0 μm.

Below this value, the yield strength (YS) increases, so the residual stress during cold working increases, and deformation due to the heating temperature during heat treatment tends to occur. Further, since the reduction of the spheroidization rate leads to the reduction of elongation and the cold workability is also deteriorated, the spheroidization rate is defined as 80% or more in the present invention.

At this time, the spheroidization rate% is defined as the occupancy ratio of the observed spherical carbides corroded by the Nital solution and having a major axis / minor axis <5 to all the observed carbides.

If the grain size of the carbide in the steel is less than 0.3 μm, YS is high and heat treatment deformation is likely to occur due to an increase in residual stress. Therefore, the grain size is set to 0.3 μm or more. Also,
If the particle size exceeds 2.0 μm, the solid solution of carbide during heat treatment and heating is restricted and hardenability deteriorates. When hardened, the uniformity of martensite is impaired and some pearlite is mixed,
In the preferred embodiment of the present invention, the grain size of carbides in steel is set to 2.0 μm or less because it leads to quenching strain.

(N) Average elongation value Elm, elongation in-plane anisotropy index ΔEl: The elongation value decreases as the C content and alloying elements increase. However, a certain amount of C, Mn, C is ensured by ensuring the metallographic structure as defined above according to the invention.
Even if it contains r and Mo, it has high elongation and is excellent in cold workability and effect of suppressing residual stress. Therefore, as a preferred embodiment of the present invention, it is decided to limit the elongation to a certain level or more in a state where these specific amounts of alloying elements are contained. At this time, the average elongation value Elm and the elongation in-plane anisotropy index ΔEl are defined as follows. Average elongation value Elm = (El ₀ +2 x El ₄₅ + El ₉₀ ) / 4) In-plane elongation anisotropy index ΔEl = (El ₀ -2 x El ₄₅ + El ₉₀ ) / 2) where El ₀ : elongation in the rolling direction El ₄₅ : Elongation in 45 ° direction in rolling direction El ₉₀ : Elongation in 90 ° direction in rolling direction Here, according to the knowledge of the present inventors , suppressing the in-plane anisotropy index ΔEl of elongation means heat treatment. It is effective in suppressing the dimensional change in the two-dimensional direction during heating. Therefore, Elm should be more than the parameter value of the specific alloy content, and ΔEl should be ±
It is limited to within 5%, and the range is specified in equations (5) and (6). 50-40 x C (%) -2 x Mn (%) -2 x Cr (%) -1 x Mo (%) <Elm (%) ・ (5) -5 <ΔEl (%) <5 ...・ (6) C. Method for producing steel material targeted by the present invention (o) hot rolling, annealing conditions: forming a metal structure of the high carbon steel strip targeted by the present invention, in particular, spheroidizing of cementite and coarse-grained structure of ferrite In order to do so, it is possible to specify the finishing temperature in hot rolling, the winding temperature, the annealing temperature after pickling, and the atmospheric conditions.

For the coarsening of ferrite and the spheroidizing of cementite, the hot rolling finishing temperature may be appropriately selected depending on the amount of C and the amount of alloy components.

In the present invention, Ac ₁ + 80 ° C. is used as a parameter with the Ac ₁ temperature varying depending on the amount of C and other alloy elements.
It was limited to the range of to Ac ₁ + 160 ° C. In a temperature range lower than this range, ferrite is excessively fine-grained even after annealing, the residual stress increases with an increase in YS, and a dimensional change occurs during heat treatment heating.

Also, when the finishing temperature is more than Ac ₁ + 160 ° C., the size of ferrite and pearlite grains before annealing is excessively coarsened, the ferrite grains are not coarsened during annealing, and the residual stress increases. Heat treatment causes dimensional changes during heating.

Regarding the coiling temperature, if it is less than 550 ° C.,
Even after annealing, the ferrite becomes excessively fine-grained and the residual stress increases as YS rises, causing dimensional changes during heat treatment and heating. Even when the temperature exceeds 700 ° C, the size of ferrite and pearlite grains before annealing is excessively coarsened, the ferrite grains are not coarsened during annealing, and dimensional changes occur during heat treatment and heating due to an increase in residual stress.

Regarding the annealing temperature, the temperature condition for obtaining an appropriate coarse-grained structure for a hot-rolled sheet is the coarse-grained ferrite specified in the present invention in a specific temperature range with respect to the parameter defined by Ac _1. since the structure is obtained, and define the temperature range _{Ac 1 -80 ℃ ~ Ac 1 +} 30 ℃ with.

At temperatures below this range, the ferrite becomes excessively fine-grained, and the residual stress increases with an increase in YS.
Heat treatment causes dimensional changes during heating. Further, if the temperature exceeds this range, excessive coarsening of ferrite and deterioration of the spheroidization rate of cementite will proceed.

The annealing temperature for the cold-rolled sheet is Ac ₁ −50 ° C. to Ac
The range is ₁ + 30 ℃. Further, the annealing atmosphere is a hydrogen atmosphere of 90% or more. Below this concentration, oxygen mixed as an impurity in the atmospheric gas causes
A decarburized layer consisting of extremely coarse ferrite grains is locally formed on the surface, which causes a decrease in elongation and the like. Therefore, the annealing atmosphere is limited to the conditions specified in the present invention.

(P) Cold rolling condition / annealing condition before cold rolling:
According to the findings of the present invention, as a method capable of further promoting and stabilizing coarsening of ferrite with respect to the manufacturing method defined in the item (o), cold rolling with an appropriate amount of reduction is performed before annealing. Is effective. At this time, the effective range of coarsening of ferrite grains by cold rolling is 5 to 30 in terms of reduction rate.
%. In the cold rolling in which the rolling reduction is less than this range, there is no difference between the condition without cold rolling and the effect of grain coarsening. On the other hand, if it exceeds 30%, increase in YS due to grain refinement, increase in residual stress, and dimensional change during heat treatment become apparent. At this time, it is also effective to make the ferrite grain size uniform by carrying out box annealing under the conditions specified in the above item (o) before cold rolling.

Next, the function and effect of the present invention will be described more specifically by way of examples.

[0074]

EXAMPLES Example 1 Steel Nos. 1 to 19 having the alloy compositions shown in Table 1 were heated to 1200 ° C.
After heating for × 1h, at the finishing temperature and winding temperature in Table 2, the plate thickness is 6.
A 0 mm steel strip was used. After descaling by pickling, cold rolling to 5.0 mm, and then 740 ° C x 12h in 100% hydrogen atmosphere
Annealing was applied to soak the heat. The metal structure and mechanical properties of the steel strip after this annealing were measured.

The elongation after annealing was used as the criterion for judging the excellent cold workability and hardenability, which were the objectives, and the satisfaction of the following two formulas was used as the criterion as defined in the present invention. 50−40 × C (%) − 2 × Mn (%) −2 × Cr (%) −1 × Mo (%) <Elm (%) <(5) −5 <ΔEl (%) <5 ... (6) In addition, the metal structure was also determined based on the conditions specified in the present invention.

Next, the cold-working test piece shown in FIG. 1 was sampled from the steel strip thus obtained and molded by a 55t crank press, and the horizontal height of the molded product was measured.

The measurement points were 7 points on the diameter shown in FIG. 1 in the rolling direction, the rolling direction 45 °, and the rolling direction 90 °, for a total of 19 points. It was placed on a horizontal surface plate and the height of each site was investigated using an LC-2320 / 2100 type laser displacement meter from KEYENCE. At this time, the horizontal steel platen was 1000 × 1000 mm, the height displacement accuracy was 10 μm / m, and the test piece was placed in the center of the platen for measurement.

In FIG. 1, FIG. 1 (a) is a plan view of the heat treatment distortion determination test piece, and FIG. 1 (b) is a sectional view thereof. In the figure, ○ indicates the measurement site, and the numbers indicate the dimensions (unit: mm).

The test piece after measuring the height was placed in an Ar atmosphere.
After heating at 870 ℃ for 30 minutes, quench in oil at 80 ℃ and continue to 42
After tempering at 0 ° C. for 40 minutes, the surface Vickers hardness after heat treatment was measured. The criterion was Hv250 or higher.

Further, subsequently, the test piece was measured by the above-mentioned laser displacement meter. The criterion for the degree of height displacement was that the height variation after heat treatment was within ± 0.2 mm.

The results are summarized in Table 3. As can be seen from these results, steel with a C content below the range of the present invention.
In No. 1, quenching deformation occurred, and the hardness after tempering was insufficient.
Steel No. 6 has an average value of elongation in three directions below the set condition.

Steel No. 7 has a Mn below the lower limit, and therefore has insufficient heat treatment hardness. Steel No. 11 has Mn above the upper limit and elongation below the invention range. In Steel No. 12, ΔEl is less than −5 because Al and conditional expression X are out of the range of the present invention. Also steel N
In o.15, Al and conditional expression X are out of the range of the present invention, so that the heat treatment strain increases. In Steel No. 16, since the conditional expression Y is out of the range of the present invention, ΔEl is less than −5, and heat treatment strain increases. In Steel No. 19, Si and X are out of the range of the present invention, so that heat treatment strain increases.

Example 2 Steel Nos. 20 to 35 having the alloy compositions shown in Table 4 were changed to 1200
After heating at ℃ × 1h, hot rolling was carried out at the finishing temperature and the coiling temperature shown in Table 5 to obtain a steel strip having a plate thickness of 6 mm. After subsequent pickling, plate thickness 5
Cold rolled to mm, 740 ° C x 16 in 100% hydrogen atmosphere
h Annealing for uniform heating was performed.

The metal structure of the obtained steel strip was observed and judged, and the mechanical properties were measured, the same test piece as in Example 1 was prepared, and the height in the horizontal direction was measured by the same method.

Subsequently, after heating in an Ar atmosphere at 870 ° C. for 30 minutes, it was quenched in oil at 80 ° C., and subsequently tempered at 420 ° C. for 40 minutes, and the surface Vickers hardness after the heat treatment was measured. The criterion was Hv250 or higher.

Further, subsequently, the test piece was measured by the above laser displacement meter. The criterion for the degree of height displacement was that the height variation after heat treatment was within ± 0.2 mm.
The criteria for determining the metal structure and mechanical properties after annealing were the same as in Test 1. The results are summarized in Table 6.

As can be seen from these results, Steel No. 23
Of Cr exceeds the range of the present invention, and the elongation Elm is below the target value. In Steel No. 27, Mo exceeds the range of the present invention, and the elongation Elm falls below the target value. Steel No. 31 contains B in an excessive amount exceeding the range of the present invention, so that the heat treatment hardness decreases and the heat treatment strain increases. Steel No. 35, Nb exceeds the scope of the present invention,
Further, since Ti is not added and B is contained alone, heat treatment hardness and heat treatment strain are large.

Example 3 Steel No. 3 in Table 1 satisfying the conditions of the present invention in chemical composition,
With respect to No. 21, as shown in Table 7, the manufacturing conditions were variously changed (processes No. 1 to No. 11), and the effects of the manufacturing conditions on the mechanical properties and heat treatment behavior were summarized.

At this time, in the annealing atmosphere, the contained element other than hydrogen is an unavoidable mixed gas atmosphere containing nitrogen as a main component and a slight amount of oxygen. The results are summarized in Table 8.

As can be seen from the results shown in Table 8, steel N
On the other hand, in Process No. 1 as compared with o.3, the finish hot rolling temperature (TF) was below the range of the present invention, and the ferrite grains became coarse.
El is out of the judgment condition, and in process No. 2, the winding temperature (TC)
Is less than the range of the present invention, and the ferrite grains are made finer, so that the quenching hardness is reduced and the heat treatment strain is also out of the judgment condition. Process No. 6 is because the rolling reduction in the cold rolling is out of the range of the present invention, so that the ferrite grains become finer,
The heat treatment strain also falls outside the judgment conditions.

Further, in the process No. 8, the hot rolling finish temperature is out of the range, in the process No. 9, the hot rolling coiling temperature is out of the range of the present invention, and the cementite particle size and the spheroidization rate are out of the conditions of the present invention. As a result, Elm is out of the judgment condition. In addition, in Process No. 11, since the annealing temperature exceeds the range of the present invention, the spheroidization rate is extremely low, and Elm falls outside the judgment conditions.

Compared with steel No. 21, in process No. 12, the finishing hot rolling temperature was below the range of the present invention, and ΔEl was out of the judgment condition because the ferrite grains became coarse, and in process No. 13, winding was performed. Since the taking temperature is below the range of the present invention and the ferrite grains become finer, the quenching hardness decreases and the heat treatment strain also falls outside the judgment conditions.

In Process No. 17, since the rolling reduction in cold rolling is out of the range of the present invention, the ferrite grains are made finer and the heat treatment strain is also out of the judgment condition. Furthermore, in process No. 19, the hot rolling finish temperature was out of the range of the present invention, and in process No. 20, the hot rolling coiling temperature was out of the range of the present invention. The condition is not met.

Further, in Process No. 22, since the annealing temperature exceeds the range of the present invention, the spheroidization rate is extremely low, and Elm is out of the judgment conditions.

[0095]

[Table 1]

[0096]

[Table 2]

[0097]

[Table 3]

[0098]

[Table 4]

[0099]

[Table 5]

[0100]

[Table 6]

[0101]

[Table 7]

[0102]

[Table 8]

[0103]

According to the present invention, it is possible to obtain a steel material having high softness and high ductility required for cold working, small dimensional variation during heat treatment performed after forming, and suitable for so-called fine blanking, By implementing the present invention, efficient production of automobile parts and the like is expected.

[Brief description of drawings]

FIG. 1 (a) is a plan view of a heat treatment distortion determination test piece, and FIG. 1 (b) is a sectional view thereof.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI C22C 38/38 C22C 38/38 (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (12)

(57) [Claims] 1. A mass ratio of C: 0.10 to 0.80%, Si: 0.005 to 0.30%, Mn: 0.20 to 1.
60%, sol.Al: 0.005 to 0.100%, N: 0.0010 to 0.0100%, T
i: 0.001 to 0.050%, and having a steel composition satisfying the expressions (1) and (2), X = 0.15 × sol.Al + N−0.29 × Ti−0.0030 ≧ 0 (1) Y = 0.15 × sol.Al + N-0.29 × Ti-0.0135 ≦ 0 (2) Average ferrite grain size d and shape e in steel are as follows (3), (4)
A high carbon steel strip that is soft and has uniform heat treatment strain, characterized by satisfying the conditions of the formula. 35 <d (μm) <100 ・ ・ ・ (3) e = n ₁ / n ₂ > 1.25 ・ ・ ・ (4) where the average ferrite grain size d is the thickness of the steel strip 1 /
4 This is the average value of the ferrite grain size observed from a micrograph of a 1.0 mm × 1.0 mm field of view taken 100 times in the region of the depth region. Shape e (n ₁ / n ₂ ) defined by the above equation (4)
Is calculated from the number of grains per unit line segment in the direction orthogonal to the splaying direction n ₁ and the number of grains per unit line segment in the splaying direction n ₂ based on the stipulated grain specifications in JIS G0552. It is a numerical value. 2. The average grain size of carbides in steel is 0.3 to 2.0 μm.
The high carbon steel strip according to claim 1, wherein the spheroidization rate ≧ 80%. However, the spheroidization rate of carbides in steel indicates the occupancy ratio of the spherical structure where the major axis / minor axis <5. 3. The average ferrite grain size d is the average value of the split lengths of crystal grains in the stretching direction and in the direction perpendicular to the stretching direction.
The high carbon steel strip according to claim 1 or 2, which has a value converted by 1.156 times. 4. The high carbon steel strip according to claim 2, wherein the carbide in the steel is mainly cementite. 5. Yield strength / tensile strength × 100 (%) (YR) is 40 to
The high carbon steel strip according to any one of claims 1 to 4, which is 80%. 6. The average elongation value Elm satisfies the equation (5) and the elongation anisotropy index ΔEl satisfies the equation (6), respectively.
High carbon steel strip according to any one of. 50-40 x C (%) -2 x Mn (%) <Elm (%) ... (5) -5 <ΔEl (%) <5 ... (6) However, Elm = (El ₀ + 2 x El ₄₅ + El ₉₀ ) / 4) ΔEl = (El ₀ -2 x El ₄₅ + El ₉₀ ) / 2) El ₀ : Elongation in rolling direction El ₄₅ : Elongation in 45 ° direction in rolling direction El ₉₀ : 90 ° in rolling direction Directional stretch 7. A mass ratio of C: 0.10 to 0.80%, Si: 0.005 to 0.30%, Mn: 0.20 to 1.
60%, Cr: 0.05-1.20%, Mo: 0.05-0.40%, Nb: 0.010-0.
050%, and B: 0.0002 to .0030% 1, two or more of, and, sol.Al:0.005 ~0.100%, N: 0.00 10 ~0.0100%, T
i: 0.001 to 0.050% is contained and has a steel composition satisfying the following formulas (1) and (2): X = 0.15 × sol.Al + N−0.29 × Ti−0.0030 ≧ 0 (1) Y = 0.15 × sol.Al + N−0.29 × Ti−0.0135 ≦ 0 (2) The average ferrite grain size d and shape e in the steel satisfy the conditions of formulas (3) and (4). High carbon steel strip that is soft and has uniform heat treatment distortion. 35 <d (μm) <100 ・ ・ ・ (3) e = n ₁ / n ₂ > 1.25 ・ ・ ・ (4) where the average ferrite grain size d is the thickness of the steel strip 1 /
4 This is the average value of the ferrite grain size observed from a micrograph of a 1.0 mm × 1.0 mm field of view taken 100 times in the region of the depth region. Shape e (n ₁ / n ₂ ) defined by the above equation (4)
Is calculated from the number of grains per unit line segment in the direction orthogonal to the splaying direction n ₁ and the number of grains per unit line segment in the splaying direction n ₂ based on the stipulated grain specifications in JIS G0552. It is a numerical value. 8. The yield strength / tensile strength × 100 (%) (YR)
The high carbon steel strip according to claim 7, which is 40 to 80%. 9. The high carbon steel strip according to claim 7, wherein the average elongation value Elm satisfies the following formula (5) and the elongation anisotropy index ΔEl satisfies the following formula (6). 50−40 × C (%) − 2 × Mn (%) −2 × Cr (%) −1 × Mo (%) <Elm (%) <(5) −5 <ΔEl (%) <5 ... (6) However, Elm = (El ₀ + 2 x El ₄₅ + El ₉₀ ) / 4) ΔEl = (El ₀ -2 x El ₄₅ + El ₉₀ ) / 2) El ₀ : elongation in rolling direction El ₄₅ : _{45 in} rolling direction Elongation in 90 ° direction El ₉₀ : Elongation in 90 ° direction in rolling direction After hot rolling at 10. finishing temperature Ac ₁ + 80 ° C. range to Ac ₁ +160 ° C., the winding range of 550 to 700 ° C., after pickling, in _{Ac 1 -80 ℃ ~Ac 1 + 30} ℃, The method for producing a high carbon steel strip according to any one of claims 1 to 9, wherein the box annealing is performed under the condition that the hydrogen concentration in the atmosphere is 90% or more by mass ratio. 11. After hot rolling at a finishing temperature Ac ₁ + 80 ° C. range ~Ac ₁ +160 ℃, 550 ~700 winding in the range of ° C., after pickling, after cold rolling at a reduction rate of 5-30% , And further Ac ₁ −50
℃ ~ Ac ₁ + 30 ℃, hydrogen concentration in the atmosphere is 90% by mass ratio
The box annealing is performed under the above conditions.
10. The method for producing a high carbon steel strip according to any one of 9 to 9. After hot rolling at 12. finishing temperature Ac ₁ + 80 ° C. range to Ac ₁ +160 ° C., the winding range of 550 to 700 ° C., after pickling, in _{Ac 1 -80 ℃ ~Ac 1 + 30} ℃, The hydrogen concentration in the atmosphere
Performed box annealing at mass ratio of 90% or more of the conditions, after cold rolling still at a reduction rate of 5-30%, subsequently Ac ₁ -50 ℃ ~Ac ₁
The box annealing is carried out at + 30 ° C. under the condition that the hydrogen concentration in the atmosphere is 90% or more by mass ratio.
The method for producing a high carbon steel strip according to any one of 1.