JP5064525B2 - High carbon steel sheet with low anisotropy and excellent hardenability and method for producing the same - Google Patents

Description

  The present invention relates to a high carbon steel sheet having small anisotropy and excellent hardenability and a method for producing the same.

In recent years, from the viewpoint of protecting the global environment, the weight reduction of automobile bodies has been promoted for the purpose of reducing CO ₂ emissions of automobiles, and the use of high-strength steel sheets has been promoted. In addition, automobile companies and parts manufacturers have manufactured parts that have been manufactured by hot forging with cold presses, such as by manufacturing processes by simplifying the process and saving energy and reducing costs. The process is more efficient.

  In particular, from the viewpoint of increasing the efficiency of the manufacturing process, hot forging of parts such as steel bars that have been used in the past and then ensuring the accuracy of the parts by cutting is omitted, and the plate material is cold forged. The so-called plate forging press production, which is manufactured by pressing, has been promoted, and a steel plate having a thickness of 2 mm or more has been used as a material to be forged.

  However, in the cold forging press of the plate material, (a) material cracking becomes more noticeable than in hot forging, and (b) The non-uniformity of formability due to the anisotropy in the plate surface is observed, and as a result, there are many problems to be solved such as the occurrence of cracks in a specific direction and the non-uniformity of the shape after pressing.

  At present, it is necessary to change the shape so that cracks do not occur, or to remove non-uniform parts (so-called ears) that have occurred after drawing, which has better workability and uniform characteristics. There is a need for materials.

  In addition, depending on the part, there are some parts that require a high Vickers hardness of 400 or more by quenching and tempering after pressing rather than material strength. Therefore, materials that require hardenability are also desired.

  Thus, in order to ensure the hardenability after forming the part, it is necessary to design the component composition of the high carbon steel so that the anisotropy is small and the hardenability can be ensured. Currently, there is no disclosure of a high carbon steel sheet having these characteristics and a method for producing the same.

  In view of the above situation, the present invention is a high-carbon steel plate for manufacturing parts used for automobile transmissions, etc., which have been conventionally manufactured by hot forging, etc., with a plate forging press (cold forming). For press-molding and quenching and tempering, for example, in a high-carbon steel sheet having a thickness of 2 mm or more, it is possible to prevent cracking during processing and to reduce workability anisotropy in the rolling direction and its perpendicular direction And

  The present inventors diligently studied a method for solving the above problems. As a result, the present inventors cannot reduce the anisotropy of the workability simply by changing the rolling conditions, and consistently optimize the component composition and the related structure control up to the hot rolling process. It was found that it is important to make it.

  Specifically, the amount of oxide and S during smelting are specified, the hot rolling conditions are optimized, and the structure is controlled to solve the above problems and to significantly improve the anisotropy. There was found.

  That is, the inventors of the present invention (i-1) the cause of the workability anisotropy in the rolling direction and the direction perpendicular thereto is particularly due to non-metallic inclusions present in the center region of the plate thickness and pearlite bands. It is a decrease in plastic deformability due to the dense state of carbide (cementite) and its size, and (i-2) the form of the dense state and its size becomes a continuous form in the rolling direction by rolling. Clarifies that the anisotropy of plastic deformability is promoted, and (ii-1) the increase in workability anisotropy can be suppressed by the relationship with the component composition, and (ii-2) It has been found that the degree of expansion and ratio in the rolling direction can be controlled by controlling the rolling conditions, the cooling conditions, and the winding conditions of the hot rolling as a series of conditions.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) By mass%, C: more than 0.2 to 0.70%, Si: 0.01 to 0.8%, Mn: 0.1 to less than 2.0%, P: 0.003 to 0. 030%, S: 0.0001 to 0.008%, Al: 0.005 to 0.07%, N: 0.0001 to 0.02%, O: 0.0001 to 0.0030%, and the balance A steel plate consisting of inevitable impurities and having an A value of 0.008 or less represented by the following formula (1), where t is the plate thickness in the plate thickness section parallel to the rolling direction,
(I) In a cross section of 4 / 10t to 6 / 10t, the area ratio of non-metallic inclusions having a length of 100 μm or more is 0.1% or less,
(Ii) at the same cross-sectional area, the area ratio of carbide carbide diameter exceeds the value indicated by the following equation (2) is not more than 10% of the carbides in the region,
A high carbon steel sheet with small anisotropy and excellent hardenability.
A value = O + S + 0.033Al (1)
O, S, Al: Content of each element B value (μm) = 1.5 × C% + 0.5 (2)

  (2) The said steel plate is the mass%, Nb: 0.003-0.1%, Ti: 0.001-0.05%, V: 0.001-0.05%, Ta: 0.01- High carbon having low anisotropy and excellent hardenability as described in (1) above, containing 0.5%, W: 0.01-0.5% or two or more steel sheet.

  (3) The said steel plate is the mass%, Cr: 0.1-2.0%, Ni: 0.01-1.0%, Cu: 0.01-1.0%, Mo: 0.005- 0.5%, B: 0.0005 to 0.01% of one type or two or more types are contained, and the anisotropy according to (1) or (2) is small and hardenability is achieved. Excellent high carbon steel plate.

  (4) The said steel plate is the mass%, Mg: 0.0005-0.003%, Ca: 0.0005-0.003%, Y: 0.001-0.03%, Zr: 0.001- One or more of 0.03%, La: 0.001 to 0.03%, Ce: 0.001 to 0.03%, containing (1) to (3) above A high carbon steel sheet having small anisotropy and excellent hardenability.

(5) In the production method for producing a high carbon steel sheet having small anisotropy and excellent hardenability according to any one of (1) to (4),
(I) The continuous cast slab or the steel ingot having the component composition according to any one of (1) to (4) is heated at 1150 ° C. or higher and 1300 ° C. or lower, and then
(Ii-1) Finish the rough rolling at 1000 ° C. or higher and 1150 ° C. or lower, perform hot rolling with a finish rolling temperature of Ae ₃ or higher, and then
(Ii-2) Take an air cooling time of 1 to 10 seconds, cool to a winding temperature at a cooling rate of 10 to 70 ° C./s, and wind at 450 to 650 ° C.
(Ii-3) A method for producing a high carbon steel sheet having small anisotropy and excellent hardenability, characterized by pickling and annealing after winding.

(6) The method for producing a high carbon steel sheet having low anisotropy and excellent hardenability according to (5), wherein the annealing is performed at a temperature of 600 ° C. or higher and Ac ₁ or lower for 8 hours or longer. .

(7) said annealing is performed at a temperature of Ac ₁ ~Ac ₁ + 50 ℃, then at 5 ° C. / time less cooling rate above, wherein the cooling to a temperature below the Ac ₁ -30 ° C. (5 The method for producing a high carbon steel sheet having small anisotropy and excellent hardenability.

  (8) The annealing is performed in an atmosphere of 95% or more of hydrogen, a dew point of up to 400 ° C. of less than −20 ° C., and a dew point of 400 ° C. or more of less than −40 ° C. The manufacturing method of the high carbon steel plate with small anisotropy in any one of-(7), and excellent in hardenability.

  (9) After the pickling, before the annealing, the high carbon steel sheet is subjected to cold rolling with a reduction rate of 10% or more, and the difference according to any one of (5) to (8), A method for producing a high-carbon steel sheet having low isotropic and excellent hardenability.

  (10) After annealing the high-carbon steel sheet subjected to cold rolling with a rolling reduction of 10% or more, the high-carbon steel sheet is again subjected to cold rolling with a rolling reduction of 10% or more, and then (6) A method for producing a high carbon steel sheet having small anisotropy and excellent hardenability, characterized by performing annealing according to any one of to (8).

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to improve the anisotropy of the high strength steel plate used as a material of automobile parts, and to manufacture the steel plate excellent in workability. As a result, in the manufacture of automobile parts, it is possible to simplify the manufacturing process and / or reduce the manufacturing cost.

It is a figure which shows the relationship between (O + S + 0.033Al) and the anisotropy ((phi) c / (phi) L) of ultimate deformability. It is a figure which shows the influence of a nonmetallic inclusion and carbide | carbonized_material on the anisotropy ((phi) c / (phi) L) of ultimate deformability.

  The present invention is described in detail below. As described above, the present inventors cannot reduce the anisotropy of workability simply by changing the rolling conditions, and consistently control the component composition and the related structure control up to the hot rolling process. We found that it is important to optimize.

  Therefore, we decided to investigate the influence of the component composition, 50kg steel ingot with 0.54% C-0.2% Si-0.6% Mn as the basic component composition and S, O and Al changed. Was melted under vacuum in a laboratory, heated at 1200 ° C., and then rolled from 100 mm thickness to 4 mm.

  The hot rolling was finished at 900 ° C., air cooled for 3 seconds, cooled to 500 ° C. at 30 ° C./s, held in a furnace at 600 ° C. for 1 hour, and then cooled in the furnace to simulate winding. A hot rolled sheet is pickled, annealed at 700 ° C. for 24 hours in an atmosphere of 100% hydrogen and dew point −55 ° C., and a round bar having a diameter of 3.5 mm from the rolling direction of the steel plate cooled in the furnace and the direction perpendicular thereto. Tensile test pieces were prepared and subjected to a tensile test.

  The ultimate deformability is calculated from the cross-sectional shrinkage of the specimen after the tensile test, the ultimate deformability in the rolling direction is φL, the ultimate deformability in the direction perpendicular to the rolling direction is φc, and the ratio (φc / φL) and components The relationship with composition was investigated.

The ultimate deformability is shown by the following formula.
Ultimate deformability φ = ln (A0 / A)
Where A0: cross-sectional area of the specimen before the tensile test
A: Cross-sectional area of fractured part after tensile test

  As shown in FIG. 1, there is a good correlation between the anisotropy of ultimate deformability (φc / φL) and (O + S + 0.033Al). When (O + S + 0.033Al) is 0.008 or less, rolling The cross-sectional shrinkage rate in the direction perpendicular to the direction approaches the cross-sectional shrinkage rate in the rolling direction, becomes a ratio of 0.9 or more, and anisotropy decreases.

  The reason for this is that when oxygen is reduced, the total amount of non-metallic inclusions is reduced, and by not adding excessive Al, coarse alumina-based non-metallic inclusions are reduced. It can be said that this is because it was suppressed together with the influence of O and Al. If this knowledge is followed, a steel plate with small anisotropy can be manufactured.

  Moreover, the component composition of 0.53% C-0.18% Si-0.31% Mn-0.022% P-0.0015% S-0.027% Al-0.0016% O manufactured by an actual machine The relationship between the component composition and the production conditions was investigated using a slab having As a result, it has been found that the presence of non-metallic inclusions and carbides during hot-rolled sheet annealing affects the anisotropy of ultimate deformability.

  In particular, in a slab manufactured by an actual machine, a high C region resulting from casting is generated at the center of the plate thickness, or the ratio of pearlite due to segregation of Mn is larger than that of the surface layer, so that spheroidization after annealing is performed. Cementite is coarsened and the abundance ratio of non-metallic inclusions such as MnS tends to be high.

  Therefore, the present inventors have investigated in detail the relationship between the anisotropy of ultimate deformability, the size of the carbide, and the existence state of non-metallic inclusions. As a result, when the sheet thickness in the sheet thickness section parallel to the rolling direction is t, the area ratio of nonmetallic inclusions having a length of 100 μm or more in the section of 4 / 10t to 6 / 10t is 0. If the area ratio of carbides of 1% or less and (y) 1.3 μm or more is 10% or more, the anisotropy of ultimate deformability (φc / φL) is less than 0.9 and the anisotropy increases. There was found.

  As the amount of coarse carbides increases, carbides are less likely to form a solution during heating before quenching, and it is difficult to shorten the heat treatment time in induction hardening, etc., so it is effective to reduce the proportion of coarse carbides. It is a technique.

  FIG. 2 shows the relationship between the anisotropy of ultimate deformability, the size of carbides, and the existence state of nonmetallic inclusions. From FIG. 2, it is understood that it is important to reduce the non-metallic inclusions such as MnS extending in the rolling direction to 100 μm or more and to keep the area ratio of coarse carbides below a certain value.

Furthermore, when the influence of the component composition was investigated, it was found that the diameter of the carbide correlates with the amount of C, and the critical diameter (B) of the carbide can be obtained by the following equation. As a result, it has become possible to determine the diameter of the influential carbide according to the amount of C.
B value (μm) = 1.5 × C% + 0.5

  The inclusions and carbides are measured by observing a plate thickness cross-sectional structure parallel to the rolling direction. About the nonmetallic inclusion, the state as grind | polished was observed by 100 to 1000 times using the optical microscope, and the area ratio was calculated | required. For carbide, lightly etched with a nital etchant, and a scanning electron microscope (SEM), 10 to 20 shots were taken at 1000 to 3000 times in an area with a plate thickness of 4 / 10t to 6 / 10t. The area ratio was obtained with an analyzer.

  First, the reason for limitation related to the component composition of the hot-rolled steel sheet of the present invention (hereinafter sometimes referred to as “the present steel sheet”) will be described. “%” Means “% by mass”.

C: Over 0.2 to 0.70%
C is an important element for securing the strength of the steel sheet. However, since the automobile member targeted by the present invention requires quenching and tempering after parts processing, hardenability and hardness after quenching are required. From the viewpoint of, it is necessary to exceed 0.2%. Depending on the required hardness, the amount of C is increased, but if it exceeds 0.70%, cracks during quenching and cracks at the welded parts during assembly become prominent, so the upper limit is 0.70%. did. Preferably it is 0.2 to 0.6%.

Si: 0.01 to 0.8%
Si is a solid solution strengthening element and can increase the strength of the steel sheet relatively inexpensively. In addition, Si needs to be added in a very small amount due to the relationship with scale wrinkles, so 0.01% or more is added, but adding more than 0.8% only saturates the addition effect. , Si was set to 0.01 to 0.8%. Preferably it is 0.05 to 0.6%.

Mn: 0.1 to less than 2.0%
Mn is a solid solution strengthening element and a hardenability improving element, and is an important element for ensuring a desired high tension in the hot-rolled steel sheet in relation to the heat treatment conditions of the steel sheet. If Mn is less than 0.1%, it is necessary to add other elements in order to ensure the required strength and hardenability, resulting in high costs. On the other hand, even if added over 2.0%, the strength Or, since the hardenability is only saturated, Mn is set to 0.1 to 2.0%. Preferably it is 0.3 to 1.5%.

P: 0.003-0.030%
P is a solid solution strengthening element and is an element that can increase the strength of the steel sheet relatively inexpensively. However, from the viewpoint of toughness after quenching and tempering, it is not preferable to add P excessively. 0.03% or less. From the viewpoint of refining, reducing P to less than 0.003% causes an increase in cost, so P was made 0.003 to 0.030%.

S: 0.0001 to 0.008%
S is a cause of lowering ductility and toughness in a steel sheet that is cold-formed, and in particular, from the viewpoint of reducing the amount of MnS that increases anisotropy, it needs to be reduced, so 0.008% or less. It was. Reducing S to less than 0.0001% significantly increases the refining cost, so S was made 0.0001 to 0.008%. Preferably it is 0.0001 to 0.003%.

Al: 0.005 to 0.07%
Al is an element added for deoxidation of steel. However, if it is less than 0.005%, the deoxidation effect is not sufficient. On the other hand, even if it exceeds 0.07%, the deoxidation effect is not enough. Not only is it saturated, but also at the time of slab bending correction during curved continuous casting, it promotes cracking due to precipitation of AlN and is economically disadvantageous, so Al is 0.005 to 0.07% did. Preferably it is 0.01 to 0.04%.

N: 0.0001 to 0.02%
When N precipitates as a nitride during slab bending correction during curved-type continuous casting, it causes cracking of the slab, so N is set to 0.02% or less. Reducing N to less than 0.0001% increases the refining cost, so N is set to 0.0001 to 0.02%.

O: 0.0001 to 0.0030%
Since O partially exists as an oxide, it affects the workability in the cold and causes the ductility and toughness to decrease. Increasing the amount of O also increases the inclusions, and when the inclusions aggregate, the ductility is significantly reduced. Since O is desirably reduced as much as possible, it is set to 0.0030%, but unavoidably 0.0001% or more exists, so O was set to 0.0001 to 0.0030%. In this invention, the fall of workability can be suppressed by satisfying the following formula.
A value = O + S + 0.033Al ≦ 0.008

Nb: 0.003 to 0.1%
Nb is an element that increases the strength of the steel sheet and improves the toughness of the steel sheet by the fine graining action, and is added as a selective element. If Nb is less than 0.003%, the effect of addition does not sufficiently appear. On the other hand, if it exceeds 0.1%, the effect of addition becomes saturated and economically disadvantageous, and the recrystallization behavior during hot rolling is not good. Because of the delay, Nb was set to 0.003 to 0.1%.

Ti: 0.001 to 0.05%
Ti is added as necessary from the viewpoint of N fixation. Ti contributes to embrittlement of the slab and stabilization of the material, but if it exceeds 0.05%, the effect of addition is saturated, while if it is less than 0.001%, the effect of addition cannot be obtained. Was 0.001 to 0.05%.

V: 0.001 to 0.05%
V is added as necessary to strengthen the hot-rolled steel sheet by precipitation of carbonitride. If it is less than 0.001%, the effect of addition is small. On the other hand, if it exceeds 0.05%, the effect of addition is saturated, so V is set to 0.001 to 0.05%.

Ta: 0.01 to 0.5%
Ta, like Nb and V, forms carbonitrides and is an element effective for preventing coarsening of crystal grains and improving toughness, and is added as necessary. If the content is less than 0.01%, the effect of addition is small. On the other hand, if the content exceeds 0.5%, the effect of addition is saturated, and the recrystallization is delayed due to an increase in cost or excessive carbide formation. Therefore, Ta is set to 0.01 to 0.5%.

W: 0.01-0.5%
W, like Nb, V, and Ta, is an element that forms carbonitride and is effective in preventing coarsening of crystal grains and improving toughness, and is added as necessary. If the content is less than 0.01%, the effect of addition is small. On the other hand, if the content exceeds 0.5%, the effect of addition is saturated, and the cost is increased and recrystallization is delayed due to excessive carbide formation. Since W increases, W was made 0.01 to 0.5%.

Cr: 0.1 to 2.0%
Cr is effective for strengthening steel sheets, and is an element that can be used as an alternative element for Mn, and is added as a selective element. If it is less than 0.1%, there is no effect of addition, whereas if it exceeds 2.0%, the effect of addition is saturated, so Cr was made 0.1 to 2.0%.

Ni: 0.01 to 1.0%
Ni is an element effective for toughness and strengthening of the steel sheet, and is added as a selective element. If it is less than 0.01%, there is no effect of addition. On the other hand, if it exceeds 1.0%, the effect of addition is saturated, so Ni was made 0.01 to 1.0%.

Cu: 0.01 to 1.0%
Cu, like Cr and Ni, is an element effective for securing the strength of the steel sheet, and is added as a selective element. If it is less than 0.01%, there is no effect of addition. On the other hand, if it exceeds 1.0%, the effect of addition is saturated, so Cu was made 0.01 to 1.0%.

Mo: 0.005-0.5%
Mo is an element effective for strengthening the structure and improving toughness, and is added as a selective element. If it is less than 0.005%, the effect of addition is small. On the other hand, if it exceeds 0.5%, the effect of addition is saturated, so Mo was made 0.001 to 0.05%.

B: 0.0005 to 0.01%
B is hard to improve hardenability when added in a small amount. Therefore, B is an element effective in reducing expensive alloy elements and lowering costs, and is added as necessary. If it is less than 0.0005%, there is no effect of addition. On the other hand, if it exceeds 0.01%, the castability deteriorates and promotes cracking of the slab, so B is set to 0.0005 to 0.01%.

Mg: 0.0005 to 0.003%
Mg is an element that is effective for controlling the form of oxides and sulfides in a small amount, and is added as necessary. If it is less than 0.0005%, the effect of addition cannot be obtained. On the other hand, if it exceeds 0.003%, the effect of addition is saturated, so Mg was made 0.0005 to 0.003%.

Ca: 0.0005 to 0.003%
Ca, like Mg, is an element that is effective for controlling the form of oxides and sulfides with a small amount of addition, and is added as necessary. If it is less than 0.0005%, the effect of addition cannot be obtained. On the other hand, if it exceeds 0.003%, the effect of addition is saturated, so Ca is set to 0.0005 to 0.003%.

Y: 0.001 to 0.03%
Y, like Ca and Mg, is an element effective for controlling the form of oxides and sulfides, and is added as necessary. If it is less than 0.001%, the effect of addition cannot be obtained. On the other hand, if it exceeds 0.03%, the effect of addition is saturated and the castability deteriorates, so Y is set to 0.001 to 0.03%. .

Zr: 0.001 to 0.03%
Zr is an element effective for controlling the form of oxides and sulfides, as with Y, Ca, and Mg, and is added as necessary. If it is less than 0.001%, the effect of addition cannot be obtained. On the other hand, if it exceeds 0.03%, the effect of addition is saturated and the castability deteriorates, so Zr is set to 0.001 to 0.03%. .

La: 0.001 to 0.03%
La, like Zr, Y, Ca, and Mg, is an element effective for controlling the form of oxides and sulfides, and is added as necessary. If it is less than 0.001%, the effect of addition cannot be obtained. On the other hand, if it exceeds 0.03%, the effect of addition is saturated and castability deteriorates, so La is set to 0.001 to 0.03%. .

Ce: 0.001 to 0.03%
Ce, like La, Zr, Y, Ca, and Mg, is an element effective for controlling the form of oxides and sulfides, and is added as necessary. If it is less than 0.001%, the effect of addition cannot be obtained. On the other hand, if it exceeds 0.03%, the effect of addition is saturated and the castability deteriorates, so Ce is set to 0.001 to 0.03%. .

  Other elements are not particularly defined, but elements such as Sn, Sb, Zn, Zr, As may be mixed as inevitable impurities from scrap. Even if these elements are mixed in an amount of about the amount of impurities, the characteristics of the steel of the present invention are not significantly affected.

  Below, the method (henceforth "the manufacturing method of this invention") which manufactures this invention steel plate is demonstrated.

In the production method of the present invention, a continuous cast slab or a steel ingot satisfying the composition of the present invention is charged directly or after cooling into a heating furnace, (i) heated to 1150 ° C. or higher, and (ii -1) Finish the rough rolling at 1000 ° C. or higher, perform hot rolling with a finishing temperature of Ae ₃ or higher, and then (ii-2) take an air cooling time of 1 to 10 seconds to obtain 10 to 70 ° C./s The steel sheet is cooled to a coiling temperature at a cooling rate of 4 to 650 ° C. and (ii-3) picked and annealed after winding.

  The continuous cast slab or steel ingot used for hot rolling is heated at 1150 ° C or higher and 1300 ° C or lower. When the heating temperature is less than 1150 ° C. (low temperature heating), the rolling temperature during hot rolling decreases, and the recrystallization behavior during rough rolling and the recrystallization behavior during air cooling after continuous hot rolling proceed. However, expanded grains remain in the steel sheet structure and anisotropy increases.

  Therefore, the heating temperature of the continuous cast slab or the steel ingot is 1150 ° C. or higher, but if the heating temperature exceeds 1300 ° C., crystal grains become coarse and anisotropy increases, so the heating temperature is 1300 ° C. or lower. Is preferred. A preferable heating temperature range is 1150 to 1270 ° C.

  In addition, when continuously cast slabs or steel ingots are directly or reheated and subjected to hot rolling, the hot rolling may be normal hot rolling, or continuous hot joining the slab by finish rolling. It may be rolled, and there is almost no difference in the properties of the steel sheet.

  In the production method of the present invention, it is desirable to utilize the recrystallization behavior during hot rolling in order to reduce the anisotropy due to rolling. In normal reverse rolling, in order to promote recrystallization behavior in rough rolling performed for 5 passes or more, the end temperature of rough rolling is set to 1000 ° C. or more, and anisotropy is reduced by recrystallization before finish rolling. When the end temperature of the rough rolling is less than 1000 ° C., the grain growth after recrystallization becomes insufficient, the longitudinal structure at the start of finish rolling becomes uneven, and the anisotropy varies.

  The higher the end temperature of the rough rolling, the better from the viewpoint of recrystallization. However, when the temperature is too high, crystal grain growth after recrystallization becomes excessively significant, and conversely, the grain size becomes coarse. If the particle size is too coarse, it adversely affects the development of expanded grains during finish rolling, and therefore it is necessary to finish rough rolling at 1150 ° C. or lower. The end temperature of rough rolling is preferably 1000 to 1100 ° C.

In finish rolling, recrystallization is promoted by setting the finish rolling temperature to Ae ₃ or higher. When Ar ₃ is used as a standard for the rolling end temperature, even if rolling is finished with an austenite structure, if the structure is in an undercooled state, recrystallization does not proceed sufficiently and anisotropy is promoted. Therefore, in the production method of the present invention, the end temperature of continuous hot rolling is set to Ae ₃ or more, an air cooling time of 1 to 10 seconds is taken in consideration of the progress of recrystallization, and then cooling is performed. When the air cooling time exceeds 10 seconds, the temperature of the steel sheet is remarkably lowered, the recrystallization behavior becomes slow, and the anisotropy improving effect is saturated.

  After air cooling, the steel sheet is cooled at a cooling rate of 10 to 70 ° C./s and wound at 450 to 650 ° C. When the cooling rate is less than 10 ° C./s, coarse ferrite and coarse pearlite are generated and annealed. Subsequent carbides become coarse, the plastic deformability itself decreases, and the anisotropy of the ultimate deformability increases, so the cooling rate is set to 10 ° C./s or more.

  When the cooling rate is more than 70 ° C./s, uneven cooling occurs in the width direction of the steel sheet. In particular, the vicinity of the edge of the hot-rolled steel sheet is supercooled and hardened, causing variations in the material of the hot-rolled steel sheet, requiring trimming of the steel sheet edge, and reducing the yield, so the cooling rate is 70 ° C./s. The following.

  Winding after the hot-rolled steel sheet is cooled is performed at 450 to 650 ° C. When the coiling temperature is less than 450 ° C., martensitic transformation occurs, the steel sheet strength increases, the workability decreases, and handling during rewinding becomes difficult.

  On the other hand, when the coiling temperature exceeds 650 ° C., C discharged during ferrite transformation is concentrated in austenite, and a pearlite structure in which ferrite and carbide are concentrated is generated, and the distribution of carbide after annealing becomes non-uniform, Since the plastic deformability varies, the winding temperature is set to 650 ° C. or lower.

  The wound hot-rolled steel sheet is then pickled according to the product sheet thickness and the required softening level, and then subjected to annealing and cold rolling. The conditions at that time will be described.

  After winding the hot-rolled steel sheet, the steel sheet is pickled and then annealed to spheroidize and soften the pearlite structure. The annealing needs to be performed at 600 ° C. or more for 8 hours or more. By this annealing, spheroidization of the carbide is promoted and ferrite grain growth is promoted to soften. If the annealing temperature is less than 600 ° C. or the annealing time is less than 8 hours, carbide spheroidization or ferrite grain growth does not proceed and workability cannot be ensured.

  Since the upper limit of the annealing time varies depending on the steel type, it cannot be limited to a specific time. However, if the annealing time is too long, the softening progresses, but the carbide becomes coarse and the ultimate deformability decreases. Therefore, practically, when the annealing temperature is 600 ° C. or higher, the annealing time is preferably 200 hours or less.

Annealing is preferably performed at a temperature of Ac ₁ or lower, but if high-temperature heating or cooling control is possible in equipment, high-temperature annealing is performed at a temperature of Ac ₁ or higher in order to promote softening in a short time. Is also possible. The high temperature annealing performed at Ac ₁ or higher will be described later.

  The annealing atmosphere can be an atmosphere mainly composed of nitrogen, but an atmosphere containing 95% or more of hydrogen is preferable, but the atmosphere should be selected in consideration of the softening level required for the steel material and hardenability. Is possible.

  However, from the viewpoint of hardenability, if the steel sheet having the component composition to which B is added is annealed in an atmosphere mainly composed of nitrogen, the hardenability of the steel sheet surface layer portion may be deteriorated due to nitrogen absorption. When annealing the steel sheet, it is preferable to use an atmosphere containing 95% or more of hydrogen, desirably 100% hydrogen.

  In the case of annealing in an atmosphere mainly composed of hydrogen, from the viewpoint of safety at the time of raising the temperature, the atmosphere is temporarily changed to an atmosphere mainly composed of nitrogen, and then the temperature is increased by substituting with hydrogen. Is −20 ° C. or lower, and the dew point is −40 ° C. or lower at 400 ° C. or higher and during holding.

  This is important from the viewpoint of reducing the solid solution B due to oxidation of the surface layer portion of the steel plate and preventing fluctuations in the composition of the surface layer portion due to decarburization. In order to improve the hardenability, the steel added with B is annealed at 660 ° C. or higher for softening, but the dew point of the atmosphere is −40 ° C. or lower.

  The manufacturing method of the present invention can take various modes as described below.

  Cold rolling is used to secure the product thickness and to efficiently perform softening in combination with annealing. When the rolling reduction is 10% or more, the spheroidization of carbide is promoted, The recrystallization without generation and the grain growth upon completion of recrystallization are likely to cause the coarsening of the crystal grains, and the softening is promoted.

  The upper limit of the rolling reduction is not particularly defined, but if the rolling reduction exceeds 60%, the uniformity of the steel sheet structure by cold rolling is further improved, but on the other hand, the recrystallized grains during annealing become finer and softened. For this reason, it must be annealed for a long time, and the anisotropy produced by cold rolling develops, so the rolling reduction is preferably 60% or less. However, the rolling reduction is appropriately determined from the viewpoint of manufacturing cost and homogenization of characteristics.

  In the manufacturing method of the present invention, after the annealing, the steel sheet may be cold-rolled again with a reduction rate of 10% or more, and then annealed again.

As described above, in the present invention, if the annealing equipment is capable of high-temperature annealing and subsequent cooling control, when the steel sheet is annealed in an atmosphere containing hydrogen of 95% or more, the range of Ac _{1 to} Ac ₁ + 50 ° C. annealing at a temperature, after annealing, following cooling rate 5 ° C. / hr, to slow cooling to Ac ₁ -30 ° C..

The reason for annealing at a temperature of Ac ₁ ~Ac ₁ + 50 ℃ is a temperature range which coexist ferrite phase and austenite phase, in order to leave the carbide in the ferrite phase. If the temperature is raised from the above temperature range and the annealing temperature is close to the austenite single phase region, the pearlite transformation during cooling cannot be prevented and it may be hardened, so annealing is performed in the above temperature range.

The reason for slow cooling to Ac ₁ -30 ° C. at a cooling rate of 5 ° C./hr or less after annealing is to promote carbide spheroidization starting from the carbides present in the ferrite phase during the annealing, and the austenite phase of ferrite grains This is in order to promote growth and further softening.

  When the cooling rate is faster than 5 ° C./hr or less, pearlite transformation occurs during cooling, and it hardens, and the grain growth of the ferrite phase to the austenite phase does not proceed, so that it does not soften.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
A 50 kg steel ingot having the composition shown in Table 1 and Table 2 (continuation of Table 1) was melted in a laboratory by vacuum melting and hot rolling shown in Table 3 and Table 4 (continuation of Table 3). Under the conditions, a steel plate having a thickness of 4 mm was manufactured. From the obtained steel sheet, a sample for observing the structure, a round bar tensile test piece for measuring the ultimate deformability, and a sample for quenching were collected. Tables 5 and 6 (continuation of Table 5) show the cold rolling conditions and annealing conditions applied to the samples.

  The structure observation is performed by polishing a cross section in the thickness direction parallel to the rolling direction, and then observing the polished state with an optical microscope, and a non-metal having a length of 100 μm or more existing in a 4 / 10t to 6 / 10t portion. The area ratio of inclusions was determined by point count.

  Further, in order to observe the state of carbides present in the same cross-sectional area, the area is lightly etched with nital, using an optical microscope and a scanning electron microscope, and the area ratio of carbides present in the center, and The carbide diameter was determined and the relationship with the B value was investigated.

  The round bar tensile test piece is a test piece having a diameter of 3.5 mm, the area of the fractured portion after the tensile test is broken, and the ultimate deformability in the rolling direction is determined according to the formula of the ultimate deformability shown above. The ratio φc / φL was determined with φL as the ultimate deformation in the direction perpendicular to the rolling direction as φc. The area ratio of the non-metallic inclusions, the area ratio of carbides having a carbide diameter equal to or greater than the B value to the carbides in the 4/10 to 6/10 t part, and the ultimate deformability ratio were determined.

  In the quenching experiment, the sample was heated to 1000 ° C. at a heating rate of 100 ° C./sec by high-frequency heating, held for 2 seconds, and then cooled with water. After cutting the sample, the structure of the cross section was observed to observe the quenched state. Those in which a defective structure such as pearlite is observed in the surface layer portion are shown in Table 7 and Table 8 (continuation of Table 7) as poor quenching characteristics.

  The above results are summarized and shown in Table 7 and Table 8 (continuation of Table 7).

  As shown in Table 7 and Table 8, those satisfying the composition of the present invention and the production conditions show a good value of an ultimate deformation ratio of 0.9 or more, and cracks are generated in a specific direction during plate forging press. As a result, anisotropy is reduced in the plastic deformability, which is an index of workability effective for prevention. Also, the hardenability is good, the anisotropy is small, and the hardenability is also good.

  On the other hand, even if the component composition is out of the scope of the present invention or the component composition is within the scope of the present invention, the steel sheet whose manufacturing conditions do not satisfy the scope of the present invention has an ultimate deformability ratio of 0.9 or less. Thus, the anisotropy cannot be improved, and when the atmosphere deviates from the conditions of the present invention, the hardenability is reduced by decarburization or deboronation of the steel sheet surface layer, and the desired steel sheet characteristics are obtained. I can't understand.

  As described above, according to the present invention, it is possible to improve the anisotropy of a high-strength steel plate used as a material for automobile parts and to manufacture a steel plate having excellent workability. As a result, in the manufacture of automobile parts, it is possible to simplify the manufacturing process and / or reduce the manufacturing cost. Therefore, the present invention also contributes to energy saving, and thus has a great applicability in the steel industry.

Claims (10)

In mass%, C: more than 0.2 to 0.70%, Si: 0.01 to 0.8%, Mn: less than 0.1 to 2.0%, P: 0.003 to 0.030%, S: 0.0001 to 0.008%, Al: 0.005 to 0.07%, N: 0.0001 to 0.02%, O: 0.0001 to 0.0030%, and the balance unavoidable impurities And the A value shown by the following formula (1) is 0.008 or less, and when the thickness in the thickness section parallel to the rolling direction is t,
(I) In a cross section of 4 / 10t to 6 / 10t, the area ratio of non-metallic inclusions having a length of 100 μm or more is 0.1% or less,
(Ii) at the same cross-sectional area, the area ratio of carbide carbide diameter exceeds the value indicated by the following equation (2) is not more than 10% of the carbides in the region,
A high carbon steel sheet with small anisotropy and excellent hardenability.
A value = O + S + 0.033Al (1)
O, S, Al: Content of each element B value (μm) = 1.5 × C% + 0.5 (2)   The said steel plate is the mass%, Nb: 0.003-0.1%, Ti: 0.001-0.05%, V: 0.001-0.05%, Ta: 0.01-0.5 %, W: 0.01-0.5% of 1 type or 2 types or more, The high-carbon steel plate having low anisotropy and excellent hardenability according to claim 1.   The said steel plate is the mass%, Cr: 0.1-2.0%, Ni: 0.01-1.0%, Cu: 0.01-1.0%, Mo: 0.005-0.5 %, B: 0.0005-0.01% of 1 type or 2 types or more, The high-carbon steel plate having low anisotropy and excellent hardenability according to claim 1 or 2.   The said steel plate is the mass%, Mg: 0.0005-0.003%, Ca: 0.0005-0.003%, Y: 0.001-0.03%, Zr: 0.001-0.03. %, La: 0.001-0.03%, Ce: 0.001-0.03% 1 type or 2 types or more are contained, The any one of Claims 1-3 characterized by the above-mentioned. High carbon steel sheet with low anisotropy and excellent hardenability. In the manufacturing method which manufactures the high carbon steel plate which the anisotropy of any one of Claims 1-4 was small and excellent in hardenability,
(I) A continuous cast slab or a steel ingot having the component composition according to any one of claims 1 to 4 is heated at 1150 ° C or higher and 1300 ° C or lower, and then
(Ii-1) Finish the rough rolling at 1000 ° C. or higher and 1150 ° C. or lower, perform hot rolling with a finish rolling temperature of Ae ₃ or higher, and then
(Ii-2) Take an air cooling time of 1 to 10 seconds, cool to a winding temperature at a cooling rate of 10 to 70 ° C./s, and wind at 450 to 650 ° C.
(Ii-3) A method for producing a high carbon steel sheet having small anisotropy and excellent hardenability, characterized by pickling and annealing after winding. The annealing at 600 ° C. or higher Ac ₁ temperature below method for producing a high-carbon steel sheets anisotropy described excellent small hardenability to claim 5, characterized in that more than 8 hours. Said annealing is performed at a temperature of Ac ₁ ~Ac ₁ + 50 ℃, then at 5 ° C. / time less cooling rate, according to claim 5, wherein the cooling to a temperature below the Ac ₁ -30 ° C. A method for producing a high carbon steel sheet having small anisotropy and excellent hardenability.   The annealing is performed in an atmosphere of 95% or more of hydrogen, a dew point of up to 400 ° C of less than -20 ° C, and a dew point of 400 ° C or more of less than -40 ° C. A method for producing a high carbon steel sheet having small anisotropy and excellent hardenability according to claim 1.   The anisotropy according to any one of claims 5 to 8, wherein after the pickling and before annealing, the high carbon steel sheet is subjected to cold rolling with a rolling reduction of 10% or more. A method for producing a high carbon steel sheet with excellent hardenability.   After annealing the high-carbon steel sheet subjected to cold rolling with a reduction rate of 10% or more, the high-carbon steel sheet is again subjected to cold rolling with a reduction ratio of 10% or more, and then any one of claims 6-8. A method for producing a high-carbon steel sheet having low anisotropy and excellent hardenability, characterized by performing annealing according to claim 1.

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