TW201708569A - Steel sheet and method for producing same - Google Patents

Abstract

Provided is a low carbon steel sheet which exhibits excellent cold forgeability and excellent impact resistance characteristics following carburizing and quenching/tempering, and which is characterized by having a prescribed constituent composition, having an average carbide particle diameter of 0.4-2.0 [mu]m, having a pearlite areal ratio of 6% or less, having a ratio of number of carbides at ferrite grain boundaries relative to number of carbides inside ferrite grains of greater than 1, and having a Vickers hardness of 100-180 HV.

Description

Steel plate and method of manufacturing same Technical field

The present invention relates to a steel sheet and a method of manufacturing the same.

Background technique

A steel sheet containing 0.1% to 0.4% by mass of carbon is being subjected to press forming, hole expanding forming, bending forming, press forming, thickening and thickness reduction, and cold forging after combining the raw materials. It is used as a material for drive system parts such as gears and clutches for automobiles. Compared with the conventional hot forging, the amount of strain accumulated in the material during cold forging becomes high, and there is a problem that the material is cracked or warped during molding, and the characteristics of the part are deteriorated.

In particular, after the carbonizing and quenching and tempering of the molding material for the wear resistance, residual stress is generated by the heat treatment, and the crack is generated and developed from the cracked portion and the warped portion. In order to use as a drive system component, it is required to have a load-bearing load that is instantaneously large due to the engagement of gears at the time of start-up, etc., and it is possible to obtain a punching resistance that does not cause brittle fracture, so that the above-mentioned steel sheet is required to have excellent cold forgeability and to ensure carbon impregnation. The punching resistance after quenching and tempering.

So far, many people have proposed to improve the cold forgeability of steel plates. A technique for resisting the punching property after carbon immersion (see, for example, Patent Documents 1 to 5).

For example, Patent Document 1 discloses a steel for machine structural use, which is a steel for machine structural use which improves the toughness by suppressing the coarsening of crystal grains by the carbon-baking heat treatment, and contains C: 0.10 to 0.30% by mass%. Si: 0.05 to 2.0%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.030% or less, Cr: 1.80 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.02 to 0.10%, N: 0.0300 Below %, the remainder consists of Fe and unavoidable impurities, and the structure before cold working is ferrite. In the Borne iron structure, the average particle size of the ferrite is 15 μm or more.

Patent Document 2 discloses a steel which is excellent in cold workability and carbon kneading property, and contains C: 0.15 to 0.40%, Si: 1.00% or less, Mn: 0.40% or less, and sol. Al: 0.02% or less. N: 0.006% or less, B: 0.005 to 0.050%, and the balance is composed of Fe and unavoidable impurities, and has a structure mainly composed of a ferrite-grained iron phase and a graphite phase.

Patent Document 3 discloses a steel material for a carbon-impregnated helical gear excellent in punching strength, a high-toughness carbon-impregnated helical gear, and a method for producing the same.

Patent Document 4 discloses a part which is subjected to cold forging after spheroidizing annealing and which is produced in a carbon-impregnated quenching and tempering step, and has excellent workability, and can also suppress coarsening of crystal grains in the subsequent carbon impregnation. Steel for carbon-impregnated parts with excellent punching resistance and crushing fatigue resistance.

Patent Document 5 discloses a steel as a cold tool steel for plasma impregnation, comprising: C: 0.40 to 0.80%, Si: 0.05 to 1.50%, Mn: 0.05 to 1.50%, and V: 1.8 to 6.0%. Further, one or more of Ni: 0.10 to 2.50%, Cr: 0.1 to 2.0%, and Mo: 3.0% or less, and the remaining portion It consists of Fe and unavoidable impurities.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2013-040376

Patent Document 2: Japanese Patent Laid-Open No. Hei 06-116679

Patent Document 3: Japanese Patent Laid-Open No. 09-201644

Patent Document 4: Japanese Patent Laid-Open Publication No. 2006-213951

Patent Document 5: Japanese Patent Laid-Open No. Hei 10-158780

Summary of invention

The steel structure for mechanical structure of Patent Document 1 is a structure of ferrite iron + wave iron, which has a larger hardness than the ferrite iron + swarf carbon iron structure, and thus cannot suppress the mold for cold forging. The loss is not called a steel for mechanical construction excellent in cold forgeability.

In the steel of Patent Document 2, the graphitization treatment of Xueming carbon iron requires annealing at a high temperature, failing to suppress a decrease in yield or an increase in manufacturing cost.

The manufacturing method of Patent Document 3 requires a hot forging after cold forging and carbon immersion, and a manufacturing method which is not fundamentally cost-effective because of hot forging.

It is not clear whether the steel for carbon-impregnated parts of Patent Document 4 has the same effect in cold forging imparting a large strain. In addition, the specific control method of the structure or organization is not clear, and it has been widely used in recent years. In the forging of a strain imparted to a large strain in a cold environment such as forging, it has not been called a steel which exhibits excellent workability.

Patent Document 5 does not disclose any observational knowledge and technique for improving the formability of steel, particularly the optimum composition and texture of cold forgeability.

The present invention is excellent in the punching resistance after cold forgeability and carbon-baking quenching and tempering in view of the actual situation of the prior art described above, and particularly provides a preferred steel sheet suitable for obtaining high-frequency gears and the like by sheet forming, and a method of manufacturing the same. As a subject.

In order to solve the above-mentioned problem, in order to obtain a steel sheet suitable for materials such as a drive component, it is known that the steel sheet containing the C required for improving the hardenability increases the particle size of the ferrite iron and bonds the carbide (mainly snow-capped carbon). Iron) is spheroidized into a suitable particle size to reduce the Borne iron structure. This is because of the following reasons.

The ferrite iron phase has low hardness and high ductility. Therefore, the structure in which the ferrite-grained iron is the main body can be improved by increasing the particle diameter thereof.

By appropriately dispersing carbides in the metal structure, it is possible to maintain the material formability and impart excellent wear resistance or rotational fatigue characteristics, so that it is an indispensable structure in the drive system parts. Further, the carbide in the steel sheet is a hard particle that prevents sliding, and the presence of carbide in the ferrite grain boundary prevents the sliding from propagating over the crystal grain boundary, thereby suppressing the formation of the shear zone and improving Cold forgeability also improves the formability of the steel sheet.

However, the stellite carbon iron is a hard and brittle structure, and when the iron and iron are in the form of a layered structure, since the steel is hard and brittle, it needs to exist in a spherical shape. Consider the cold forging property, or the crack generated during forging, which is required to be in the proper range.

However, the manufacturing method for realizing the aforementioned organization has not been disclosed so far. Therefore, the inventors of the present invention have intensively studied a manufacturing method capable of realizing the aforementioned structure.

As a result, in order to make the metal structure of the steel sheet after the hot rolling is rolled up, a fine wave having a small layer interval is formed, and the tough iron structure of the ferritic carbon iron is dispersed in the iron or the fine ferrite iron. Low temperature coiling (400 ° C ~ 550 ° C). By taking it at a lower temperature, the smectite carbon iron dispersed in the ferrite iron also becomes easy to spheroidize. Next, in the first-stage annealing, annealing is performed at a temperature lower than the Ac1 point to partially spheroidize the sulphur carbon. Then, in the second-stage annealing, the temperature between the Ac1 point and the Ac3 point (ie, the two-phase domain of the ferrite iron and the Vostian iron) is annealed, and a part of the ferrite particles are left, and a part of the ferrite is transformed into a worth. Tian Tie. Then, the slow cooling is carried out to grow the residual ferrite particles, and the Worthite iron is transformed into a fermented iron and iron metamorphism, thereby obtaining a large ferrite iron phase and precipitating the snow-light carbon at the grain boundary. Iron, found to be able to achieve the aforementioned organization.

In other words, it has been found that only a single study of hot rolling conditions or annealing conditions, etc., which is difficult to achieve at the same time satisfying the hardenability and formability of the steel sheet by hot rolling. A so-called series of steps such as an annealing step can be achieved by optimizing the steps.

Further, the improvement of the press formability at the time of cold forging is required to reduce the plastic anisotropy, and it has been found that the adjustment of the hot rolling conditions is important for the improvement.

The present invention has been constructed in view of the knowledge gained from such observations, and the gist thereof is as follows.

(1) A steel sheet having a composition of C: 0.10 to 0.40%, Si: 0.01 to 0.30%, Mn: 0.30 to 1.00%, and Al: 0.001 to 0.10%, Cr: 0.50 to 2.00%, Mo: 0.001 to 1.00%, P: 0.020% or less, S: 0.010% or less, N: 0.020% or less, O: 0.020% or less, Ti: 0.010% or less, B : 0.0005% or less, Sn: 0.050% or less, Sb: 0.050% or less, As: 0.050% or less, Nb: 0.10% or less, V: 0.10% or less, Cu: 0.10% or less, W: 0.10% or less, Ta: 0.10 % or less, Ni: 0.10% or less, Mg: 0.050% or less, Ca: 0.050% or less, Y: 0.050% or less, Zr: 0.050% or less, La: 0.050% or less, and Ce: 0.050%, and the balance is Fe and a low-carbon steel sheet with impurities; the metal structure of the low-carbon steel sheet satisfies: a carbide particle size of 0.4 to 2.0 μm, a brite iron area ratio of 6% or less, and a number of carbides in the ferrite grain boundary with respect to the fat grain The ratio of the number of carbides in the iron particles is more than 1; the Vickers hardness of the low carbon steel sheet is 100 HV or more and 180 HV or less.

(2) A method for producing a cold forge and a steel sheet according to the above (1), wherein the steel sheet having the composition of the component (1) is subjected to hot rolling to form a hot rolled steel sheet, and The hot rolling is performed in a temperature range of 650 ° C or higher and 950 ° C or less to terminate the final hot rolling; the hot rolled steel sheet is taken up at 400 ° C or higher and 600 ° C or lower; and the hot rolled steel sheet after coiling is subjected to pickling, and then The hot-rolled steel sheet after pickling is heated to an annealing temperature of 650 ° C or higher and 720 ° C or lower at a heating rate of 30 ° C / hour or more and 150 ° C / hour or less, and then subjected to the first stage of maintaining for 3 hours or more and 60 hours or less. Annealing; then heating the hot-rolled steel sheet to an annealing temperature of 725 ° C or higher and 790 ° C or lower at a heating rate of 1 ° C / hour or more and 80 ° C / hour or less, and then performing the second-stage annealing for 3 hours or more and 50 hours or less. The annealed hot-rolled steel sheet was cooled to 650 ° C at a cooling rate of 1 ° C / hour or more and 100 ° C / hour or less.

According to the present invention, it is possible to provide a steel sheet excellent in cold forging property and carbon-impregnated tempering, and particularly a steel sheet obtained by forming a high-frequency gear or the like by sheet forming, and a method for producing the same.

1‧‧‧Disc test material

2‧‧‧ cup test material

3‧‧‧ crack

4‧‧‧sample

5‧‧‧ Drop hammer

L‧‧‧ maximum length of crack

Fig. 1 is a view schematically showing an outline of a cold forging test and a state of crack introduced during cold forging. Fig. 1(a) shows a disc-shaped test material cut out from a hot-rolled steel sheet, Fig. 1(b) shows the shape of the test material after cold forging, and Fig. 1(c) shows a cross-sectional aspect of the test piece after cold forging. .

Fig. 2 is a view schematically showing an outline of a drop weight test for evaluating the impact resistance characteristics of a sample subjected to carbon immersion quenching and tempering.

Fig. 3 is a graph showing the relationship between the ratio of the number of grain boundary carbides to the number of intragranular carbides, the crack length of the cold forged test piece, and the impact resistance characteristics after the carbon-impregnated tempering.

Fig. 4 is a graph showing the relationship between the ratio of the number of grain boundary carbides to the number of intragranular carbides, the crack length of the cold forged test piece, and the impact resistance characteristics after the carbon hardening and tempering.

Form for implementing the invention

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Here, the "%" of the component composition means "% by mass".

[C:0.10~0.40%]

C system forms carbides in steel, strengthens steel and fines iron particles The effective element. In order to suppress the production of pear skin by cold working and to ensure the appearance of the surface of the cold forged part, it is necessary to suppress the coarsening of the iron particle size of the ferrite, but when it is less than 0.10%, the volume fraction of the carbide is insufficient, and the coarsening of the carbide in the annealing cannot be suppressed. Therefore, C is set to 0.10% or more. It is preferably 0.11% or more.

On the other hand, when it is more than 0.40%, the volume fraction of the carbide increases, and when the load is instantaneously loaded, a large number of cracks which are the starting point of the fracture are generated, and the punching resistance is deteriorated. Therefore, C is set to 0.40% or less. It is preferably 0.38% or less.

[Si: 0.01~0.30%]

The Si system acts as a deoxidizer and, in addition, an element that affects the morphology of the carbide. In order to reduce the number of carbides in the ferrite particles of the available deoxidation effect and increase the number of carbides on the ferrite grain boundary, the Worthite iron phase is formed in the annealing by the two-stage annealing. After the carbide is melted and then cooled slowly, it is necessary to promote the formation of carbides in the ferrite grain boundary.

When Si is more than 0.30%, the ductility of the ferrite-grained iron is lowered, and cracks are likely to occur during cold forging. Since the cold-forgeability and the carbon-resistant quenching and tempering resistance are deteriorated, the Si is set to 0.30% or less. It is preferably 0.28% or less.

Si is preferably as small as possible, but when it is reduced to 0.01%, the refining cost is greatly increased, so Si is made 0.01% or more. More preferably 0.02% or more.

[Mn: 0.30~1.00%]

Mn is an element that controls the morphology of carbides in a two-stage annealing. When it is less than 0.30%, since the carbide is formed on the grain boundary of the ferrite grain in the slow cooling after the second-stage annealing, Mn is set to 0.30% or more. More than 0.33% is preferred.

On the other hand, when it is more than 1.00%, the toughness after tempering by carbon immersion is lowered, so Mn is made 1.00% or less. It is preferably 0.96% or less.

[Al: 0.001~0.10%]

Al acts as a deoxidizer for steel to stabilize the elements of ferrite. When it is less than 0.001%, since the effect of addition is not sufficiently obtained, Al is made 0.001% or more. More preferably 0.004% or more.

On the other hand, when it is more than 0.10%, the ratio of the number of carbides on the grain boundary is lowered, and the crack length at the time of cold forging is increased, so that Al is made 0.10% or less. It is preferably 0.09% or less.

[Cr: 0.50~2.00%]

Cr and Mo can enhance the toughness of the elements. Cr is an element that contributes to the stabilization of carbides during heat treatment. When the amount is less than 0.50%, carbides are less likely to remain during carbon immersion, and the particle size of the Worthite iron in the surface layer is coarsened, and the punching resistance is deteriorated. Therefore, Cr is set to 0.50% or more. It is preferably 0.52% or more.

On the other hand, when it is more than 2.00%, the amount of Cr in the carbide increases, and a large amount of fine carbide remains in the iron phase of the Worth field formed in the two-stage annealing, so that carbides are also present in the grains after the slow cooling. As a result, the hardness increases and the ratio of the number of grain boundary carbides decreases, and the cold forgeability decreases. Therefore, Cr is set to 2.00% or less. It is preferably 1.94% or less.

[Mo: 0.001~1.00%]

Mo is an element that helps control the morphology of the carbide. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so Mo is made 0.001% or more. More preferably 0.017% or more.

On the other hand, when it is more than 1.00%, it is rich in Mo in the carbide. The formation of stable carbides in the iron phase of Vostian also increases. Therefore, carbides are also present in the grains after slow cooling, resulting in a decrease in hardness and a decrease in the ratio of grain boundary carbides, and a decrease in cold forgeability. Set to 1.00% or less. It is preferably 0.94% or less.

The following elements are impurities and need to be controlled below a fixed amount.

[P: 0.020% or less]

P is segregated at the ferrite grain boundary and inhibits the formation of grain boundary carbides. The less the better. The P content may be 0, but it is required to be highly purified when it is less than 0.0001% in the refining step, and refining takes a long time, and the manufacturing cost is greatly increased, so the lower limit of the substance is 0.0001 to 0.0013%.

On the other hand, when it is more than 0.020%, the ratio of the number of grain boundary carbides decreases, and the cold forgeability falls, so P is set to 0.020% or less. It is preferably 0.018% or less.

[S: 0.010% or less]

S forms an impurity element of a non-metallic inclusion such as MnS. Non-metallic inclusions will become the starting point for cracking during cold forging, so S is preferably as small as possible. The S content may be 0. However, when S is reduced to less than 0.0001%, since the refining cost is greatly increased, the substantial lower limit is 0.0001 to 0.0012%.

On the other hand, when it is more than 0.010%, since the crack length at the time of cold forging increases, S is set to 0.010% or less. It is preferably 0.009% or less.

[N: 0.020% or less]

N segregates toward the ferrite grain boundary, and suppresses the formation of carbides on the grain boundaries. The less the better. The N content may also be 0, but when it is reduced to less than 0.0001%, since the refining cost is greatly increased, the lower limit of the essence is 0.0001~0.0006%.

On the other hand, when it is more than 0.020%, even if 2-phase annealing and slow cooling are performed, the ratio of the number of carbides on the ferrite grain boundary to the number of carbides in the ferrite grain is still less than 1, cold forging Since the property is lowered, N is set to be 0.020% or less. It is preferably 0.017% or less.

[O: 0.0001~0.020%]

O is an element that forms an oxide in steel. Since the oxide existing in the ferrite grains is a place where carbides are formed, the smaller the better, the better. The O content may be 0. However, when O is reduced to less than 0.0001%, since the refining cost is greatly increased, the substantial lower limit is 0.0001 to 0.0006%.

On the other hand, when it is more than 0.020%, the ratio of the number of carbides on the ferrite grain boundary to the number of carbides in the ferrite grain will be less than 1, and the cold forgeability is decreased, so O is set to 0.020%. the following. It is preferably 0.017% or less.

[Ti: 0.010% or less]

The Ti-based element is an important element for controlling the form of the carbide, and the element which is formed in a large amount to promote the formation of carbides in the iron particles of the ferrite is preferably as small as possible. The Ti content may be 0, and when it is reduced to less than 0.0001%, since the refining cost is greatly increased, the substantial lower limit is 0.0001 to 0.0006%.

On the other hand, when it is more than 0.010%, the ratio of the number of carbides on the ferrite grain boundary to the number of carbides in the ferrite grain will be less than 1, and the cold forgeability is lowered, so Ti is set to 0.010%. the following. It is preferably 0.007% or less.

[B: 0.0005% or less]

B is an element that effectively controls the sliding of the cold forging time difference row. Since it is contained in a large amount, the activity of the sliding system is limited, so B is preferably as small as possible. B contains The amount can also be zero. Detection of less than 0.0001% of B requires careful attention and falls below the detection limit of the analytical device.

On the other hand, when it is more than 0.0005%, the slippage of the difference between the shear bands formed by cold forging is suppressed, and the strain is locally concentrated to cause cracks, so B is made 0.0005% or less. It is preferably 0.0005% or less.

[Sn: 0.050% or less]

Sn is an element mixed with steel raw materials (waste), and the smaller the better, the better. The Sn content may be 0. When the amount is reduced to less than 0.001%, since the refining cost is greatly increased, the lower limit of the substance is 0.001 to 0.002%.

On the other hand, when it is more than 0.050%, since the ferrite is iron embrittled and the cold forgeability is lowered, Sn is set to 0.050% or less. It is preferably 0.048% or less.

[Sb: 0.050% or less]

Sb is the same as Sn, and is an element mixed with steel raw materials (waste). Sb segregates at the grain boundary, and the ratio of the number of grain boundary carbides is lowered, so that the smaller the better. The Sb content may be 0. When the amount is reduced to less than 0.001%, since the refining cost is greatly increased, the lower limit of the substance is 0.001 to 0.002%.

On the other hand, when it is more than 0.050%, since cold forgeability falls, Sb is set to 0.050% or less. It is preferably 0.048% or less.

[As: 0.050% or less]

As is the same as Sn and Sb, and is an element mixed with a steel raw material (waste). As is segregated at the grain boundary, the ratio of the number of grain boundary carbides is lowered, so that the smaller the better, the better. The As content may be 0, and when it is reduced to less than 0.001%, since the refining cost is greatly increased, the lower limit of the substance is 0.001 to 0.002%.

On the other hand, when it is more than 0.050%, the number of carbides in the grain boundary Since the ratio is lowered and the cold forgeability is lowered, As is set to 0.050% or less. It is preferably 0.045% or less.

The steel sheet of the present invention has the above-mentioned elements as a basic element, and further has the following elements for the purpose of improving cold forgeability or other characteristics. The following elements are not necessary for obtaining the effects of the present invention, and therefore the content may be zero.

[Nb: 0.10% or less]

The Nb system effectively controls the elements of the carbide form, and the microstructure can be refined to help improve the toughness. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so Nb is preferably 0.001% or more. Preferably, it is 0.002% or more.

On the other hand, when it is more than 0.10%, fine Nb carbides are precipitated in a large amount, and the strength is excessively increased. Further, since the ratio of the number of grain boundary carbides is lowered, the cold forgeability is lowered, so that Nb is made 0.10% or less. . It is preferably 0.09% or less.

[V: 0.10% or less]

V is also the same as Nb. It is an element that effectively controls the form of carbides. It also refines the structure and helps to improve the elements of toughness. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so it is preferable to set V to 0.001% or more. Preferably it is 0.004% or more.

On the other hand, when it is more than 0.10%, fine V carbides are precipitated in a large amount, and the strength is excessively increased. Further, since the ratio of the number of grain boundary carbides is lowered, the cold forgeability is lowered, so V is made 0.10% or less. . It is preferably 0.09% or less.

[Cu: 0.10% or less]

Cu forms fine precipitates and contributes to the strength of the elements. When the amount is less than 0.001%, the effect of improving the strength is not sufficiently obtained, so Cu is preferably 0.001% or more. Preferably, it is 0.008% or more.

On the other hand, when it is more than 0.10%, red hot brittleness is exhibited in hot rolling, and since productivity is lowered, Cu is made 0.10% or less. It is preferably 0.09% or less.

[W: 0.10% or less]

W is also the same as Nb and V, and is an element that effectively controls the morphology of carbides. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so W is preferably 0.001% or more. Preferably, it is 0.003% or more.

On the other hand, when it is more than 0.10%, a large amount of fine W carbide is precipitated, and the strength is excessively increased. Further, since the ratio of the number of grain boundary carbides is lowered, the cold forgeability is lowered, so W is made 0.10% or less. It is preferably 0.08% or less.

[Ta: 0.10% or less]

Ta is also the same as Nb, V, and W, and is an element that effectively controls the morphology of carbides. When it is less than 0.001%, since the effect of addition is not sufficiently obtained, Ta is preferably 0.001% or more. More than 0.007% is preferred.

On the other hand, when the amount is more than 0.10%, the fine W carbide is precipitated in a large amount, and the strength is excessively increased. Further, since the ratio of the number of grain boundary carbides is lowered, the cold forgeability is lowered, so that Ta is made 0.10% or less. It is preferably 0.09% or less.

[Ni: 0.10% or less]

Ni is an element that effectively improves the punching resistance of parts. Less than 0.001% In the case where the effect of addition is not sufficiently obtained, it is preferable to set Ni to 0.001% or more. Preferably, it is 0.002% or more.

On the other hand, when the ratio is more than 0.10%, the ratio of the number of grain boundary carbides decreases, and the cold forgeability decreases. Therefore, Ni is made 0.10% or less. It is preferably 0.09% or less.

[Mg: 0.050% or less]

The addition of a trace amount of Mg to control the element of the sulfide form. When the amount is less than 0.0001%, the effect of addition is not sufficiently obtained, so that Mg is preferably made 0.0001% or more. Preferably, it is 0.0008% or more.

On the other hand, when it is more than 0.050%, since the ferrite is iron embrittled and the cold forgeability is lowered, Mg is set to 0.050% or less. It is preferably 0.049% or less.

[Ca: 0.050% or less]

Ca is the same as Mg, and it is an element that controls the form of sulfide by adding a trace amount. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so it is preferable to set Ca to 0.001% or more. Preferably, it is 0.003% or more.

On the other hand, when it is more than 0.050%, coarse Ca oxide is formed, and when cold forging is the starting point of cracking, Ca is set to 0.050% or less. It is preferably 0.04% or less.

[Y: 0.050% or less]

Y is the same as Mg and Ca, and is added to a small amount to control the element of the sulfide form. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so Y is preferably 0.001% or more. Preferably, it is 0.003% or more.

On the other hand, when it is more than 0.050%, a coarse Y oxide is formed, and when cold forging is a starting point for cracking, Y is set to 0.050% or less. Take Below 0.031% is preferred.

[Zr: 0.050% or less]

Zr is the same as Mg, Ca, and Y, and is added to a small amount to control the element of the sulfide form. When the amount is less than 0.001%, the effect of addition is not sufficiently obtained, so it is preferable to set Zr to 0.001% or more. Preferably it is 0.004% or more.

On the other hand, when it is more than 0.050%, a coarse Zr oxide is formed, and when cold forging is a starting point for cracking, Zr is set to 0.050% or less. It is preferably 0.045% or less.

[La: 0.050% or less]

The addition of a trace amount of La is effective for controlling the element of the sulfide form, and is an element which segregates at the grain boundary and lowers the ratio of the number of grain boundary carbides. When the amount is less than 0.001%, the form control effect is not sufficiently obtained, so that La is preferably 0.001% or more. Preferably, it is 0.003% or more.

On the other hand, when it is more than 0.050%, the ratio of the number of grain boundary carbides is lowered, and the cold forgeability is lowered. Therefore, La is set to 0.050% or less. It is preferably 0.047% or less.

[Ce: 0.050% or less]

Ce is the same as La, and is an element which controls the form of sulfides by adding a trace amount, and is an element which segregates at the grain boundary and lowers the ratio of the number of grain boundary carbides. When the amount is less than 0.001%, since the form control effect is not sufficiently obtained, it is preferable to set Ce to 0.001% or more. Preferably, it is 0.003% or more.

On the other hand, when it is more than 0.050%, the ratio of the number of grain boundary carbides decreases, and the cold forgeability falls, so Ce is set to 0.050% or less. It is preferably set to 0.046% or less.

Further, the remainder of the composition of the steel sheet of the present invention is Fe and unavoidable impurities.

Next, the steel sheet structure of the present invention will be described.

The structure of the steel sheet of the present invention is essentially a structure composed of ferrite iron and carbide. In addition to ferritic carbon iron (Fe ₃ C), which is a compound of iron and carbon, the carbide is also a compound or alloy carbide (M ₂₃ C ₆ , M _{6 )} in which Fe atoms in ferritic carbon iron are replaced by Mn, Cr or the like. C, MC, etc., M system Fe and other metal elements).

When the steel sheet is formed into a predetermined part shape, a shear band is formed in the giant structure of the steel sheet, and sliding deformation is concentrated in the vicinity of the shear band. The sliding deformation grows in the vicinity of the shear band to form a region having a high difference in density. As the strain amount imparted to the steel sheet increases, the sliding deformation is promoted, and the difference in density is increased.

In cold forging, the equivalent strain is more than 1 strong processing. Therefore, in the conventional steel sheet, it is not possible to prevent the occurrence of voids and/or cracks due to an increase in the difference in the discharge density, and it is difficult to improve the cold forgeability.

In order to solve the problem of this difficulty, it is effective to suppress the formation of the shear band at the time of molding. From the viewpoint of microstructure, the formation of shear bands can be understood as a phenomenon in which the sliding of a certain particle propagates continuously beyond the grain boundary to adjacent particles. Thereby, in order to suppress the formation of the shear band, it is necessary to prevent the spread of the sliding across the crystal grain boundary.

The carbide in the steel sheet is a strong particle that prevents slippage, and if the carbide exists in the ferrite grain boundary, the formation of the shear band can be suppressed, and the cold forgeability can be improved.

In order to achieve such an effect, it is necessary to disperse an appropriately sized carbide in the metal structure. Therefore, the average particle diameter of the carbide is set to be 0.4 μm or more and 2.0 μm or less. When the particle diameter of the carbide is less than 0.4 μm, the hardness of the steel sheet is remarkably increased, and the cold forgeability is lowered. Preferably, it is 0.6 μm or more.

On the other hand, when the average particle diameter of the carbide is more than 2.0 μm, the carbide becomes a starting point of cracking during cold forming. Preferably, it is 1.95 μm or less.

Further, the iron carbide, that is, the hard and brittle structure of the stellite carbon iron, is hard and brittle when it exists in the form of a ferrite with a layered structure of the ferrite iron. Therefore, it is necessary to minimize the amount of the iron, and the steel sheet of the present invention is set to have an area ratio of 6% or less.

Because of its unique layered structure, the Borne iron can be discerned by SEM and optical microscopy. The area ratio of the Borne iron can be obtained by calculating the layered structure region in an arbitrary cross section.

According to the theory and principle, it is known that the cold forgeability is strongly influenced by the coverage of carbides at the ferrite grain boundary. Although the measurement of high precision can be obtained, the coverage of carbide on the grain boundary of the ferrite grain in the 3-dimensional space is The measurement requires continuous SEM observation or 3-dimensional EBSP observation of the sample cutting and observation by FIB in a scanning electron microscope, which requires a large measurement time and accumulation of technical knowledge.

The inventors of the present invention have found that the ratio of the number of carbides in the ferrite grain boundary to the number of carbides in the ferrite grains is found as a result of the evaluation of the evaluation index which is simpler and more accurate. As an indicator, the cold forgeability can be evaluated. When the ratio of the number of carbides in the ferrite grain boundary to the number of carbides in the ferrite grain is greater than 1, the cold will be significantly improved. Forging.

Furthermore, the warpage, the pinching, and the folding of the steel sheet generated during cold forging are caused by the localization of the strain caused by the formation of the shear band, and similarly, the carbide exists in the ferrite grain boundary. The formation of the shear band and the localization of the strain can alleviate the occurrence of warpage, pinching, and folding.

The observation of the carbide was carried out by a scanning electron microscope. Before the observation, the sample for observation of the structure was polished by wet grinding of the sandpaper and the diamond abrasive grains having an average particle size of 1 μm, and the observation surface was mirror-finished, and then the structure was etched in the saturated picric acid-alcohol solution.

The observation magnification was set to 3000 times, and a field of view of 30 μm × 40 μm in 1/4 layer of 8 sheet thicknesses was randomly taken. The area of each carbide contained in the area was measured in detail by the image analysis software represented by Winroof Co., Ltd. (Win ROOF). A circle equivalent diameter (= 2 × √ (area / 3.14)) was obtained from the area of each carbide, and the average value thereof was defined as the carbide particle diameter.

Further, in order to suppress the influence of the measurement error caused by the head, the carbide having an area of 0.01 μm ² or less was excluded from the evaluation target.

Calculate the number of carbides present on the grain boundary of the ferrite grain, and subtract the number of carbides on the grain boundary from the total number of carbides to obtain the number of carbides in the ferrite grain. Based on the number of measurements, the ratio of the number of carbides in the ferrite grain boundary to the intragranular carbide was calculated.

The cold forgeability can be improved by setting the grain size of the ferrite iron to 3.0 μm or more and 50.0 μm or less as the structure after annealing. When the particle size of ferrite is less than 3μm, the hardness increases, and cracks or cracks are likely to occur during cold forging. The iron particle diameter is preferably 3.0 μm or more. Preferably, it is 7.5 μm or more.

On the other hand, when the particle size of the ferrite iron is more than 50.0 μm, the number of carbides on the crystal grain boundary which suppresses the sliding propagation is reduced, and the cold forgeability is lowered. Therefore, the particle size of the ferrite grain is preferably 50.0 μm or less. Preferably, it is 37.9 μm or less.

After observing the observation surface of the sample for observation of the tissue into a mirror surface in the above-described order, the structure of the observation surface etched by the 3% nitric acid-alcohol solution was observed by an optical microscope or a scanning electron microscope, and the line was used for the captured image. The method measures the particle size of the ferrite.

By setting the Vickers hardness of the steel sheet to 100 HV or more and 180 HV or less, the cold forging property and the punching resistance characteristics after the carbon-impregnated tempering can be improved. When the Vickers hardness is less than 100 HV, warpage is likely to occur in cold forging, and the warp portion is caught and folded, and the punching resistance is lowered. Therefore, the Vickers hardness is set to 100 HV or more. It is preferably 110 HV or more.

On the other hand, when the Vickers hardness is more than 180 HV, the ductility is lowered, and internal cracking is likely to occur during cold forging, and the punching resistance is deteriorated. Therefore, the Vickers hardness is set to 180 HV or less. It is preferably 170 HV or less.

Next, a method of evaluating cold forgeability will be described.

Figure 1 is a schematic view showing the outline of the cold forging test and the cracking introduced by the cold forging. Fig. 1(a) shows a disk-shaped test material cut out from a hot-rolled steel sheet, Fig. 1(b) shows the shape of the test piece after cold forging, and Fig. 1(c) shows a cross-sectional aspect of the test piece after cold forging.

As shown in Fig. 1, a disk-shaped test material 1 having a diameter of 70 mm was cut out from a hot-rolled steel sheet having a thickness of 5.2 mm (refer to Fig. 1 (a)), and the bottom surface was straightened by deep drawing. A cup-shaped test material having a diameter of 30 mm (not shown). Then, using a single-shot forming press manufactured by Mori Iron Works, the vertical wall portion of the cup-shaped test material was formed by thickening at a thickness ratio of 1.54 (=8 mm/5.2 mm) to produce a diameter of 30 mm, a height of 30 mm, and a longitudinal direction. A cup-shaped test material 2 having a wall thickness of 8 mm (see Fig. 1 (b)).

The cup-shaped test material 2 subjected to the thickening molding was cut by a wire-cut electric discharge machine manufactured by FANUC, and the cross section of the diameter portion was exposed (see FIG. 1(c)). The mirror-polished cross section confirms the presence of the crack 3 in the cut section, and measures the ratio of the maximum length L of the crack existing in the vertical wall portion to the thickness of the vertical wall portion after the thickening (= the maximum length of the crack L/thickness The thickness of the vertical wall portion is 8 mm). The cold forgeability was evaluated by the measured value.

In addition, when the initial thickness is 5.2 mm or less, the diameter of the cut disc-shaped test material is adjusted so that the height of the vertical wall after thickening is 30 mm, and then formed by the same thickness ratio of 1.54, The evaluation result can be reproduced by the influence of the initial thickness. Therefore, the hot-rolled steel sheet to which the present invention is applied is not limited to a hot-rolled steel sheet having a thickness of 5.2 mm. The invention can still improve the punching resistance characteristics after the cold forging property and the carbon-impregnated quenching and tempering in the hot-rolled steel sheet with a general plate thickness (2-15 mm).

Next, the production method of the present invention will be described. The technical idea of the manufacturing method of the present invention is to continuously control the hot rolling conditions and the annealing conditions when manufacturing the steel sheet from the steel sheet composed of the above-mentioned components, and to improve the punching resistance characteristics after cold forgeability and carbon-impregnated tempering.

The features of the manufacturing method of the present invention are illustrated.

[Features of hot rolling]

Continuous casting of molten steel having the required composition to form a flat steel embryo, the flat The steel blank is directly used for hot rolling, or temporarily cooled, and then heated for hot rolling, and the final hot rolling is finished at a temperature range of 650 ° C or higher and 950 ° C or lower. The hot rolled steel sheet after the final rolling is cooled on the ROT, and then coiled at a coiling temperature of 400 ° C or more and 600 ° C or less.

[Characteristics of annealing]

The hot-rolled steel sheet is subjected to two-stage annealing maintained in two temperature domains after pickling. At this time, in the first-stage annealing, the hot-rolled steel sheet is heated at a heating rate of 30 ° C / hour or more and 150 ° C / hour or less. The annealing is carried out for 3 hours or more and 60 hours or less in the temperature range of 650 ° C or more and 720 ° C or less to the annealing temperature.

In the subsequent second-stage annealing, the hot-rolled steel sheet is heated to an annealing temperature at a heating rate of 1° C./hour or more and 80° C./hour or less, and is maintained in a temperature range of 725° C. or more and 790° C. or lower for 3 hours or more. Annealing below 50 hours.

Next, the hot-rolled steel sheet after annealing was cooled to 650 ° C at a cooling rate of 1 ° C / hour or more and 100 ° C / hour or less, and then cooled to room temperature.

By using the hot rolling conditions and the annealing conditions in combination, it is possible to obtain a low carbon steel sheet excellent in cold forging resistance and after kneading quenching and tempering.

Hereinafter, the step conditions of the production method of the present invention will be specifically described.

[hot rolling delay]

Final hot rolling temperature: 650 ° C or more and 950 ° C or less

Coiling temperature: 400 ° C or more and 600 ° C or less

Continuous casting of molten steel with the required composition, direct heat After rolling, or after cooling, the film is heated and then hot rolled, and the final hot rolling is completed in a temperature range of 650 ° C or higher and 950 ° C or lower, and the hot rolled steel sheet is wound up at 400 ° C or higher and 600 ° C or lower.

The heating temperature of the flat steel embryo is preferably 1300 ° C or less, and the heating time for maintaining the surface temperature of the flat steel embryo at 1000 ° C or more is preferably 7 hours or less.

When the heating temperature is greater than 1300 ° C, or the heating time is more than 7 hours, the decarburization of the surface layer of the flat steel becomes remarkable. The Worthite iron particles in the surface layer will grow abnormally when heated before quenching, and the erosion resistance is lowered, so the heating temperature is lowered. The temperature is preferably 1300 ° C or less, and the heating time is preferably 7 hours or less. Preferably, the heating temperature is 1280 ° C or less, and the heating time is 6 hours or less.

The final hot rolling is terminated at a temperature of 650 ° C. or higher and 950 ° C or lower. When the final hot rolling temperature is less than 650 ° C, the deformation resistance of the steel material increases, the rolling load is remarkably increased, and even the roll abrasion amount is increased, and the productivity is lowered. Therefore, the final hot rolling temperature is set to 650 ° C or higher. It is preferably 680 ° C or higher.

On the other hand, when the final hot rolling temperature is greater than 950 ° C, a thick scale will be produced when passing the ROT (Run Out Table), because the scale will cause flaws on the surface of the steel sheet, during cold forging, and/or dip When the load is applied after the carbon quenching and tempering, the crack is generated starting from the crucible, and the punching resistance is lowered. Therefore, the final hot rolling temperature is set to 950 ° C or lower. It is preferably 920 ° C or less.

The cooling rate when cooling the hot rolled steel sheet on the ROT is preferably 10 ° C / sec or more and 100 ° C / sec or less. When the cooling rate is less than 10 ° C / sec, thick scale will be formed on the way to cooling, and the surface of the steel sheet will not be inhibited. After the birth, the resistance to smashing is reduced, so the cooling rate is preferably 10 ° C / sec or more. Preferably it is 20 ° C / sec or more.

On the other hand, when the hot-rolled steel sheet is cooled at a cooling rate of more than 100 ° C / sec from the surface layer of the steel sheet to the inside, the outermost layer portion is excessively cooled, and a low-temperature metamorphic structure such as toughened iron or 麻田散铁 is generated in the outermost layer portion.

After the coiling, when the hot-rolled steel sheet of 100 ° C to room temperature is taken out, micro cracks are generated in the low-temperature metamorphic structure, and cracks are not easily removed in the subsequent pickling step and the cold rolling step, during cold forging and/or carbon-quenching. When the smashing load is applied after the tempering, the crack due to the crack is increased, and the impact resistance is lowered. Therefore, the cooling rate is preferably 100 ° C / sec or less. Preferably, it is 80 ° C / sec or less.

In addition, the cooling rate refers to the cooling equipment received from each water injection section after the hot-rolled steel sheet after the final hot rolling passes through the water-free section, when the water is cooled from the water injection section until the target temperature of the coil is cooled on the ROT. The cooling energy does not refer to the average cooling rate from the start of the water injection point to the temperature taken up by the coiler.

The coiling temperature is set to 400 ° C or more and 600 ° C or less. This is a lower temperature than the usual coiling temperature. By winding the hot-rolled steel sheet produced under the above conditions in this temperature range, the steel sheet structure can be used as a toughened iron structure in which carbides are dispersed in the fine ferrite iron.

When the coiling temperature is less than 400 °C, the Worthite iron which is not metamorphosed before the coiling will be transformed into a hard ramie loose iron. When the hot-rolled steel sheet after coiling is taken out, cracks are generated due to the surface layer, and the punching resistance is lowered. The coiling temperature is set to 400 ° C or higher. It is preferably 430 ° C or higher.

On the other hand, when the coiling temperature is more than 600 ° C, iron having a large interlayer interval is formed to form a needle-like carbide having a high thermal stability, and the needle-like carbide remains after the two-stage annealing. In the case of cold forging, cracks originating from the needle-shaped carbide are generated and the coiling temperature is increased. Therefore, the coiling temperature is set to 600 ° C or lower. It is preferably 570 ° C or less.

The hot-rolled steel sheet produced under the above conditions was subjected to two-stage annealing maintained in two temperature domains after pickling. By performing a two-stage type annealing on the hot-rolled steel sheet, the stability of the carbide can be controlled, and the formation of carbides at the ferrite grain boundary is promoted.

First, the technical idea of the two-stage annealing will be explained.

The first-stage annealing is performed in a temperature range below the Ac1 point to coarsen the carbide and to concentrate the alloying element, thereby improving the thermal stability of the carbide. Thereafter, the temperature is raised to a temperature range of Ac1 or higher, and Worthite iron is formed in the structure, and carbides in the fine ferrite grains are melted in the Worthite iron, and coarse carbides remain in the Worthite iron.

With the subsequent slow cooling, the Worthite iron is transformed into fertilized iron to increase the carbon concentration in the Worthfield iron. By slow cooling, carbon atoms are adsorbed on the residual carbides in the Worthite iron. The carbides and the Worthite iron will coat the grain boundaries of the ferrite grains, and finally form a large amount of carbide-forming structures in the ferrite grain boundary. . From this, it can be seen that the structure defined by the present invention cannot be formed by simple annealing.

Hereinafter, specific annealing conditions will be described.

[Phase 1 Annealing]

Heating rate to annealing temperature: 30 ° C / hour or more and 150 ° C / hour

Annealing temperature: 650 ° C or more and 720 ° C or less

Annealing temperature retention time: 3 hours or more and 60 hours or less

The heating rate to the first-stage annealing temperature is set to 30 ° C / hour or more and 150 ° C / hour or less. When the heating rate is less than 30 ° C / hour, the productivity is lowered due to the temperature rise time, so the heating rate is set to 30 ° C / hour or more. It is preferably 40 ° C / hour or more.

On the other hand, when the heating rate is more than 150 ° C / hour, the temperature difference between the peripheral portion and the inside of the coil increases, and friction or burning occurs due to the difference in thermal expansion, and irregularities are formed on the surface of the steel sheet. In the case of cold forging, cracks having the irregularities as a starting point are generated, and the cold forgeability is lowered and the punching resistance characteristics after the carbon-baking quenching and tempering are lowered. Therefore, the heating rate is set to 150 ° C /hr or less. It is preferably 120 ° C / hour or less.

The annealing temperature (first-stage annealing temperature) of the first-stage annealing is 650 ° C or more and 720 ° C or less. When the first-stage annealing temperature is less than 650 ° C, the stability of the carbide is insufficient, and in the second-stage annealing, carbides in the Worstian iron are not easily formed, so the first-stage annealing temperature is 650 ° C or higher. It is preferably 670 ° C or higher.

On the other hand, when the annealing temperature is more than 720 ° C, the Worthite iron is formed before the stability of the carbide is increased, and the above-described structural change is not controlled, so the annealing temperature is 720 ° C or lower. It is preferably 700 ° C or less.

The holding time of the first-stage annealing (holding time of the first stage) is set to 3 hours or more and 60 hours or less. When the holding time in the first stage is less than 3 hours, the stabilization of the carbide is not sufficient, and in the second-stage annealing, carbides are less likely to remain, so the holding time in the first stage is set to 3 hours or longer. Take More than 10 hours is better.

On the other hand, when the holding time of the first stage is more than 60 hours, the stability of the carbide is not further improved, and the productivity is lowered. Therefore, the holding time in the first stage is set to 60 hours or less. It is preferably 50 hours or less.

[Phase 2 Annealing]

Heating rate to annealing temperature: 1 ° C / hour or more and 80 ° C / hour

Annealing temperature: 725 ° C or more and 790 ° C or less

Annealing temperature retention time: 3 hours or more and 50 hours or less

After the completion of the first-stage annealing, the hot-rolled steel sheet is heated to an annealing temperature at a heating rate of 1 ° C / hour or more and 80 ° C / hour or less. When the second-stage annealing was not performed and the cooling was performed, the particle size of the ferrite-grain iron did not become large, and an ideal structure could not be obtained.

In the second-stage annealing, the Worthite iron is formed from the ferrite grain boundary. By slowing down the heating rate, the formation of the nucleus of the Worthite iron can be suppressed, and the grain boundary coverage of the carbide can be improved in the structure obtained after the cooling. Therefore, the heating rate of the second-stage annealing is preferably small.

When the heating rate is less than 1 ° C / hour, the productivity is lowered due to the time required for heating, so the heating rate is set to 1 ° C / hour or more. It is preferably 10 ° C / hour or more.

On the other hand, when the heating rate is more than 80 ° C / hour, the temperature difference between the peripheral portion and the inside of the coil increases, and the thermal expansion is greatly deteriorated due to the deformation, and friction or burning occurs, and irregularities are formed on the surface of the steel sheet. In cold forging, cracks originating from the unevenness are generated, resulting in a decrease in cold forgeability and carbonization quenching. After the fire, the punching resistance is lowered, so the heating rate is set to 80 ° C / hour or less.

The annealing temperature (second-stage annealing temperature) of the second-stage annealing is 725 ° C or more and 790 ° C or less. When the second-stage annealing temperature is less than 725 ° C, the amount of formation of Worthite iron is small, and after cooling in the second-stage annealing, the ratio of the number of carbides on the ferrite grain boundary is decreased, and the grain size of the ferrite is changed. small. Therefore, the second-stage annealing temperature is set to 725 ° C or higher. It is preferably 735 ° C or higher.

On the other hand, when the second-stage annealing temperature is more than 790 ° C, it is not easy to retain carbides in the Worthite iron, and it is difficult to control the above-described structural change. Therefore, the second-stage annealing temperature is set to 790 ° C or lower. It is preferably 780 ° C or less.

The holding time of the second-stage annealing (holding time of the second stage) is set to 1 hour or more and 50 hours or less. When the holding time of the second stage is less than 1 hour, the amount of iron in the Vostian is small, and the carbides in the ferrite grains are not sufficiently melted, and it is difficult to increase the ratio of the number of carbides on the ferrite grain boundary. Since the particle size of the ferrite iron is small, the holding time of the second stage is set to be 1 hour or longer. More than 5 hours is preferred.

On the other hand, when the holding time of the second stage is more than 50 hours, it is not easy to retain carbides in the Worthite iron, so the holding time in the second stage is set to 50 hours or less. It is preferably less than 45 hours.

[Cooling after annealing]

Cooling stop temperature: 650 ° C

Cooling rate: 1 ° C / hour or more and 100 ° C / hour or less

After the second stage annealing is completed, the temperature is 1 ° C / hour or more. The cooling rate after cooling at 100 ° C / hour or less is gradually cooled to 650 ° C. When the stop temperature of the slow cooling is greater than 650 ° C, and then to room temperature by a cooling rate of more than 100 ° C / hour, the untransformed Worth iron will be transformed into wave iron or toughened iron, and the hardness increases the cold forgeability. Therefore, the cooling stop temperature was set to 650 °C.

The Worthite iron formed in the second-stage annealing is cooled to make it metamorphose into ferrite iron, and the carbon is adsorbed to the carbide remaining in the Worthite iron, and the cooling rate is preferably slow. When the cooling rate is less than 1 ° C / hour, the time required for cooling increases, and the productivity is lowered. Therefore, the cooling rate is set to 1 ° C / hour or more. It is preferably 10 ° C / hour or more.

On the other hand, when the cooling rate is higher than 100 ° C / hour, the Instron iron is transformed into a wave of iron, and the hardness of the steel sheet is increased, resulting in a decrease in cold forgeability and a decrease in the impact resistance after carbon-quenching and tempering, so that cooling is performed. The speed is set to 100 ° C / hour or less. It is preferably 90 ° C / hour.

Here, the cooling stop temperature is a temperature controllable at the cooling rate, and cooling at 650 ° C or lower is not particularly limited as long as it is cooled to 650 ° C at a cooling rate of 1 ° C / hour or more and 100 ° C / hour or less.

Furthermore, the annealing gas environment is not limited to a specific gaseous environment. It may be, for example, a nitrogen atmosphere of 95% or more, a hydrogen atmosphere of 95% or more, and an atmospheric gas atmosphere.

As described above, the method for producing the structure control of the steel sheet by the hot rolling conditions and the annealing conditions of the present invention is consistently managed, and the cold forging property which is excellent in the cold forging which is combined with the press forming and the thick forming can be exhibited. It is also a low carbon steel sheet with excellent punching resistance after carbon hardening and tempering.

Example

Next, the examples will be described. The level of the examples is an example of the conditions for carrying out the workability and effects of the present invention, and the present invention is not limited by the conditions. The invention may be used in various forms without departing from the spirit and scope of the invention.

A continuous casting slab (steel block) having the composition shown in Table 1 was heated at 1240 ° C for 1.8 hours, and then hot rolled. The final hot rolling was completed at 890 ° C, and cooled to 520 ° C at a cooling rate of 45 ° C / sec on the ROT, and taken up at 510 ° C to produce a hot rolled coil having a thickness of 5.2 mm.

The hot-rolled coil is pickled, and the coil is placed in a box-type annealing furnace. After controlling the gas atmosphere to be 95% hydrogen-5% nitrogen, it is heated from room temperature to 705 ° C at a heating rate of 100 ° C / hour, and kept at 705 ° C for 36 hours. Thereafter, the temperature distribution within the coil is homogenized. Thereafter, the mixture was heated to 760 ° C at a heating rate of 5 ° C / hour, further kept at 760 ° C for 10 hours, and then cooled to 650 ° C at a cooling rate of 10 ° C / hour, and then cooled to room temperature in the furnace to prepare a test characteristic. kind.

The structure of the sample was observed by the aforementioned method, and the crack length in the sample after cold forging was measured by the aforementioned method.

The carbon impregnation of the sample after thickening is carried out by gas carburization. In order to allow the carbon to diffuse into the steel from the surface of the sample in the furnace atmosphere, the carbon potential is controlled in a furnace of 0.5% by mass C, and the treatment is carried out at 940 ° C for 120 minutes, and then cooled to room temperature in the furnace. .

Then, after heating from room temperature to 840 ° C, it was kept for 20 minutes and quenched in oil at 60 ° C. The quenched sample was held at 170 ° C for 60 minutes, and then air-cooled and tempered to prepare a carbon-impregnated tempered sample.

The impact resistance of the carbon-impregnated tempered sample was evaluated by the drop weight test. Fig. 2 is a view schematically showing an outline of a drop weight test for evaluating the impact resistance characteristics of a sample after carbon-quenching quenching and tempering. Fix the cup bottom of the cup-shaped sample 4 subjected to carbon-immersed quenching and tempering with a clamp, and freely drop the weight of 2 kg from the upper side of the cup at a distance of 4 m on the side of the cup (upper side width: 50 mm, lower side width: 10 mm, height: 80 mm) Length: 110 mm), about 80 J was applied to the vertical wall portion of the sample 4, and the presence or absence of the crack of the sample was evaluated for the punching resistance.

As a result of free fall, samples of no cracks or damage were found A score of "OK" excellent in the characteristics of the punching resistance was marked, and a sample having cracks or damage was marked with a score of "NG" which was poor in punching resistance.

Table 2 shows the carbide diameter, the Wolla area ratio, the ferrite iron particle size, the Vickers hardness, and the number of carbides in the ferrite grain boundary relative to the carbide in the ferrite grain. The ratio of the number, the ratio of the maximum crack length to the thickness of the vertical wall portion, and the measurement results and evaluation results of the impact resistance.

As shown in Table 2, the invention steels A-1, B-1, C-1, D-1, In E-1, F-1, G-1, H-1, I-1, J-1, and K-1, the number of carbides in the ferrite grain boundary is relative to the carbide in the ferrite grain. The ratio of the number is more than 1, and the Vickers hardness is 100 HV or more and 180 HV or less, and the cold forging property and the carbon-impregnated tempering are excellent in the punching resistance.

On the other hand, in the comparative steel L-1, since the amount of C is low and the hardness before cold forging is less than 100 HV, the cold forgeability is low. The comparative steels M-1, P-1, and Z-1 excessively contain P, Al, and N, and during the second-stage annealing, since the segregation amount to the γ/α interface is large, formation of carbides in the grain boundaries is suppressed.

Since the comparative steel S-1 contains Si excessively, the ductility of the ferrite iron is low, so the cold forgeability is low. Since the comparative steels N-1 and T-1 contained Mo and Cr excessively, the carbides were finely dispersed in the ferrite grains and the hardness was more than 180 HV. Comparative steel Q-1 has a significantly low impact resistance after immersion in carbon quenching and tempering due to excessive Mn content.

Compared with steel O-1, the amount of Cr is small, and the Worthite iron particles in the surface layer are abnormally coarsened when carbon immersed, so that the punching resistance is low. Since the comparative steel R-1 contains S excessively, coarse MnS is formed in the steel, and the cold forgeability is low. Since the comparative steel U-1 contains C excessively, coarse carbides are formed inside the steel thickening, and coarse carbides remain after the carbon hardening, so that the punching resistance is low.

Compared with the steel V-1, the amount of Mn is small, and it is difficult to improve the stability of the carbide. Therefore, the cold forging property and the carbon-impregnated tempering resistance are low. Comparative steels W-1 and X-1 contain O and Ti in excess, so oxides present in the ferrite grains and TiC become carbides in the slow cooling after annealing in the 2-phase domain. It inhibits the formation of carbides at the grain boundaries and has low cold forgeability. Since the comparative steel Y-1 contains B excessively, the cold forgeability is low.

Next, in order to investigate the influence of the manufacturing conditions, the flat steel embryos having the compositions of A, B, C, D, E, F, G, H, I, J, and K shown in Table 1 are as shown in Table 3. The hot rolling conditions and annealing conditions shown were prepared into a hot rolled sheet annealed sample having a thickness of 5.2 mm.

Table 4 shows the carbide diameter, the Wolla area ratio, the ferrite iron particle size, the Vickers hardness, and the number of carbides in the ferrite grain boundary relative to the carbide in the ferrite grain. The ratio of the number, the ratio of the maximum crack length to the thickness of the vertical wall portion, and the measurement results and evaluation results of the impact resistance.

Compared with steel E-3, the final hot rolling temperature is low, the rolling load is increased, and the productivity is low. Comparative Steel D-2 has a high hot rolling temperature and produces scales on the surface of the steel sheet. Therefore, when it is used in the abrasion resistance test after quenching and tempering, cracks and peeling will occur as the starting point of the scale, and the wear resistance will be degraded. . Comparing the slow cooling rate of the ROT (Run Out Table) in the steel F-2 will result in a decrease in productivity and the production of scales.

In the comparative steel C-4, the outermost layer portion of the steel sheet was excessively cooled by the cooling rate of ROT of 100 ° C / sec, and fine cracks were formed in the outermost layer portion. The coiling temperature of the comparative steel C-2 is low, and a large amount of low-temperature metamorphic structure such as toughened iron or granulated iron is embrittled, and cracks are frequently generated when the coil is rolled, and the productivity is lowered. In addition, the sample taken from the cracked piece has low wear resistance characteristics.

The coiling temperature of the comparative steel G-2 is high, and a layered interval thick layer of iron is formed in the hot rolled structure, and the needle-shaped coarse carbide has high thermal stability, and the above carbide is formed after the two-stage annealing. It remains in the steel sheet, so the machinability is low. The comparative steel H-4 has a low heating rate due to the slow heating rate of the first-stage annealing of the two-stage type annealing, so that the productivity is low.

In the comparative steel E-3, since the heating rate of the first-stage annealing is fast, the temperature difference between the inside of the coil and the inner and outer peripheral portions becomes large, and frictional entanglement and burning due to the difference in thermal expansion occur, and is used for resistance after quenching and tempering. In the evaluation test of the wear characteristics, cracks and peeling occurred from the crotch portion, and the wear resistance was lowered.

The holding temperature (annealing temperature) of the first-stage annealing of the comparative steel G-4 is low, the coarsening treatment of the carbide below Ac1 point is insufficient, and the thermal stability of the carbide is insufficient, thereby remaining in the second-stage annealing. The carbide is reduced, and the structure after the slow cooling does not suppress the metamorphism of the ferrite, so the machinability is low.

Comparing the holding temperature (annealing temperature) of the first stage annealing of steel D-4 is high, the formation of Worthite iron during annealing fails to improve the stability of the carbide, so that after the annealing, the generated iron is generated, and the Vickers hardness is greater than 180 HV. Low machinability. The retention time of the first-stage annealing of the comparative steel J-4 was short, the stability of the carbide was not improved, and the machinability was low.

The first stage annealing of Comparative Steel F-2 has a long holding time, low productivity, and burnt defects, and low wear resistance. In the second-stage annealing of the comparative steel B-4, the second-stage annealing has a slow heating rate, so the productivity is low. Comparing the heating rate of the second-stage annealing of steel A-3 is fast, the temperature difference between the inner and outer peripheral portions of the coil becomes large, and the frictional enthalpy and the burning of the large thermal expansion difference due to the metamorphosis occur, and the wear resistance after quenching and tempering low.

The holding temperature (annealing temperature) of the second-stage annealing of the comparative steel K-2 is low, and the amount of formation of the Worthite iron is small, and the ratio of the number of carbides at the ferrite grain boundary is not increased, so that the machinability is low. Comparing the holding temperature (annealing temperature) of the second-stage annealing of steel C-4 is high, and promoting the dissolution of carbides during annealing, so that grain boundary carbides are not easily formed after slow cooling, and in addition, Borne iron is formed, and Vickers hardness is greater than 180HV, low machinability.

Since the second-stage annealing of the comparative steel J-3 has a long holding time and promotes dissolution of the carbide, the machinability is low. Comparing steel D-3 from the second stage annealing to 650 ° C, the cooling rate is slow, the productivity is low, and the coarsely formed carbide is formed in the structure after the slow cooling, and the crack is generated by the coarse carbide as the starting point in the cold forging. Cold forgeability is reduced. Comparing steel I-3 from the second stage annealing to 650 ° C, the cooling rate is fast, and the cooling of the Borne iron metamorphic hardness increases, so the cold forgeability is low.

Next, in order to investigate the allowable content of other elements, a continuous casting slab (steel block) having the composition shown in Table 5 and Table 6 (continued Table 5) was heated at 1240 ° C for 1.8 hours, and then hot rolled. Final hot rolling at 890 ° C, Further, it was cooled to 520 ° C at a cooling rate of 45 ° C / sec on the ROT, and taken up at 510 ° C to produce a hot rolled coil having a thickness of 5.2 mm.

Pickling the hot rolled coil, charging the hot rolled coil into the box annealing furnace, controlling the gas atmosphere to 95% hydrogen to 5% nitrogen, heating from room temperature to 705 ° C at a heating rate of 100 ° C / hour, and maintaining at 705 ° C After 36 hours, the temperature distribution in the coil was homogenized, and then heated to a heating rate of 5 ° C / hour to After maintaining at 760 ° C for 10 hours at 760 ° C, the film was cooled to 650 ° C at a cooling rate of 10 ° C / hour, and then cooled to room temperature in the furnace to prepare a sample for evaluation characteristics.

Further, the structure of the sample was observed by the aforementioned method, and the crack length in the sample after cold forging was measured by the aforementioned method.

Table 7 shows the carbide diameter, the Wolla area ratio, the ferrite iron particle size, the Vickers hardness, and the number of carbides in the ferrite grain boundary relative to the carbide in the ferrite grain. The ratio of the number, the ratio of the maximum crack length to the thickness of the vertical wall portion, and the measurement results and evaluation results of the impact resistance.

As shown in Table 7, the invention steels AA-1, AB-1, AC-1, AD-1, AE-1, AF-1, AG-1, AH-1, AI-1, AJ-1, AK- 1. In AL-1, AM-1, AN-1, AO-1, AP-1, and AQ-1, the number of carbides in the ferrite grain boundary is relative to the number of carbides in the ferrite grain. The ratio When it is more than 1, the Vickers hardness is 100 HV or more and 180 HV or less, and the cold forging property and the carbon-impregnated tempering are excellent in the punching resistance.

On the other hand, the comparative steels AR-1, AS-1, AW-1, AZ-1, BB-1, and BF-1 each contain La, As, Cu, Ni, Sb, and Ce in excess, and the second stage. Since the amount of segregation to the γ/α interface increases during annealing, the formation of carbides in the grain boundaries is suppressed. Since the comparative steel BG-1 contains Si excessively, the ductility of the ferrite iron is low, so the cold forgeability is low.

Comparative steels AT-1, AV-1, BA-1, BC-1, BH-1, and BJ-1 are excessively contained in Mo, Nb, Cr, Ta, W, and V, so that the carbide is finely dispersed. In the ferrite grains, the hardness is greater than 180 HV. Since the comparative steel BF-1 contains Mn excessively, the impact resistance characteristics after carbon immersion quenching are remarkably low.

Comparative steels AU-1, AX-1, AY-1, and BE-1 each contain Zr, Ca, Mg, and Y, respectively, to form coarse oxides or non-metallic inclusions in the steel, which will be produced during cold forging. A large oxide or a coarse non-metallic inclusion is used as a starting point for cracking, and cold forgeability is lowered. Comparative steel BD-1 contains Sn excessively, and the ferrite is embrittled and has low cold forgeability. Comparative steel BK-1 contains C in excess, and coarse carbides are formed inside the thickened steel. After the carbon hardening, coarse carbide remains, and the punching resistance is lowered.

Next, in order to investigate the influence of manufacturing conditions, AA, AB, AC, AD, AE, AF, AG, AH, AI, AJ, AK, AL, AM, AN, AO, AP, and The flat steel blank of the composition of the AQ component was prepared into a hot-rolled sheet annealed sample having a thickness of 5.2 mm under the hot rolling conditions and annealing conditions shown in Table 8.

Table 9 shows the carbide diameter, the Wolla area ratio, the ferrite iron particle size, the Vickers hardness, and the number of carbides in the ferrite grain boundary relative to the carbide in the ferrite grain. The ratio of the number, the ratio of the maximum crack length to the thickness of the vertical wall portion, and the measurement and evaluation of the impact resistance Price result.

The final hot rolling temperature of the comparative steel AC-2 is low and the productivity is low. The final hot rolling temperature of the comparative steel AN-4 is high, and the scale is formed on the surface of the steel sheet. When cold forging and carbon-impregnated quenching and tempering give the punching load, the crack will be generated from the crotch and the impact resistance will be degraded.

Inventive steel AB-3 has a slow cooling rate due to ROT, resulting in a decrease in productivity and the production of scales. The cooling rate of the inventive steels AJ-3 and AD-4 at ROT was 100 ° C / sec, and the outermost layer portion of the steel sheet was excessively cooled, and fine cracks were formed in the outermost layer portion.

Comparative steel AN-3 has a low coiling temperature, and a large amount of low-temperature metamorphosed structure such as toughened iron or granulated iron is embrittled, and cracks are frequently generated when the coil is rolled, and productivity is lowered. In addition, the cold-forging and carbon-impregnated tempering of the sample taken from the cracked piece was poor after the tempering.

Compared with steel AH-3, the coiling temperature is high, and the layered interval is thick and thick in the hot-rolled structure, and the needle-like coarse carbide has high thermal stability. The carbide is formed after the 2-stage annealing. It remains in the steel sheet, so the cold forgeability is low.

Comparative steel AF-4 has a low heating rate in the first-stage annealing of the two-stage type annealing, so productivity is low. Compared with the first stage of annealing, the comparative steel AG-2 has a high heating rate, so the temperature difference between the inner and outer peripheral portions of the coil becomes large, resulting in friction entanglement and burning due to the difference in thermal expansion, and resistance after cold forging and carbon tempering and tempering. The rushing characteristics are degraded.

The holding temperature (annealing temperature) of the first-stage annealing of the comparative steel AA-2 is low, the coarsening treatment of the carbide below the Ac1 point is insufficient, the thermal stability of the carbide is insufficient, and the carbide remaining during the second-stage annealing Reduced, the structure after slow cooling failed to inhibit the wave iron metamorphosis, and the cold forgeability decreased.

Compare the holding temperature of the first stage of steel AM-3 (annealing temperature) High, the formation of Worthite iron in the annealing, failed to improve the stability of the carbide, cold forging and carbonization quenching and tempering after the aging resistance decreased. The first stage annealing of the comparative steel AF-2 has a short holding time, fails to improve the stability of the carbide, and has low cold forgeability. The first stage annealing of the comparative steel AO-4 has a long holding time and low productivity.

In the second-stage annealing of the comparative steel AP-4, the second-stage annealing has a slow heating rate, so the productivity is low. Comparing the heating speed of the second-stage annealing of steel AI-3 is fast, the temperature difference between the inner and outer peripheral portions of the coil becomes large, and the frictional enthalpy and the burning of the large thermal expansion difference due to the metamorphosis occur, and the carbon immersion quenching and tempering is imparted. When the load is washed, cracks will be generated from the crotch and the impact resistance will be degraded.

The holding temperature (annealing temperature) of the second-stage annealing of the comparative steel AL-3 was low, and the amount of formation of the Worthite iron was small, and the ratio of the number of carbides at the ferrite grain boundary was not increased, and the cold forgeability was lowered. Compared with the second stage of annealing of steel AD-2, the holding temperature (annealing temperature) is high, and the melting of carbides is promoted during annealing. Therefore, grain boundary carbides are not easily formed after slow cooling, and the resistance after cold forge and carbon tempering and tempering The rushing characteristics are degraded.

The second-stage annealing of the comparative steel AJ-4 has a long holding time and promotes melting of the carbide, so that the cold forgeability is low. Comparing steel AQ-3 from the second stage annealing to 650 ° C, the cooling rate is slow, the productivity is low, and the coarsely formed carbide is formed in the structure after the slow cooling, and the crack is generated by the coarse carbide as the starting point in the cold forging. Cold forgeability is reduced. Comparing the steel AP-2 from the second stage annealing to 650 ° C, the cooling rate is fast, and the wave will be metamorphosed upon cooling, and the hardness is increased, so the cold forgeability is lowered.

Here, the relationship between the number of grain boundary carbides and the number of intragranular carbides is shown in Fig. 3, and the relationship between the crack length of the cold forged test piece and the impact resistance after the carbon-impregnated tempering.

As can be seen from Fig. 3, when the number ratio (=number of grain boundary carbides/number of intragranular carbides) is more than 1, the ratio of crack length introduced during cold forging can be suppressed, and after carbon immersion quenching and tempering, Excellent resistance to punching.

Further, Fig. 4 shows the relationship between the number of grain boundary carbides and the number of intragranular carbides, the crack length of the cold forged test piece, and the impact resistance characteristics after the carbon-impregnated tempering. Fig. 4 is a view showing that the crack length can be suppressed even in a steel sheet to which an additive element is added.

As can be seen from Fig. 4, even when the steel sheet is added with an appropriate range of elements, the number ratio (=number of grain boundary carbides/number of intragranular carbides) is more than 1, the turtle introduced during cold forging can be suppressed. The ratio of crack length, excellent resistance to punching after carbon quenching and tempering.

Industrial availability

As described above, according to the present invention, it is possible to provide a low carbon steel sheet excellent in cold forging property and after punching and quenching and tempering, and a method for producing the same. The steel sheet of the present invention is suitable for use as a material for forming a high-frequency gear or the like after cold forging such as sheet forming, and therefore has high industrial applicability.

Claims (2)

A steel sheet whose composition is in mass %, including: C: 0.10 to 0.40%, Si: 0.01 to 0.30%, Mn: 0.30 to 1.00%, Al: 0.001 to 0.10%, Cr: 0.50 to 2.00%, Mo: 0.001 to 1.00%, P: 0.020% or less, S: 0.010% or less, N: 0.020% or less, O: 0.020% or less, Ti: 0.010% or less, B: 0.0005% or less, Sn: 0.050% or less, Sb: 0.050 % or less, As: 0.050% or less, Nb: 0.10% or less, V: 0.10% or less, Cu: 0.10% or less, W: 0.10% or less, Ta: 0.10% or less, and Ni: 0.10% or less. Mg: 0.050% or less, Ca: 0.050% or less, Y: 0.050% or less, Zr: 0.050% or less, La: 0.050% or less, and Ce: 0.050%, and the balance is Fe and impurities; the low carbon steel sheet is characterized by The metal structure satisfies: the carbide particle size is 0.4 to 2.0 μm, the brite iron area ratio is 6% or less, and the ratio of the number of carbides in the ferrite grain boundary to the number of carbides in the ferrite grain. It is larger than 1; the Vickers hardness of the low carbon steel sheet is 100 HV or more and 180 HV or less. A method for producing a steel sheet, which is characterized in that the steel sheet according to claim 1 is characterized in that the steel sheet having the composition of claim 1 is subjected to hot rolling to form a hot rolled steel sheet, and the hot rolling is carried out Final hot rolling is completed in a temperature range of 650 ° C or higher and 950 ° C or lower; the hot-rolled steel sheet is taken up at 400 ° C or higher and 600 ° C or lower; the hot-rolled steel sheet after coiling is pickled, and then at 30 ° C / hour or more And heating the hot-rolled steel sheet after pickling to an annealing temperature of 650 ° C or more and 720 ° C or less at a heating rate of 150 ° C / hour or less, and then performing the first-stage annealing for 3 hours or more and 60 hours or less; Next, the hot-rolled steel sheet is heated to an annealing temperature of 725° C. or more and 790° C. or less at a heating rate of 1° C./hour or more and 80° C./hour or less, and then subjected to a second-stage annealing which is maintained for 3 hours or more and 50 hours or less; The annealed hot-rolled steel sheet was cooled to 650 ° C at a cooling rate of 1 ° C / hour or more and 100 ° C / hour or less.