JP5521931B2 - Soft medium carbon steel plate with excellent induction hardenability - Google Patents

Description

  The present invention relates to a soft medium carbon steel plate excellent in induction hardenability.

  Medium carbon steel plates are widely used as materials for chains, gears, clutches, saws, blades and the like. When a product is produced from a medium carbon steel plate, it is usually cured by forming and then subjected to a heat treatment such as quenching and tempering. Therefore, the medium carbon steel sheet is required to have workability to withstand complicated and severe processing. Furthermore, in recent years, in comparison with conventional furnace heating, induction hardening that can save the heat and save energy in a short time is often used. In addition to being soft, the medium carbon steel plate is also required to be induction hardened so as to ensure sufficient hardenability even with rapid heating.

  Conventionally, many investigations have been made on the relationship between workability and induction hardenability of medium carbon steel sheets (for example, see Patent Documents 1 to 4). Sufficient hardenability is achieved in rapid heating at 100 ° C./s or higher. No examples have been reported.

  For example, Patent Document 1 is made of hypoeutectoid steel with C: 0.1 to 0.8% by mass and S: 0.01% by mass or less, and carbides have a carbide spheroidization rate of 90% or more. A medium-high carbon steel sheet which is dispersed in ferrite and has an average carbide particle size of 0.4 to 1.0 μm and whose ferrite crystal particle size is adjusted to 20 μm or more as required is disclosed. However, the present invention has not proposed an invention that can ensure good hardenability even in the high-frequency heat treatment in which the medium / high carbon steel sheet is quenched at a heating rate of 100 ° C./s or more.

Japanese Patent Laid-Open No. 11-80884 JP-A-9-268344 JP 2001-329333 A JP 2001-355047 A

  Then, in view of the said situation, this invention makes it a subject to provide the medium carbon steel plate excellent in induction hardenability, soft, and excellent in workability.

The inventors of the present invention have intensively studied a method for solving the above-described problems. As a result, when high-frequency heating was performed at a heating rate of 100 ° C./s or more, it was found that the austenitizing temperature strongly depends on the ferrite grain size together with the steel plate components.
This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) In mass%,
C: 0.30 to 0.65%,
Si: 0.05-0.4%
Mn: 0.2 to 2.0%,
P: 0.005 to 0.03%,
S: 0.0001 to 0.006%,
Al: 0.005 to 0.10%,
N: 0.001 to 0.01%
A soft medium carbon steel sheet excellent in induction hardenability, characterized in that the remainder is made of Fe and inevitable impurities, and has a Vickers hardness of 160 HV or less and a ferrite particle size of 10 μm or more.

(2) In mass%,
Cr: 0.05-1.0%,
Ni: 0.01 to 0.5%,
Cu: 0.05 to 0.5%,
Mo: 0.01 to 0.5%
A soft medium carbon steel sheet excellent in induction hardenability as described in (1) above, comprising one or more of the following.

(3) In mass%,
Nb: 0.01-0.5%
V: 0.01-0.5%
Ta: 0.01 to 0.5%
B: 0.001 to 0.01%,
One or above, wherein the containing two or more (1) or superior soft in carbon steel to a high-frequency hardenability according to (2) of the.

(4) In mass%,
Sn: 0.003-0.03%,
Sb: 0.003 to 0.03%,
As: 0.003 to 0.03%
A soft medium carbon steel sheet excellent in induction hardenability according to any one of the above (1) to (3), characterized by containing one or more of the following.

  According to the present invention, a soft medium carbon steel sheet excellent in induction hardenability can be provided by controlling the ferrite grain size to 10 μm or more even at a heating rate of 100 ° C./s or more.

It is a figure which shows the relationship between the austenitizing temperature and hardness of this invention steel and a comparison steel, and a ferrite particle size. It is a figure which shows the relationship between the quenching hardness of this invention steel and comparative steel, and carbon content.

  The soft medium carbon steel sheet of the present invention is in mass%, C: 0.30 to 0.65%, Si: 0.05 to 0.4%, Mn: 0.2 to 2.0%, P: 0.00. 005 to 0.03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.0010 to 0.01%, the balance being Fe and inevitable A steel plate made of a general impurity has a Vickers hardness of 160 HV or less and a ferrite particle size of 10 μm or more.

  First, the reason for limitation relating to the component composition of the soft medium carbon steel sheet of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”) will be described. Hereinafter, “%” means “mass%”.

C: 0.30 to 0.65%
C is an important element for securing the quenching strength of the steel sheet, and 0.30% or more is added to ensure the required strength. If it is less than 0.30%, the hardenability is lowered and the strength as a high-strength steel sheet for mechanical structures cannot be obtained, so the lower limit is made 0.30%. If it exceeds 0.65%, the toughness decreases, so the upper limit is made 0.65%. Preferably, it is 0.35 to 0.55%.

Si: 0.05-0.4%
Si acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.05%, the effect of addition cannot be obtained, so the lower limit is made 0.05%. If it exceeds 0.4%, the surface properties are deteriorated due to scale wrinkling during hot rolling, so the upper limit is made 0.4%. Preferably, it is 0.10 to 0.3%.

Mn: 0.2 to 2.0%
Mn acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.2%, the effect of addition cannot be obtained, so the lower limit is made 0.2%. If it exceeds 2.0%, impact characteristics after quenching and tempering are promoted, so the upper limit is made 2.0%. Preferably, it is 0.5 to 1.5%.

P: 0.005 to 0.03%
P is a solid solution strengthening element and is an element effective for the strength of the steel sheet. Since excessive inclusion will inhibit toughness, the upper limit is made 0.03%. Reduction to less than 0.005% causes an increase in refining cost, so the lower limit is made 0.005%. Preferably, it is 0.007 to 0.02%.

S: 0.0001 to 0.006%
S is contained as an impurity in the steel, forms non-metallic inclusions, and becomes a cause of hindering workability and toughness after heat treatment, so the upper limit is made 0.006%. Reducing to less than 0.0001% causes a significant increase in refining costs, so the lower limit is made 0.0001%. Preferably, it is 0.001 to 0.004%.

Al: 0.005-0.10%
Al acts as a deoxidizing agent and is an element effective for fixing N. If it is less than 0.005%, the effect of addition cannot be sufficiently obtained, so the lower limit is made 0.005%. If it exceeds 0.10%, the effect of addition is saturated and surface flaws are likely to occur, so the upper limit is made 0.10%. Preferably, it is 0.01 to 0.05%.

N: 0.001 to 0.01%
N is an element that forms a nitride. If nitride precipitates during slab bending correction in curved continuous casting, the slab may crack, so the upper limit is made 0.01%. A smaller amount is preferable, but a reduction to less than 0.001% leads to an increase in refining costs, so the lower limit is made 0.0010%. Preferably, it is 0.004 to 0.007%.

  In order to enhance the mechanical properties of the steel sheet of the present invention, a required amount of one or more of Cr, Ni, Cu, and Mo may be added.

Cr: 0.05-1.0%
Cr is an element effective for improving hardenability. If it is less than 0.05%, there is no effect of addition, so the lower limit is made 0.05%. If it exceeds 1.0%, the effect of addition is saturated, so the upper limit is made 1.0%. Preferably, it is 0.07 to 0.7%.

Ni: 0.01 to 1.0%
Ni is an element effective for improving toughness and hardenability. If it is less than 0.01%, there is no effect of addition, so the lower limit is made 0.01%. If it exceeds 1.0%, the effect of addition is saturated and the cost is increased, so the upper limit is made 1.0%. Preferably, it is 0.05 to 0.5%.

Cu: 0.05 to 0.5%
Cu is an element effective for ensuring hardenability. If it is less than 0.05%, the effect of addition is insufficient, so the lower limit is made 0.05%. If it exceeds 0.5%, it becomes too hard and the cold workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.08 to 0.2%.

Mo: 0.01 to 1.0%
Mo is an element effective for improving hardenability and improving resistance to temper softening. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 1.0%, the effect of addition is saturated, so the upper limit is made 1.0%.

  In order to further enhance the mechanical properties of the steel sheet of the present invention, one or more of Nb, V, Ta, B, and W may be added in a required amount.

Nb: 0.01 to 0.5%
Nb is an element that forms carbonitride and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so the lower limit is made 0.01%. If it exceeds 0.5%, the effect of addition is saturated, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

V: 0.01 to 0.5%
V, like Nb, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

Ta: 0.01 to 0.5%
Ta, like Nb and V, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

B: 0.001 to 0.01%
B: It is an element effective for enhancing hardenability by adding a small amount. If it is less than 0.001%, there is no effect of addition, so the lower limit is made 0.001%. If it exceeds 0.01%, the castability deteriorates, and a B-based compound is generated to reduce toughness. Therefore, the upper limit is made 0.01%. Preferably, it is 0.003 to 0.007%.

  When scrap is used as a raw material for the steel sheet of the present invention, one or more of Sn, Sb, and As are inevitably mixed in by 0.003% or more, and any of them may be 0.03% or less. In the steel sheet of the present invention, Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0, since the induction hardenability and hardenability of the steel sheet of the present invention are not impaired. 0.001 to 0.03% of one kind or two kinds or more are allowed.

  In the steel sheet of the present invention, the amount of O is not specified, but when the oxide aggregates and coarsens, the ductility decreases, so O is preferably 0.0025% or less. A smaller amount of O is preferable, but since it is technically difficult to reduce it to less than 0.0001%, a content of 0.0001% or more is allowed.

  When scrap is used as a melting raw material of the steel sheet of the present invention, elements such as Zn and Zr are mixed as unavoidable impurities. Allow mixing. In addition, elements other than Zn, Zr and the like are allowed to be mixed as long as the characteristics of the steel sheet of the present invention are not impaired.

  The steel sheet of the present invention is formed into a product shape by processing such as cold plate forging. The higher the hardness of the material, the greater the resistance at the time of compressive deformation. In order to ensure the workability, it is effective to soften the steel sheet. In particular, when the Vickers hardness is 160 HV or less, the workability is ensured, so the hardness of the steel sheet is specified to 160 HV or less.

  As described above, the steel sheet of the present invention is characterized in that, in addition to the component composition, the ferrite grain size is 10 μm or more, and the Vickers hardness before quenching is 160 or less.

  It is a novel finding that the present inventors have found that the induction hardenability of the steel sheet is remarkably improved when the ferrite grain size is 10 μm or more.

  The observation of the tissue is preferably performed with a scanning electron microscope. Four or more regions containing 200 or more ferrite grains are selected on the observed structure, and the number of ferrite grains contained in the region is counted. One ferrite grain that can be included in the region and 0.5 ferrite grain that cannot be included. The area per ferrite grain is determined by dividing the area of the area by the number of ferrite grains in the area. The square root of the obtained area is defined as the ferrite particle size, and the average value is defined as the average ferrite particle size. If the ferrite particle size is less than 10 μm, it is difficult to ensure the required induction hardenability.

  FIG. 1 shows the relationship between the austenitizing temperature and hardness of the invention steel and the comparative steel and the ferrite grain size.

The linear thermal expansion coefficient of the sample is measured at a heating rate of 100 ° C./s or 200 ° ^C./s , and after transformation shrinkage, the linear thermal expansion coefficient becomes 1.9 × 10 ⁻⁵ K ⁻¹ or more. Was defined as the austenitizing temperature. In the case of short-time heating performed at a high frequency, austenite is generated mainly on grain boundaries rather than on carbides. If the ferrite particle size is less than 10 μm, the austenitizing temperature becomes 1000 ° C. or more, and the hardenability is lowered. If the ferrite grain size is 10 μm or more, the austenitizing temperature is less than 1000 ° C., and it is possible to ensure hardenability.

  FIG. 2 shows the quenching hardness of the steel of the present invention and the comparative steel.

  The sample was heated to 1200 ° C. at a heating rate of 100 ° C./s or 200 ° C./s, then quenched with water and measured for Vickers hardness. When the C content is less than 0.3% by mass, the quenching hardness is less than 600 HV and sufficient strength cannot be obtained. When the C content is 0.3% or more, a sufficient quenching hardness of 600 HV or more can be obtained.

  Next, a manufacturing method of the steel sheet of the present invention (hereinafter referred to as “the manufacturing method of the present invention”) will be described.

  For continuous cast slabs (cold slabs) used for hot rolling, the heating conditions are 1300 ° C. or less and a soaking time of 90 minutes or less. If heating exceeds 1300 ° C. or the soaking time is increased to 90 minutes or more, the surface layer portion of the slab becomes prominent in the heating process, and the heating temperature is 1300 ° C. to deteriorate the hardenability of the steel sheet surface. Hereinafter, the heating time is 90 minutes or less. From the viewpoint of suppressing de-C, the heating temperature is preferably 1200 ° C. or less, and the soaking time is preferably 60 minutes or less.

  In addition, although a continuous cast slab is directly heated or subjected to hot rolling by reheating the continuous cast slab, there is almost no difference in steel sheet characteristics between direct rolling and rolling after reheating. .

  Hot rolling may be either normal hot rolling or continuous hot rolling in which slabs are joined in finish rolling. The hot rolling finish temperature (hot rolling finish temperature) is not only in terms of productivity, sheet thickness accuracy and anisotropy improvement, but also in terms of surface flaws. If it is higher than ° C., the frequency of occurrence of wrinkles due to scale increases, and the product yield is lowered and the cost is increased.

  After hot rolling, the steel sheet is cooled to 650 ° C. at a cooling rate of 30 ° C./second or more after finish rolling, and subsequently, at a cooling rate of 20 ° C./second or less, a coiling temperature of 400 to 600 ° C. Allow to cool slowly.

  The reason for cooling to 650 ° C after hot rolling at a cooling rate of 30 ° C / second or more is that if the cooling rate is slower than this, a pearlite band is generated due to segregation, and coarse carbides exist even after annealing. It is easy to lead to deterioration of workability. From the viewpoint of preventing this, cooling is performed at 30 ° C./s or more. In addition, the reason for the slow cooling at a cooling rate of 20 ° C./s or less from 400 to 650 ° C., which is the cutting temperature, is to promote uniform pearlite transformation and bainite transformation of the pearlite structure. This is because, when the range is rapidly cooled, the yield decreases greatly, for example, wrinkles due to the coiling shape disorder caused by the supercooling γ occur.

The reason for winding at a coiling temperature of 400 to 650 ° C. is that when it is less than 400 ° C., some martensitic transformation occurs, the strength of the steel sheet increases, handling becomes difficult, and the structure when cold-rolling is performed. This is because yield hunting is caused by non-uniform gauge hunting. On the other hand, when high-temperature cutting exceeding 650 ° C. is performed, not only the scale of the hot-rolled sheet becomes thick and the pickling property decreases, but also the oxidation of the surface layer part and the grain boundary oxidation progress.
After the steel plate is pickled and the surface is cleaned, the steel plate is softened and annealed. In the production method of the present invention, the steel sheet is subjected to softening box annealing to improve workability.

Softening box annealing is performed by heating the steel sheet from room temperature to 600 ° C. to 750 ° C. and holding for 5 hours or more. By holding for 5 hours or more, the ferrite grains are coarsened and softened.
After holding for 5 hours or longer, it is slowly cooled so as not to generate new pearlite.

The box annealing is preferably performed in an atmosphere of 95% or more of hydrogen, a dew point up to 400 ° C. of less than −20 ° C., and a dew point of over 400 ° C. of less than −40 ° C.
In addition to the purpose of making the temperature distribution in the coil uniform, annealing is performed in an atmosphere of 95% or more of hydrogen in order to suppress a decrease in hardenability due to nitrogen penetration. In order to suppress decarburization during annealing, the dew point up to 400 ° C was set to less than -20 ° C, and the dew point above 400 ° C was set to less than -40 ° C.

  As long as the structure of the steel sheet satisfies the scope of the claims of the present invention, cold rolling and subsequent softening annealing may be added.

  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)
The steel particles having the component compositions shown in Table 1 were examined for product properties (hardness and induction hardenability) by controlling the ferrite grain size by cold rolling and annealing. The results are shown in Table 2.

In Table 2, No. 1A and No. 1 1B (using the steel plate No. 1 in Table 1) is a reference example of an austenitizing temperature exceeding 1000 ° C. and insufficient hardenability and insufficient quenching hardness of less than 600 HV. No. 1C, 1D, 1E, and 1F are reference examples of insufficient quenching hardness of less than 600 HV, although the austenitizing temperature is less than 1000 ° C. and the hardenability is ensured. No. 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, and 6B are comparative examples in which the austenitizing temperature exceeds 1000 ° C. and the hardenability is insufficient.

  In contrast, no. 2C, 2D, 2E, 2F, 3C, 3D, 3E, 3F, 4C, 4D, 4E, 4F, 5C, 5D, 5E, 5F, 6C, 6D, 6E, and 6F are examples of the invention.

  As described above, according to the present invention, it is possible to provide a soft medium carbon steel plate excellent in induction hardenability. Therefore, this invention expands the use of the medium carbon steel plate using induction hardening, and has high applicability in the steel product manufacturing industry.

Claims (4)

% By mass
C: 0.30 to 0.65%,
Si: 0.05-0.4%
Mn: 0.2 to 2.0%,
P: 0.005 to 0.03%,
S: 0.0001 to 0.006%,
Al: 0.005 to 0.10%,
N: 0.001 to 0.01%
A soft medium carbon steel sheet excellent in induction hardenability, characterized in that the remainder is made of Fe and inevitable impurities, and has a Vickers hardness of 160 HV or less and a ferrite particle size of 10 μm or more. In mass%,
Cr: 0.05 to 1.0%,
Ni: 0.01 to 0.5%,
Cu: 0.05 to 0.5%,
Mo: 0.01 to 0.5%
The soft medium carbon steel plate excellent in induction hardenability according to claim 1, comprising one or more of the following. In mass%,
Nb: 0.01-0.5%
V: 0.01-0.5%
Ta: 0.01 to 0.5%
B: 0.001~0.01%,
The soft medium carbon steel plate excellent in induction hardenability of Claim 1 or 2 characterized by containing 1 type, or 2 or more types of these. In mass%,
Sn: 0.003-0.03%,
Sb: 0.003 to 0.03%,
As: 0.003 to 0.03%
A soft medium carbon steel sheet excellent in induction hardenability according to any one of claims 1 to 3, characterized by containing one or more of the following.