JP5333533B2 - Hot rolled steel sheet for carburizing and quenching - Google Patents

Description

  The present invention relates to a carburizing and quenching steel sheet, and more particularly to a carburizing and quenching steel sheet that is excellent in carburizing hardenability and excellent in workability and surface properties such as punching and burring.

  In general, small parts such as automobiles and machines are often manufactured by punching a steel plate as a raw material. However, such a component is required to have a certain degree of hardness because it is excellent in dimensional accuracy and has both strength and wear resistance. However, increasing the hardness of the material impairs the punching workability and causes wear and damage to the punching die. Therefore, as a material used for the punching process, TS uses a soft material of 440 MPa class to improve the punching processability, while the required hardness required for the product is carburizing and quenching after the punching process. It has been made up with.

  As steel used by carburizing and quenching, for example, SCr steel, SCM steel, SNCM steel and the like special steel strip (skin-hardened steel) defined in JIS G3311 are known. However, since these steel sheets contain a large amount of C at 0.1 mass% or more, it is necessary to subject them to spheroidizing annealing and soften them, and to provide a punching process, and because the strength is too high, It cannot be said that it is suitable for applications such as punching and burring.

  Therefore, a steel plate for carburizing and quenching with a reduced amount of C has been proposed. For example, Patent Document 1 discloses a steel material of C: 0.05 to 0.35 mass%, and Patent Document 2 discloses a steel plate of C: 0.05 to 0.50 mass%.

JP 59-123714 A JP 61-106715 A

  However, although the amount of C in these steels has been reduced, it is still high. Therefore, when the above steel sheet is punched, cracks may occur in the flange portion in burring, which is one of the stretch flange moldings. As a countermeasure, precision punching must be employed. is there. In addition, since the steel sheet contains a relatively large amount of Si, scale defects (red scale) are induced, resulting in variations in the surface roughness of the steel sheet, and steel sheet feeding and punching during punching. There has been a problem that deviations in the feeding of parts occur, which tends to cause dimensional defects in units of microns.

  Then, the objective of this invention is providing the steel plate for carburizing hardening which aimed at the improvement of workability and surface property, maintaining carburizing hardenability.

  The inventors have repeatedly studied for the development of a carburized and quenched steel sheet having a tensile strength of 440 MPa class and excellent carburizing hardenability, and excellent workability, particularly burring workability and surface properties. As a result, in order to improve workability, it is effective to reduce the amount of C and S, and to reduce the hardenability accompanying the decrease in the amount of C, it is necessary to add appropriate amounts of Mn and Cr. In addition, in order to improve the surface properties, it has been found that reduction of the Si amount is effective, and the present invention has been completed.

That is, the present invention is C: 0.03-0.10 mass%, Si: 0.05 mass% or less, Mn: 1.00-1.80 mass%, S: 0.007 mass% or less, Al: 0.010 Contains 0.060 mass%, N: 0.0060 mass% or less, Cr: 0.20 to 0.50 mass%, the balance is made of Fe and inevitable impurities , the tensile strength is 440 to 490 MPa, and the critical hole expansion rate Is a hot-rolled steel sheet for carburizing and quenching, characterized by being 85% or more .

  In addition, the hot-rolled steel sheet for carburizing and quenching according to the present invention has a thickness of 4.0 to 6.0 mm.

  According to the present invention, it is possible to provide a carburized and hardened steel sheet having excellent workability and surface properties. Therefore, the steel sheet is manufactured by carburizing and quenching after punching or burring. It can be suitably used as a material for small parts such as.

The reason for limiting the component composition of the steel sheet according to the present invention will be described.
C: 0.03-0.10 mass%
C is a component necessary for ensuring the strength of the material before carburizing and quenching, and in order to obtain the tensile strength TS ≧ 440 MPa, it is necessary to add at least 0.03 mass%. However, when the C content exceeds 0.10 mass%, the strength of the steel sheet increases, and the workability, particularly the stretch flangeability decreases, and cracking occurs in burring. Therefore, C is set to a range of 0.03 to 0.10 mass%. Preferably it is 0.04-0.07 mass%.

Si: 0.05 mass% or less Si is an element added as a deoxidizer and to increase the strength of steel. However, since Si generates scale flaws called red scale in hot rolling and makes the surface of the steel sheet uneven, it is preferable that the Si content be 0.05 mass% or less in the present invention. Preferably, it is 0.03 mass% or less.

Mn: 1.00-1.80 mass%
Mn has an effect of increasing material strength and an effect of improving hardenability. In the present invention, since the C content is reduced, 1.00 mass% or more is added in order to compensate for this. On the other hand, if the amount of Mn exceeds 1.80 mass%, not only the toughness is lowered, but also a layered structure is generated and the hole expandability is lowered, so the upper limit is made 1.80 mass%. Preferably it is the range of 1.50-1.60 mass%.

S: 0.007 mass% or less S is an impurity component inevitably mixed in steel, and is a harmful element that forms sulfide inclusions and reduces stretch flangeability. Therefore, it is desirable that S be small, and in the present invention, it is 0.007 mass% or less. Preferably, it is 0.003 mass% or less.

Al: 0.010-0.060 mass%
Al is a component added as a deoxidizer, and in particular, in the component system of the present invention, it is an important component as a deoxidizer that replaces Si. In order to obtain this effect sufficiently, it is necessary to make it 0.010 mass% or more. On the other hand, when the amount of Al exceeds 0.060 mass%, it combines with N to form AlN, suppresses the grain growth of austenite, and inhibits hardenability. In addition, the addition of a large amount of Al causes an increase in Al ₂ O ₃ -based inclusions, leading to a decrease in surface properties and workability. Therefore, in the present invention, the content of Al is set to 0.010 to 0.060 mass%. Preferably, it is in the range of 0.010 to 0.050 mass%.

N: 0.006 mass% or less N is an impurity that is inevitably mixed and forms AlN by combining with Al, and suppresses grain growth of austenite and lowers hardenability, so 0.006 mass% or less Limit to. In addition, excessively reducing N causes an increase in steelmaking cost, so the lower limit is preferably about 0.0020 mass%. More preferably, it is N: 0.0030-0.0060mass%.

Cr: 0.20 to 0.50 mass%
Cr is an element that improves hardenability. In the present invention, Cr is an important component for compensating for hardenability due to C reduction, like Mn. In order to obtain the effect, it is necessary to add at least 0.20 mass%. However, if it exceeds 0.50 mass%, the hardenability becomes too high, leading to a decrease in toughness and an increase in raw material cost. Therefore, in the present invention, the Cr content is in the range of 0.20 to 0.50 mass%. Preferably, it is 0.30 to 0.40 mass%.

  In the steel sheet of the present invention, the balance other than the above components is preferably composed of Fe and inevitable impurities. Examples of the inevitable impurities include P. P is preferably about 0.03 mass% or less. In addition, this invention does not refuse containing a component other than the above, if it is a range which does not injure the effect of this invention.

Next, preferable characteristics of the carburized and quenched steel sheet according to the present invention will be described.
Tensile strength TS: 440 to 490 MPa
Since the steel sheet of the present invention is used for small parts such as automobiles and machines, it is necessary to ensure a certain level of strength in a state before carburizing and quenching. In addition, it is desirable to have a certain degree of hardness from the viewpoint of preventing sagging and burrs in the punching process. In order to satisfy these requirements, it is preferable that the tensile strength TS of the steel sheet as the material is 440 MPa or more. However, if the strength becomes too high, the punchability and workability are impaired, the wear of the punching die is accelerated, and damage is caused. Therefore, the upper limit is preferably 490 MPa or less.

Limit hole expansion ratio λ: 85% or more It is desirable to have excellent stretch flange characteristics so as to be excellent in workability, and particularly in order to prevent cracking in burring after punching. This stretch flange characteristic is generally known to correspond to the critical hole expansion rate λ defined by the following formula obtained from the hole expansion test, and in the steel sheet of the present invention, this λ is 85% or more. It is preferable. More preferably, λ is 100% or more.
Limit hole expansion rate λ = {(d ₁ −d ₀ ) / d ₀ } × 100 (%)
Here, d ₁ : hole diameter at the time of fracture occurrence (mm), d ₀ : initial hole diameter before drawing (mm)

Plate thickness: 4.0-6.0mm
The steel plate of the present invention preferably has a plate thickness in the range of 4.0 to 6.0 mm. This is because this thickness range is most often used for applications that are carburized and quenched after punching.

Next, the preferable manufacturing method of the steel plate of this invention is demonstrated.
Slab production, slab heating Steel that satisfies the above composition is melted by a generally known method such as a converter or an electric furnace, and a steel slab is obtained by a generally known method such as a continuous casting method or an ingot-bundling rolling method. . Next, this steel slab is preferably charged into a heating furnace, reheated to about 1200 to 1250 ° C., and then hot-rolled to obtain a hot-rolled steel sheet. In addition to the usual method of charging the cooled slab into the heating furnace, the slab is reheated, after continuous casting, after charging the slab into the heating furnace without lowering it to room temperature, You may employ | adopt the warm piece charging method with which it uses for rolling. Moreover, if the finish rolling completion temperature mentioned later can be ensured, you may use the direct rolling method or direct feed rolling method which uses for hot rolling immediately after continuous casting, without cooling a slab.

Hot rolling The steel slab is hot-rolled with a finish rolling finish temperature FDT of 800-880 ° C, and preferably a hot-rolled steel plate with a plate thickness of 4.0-6.0 mm. When the FDT is less than 800 ° C., a ferrite structure with low ductility is obtained, and the elongation El decreases. On the other hand, when the temperature exceeds 880 ° C., surface defects are likely to occur. Moreover, coil winding temperature CT after hot rolling shall be 500-600 degreeC. This is because if the CT is less than 500 ° C., the structure becomes hard and the ductility decreases, while if it exceeds 600 ° C., the strength cannot be secured.

  The steel sheet after hot rolling is used as it is or subjected to descaling treatment such as pickling or shot blasting to obtain a carburized and quenched steel sheet for products.

  The carburized and quenched steel sheet of the present invention is carburized by a generally known method after forming such as punching or burring, and is hardened to obtain a product having a desired hardness. The carburizing method is preferably a gas carburizing method. In this case, as carburizing conditions, for example, after carburizing at a temperature of 900 to 930 ° C. for 90 to 150 minutes, a diffusion treatment of about 30 minutes is performed. preferable. Moreover, it is preferable that a quenching process is air-cooled after heating for 30 to 40 minutes at 830-850 degreeC. In addition, you may perform a tempering process after hardening as needed.

A steel having the composition shown in Table 1 is melted in a converter and made into a steel slab by a continuous casting method. After this steel slab is reheated to 1230 ° C, the finish rolling finish temperature is 860 ° C. After rolling, it was wound up at a temperature of 570 ° C. to obtain a hot rolled sheet having a sheet thickness of 4.0 to 6.0 mm. This hot-rolled sheet is passed through the pickling line and descaled, and then the steel sheet surface is observed on the exit side of the line, the state of generation of scale defects due to Si is observed, and the scale defects due to Si (red scale) are observed. The case where there was an occurrence was evaluated as x, and the case where there was no occurrence was evaluated as ◯. Moreover, the sample was extract | collected at the pickling line exit side, and it used for the following test.
<Tensile test>
A JIS No. 5 tensile test piece was taken in a direction parallel to the rolling direction, a tensile test was performed in accordance with JIS Z2241, a tensile strength TS was measured, and a sample having a TS of 440 to 490 MPa was accepted.
<Measurement of critical hole expansion rate λ>
In accordance with JFST-1001-1996 “Hole Expansion Test Method”, a disk sample having an outer diameter of 130 mm × 130 mm and a hole of ₁₀ mmφ (d ₀ ) in the center was collected from the sample, and 60 ° push the conical punch, stop to the rupture hole edge occurs, the hole diameter d ₁ measured at this time, according to the following formula to determine the critical hole expansion ratio lambda. As a result of the above measurement, a sample with λ ≧ 85% was regarded as acceptable.
Limit hole expansion rate λ = {(d ₁ −d ₀ ) / d ₀ } × 100 (%)
<Evaluation of carburizing hardenability>
A 50 mm × 100 mm test piece is taken from the above sample, and the test piece is subjected to diffusion treatment after carburizing at 900 ° C. for 120 minutes by a gas carburizing method, and then heated to 850 ° C. for 30 minutes and then cooled by air. Then, the Vickers hardness Hv on the sample surface was measured. In addition, the sample is cut, the Vickers hardness in the thickness direction is measured according to JIS G0557, a hardness transition curve is created, and the effective hardened layer depth (distance from the surface to Hv550) is determined from the curve. Asked. As a result of the measurement, a surface Vickers hardness Hv of 750 to 880 and an effective hardened layer depth of 500 μm or more were evaluated as acceptable.

  The results of the above measurements are shown together in Table 1. From Table 1, the steel sheet satisfying the component composition of the present invention is excellent in all of tensile strength, workability, and surface characteristics. On the other hand, it can be seen that the comparative steel that does not satisfy the component composition of the present invention is inferior in any one or more of the above characteristics.

Claims (2)

C: 0.03-0.10 mass%,
Si: 0.05 mass% or less,
Mn: 1.00-1.80 mass%,
S: 0.007 mass% or less,
Al: 0.010 to 0.060 mass%,
N: 0.0060 mass% or less,
Cr: 0.20 to 0.50 mass% is contained,
A hot-rolled steel sheet for carburizing and quenching, wherein the balance is Fe and inevitable impurities , the tensile strength is 440 to 490 MPa, and the critical hole expansion rate is 85% or more . The hot rolled steel sheet for carburizing and quenching according to claim 1, wherein the steel sheet has a thickness of 4.0 to 6.0 mm.