JPH0892633A - Production of high strength and high toughness steel - Google Patents

Abstract

PURPOSE: To produce a high strength steel improved in crack propagating resistance. CONSTITUTION: A steel having a compsn. contg. 0.3 to 0.8% C, 0.5 to 2.0% Mn and 0.01% P and contg. one or >= two kinds of 0.005 to 0.02% N and 0.01 to 0.1% V, 0.01 to 0.1% Nb and 0.01 to 0.1% Ti is heated to the Ac3 point or above, is austenitized, is thereafter rapidly cooled to the temp. range of the martensite forming temp. or below and is held to the same temp. range. The steel to be used may contain 0.2 to 2.0% Cr, 0.2 to 2.0% Ni and 0.1 to 2.0% Mo as well and is used as various machine parts such as gears and chains, cutting tools or the like excellent in durability.

Description

Detailed Description of the Invention

[0001]

The present invention has a high crack propagation resistance,
The present invention relates to a method for producing high strength and high toughness steel having suitable strength and toughness as various mechanical parts such as blades, tools, chains, and gears.

[0002]

2. Description of the Related Art High hardness materials such as high carbon steel are used in various fields such as various machine parts, blades and tools. The mechanical properties required for this type of part include high hardness, high strength, high toughness, high fatigue strength, and wear resistance. Fatigue properties and wear resistance are generally improved by increasing hardness and strength. However, when the hardness and strength are increased, the toughness is reduced accordingly, and the problem caused by the increase in notch sensitivity becomes more serious. Many members used for various machine parts and the like are first heat-treated after being formed into a member shape by punching, cutting or the like from a raw material in a manufacturing process. At that time, it is very difficult to guarantee surface flaws in an industrial mass production line. For example, the link plate of a chain is usually formed by high-speed stamping of a coil-shaped raw material plate. After the punching, a secondary sheared surface and mussles associated therewith frequently occur on the end surface of the material, but it is usually used as a product without removing them.

The generated secondary shear planes and mussles are notches,
It acts as an initial crack or the like, which causes a significant decrease in the toughness of the link plate. Further, when the hardness of the link plate material is increased in order to increase the strength of the chain, the notch susceptibility due to the properties of the punched end face becomes even higher, increasing the risk of brittle fracture. Steel for machine parts with tensile strength of 1500 N / mm ² or more or hardness of HRC
When obtaining a high-strength material of 45 or more, it is general to heat-treat after molding the parts. However, the metal structure obtained by quenching and tempering is a tempered martensite structure and has a low crack propagation resistance. A method of isothermal transformation treatment has been developed as a method of strengthening high-strength steel instead of quenching / tempering treatment. For example, Japanese Patent Publication No. 51-2949
According to Japanese Patent Laid-Open No. 2 (1993), a steel sheet having a bainite structure is manufactured by keeping a low-alloy carbon steel at a temperature above the martensitic transformation temperature. The obtained steel sheet has a hardness HV.
High ductility at 473 and a tensile strength of 1533 N / mm ² .

[0004]

In order to further improve the tensile strength by bainizing, it is necessary to lower the isothermal transformation temperature. However, in this method, since the isothermal holding temperature is limited to the martensitic transformation temperature or higher, the tensile strength to be obtained has its own limit. Therefore, the tensile strength of this method is 1500 N / mm.
Steel sheets exceeding ² cannot be obtained. As a method for increasing the strength by a constant temperature transformation treatment, Japanese Patent Publication No. Sho 64-8051 discloses a method called "pulling austempering method". According to this method, the austenitized steel is once quenched below the martensitic transformation point and then reheated to the bainite transformation temperature to mix martensite in the structure to increase the strength. However, this method requires three stages of heat treatment, and since temperature control and time control are strict, production efficiency is poor except for a continuous heat treatment line. The present invention has been devised to solve such a problem, and the tensile strength of 1500 N can be obtained by the combination of the specified component adjustment and the constant temperature holding temperature.
The object is to obtain a high-strength and high-toughness steel having a crack resistance of more than 1 / mm ² and excellent crack propagation resistance.

[0005]

In order to achieve the object, the production method of the present invention is C: 0.3 to 0.8% by weight, M
n: 0.5 to 2.0 wt% and P: 0.01 wt% or less, and N: 0.005 to 0.02 in the basic composition of the balance Fe.
% By weight, V: 0.01 to 0.1% by weight, Nb: 0.01
~ 0.1 wt% and Ti: 0.01-0.1 wt% 1
, Or steel containing two or more kinds is heated to a temperature of Ac ₃ point or more to austenite, and then rapidly cooled to a temperature range of 220 ° C. or higher and a temperature of martensite formation temperature or lower, and the temperature range is 10 to 90. It is characterized by holding minutes. The steel material used in the present invention includes, in addition to the above-mentioned basic components, Cr:
If necessary, one or more of 0.2 to 2.0% by weight, Ni: 0.2 to 2.0% by weight, and Mo: 0.1 to 2.0% by weight may be contained.

[0006]

In the present invention, a steel material containing C-Mn steel as a basic component, reducing the P content and adding necessary alloying elements is used. When this steel material is subjected to a heat treatment characterized by being kept at a temperature below the martensitic transformation temperature, the crack propagation resistance increases due to the toughness of the former austenite grain boundaries and the refinement of the former austenite grain size. The present invention has been completed based on this finding, and provides a composite structure exhibiting high strength and high toughness by a specified combination of component adjustment and constant temperature holding conditions. The reason why the steel produced according to the present invention exhibits excellent crack propagation resistance is presumed as follows. That is, when the content of the alloy component is appropriately adjusted and the isothermal holding is performed in the temperature range below the martensitic transformation point, a metal structure having a lower bainite phase as a main phase is generated. This metallic structure is
Effectively suppress brittle fracture due to former austenite grain boundary fracture.

The action of the metallic structure has been confirmed by experiments by the present inventors, as will be apparent from the examples described later. That is, the fatigue precracked test piece was subjected to a tensile test to investigate the effect of heat treatment on the crack propagation resistance. When compared at the same hardness, it was revealed that the steel sheet subjected to the isothermal holding treatment had a lower occurrence rate of the former austenite intergranular fracture and a higher crack propagation resistance than the steel sheet subjected to the quenching / tempering treatment. The isothermal holding treatment in which the holding temperature is lower than the martensitic transformation point of steel is called martemper, and when it is rapidly cooled below the martensitic transformation point, martensite is generated according to the degree of supercooling from the martensitic transformation point. To do. The martensite transformation does not proceed during the isothermal holding because it is a non-isothermal transformation, and the produced martensite is immediately tempered to become tempered martensite. At this time, lower bainite is isothermally transformed from untransformed austenite.

At the isothermal holding temperature just below the martensite transformation point, the amount of martensite transformation is relatively small, and the bainite transformation is accelerated with the produced martensite as a nucleus. As a result, a structure having lower bainite as a main phase tends to be formed. As described above, the metal structure obtained by the martempering treatment is a metal structure containing a large amount of bainite in the carbon steel targeted by the present invention, although it depends on the composition of the steel material. It is not clear why the steel sheet that has been subjected to the isothermal holding treatment according to the present invention exhibits superior crack propagation resistance as compared with the steel sheet that has been subjected to the quenching and tempering treatment. However, since tempered martensite and lower bainite differ in the composition and shape of the precipitated carbide and the orientation of the precipitation plane, it is speculated that the difference in carbide morphology affects the improvement in toughness. It is also considered that the former austenite intergranular cracks were suppressed due to less precipitation of intergranular carbides in the isothermal holding treated material.

In order to obtain a bainite structure having a tensile strength of 1500 N / mm ² or more in the isothermal holding treatment, it is necessary to adjust the holding temperature. In the case of low alloy carbon steel, the relationship between the tensile strength and the holding temperature of the steel material subjected to the isothermal holding treatment is almost constant regardless of the component system. Specifically, 1500N
Above / mm ² , it is required to be about 350 ° C or lower. The martensitic transformation point of steel differs depending on the component system, and when the holding temperature is just above the martensitic transformation point, the bainite transformation rate is significantly reduced. If the isothermal holding treatment is performed in this temperature range, the bainite transformation becomes insufficient, and the untransformed austenite is likely to undergo the martensite transformation in the cooling stage after the holding treatment. The martensite formed at this time is called an MA phase accompanied by unstable retained austenite, and has poor toughness. That is, a material kept at a constant temperature just above the martensitic transformation point becomes a material having poor toughness.

On the other hand, when isothermal holding is carried out at a temperature just below the martensite transformation point, the martensite produced when quenching to the holding temperature is immediately tempered and the toughness is improved. In addition, untransformed austenite rapidly transforms into lower bainite. Therefore, by maintaining a constant temperature just below the martensitic transformation point, a material having high toughness can be obtained. Looking at the influence of the component system, the reduction of the P content increases the strength of the former austenite grain boundary and suppresses the grain boundary destruction. However, improvement of the crack propagation resistance is insufficient only by reducing P. In this regard, if N, V, Nb, and Ti are added to reduce the grain size of the prior austenite, the maximum effect on the crack propagation resistance is exhibited. That is, the crack propagation resistance is significantly improved by refining the former austenite grain size strengthened by reducing P. Further, since the bainite transformation is promoted by the refinement of the grain size of the former austenite, the heat treatment time is shortened.

The alloy components contained in the steel material used in the present invention, heat treatment conditions, etc. will be described below. C: 0.3 to 0.8% by weight An alloying element effective in improving the strength of the steel sheet, and 1500 N /
In order to obtain a tensile strength exceeding mm ² , a C content of 0.3% by weight or more is necessary. However, the C content is 0.8
If the content exceeds 100% by weight, grain boundary cementite is inevitably precipitated and the toughness is reduced. Mn: 0.5 to 2.0 wt% It is an alloying element necessary to secure the hardenability of the steel sheet, and a Mn content of 0.5 wt% or more provides a sufficient hardenability improving effect. However, when a large amount of Mn exceeding 2.0% by weight is contained, not only the workability of the hot-rolled sheet or the cold-rolled sheet is lowered, but also the martensite transformation point is lowered and the bainite transformation during the isothermal holding is remarkable. Suppressed. As a result, the region where the structure having the lower bainite as the main phase is generated becomes extremely narrow, and strict temperature control is required for the heat treatment.

P: 0.01% by weight or less It is desirable to reduce the P content as much as possible, since it segregates at the austenite grain boundaries and promotes grain boundary destruction. However, an excessively low P content causes a rise in manufacturing cost. Therefore, the range that does not have a substantial adverse effect on the reduction in toughness was investigated, and the upper limit of the P content was set to 0.01% by weight. Cr: 0.2 to 2.0% by weight It is an alloying element added according to the target characteristics, and has the function of preventing graphitization during annealing and enhances the hardenability of the steel sheet to improve the strength. Such an effect becomes remarkable when the Cr content is 0.2% by weight or more. However, when a large amount of Cr exceeding 2.0% by weight is contained, such effects are lost, spheroidizing annealing becomes difficult, and the workability of the annealed material deteriorates. A large amount of Cr content also exerts an action of remarkably suppressing bainite transformation during holding at a constant temperature, and makes a region where a structure having a lower bainite as a main phase is very narrow. As a result, strict temperature control is required for heat treatment.

Ni: 0.2 to 2.0% by weight This is an alloying element added according to the target characteristics and has the effect of enhancing the hardenability of the steel sheet and improving the strength. The effect of adding Ni becomes remarkable at 0.2% by weight or more. But,
Addition of a large amount of more than 2.0% by weight lowers the martensitic transformation point and remarkably suppresses bainite transformation during constant temperature holding. As a result, the region where the structure having the lower bainite as the main phase is generated becomes extremely narrow. Mo: 0.1 to 2.0% by weight It is an alloying element added according to the target characteristics, and exhibits an action of improving strength. The effect of adding Mo becomes remarkable at a content of 0.1% by weight or more. However, if the Mo content exceeds 2.0% by weight, the workability of the hot-rolled sheet and the cold-rolled sheet deteriorates.

N: 0.005-0.02% by weight, V:
0.01 to 0.1% by weight, Nb: 0.01 to 0.1% by weight and Ti: 0.01 to 0.1% by weight, one or more kinds N, V, Nb and Ti are It is the most important alloying element in the present invention, and has an action of refining the prior austenite grain size during austenitization and increasing the crack propagation resistance of the steel sheet subjected to the isothermal holding treatment. At least N: 0.005% by weight, V: 0.0 for refining the former austenite grain size
1% by weight, Nb: 0.01% by weight, and Ti: 0.01% by weight are required, and it is possible to add these alloying elements in combination. However, if these alloying elements are added in an amount more than necessary, not only the crack propagation resistance is saturated, but also the workability of the material is deteriorated. Therefore,
The upper limit of each alloying element is N: 0.02% by weight, V: 0.1
%, Nb: 0.1% by weight, Ti: 0.1% by weight, respectively.

Constant temperature holding treatment condition: The material to be heat treated is
It is manufactured by a process similar to a normal high carbon steel manufacturing process. In the heat treatment, after heating to a temperature of Ac ₃ point or higher to austenite, it is held at a temperature range of 220 ° C. to the martensite transformation point for 10 to 90 minutes. At a heating temperature below the Ac ₃ point, the target strength cannot be obtained due to insufficient austenitization. In order to sufficiently promote the austenitization, it is preferable to heat at a temperature of (Ac ₃ + 30 ° C.) or higher for 5 minutes or longer. When the isothermal holding temperature is higher than the martensite transformation point, the amount of martensite produced is small,
The effect of promoting bainite transformation cannot be obtained. Therefore, it takes a long time to complete the bainite transformation. On the other hand, at a constant temperature holding temperature that does not reach 220 ° C., the amount of martensite generated during cooling increases and the amount of bainite decreases, so the crack propagation resistance decreases. Retention time 1
A sufficient amount of bainite can be obtained by keeping the temperature constant for 0 minutes or more. The effect of the constant temperature holding is saturated at the holding time of 90 minutes, and even if the holding time is longer than that, no substantial improvement in the characteristics is observed.

[0016]

EXAMPLE A steel material having a composition shown in Table 1 and a plate thickness of 1.6 mm was subjected to the heat treatment shown in Table 2, and the effects of the components and the heat treatment conditions on the properties of the steel material were investigated. Group A in Table 1 is the carbon steel used for comparison with group B steel according to the present invention. As shown in Table 2, when each steel material was subjected to heat treatment under different conditions, the characteristics of the steel material subjected to the isothermal holding treatment exhibited different characteristics and metallographic structures depending on the processing conditions, as shown in Table 3. For the evaluation of crack propagation resistance, a test piece of 45 mm × 180 mm was cut out from a material steel plate, and a hole formed in the central portion with a dimension shown in FIG. 1 was provided with a groove portion by electric discharge machining. By repeatedly applying tensile load to this test piece with a hydraulic fatigue tester,
Fatigue pre-crack was added. Then, it heat-processed and tempered and used for the tensile test. As the crack propagation resistance value, a value obtained by dividing the maximum load until breakage in the tensile test by the initial cross-sectional area was used. The isothermal holding temperature exerted the effect as shown in FIG. 2 on the crack propagation resistance value.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

When a steel material of Group A containing a large amount of P and containing no N, V, Nb or Ti is kept at a constant temperature,
As shown in Table 3 as Comparative Example I, the crack propagation resistance value is low. The low crack propagation resistance comes from the fact that the austenite grain size grows large during heat treatment and the strength of the former austenite grain boundaries is not sufficient. On the other hand, B satisfying the conditions concerning the components specified in the present invention
When the steel members of the group were subjected to the constant temperature holding treatment of Condition 1 or 2, high crack propagation resistance values were obtained.

Even if the components satisfy the conditions of the present invention, as shown in Table 3 as Comparative Example II, the conditions 3 to 5 are satisfied.
When subjected to the heat treatment of No. 1, neither of the desired high strength and high toughness was obtained. That is, in the case where the steel material B1 was heat-treated under the condition 3, since the holding temperature was too low, martensite became the main phase and the crack propagation resistance value was low. In the case where the steel material B1 is heat-treated under the condition 4,
The holding temperature was too high, indicating low tensile strength. Further, in the case where the steel material B1 was heat-treated under the condition 5, the austenitizing temperature was low, so that the austenitizing was insufficient and the tensile strength was low. Table 3
As is clear from the above, it was confirmed that the combination of the specific components and the heat treatment under the specific conditions is effective in order to obtain a steel material having both high strength and toughness.

[0022]

As described above, in the present invention, a steel material in which the P content is reduced and the former austenite grain size is refined with N, V, Nb, and Ti is used at a temperature below the martensitic transformation point. By maintaining a constant temperature, the toughness is improved while maintaining high strength, and the metal structure has reduced notch sensitivity. The steel material obtained has a tensile strength of 1500.
Since it exhibits excellent crack propagation resistance at N / mm ² or more,
It is used in a wide range of fields such as various machine parts and blades.

[Brief description of drawings]

[Fig. 1] Specimen for investigating crack propagation resistance

FIG. 2 is a graph showing the effect of constant temperature holding temperature on crack propagation resistance.

Claims (2)

[Claims] 1. C: 0.3 to 0.8% by weight, Mn: 0.
5 to 2.0 wt% and P: 0.01 wt% or less, and the balance of the basic composition of Fe is N: 0.005 to 0.02 wt%,
V: 0.01 to 0.1% by weight, Nb: 0.01 to 0.1
Weight% and Ti: 0.01 to 0.1% by weight of one or two
The steel containing at least the seeds is heated to a temperature of Ac ₃ point or higher to austenite, and then rapidly cooled to a temperature range of 220 ° C. or higher and a temperature of martensite generation temperature or lower, and the temperature range is 10 to 10
A method for producing a high-strength and high-toughness steel that holds 90 minutes. 2. The steel according to claim 1, further comprising Cr: 0.2 to
2.0% by weight, Ni: 0.2 to 2.0% by weight and Mo:
A method for producing a high-strength and high-toughness steel containing 0.1 to 2.0% by weight of one type or two or more types.