JP2010043331A - Method for manufacturing seamless steel pipe for high-strength carburized part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a seamless steel pipe as a material for a part having excellent bending fatigue strength and rolling fatigue life in an environment containing foreign substances at a low cost without deteriorating the quality due to surface flaws and decarburization.
SOLUTION By mass%, C: 0.10 to 0.35%, Si: 0.35 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% or less, S : 0.030% or less, Cr: 1.50 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.01 to 0.10%, N: 0.003 to 0.025% containing, steel balance consisting of Fe and unavoidable impurities, and the steel pipe by performing the piercing and rolling heating temperature 1240 ° C. or more, before rolling finish piercing rolled steel pipe by between this heat, the following _point Ar 0 .High-strength carburizing with excellent bending fatigue strength and rolling fatigue life in an environment containing foreign matter, after cooling at a rate of 0.05 ° C./s or more and then reheating to 900 to 1080 ° C. and finish rolling. A method of manufacturing seamless steel pipes for parts.
[Selection] Figure 1

Description

  The present invention uses, for example, a ring-shaped carburized part manufactured by cutting or cold forging using a seamless steel pipe such as a cage of a constant velocity joint of an automobile or a bearing race, or a seamless steel pipe. It relates to the production of cold rolled or cold drawn steel tubes.

  High-strength, high-toughness case-hardened steel used in applications such as cages that are components of automotive constant velocity joints reduces the abnormal surface layer that occurs during carburizing by optimizing the components and impurities, and strengthens bending fatigue. Manufactured to improve the height. Also, in rolling parts that are used by polishing the raceway surface like a bearing, especially when used in an environment where foreign matter is mixed in or when the film thickness of the lubricating oil is not sufficient, the surface starting type is applied to the steel material. The peeling form is shown. For this reason, as measures to prevent the separation, improvement of rolling fatigue life and improvement of surface pressure fatigue strength by optimization of the amount of retained austenite by carburization and carbonitriding and adjustment of components are attempted. In addition, the parts made of these steel materials are intended to have a long life in combination with reduction of non-metallic inclusions by high cleanliness operation.

  Furthermore, the applicant has developed a high cleanliness steel to which about 0.15% of Ti is added as a high-strength case-hardened steel excellent in cold workability and crystal grain size characteristics. (See, for example, Patent Document 1 and Patent Document 2). However, these patent documents are materials and manufacturing methods that focus on improving the cold workability and grain size characteristics of high-strength case-hardened steel. However, in order to satisfy the requirements with an emphasis on the intended bending fatigue strength and rolling fatigue life in the presence of foreign matter, different types of steel grades are used and industrially high quality materials are reduced. It was necessary to develop a manufacturing method suitable for the supply at a low cost.

JP-A-11-92824 JP 2005-105379 A

  The problem to be solved by the present invention is to produce a seamless steel pipe as a material of a part having excellent bending fatigue strength and rolling fatigue life in an environment containing foreign matter at low cost without deteriorating the quality due to surface flaws and decarburization. Is to provide a method.

The means of the present invention for solving the above-mentioned problems is that, in the invention of claim 1, in mass%, C: 0.10 to 0.35%, Si: 0.35 to 1.00%, Mn: 0 20 to 0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 1.50 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.0. 01 to 0.10%, N: 0.003 to 0.025% is contained, and the balance is Fe and inevitable impurities, and a steel pipe is obtained by performing piercing and rolling at a heating temperature of 1240 ° C. or higher. Bending fatigue characterized by performing finish rolling by reheating to 900 to 1080 ° C. after cooling at a rate of 0.05 ° C./s or less to Ar ₁ point or less before finish rolling the pierced and rolled steel pipe High-strength carburized parts with excellent strength and rolling fatigue life in the presence of foreign matter It is a method for producing a seamless steel pipe.

In the invention of claim 2, in mass%, C: 0.10 to 0.35%, Si: 0.35 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% Hereinafter, S: 0.030% or less, Cr: 1.50 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.01 to 0.10%, N: 0.003 to 0 0.025%, further containing B: 0.0003 to 0.0050%, Ti: 0.01 to 0.05%, with the balance being Fe and inevitable impurities, with a heating temperature of 1240 ° C or higher. Piercing and rolling to obtain a steel pipe, and after finishing the hot piercing and rolling steel pipe, after cooling at a rate of 0.05 ° C./s or higher to Ar ₁ point or less, reheating to 900 to 1080 ° C. Bending fatigue strength, which is characterized by performing finish rolling, and rolling in an environment containing foreign matter It is a manufacturing method of Ri tired high-strength carburizing parts for seamless steel pipe with excellent life.

In the invention of claim 3, in addition to the components in the means of claim 1 or claim 2, two elements of Ni: 0.10 to 2.00%, Mo: 0.05 to 0.50% in mass% One or two of them are contained, and the balance is made of steel made of Fe and unavoidable impurities at a heating temperature of 1240 ° C. or more and subjected to piercing and rolling to form a steel pipe. Bending fatigue strength and rolling fatigue life in an environment containing foreign matter, characterized by cooling to a temperature of 0.05 ° C / s or more to _one point or less, then reheating to 900 to 1080 ° C and performing finish rolling This is a method for producing a seamless steel pipe for high strength carburized parts.

  The reasons for limiting the steel components in the present invention will be described below. In addition,% shows the mass%.

C: 0.10 to 0.35%
C is an element necessary for securing the core strength after carburizing or carbonitriding as a machine structural component. However, if C is less than 0.10%, the effect cannot be obtained sufficiently. If it exceeds 0.35%, the toughness is reduced and the hardness of the material is increased to deteriorate the workability. Therefore, C is 0.10 to 0.35%, preferably 0.13 to 0.20%.

Si: 0.35-1.00%
Si is an element effective for deoxidation of steel, and is an element effective for imparting necessary strength and hardenability to steel, improving temper softening resistance, and ensuring surface pressure strength. However, if the Si content is less than 0.35%, the strength improvement effect cannot be ensured. If the Si content exceeds 1.00%, the toughness decreases, the material hardness increases, and the workability deteriorates. Therefore, Si is 0.35 to 1.00%, preferably 0.40 to 0.80%.

Mn: 0.20 to 0.60%
Mn is an element that improves the hardenability of steel. However, if Mn is less than 0.20%, improvement in hardenability cannot be ensured, and manufacturability deteriorates. If it exceeds 0.60%, the hardness of the material increases and workability deteriorates. To do. Therefore, Mn is set to 0.20 to 0.60%.

P: 0.030% or less P is an element that segregates at a grain boundary to lower toughness and fatigue strength and lower part strength. Therefore, in order to solve these problems by reducing P as much as possible, it is set to 0.030% or less.

S: 0.030% or less S is an element that segregates at the grain boundary to cause embrittlement of the grain boundary and deteriorates cold workability and toughness. Therefore, in order to solve these problems by reducing S as much as possible, the content is made 0.030% or less.

Cr: 1.50 to 3.00%
Cr is an effective element for imparting hardenability and strength to the steel, and for improving the temper softening resistance and ensuring the surface pressure strength. However, if Cr is less than 1.50%, the effect is not sufficiently obtained. If it exceeds 3.00%, the hardness is increased and the workability is deteriorated. Therefore, Cr is 1.50 to 3.00%, preferably 1.80 to 2.50%.

Al: 0.005 to 0.050%
Al is an element necessary for deoxidation, and is an effective element for securing the effect of suppressing the coarsening of crystal grains by forming AlN by combining with solid solution N, but Al is 0.005. If it is less than%, the effect is not sufficient, and if there is too much Al, the effect is saturated, and further, non-metallic inclusions are generated and the fatigue strength decreases. Therefore, Al is made 0.005 to 0.050%.

Nb: 0.01 to 0.10%
Nb is an element necessary for forming fine NbC and fine NbCN to ensure the effect of suppressing the coarsening of crystal grains during carburization. However, if Nb is less than 0.01%, the effect is not sufficient. When Nb exceeds 0.10%, the effect is saturated and the cost is increased. Therefore, Nb is set to 0.01 to 0.10%.

N: 0.003 to 0.025%
N is an effective element for ensuring the effect of suppressing the coarsening of crystal grains during carburization by forming Al or Nb and AlN or NbCN in steel, but if N is less than 0.003%, The effect is not sufficient, and even if it exceeds 0.025%, the effect is saturated and hot workability is deteriorated. Therefore, N is set to 0.003 to 0.025%. However, although it mentions later, when adding B, it is necessary to make B exist freely in steel. In that case, it is necessary to reduce the amount of N, and further add Ti to fix N as TiN, thereby suppressing the formation of BN and ensuring the effect of improving the strength and hardenability of B. Therefore, N in the case of adding B is preferably 0.010% or less.

B: 0.0003 to 0.0050%
B may not be added. However, when the solid solution B is present in the steel, the effect of improving the strength and hardenability can be obtained. If B is less than 0.0003%, the effect is not sufficient, and even if it exceeds 0.0050%, the effect is saturated. Therefore, B is set to 0.0003 to 0.0050%.

Ti: 0.01 to 0.05%
Ti is added when B is added or when grain coarsening is suppressed as in the case of Nb. When Ti is added, it is an effective element for fixing solid solution N as TiN to suppress BN formation and ensuring solid solution B. However, if less than 0.01%, the effect is not sufficient. However, if it exceeds 0.05%, the workability deteriorates. Therefore, Ti is set to 0.01 to 0.05%.

Ni: 0.10 to 2.00%
Ni need not be added. However, it is an element effective for improving the hardenability and toughness of steel. When Ni is less than 0.10%, these effects are not sufficient, and when it exceeds 2.00%, the hardness of the material increases too much. The workability is deteriorated and the cost is further increased. Therefore, Ni is set to 0.10 to 2.00%.

Mo: 0.05 to 0.50%
Mo may not be added. However, it is an element effective for improving the hardenability and toughness of steel. If Mo is less than 0.05%, these effects are not sufficient. Deteriorates the cost and further increases the cost. Therefore, Mo is set to 0.05 to 0.50%.

  Ni and Mo can selectively contain 1 type or 2 types in the range of said each component.

Furthermore, the reason for limitation in the manufacturing method will be described.
When the heating temperature during piercing and rolling is less than 1240 ° C., Nb carbide and carbonitride that existed in the raw material state before heating further grow, and the number decreases. As a result, when the parts are carburized, the austenite crystal grains become coarse, that is, cause mixed grains. Therefore, the heating temperature at the time of piercing-rolling shall be 1240 degreeC or more. If the heating temperature is 1240 ° C. or higher, carbides and carbonitrides of Nb existing before that are once dissolved, and the solid solution Nb is reprecipitated as fine carbides and carbonitrides by the subsequent heat treatment. It works effectively to suppress austenite grain coarsening during carburizing.

The reason for setting the cooling temperature before reheating to Ar ₁ point or less is to precipitate the above-dissolved Nb carbides and carbonitrides. Desirably, the cooling temperature before reheating is set to Ar ₁ point-100 ° C. The following.

  As for the cooling rate to the above temperature, if it is less than 0.05 ° C./s, when Nb carbide and carbonitride reprecipitate, the growth proceeds at the same time. The cooling rate is set to 0.05 ° C./s or more.

  The reason for setting the reheating temperature before finish rolling to 900 to 1050 ° C. is to minimize the growth of Nb carbides and carbonitrides already precipitated by setting the reheating temperature to 900 ° C. or higher. This is because the growth of fine Nb carbides and carbonitrides and the re-dissolution of precipitates start when the temperature exceeds 1050 ° C.

  The present invention is a seamless steel pipe that is a material for parts excellent in bending fatigue strength and rolling fatigue life in a foreign matter mixed environment by using each of the above means, without damaging the quality due to surface flaws and decarburization, The invention according to each claim of the present application, such as being able to be manufactured without any particular cost, is a method that exhibits an excellent effect that has not been achieved in the past.

  BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described through respective steel components and tests of respective inventive examples and comparative examples with reference to tables and drawings.

  First, the manufacturing process of the test piece applied to the test of this invention is demonstrated. In the 1-t vacuum melting furnace, steels comprising the chemical components of the inventive steel and comparative steel shown in Table 1 were melted and cast into steel ingots. These steel ingots were decomposed and rolled into 120 mm diameter steel bars to obtain steel pipe base materials made of steel bars. In Table 1, No. of the steel for invention is shown. 2 and no. The contents of Ni and Mo of 3 are contained as inevitable impurities during factory production, and are less than the respective component ranges in the present invention.

The steel pipe base material obtained by cracking and rolling as described above was punched into a steel pipe according to the manufacturing conditions of the piercing and rolling heating temperature, the temperature before reheating, the cooling rate, and the heating temperature before finishing rolling shown in Table 2. In this case, the steels for invention and comparative steels in Table 2 and their No. No. of each of the steels for invention and comparative steels in Table 1. It corresponds to the steel of the chemical composition. More specifically, the φ120 mm steel bar, which is the steel pipe base material, is subjected to hot piercing and rolling at the respective piercing and rolling heating temperatures, which are the production conditions shown in Table 2, and then from this piercing and heating temperature, Ar ₁ Cooling to the following temperature at each cooling rate to each pre-heating temperature, then reheating from each of these pre-heating temperatures to each of the pre-rolling reheating temperatures, finish rolling, and having a thickness of 72 mm The steel pipe was 18 mm. Furthermore, these steel pipes were formed into cold rolled steel pipes having a diameter of 45 mm and a thickness of 12.5 mm by cold rolling.

  Annealed hardness measurement test piece, Ono-type rotary bending fatigue test piece, and thrust type rolling fatigue test piece were produced from the steel pipe thus manufactured by machining.

  Annealing hardness measurement specimens were annealed at a maximum temperature of 750 ° C. for a total of 10 hours, and then the central part in the specimen thickness direction was cut out and the annealing hardness was measured by HRB. The hardness after annealing is an index for evaluating the cold workability.

The Ono-type rotating bending fatigue test pieces are notched test pieces having a parallel portion of φ8 mm and a shape factor α = 1.93. Carburizing and tempering these test pieces under the carburizing conditions shown in the heat treatment pattern of FIG. After that, the notched portion was subjected to the test while being heat-treated. The fatigue limit was the fatigue strength at 10 ⁷ cycles.

  Thrust-type rolling fatigue test pieces were cut into 9 mm width steel pipes, carburized and tempered under the carburizing conditions shown in the heat treatment pattern of FIG. It was subjected to a thrust type rolling fatigue test in the environment. As a foreign matter, 1 g of powder high speed of 100 to 150 μm per liter of lubricating oil was added. The maximum contact surface pressure in the thrust type rolling fatigue test was 5.3 GPa.

Table 3 shows the test results of the present invention and the results of the comparative example. No. of the steel for invention of the invention example which manufactured the steel which consists of a component of steel for this invention on the invention conditions. 1-No. In the case of 5, the annealing hardness was 85HRB or less, and it was judged that the cold workability was excellent. Furthermore, the fatigue strength of 10 ⁷ cycles of rotational bending is 450 MPa or more, the rolling fatigue life L ₁₀ in a foreign matter-contaminated environment is 0.30 × 10 ⁶ cycles or more, and these steels are steels for subject parts of the present invention. Both the bending fatigue strength and the rolling fatigue life required as are good.

  On the other hand, as shown in the comparative example, when the chemical composition of the comparative steel deviates from the composition of the steel of the present invention or the composition of the steel of the present invention, and the manufacturing condition of the steel pipe deviates from the range of the inventive condition, In one or more items of annealing hardness, rotational bending fatigue strength, or thrust type rolling fatigue life in a foreign matter mixed environment, as shown in Table 3 with hatching, it is inferior to a certain value or less. It is the result.

It is a figure which shows the pattern of carburizing and heat processing.

Claims (3)

In mass%, C: 0.10 to 0.35%, Si: 0.35 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% or less, S: 0.030 %: Cr: 1.50 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.01 to 0.10%, N: 0.003 to 0.025%, and the balance Piercing and rolling a steel material comprising Fe and inevitable impurities at a heating temperature of 1240 ° C. or higher to form a hot rolled steel pipe, and 0.05 ° C./s or higher to Ar ₁ point or lower before finish rolling the hot rolled steel pipe After cooling at a speed of, it is reheated to 900 to 1080 ° C. and finish-rolled, and it is seamless for high strength carburized parts with excellent bending fatigue strength and rolling fatigue life in an environment containing foreign matter Steel pipe manufacturing method. In mass%, C: 0.10 to 0.35%, Si: 0.35 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% or less, S: 0.030 % Or less, Cr: 1.50 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.01 to 0.10%, N: 0.003 to 0.025%, and , B: 0.0003 to 0.0050%, Ti: 0.01 to 0.05%, the balance being hot rolled steel pipe by piercing and rolling a steel material composed of Fe and inevitable impurities at a heating temperature of 1240 ° C or higher The hot-rolled steel pipe is cooled at a rate of 0.05 ° C./s or higher to the Ar ₁ point or lower before finish rolling, and then heated again to 900 to 1080 ° C. to perform finish rolling. High-strength carburized parts with excellent bending fatigue strength and rolling fatigue life in the presence of foreign matter Method of manufacturing a seamless steel pipe. In addition to the component of Claim 1 or Claim 2, it contains one or two of two elements of Ni: 0.10 to 2.00% and Mo: 0.05 to 0.50% by mass%. The balance of the steel material comprising Fe and inevitable impurities is pierced and rolled at a heating temperature of 1240 ° C. or more to form a hot rolled steel pipe, and this hot rolled steel pipe is 0.05 ° C./s below Ar ₁ point before finish rolling. After cooling at the above speed, re-heating to 900 to 1080 ° C. and finish rolling is performed, and the joint for high-strength carburized parts excellent in bending fatigue strength and rolling fatigue life in a foreign matter mixed environment No steel pipe manufacturing method.

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