JP4145764B2 - Method for producing boron-added case-hardened steel pipe excellent in cold workability and grain size characteristics - Google Patents

Description

  The present invention relates to 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 cold rolling or cold drawing using a seamless steel pipe. It relates to steel pipes.

For high-strength, high-toughness case-hardened steel used in applications such as cages for constant velocity joints in automobiles, by optimizing the components and impurities, surface abnormal layers generated during carburizing are reduced and fatigue strength is improved. Yes. Furthermore, the high cleanliness operation is combined with the reduction of inclusions to extend the service life. Furthermore, the applicant has developed a high cleanliness steel containing 0.15% Ti, which produces a steel ingot by continuous casting, a rolling process to a steel slab, and a subsequent rolling process and manufacturing to a steel bar. In the forging step, after heating to 1250 ° C. to 1400 ° C., one end is cooled to room temperature, then reheated to Ac _{3 to} 1050 ° C. and rolled to the desired size to obtain a bar steel material (see Patent Document 1) ). By the way, this process is the manufacture of steel bars and parts and does not match the manufacturing method of steel pipes. Since the manufacture of steel pipes is a special manufacturing process such as a piercing and rolling process, the workability deteriorates in a normal steel material process, and it is necessary to add a heat treatment after rolling.

Furthermore, for example, a general seamless that is heated to 1200 ° C., pierced and rolled, then roughly rolled, and re-heated from 900 ° C. to 1000 ° C. and finish-rolled before the temperature is sufficiently lowered to Ac ₁ point or less. In the steel pipe manufacturing method, the hardness after finish rolling is high, and re-heat treatment is required after rolling, resulting in an increase in cost. Further, decarburization also proceeds due to reheat treatment, and there is a problem that the quality is deteriorated and the grain size characteristics are not good.

  On the other hand, after hot forming a steel component containing 0.01 to 0.3% Ti into a pipe, cold working with a cross-sectional area reduction rate of 50% or less and annealing at 650 to 950 ° C. There exists a method (refer patent document 2) which manufactures the steel pipe for case hardening. However, in this method, no technical consideration is given to the characteristic treatment process after hot rolling and before cold working.

JP-A-11-92824 JP-A-9-53250

  The problem to be solved by the present invention is to provide a method for producing a seamless steel pipe excellent in crystal grain size characteristics and excellent in cold workability at a low cost without impairing the quality of surface defects and decarburization. .

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: 0.05 to 0.35%, Mn: 0 20 to 1.00% (preferably 0.20 to 0.60%), Cr: 0.80 to 2.00% (preferably 0.80 to 1.50%), B: 0.0005 to 0 .0050%, N: 0.010% or less, Ti: 0.05 to 0.20% (desirably 0.10 to 0.20%), Nb: 0.02 to 0.20% , V: Hot steel pipe which has been pierced and rolled at a heating temperature of 1250 ° C. or more, containing one or more of three elements of 0.02 to 0.20%, the balance being Fe and inevitable impurities Was cooled to Ar ₁ point or less (preferably Ar ₁ point −100 ° C. or less) at a rate of 0.1 ° C./s or more before finish rolling. After a cold workability and a manufacturing method excellent hardening steel seamless steel pipe on the crystal grain size characteristics and performing a finish rolling, and then re-heated from the Ac ₃ point or more to 1050 ° C..

In the invention of claim 2, in mass%, C: 0.10 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.20 to 1.00% (desirably 0.20 to 0.60%), Cr: 0.80 to 2.00% (desirably 0.80 to 1.50%), B: 0.0005 to 0.0050%, N: 0.010% or less Furthermore, Ti: 0.05 to 0.20% (desirably 0.10 to 0.20%), Nb: 0.02 to 0.20%, V: 0.02 to 0.20%, three elements 1 type or 2 types or more among them, further containing 1 type or 2 types out of 2 elements of Ni: 0.05 to 1.00%, Mo: 0.03 to 0.50%, and the balance Fe and Ar ₁ point below the steel consisting of unavoidable impurities before finish rolling steel pipe between the piercing was performed heat heating temperature 1250 ° C. or more (preferably After cooling in Ar ₁ point -100 ° C. or less) to 0.1 ° C. / s or faster, and re-heated from the Ac ₃ point or more to 1050 ° C. cold and performing finish rolling processability and crystallinity This is a method for producing a case-hardened steel seamless steel pipe having excellent particle size characteristics.

  The reason for limiting the components of the steel material will be described. In addition,% is the mass%.

C: 0.10 to 0.35%
C is an element necessary for securing the core strength after carburizing treatment as a machine structural component. If the content is less than 0.10%, the effect cannot be sufficiently obtained. If the content exceeds 0.35%, the core is obtained. Reduce toughness. Therefore, C is set to 0.10 to 0.35%.

Si: 0.05 to 0.35%
Si is an element added as a deoxidizer, but if it is less than 0.05%, a sufficient deoxidation effect cannot be obtained, and if it exceeds 0.35%, the workability is lowered and the grain boundary during carburization is reduced. Promotes the formation of an oxide layer and further reduces fatigue properties. Therefore, Si is set to 0.05 to 0.35%.

Mn: 0.20 to 1.00% (desirably 0.20 to 0.60%)
Mn is an element necessary for ensuring hardenability, but if it is less than 0.20%, the effect cannot be sufficiently obtained, and if it exceeds 1.00%, workability is lowered. Therefore, Mn is 0.20 to 1.00%, preferably 0.20 to 0.60%.

Cr: 0.80 to 2.00% (preferably 0.80 to 1.50%)
Cr is an element that improves hardenability and carburization, but if it is less than 0.80%, its effect is not sufficient, and if it exceeds 2.00%, coarse carbides are formed in the carburized layer. Deteriorates mechanical properties and fatigue properties, and deteriorates workability. Therefore, Cr is 0.80 to 2.00%, preferably 0.80 to 1.50%.

B: 0.0005 to 0.0050%
B is an element that remarkably improves the hardenability of steel by adding a very small amount. If it is less than 0.0005%, the effect is not sufficient, and if it exceeds 0.005%, the hardenability is lowered. Therefore, B: 0.0005 to 0.0050%.

N: 0.010% or less When N exceeds 0.010%, TiN increases and adversely affects fatigue properties. Therefore, N is set to 1.0010% or less.

Ti: 0.05 to 0.20% (desirably 0.10 to 0.20%)
Ti fixes free-N in steel, improves the hardenability effect of B, and finely precipitates Ti carbide, composite carbide containing Ti, and Ti nitride, so that when carburizing instead of AlN This element is necessary to suppress the coarsening of the austenite crystal grain size. In particular, nano-order TiC finely dispersed in steel suppresses the growth of crystal grains. If Ti is less than 0.05%, the effect is not sufficient, and if it exceeds 0.2%, precipitates become excessive and workability is lowered. Therefore, Ti is set to 0.05 to 0.20%, preferably 0.10 to 0.20%.

Nb: 0.02 to 0.20%
Nb forms carbides or nitrides and has the effect of suppressing coarsening of the austenite crystal grain size, similar to Ti. In particular, nano-order NbC finely dispersed in steel suppresses the growth of crystal grains. If Nb is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.20%, the amount of precipitates becomes excessive and the workability is lowered. Therefore, Nb is made 0.02 to 0.20%.

V: 0.02 to 0.20%
V forms carbides and, like Ti, has the effect of suppressing the coarsening of the austenite grain size. In particular, nano-order VC finely dispersed in steel suppresses the growth of crystal grains. If V is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.20%, the amount of precipitates becomes excessive and workability is lowered. Therefore, V is 0.02 to 0.20%.

  Among the above three elements of Ti, Nb, and V, one or more of them are selectively contained as essential elements in the invention according to claim 1.

Ni: 0.05-1.00%
Ni is an element that improves hardenability and toughness, but if it is less than 0.05%, the effect cannot be obtained, and if it exceeds 1.00%, it becomes a bainite or martensite structure after rolling or forging, and the workability Significantly lowers the cost. Therefore, Ni is set to 0.05 to 1.00%.

Mo: 0.03-0.50%
Mo is an element that improves hardenability and toughness, but if it is less than 0.03%, its effect cannot be obtained, and if it exceeds 0.50%, it will be bainite or martensite after rolling or forging as with Ni. It becomes an organization and the workability is significantly reduced, leading to an increase in cost. Therefore, Mo is set to 0.03 to 0.50%.

  The above two elements of Ni and Mo are selectively contained as essential elements in the invention according to claim 2 selectively.

Furthermore, the reason for limitation in the manufacturing method will be described.
The reason for setting the heating temperature to 1250 ° C. or higher for piercing and rolling is to dissolve carbonitrides such as Ti, Nb, and V.

The reason for setting the cooling temperature before reheating to Ar ₁ point or less is to precipitate the carbonitrides such as Ti, Nb, and V dissolved in the above, and preferably the cooling temperature before reheating is set to Ar ₁ point. It shall be -100 degrees C or less.

  The reason why the cooling rate to the above cooling temperature is 0.1 ° C./s or more is to finely precipitate the carbonitrides such as Ti, Nb, and V dissolved in the above.

The reason why the reheating temperature before the finish rolling is set to Ac ₃ point or higher to 1050 ° C. is that by setting the Ac ₃ point or higher, the consistency between the precipitate already precipitated by re-austenite and the matrix is lost and softened. If the temperature exceeds 1050 ° C., the precipitate is re-dissolved, so the temperature is set to 1050 ° C. or lower.

In the invention of the present application, a hot steel pipe formed by piercing and rolling a case-hardening steel material containing boron is heated to a temperature of Ar ₁ point or lower, preferably Ar ₁ point to 100 ° C. or lower before finishing rolling, and 0.1 After cooling at a rate of ℃ / s or higher, reheating to Ac ₃ points to 1050 ° C and performing finish rolling, by subsequent cold working, for example, a constant velocity joint retainer, ring-shaped carburized parts, etc. That can produce car parts such as carburized steel seamless steel pipes with excellent grain size characteristics and excellent workability without deteriorating the quality of decarburization, etc. The invention has an excellent effect.

  The best mode for carrying out the present invention will be described through tables and examples.

  The steel components of the present invention shown in Table 1 are designated as invention Nos. Nos. 1 to 3 are steel compositions of comparative examples. 1-6. Steel of each component shown in Table 1 is melted in a 1 ton VIM (vacuum induction melting furnace), continuously cast, and the obtained slab is rolled into pieces to obtain a steel pipe base metal of φ80 mm steel bar.

The obtained steel bar is manufactured into a steel pipe by a steel pipe manufacturing process under the hot rolling conditions of the manufacturing conditions shown in Table 2. Inventive steels and comparative steels of the steel types in Table 2 and their No. No. in comparison with the invention in Table 1. The steel of the component is shown. That is, No. of invention steel in the comparative example of Table 2. Nos. 7 to 11 are Nos. It is a comparative example in which the manufacturing conditions of the steel of component 1 are different from the manufacturing conditions of the present invention. For example, no. No. 7 has a piercing and rolling temperature of 1210 ° C., which is lower than the piercing and rolling temperature of the present invention of 1250 ° C. or higher. No. _No. 8 has a temperature before reheating (cooling temperature before reheating) higher than Ar ₁ + 15 ° C., which is lower than the Ar ₁ point of the temperature before reheating of the present invention. No. No. 9 has a cooling rate of 0.01 ° C./s, which is slower than the cooling rate of 0.1 ° C./s or more of the present invention. No. No. 10 has a heating temperature before finish rolling of Ac ₃ −40 ° C. and a heating temperature before finish rolling of the present invention lower than Ac _{3 to} 1050 ° C. No. No. 11 has a heating temperature before finish rolling of 1090 ° C., and the heating temperature before finish rolling of the present invention is higher than Ac _{3 to} 1050 ° C.

  The above-mentioned steel pipe base metal φ80 mm steel bar is hot rolled under the manufacturing conditions shown in Table 2 on a specimen having a diameter of 55 mm and a thickness of 12 mm, and further subjected to cold rolling to obtain a specimen having a diameter of 32 mm and a thickness of 7 mm. Table 2 shows the hardness after hot rolling and the grain size characteristics of the cold rolled steel pipe, which are maintained at 925 to 1050 ° C. for 6 hours and then cooled with water and then cooled. No. of the example of the present invention. 1, no. 2, no. As shown in Table 2, the hardness of No. 3 is 78HRB, 83HRB, 79HRB in the same order, and it can be seen that these are the optimum processing hardness when cold-working various products. Further, in the same order, the coarsening temperatures are over 1050 ° C., 1025 ° C., and 1000 ° C., both of which are 1000 ° C. or higher. On the other hand, the comparative example 1 has a hardness of 84HRB which is appropriate for the comparative example 1, but the coarsening temperature is low, and the other comparative examples 2 to 6 have a cold workability of 95HRB or more.

The φ80 mm steel bar obtained by rolling to the steel pipe base material described above is heated to 1250 ° C. or higher under the production conditions shown in Table 2, pierced and rolled by a piercing mill, and then roughly rolled by an elongator and brought to a temperature before reheating. Cool at the cooling rate shown in Table 2, reheat to Ac ₃ or higher of the pre-finishing heating temperature, which is the most characteristic feature of the present invention, adjust the dimensions with a sinking mill, finish rolling, and adjust the roundness with a rotary sizer. And finish rolling. The seamless steel pipe obtained by rolling is further subjected to cold forging, cold rolling or cold drawing according to the use of each part to be produced.

Claims (2)

In mass%, C: 0.10 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.20 to 1.00%, Cr: 0.80 to 2.00%, B: 0.0005 to 0.0050%, N: 0.010% or less, Ti: 0.05 to 0.20%, Nb: 0.02 to 0.20%, V: 0.02 A hot steel pipe that has been subjected to piercing and rolling at a heating temperature of 1250 ° C. or more containing one or more of 0.20% of the three elements and the balance Fe and inevitable impurities is subjected to Ar ₁ before finish rolling. The skin excellent in cold workability and crystal grain size characteristics, characterized in that after cooling at a rate of 0.1 ° C./s or less below the point, it is reheated from Ac ₃ point to 1050 ° C. and finish rolling is performed. A method for producing baked steel seamless steel pipes. In mass%, C: 0.10 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.20 to 1.00%, Cr: 0.80 to 2.00%, B: 0.0005 to 0.0050%, N: 0.010% or less, Ti: 0.05 to 0.20%, Nb: 0.02 to 0.20%, V: 0.02 It contains one or more of 0.20% of the three elements, and further includes one of the two elements of Ni: 0.05 to 1.00% and Mo: 0.03 to 0.50% or A hot steel pipe containing two types, the balance Fe and unavoidable impurities and subjected to piercing and rolling at a heating temperature of 1250 ° C. or more, at a rate of 0.1 ° C./s or less to Ar ₁ point or less before finish rolling. Cold workability and grain size characteristics characterized by performing re-rolling by reheating from Ac ₃ points or higher to 1050 ° C. after cooling Of excellent case-hardened steel seamless steel pipe.