JPH0112815B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a manufacturing method for producing low alloy steel with excellent cold workability as hot-rolled without performing heat treatment after hot-rolling. Alloy steels for machine structures, such as chrome steel and chrome molybdenum steel, are generally formed by hot rolling, then hot forging, and then finished by machine cutting. It was quenched, carburized and quenched, or formed by cold processing such as hot rolling and cold forging, and then finished by machine cutting, quenched and tempered, and then surface hardened and tempered. . In the latter cold working method, the hot rolled steel material is annealed to ensure cold workability. This invention targets low alloy steel for machine structures, which is subjected to quenching and tempering treatment or surface hardening treatment after processing and is used extensively as bolts, shafts, gears, and other machine parts, and in the production thereof, This paper proposes a manufacturing method for producing low-alloy steel with fine ferrite/pearlite structure as hot-rolled without annealing and with excellent cold workability. It has cold workability equivalent to or better than that obtained by annealing, and by omitting annealing before cold working, the manufacturing process can be simplified and productivity can be improved. . That is, carbon 0.13-0.48%, silicon 0.33% or less, manganese 0.30-1.80%, sulfur 0.025% or less,
Steel containing 0.025% or less of phosphorus, 0.10% or less of acid-soluble aluminum, or 1.50% of chromium in addition to the above composition
Those containing the following, and those containing both chromium 1.50% or less and molybdenum 0.45% or less, and vanadium 0.10% or less and niobium 0.05 in the above composition.
% or less is heated to a temperature above the Ac ₃ transformation point, and the steel containing 1 or 2 types of % or less is heated to a temperature above the Ac 3 transformation point and above the Ar ₃ transformation point - Ar ₃ transformation point +
The gist is to carry out finish rolling with a total area reduction of 40% or more in a temperature range of 200°C or less, and then slowly cool it at a cooling rate of 0.05 to 0.29°C/sec. According to this invention, the austenite crystal grain size immediately after hot rolling becomes fine grain size 6 or more, and after Ar ₁ transformation, a fine and uniform structure of ferrite and pearlite is obtained. The reason for limiting the chemical composition of the target steel in this invention is as follows. Since this invention is mainly intended for case-hardening steel and strong steel, the basic components carbon, silicon, manganese, chromium, and molybdenum are based on the applicable components of the Japanese Industrial Standards, with carbon 0.13 to 0.48% and manganese 0.30 to 1.80%. %,
Chromium was 1.50% or less and molybdenum was 0.45% or less. Phosphorus and sulfur are harmful to cold workability, and in order to obtain sufficient cold workability, it is necessary to suppress both phosphorus and sulfur to 0.025% or less. Acid-soluble aluminum is effective in refining crystal grains and adjusting the texture, but if it exceeds 0.10%, toughness will be impaired, so it was set to 0.10% or less. Since vanadium and niobium have the effect of refining crystal grains, they are effective in inducing ferrite transformation and suppressing bainite structure by refining austenite grains during the rolling process.
Particularly suitable for chromium-molybdenum steel with high hardenability. In this case, vanadium is
Even if the amount of niobium is increased beyond 0.10% and niobium is increased beyond 0.05%, no improvement in the content effect is observed, so vanadium was limited to 0.10% or less and niobium was limited to 0.05% or less. Note that when containing either vanadium or niobium, which has a grain refining effect, addition of aluminum, which has a similar grain refining effect, can be omitted. The hot rolling conditions were determined for the following reasons. _The heating temperature should be set at or above the Ac ₃ transformation point, as the structure before rolling becomes two-phase austenite and ferrite, and if rolled in this two-phase structure, a uniform ferrite/pearlite structure cannot be obtained after rolling. shall be. Regulating the rolling temperature in finish rolling is necessary to adjust the austenite grain size to a finer grain size of No. 6 or higher, but if the temperature is lower than the Ar ₃ transformation point, the two austenite and ferrite
This is undesirable because processing occurs in a phase region, and a uniform fine ferrite/pearlite structure cannot be obtained after rolling, resulting in a partially axial ferrite/bainite structure. If the temperature is too high, exceeding the Ar ₃ transformation point +200°C, the crystal grains will become coarse and grain refinement cannot be expected, so the temperature range is from the Ar ₃ transformation point to the Ar ₃ transformation point +200°C. The reason why we set the total area reduction rate in finish rolling to 40% or more is because if the area reduction rate is less than 40%, there will be less lattice defects and strain energy accumulation caused by hot working, and fine austenite grains before transformation cannot be obtained. It's for a reason. If the cooling rate after hot rolling is less than 0.05℃/sec, little improvement in the structure will be observed and productivity will decrease, and if it exceeds 0.29℃/sec, a bainite structure will occur and cold workability will deteriorate. ,0.05~0.29
℃/sec. Note that the slow cooling at the above cooling rate is preferably slow cooling with a cover on a cooling bed or slow cooling using a heat retention furnace. In order to suppress scale formation during slow cooling as much as possible, it is effective to carry out the cooling in an inert gas atmosphere such as argon gas or nitrogen gas. Next, embodiments of the invention will be described. JISG4105SCM system with chemical components shown in Table 1,
JISG4104SCr high manganese steel is melted in a 70t converter,
The steel ingot was bloomed into a 180 mm square and hot rolled under the rolling conditions shown in Table 2 to produce a 30 mm diameter steel bar. For comparison, a sample hot-rolled by a conventional method is shown as a comparative example. After hot rolling, a heat insulating cover was provided on the cooling bed, and the sheet was slowly cooled while being conveyed at a speed of 0.5 to 0.7 m/min.

【table】

[Table] Samples were taken from each of the above steels and tested for mechanical properties and cold workability. The results are shown in Table 4. The comparative examples include a steel bar as hot rolled (comparative example) and a steel bar subjected to softening annealing treatment (heating temperature 780°C, holding time 3 hours, cooling in the furnace) after hot rolling (comparative example). ) was tested. In addition, an extrusion cracking test for determining cold workability was carried out under the conditions shown in Table 3 by repeating extrusion until internal cracking occurred.

【table】

[Table] From the above results, the steel bar produced by implementing the present invention has cold workability equal to or better than that of the steel bar subjected to softening annealing treatment by the conventional method, and the steel bar produced by implementing the present invention has cold workability equal to or better than that of the steel bar subjected to softening annealing treatment by the conventional method. It can be seen that the properties are significantly improved compared to the steel bar of the comparative example.

Claims (1)

[Claims] 1. Carbon 0.13-0.48%, silicon 0.33% or less, manganese 0.30-1.80%, acid-soluble aluminum 0.10%
Hereinafter, a steel consisting of the balance iron and unavoidable impurities, with 0.025% or less of sulfur and 0.025% or less of phosphorus in the impurities, is heated to a temperature above the Ac ₃ transformation point and hot rolled, and then heated to a temperature above the Ac ₃ transformation point to Ar _3. After finish rolling with a total area reduction of 40% or more in the temperature range below the transformation point + 200℃, slow cooling is performed at a cooling rate of 0.05 to 0.29℃/sec. Method of manufacturing alloy steel. 2 Consisting of 0.13 to 0.48% carbon, 0.33% or less silicon, 0.30 to 1.80% manganese, 1.50% chromium or less, 0.10% or less acid-soluble aluminum, the balance being iron and unavoidable impurities, sulfur in the impurities being 0.025% or less, phosphorus is 0.025% or less, heated to above Ac ₃ transformation point and _hot rolled _,
After finish rolling with a total area reduction of 40% or more in the temperature range below the transformation point +200℃, the cooling rate is 0.05 to 0.29.
A method for producing low-alloy steel with excellent cold workability, which is characterized by slow cooling at a rate of °C/second. 3 Consists of 0.13 to 0.48% carbon, 0.33% or less silicon, 0.30 to 1.80% manganese, 1.50% chromium or less, 0.45% or less molybdenum, the balance being iron and unavoidable impurities, including sulfur 0.025% or less, phosphorus
Steel with a content of 0.025% or less is heated to a temperature above the Ac ₃ transformation point and then hot-rolled to form a material with an Ar ₃ transformation point or above - Ar ₃ transformation point +200.
Manufacture of low alloy steel with excellent cold workability, characterized by finishing rolling with a total area reduction of 40% or more in a temperature range below ℃, followed by slow cooling at a cooling rate of 0.05 to 0.29℃/sec. Law. 4 Carbon 0.13-0.48%, silicon 0.33% or less, manganese 0.30-1.80%, chromium 1.50% or less, molybdenum 0.45% or less, one or two of vanadium 0.10% or less and niobium 0.05% or less, acid-soluble aluminum 0.10 % or less, the balance is iron and unavoidable impurities, and the impurities include sulfur of 0.025% or less and phosphorus of 0.025% or less. The steel is heated to a temperature above the Ac ₃ transformation point and then hot-rolled to form a steel with an Ar ₃ transformation point or above. Excellent cold workability characterized by finishing rolling with a total area reduction of 40% or more in the temperature range below Ar ₃ transformation point + 200℃, followed by slow cooling at a cooling rate of 0.05 to 0.29℃/sec. Manufacturing method of low alloy steel.