JPH0140901B1 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for manufacturing structural steel having excellent toughness without heat refining, that is, without performing heat refining treatment such as quenching and tempering while hot working.

[Conventional technology]

Generally, products are manufactured from structural steel by hot working, tempering, and cutting, but if heat refining can be omitted, costs can be reduced and , it can also meet the demand for energy conservation. Therefore, research on such steels has been actively attempted, and in particular, it has been found that these carbides and nitrides precipitate in the ferrite + pearlite structure during the cooling process after hot working of steels to which V and Nb have been added. Steels that are strengthened using the When rolling is performed as hot working, the steel material is strengthened as it is, or is subjected to cold or warm plastic working and then cutting to form the final product. When manufacturing, the rolled steel material is die-forged and the hardened rough material is cut into products. However, the product obtained in this way has a disadvantage in that its toughness is lower than that of products that undergo ordinary heat treatment.

[Problems to be solved by the invention]

An object of the present invention is to overcome this drawback and provide a method for producing non-tempered structural steel that is comparable in color to tempered steel. Composition of the invention

[Means for solving the problems] The method for manufacturing non-tempered structural steel of the present invention includes C:
0.30~0.60%, Si: 0.10~2.0%, Mn: 0.20~2.5
%, A: 0.005-0.10% and N: 0.005-0.030
%, and one or both of V: 0.5% or less and Nb: 0.5% or less, with the remainder essentially consisting of Fe, according to the following limits depending on the N and Nb content. It is characterized by performing hot working at the following heating and finishing temperatures.

【table】

[Effect]

The role of each component in the alloy composition adopted in the present invention and the reason for its limitation are as follows. C: 0.30 to 0.60%, the presence of 0.30% or more is essential to ensure the necessary strength. The upper limit was determined from the viewpoint of toughness. Si: 0.10-2.0%, preferably 0.15-1.0% Si is an element necessary as a deoxidizer, and 0.10%
However, non-tempered structural steel is usually manufactured into a product by machining after hot working, and it is desired that the material has good machinability. Therefore, Si should be kept at 2.0% or less, preferably 1.0% or less. Mn: 0.20 to 2.5%, preferably 0.5 to 1.8% Needless to say, Mn also acts as a deoxidizing agent, which is important in obtaining high toughness in non-tempered steel. In the present invention, a relatively high content, typically around 1.5%, is used. However, from the viewpoint of machinability, it is best to avoid adding too much. A: 0.005 to 0.10%, preferably 0.01 to 0.05% It also has a deoxidizing effect, but its main role is to refine the crystal grains. Since A also has a negative effect on machinability, it is limited to 0.10% or less. N: 0.005 to 0.30%, preferably 0.23% One of the effects of N is grain refinement, and the other is precipitation strengthening due to the formation of nitrides with V or Nb. Too much of this also impairs toughness. V: 0.5% or less, Nb: 0.5% or less (total 0.5% or less when used together) both form nitrides and contribute to precipitation strengthening and crystal grain refinement. However, Nb has a large influence on the processing conditions of steel materials, and the higher the content, the wider the upper limit of the appropriate processing temperature. The upper limit was set for the reason of deterioration in hot workability, but since both V and Nb are expensive materials, the amount added should be determined in harmony with the effect of addition and economical efficiency. In carrying out the present invention, the contents of Nb and N are determined by taking into consideration the allowable cost of the intended non-tempered structural steel and the practical requirements of the field work. In particular, Nb is important because it is a highly effective material to add and also has a large impact on cost. Here, the practical requirement for on-site work is the requirement to minimize the waiting time associated with the use of soaking furnaces in factories that handle various steel materials with different processing conditions. The time to be kept in the soaking furnace varies depending on the processing conditions of each type of steel.
By judiciously combining them, latency is minimized. When using the non-tempered structural steel of the present invention as one of various steel materials, a more advantageous combination can be achieved by selecting the Nb and N contents and changing the upper limit of the heating temperature.
The above requirements can be met. Regardless of whether the steel is heat-refined or non-heat-refined, it is unavoidable that anisotropy in mechanical properties exists to varying degrees. In the steel of the present invention, especially when it is desired to lower the directionality of toughness, it is recommended to contain S: 0.40% or less and Te: 0.10% or less at a ratio of Te/S: 0.04 or more. S is, of course, an element that helps improve machinability, but here, rather, by combining appropriate amounts of S and Te,
Controls the morphology of sulfide inclusions in steel,
This has the effect of reducing toughness anisotropy. The upper limits of S and Te were determined mainly based on hot workability. In addition, the following elements may be added within specific ranges to the steel of the present invention, depending on the desired properties and uses, with the expectation of various effects.・ When you want to further refine the grain size...Ti:
0.5% or less, Ta: 0.5% or less, or Zr: 0.5%
Below - When you want to improve weather resistance...Cu: 2.0% or less - When you want further improvement in machinability...Pb: 0.4%
Below, Bi: 0.4% or less, Se: 0.4% or less, Ca:
0.01% or less・When you want to improve fatigue strength...0: 0.0030% or less・When you want to increase strength...Cr: 5% or less,
Ni: 5% or less, Mo: 3% or less, as mentioned above, the major feature of the steel of the present invention is that it has a fine grain structure with prior austenite crystal grains having an average grain size number of 5 or more. .
Needless to say, "prior austenite" grains are ferrite grains with a ferrite-to-pearlite structure that are generated by cooling to room temperature after hot working.
It exists around the existing austenite grain boundaries at high temperatures and indicates the size of the previous grains. This measurement is based on JIS
Perform according to the slow cooling method specified in GO551. Considering that the structure of conventional non-tempered steel generally has a grain size number of 2 to 3, the structure selected in the present invention is as follows:
It is extremely unique. In order to obtain structural steel with high toughness as hot-worked, the processing conditions when hot-working and cooling the steel alloy with the above composition must be selected so that a predetermined grain size number is achieved. It won't happen. N and Nb
The present invention determines the upper limits of the heating temperature and finishing temperature for hot working that give good results depending on the content of , and the values are as described above. The actual operating conditions are within the above limits, and the higher the temperature, the lower the resistance to plastic working, but relatively low temperatures are preferred as far as the toughness of the resulting steel is concerned.
Again, the balance between the two must be selected depending on each case.

【Example】

Steel having the alloy composition shown in Table 1 was melted and cast in a 2 ton arc furnace. (No. ^* is comparison steel)

[Table] An ingot was rolled to make a 100 mm square billet, and the heating temperature and finishing temperature were set to the conditions shown below for forging, resulting in a material with a cross section of 20 mm x 60 mm.
Comparison materials were further forged at 850 degrees, oil cooled at -600 degrees Celsius,
Quenching and tempering were performed under air cooling conditions.

[Table] Table 2 shows the results of various tests performed on the materials obtained as described above. In the table, the numbers marked with * are comparative examples, and the numbers without marks are examples of the present invention. "-" means not measured. The Charpy impact value was measured at room temperature using a JIS No. 3 test piece. From the data in Table 2, it can be seen that the combination of alloy composition and processing conditions according to the present invention results in a prior austenite grain size of a predetermined size, and that the steel material satisfying the conditions has a hardness, toughness, and toughness direction. It was found that the steel had a value comparable to that of conventional products that had undergone heat refining, and furthermore, it was confirmed that the directionality of toughness was significantly improved when appropriate amounts of S and Te were contained.

[Table] Effects of the Invention According to the present invention, steel having high strength and toughness can be obtained as it is hot-worked from structural steel widely used in manufacturing industrial machinery and automobiles.
Therefore, it is now possible to omit the heat treatment process when manufacturing various mechanical parts that previously required heat treatment such as quenching and tempering, such as crankshafts, connecting rods, axle shafts, spindles, and steering racks. This will greatly increase productivity. This benefit is even more pronounced for mass-produced products.

Claims (1)

[Claims] 1 C: 0.30-0.60%, Si: 0.10-2.0%, Mn:
0.20-2.5%, A: 0.005-0.10% and N:
0.005-0.030%, and V: 0.5% or less and
Nb: A steel alloy containing 0.5% or less of one or two types, with the remainder substantially consisting of Fe, is heated at a heating temperature and finishing temperature below the following limits, depending on the content of N and Nb. A method for manufacturing non-temperature structural steel, which is characterized by performing temporary processing. 【table】