Effects of chemical composition and hot working condition on the toughness of microalloyed steels for machine structural use were investigated. The results obtained were as follows.
(1) Decrease of C content, increase of Mn and Cr content, refining of prior austenite grain size and increase of cooling rate improve the toughness of microalloyed steel.
(2) Improved toughness by grain refining is mainly due to decrease of cleavage facet size. It is assumed that decrease of it lowered the brittle ductile transition temperature.
(3) Improved toughness by increase of Mn and Cr content is mainly due to decrease of cementite lath thickness. It is assumed that decrease of it is owing to “pearlite dilution”.

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Alloy-designing of a nitrocarburizing steel for crank shafts of automobile engines was studied in order to increase the fatigue strength.
This crank shaft has been made of a free-machining carbon steel, and it was required to increase the fatigue strength by 20%.
It has been known that fatigue strength of nitro-carburized steels was controlled by mainly surface hardness and case depth, so contents of Cr and Al were optimized in view points those two factors. Through the laboratory experiments, optimum compositions were determined as follows;
0.2%C-0.7%Si-1%Mn-1%Cr-0.15%Al-0.08%S-0.1%S-0.1%Pb.
This new steel was produced in an industrial quantity and practical properties such as hot-forgeability, machinability, fatigue strength of a crank shaft after nitrocarburizing and so on were investigated in an automobile producer's shop. As a result, the new steel was found to satisfy the above requirements.

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Influences of the microstructures on the fatigue properties of the medium carbon microalloyed steels were investigated under the rotating bending fatigue tests. Results obtained are as follows: (1) Under fatigue conditions surface relief made of slip bands are found to be generated largely on ferrite in the medium carbon ferrite pearlite steels. The fatigue crack initiates along the surface relief and propagates along the ferrite/pearlite boundary in the early stage of the fatigue crack propagation. (2) Increase in vanadium contents raises the slip band initiation stress and the fatigue strength through strengthening ferrite. Increase in carbon or manganese contents raises the fatigue strength through increasing pearlite volume fraction and refining ferrite grain size. Increase in vanadium contents are more effective to improve the fatigue limit ratio. (3) Coarsening of prior austenite grain size reduces the fatigue limit. This is because the ferrite precipitated along austenite grain boundary are regarded as small fatigue cracks in medium carbon ferrite pearlite steels with fine prior austenite grain size.

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Comparing the rolling contact fatigue strength of SCM22YM and S48CYM which are calcium bearing free-machining steels in which a few quantity of both Pb and S are added, with that of the base steels of SCM22, S48C and leaded free-machining steels of SCM22F and S48CF, the following results were obtained.
1. The said strength of YM free-machining steels of SCM22YM and S48CYM cannot compete with that of the base steels, but they are better as many times as 3 to 5 than leaded free-machining steel of SCM22F and 10 to 30 than S48CF respectively.
2. The said strength of SCM22YM and S48CYM is found better if the Pb content is smaller. Also, in the case of SCM22YM if the S is composite-added as much as 0.012 to 0.080% it contributes more or less to the improvement of the said strength.

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The austenite recrystallization behaviors during the hot-deformation and the characteristics of the ferrite-pearlite structures transformed during continuous cooling from the hot-deformed austenite were investigated for the medium carbon steels containing 0.4% carbon with 0 to 0.3% vanadium. The results are as follows: (1)The vanadium additions inhibit the recrystallization of hot-deformed austenite and elevate the austenite recrystallization temperatures in the medium carbon steels, more effectively than those in the low carbon steels. The austenite recrystallization-stop temperature of 0.4% carbon steel added 0.3% vanadium is over 200K higher than that of 0.4% carbon steel. This is considered to be due to the increase of the strain induced precipitation of VC in deformed austenite, through the increase of the super saturation of VC with the increase in the carbon content. (2)The ferrite-pearlite structures of 0.4%C-0.3%V steels hot-deformed at non-recrystallization temperature (below 1223K) are refined remarkably, through the increase of the ferrite nucleation site consisted of the flattend austenite grain boundary and the annealing twin formed in the austenite grain. (3)0.4%C-0.3%V steels hot-rolled at non-recrystallization temperature on the practical mill have superior toughness in spite of high tensile strength over 900N/mm2 because of the microstructure refinement through the non-recrystallization austenite deformation.

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Li-Mn-Zn-ferrite (Li0.33 Mn0.42 Zn0.20 Fe2.05 O4+δ) was heat-treated at various temperatures between 450 and 1200°C, and the changes of cation valence, crystal lattice constant and oxygen content in the ferrite by this heat-treatment were measured. The oxygen and Mn3+ ion contents in the ferrite decreased with increasing temperature, while the crystal lattice constant increased. The stoichiometric ferrite (δ=0) was obtained by heat-treating at about 900°C in oxygen and air, and at about 700°C in nitrogen.
The relationship between the crystal lattice constant a0 and an oxygen content deviation from the stoichiometry δ, was found to be as follows:
a0=8.371-0.37δ forδ>0
a0=8.371-0.37δ forδ>0.

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The strength of microalloyed medium carbon steels is significantly affected by chemical compositions and the conditions in hot forging. The effect of chemical compositions on the hardness of medium carbon steels is first investigated and is introduced as “Hardness Equivalent Parameter”, Heq. by the authors. According to this parameter, it becomes possible to design the steel for automobile components with various strengths. The effect of hot working conditions on the hardness of the microalloyed steels is examined and the proper processing conditions for them has been obtained.
After research programs have been conducted, the running test of engine and suspension components built-in a passenger car are carried out, and the authors have certified that the microalloyed steels possess satisfactory properties as forged.

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S48Cをベースとして,これにCa, PbおよびSの快削成分を2種または3種複合添加した快削鋼について,この鋼を一体鍛造クランクシャフトに使用する場合に,要求される各種の強度特性が基本鋼に対して,どの程度の水準にあるかを試験した。
また,複合快削鋼製クランクシャフトの曲げ疲労試験を行ない,丸棒試験片による結果と比較した。
その結果Ca, Pb, Sを複合添加した快削鋼の疲労強度は基本鋼に劣るが,その差は大きくなく設計技術,製造技術によって充分にカバーしうることが明らかとなった。
Ca-Pb-S鋼は,すぐれた被削性を有しており強度が多少基本鋼より劣る点をカバーできればクランクシャフトの用材として実用化しても何ら問題はない。

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Relation between fatigue strength and static strength, and the relation between fatigue crack initiation and microstructure in bainitic microalloyed steels were investigated, using various bainitic steels changed in heat treatment conditions. Results obtained are as follows : (1) Lowering austenitizing temperature raises the tensile strength (σ_B), 0.2% yield strength (σ_y) and fatigue limit (σ_w) of the air-cooled bainitic steels by reason of the refinement of packet size. An increase in cooling rate raises σ_B, σ_y and σ_w of the air-cooled bainitic steels by reason of lowering bainite transformation temperature. (2) The fatigue limit ratios (σ_w /σ_B) of the air-cooled bainitic steels are almost the same as those of the tempered martensitic steels and the tempered bainitic steels except the bainitic steels austenitized at a higher temperature (1603K). (3) Under fatigue conditions, linear surface reliefs made of slip bands are found to be generated mainly along bainitic ferrite lath in the air-cooled bainitic steels, and a fatigue crack initiates along a linear surface relief. (4) Coarsening of prior austenite grain size over about 100μm reduces the fatigue limit of the air-cooled bainitic steels. This is because the longer bainitic ferrite laths in coarser prior austenite grains are regarded as small fatigue cracks in the bainitic steels with fine prior austenite grains.

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The crankshafts of a motor vehicle is one of these parts which require such properties of the highest levels with respect to materials used as the machinability including the forgeability, the strength and so on.
In the case of crankshafts, it is essential to produce crankshafts that are reliable as to the quality characteristics at the lowest possible cost. However, the free cutting steels containing 0.2% lead has problems such that as its amount used increases and when billets are heated by induction heating in the process of hot forging, the lead contained in the steel evaporates as fume to damage the heating equipment.
To solve the problem, we conducted researches on the technical art of materials which make it possible to reduce the lead content to such an extent as necessary for solving the problem and to offset the resultant lowering in the machinability without deteriorating the strength properties by changing the quantities of various elements, such as carbon, sulfur and so forth, so as to ensure a good and steady machinability.

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The essential policy for the product development is to provide all customers the highest satisfaction.
In order to fulfil this goal, materials have been actively involved in technological development in many areas.
Selecting automobile materials has shown enormous changes based upon both technological direction to develop new models and technological needs for the materials to meet any requirements of the development.
Materials for automobile applications have diversified considerably in consequence of the development and utilization of various types of new materials to meet major technical objectives, such as exhaust gas emission control, substantial improvement of fuel economy and weight reduction.
As a result of these efforts, material technologies have made a great contribution to the advancement of the technolgies in the auto industry.
Tehnological trends and design concept in developing new models are the key factors in determing the direction of automobile materials.
Cost/performance balance of the final products is the crucial key to selecting the most suitable materials from a number of available materials.
In addition, production of cars in the U.S. has been rapidly expanding in these days, which has resulted in the strong requirement for taking careful consideration of the availability of any materials for models assembled there.
Taking these circumstances into consideration, trends of materials, as well as wants and needs for the future development of cars, will be explained.

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