Ancient Japanese Steelmaking Processes

By Shinnosuke Yamamoto *

Methods of direct steelmaking were evolved in Japan several thousand years ago. The steel prod uced was used to make famous Japanese swords. Many archaeologists and scientis t s 1 ):'!):!} have carried out resear ch on these ancient iron and steelmaking processes. It cannot be said, h owever, that their origin and theoretical aspects have been thoroughly and fu lly investigated. At the present time, there is strong proof to support opinions that the origin can be traced back to about 2,000 years , ago. The iron and steel making process of that time is generall y call ed the "Tatara" furnace process, and the process was different from that used in the Asia n a rea. This furnace was r ectangu lar in Photo 1. Prototype of the "Tatara" furnace shape with a low shaft, and the furnace body was made of clay with heat-resistant properties. This furnace is s hown in Photo l. This pictures a type of steelmaking furnace, and there was another kind of furnace for making pig iron. This type of furnace is 850 mm X 2,100 mm in diameter and 1,150 mm hig h. Its des ign is shown in Fig. l. There are 18-20 tuyeres on both longitudinal s ides of the furnace, and these tuyeres come out of the bell ows on both sides. The fou ndation part of the furnace does not appear above the ground, but t his foundation is three times as high as the furnace itself so that it can keep complete heat-insulation • and prevention of moisture penetration. As a raw material, only iron sand was cheuged. This, how­ ever, was a type of magnetite containing titan ium. Its chemical composition is shown in Table l. As can be seen from this table, the impurities con­ tained in t his iron sand were very few. It con­ tained on ly sm a ll amounts of s ulphur, phosphorus, vanadium a nd titanium, but the amount of re­ mainder of these impurities in the finish ed blister Fig. 1. " Tatara" furnace • steel was extremely small. This is shown in Table 2. The life of the furnace, i.e., one cycle, was about During one cycle, the majority of the pig iron 60 hours, and during that time, it was norma l to flows out of a few holes bored in t he lower front obtain 2,900 kilograms of b lister steel and 2,250 pa r t of the furnace, but ordinarily t he rema inder kilograms of pig iron. In order to produce these stays within the "Ker a" lumps which are the blister products, 16,900 kilograms of iron sand and 16,000 steel. Consequently, the "Kera" is a mixture of kilograms of charcoal were cha r ged. The blistel· blister steel, pig iron, acid slag and a small a mount steel was called "Kera" and the pig iron, "Zuku". of charcoal. This "Kera" is s hown in Photo 2.

Table 1. Chemical analysis of a typical iron sand (%)

T. Fe Fe203 FeO S i02 CaO MgO Ab0 3 Ti02 MilO V20S P S .. Masa " iron sand 59.00 I ~ 24.72 8.40 2.24 1.54 2.34 1. 27 0.05 0.258 0.064 0.009 " A kome" iron sand 52 .07 52.71 19.55 14.50 2.68 0.94 4.98 5.32 0.34 0.369 0.095 0.026 I - Beach iron sand 59 .00 I~ 24.72 4.90 2.36 0.37 1.79 6.98 0.03 0.295 0. 121 0.032 River iron sand 62.55 64.84 22.13 2.24 0.50 1.10 4.51 5.23 Nil. I 0.243 I 0.061 0.014

* Head of the Technical Department and Managing Director, Hitachi Metal Industries, Ltd.

49 Tetsu-to-Hagane Overseas Vol. 1 No.2 September 1961

Table 2. Chemical analysis of blister steel (?o )

C S i Mn P S Ni Cr V Ti Cu As Sn

"Kera .. No. I 2.6·1 0.03 tr. 0.015 O.O I ~ nil. 0.05 tr. 0.012 0.003 0.002 tr.

"Kera .. No.2 1.44 0.21 0.02 0.005 0.003 nil. tr. tr. 0.014 0.002 0.001 tr.

.. Tamahagane " No. 1* 1.56 0.06 tr. 0.023 0.005 nil. tr. tr. 0.024 0.001 0.001 tr. * .. Tamahagane " means a selected bliste r steel.

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Photo 2. A raw block of the blister steel" Kera " Photo 3. Pearlite and cementite structure in the blister This piece is 3,000 mm x l,OOO mm and 300 mm thick. steel " Kera" (x 125) (5 6) This piece was taken out after the operation of the "Tatara" furnace had been completed and the re­ • maining furnace walls were tOl"ll down. The piece wa::; broken into pieces as large as 50-100 mm by a drop hammer pulled up by a water-wheel and then placed in classifications according to quality for different uses. The special feature of the "Ta­ tara" furnace was that blister steel of high quality could be obtained directly from iron ore, and it was composed of pearlite and cementite structure as shown in Photo 3. The structure of the small • amount of pig iron in the "Kera" is as s hown in Photo 4 and ledeburite is present.

" Kera" Refining Theory

The first stage or the first period of the opera­ Photo 4. Ledeburite structure in the blister steel " Kera " tion is the stage of regeneration, and it was begun (x 125) (5'6) by charging the furnace with a large amount of charcoal. Appropriate air blasting from the tuyere charged instead of the iron sand. This kind of group graduall y makes the furnace temperature iron sand containing titanium has such a low­ high€l", and when it gets to a designated tempera­ melting point that it melts easily. Soon after it ture, about 7.5 kilogra ms per unit of iron sand are melts, it is reduced to metallic iron having contact charged. The iron sand is charged in layer form with the white hot charcoal, and it changes into alternately with the charcoal, and the amount of molten pig iron by absorbing carbon. At the same charcoal is more than three times that of iron sand. time acid s lag containing iron oxide is formed. The charging work is carried out continuously for These molten materia ls collect in the furnace 6-8 hours in order to in crease the total charged bottom. Then heat is applied to the furnace bottom volume. The iron sand charged in this stage is accelerating the regeneration and the molten the kind which contains titanium and which is materials flow out of the holes in the lower front called "Akome". Sometimes beach iron sand is of the furnace.

50 Tetsu-to-Hagane Overseas Vol. 1 No.2 September 1961

The second stage is t he intermediate period; the proceeding very well , most of the phosphorus is t r ansition period between t h e above-mentioned carried away with t he melted pig iron. stage a nd the third stage, which is the initial step The "Kera" shaping process can be explained in shaping the "Kera". The total ch arged vo lume mo st easily in refere nce to t he well-known F e-C during this stage is more t han three times t hat in equ ilibrium diagram: when the iron sand is direct­ t he first stage. And in this case, t h e charged iron ly reduced by the white hot charcoal, t he iron sand containing less t itanium in it than in the easily absorbs carbon a nd becomes molten pig iron. first stage is cha rged. In this stage also, in the Tn the equilibrium diagram, Fig. 2, for example, if first half of the operation, practically all of the the molte n iron at .r point drops on top of the molten pig iron and s lag are di scha rged outs ide t h e "Kera" lump, it is cooled and finally arrives at 11 fu rnace. However, t he molten pig iron gradually point on the E-F line. During t his process, for coll ects in the furnace bottom, becomes cooled, and instance, "Ker a" with a car bon co ntent of about turns into half-melted lumps. Because of the 1.15';, is separated at t he? point on the D-F line equilib l' ium r atio, it r eleases carbon to some extent. and "Kel'a" with a carbon co ntent of about 1.43" ;, In the latter half of the process, practically a ll the is separated when the molten iron drops f urther pig iron remains at t he bottom of the furnace. in temperature and reaches II' point. From t hi s This fact is caused by the cooling eff ect of t h e phenomena, it is known why the composition of the "bear" made in the beginning a nd t he widening of "Kera" lump is not uniform. However, it is con­ the space between the t uyeres which face each sidered t hat unifo rmity can be attained by diffusion other. The reason for widening of this space is as of t he carbon in the high temperature f urnace. foll ows : along with t he rise of f urnace temper a­ Indil'ect reduction wit h CO gas does not seem to ture, the walls of t he fu rnace a r e eroded by t h e be so predominant in t hi s operation. Some iron acid slag which contains much iron oxide, so t hat sand seems to be red uced while passing through the furnace space widens about 50 mm on each s ide. the high temperature CO gas zone and absorbs Consequently t he points of t he tuyeres also melt carbon. However, there cannot be adequate absorp­ and become shorter. Therefore, it becomes diffi c ult tion of carbon because of the excessive a ir blast for the air blast from the tuyeres to reach t he volume. Because of this excessive a ll' blast center part of t he furnace, and the temperature of vo lume, the r eduction powel' of the gas is weak. this part drops. Shaping the half melted lump in Fudhel', t he ox id e content of the slag rises and the bottom of t he furnace plays t he most important t he dephosphorization process is added, as outlined rol e in the "Tatara" operation. above. Twelve hours after the beginning of the first operation, the process enters the third stage. The ( cr------,------,------,------,------, walls on both s ides together wit h the tuyeres have

become about 100 mm t hinner. The iron sand 1600 charged in this s tage must have less titanium in 1) I I it than t he iron s and charged in the second stage. 1.iquid +I • I The molten iron formed in the high t emperature 1.00 -+------+------+----- A

zone of the furnace drips down, comes in contact / I.i{luid I , with the above-mentioned cold ump, a nd becomes I r{' me nt i l £' a low-carbon solid. The growth speed of t he "Ke­ 1200 ra" during the 12- 14 hour stage is quite fast. .\ us lt'n i t ('

The fin al stage is the longest one, lasting 30-40 1000 t------~----c{---,------'-'-'-=--:c"'-___"r:=cc..:.::...-- hours from the end of the third stage. The iron sand charged in this stage is the highest quality sand call ed "Masa." As shown in Table 1, this iron sand has the highest iron content and the Fig. 2. Iron -carbon equilibrium diagram lowegt TiO" content. The amount to be charged at on e time is 60 kilogra ms of iron sand, the la rgest As a by-product of this "Kera"-manufacturing amount. This pure iron sand " Mas a" is easier to process, pig iron is obtained. But t here is another r ed uce than any other iron sand. Also, most of "Tatara" method which can be called an indirect the molten pig iron a nd slag, as in the previous iron making process. The fu rnace for this process stage, are r eleased outside the f urnace every 3-4 is simi lar to the "Ker a" -making f urnace in shape, hours . In this final stage, the gr owth speed of the but the difference li es in the fact that the tuyeres "Kera" is at maximum. The iron oxide in the s lag are placed closer to each other and that iron sand is mll1lmum in quantity compared to the oth er containing titanium is mainly charged. With this stages, and even though it proves that reduction is pig iron as the raw material, low-carbon wrought

51 Tetsu-to-Hagane Overseas Vol. 1 No.2 September 1961

iron was made in small f urnaces with handworked bellows. In this process, pig iron a nd charcoal were piled on top of each other. Excessive air was pumped in from below to carry out decarbonization and the pig iron converted into a h a lf-melted state. This was how wrought iron was produced. Famous J apanese swords, other weapons and iron tools were a ll made from this "Kera" and wrought iron. It is a well-known fact that they have s uperior qualities because of their virginity and h ereditary nature. REFERENCES 1) Kuniichi Tawara: Ancient Iron Sand Refining Method (1933), Maruzen Co., Ltd. 2) Kuniichi Tawara: Scientifi c Study of the Japanese Sword (1953), Hitachi Hyoron-Sha 3) Rokuro Maeda: Japanese Steel, Japanese Iron (194 3), Kawade Shobo

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