METHOD FOR MANUFACTURING LOW-ALLOY, HIGH-STRENGTH STEEL SHEETS WITH SUPERIOR LOW-TEMPERATURE TOUGHNESS.

Description

TITLE METHOD FOR THE MANUFACTURE OF LOW STEEL SHEETS,GA = HIGH STRENGTH, WITH SUPERIOR TOUGHNESS AT LOW TEMPERATURE.INV. D ES OUC4I CHIAKI, MOTO KAZUAKI OKITA TOIMOYOSHIr MATS PRIORITY JAPAN DONI. PATENT OF JULY 1977 No. 85980 / 1977 THE DIRECTOR Rome, / / .............3-4- - AUG-1985-----------Ch <CC· TlVOH Record A Protocol No. 25785 χ / 78 Ministry of Industry, Trade and Crafts Provincial Office of Industry, Commerce and Crafts of Milan COPY OF THE MINUTES OF FILING FOR INDUSTRIAL INVENTION PATENT The year -9? 8 on the day SEVENTEEN at eleven hours and minutes of the month of zero eight JULY the Company NIPPON KOKAN KABUSHIKI KAISHA xb&gnac of Japanese nationality TOkyo (Japan) Via through the sole agent Brevetti ing. GALVANI.salvi & VERONELLI and with domicile for legal purposes in Milan - Piazza Duca d'Aosta 4 at the office of the agent has presented to me, the undersigned: PRINCIPAL - Stamped application for the granting of a PATENT FOR INDUSTRIAL INVENTION COSJfiCCIXSiX entitled: METHOD FOR MANUFACTURING HIGH-STRENGTH, LOW-ALLOY STEEL SHEET METAL WITH SUPERIOR TOUGHNESS AT LOW TEMPERATURE Designated inventors: OUCHI Chiaki, OKITA Tomoyoshi, MATSUMOTO Kazuaki Priority of patent application in: JAPAN No. 85980 / 1977 dated July 20, 1977 supplementary to patent no. (application no.) / / filed on / / granted on / / accompanied by - Duplicate description of π, 3θ pages of writing. * Drawings, plates n, TWO in duplicate. - Letter of appointment - Priority document and Italian translation (with reservation) 1 / - Ao3£KCZ2aC<«DeOC2It0UIKÌieSS}aCéX - Declaration of consent of the inventor to be mentioned in the patent. - Proof of payment on the postal account no. 00668004 in the name of the Tax and Concession Registry Office of Rome of L, 1 2?. 5C£ issued by the Post Office of Milan 40 on 17 July 1978 no. - Revenue stamp of L.tfjgSQai 2,000.= The application, descriptions and drawings listed above have been signed by the applicant and countersigned by me and stamped with the office stamp *r « THE DEPOSITOR ;r copy identical to the original OFFICER ROGANT / Russian Idyll p. the Direct (Federico N the office clerk (Russian idyll) Qn. MINISTRY OF INDUSTRY, TRADE & CRAFTS Central Patent Office____ — ROME — The : NIPPON KOKAN KABUSHIKI KAISHA of nationality . Japanese, with registered office in Tokyo (Japan), by _mandate-, datary Patent Office - ING. CALVANI, SALVI & VERONELLI and with elective domicile for legal purposes__ __ .in Milan. (P.zza Duca D'Aosta, 4) applies for the granting of a main patent for 1 * invention having as title: _ ______ ___ METHOD FOR MANUFACTURING STEEL SHEETS LOW ALLOY, HIGH STRENGTH, TOUGH— LOW TEMPERATURE HIGHER TA Inventors: OUCHI Chiaki, OKITA Tomoyoshi, MATSUMOTG Kazuaki PRIORITY: The right of priority is claimed arising from: _ _ _ a) previous patent application filed in JAPAN on July 20, 1977 No. 85980 / 1977 Attached documents: a) description in duplicate^ of η. pages of writing; ___ ___ b) drawings, n. 2 duplicate plates; _ _ c) letter of appointment; _d) Priority document with Italian translation_(subject to reservation) e) inventors' declaration Ϊ3 f) proof of payment into the postal account............. n ·.6 68004 registered in the name of * Registry Office, . taxes and concessions of Rome of.....L. 127,500.= issued I ' from the 1st Post Office of Milan dated 17 July 1078 σ) revenue stamp of L. 2,000,=_____________ _____________________________ K CM.WW SALVI t VDfflNELU DESCRIPTION of the * Invention having as title: *' METHOD FOR MANUFACTURING STEEL SHEETS LOW ALLOY AND HIGH STRENGTH, WITH TOUGHNESS SUPERIOR LOW TEMPERATURE TYPE in the name of: NIPPON KOKAN KABUSHIKI KAISHA in Tokyo (Japan) ..... • 4 444 ^located on I · LVW· With j.1 No. taro • *### The present invention relates to a method for the manufacture of low alloy steels and eie- ί··'·· 4 4 · high strength with low temperature toughness, *'* and more specifically a method capable of considerably improving strength and toughness by means of an accelerated cooling treatment following hot rolling. Several methods have already been proposed to improve the strength and toughness of low-alloy and high-strength steels. Among these methods, thermomechanical treatment of low-alloy steels is important, given the economics and product characteristics, but accelerated cooling following hot rolling of thick sheet metal is also important. Furthermore, the controlled rolling technique is widely used to improve the strength and toughness of low-alloy steels, and it is 2· |& CIUMU SALVI c VMEIh It has been known that accelerated cooling following controlled rolling is effective in further improving strength without affecting toughness. Using these known methods, however, it has proven difficult to simultaneously improve strength and toughness by accelerated cooling after hot rolling. Especially when a large improvement in strength is expected from the accelerated cooling treatment, one has been forced to sacrifice toughness. Accordingly, it is an object of the present invention to provide an improved method for manufacturing low alloy steel sheet having high tensile strength and toughness characteristics. According to the present invention, there is provided a ·· ·· • 4 *44* ··♦ • 4 4 4 · .♦·· 4 4 4 4 4 4 4 4 4 4 4 4 4 4 • * 4 » φ 4 4 44* 4* 4 4 Method for the manufacture of high tensile steel and high tenacity steel sheets, characterized in that a steel alloy or ingot containing 0.03 to 0.15 wt. % C, 0.05 to 0.60 wt. % Si, 0.60 to 2.5 wt. % Mn, 0.010 to 0.15 wt. % Nb, 0.005 to 0.10 wt. % soluble Al and the balance iron, is heated to an ΐ-' zone -3ING. GALVANI SILVI ( VOTIil of temperatures above 105CP® and below the reheating temperature zone where the austenitic grain size should be 150 μ ϊ by the fact that the heated steel slab or ingot is hot rolled with an overall rolling reduction greater than 40% compared to a finished thickness, in the non-recrystallized austenitic zone by the fact that the hot-rolled products are subjected to accelerated cooling at a rate of 5-20°C / sec from a temperature above the Ai / transformation point to a temperature between 550 and 650°C; and by the fact that the products are air-cooled. If desired, the steel slab or ingot may also contain one or more of the following elements in the following quantities: less than 0.025% by weight Ti, less than 0.5% by weight Cu, less than 0.60% ·♦ · · » a ·· · · • · · 4th 4 4 4 4 ·*·· 4 4 *4 4· 4 4 4, 4 4 4 4 4 4 4 ·.·· 4 4 4* 4 4 4 wt% Mo and less than 0.50 wt% Ni. The invention will now be described in more detail with reference to the attached drawings in which: Figures 1A, 1B, and 1C are graphs illustrating the correlations between slab heating temperature, strength, and toughness of as-rolled products and products subjected to accelerated cooling. -4MB. CALVARI SALVI c «Oli Fig. 2 is a graph illustrating the correlation between austenite grain size at the reheating temperature of the slab and toughness. Fig. 3 is a graph illustrating the correlation between the starting temperature of accelerated cooling and the strength of simply rolled products and products subjected to accelerated cooling. Fig. 4 is a graph illustrating the correlation between the accelerated cooling termination temperature and the resistance. As a result of extensive research by the Applicant, it has been found that it is possible to simultaneously improve strength and toughness by accelerated cooling following hot rolling only when a particular combination of heating, rolling and cooling conditions is used. .· · · fi fi • fi·· fi·· «fifi fi fi fi··* • fi fi • fi fi fi fififi fi fi fi fi fi ♦••fi fi fi fi fi fi •fi·· fi fi fi fi fi fi • fi · fi fi fi fi , fi fi fi fi · fi fi fi fi fi fi fi fi • . fi · fi · fi fi fi More specifically, a substantial increase in strength through accelerated cooling can be achieved by forming a two-phase structure of extremely fine ferrite and bainite. In general, when the rate of accelerated cooling after rolling is relatively low, no significant increase in strength occurs. IIHL Miti SALTI and «BHHU radically change the structure but the ferrite grains become finer, thereby improving the notch toughness. In this case, however, the increase in tensile strength due to accelerated cooling is small, so it is impossible to reduce the alloying elements and carbon content to improve weldability and significantly increase the tensile strength. To achieve these goals, it is necessary to radically change the microstructures from errite-pearlite in the as-rolled condition to ferrite-bainite in accelerated cooling that effectively increases the strength of the steel. As a result of accelerated cooling tests carried out on different types of steel, it was found that it is necessary to use accelerated cooling at a rate of at least 5°C / sec from 4 to 100°C. 44444 4 · ···♦ « 4 · 4 φ e • 4 4 4 * 4 4 4* · · 4 4 a temperature above the ATg transformation point in order to achieve the previously described microstructural modification. According to the prior art, accelerated cooling, which results in a large increase in strength and a structural modification, results in low-temperature toughness comparable to or lower than that of rolled-only products. However, when using a combination of an appropriate heating temperature and suitable rolling conditions, it is possible to disperse not only the ferrite but also the bainite structure extremely uniformly and finely, thus improving strength and toughness. The steel according to the present invention has the chemical composition previously described and; The ranges of variation of the respective elements are determined due to the following reasons. More specifically, a C content lower than 0.03 wt% is not sufficient to increase the tensile strength, while when the amount of C increases above 0.15 wt%, the weldability is negatively affected. Although the amount of Si should be greater than 0.05 wt% to achieve deoxidation, however, a Si content greater than 0.60 wt% reduces the weldability and formability characteristics. Similar to C, at least 0.60 wt% of Mn is required to increase the tensile strength, while a Mn content greater than 2.50 wt% decreases the weldability. Nb is an indispensable element according to this in— ·· ·· • · ···· ♦ ·*♦ »* · ·· ·· • · · * :· 4 4 4 4 4 ·*·* •I · *»44 • 4 4 4 «44· 4 4 4 4 .. · 4 4 · 4 4 4 • 4 . >·* · 4 4 invention and its quantity must be greater than 0.010% by weight in order to form non-recrystallized austenite, however when its quantity exceeds 0.15% by weight the weldability decreases. -7· Ufi. CALVARI àLH « WWflU Because 1»A1 is used to deoxide the steel, to bind N, and to refine the microstructure, its amount must be greater than 0.005 wt% in terms of acid-soluble aluminum and less than 0.1 wt% to maintain solution clarity. Ti, which forms a stable TiN with N in austenite, increases the temperature at which the austenitic structure coarsens and is effective in refining the grain size of the austenitic structure, and its amount must be less than 0.025 wt% to display these desirable characteristics. Cu, Mo, and Ni are effective in increasing tensile strength because they form solid solutions with Pe, thus hardening the steel. Ni, in particular, serves to increase toughness at low temperatures.Cu less than 0.5%, Mo less than 0.6%, and Ni less than 0.5%, all by weight, can serve their purpose, and these elements are incorporated singly or together. The heating temperature of the alloy must be higher than 1050°C, since at a temperature lower than 1050°C the Nb does not form a solid solution and the recrystallization temperature of the austenite .· · · • fi • fi fi * • · ·* . ··* ···:. t fi fi fi fi » fi fi fi fi ► ♦·· fi '••fi •fi·· fififi -8MG. CM.VIW Stt.H ( YEHMEUi does not increase, so it is impossible to increase the total rolling reduction in the desired non-recrystallized austenitic zone and because it is impossible to improve the precipitation hardening property caused by accelerated cooling. Furthermore, by decreasing the hardening capacity, the fine bainitic structure is not formed, so the mixed two-phase structure of fine ferrite and bainite which is essential for increasing the strength and toughness is not formed. According to the present invention, the slab or ingot is subjected to heating. • · ·©·· • « · subsequent up to a temperature above 1050°C and below the reheating temperature zone that gives rise to austenite grain sizes of 1 50 μm at a given reheating time, e.g. one hour. Considering this correlation in more detail, three steel samples A, B and C were prepared as shown in Table 1 below and the slabs were heated to the same temperature and rolled under the same conditions (i.e. a slab heating temperature of 1100-1280°C and a finishing temperature of 800 to 810°C). One sample was rolled and then air-cooled to obtain a rolled-only product, while the other sample was subjected to accelerated cooling at a rate of 8-12°C / sec between 790°C and 600°C for a further heating time, e.g. one hour. obtain accelerated cooled sheets. Table 1 ·· · · • · ·· ·· • **· · · · * · M · ·· '· ·· k 4 4 · TYPE C si Mn PS Mo Nb Ti soi.aX; 4 « ..... a 4 4 4 4 4 ::: : tn '···.....· . · ABC 0.09 0.09 0.12 0.21 0.22 0.25 1 .33 1 .42 1 .35 0.013 0.014 0.01 8 0.007 0.008 0.003 0.13 0.019 0.038 0.036 0.017 4 0.017 4 ' 0.010 . « 4 0.020* « 4 4 ι • • Ó:pO63 , ··· ·* , · · · • · · · ; ®»0G6o ...· .··.·· • · · · · —· Q.0058 • · · · · • 4 4 4 4 »444 4 .......1 The strength and Charpy fracture transition temperature of rolled steel sheets and sheet metal— accelerated cooling rates in different steel grades were measured and the results are shown; in figure 1 » In this figure, figures 1A, 1B, and 1C. refer to samples A, B, and C respectively. In each graph, the abscissas show the temperatures in °C of subsequent heating of the slab, while the ordinates, from 0 to 10, show the temperature differences in °C, the resistance differences in kg / mm2 and the resistances in kg / mm2 respectively. The symbols shown indicate: o: TS rolled only; ·: TS with accelerated cooling; Δ: YS rolled only; A: YS with accelerated cooling. The accelerated cooled products of the three steel samples show a resistance approximately 5-10 kg / mm ​​higher than the rolled only sheets, regardless of the subsequent heating temperature. However, the low-temperature toughness of the accelerated cooled sheets is greater or less than that of the rolled only sheets, regardless of the subsequent heating temperature of the slab.From the correspondence between the austenitic grain size and the change in toughness, which are reported at the respective measuring points indicated in figure 1, as well as from the correlation between the austenitic grain size at the respective reheating temperatures of the slab and the difference between the toughness of the accelerated cooled products and the rolled-only products as indicated in figure 2, ♦ · ·♦ • 4 ••4* ··· »••44 ··· ·· 4». 44444 » 4444 ··*· I * · ·· ·· •44* • 44», 4 4 4 4 ' » · 4 »44 ' • 44 It can be seen that the improvement in toughness caused by accelerated cooling can be obtained in cases where the austenitic grain size reached on reheating is less than 150 μ. As can be seen from the upper curve of Figure 1, toughness is generally improved by 10°C or more when the grain size is less than 120 μ. It is also evident from these figures that the increase in strength is approximately 5-10 kg / mm'' -111HL CALMI SALVI t VERONELU It is obtained by accelerated cooling of these steels. The size of the austenitic grains at the reheating temperature of the slab can be varied depending on the heating temperature, the composition, and especially the microalloying elements such as Al, Nb, or Ti. In samples A, B, and C described above, the heating temperature at which the grain size becomes larger than 1.50 μ varies depending on the amounts of Nb or a small amount of Ti addition. This temperature is assumed to be approximately 117O eC for sample A, Π80°C for sample B and 1280°C for sample C. From the correlation described above it can be seen that, in order to improve toughness by accelerated cooling, it is essential to use a reheating temperature of the slab at which the austenite grain size becomes smaller than 150 μ due to the chemical composition of the steel. The importance of the austenitic grain size under reheating conditions has been clarified by systematic research on the transformation structure of accelerated cooled products, more specifically, under conditions where the rate of a 1 »·* *··· ··♦· • *·4 β 4 4 44* · 4 4· >·· 4 * 4 4 12' B CALVA» SUD and VEBBHJL1 accelerated cooling is greater than 5°C / sec (in the case illustrated in Figure 1, 8-12 °C / sec) even when the total cumulative rolling reduction of the austenite non-recrystallization zone is sufficiently high. When the austenite grain size is greater than 150 ja, the transformation structure becomes a coarse-grained bainitic structure in which the initially transformed ferrite has grown along the austenite grain interfaces extending in the rolling direction, while when the grain size is less than 150 and in addition the cumulative rolling reduction is greater than 40%, a mixed structure of extremely fine ferrite and bainite is obtained, with simultaneous improvement in strength and toughness. As illustrated in Figure 2 and subsequent embodiments, even when a slab reheating temperature is used that ensures an austenitic grain size of less than 1 50 μm, when the total reduction in the lamination of non-recrystallized austenite is less than 40%, the level of toughness alone is lower, so that it is impossible to improve the toughness by accelerated cooling. In -13' INO. CALVAW SILVI t VEM In the said figure 2, the size in fi of the austenitic grains is shown on the abscissa, and the temperature difference on the ordinate for the cases of only rolled or rapidly cooled products. Above, the total reduction ratio is indicated. From the above, it is clear that it is essential to use a cumulative total rolling reduction greater than the 40% in the non-recrystallized austenitic zone in order to obtain a mixed structure of bainite and fine ferrite. (The temperature at which austenite ; assumes the non-recrystallized condition during rolling depends mainly on the amount of ; Nb and is about 860°C for sample A which contains 0.02 wt% Nb, about 900°C for sample B which contains 0.04 wt% Nb and also about 900°C for sample C which contains 0.036 wt% Nb, which shows that temperatures increase with increasing Nb content). With reference to the accelerated cooling condition, it is essential that the accelerated cooling starting temperature is above the transformation point Ar · Therefore, even when the slab is hot rolled under the conditions previously described for the present invention, if the accelerated cooling starting temperatures are • rare· * *·*♦ « ·· ·. rare · • rare • ••rare • ••rare • rare ra ·* · · • ♦ · ··· ra· • · rare··· ra · · » . 14INt CM.VJN SILVI « fiefiil 5 !· t....... iP ¢£ : irΐ· t:·· lower than the transformation point Ar as shown in figure 3, the increase in resistance becomes lower than that of the rolled sheets for several kg / mm ​​. (Figure 3 shows on the x-axis, for sample b, the temperature in °C at the start of accelerated cooling while on the y-axis are reported the variations in resistance and the resistance in kg / mm ​​2. The cooling rate and the end-of-cooling temperature are 8-10°C / sec and 600°C, respectively. This means that below this point it is impossible to achieve the aims of the invention. (The results shown in Figure 1 were obtained with a rolling end temperature of 800-810°C and a cooling start temperature of 79O°C, while the Ar transformation points of samples A, B and c were at 75O-76O°C.) Especially when the hot rolled product is cooled from the transformation point temperature Ar «J plus 20 eC, the strength of the same can increase by about 10 kg / mm. The lower limit of the cooling rate is c;5°c / sec, while the upper limit is 20°c / sec when the accelerated cooling is carried out at a rate higher than 20°C / sec the toughness cannot be improved and furthermore there is an increase in the non-uniformity of the structure and in the ·· * · » ·· ·· V·*·*· a · a · · • ··· ·· ·· 4 4 4 « • 4 to 4 4 « « 4 ····< • a · and 1 aa · • 44 · 4 • · • 44 ·.· · aa·· 4 · 4 4 Μ SALVI « ΟB11 hardness in the direction of the sheet thickness. The temperature range at which accelerated cooling must be stopped is between 550 and 650°C. The steel of the present invention has a fine ferrite and bainite structure containing more than 5% of fine, uniformly dispersed bainitic structure. By stopping accelerated cooling at this temperature, a self-feeding effect occurs during subsequent air cooling, with simultaneous improvement in strength and toughness. Precipitation of Nb and ν' occurs much more effectively at the said stopping temperatures. However, if the steel product is continuously cooled to a temperature below 550°C and then air-cooled, sufficient precipitation does not occur during the subsequent cooling. ···· *1 · ·· · · © © • · ·· · · · • · · © © · © ♦ · ·.·· • « · • · · · Φ © · ·« · cooling, while when the cooling is stopped at a temperature above 650°C, the increase in strength due to the accelerated cooling decreases rapidly as illustrated in figure 4, in which case the amount of bainite becomes less than 5%» indicating that the object of the present invention cannot be achieved. (The graph in figure 4 shows on the abscissa the temperature in *C at the end of the cooling and on the ordinate Hit CMVMB SILVI ϊ VEBMOB tF...... •Wr2 the resistance in kg / mm, the conditions being: cooling start temperature 750°C; cooling rate 8-10°c / sec). As illustrated in Figure 1, accelerated cooling decreases toughness in one case, but increases it in the other case depending on the reheating temperature. The amounts of N in the solid solution condition at the temperature higher or lower than the said critical temperatures. , 3. . . of the reheating temperatures are 0.0053 wt.% and 0.0053 wt.% at 1200°C and 1150°C, respectively, for sample A, 0.0052 wt.% and 0.0046 wt.%, respectively, at the same temperatures for sample B, and 0.0027 wt.% and 0.0021 wt.% at 1280°C and 1250°C, respectively, for sample C. These data show that the steel according to the present invention is not affected by the amount of N, In the embodiments that will be described later, steels H and I are samples containing ·· · · • 4 ···· Ύζ·* * • ··· ···:. » 4 · 4 V.··· 4444 4 ·· · · « 4 • · ••4» * 4 4 4 4 • 4 * · 4 444 • 444 » 4 4 44 * different quantities of N, but as can be seen from the characteristics of these samples, the method according to the present invention allows to produce steel sheets having excellent toughness properties even when the K content is relatively 17WG. CO» SO * r......... high. For a better understanding of the invention, the following examples are given, but it should be understood that the invention is not limited to such specific examples. Example 1 Steel samples A, B, and C having the compositions shown in Table 1 were heated to a temperature of 1100 to 1150°C, and the mechanical properties of the rolled products and of the products subjected to accelerated cooling after rolling were measured. The mechanical properties were measured in all examples on quality samples in a direction transverse to the rolling direction. Additionally, the strength and toughness of the rolled-only products and of the accelerated-cooled and rolled-under-the-same-condition products were also measured. The measurement results are shown in Table 2 below. ·· ·< ·· ·. fi fi • fi « fi fi fi fi fi fi fi •fi·· fi - - fi fi • fi fi · fi fi fi fi ♦ fi fi fi fi fi fi fi fi fi * fi Table 2 Sample or present invention X comparison Slab heating temperature (°C) Austenitic grain size at slab reheating temperature (°C) Rolling rate (%) in the non-recrystallization zone • Accelerated cooling rate,..(°C / sec.) • -1 * •· IlII *··· 4 A-1 X 1 200 300 65 • '•ÌsòiL*ó 'la- 2 X « II II • ::.¼^ 4 1··· . . . 3 X 1140 115 65 4 • 4 4·· 'soltxJLa- ...min&to 4 σ ir II II • '· :;1.Ί .5 fl . ···· •j • · · .· 5 X 1100 70 65 4 ” only «laminated »··· • miASr&o 6 o It It II 4 4 B-1 X 1 200 220 60 4 'only laminated- 2 X II II II mÌS5° 3 X 1150 105 60 only^laminated 4 ό II 11 It 8.5 5 X II II tl 2.3 6 X 1150 105 35 only, laminated 7 X « 11 II 12.8 8 X 1100 63 60 only laminated 9 o It II II 9.8 C-1 X 1250 130 65 only laminated 2 0 II II 11 9.8 3 X 1200 95 65 only laminated 4 o II II II 12.5 5 X 11 II II 23.5 6 X 11 00 65 65 only, laminated 7 or II II II 11.5 8 X 1000 44 65 only^la? copper 9 X II tt II 10.6 9Nt tOB «M· 11 * Table 1a 2 (continued) 1 1 Tensile test bells — ------- ..................« 2V Test of s Quotes py Tensile strength (YS) (Kg / ram ) Tensile strength (TSl (kg / mm ​​) AYS 2 (kg / mm ​​) energy at 0°C >Eo) (kg-m) VTS C-c) 53.9 12..8 -64 2 51.9 61.7 6.8 10.3 -54 ·· • ·· •.10 • 3 43.5 52.5 - 12.5 -75 • · rare· « « ............... — · · · · - rare ·· 4 50.0 59.87. a ···· «ara 5 42.8 51.7 12.2 -83 • ara • a « :· ·· . · . · ... τΛ· 6 48.6 57.4 5.8 10.9 -92 ra aa ·· • » · rv • -3⁄4. ··. .·· B-1 48.4 54.5 12.2 -76 •ra • aa ·: *·· : 2 53.8 60.5 5.4 9.6 -64 aa ··« •,iv.;: 5 53.0 47.6 59.9 54.4 6.6 1.8 12.1 12.7 -95 -87 • ara rara· • rara -12 £4 6 45.8 53.0 13.2 -70 7 53.0 60.1 7.2 11.5 34.5 34.3. 13.0 -74 — 9 52.1 59.0 7.8 10.8 -107 -13 3-1 48.5 58.0 17.2 -60 — 2 57.1 67.2 8.6 1 5.8 -66 -6 3 48.0 57.8 -86 -56. 66.2 8.4 15.6 -99 13 5 59.1 80.2 11 .1 7.9 -65 20 6 46.5 57.2 - 16.1 -11 8 - 7 55.8 65.3 9.3 14.7 -129 -11 8 42.7 52.3 - 17.2 -123 Μ1* 9 49.4 53.5 1.7 16.4 -139 -8. 20— WG. CALVARI SALVI and VOHMIQU Most of the data in Table 2 are reported in Figure 1. As already pointed out, it is essential to limit the austenitic grain size below 150 μ in order to significantly improve the tensile strength and low-temperature toughness when the total rolling reduction of the non-recrystallized austenite zone is high, i.e., 60–65%. Thus, in samples A-1 and B-2, since the grain size is larger than 150 μ, it is impossible to obtain high toughness.In sample B-5, although the grain size is 1.50 p and the total rolling reduction is 60%, which are values ​​within the scope of the invention, the accelerated cooling rate is 2.3°C / sec, which is less than 5°C / sec, and therefore the increase in strength is only 2 kg / mm, so there is practically no increase in strength. In sample C, the cooling rate is 23t5°C / sec, which is outside the scope of the invention. In this case, although the tensile strength increases significantly, the vTs value is higher than that of the product rolled only for 20°C, and therefore the toughness decreases. Thus, regardless of the steel grades, it is essential to use a rate of *· · · • 4 ···* *·?? / . ·· ·· • 4 4 · 4 «••44 • 4 « 4 · • ♦ • 44 ··:· ♦··· « « • 1 ···· 4·4 • ·• · · 4 and • 4 ·· * -21iHB. CALVARI SALVI « VQfflHEUl cooling from 5 to 20°C / sec to significantly increase both tensile strength and toughness. In sample B-7, although the heating condition and the accelerated cooling conditions are within the scope of the present invention, nevertheless the total reduction of rolling in the zone ; The non-recrystallized austetinite content is 35%, i.e., less than 40%. In this case, because coarse-grained bainite is formed, tensile strength is increased, but toughness is decreased. This indicates the reason for the limitation of the total rolling reduction. In sample C-9, the heating temperature is 1000°C, i.e., lower than the 1050°C indicated according to the invention. In this case, although the toughness is higher than in the case of the rolled-only product, the increase in strength is small. Example 2 Samples of steel D, E, P and G were prepared having the chemical compositions indicated in the table. · · · • · · · fi · · ·· fi fi · fi · fi fi • fi · fi fi fi • fi fi fi fi fi fifi fi fi !· fi · fi fi fi fi fi ' fi • fi·· fi · fi fi fi fi · • · fi fi • · fi fi fi fi • fi fi fi t fi fi fi fi · following table 3 ME. CALVARI SALVI (VERONELLI -22' Table 3 Sample C Si Mn PSD 0.10 0.30 1.50 0.008 0.006 .· · · • · • ·. · · .·· • « * E 0.05 0.17 1.77 0.012 0.009 • ··· « . · *· ·· • • · · F 0.06 0.24 2.12 0.006 0.006 • • · · · · ;· • · · • • · G 0.06 0.25 1 .53 0.009 0.008 • · · · · .··:·· ··.. - • .· • • · • · · · · · · · · · · * Sample Mo Nb V Ni Sol. Al -W» ,γτ.- ·© * * · · © » · · · · W; • · · · ~ D ·*· 0.036 0.08 0.23 0.032 ΟΪ0δ*62 E 0.16 0.033 - - 0.011 0.0065 F - 0.056 - 0.25 0.015 0.0061 - G 0.48 0.052 — 0.27 0.022 0.0087 The mechanical properties of the rolled-only products and the accelerated-cooled products among the above samples, prepared under different heating conditions and under different rolling conditions, were measured together with the strength and toughness of the cooled-only products and the rolled-only products, the results being reported in the following Table 4. The accelerated cooling was started at 790-780°C and stopped at 600°C, within •23ING. OLVMI SALVI t VBUEUJ r Table 4 Sample the present invention X comparison Temp. of heating of the slab (°C) FSize of austenitic granules at the subsequent heating temperature of the slab ( μ ) Total reduction of lamination (%) in the zone of non-recrystallization ·· 4 4 4 .gol©» .1 a- • · •a. ·* 6 X n tl It j . ··*· » a • 4 4 · aaa 4 4 3-1 X 1140 125 60 4 4 .solo^laminated 2 o It ,1 IV 4 4· '' 6.2 3 on tl VI 1 1 1 X 5.6 P- 1150 95 II 10.2 3 o IV It tl 12.4 4 o 1100 56 11 10.5 G-1 X 10.3 6 X 1 080 52 30 only 7 X ​​tl 11 tt mixed 8 X 1000 35 65 only laminated 9 X tl ir IV t______ 24ING. CALVARY SAVE t SEE HIM Table 4 (continued) Tensile test specimen 2V Charpy test ΥΞ (kg / mm ​​) TS. 2 ΔΥ3 2 (kg / mm ​​) ÙTÌ νΕο vTs IvTs (kg / mm ​​) (kg / mm ​​) (kg-m) (°c) (°c) D-1 51.7 61.2 - - 10.1 “73 - 2 59.3 71.9 7.6 10.7 8.4 -64 9 3 50.7 7.2 — — 9.2 -114* · • 4 « • · “ 4 58.8 68.9 7.9 9.7 7-2 -125.\ • 44 Γ ' 5 52.9 72.5 12.2 13.3 6.8 “1 21 ,j ··· •4 ^7·· 6 62.7 61 .0 2.0 1.8 8.8 -115 • · · · • 4 4 4 4 · E-1 40.6 55.4 12.4 -94 Ϊ • 4 · 4 « 4 V · · 4 2 45.6 60.5 5.0 5.1 13.7 -110 3 50.3 63.5 9.7 8.1 12.5 -108 ·: 4 .:.-m · F-1 57.1* 73.2 5.9 7.3 9.6 -75 .. • ^ » · · ··· 2 55.6* 73.6 6.5 7.8 9.7 -132 . · 3 56.3* 74.3 5.2 8.5 9.6 -128 ·· •'•*-14 4 59.8* 66.5 2.4 6.0 10.6 -135 *· 4 4 •>25 4 4 * G-1 43.1* 66.0 — 8.0 -103 2 48.7* 73.6 4.8 7.6 8.0 -115 -12 3 51 .7* 77.0 7.8 11 .0 7.8 -116 -13 4 42.3 62.6 - - 9.2 -114 - 5 52.9* 79.1 10.6 6.5 7.6 -150 -36 6 41 .6 62.2 - - 9.0 -81 - 7 48.8* 71.3 7.2 9.1 7.2 -70 11 8 40.3 58.1 - - 8.7 -118 - 9 42.5 59.8 2.2 1.7 8.1 -1 36 -18 The * sign indicates a 0.2% phase shift in flow stress and the other data in the YS column indicates a lower yield strength. 25IHt CALVARI SALVI and VERONELLI The heating condition, rolling condition, and cooling condition of samples D-4, Ε>-5,β-2, B-3, P-2, P-3, P-4, G-2, G-3, and G—5 are within the limits of the present invention, and the changes in strength and toughness in the accelerated-cooled sheets compared to the rolled-only sheets demonstrate that the strength increased by several kg / mm, while the vTs decreased by 10*C or more. These data show that the method of the present invention permits the production of steel sheets having excellent tensile strength and high toughness. On the other hand, the austenitic grain size under heating condition of samples D-2 and P—1 is larger than 1 50 jx, which shows that only the tensile strength increases, while the toughness decreases even if the rolling and cooling conditions are satisfied. In the sample D-6 the cooling rate is slow and therefore, while the toughness is equal to or greater than that of the rolled product alone, the increase in strength is small, i.e. less than several kg / mm, thus reducing the advantage of accelerated cooling. The total rolling reduction in the non-recrystallized austenite zone of sample S-7 is ···· « 4 ··»· 444 4 < • · 4 · l *4 4 J • 4 4 4 4444 • 444 :· 4 4· 4* · *· « ··· 4 4 4 4 4 4 4 4 4 φ 4444* •26WG. WITH SALVI and VEMMEUA 'less than 40%, i.e., below the limits specified by the invention. In this case, although the tensile strength increases, the toughness becomes considerably poor for the reason previously described. In sample G-9, the reheating temperature is 1,000°C lower than 1,050°C. Consequently, even when all other conditions are met, due to the decrease in the amount of Nb in the solid solution condition, the possibility of quenching decreases, so that fine bainite does not form. Furthermore, the effect of precipitation quenching due to accelerated cooling does not increase, so that the increase in strength is smaller. * ... 7* *' fi «fi fi fi fi··· •i · ·♦ ·. # · « · · :· ···· • · ···· • · « · ··♦ ····: • « · · · • · · · < · · ·.·· fi fi fi ··· fi fi fi fi fi Example 3 THE Two HLe samples containing different amounts of N were prepared as shown in the following table 5 Table 5 Sample C Si Mn P s Mo Nb Sol.Al TN H 0.09 0.26 1 .44 0.012 0.007 0.14 0.024 0.033 0.0021 I 0.09 0.28 1 .48 0.015 0.007 0.15 0.027 -1 0.017 0.0059 These samples were heated to the temperature indicated in the following table 6, laminated with a re—2 7— |Ht CALVMU SALVI and V 55% total rolling reduction in the non-crystallized austenitic zone, finished to 20 mm at 800°C, then air-cooled after rolling or accelerated quenching to 600°C to obtain the corresponding products. The characteristics of these products are shown in the right half of Table 6. In these two samples, when their slabs were heated to 1200°C, where the austenite grain size becomes greater than 1.50 ji, accelerated quenching increases the tensile strength but does not improve the toughness. On the other hand, when the austenite grain size is smaller than 150 ji, the method of the present invention significantly increases the tensile strength and improves the toughness compared to rolled-only products, regardless of the steel grades, thereby producing high-quality steel sheets. As previously described, the method according to the present invention allows to produce sheets of steel having tensile strength greater than that of only rolled products by several kg / mm, and at the best characteristic temperatures. Fi I & Λ L £>1 OCX ω E> sr t— ING. CALVARY ον I SAVE t VQDgQU to rt table o β PL, > OJ and β or Ή Court P β Ό rt > c β P«4 4C Heart X with a tJ3 ,X The β ° 1 •The higher the H > Ή fu in in a> Φ rt β Pi « cn <—OJ co £ 4J ><\ β Cn Φ Λί £ % <& Φ β Ό Ο Ό Φ β Ό at rd Φ φ A & υ u rt and Φ tn o the 1) H 1 β •ri M Φ rd Φ φ ; φ tn cn β p •ri β4 ±J β σι 0 ε β ε τί <3 Φ Ο •H rt ri φ <ά Ο φ ι—1 in β β -Ρ <Ρ φ β & Φ β rt β Φ Φ β rt β Φ cn Ο e Η tn r4 β ω •ri QJ β ι—1 rt •Η ο 0 o rt) rt iti β β -Ρ > 1 IH φ rt rd φ o cn cn β » •ri β ο β β φ rd χ—\ ε β Η ο φ •ri ?3 Φ ο 0 F XJ τ3 & '— QJ b φ o 'l··' rd β « (V CP F b b o +j b the b the the c- η to О ω in r- ο > 1 I co to οι σ to to co σ liti to tO to <0 oo y <— <cf co σ co to 4 • • • • . • * <Γ>θ'- oo 00 σ\ to r- rt· ΙΓ\ to tn to in to to to OJ σ r- Γ- OJ o 00 * • -4 * 4 • * 4 in o CO σ o T - co s|- IO rt- IO l H O | J The fOM The S · The *rD >» tn e A or y Q IVSj— h O •rC' ω e rd 0 y O rt oo rd b · O -ri CO tn g rt rd O b O rt co rd b · O 00 tn £ o s O.J. Oh £ OJ IO this O rt' to = to OJ OJ Φ β Ο •Η Ρ Oh what OJ WHAT ♦· · · · 4 » 4 .·· « ··· ···:. « 4 4 *4 4 4 4 4 4 *4 4 · 4 4*, * * * 4 4 4 4 4 :::· 4 4 ♦ 4 * * 4 4 • · * at.

Claims

1. CLAIMS WG. CALVARI SALVI t VERONELLI 1. Method for the manufacture of steel sheets having high tensile strength and high toughness, comprising the steps of successively heating a steel slab or ingot containing 1 I ί O»Q3 -0.15% by weight of C, 0.05-0.60% by weight of Sij, 1 I 0.60-2.5% by weight of Mn, 0.010-0.15% by weight of Nb, 0.005-0.10% by weight of soluble Al and the remainder; of iron, up to a temperature zone above 1050°C and below a reheating temperature at which the austenite grain size would be greater than 150 microns, hot roll the heated steel slab or ingot with a total rolling reduction of more than 40% with respect to a finishing thickness in a zone of non-recrystallized austenite temperatures, accelerated cooling of the rolled products at a rate of 5-20°C / sec from a temperature above .·· fi·« fi fi • ·· fi fi fi fi * -fi·· • fi • fi ,··:· ·· fi fi fi fi fi fi •fi·· fi · fi fi fi fi fi fi fi fi fi .·· , above the transformation point Ar^ at a temperature between 550 and 650°C, and then cool the products in air.

2. The method of claim 1, wherein said steel slab or ingot contains one or more of the following elements, in the following quantities: less than 0.025 wt.% Ti, less than 0.50 wt.% Cu, less than 0.60 wt.% Mo, and less than 0.50 wt.% Mi.

3. The method of claim 1 wherein said austenite grain size is less than 120 p.

4. The method of claim 1, wherein said hot-rolled products are cooled from a temperature above the Ar transformation point plus 20°C.

5. The method of claim 3, wherein said hot-rolled products are cooled from a temperature above the transformation point. · · · • · ♦ · · · · · · • · · • © · · © ·« · © © • · · · · · · · • © · · » 9 · · Γ' Table No. 2