FI114484B - Hot rolled strip steel and its manufacturing process - Google Patents

Abstract

A hot-rolled steel strip rolled to a final thickness that is at least 2 mm but no more than 12 mm, with a microstructure comprising at least 95 % martensite and/or bainite. The steel contains, in percentages by weight: 0.08 % - 0.16 % C, 0.5 % - 1.5% Cr and/or 0.1 % - 0.5 % Mo, ≤ 0.015 % S and ≤ 0.03 % P, 0.01 % - 0.08 % Al, as well as 0.6 % - 1.1 % Mn, and 0.1 % - 0.3 % Si, the rest being Fe and unavoidable impurities. The tensile strength of the steel strip is 700 Mpa - 1500 Mpa at a tensile elongation that has an A5 value of at least 6 %, and the yield strength is 600 Mpa - 1400 Mpa.

Description

114484

The present invention relates to a steel strip which is hot-rolled to a final thickness of not less than 2 mm and not more than 12 mm and whose microstructure comprises at least 95% by weight of martensite and / or bainite: , 08% -0.16% C, 0.5% -1.5% Cr and / or 0.1% -0.5% Mo, <0.015% S and <0.03% P, 0.01% 0.08% Al plus the remaining Fe and unavoidable impurities. The invention also relates to a method of making such hot rolled steel strip.

Traditionally, hard steels have been made by hardening, but this has not achieved mm. optimum surface quality and no impact toughness. Similarly, the manufacturing costs have been high.

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GB-2 195 658 discloses a forging steel containing in its preferred embodiment 0.05% to 0.08% carbon, 0.1% to 0.5% silicon, 0.5% to 1.6% manganese, 0 , 5% to 1.5% chromium, up to 0.05% titanium, up to 0.1% niobium, 0.005% to 0.012% nitrogen, up to 0.06% aluminum and 0.002% to 0.005% bo-20. Further, according to the publication, the forging is started at a temperature of 1200 ° C to 1275 ° C and the forged body is extinguished in a bath whereby the temperature of the body is measured continuously. . and the quenching is interrupted before the transformation to martensitic is complete. This results in a tensile strength of 700-1100 N / mm2 while providing satisfactory '·' · * impact strength and a PS / TS ratio of about 0.75 without "tempering" or: '25 other heat treatments.

Unlike steels used as such forged steels, known high-strength strip steels, ie: *** steels used in rolling are of high manganese and often also relatively high carbon, such as the hot strip described in US-6,284,063. : 30 with a thickness of less than 5 mm. The steel in the publication contains 0.08%, ··· ', 0.25% carbon, 1.2% to 2.0% manganese, 0.02% to 0.05% aluminum and less than 0.07% silicon by weight, and Not more than 0,015% of phosphorus and not more than 0,003% of sulfur, the hot strip containing more than 95% of martensite. The steel may also additionally contain up to 1.0% chromium, up to 0.1% copper, up to 0.5% molybdenum, up to 0.1% nickel, up to: 0.009% nitrogen, up to 0.0025% boron and optionally titanium in a stoichiometric ratio, Ti = 3.4% N, to the amount of nitrogen. The ingot is first heated to 1000 ° C to 1300 ° C, pre-rolled in a temperature range of 950 ° C to 1150 ° C and finalized at a final rolling temperature above Ar3. The thus obtained hot, 114484 strip is cooled to a winding temperature of 20 ° C below martensitic winding. to reduce the content of phase forms other than martensite to less than 5%. According to the publication, cooling to the winding temperature is preferably effected in such a way that the cooling between 800 ° C and 500 ° C occurs in less than 10 seconds. In this way, a tensile strength of between 800 N / mm 2 and 1400 N / mm 2 is obtained for the final 5 product.

U.S. Pat. No. 4,406,713 discloses a process for producing high strength, tough, easy to forge and weld steel containing from 0.005% to 0.3% carbon, 0.3% to 2.5% manganese, up to 1.5% silicon, up to 0.1% niobium, 10 up to 0.15% vanadium, up to 0.3% titanium and up to 0.3% zirconia. According to the process, austenitization is carried out at a temperature of 1000 ° C to 1300 ° C followed by, for example, hot rolling at a temperature range of Ar3 to 930 ° C, whereby the recrystallization of the austenite is clearly retarded and to a degree of surface shaping. Such shaping causes considerable stresses on the austenite, which shifts the ferrite phase precipitation temperature in the normal continuous cooling chart to higher temperatures and shorter times. In post-process cooling, carbon is concentrated to untransformed austenite as ferrite precipitation progresses. After the ferrite forms 5 to 65% of the steel, it is rapidly quenched below the Ms temperature to provide the steel with a two-phase structure consisting of fine-grained ferrite and high-carbon martensite.

The object of the invention is to provide a hot-rolled strip steel and a process for making it such that the steel is not critical to the local winding of the strip for the thermal "25" bearings, is highly weldable, suitable for thermal cutting, edging and high impact strength. the object is to provide such hot rolled strip steel and its roll: "; a production process that would allow low manufacturing costs.

* »·. *. According to the first principle of the invention, the hot rolled strips * · · ··· * initially defined further comprise 0.6% to 1.1% Mn, and 0.1% to 0.3% Si, and the tensile strength of the steel strip is 700 MPa to 1500 MPa with a tensile elongation at least 6% A5 and a yield strength of 600 MPa to 1400 MPa. According to another principle of the invention, such a steel strip is produced by a process comprising the steps of: steel strip, · 35 hot rolling at a temperature between 860CC and 960 ° C to said final thickness; and, "direct quenching" of this hot rolled steel strip with a delay of not more than 15 seconds from the last rolling point to a winding temperature between 114484 ° C and 20 ° C to 520 ° C with a cooling rate of at least 30 ° C / s. No emission annealing is performed.

The inventive idea is based on the fact that by reducing the amount of manganese and carbon 5 and by doping chromium and / or molybdenum and, if necessary, boron, the "hardening" can be maintained and the following advantages are achieved. The structure of the steel is not critical for manganese and carbon drainage during casting due to the low content of manganese and carbon. The properties of the steel are not critical to the local winding temperature differences of the strip, which facilitates the production of the steel and advantageously affects the homogeneity of the mechanical properties, which in turn has a beneficial effect on the flatness and residual stresses of the end product. The steel sheet has good weldability as well as laser cutting and at the same time good fatigue resistance despite such thermal treatments. Further, the steel sheet has excellent edging properties, good impact strength as well as good impact resistance.

By producing this type of steel instead of conventional furnace quenching by direct quenching by hot rolling, excellent impact toughness is achieved since the phase conversion to martensitic and / or bainite occurs from the finely-formed 20 austenitic. Likewise, the surface quality is improved because the oven dander is washed with gentle washing before molding. Manufacturing costs also decrease with streamlining of the process. The strip rolling line typically uses high furnace heating; Y temperature, for example between 1000 ° C and 1300 ° C, and a long holding time, for example 2 · · · h -10 h. Dissolution of special carbides, e.g. Cr and Mo carbides, and structure: "25 homogeneity is thus as complete as possible. On the other hand, granule growth of austenite at high heating temperature does not cause the final product to become brittle because the austenite is comminuted during hot working. This results in: "": excellent hardness combined with excellent impact strength.

The hot rolled strip of the invention according to the invention can be manufactured to be durable and of various hardness, typically in the range of 300 HB to 400 HB, so called hot rolled strip. as a wear plate by the same manufacturing method, only by altering the analysis and / or post-rolling strip cooling rate and / or temperature before winding within the scope of the invention. Such abrasive steel is possible; · 35 is also used in applications where the structure typically requires structural steel properties such as good formability, weldability, and impact strength, so that the hot-rolled strip steel of the invention is also useful as structural steel. In the following analysis of steel, all percentages of pi-114484 4 are by weight, and the otherwise specified portion of steel is, of course, iron Fe and unavoidable impurities.

First of all, the steel according to the invention has a relatively low carbon content of 5, that is, at least 0.08% C and at most 0.16% C due to its good impact strength, flangability and weldability. The content of impurity phosphorus P shall not exceed 0,03% and of sulfur S 0,015% respectively, the amount of which shall be limited to achieve a good impact strength and a non-flattening effect. Further, the properties can be further improved by melt Ca or CaSi treatment, if necessary. The sealant 10 is aluminum, which in the final product may be at least 0.01% Al and at most 0.08% Al. Chromium at least 0.5% Cr and at most 1.5% Cr and / or molybdenum at least 0.1% Mo and at most 0.5% Mo are alloyed to increase hardening and release resistance. In this case, "Precipitation" is possible at higher winding temperatures, e.g. reduces and even prevents a decrease in strength 15 and mitigates local temperature differences during coil cooling

Unlike other high-strength steels of the same type, manganese has a minimum of only 0.6% Mn and a maximum of only 1.1% Mn. In this case, steel is less prone to segregation of manganese and carbon 20, which improves the homogeneity of the microstructure. It has been found in the tests that this achieves good edging and uniform mechanical properties in different directions as well as a high quality thermally cut surface. Silicon again · · The steel of the invention has a Killing agent and a solid solution (V) to strengthen at concentrations of at least 0.10% Si and at most 0.30% Si, mi- impact strength and formability.

The steel of the invention can be thermally cut, e.g. laser cut, precision-sized parts. A laser-cut body has been found to achieve a fairly smooth and »cutting surface. On the other hand, it has been found that the difference in strength between the base material and the soft zone formed in the technical cutting, which is between the hardened zone and the hardened zone, is relatively small. Together, these preferably affect fatigue strength. In addition, low carbon content lowers the peak hardness of the hardened zone so that the cutting surface is not sensitive to ...! (c) embrittlement and cracking during the molding process; * 35 under operating conditions.

♦ * The test assays presented herein did not contain appreciable amounts of copper, but other non-reported assays may lead to a reduction in the copper content of less than 0.3% Cu to ensure excellent surface quality of the hot rolled strip. If the copper content exceeds 0.3%, it is advisable to also dop the nickel at least 0.25 times the copper content. Even if copper is not present in the alloy, the amount of nickel is limited to <1.5% Ni.

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Typically, boron may be doped with at least 0.0005% B and at most 0.005% B to reduce grain size and increase hardening. Typically, titanium from its alloy may be at least 0.01% Ti and at most 0.1% Ti to bind nitrogen N and prevent the formation of boron nitrides BN, since boron nitride reduces the potency of boron as a hardener and grain size reducer.

Especially at the lower carbon content limit, the steel according to the invention is highly edgeable, i.e. bendable and weldable, for example, by means of high-frequency welding, i.e. HF welding, without additive. In addition, it has been found in test fabrication 15 that the material is well suited for the manufacture of both open sections and HF-welded pipe beams.

According to the invention, the steel is produced at a final rolling temperature of between 860 ° C and 960 ° C for a final thickness of 2 mm to 12 mm. Cooling of the strip is started 20 no later than 15 seconds after the last rolling mill and cooled rapidly, with a cooling rate of at least 30 ° C / s, to a low winding temperature of 20 ° C to 520 ° C. The result is typically an almost completely bainitic and / or martensitic microstructure with at least 95% by volume of bainitic and / or martensitic. In the winding temperature range 20 ° C to 100 ° C, martensite is unleashed when: 'j25 whereas at a winding temperature of 100 ° C or more, martensite is discharged; | for example, in the range of 100 ° C to 200 ° C, the martensite is slightly discharged and in the winding temperature range of about 200 ° C to 520 ° C, the martensite is released and precipitated.

··, Although the winding was carried out in the lower emission brittle range, 200 ° C to 400 ° C, or the cooling was carried out through the range, the brittleness was not observed with this combination of manufacturing method and composition. The tensile strength Rm becomes about 700 MPa to 1500 MPa and the yield strength Rp0.2, i.e., the strength at 0.2% elongation is about 600 MPa to 1400 MPa. The elongation at break, A 5, which is a fixed: f, a measurement length relative to the thickness of the sample rod, is then about 6% to about 18%. The yield ratio is typically in the range of 0.8 to 0.96.

.35v [In particular, if desired for "wear resistant" surface hard plates, the carbon content of the MM f may be provided in the range of 0.12% to 0.16% C and the hot rolled steel strip will then directly quench to a winding temperature of 20 ° C. ° C - 400 ° C.

114484 6

The quenching can be performed either at low winding temperatures of 20 ° C to 100 ° C, or preferably at winding temperatures above 100 ° C but still below 400 ° C, whereby residual stresses are reduced or eliminated without affecting the hardness of the wear steel. Thus, a relatively low winding temperature range of 5,100 ° C to 200 ° C may be applied, for example, to thinner tapes, or a slightly higher winding temperature range of 200 ° C to 400 ° C, e.g. to thicker tapes. If, on the other hand, more structural steel-like properties are desired, the carbon content of the steel is monitored between 0.08% and 0.12% C and the hot rolled steel strip directly quenches to a winding temperature of 20 ° C to 520 ° C. In this case as well, quenching can be performed at low winding temperatures between 20 ° C and 100 ° C, or for the same reason as above, preferably at winding temperatures above 100 ° C but still below 520 ° C. For example, a relatively low winding temperature range of 100 ° C to 200 ° C may be applied to thinner tapes and, for example, a slightly higher winding temperature range of 200 ° C to 520 ° C for thicker tapes. However, in the case of "structural steel", i.e., in the carbon content range of 0.08% to 0.12%, variations of the order of magnitude in the winding temperature have a relatively minor effect on the properties of the steel strip, while remaining good regardless of the winding temperature.

20 Sample section

Example 1. In the laboratory, conventional tempering tests were carried out with composition ai, Y: see Table 1, by heating samples 8x100x250 mm in an oven for 20 minutes and at 900 ° C. The samples were quenched in water and the effluent was spawned for 2 h at different temperatures. The results are shown in Table 2. The results indicate that the material has a low toughness range at temperatures between 250 ° C and 350 ° C.

. ·,: On the other hand, the elongation at well above 400 ° C is significantly increased, whereby the strength also begins to drop.

30 Table L Test compositions

1 le I Si Μη 1P Is I AI I N 1 Cr 1 Mo 1 Ti I B

, .. 'I steel A! lal 0.098 0.22 0.71 0.008 0.004 i 0.030 0.005 0.94 0.20 0.032 0.002: | a2 0.086 0.28 0.77 0.008 0.003 0.024 0.005 0.82 0.27 0.032 0.002 Y | _a3_ 0.083 0.21 0.77 0.010 0.003 0.033 0.005 1.04 0.27 0.036 0.002 •.,. · j steel B i

! bl 0.140 0.26 0.81 0.110 0.003: 0.027 0.006 0.65 0.21 0.038 0.002 I

V: b2 0.146 0.23 0.82 0.006 0.003 0.032 0.007 0.88 0.27 0.036 0.002 j b3 0.135 0.23 0.90 0.009 0.004 1 0.035 0.006 0.88 0.27 0.038 0.002: '': b4 [ 0.130 0.25 0.84 0.008 0.002 10.032 0.005 1.06 0.28 0.037 0.002 | 114484 7

Table 2. Emission test results for composition ai.

Emission Time Rp0.2 Rm A5 Charpy-V, J / cm2 toughness,% _ _ jb_N / mm2 N / mm2% _ (-20 ° C) (-40 ° C) (-20 ° C) | (-40 ° C) *) 972 1072 12.6 20 5 100 2 897 1123 11.7 133 85 40 15 150 2 913 1125 12.0 172 72 65 10 200 2 922 1113 12.4 122 50 40 10 250 2 938 1112 12.2 36 26 10 10 300 2 928 1086 11.7 55 28 10 5 350 2 963 1064 11.8 115 27 40 10 400 2 971 1049 12.6 93 58 20 15 450 2 911 960 14.2 218 85 80 15 500 2 822 901 15.1 251 216 98 80 600 2,741 773 17.3 334 329 100 98 [700 | 2 [430 [528 21.2 430_ [451 _ [_ lp0_ [l00_ *] only shut down 5

Production scale direct fire extinguishing tests at low carbon level

Example 2. Tape roll was rolled with a2 6 mm thick moon-10 manna by direct quenching to the winding temperature TCO1. The results are shown in Table 3.

The results show that, even when winding, as shown in Example 1, in the brittle temperature range of 300 ° C, excellent ductility is still obtained. The strength and elongation do not differ much from Example 1. The material edging test results are shown. .15 in Table 4.

• ·

Example 3. Tape rolling mill roll was rolled with a2 composition 3mm thick hot tub by direct quenching to a winding temperature TCoil. The results are shown in Table * 3.

The results show that the same mechanical properties as in Example 2 were, however, achieved even at winding at a significantly higher temperature * ... · 450 ° C.

Example 4. Tape rolling mill was rolled with a2 composition by direct quenching of 4 mm hot roll to TCoil · The results are shown in Table -255.

The results show. however, even when winding at a significantly lower temperature, i.e., 100 ° C, the same mechanical properties as in the example T and 2 were achieved.

114484 8

It can be concluded that this steel composition and method of production achieves a homogeneous material which is insensitive to winding temperature variations.

Example 5. 5 strips of 10 mm thickness were rolled with a3 roller by direct quenching to a TCoil winding temperature. The results are shown in Table 3.

The results show that the strength and impact strength are slightly reduced, but the properties are still excellent provided the winding temperature does not exceed about 500 ° C.

10 Table 3. Mechanical properties of the strip obtained in rolling tests _______ Longitudinal ___ Transverse _ Steel Thick. width TCoil Rp0.2 Rm Y / T A5 HB ChV Rp0.2 Rm A5 mm mm ° CN / mm1 N / mm2% -40 ° C, N / mm N / mra% _________ j / cm2 2 _: __ ai 8 a1 971 1049 0.93 12.6 57 ai 8 b1 897 1123 0.80 11.7 25 a2 3 1000 460 958 1030 0.93 10.9 304 925 1016 10.5 a2 3 1000 450 971 1014 0.96 11.8 299 977 1056 9.9 a2 4 1000 100 977 1117 0.87 13.3 329 987 1130 11.6 a2 6 1000 200 934 1078 0.87 12.8 240 920 1070 9.9 a3 10 1250 520 748 874 0.86 13.0 71 819 899 11.8 a3 10 1250 510 836 901 0.93 13.0 133 896 957 11.0 a3 10 1250 370 853 965 0.88 11.5 171 898 975 9.5 a3 10 1250 320 858 979 0.88 11.1 165 914 1005 10.8 bl 4 1300 470 980 1031 0.95 10.0 304 1051 1071 8.4 b2 4 1500 515 860 1000 0.86 12.4 295 974 1006 9.9 b2 4 1500 530 702 853 0.82 17.4 252 747 847 13.8 b2 4 1500 100 1179 1347 0.88 8.9 396 1189 1308 6.9 b3 4 1250 380 1163 1275 0.91 9.6 375 1162 1294 6.8: Y: b3 4 1250 200 1125 1317 0.85 11.5 387 1130 1333 8.9 [b4 | 6 11250 | 200 1125 11295 0.87 9.5 384 __ | ___ '' a1 Traditional laboratory test: dissolution, quenching in water, p 400 ° C, 2h ·; ·· .1 b1 Conventional laboratory test: leaching, quenching in water, discharge 100 ° C, 2h;. [i5 '···' Production scale direct quenching tests at high carbon level: Example 6: Rolling mill with higher carbon level, composition - • »with b2 and b3 4 mm hot strip by direct quenching to winding temperature '. Coiling temperatures of 100 ° C, 200 ° C and 380 ° C were used in the Tcoil experiments. The results are shown in Table 3.

·; · 1 The results are indicated. that the strength and hardness decrease slightly as the winding temperature rises>, · ί, but the properties are still in the same range as long as the winding temperature does not exceed ·: · 1: about 400 ° C.

25 114484 9

It can be concluded that this steel composition and method of production achieves a homogeneous material which is insensitive to winding temperature variations.

Example 7. The strip mill was rolled with a higher carbon level, 5 bbl and b2 composition hot strip by direct quenching to a Tcoil winding temperature of 470 ° C, 515 ° C and 530 ° C. The results are shown in Table 3.

The results show that the strength and hardness decrease, whereas the elongation increases significantly as the winding temperature rises.

10

Table 4. Edging tests with composition a2, winding temperature 300 ° C

R = __ Longitudinally Waltz. in the direction of__Overall Waltz. direction_ 3h__ok__ok_ 2.5h__ok__ok_ 2h__ok __ (ok), low surface cracking_l, 5h__ok__deep cracking_

It __ (ok), low surface cracking__ deep cracking_ 0,7t 1 (ok), low surface cracking _

Bending Radius = R, Plate Thickness = t 15 »

Claims (6)

10 1 1 4484 1. For a final thickness of not less than 2 mm and not more than 12 mm, hot-rolled steel strip with a microstructure of at least 95% martensite and / or bainite and containing by weight: 0.08% to 0.16% C, 0.5% to 1.5% Cr and / or 0.1% to 0.5% Mo, 0.6% to 1.1% Mn, and 0.1% to 0.3% Si, <0.015% S and <0 , 03% P, 0.01% to 0.08% Al, and the remainder of Fe and non-avoidable impurities, with a tensile strength of the steel strip of 700 MPa to 1500 MPa with an elongation at break of 6% and a yield strength of 600 MPa - 1400 MPa. Hot rolled steel strip according to claim 1, characterized in that the steel further comprises 0.0005% to 0.005% B and / or 0.01% to 0.1% Ti. Hot rolled steel strip according to claim 1, characterized in that it has a yield ratio in the range of 0.8 to 0.96. 15 A process for the production of hot rolled steel strip having a final thickness of at least 2 mm and at most 12 mm comprising by weight: 0.08% to 0.16% C; 0.5% to 1.5% Cr and / or 0.1% to 0.5% Mo; 0.01% to 0.08% AI; 0.6% to 1.1% Mn; 0.1% to 0.3% Si; and the remaining Fe and unavoidable impurities, the process comprising the steps of: - hot rolling a steel strip at a temperature between 860 ° C and 960 ° C to said final thickness; «- direct quenching of this hot rolled steel strip with a delay of no more than 15 seconds •. ' from the last rolling point to a winding temperature between 20 ° C and 520 ° C such that the cooling rate of ':: 25 in direct quenching is at least 30 ° C / sec. »* * · A process according to claim 4, characterized in that the carbon content of the steel is between 0.12% and 0.16% C and the hot-rolled steel strip is quenched directly to a winding temperature of 20 ° C to 400 ° C, or a winding temperature: ·. ·. 30 ° C to 20 ° C to 100 ° C, or 100 ° C to 200 ° C, or 200 ° C to 400 ° C. '!' 6. A method as claimed in claim 4, characterized in that a method is provided. a carbon content of 0.08% to 0.12% C and direct quenching of said hot rolled steel strip to a winding temperature of 20 ° C to 520 ° C. * · ·. From 20 ° C to 100 ° C, or from 100 ° C to 200 ° C, or from 200 ° C to 520 ° C. 114484