CS196236B2 - Method for thermal treatment of weldable constructional steel with high tensile strength - Google Patents

Abstract

1,253,552. Heat treating alloy steel. MITSUBISHI JUKOGYO K.K. Jan.31, 1969 [Jan.31, 1968], No. 5364/69. Heading C7A. A steel containing in percentage by weight:- is heated above the A 3 temperature; when the combined content of Mo + Cr + Mn + Ni is less than 3.2% the steel is cooled from 800‹ to 500‹C. in 2.1 to 54 seconds followed without interruption by cooling from 500‹ to 200‹C. in more than 15 seconds; when the combined contents are from 3.2% to less than 3.8% the corresponding periods are 2.3 to 70 seconds and more than 20 seconds, and when the combined contents are 3.8% or more the periods are 2.4 to 80 seconds and more than 24 seconds. The steel may then be tempered.

Description

The invention relates to a method of heat treating a weldable structural steel of high tensile strength having a composition in% by weight of 0.05 to 15% carbon, 0.05 to 0.6% silicon, 0.1 to 1.4% manganese, 0.5 up to 4.5% nickel, 0.1 to 1.4% chromium, 0.1 to 0.8% molybdenum, the sum of the amounts of manganese, nickel, chromium and molybdenum being in the range of 1.6 to 4.2%, and further comprising 0.01 to 0.09% aluminum and 0.001 to 0.15% titanium individually or in combination with each other and optionally traces up to 0.12% vanadium, traces up to 0.04%. niobium and traces of up to 0.005% boron, individually or in combination, with the remainder being iron and impurities.

As illustrated in Table 1, high tensile strength steels for welded structures are typically obtained by quenching and tempering after heat treatment, thereby increasing their tensile strength and notch toughness, and their structure is tempered martensite.

These steels with high tensile strength have a high elongation ratio, i.e. the ratio of the yield point to the tensile strength in this conventional quenching and tempering treatment, and after reaching the yield point, their deformation and absorbed energy are greatly reduced until rupture, resulting in little o2 Strength against concentrated stress on structures.

If the degree of safety is evaluated on the basis of the stretching ratio, a high degree of safety by increasing the stretching ratio should be ensured in practical use.

However, if the yield ratio increases, the permissible stresses of such steel must be low, while at the same time the thickness of the plates of the welded steel structures must be very large so that the structures are heavy. Such steel, with high tensile strength obtained by hardening and tempering, must be further tempered at high temperatures, for example above 600 ° C, in order to increase its notched toughness.

As a result of this tendency, the alloying element content of such steel must be high relative to the required strength.

Thus, as the steel strength increases, the carbon equivalent, hereinafter referred to as Cekv, is increased, and for which Cekv == = CJl / 24 Si + 1/6 Mn + 1/40 Ni + 1/5 Cr + + 1/4 Mo- j-1/14 V.

As the tendency of the heat-affected zone to harden increases, the sensitivity to cracking in the weld also increases, with the result that the pre-heating temperatures for such steel must also be high, as shown in Table 1, to avoid cracking in the weld.

The above-mentioned disadvantages of the prior art steels are eliminated by a method of heat treatment of steel consisting of 0.05 to 0.15% C, 0.05-0.6% Si, 0.1-1.4% Mn, 0.5- 4.5% Ni, 0.1 - 1.4% Cr and 0.1 - 0.8 why. Mo, and in addition 0.01% to 0.09% Al and / or 0.001% to 0.15% Ti, the Mn- | -Cr + Mo value being in the range 1.6-4.2% and the remainder being Fe and unavoidable impurities according to the invention, characterized in that the steel is heated to 850 to 980 ° C above the transition point Аз and then cooled in a period of 2.1 to 80 seconds from 800 to 500 ° C and thereafter at least Cool for 15 seconds, maximum 1200 seconds, from 500 to 200 ° C.

According to another embodiment of the invention, the steel is then tempered at temperatures of 500 to 680 ° C below the transformation point Ai.

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2) Cekv = C + 1/24 Si + 1/6 Мц + 1/40 Ni-ψ 1.5 Cr + 1/4 ΜοψΙ / 14 V.

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This results in a steel with a high tensile strength bainitic structure for welded structures. .....

Since the invention has overcome the known discovery that it is difficult to produce steel with a wide range of tensile strength due to the assumed low notch toughness of the bainitic structure, the invention is progressing to produce a high tensile, bainitic, high-strength steel. notch toughness .; and with a low tendency to form cracks in the weld.

The steels heat treated according to the invention can of course be used, for example, for the production of steel plates, wrought iron products, cast steel, shaped steel such as steel tubes, rod steel and steel wires.

The composition of the steel and its heat treatment according to the invention will be explained in more detail with reference to the accompanying drawings. ?

Fig. 1 is a graph showing the relationship between the amount of Ni and the energy of the treated steel absorbed in the Charpy Notch V-Notch Impact Test; Fig. 2 is a graph showing the relationship between Mn + Ni -j-Cr-J-Mo Fig. 3 is a graph showing the relationship between the amount V, yield strength and tensile strength of the steel treated according to the invention; Fig. 4 is also a graph showing the relationship between The amount of V and the absorbed energy of the steel treated according to the invention in the Charpy Notch Impact Test / Fig. 5 is a diagram for converting the steel by continuous cooling following heat treatment according to the invention; Fig. 6 is a graph showing the relationship between f-Ni-f- Cr + Mo and between the cooling time of the treated steel according to the invention, Fig. 7 is a graph showing the relationship between the carbon equivalent and the mechanical 8 and 9 are images showing the microstructure of the steel treated according to the invention.

The steel treated according to the invention contains in weight% 0.05 to 0.15% C, 0.05 to 0.6% Si, 0.1 to 1.4% Mn, 0.5 to 4.5% Ni 0.1 to 1.4% Cr, and 0.1 to 0.8% Mo, and in addition 0.01 to 0.09% Al or 0.001 to 0.1% Ti, and, if necessary, an additive or two of these elements not more than 0.12% V, not more than 0.04% Nb and not more than 0.005% B. ·

When the C content is greater than 0.15% by weight, the zone affected by the welding heat can harden, cracks in the weld and a martensitic structure is formed as a result of the heat treatment, for which reason the C content must be less than 0.15% and greater than 0.05. bearing in mind the need for high tensile strength. .

When the Si content is in excess of 0.6% by weight, the weldability of the steel deteriorates, taking into account the

In general, greater than 0.05%, the amount will be. Ranging from '0.05' to 0.6 percent. ^:

Similar to C, Mn is an effective alloying element for promoting the tensile strength of steel, but increasing its content deteriorates the weldability of the steel as well as increasing the carbon content.

For this reason, the Mn content in terms of attainable tensile strength must be greater than 0.1% and at the same time in terms of weldability - OT - less than 1.4% to avoid martensitic formation. structures during heat treatment. 4

Although 'Ni', 'how'. It is known that an alloying element is effective for increasing. . notch toughness, its effective amount has a certain limit.

Giant. 1 shows the effect of Ni content on notch toughness. · After the heat treatment according to the invention. .

In this drawing, the abscissa indicates the Ni content by weight ·% and the ordinate is the absorbed energy VEO in the J Charpy notch test; · Impact at notch V with a depth of · 2 'mm. From the graph it can be seen that the addition of Ni in an amount of more than 0.5% acts to increase notch toughness, but that. the additive in an amount greater than 4.5% is in. 'this' look back 'ineffective as follows from the bottom'. curve, so that the content of µM1 is determined by the range 0.5 'to 4.4%. · '

The Cr content must be greater than 0.1% by weight to be. · Created a 'bainitic structure and increased' strength. strength, but greater amounts meso '1.4'% 'would cause the' deterioration svařitel.ncštt 'steels and besides' this can not be expected. ·'_increases; the tensile strength of the heat treatment according to the invention in this case, so that the Cr content is determined by an interval of 0.1 to 1.4%.

The Mo content must also be greater than 0.1% by weight in order to create a bainltic structure and to increase the tensile strength, but an amount greater than 0.8% Mo would cause the zone to harden. . affected by the welding heat and to the deterioration of the weldability, and at the same time, as in the case of Cr, ηθίζθ expected at heat. treatment according to the invention increases the tensile strength so that the Mo content will be in the range 0.1 to 0.8 percent. .

Giant. 2 shows the influence of the composition of the steel on the strength, the abscissa being the percentage by weight of the amount of Mn 4 Ni-J-Cr-J + Mo as a parameter and the ordinate indicates the yield point and tensile strength. The curve interlaced with empty circles indicates the tensile strength curve, the curve interlaced with empty triangles indicates the yield strength curve. As can be seen from this relationship, the sum-in% by weight of the Mn-j-Ni + + Cr + Mo must be greater than 1.6%. optionally greater than 2%; Higher than 3.8%, resp.

higher than 4.2% in order to achieve a tensile strength higher than 686 · MPa, respectively. taller than

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785 MPa, resp. higher than 883 MPa, resp. higher than 981 MPa.

The Al content must be in ° / weight greater than 0.01-0.09 o / o in order to dehumidify and produce fine crystalline particles in steel production, but a content greater than 0.09 O / o AI would result in a reduction in the notched toughness contrary to the original purpose, whereas less than 0.01% Al would have no effect at all.

Ti with almost the same effect as Al can also be used to achieve deoxidization and formation of fine crystalline particles. In this case, the Ti content preferably remains in the range of 0.001 to 0.15%. It is also possible to use combinations of both Al and Ti elements for this purpose.

Although the element V acts to increase the tensile strength, too much of the element can reduce too much notchiness.

In Fig. 3, the abscissa indicates the V content by weight and the ordinate indicates the yield point and tensile strength in MPa, and it can be seen that both the yield point and tensile strength increase as the amount V increases. in tension, a curve interlaced with empty triangles indicates the yield point.

Giant. 4 illustrates the relationship between the V-content and the notch toughness in the curve interlaced with the empty circles, where the line indicates the V-content by weight and the ordinate indicates the absorbed VEO energy (J) in the Charpy Notch Impact V notch test at 2 mm. It follows from this relationship that the V content, which does not induce a reduction in notch toughness, must be less than 0.12% by weight, for which reason this 0.12% content is considered to be adequate for the purposes of the invention.

Also, the elements Nb and В act to increase the tensile strength, but their too high content would reduce the notch toughness, so that the Nb content is determined to be less than 0.4 wt. % and a V content of less than 0.005 wt. ° / o, which are the limits for preventing a significant reduction in notch toughness.

Some unavoidable impurities are also present in the steel composition.

The heat treatment of the steel according to the invention will now be explained in detail.

Giant. 5 is a graph showing a CCT transformation diagram for continuous cooling of 900 tet. Celsius for steel containing 0.11% C, 0.20% / Si, 0.28% Mn, 2.51% Ni, 1.12% Cr, 0.28% Mo and 0.025% AI. On the line, the cooling time is applied in seconds, in logarithmic division, from 800 ° C and the ordinate indicates temperatures in degrees Celsius, linear division, for the steel transformation regions, A being the austenite region, F the initial ferrite region, V the bainite structure region, and The M region of the martensitic structure, the line a-b-c shows the start point of the martensitic structure transformation, the line d-e approximately the end point of the martensitic structure transformation, and the e-f curve the end point of the bainitic structure transformation.

The cooling curve 1 represents the critical cooling curve to form the initial ferrite, the cooling curve 2 represents the critical cooling curve for the conversion of the entire structure to bainlitter, and the cooling curve 3 represents the critical cooling curve for the conversion of the entire structure to martensitic.

From this CCT transformation diagram it can be seen that the initial ferrite is formed at cooling slower than the cooling curve 1, the tensile strength and the notch toughness decrease, that the entire structure is converted to martensite at cooling faster than the cooling curve! it will have a high tensile strength but considerably reduce notch toughness, and that the bainite and martensite structure will be formed by cooling between curves 2 and 3, but no increase in notch toughness due to martensite admixture can be expected.

In order to provide both satisfactory tensile strength and simultaneously desirable notch toughness, it is important to form a bainitic structure, whereby the steel must be cooled within a cooling curve of 1 and 3 to 500 ° C, which is approximately the temperature of the initial martensitic point.

The cooling time for cooling from 800 ° C to 500 ° C must be between S3 seconds and seconds. Cooling conditions above 500 ° C below must, as is well known, have a relationship to martensitic conversion. It has thus been found that from the critical cooling curve 2 in order for the entire structure to become bainite, the cooling time for cooling from 500 ° C to 200 ° C must be S2 seconds, so that during cooling longer than S2 seconds from 500 ° C to 200 ° C creates no martensitic structure at all.

Due to the above conditions, the cooling time must be from 800 to 500 ° C. C between S3 and Si sec, and then the cooling time from 500 to 200 ° C must run for more than seconds to convert the entire structure to bainitic by appropriate conversion.

These critical cooling time data are related to the composition of the steel; Table 2 therefore shows the values of S1, S2 and S3 and the composition for the different steels of the invention as obtained from the CCT transformation diagram for continuous cooling.

Giant. 6 shows the relationship between the chemical composition of the steel and the periods S1, S2 and S3, where the line defines as a chemical composition parameter the percentage value of the sum of Mn + Ni + Cr + Mo and denotes in seconds the time of Si, S2 and Ss in logarithmic division relationship.

From the relationship between the percentage

Mn -j- N1 -f- Cr -j- Mo and tensile strength,

19.9.23.6, as shown in FIG. 2, the cooling conditions required to form the bainitic are obtained. % of the structure in the ratio to the percentage range in the sum of Mn, -J-Ni Cr, -4 Mo as follows:.

If the sum of · Mn + Ni - Cr + Mo, · is in the range of 1.6 to 3.2 wt. . then the cooling time from 800 to 500 ° C will be in the range of 2 to 1. 54 · seconds' and · continuous cooling time, from 500 ° C to 200 ° C will be greater than 16 seconds ....

If the sum of Mn. · + Ni · 1 H-Cr-1 · m is in the range of 3.2 to 3.8 wt%, cooling time from 800 to 500 ° C will be in the range of 2.3 - 70 seconds and a continuous cooling period from. 500 at 200 ° C will be higher than 20 seconds. ·

If the sum of Mn -j-Ni- · Cr -j- · Mo is in the range 3.8 to 4.2% by weight, the cooling time will be for cooling. . ' at 500 ° C · · v, ·. the range of 2.4 to 80 seconds and the continuous cooling time from 500 to 200 ° C will be longer than 24 seconds. :.

Thus, heat treatment according to the invention can be achieved. Fine b.Altimal structures · Steel and · Guarantee. sufficient tensile strength as well as notched toughness. ''.

If greater notch toughness is required, this is achieved by tempering the steel at temperatures below the transformation point Ai. ,. . . ... . ·.

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Thus, it is apparent that similar tempering treatments may be employed if desired.

Several exemplary embodiments of the invention will now be explained in more detail.

Table 3 contains the chemical composition, heating conditions and mechanical properties for the various types of steel according to the invention.

Although the steels according to Table 3 are heat treated under the conditions specified by the invention, none is carried out. tempering in order to clearly show that high tensile strength and high notch toughness can be achieved by the mere processing of the invention.

In contrast, Table 4 contains several embodiments in which the steels are cooled by the process of the invention and then tempered.

As is apparent from these embodiments, notch toughness can be improved by tempering and, moreover, it is possible to improve notch toughness without reducing the tensile strength, depending on the temperatures used for tempering.

Giant. 7 depicts the relationship between the Cekv carbon equivalent. and the yield point, the tensile strength, and the yield ratio, with a segment of · Cekv. And, on the left ordinate, the yield point, tensile strength, and on the right ordinate in unnamed units, the elongation ratio to both demonstrate the relationship between them and to make a comparison with high tensile strength steels which have been hardened and tempered according to conventional procedures.

The curve 1 in FIG. 7 shows the yield strength of the steel treated according to the invention, the curve 2 the tensile strength of this steel and the curve 3 the yield ratio for the same steel, while the curve 1 'shows the yield strength of conventional steel; steel and curve 3 'shows the stretching ratio for the same steel.

Of these relationships is at the same Cekv. see that the yield point and tensile strength of the steel treated according to the invention is higher than that of ordinary steel, whereas the yield ratio of the steel according to the invention is higher than that of ordinary steel. of the invention is. lower than the stretching ratio of ordinary steel. ^: ·

It is further seen that Cekv. for steel treated according to the invention it may be less than Cekv. for ordinary steel, the same tensile strength.

In other words, at the same degree of tensile strength, the zone of steel treated according to the invention which has been affected by the welding heat will be less cured by the welding heat, and thus its tendency to crack in the weld will be less. Than ordinary steel, whereby the steel treated according to the invention will have a better weldability than ordinary steel.

The fact that the yield ratio of the steel treated according to the invention is lower than that of ordinary steel is the reason that the steel treated according to the invention is more reliable at concentrated stress compared to ordinary steel so that a lower safety factor can be achieved and · higher permissible stresses · when designing structures. |

To give an example of the microstructure of the steel, the structure of steel No. 8 and the steel No. 18 shown in Table 3 are shown in Figures J3 and 9, respectively, at 500 magnification.

It is clear from these microstructures that the steel treated according to the invention has a fine bainitic structure.

Table 5 shows the maximum hardness ... of the zone affected by the heat of welding for steel No. 18 at a tensile strength of 981 MPa and given the percentage of root crack cases in the weld cracking test limited to groove Y.:

These results immediately indicate that the hardening properties of such a zone affected by the welding heat are considerably low to a high tensile strength of the order of 981 MPa, so that a crack in the weld can be perfectly prevented if it is carried out. Preheating · at 70 ° C using normal arc welding.

In comparison with conventional tensile strength steels of the order of magnitude of 981 MP.a according to Table 1, the steel according to the invention has a considerably better weldability.

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Table 3 - continued

Marking of steel T ¹ ) (° C) N _A ² ) (s) ⁵ ) (s) Yield stress MPa Tensile strength MPa Prodlou- Kontrak- VEO (J) country (%) ⁴ ) ce ( ^u / o) 1 900 16 328 600 720 32.3 68.0 116 2 900 12 460 621 718 31.5 - 212 3 900 15 Dec 362 572 764 31.5 - 149 4 900 21 152 615 746 18.0 65.2 91 5 900 25 257 587 696 19.5 67.5 94 6 900 18 128 670 771 18.0 65.2 94 7 900 35 92 660 767 15.5 69.7 147 8 900 42 181 627 758. 17.5 70.8 144 9 900 48 257 607 730 17.0 73.9 171 10 900 28 282 657 779 15.5 69.7 138 11 900 32 880 801 916 14.0 63.9 119 12 900 26 780 972 911 14.0 69.7 144 13 900 32 910 795 927 15.0 66.4 133 14 900 49 460 643 715 15.0 68.6 122 15 Dec 900 45 481 683 775 17.0 68.6 133 16 900 36 252 942 1068 14.0 66.4 161 17 900 42 322 837 1023 29.3 - 65 18 900 35 980 851 1003 19.5 67.3 133 1.1 T: temperature of austenitization, ² ) S _A : time cooling from 800 to 500 ° c, 3) S _B : time cooling from 500 up to 200 ^e C ⁴ ) length of the tested person sample = = 50 mm. Table 4 ' no. C Si Μη P WITH N1 Cr Mo AI Cekv. T ¹ ) Sa ² ) S _b 3) (° C) 18 · .11 .20 .28 0.12 .006 2.51 1.12 .28 0.25 i .47 900 35 980

continued ČÍS. Tempering temperature (° C) Yield stress MPa Tensile strength MPa . Elongation (%) Kontrak- ce (° / o) VEO (J) VE-50 (J) 18 580 739 8®4 · 27: 5 · 71.0 223 226 500 830 983 22.3 68.5 150 57

(i) T: austenitization temperature; ( ² ) Sa: cooling time from 800 to 500 ° C;

³⁾ _B: the cooling time from 500 to 200 ° ^C: the sample to test for tensile strength had diameter 14 mm, length 50 mm. z z;  ”· - Table 5 Ref. Cekv. Limit Pev- For- Kon- Maxlmál- Preheating temperature a Pre- through- nost dl- hardness percentage of cracks- hřívací tažnos- in turn getting married . (%) bands in the test for temperature ti MPa (.%) zasaže- Y groove pro za- MPa of the without over 50 ° C 75 ° C mezení I say deh warming cracks in e- · - -heat . '·· root Hv 18 .47 851 1003 19.5 67.3 358 100 20 0 75

The tensile test sample had a length of 50 mm.

Claims (3)

A method of heat treating a weldable structural steel having a high tensile strength of 0.05 to 15 wt% carbon, 6.05 to 0.6 wt% silicon, 0.1 to 1.4 wt% manganese, 0.5 4.5 weight% nickel, 0.1 to 1.4 wt · ^l0 / o chromium, · 0.1 to 0.8 wt% molybdenum, with the sum of the amounts of manganese, nickel, chromium and molybdenum is in the range 1 6 to 4.2% by weight and further comprising 0.01 to 0.09% by weight of aluminum and 0.001 to 0.15% by weight of titanium individually or in combination with each other and optionally traces up to 0.12% by weight of vanadium, traces up to 0.04 wt% niobium and traces up to 0.005 wt% boron, singly or in combination with each other, the remainder being iron and impurities, characterized in that the steel is heated to 850-980 ° C above the conversion point As and then cooled in for a time of 2.1 to 80 seconds from a temperature of 800 to 500 ° C and then, at a time at least 15 seconds, maximum 1200 seconds, are cooled from 500 to 200 ° C. 2. A method according to claim 1, characterized in that the steel is then tempered at temperatures of 500 to 680 [deg.] C. below the transformation point Ai. 4 sheets of drawings