KR890002034B1 - Producing method of high tension steel - Google Patents

Abstract

High tension steel with 50 kg/mm2 grade tensile strength is manufactured by hot rolling the steel slab, which consists of 0.05- 0.20 wt.% C, 0.20-0.50 wt.% Si, 1.0-1.6 wt.% Mn, 0.010-0.025 wt.% P, 0.002-0.01 wt.% S, 0.015-0.050 wt.% V, 0.010-0.020 wt.% Ti, 0.0005- 0.0050 wt.% Ca, 0.01-0.080 wt.% Al, 0.0030-0.0070 wt.% N and balance Fe, on condition of a rolling temperature lower than 900 deg.C, total reduction rate more than 50% and finishing rolling temperature lower than 800 deg.C.

Description

Manufacturing method of 50kg / mm2 grade non-tensionable high tensile strength steel

1 is a regenerated thermal cycle curve diagram corresponding to a single layer welding bond portion having a welding heat input amount of 178 KJ / cm.

2 shows welding conditions for electrogas welding with a welding heat input of 190 KJ / cm.

3 is a 2 mm V-shape transition curve in the rolling direction and the dark vertical direction in the base material.

More specifically, the present invention relates to a method of manufacturing a non-coarse high tensile steel having a tensile strength of 50kg / mm2, which is superior in heat input welding characteristics and has no impact toughness.

With recent advances in industrial technology, steel structures such as ships and crude oil tanks are becoming larger and larger, and high-temperature heat welding methods for welding the entire thickness of steel sheets in a single layer for the purpose of high efficiency construction during welding processing of these structures (one-sided SAW, EGW). Adoption, ESW, etc.) is increasing.

However, when the high heat input welding is applied to a conventional high tensile steel sheet, the toughness of the weld heat affecting part, in particular, the bond part is reduced, resulting in a safety problem of the structure. This is because the austenite grains in the bond portion become extremely coarse due to excessive heat input of the weld, and the cooling rate after welding is low, resulting in an intermediate structure having a weak structure.

Therefore, in order to secure bond toughness, development of large heat input welding steel that has limited the welding cooling rate, that is, the amount of heat input for welding, does not deteriorate the bond part toughness even if the heat input welding is performed from the aspect of high efficiency of welding construction. This has been urgently requested.

According to the known technology, the basic manufacturer of high heat input welding steel suppresses the growth of austenite grains by using stable precipitates even at high temperature, and at the same time, these precipitates are used as ferrite nucleus growth plant to promote the ferrite transformation by cooling. Instead of vulnerable mid-level tissues, they are switching to ferrite + pearlite tissues.

For such high heat input welding steels, Tin, BN, etc. are currently used, and in the rolling condition, annealing or quenching is performed in a rolled state, but in this case, the welding bond part is extremely hot at 1350 ° C or higher. Since it loses its performance due to partial dissolution of stable nitrides such as Tin and BN due to heating, there is a disadvantage in that it is not possible to obtain excellent and stable high-input welding properties.

Accordingly, the present invention forms a more stable precipitate that does not dissolve even at high temperatures in addition to nitrides such as Tin and BN by adversely affecting the components of V, Ti, Ca, Al and N and hot rolling conditions, and adversely affects impact toughness. The main purpose is to produce 50kg / mm2 non-tensioned high tensile strength steel with excellent high-intensity welding characteristics and no impact toughness by reducing the amount of MnS.

Hereinafter, the present invention will be described in detail.

The present invention is a method for producing a tensile strength 50kg / mm-class non-tensile high strength steel, the weight% V: 0.015-0.052%, Ti: 0.010-0.020%, Ca: 0.0005-0.0050%, Al: 0.01-0.080% and N: Slab of high tensile strength steel containing 0.0030-0.0070% with 50% cumulative reduction rate at hot rolling temperature below 900 ℃ and hot rolling at finishing temperature below 800 ℃ It is about a method.

In general, the tensile strength of 50kg / mm2 non-coarse high tensile steel is C: 0.05-0.20%, Si: 0.20-0.50%, Mn: 1.0-1.6%, P: 0.010-0.025%, and S: 0.002-0.01% Although the present invention basically includes, V, Ti, Ca, Al and N as appropriately described as to explain the reason for numerical limitation of each of these components as follows.

V is an indispensable element in the present invention with controlled rolling as a whole, and in order to obtain a finer effect by controlled rolling, at least 0.015% by weight (hereinafter referred to as "%") is required to be added, and exceeds 0.05%. If it is lower in weld hardenability, it is preferable to limit it to 0.015-0.052%.

Since Ti inhibits the growth of austenite grains in welding by heating TiN in steel, it is necessary to disperse the fine TiN as much as possible.Addition of at least 0.010% is required to secure the deposition amount of fine TiN. If it exceeds%, coarse TiN precipitates and damages toughness, so it is preferable to limit the component range to 0010-0.020%.

Ca makes extremely stable compounds at high temperatures such as CaO and CaS in steel to inhibit austenite grain growth, and at the same time, promotes ferrite transformation by using these micro-inclusions as nucleation-growing elements during cooling, and in particular, CaS substrates formed instead of MnS inclusions. Water does not stretch in the rolling direction during rolling and maintains a spherical shape to reduce the anisotropy of impact toughness. However, if it is less than 0.0005%, there is no effect of microstructure of bonded part during welding by CaO formation and improvement effect of S-type inclusions. If it exceeds 0.0005%, the amount of inclusions increases significantly, which damages the toughness and cleanliness of the steel sheet. It is preferable to limit to%.

Al is a deoxidation phase element which is essentially contained in the same steel as the present invention steel, and less than 0.01% deoxidation is insufficient, so the toughness of the base material decreases, so the lower limit is 0.01%. Since the toughness decreased, the upper limit was made 0.08%.

N is combined with Ti to precipitate as TiN to inhibit austenite formation growth during welding heating. For fine TiN deposition, the content of N is 0.0030- in the Ti component range of 0.010-0.020% of the present invention steel. It is preferable that it is 0.0070%.

Hereinafter, the hot rolling conditions will be described.

In the hot rolling of the present invention, the cumulative reduction rate is 50% or more at 900 ° C. or lower. As the cumulative reduction ratio at 900 ° C. or lower is increased, the fineness of the ferrite grains is promoted and the toughness is improved, so the lower limit thereof is limited to 50%. The upper limit is limited based on the load of the rolling mill and the occurrence of surface defects.

Here, the cumulative reduction ratio is expressed as [(to-t) / t] × 100 (%) when the plate thickness is at 900 ° C. and the plate thickness in the final product state is t after finishing rolled paint. do.

In addition, the finishing temperature is as low as possible, the ferrite grains are fine grained to improve the low-temperature toughness, so the upper limit is set to 800 ℃, the lower limit is limited so that hot rolling is carried out in the ferrite and austenite two-phase region.

The steel of the present invention manufactured as described above uses Ca-based inclusions by adding Ca as a complement factor of TiN precipitates partially dissolved in the high temperature region (1,350 ° C or higher), and Ca is added as CaC ₂ or Ca-Si in molten steel. If Ca is added first, it is used for deoxidation and slack reduction of molten steel to produce Ca, and Ca bubble (Ca medium atmospheric pressure is higher than atmospheric pressure at molten steel. Pca = 1.8tm at 1600 ° C) is desulfurized while floating The reaction takes place and finally forms CaO and CaS inclusions.

These inclusions do not elongate during hot rolling and remain spherical and fine in size up to 10μ.

The stability at high temperatures of these CaO inclusions can be described as standard free energy.

In other words, standard free energy between 1000 ℃ and 1500 ℃ is -51--41Kcal / mol for TiN, whereas CaO is -120--106Kcal / mol, which is about 2.4-2.6 times smaller than TiN. It can be seen.

Therefore, assuming that the N, O and Ti and Ca activity coefficients are the same, the solubility of CaO = (Solubility of TiN 2.4-2.6).

Since the solubility of TiN at 1350 ° C. is about 10 ⁻⁶ , the CaO solubility at that temperature is very small at about 10 ^{−12, which} is more stable than that of TiN at high temperatures.

Therefore, even in the weld bond portion heated to 1350 ° C. or more, CaO does not dissolve but complements TiN, which partially dissolves, thereby suppressing austenite grain growth and simultaneously acting as a nucleus of ferrite transformation during cooling to promote ferrite transformation.

In general steel, sulfide inclusions are mostly present as MnS, and the MnS inclusions are elongated in the rolling direction when the steel sheet is rolled, and finally the impact toughness in the perpendicular direction of rolling is significantly lowered, resulting in the impact toughness. Acts as.

Therefore, improvement of the impact toughness anisotropy of the steel of the present invention is achieved by converting the MnS inclusions into CaS inclusions by adding Ca, thereby reducing the inclusions of MnS.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

The composition of Table 1 is hot-rolled under the hot rolling conditions of Table 2 and measured mechanical properties are shown in Table 2 below.

TABLE 1

TABLE 2

In the above, the welding part impact toughness test of each steel material is measured after the impact toughness of the base material, in the case of the invention material and comparative materials 1, 2, 3, 1350 ℃ corresponding to the weld bond part by using the welding heat cycle reproducing apparatus After rapid heating, it cools at the cooling rate equivalent to 178KJ / cm of welding heat input, and reproduces the heat input welding.In the case of comparative materials 4 and 5, the actual heat input welding is performed with the welding heat input of 190KJ / cm by electrogas welding. After the test, a 2 mm Vyscharpy impact specimen was produced from each steel and subjected to an impact test.

The welding heat cycles applied to the invention and the comparative materials 1, 2 and 3 and the welding conditions applied to the comparative materials 4 and 5 are shown in FIGS. 1 and 2, respectively.

As shown in Table 2, the present invention has a tensile strength of 50kg / ㎜ or more, it can be seen that the impact toughness and the weld impact impact toughness of the base material compared to the comparative material.

In addition, the V Charpy transition curve of 2 mm in the rolling direction and the rolling right direction of the invention and the comparative materials 4 and 5 in the steel of Table 2 was measured and shown in FIG.

Comparative materials 4 and 5 shown in FIG. 3 show great anisotropy, while the present invention has little anisotropy.

As described above, the present invention is to control the composition of the steel and the hot rolling conditions, the tensile strength of 50kg / ㎜ or more, but also has excellent effects of heat input welding characteristics and has the effect of producing non-tensioned high tensile strength steel without impact toughness.

Claims (1)

Tensile strength 50kg / mm2 class of non-coarse high tensile steel, in weight%, C: 0.05-0.20%, Si: 0.20-0.50%, Mn: 1.0-1.6%, P: 0.010-0.025% and S: 0.002 High tensile steel formed by adding V: 0.015-0.050%, Ti: 0.010-0.020%, Ca: 0.0005-0.0050%, Al: 0.01-0.080%, and N: 0.0030-0.0070% to a common high tensile steel including -0.01% The slab of the hot rolling temperature of 900 ℃ or less has a cumulative reduction rate of more than 50% and hot rolling to a finishing temperature of 800 ℃ or less, characterized in that the tensile strength 50kg / ㎠ class non-coarse high strength steel manufacturing method.

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