KR100415671B1 - A TENSILE STRENGTH 80kg/㎟ GRADE HOT ROLLED STEEL SHEET WITH SUPERIOR FATIGUE PROPERTY AND A METHOD FOR MANUFACTURING IT - Google Patents

Abstract

The present invention relates to a high-strength hot-rolled steel sheet and a method of manufacturing the same, and by appropriately controlling the steel component and the final structure, tensile strength of 80 kg / mm2 class with excellent fatigue characteristics simultaneously with excellent strength and durability (fatigue limit / tensile strength) It is an object of the present invention to provide a hot rolled steel sheet and a method of manufacturing the same.

In the present invention, C: 0.05 to 0.12%, Si: 0.01 to 0.3%, Mn: 1.0 to 2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.01 to 0.05%, Ti: 0.05 to Tensile strength 80kg / mm2 hot rolled steel sheet with excellent fatigue properties containing 0.20%, Nb: 0.02 ~ 0.035%, Mo: 0.1 ~ 0.3%, N: 80 ~ 110ppm, balance Fe and other unavoidable impurities, This is a technical point.

Description

Tensile strength 80 ㎏ / ㎠ class hot rolled steel sheet with excellent fatigue characteristics and manufacturing method {A TENSILE STRENGTH 80 kg / ㎠ GRADE HOT ROLLED STEEL SHEET WITH SUPERIOR FATIGUE PROPERTY AND A METHOD FOR MANUFACTURING IT}

The present invention relates to a hot rolled steel sheet having excellent fatigue characteristics applied to structural materials such as automobile wheels and frames, and a method for manufacturing the same. More specifically, by appropriately adjusting the steel component and the final structure fraction, an excellent durability ratio of 0.6 or more can be achieved. Degree / tensile strength) and tensile strength of 80kg / mm2 grade hot rolled steel sheet having excellent fatigue properties and a method of manufacturing the same.

Hot rolled steel sheet used as structural material for automobiles is mainly used for tensile strength 60㎏ / ㎠ class hot rolled steel sheet. In this structural hot rolled steel sheet, in addition to strength, bending characteristics in frame processing and fatigue characteristics to maintain stability as a structure during operation are required. It is becoming.

Recently, the tensile strength of hot-rolled steel sheets used in automobile structures is required up to 80 kg / mm2 class in order to achieve lightweight automobiles, because such high strength manufacturing makes it possible to reduce automobile weight due to thickness reduction.

However, the conversion to a high strength steel sheet can be achieved by controlling the chemical composition and the microstructure, but since the fatigue properties are not improved as much as the increase in tensile strength, it is necessary to improve the fatigue properties with the increase of the strength.

On the other hand, automotive wheels are manufactured by press molding and welding of rims and disks. In these processes, factors that can affect the fatigue properties of materials are included in the inclusions and precipitates inside the tissue during molding. This can be summarized as the occurrence of microcracks, residual stresses during molding or welding. Since the microcracks formed by inclusions and precipitates in the double tissue may be different depending on the strength and the microstructure, their influence on the fatigue properties of the material.

In general, it is important to consider the effect of inclusions and precipitates on fatigue limit because the fatigue limit of hot-rolled steel sheet is determined not by the limit stress at which cracks occur but rather by the limit stress at which cracks propagate little by little after propagating the matrix structure. will be.

When the inclusions and precipitates are the starting point of destruction, cracks are initially formed at the interface between the inclusions and the matrix structure, or after the inclusions are destroyed, they become cracks, which are then expanded and propagated in the matrix structure. Also in this case, the fatigue limit can be said to be a state where these cracks are stopped after propagating little by little in the matrix structure.

Therefore, the equation of the fatigue limit using the square root √ (area) of the projected area in the maximum principal stress direction of the Vickers hardness HV, inclusions, and microdefects of the matrix structure and the stress ratio R in terms of being present on the free surface of the inclusions is given by the following equation. It is announced as 1.

σ w (kg / mm 2) = 1.43 (HV + 120) / √ (area) ^1/6 [(1-R) / 2] ^α

(σw (fatigue limit): MPa, HV: kgf / mm2, √ (area): μm, R (stress ratio) = σmin / σmax, α = 0.026 + HV * 10-4)

Analyzing what is shown in Equation 1, it can be predicted that the fatigue properties are the worst when the inclusions on the surface and the product surface state has a direct correlation with the fatigue properties.

In addition, the prediction of the fatigue limit from the viewpoint of the size and distribution of the precipitate is mainly summarized by the following equation (2).

σw (kg / ㎡) = A + Bσ _ss + Cσ _ppt + Dσ _pert + Eσ _gr + Fσ _dis

(A ~ F: material constant, σ _ss : employment hardening, σ _ppt : precipitation hardening, σ _pert : pearlite hardening, σ _gr : particle hardening, s _dis : dislocation hardening)

An important item here is the amount of precipitation intensification, because the precipitates disperse the pile-up of dislocations formed during repeated loading.

As described above, it can be seen that the fatigue limit of the high-strength steel does not improve the fatigue property regardless of the increase in the constant strength, so that the influence of inclusions and precipitates is examined as a means for analyzing the fatigue limit.

Therefore, it is necessary to adjust the size and distribution of precipitates and reduce the inclusions in order to improve the fatigue properties in high strength steel with tensile strength of 80㎏ / ㎠ or more. It is necessary to examine the effects closely.

Recently, in the manufacturing method of high strength hot rolled steel sheet with excellent fatigue characteristics of tensile strength of 780 MPa, the fatigue characteristics have been improved by controlling the microstructure, but the bending durability (fatigue limit / tensile strength) level is 0.6 or less. There is a need for improvement. As examples, Japanese Patent Laid-Open No. 9-87798 and Japanese Patent Laid-Open No. 7-166236 are representative.

Residual austenitic steel (TRIP steel) disclosed in Japanese Patent Application Laid-Open No. 9-87798 has better strength-elongation balance characteristics than precipitation-reinforced steel, but has a durability ratio (fatigue limit / tensile strength) of 0.6 or less. It is known to be somewhat lower than precipitation hardened steel.

In addition, Japanese Patent Laid-Open No. 7-166236 proposes a three-phase steel of ferrite + bainite + martensite having an endurance level of 0.54.

However, the three-phase steel (Tri-phase steel) or residual austenite steel (TRIP steel) is very excellent in terms of strength × elongation balance, but Si is added in the range of 0.5 to 2.5wt% for early deposition of ferrite after hot rolling. This is likely to cause surface oxide scale defects. In addition, due to the large hardness difference between the ferrite and the second phase of the soft phase, the stretch flange has a lower disadvantage than that of the conventional ferrite-perlite tissue steel, and the fatigue characteristic is also lower than the endurance level of 0.6. This is necessary.

Accordingly, the present inventors conducted research and experiments to solve the above problems, and proposed the present invention based on the results. The present invention provides excellent strength and durability by appropriately controlling the steel component and the final structure. It is an object of the present invention to provide a tensile strength of 80 kg / mm 2 hot rolled steel sheet having excellent fatigue properties and a method of manufacturing the same.

In the present invention, C: 0.05 to 0.12%, Si: 0.1 to 0.3%, Mn: 1.0 to 2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.01 to 0.05%, Ti: 0.05 to The present invention relates to a tensile strength 80kg / mm2 high strength hot rolled steel sheet having excellent fatigue properties containing 0.20%, Nb: 0.02 to 0.035%, Mo: 0.1 to 0.3%, N: 80 to 110 ppm, residual Fe and other unavoidable impurities.

In the present invention, C: 0.05 to 0.12%, Si: 0.1 to 0.3%, Mn: 1.0 to 2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.01 to 0.05%, Ti: Steel slab containing 0.05 ~ 0.20%, Nb: 0.02 ~ 0.035%, Mo: 0.1 ~ 0.3%, N: 80 ~ 110ppm, balance Fe and other unavoidable impurities, after reheating, hot-rolled and finished in austenite zone It relates to a method of manufacturing a high strength hot rolled steel sheet with a tensile strength excellent in fatigue characteristics characterized in that it is wound in the temperature range of 600 ~ 630 ℃ after.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, in the manufacture of hot-rolled steel sheet for automotive structural of 80kg / ㎜ class tensile strength, carbon, manganese, titanium, niobium, The hot rolling condition is controlled to properly control the content of molybdenum and, in particular, to add carbon in an appropriate amount of nitrogen in order to precipitate the carbonitride on the ferrite single phase to improve fatigue characteristics.

Hereinafter, the reason for controlling the steel component and the manufacturing conditions will be described.

The carbon (C) is a basic element to obtain the strength of the hot-rolled steel sheet, in the present invention steel in order to control the minimum amount necessary for the precipitation of titanium and niobium carbide in order to reduce the ferrite single phase and grain boundary cementite, the content is 0.05 It is desirable to limit to 0.12%.

The manganese (Mn) is an element similar to molybdenum (Mo), and decreases the Ar ₃ temperature and delays the precipitation of titanium-niobium-nitrogen-carbon precipitated in austenite to increase the solubility in steel. It is an effective element for strengthening. However, if the content is less than 1.0%, the expected strength is not obtained. If it exceeds 2.2%, the strength is obtained but the elongation is drastically reduced. That is, when the manganese content is increased, the hardenability increases due to the decrease in Ar ₃ temperature, and thus the possibility of acicular ferrite is increased, and the precipitation strengthening effect is reduced by inhibiting the precipitation of titanium carbide. Therefore, the content is preferably set to 1.0 to 2.2%.

The silicon (Si) acts as an element for promoting coarse ferrite and promotes niobium-nitrogen-carbon precipitation in austenite. Therefore, the expected effect of silicon addition is opposite to that of manganese (Mn), and the change in strength is very significant in view of promoting deposition of niobium-nitrogen-carbon in austenite and coarse ferrite. Has a feature that grows. However, in terms of encouraging coarse ferrite, it leads to a decrease in strength and does not contribute significantly to fatigue characteristics. In addition, since silicon causes an oxide scale defect on the surface of the hot rolled steel sheet, it is preferable to set the addition amount at 0.1 to 0.3% in order to obtain proper strength and good surface properties.

The niobium (Nb) is the most important element for securing strength, and induces an increase in strength because it suppresses the formation of austenite and formation of equiaxed ferrite, and also improves fatigue characteristics. This is because niobium is an element that finely forms precipitates in the form of niobium-carbon-nitrogen upon coil winding. However, if the addition amount is excessive, the possibility of generating slag cracks in the manufacturing process of the continuous casting slab increases, so it is preferable to set it at 0.020 to 0.035%.

The titanium (Ti) is an element that precipitates as titanium-niobium carbide or titanium-niobium-molybdenum composite carbide in the ferrite structure in steel and contributes to strengthening of the ferrite matrix. When the titanium content is small, the precipitation strengthening effect cannot be sufficiently expected. On the contrary, when the titanium content exceeds 0.1%, the precipitation strengthening effect is saturated, and therefore, the amount of titanium added is preferably limited to the range of 0.050 to 0.20%.

The molybdenum (Mo) is an element that increases the hardenability of the steel by reducing the Ar ₃ transformation point together with Mn, and contributes to precipitation strengthening together with titanium and niobium. In the present invention, it is preferable to add the tensile strength in the range of 0.10 to 0.30% in consideration of the strength level and weldability of 80 kg / mm2 grade hot rolled steel sheet, because when the addition amount exceeds 0.3%, the increase in the hardenability by molybdenum This is because the likelihood of dysfunction, that is, the occurrence of weld cracking, increases.

The aluminum (Al) is added as a deoxidizer, and in particular, increases the ductility of the hot rolled steel sheet, but when added in large amounts, it promotes precipitation of AlN and decreases the amount of solid solution N during hot rolling. desirable. On the other hand, the function as a deoxidizer is usually sufficient at a content of 0.05% or less.

The nitrogen (N) is the most important element in the present invention, by forming a precipitate of titanium-nitrogen and titanium-niobium element and carbide in the steel to induce the dispersion (dislocation) during repeated loading stress distribution It is an element that greatly improves fatigue characteristics because it acts as an element that causes.

Nitrogen is precipitated with AlN in the hot rolled steel sheet to increase the ductility of the hot rolled steel sheet, but in order to increase the fatigue property which is most important in the automotive hot rolled steel sheet, it is preferable to adjust the amount with the addition of Al.

In other words, in order to improve the fatigue characteristics under repeated loads, the precipitate such as carbonitrides acts as the most important factor, so the action of excess nitrogen (N) remaining after precipitation with AlN is very important. It is to disperse the stress by disturbing the local pile-up of dislocations that multiply at.

Such nitrogen is precipitated in the form of titanium-nitrogen, titanium-niobium, and based on the bending fatigue test, the content is preferably set to 80 to 110 ppm. The reason is that if the content is more than 110 ppm, the precipitated titanium-nitrogen precipitate and titanium-niobium carbide and the remaining nitrogen in the steel cause rapid work hardening under repeated loads, thereby reducing the fatigue characteristics. In other words, the action of nitrogen in the steel basically forms carbonitrides to inhibit dislocation movement, thereby inhibiting recrystallization growth and inducing dispersal of dislocation upon repeated loading, but the amount of nitrogen is 110 ppm. If exceeded, it will be precipitated as carbonitrides and the amount of remaining dissolved nitrogen will increase, so it is employed in the river. Nitrogen dissolved in the steel contributes to the increase in strength of the steel, but it causes work hardening by interaction with existing carbonitrides, which promotes internal crack formation during repeated loading.

Finish rolling and winding at 840 ° C. or more, which is Ar ₃ or more, using the steel slab formed as described above, the winding temperature is preferably limited to 600 ~ 630 ° C. In other words, if the coil is wound in this temperature range, the precipitate strengthening element precipitates as fine carbides, thereby contributing to the increase in strength.

The hot rolled steel sheet of the present invention has a microstructure of ferrite 75 to 85% and pearlite 25 to 15% in the final structure, and has a tensile strength of 80 kg / mm 2 or more, a yield strength of 70 kg / mm 2 or more, and a bending durability (fatigue limit). Tensile strength) can provide excellent properties of 0.6 or more.

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

(Example 1)

The steel slab prepared by the chemical components shown in Table 1 was hot rolled, and then wound and cooled at 620 ° C. to prepare a hot rolled steel sheet having a final plate thickness of 3.0 mm.

Then, in order to measure the mechanical properties of the produced hot-rolled steel sheet, a JIS No. 5 tensile test piece was prepared in a direction perpendicular to the rolling direction, and tensilely evaluated at a rate of 10 mm / min at room temperature.

Fatigue test specimens used an hourglass specimen with a size of 30 W × 200 L × t (mm) in the direction perpendicular to the rolling. The test conditions were carried out under 20 Hz and sine waveform conditions.

The load mode was fully positive bending mode and the fatigue limit was determined based on 10 ⁷ cycles.

Steel grade Chemical composition (wt%) C Mn Si P S Al Ti Nb Mo Cr N (ppm) Comparative Steel 1 0.073 1.69 0.05 0.017 0.0021 0.034 0.091 0.030 0.21 － 28 Comparative Steel 2 0.075 1.70 0.05 0.019 0.0020 0.033 0.088 0.030 0.19 35 Comparative Steel 3 0.071 1.68 0.05 0.016 0.0024 0.034 0.084 0.030 0.18 46 Comparative Steel 4 0.076 1.71 0.05 0.017 0.0022 0.032 0.080 0.030 0.20 57 Inventive Steel 1 0.073 1.69 0.05 0.018 0.0023 0.034 0.081 0.030 0.19 82 Inventive Steel 2 0.072 1.69 0.05 0.017 0.0020 0.034 0.088 0.030 0.20 110 Comparative Steel 5 0.12 1.53 1.17 0.006 0.001 0.033 － － － 1.01 － Comparative Steel 6 0.25 1.50 0.97 0.008 0.003 0.009 － 38

Steel grade Thickness (mm) Mechanical properties Fatigue Limit (MPa) Durability (fatigue limit / tensile strength) √ (area) (μm) Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Comparative Steel 1 3 632 810 24 410 0.51 0.028 Comparative Steel 2 664 817 23 420 0.52 0.026 Comparative Steel 3 673 822 22 430 0.52 0.022 Comparative Steel 4 688 835 22 450 0.54 0.023 Inventive Steel 1 767 840 21 510 0.63 0.020 Inventive Steel 2 740 828 20 510 0.61 0.019 Comparative Steel 5 3.5 570 820 24 440 0.54 － Comparative Steel 6 3 610 822 32 477 0.58

As shown in Table 2, all of the present invention steel can be confirmed that the endurance ratio (fatigue limit / tensile strength) is 0.6 or more.

Comparative steels (5) and (6) are three-phase steels (Tri-phase steels) and residual austenite steels (TRIP steels), respectively.The tensile strength × elongation balance is 15000 MPa ·% or more, but the endurance level is required in the present invention. Was lower than 0.6.

On the other hand, it is characteristic that the size of √ (area) of the titanium-niobium-nitrogen-carbon precipitates of 20 nm or less, which contributes to the strength, is reduced with the addition of nitrogen (N), and the invention steels (1) and (2) In the case of 20 nm or less of the precipitate of titanium-niobium-nitrogen-carbon precipitate (area) is 0.02㎛ or less and the bending durability is also 0.6 or more. That is, it can be seen that the action of the precipitates of titanium-niobium-nitrogen-carbon having a thickness of 20 nm or less contributes to dispersing the pile-up of dislocations under repeated loads.

Nitrogen-related precipitates were also present as titanium-niobium-nitrogen-carbon as well as titanium-nitrogen precipitates, but their average size was 80 nm or more, which was evaluated as a precipitate that did not contribute to strength.

As described above, the present invention provides an excellent tensile strength 80kg / mm2 hot rolled steel sheet having excellent fatigue properties that can be used as a material for automobile wheels by providing a good resistance to crack formation under repeated loads by appropriately controlling the steel components It can be effective.

Claims (5)

By weight%, C: 0.05-0.12%, Si: 0.01-0.3%, Mn: 1.0-2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.01-0.05%, Ti: 0.05-0.20% , Nb: 0.02 ~ 0.035%, Mo: 0.1 ~ 0.3%, N: 80 ~ 110ppm, Tensile strength with excellent fatigue property containing residual Fe and other unavoidable impurities 80kg / mm2 hot rolled steel sheet According to claim 1, The hot rolled steel sheet has a tensile strength of 80kg / ㎜ or more, yield strength of 70kg / ㎜ or more, the bending durability (fatigue limit / tensile strength) is 0.6 or more tensile strength excellent tensile strength, characterized in that more than 0.6kg /. ㎜ grade hot rolled steel sheet According to claim 1 or 2, wherein the final structural fraction of the hot-rolled steel sheet is 75 ~ 85% ferrite and pearlite 25 ~ 15% excellent tensile strength tensile strength 80kg / mm2 hot rolled steel sheet By weight%, C: 0.05-0.12%, Si: 0.01-0.3%, Mn: 1.0-2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.01-0.05%, Ti: 0.05-0.20%, Steel slab containing Nb: 0.02 ~ 0.035%, Mo: 0.1 ~ 0.3%, N: 80 ~ 110ppm, Remnant Fe and other unavoidable impurities, after reheating, hot-rolled at the austenite temperature range, cooling and winding Method for manufacturing 80kg / mm2 grade hot rolled steel with excellent tensile strength The method for producing a hot-rolled steel sheet having a tensile strength of 80 kg / mm 2 class, characterized in that the winding temperature is 600 ~ 630 ℃.