JP5890735B2 - Method for producing hot-rolled steel sheet having both pickling and workability - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel sheet having good pickling properties and excellent workability.

  In recent years, in order to increase the strength of steel sheets, the amount of elements such as Si and Mn, which increase the strength of steel, has increased, and this has increased the strength of hot-rolled steel sheets obtained by hot rolling. It has become difficult to process cold rolling.

  In order to reduce the strength of the hot-rolled steel sheet in order to ensure workability, it is necessary to wind at a high temperature. However, if the coiling temperature is raised, grain boundary oxidation occurs deeply and grain boundaries due to pickling It takes a long time to remove the oxide layer, that is, there is a problem that the pickling property is deteriorated and the productivity is lowered.

  Then, the manufacturing method of the hot-rolled steel plate which can ensure workability, maintaining favorable pickling property is desired, and various proposals have been made in the past.

  For example, Patent Document 1 discloses a method of manufacturing a hot-rolled steel sheet that is hot-rolled so that the volume ratio of austenite on the final stand exit side is 20 to 70%, and then wound at a temperature of 300 to 650 ° C. (Refer to claim 1) It is said that the grain boundary oxidation is suppressed by reducing the transformation recuperation after winding by hot rolling under such conditions.

  Further, in Patent Document 2, after hot rolling at a finish rolling temperature of 800 to 950 ° C., a hot rolled steel sheet that is wound up at a coiling temperature of 650 to 780 ° C. in 40 seconds or less and wound at the above temperature is disclosed. A method is disclosed (refer to claim 1), and by hot rolling under such conditions, the hot-rolled material is softened and the oxide scale and grain boundary oxide layer on the surface of the steel sheet are not made too thick.

  Patent Document 3 discloses a hot rolling method in which a hot rolled strip is hot-rolled at a final hot rolling temperature of 800 to 950 ° C. and then wound at a suitable winding temperature of 530 to 580 ° C. A method for producing a steel sheet is disclosed (see paragraph [0037]), and hot rolling is performed under such conditions to ensure workability while suppressing grain boundary oxidation.

  Patent Document 4 discloses that hot rolling is performed under conditions of a finishing temperature: 830 to 900 ° C., an average cooling rate: 30 to 45 ° C./s, and a winding temperature: 500 to 680 ° C. 3), and hot rolling under such conditions, it is supposed to ensure workability while suppressing grain boundary oxidation.

  However, in the methods described in Patent Documents 1 to 4, as described later, an oxide scale is further formed on the oxide scale formed during hot rolling during the period from finish rolling to winding. The thickness increases, and this oxide scale becomes an oxygen source in the coil during the cooling of the coil after winding and causes grain boundary oxidation, so the grain boundary oxidation cannot be reliably suppressed, There is a problem that the pickling property cannot be improved.

Japanese Patent Laid-Open No. 3-20407 JP 2009-197256 A Special table 2010-535946 JP 2009-24233 A

  The present invention has been made paying attention to the above circumstances, and its purpose is to provide a method for producing a hot-rolled steel sheet that has excellent workability while maintaining excellent pickling properties by suppressing grain boundary oxidation. It is to provide.

The invention according to claim 1 is in mass% (hereinafter the same for chemical components), C: 0.04 to 0.20%, Si: 1.0 to 3.0%, Mn: 0.5 to 3.0%, P: 0.02% or less (not including 0%), S: 0.004% or less (not including 0%), N: 0.01% or less (not including 0%) In addition, after hot finish rolling a steel material having a composition composed of iron and inevitable impurities in the balance, the oxide scale is removed using high-pressure water or mechanical means, and the temperature T ₁ (° C. at the time of completion of this scale removal is obtained. ) To winding temperature: 600 to 750 ° C. by air cooling or water cooling at a time t (s) satisfying the relationship of the following formula (1), and winding at the winding temperature to obtain the tensile strength of the obtained hot rolled steel sheet Pickling property, characterized in that the strength is 800 MPa or less and the grain boundary oxide layer depth is 10 μm or less. It is a manufacturing method of the hot-rolled steel plate which has workability.
1.5 × 10 ⁶ · √t · exp [-149000 / {8.31 × (T ₁ +273)}] ≦ 1.1 Formula (1)

  In the invention according to claim 2, the component composition further includes Ni: 2% or less (not including 0%), Cu: 2% or less (not including 0%), Mo: 2% or less (0%) B: 0.01% or less (not including 0%), Cr: 2% or less (not including 0%), Nb: 1% or less (not including 0%), V: 1% or less (Not including 0%), W: 0.3% or less (not including 0%), Al: 0.06% or less (not including 0%), Ti: 0.1% or less (including 0%) The method for producing a hot-rolled steel sheet having both pickling property and workability according to claim 1, comprising one or more selected from the group consisting of:

  In the invention according to claim 3, the component composition further includes: Ca: 0.03% or less (not including 0%), Mg: 0.003% or less (not including 0%), REM: 0.03 The method for producing a hot-rolled steel sheet having both pickling property and workability according to claim 1 or 2, comprising one or more selected from the group consisting of 1% or less (not including 0%). is there.

According to the method of the present invention, after hot finish rolling a steel having a predetermined component composition, the oxide scale is removed using high-pressure water or mechanical means, and the temperature T ₁ (° C.) at the time of completion of the scale removal is removed. Winding temperature: By cooling from 600 to 750 ° C. for a predetermined time t (s), the thickness of the oxide scale at the start of winding can be reduced to 1.1 μm or less. The hot-rolled steel sheet obtained by post-cooling can have a tensile strength of 800 MPa or less and a grain boundary oxide layer depth of 10 μm or less. It can be manufactured.

  In order to achieve both pickling and workability in the hot-rolled steel sheet, the present inventors believe that it is important to find a measure for suppressing grain boundary oxidation even when the coiling temperature is increased. The study was based on the mechanism.

  That is, grain boundary oxidation (generation of Si—Mn oxide at the grain boundary) occurs after winding of the hot rolled coil. And when winding the coil, the steel strip is wound tightly, so that the inside of the coil is in an oxygen-free state, and oxygen supply from the outside is established, but it is already formed on the surface of the steel strip at the time of winding. It is known that oxide scale (Fe oxide; hereinafter, also simply referred to as “scale”) is an oxygen source for grain boundary oxidation.

  Therefore, the inventors thought that if the oxide scale thickness on the surface of the steel strip at the time of winding is suppressed, the oxygen source in the coil is reduced, so that the grain boundary oxidation can also be suppressed.

  Next, an index for evaluating the pickling property and workability of a hot-rolled steel sheet obtained by cooling after coil winding and its target value were determined by known knowledge and experiments.

  As an index for evaluating the workability of the hot-rolled steel sheet, the tensile strength (TS) of the hot-rolled steel sheet is used, and the target value is 800 MPa or less (corresponding to 270 or less in Vickers hardness (Hv)). In order to realize the above, it has been found that the coiling temperature is required to be 600 ° C. or higher. However, if the coiling temperature is too high, it takes time to cool the coil and the productivity deteriorates. Therefore, the coiling temperature is set to 750 ° C. or lower.

  On the other hand, as an index for evaluating the pickling property of the hot-rolled steel sheet, the grain boundary oxide layer depth of the hot-rolled steel sheet is used, and the target value is set to 10 μm or less. From the example, it was found that the oxide scale thickness of the hot-rolled steel sheet needs to be 1.1 μm or less.

  Here, since the oxidized scale of about 10 μm is already generated by finish rolling usually performed at about 900 to 1100 ° C., it has been thought that the oxidized scale may be removed before winding. Oxide scale is also generated during the air cooling or water cooling from the scale removal to winding, but the thickness and thickness of the hot rolled steel sheet are adjusted by adjusting the temperature and time during the air cooling or water cooling. Was considered to be 1.1 μm or less.

  Based on the above findings, further studies have been made and the present method has been completed.

  The processing conditions after finish rolling that characterize the method of the present invention will be described first.

(Remove oxide scale using high pressure water or mechanical means after finish rolling)
As described above, an oxide scale of about 10 μm has already been generated by finish rolling usually performed at about 900 to 1100 ° C., and therefore this oxide scale is removed on the run-out table before winding. Since it is necessary to remove the oxide scale at a high temperature, high-pressure water descaling using high-pressure water or mechanical descaling using a brush or a grinder as mechanical means may be employed. In addition, when using high pressure water, you may serve as a descaling and cooling to coiling temperature.

[Winding temperature: 600 to 750 ° C.]
As described above, in order to set the tensile strength (TS), which is an index of workability of the hot-rolled steel sheet, to 800 MPa or less, the winding temperature is set to 600 ° C. or more. However, if the coiling temperature is too high, it takes time to cool the coil and the productivity deteriorates. Therefore, the coiling temperature is set to 750 ° C. or lower.

[Air-cooling or water-cooling from the temperature T ₁ (° C.) at the completion of scale removal to the coiling temperature at time t (s) satisfying the relationship of the following formula (1) 1.5 × 10 ⁶ · √t · exp [-149000 /{8.31×(T ₁ +273)}] ≦ 1.1 Formula (1)]
After removing the scale, the thickness of the oxide scale generated during air cooling or water cooling until winding is reduced to 1.1 μm or less, thereby reducing the oxygen source for grain boundary oxidation in the coil as much as possible. This is because the depth of the grain boundary oxide layer formed in the above is 10 μm or less.

  Here, the left side of the above equation (1) is an equation for predicting the oxide scale thickness and is derived as follows.

  That is, as shown in the following formula (2), the oxide scale thickness y is an exponential function with respect to the absolute temperature T, and is thick with a square root with respect to the elapsed time t. This is well-known from the theory and experiment which considered the oxidation reaction as a thermal diffusion phenomenon.

y = C ₁ · √t · [exp (−C ₂ / RT)] Equation (2)
Here, C ₁ and C ₂ are constants, and R is a gas constant.

Then, in Experiment 1 in the examples described later, the steel sheet after finish rolling is heated in an air atmosphere with various changes in the absolute temperature T and time t, and the thickness y of the oxide scale formed on the steel sheet surface is changed. Was measured, C ₁ and C ₂ were obtained by performing data regression on the relationship between y, T, and t, and the left side of the above formula (1) was obtained. In the left side of the above formula (1), “8.31” is a gas constant (unit: kJ / (K · mol)), and “+273” is for conversion from Celsius temperature (° C.) to absolute temperature (K). It is a constant, and the value of the entire left side and the unit of “1.1” on the right side are both μm.

Note that the left side of the above formula (1) predicts the thickness of the oxide scale that is generated under the condition that the temperature T ₁ at the time of scale removal completion is constant. Since it decreases due to air cooling or water cooling until the start of winding, the predicted value by the left side of the above formula (1) is always larger than the thickness of the actual oxide scale generated during the air cooling or water cooling. . That is, the left side of the above formula (1) predicts the actual oxide scale thickness generated during the air cooling or water cooling on the safer side (larger side), and the predicted value according to the above formula (1) If the condition of 1.1 μm or less is satisfied, it means that the grain boundary oxide layer depth can be reliably realized to be 10 μm or less.

  The method of the present invention defines the processing conditions after finish rolling, but the hot rolling process may be performed according to a conventional method. For example, the heating temperature when heating the steel material is preferably 1000 to 1300 ° C. from the viewpoint of securing the finishing temperature.

  In the method of the present invention, by appropriately controlling the processing conditions after hot finish rolling, a hot rolled steel sheet having both pickling property and workability is obtained. What is sufficient is just to satisfy the characteristic as a high-strength steel plate. From such a viewpoint, the reason for limitation of the chemical component composition of the steel material used in the method of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

[Chemical composition of steel]
C: 0.04 to 0.20%
C is an important element for increasing the strength of the steel and needs to be contained in an amount of 0.04% or more. However, if it exceeds 0.20%, the cold workability is lowered.

Si: 1.0-3.0%
Si is an important element capable of ensuring ductility and workability while developing strength in steel. The lower limit of the Si content necessary for the high-strength steel sheet is 1.0%, preferably 1.2%. However, if it is excessively contained, the ductility is impaired, so the upper limit is made 3.0%, preferably 2.5%.

Mn: 0.5 to 3.0%
Mn is an important element capable of ensuring the strength and toughness of the steel, and the lower limit is 1.0%, preferably 0.8%, as the minimum Mn content necessary for the steel-strength steel plate. However, if excessively contained, the ductility is impaired in the same manner as Si, so the upper limit is made 3.0%, preferably 2.5%.

P: 0.02% or less (excluding 0%)
P is an element inevitably contained, but the presence of a trace amount of P delays precipitation of cementite and suppresses transformation. However, if excessively contained, the ductility deteriorates and the plating adhesion deteriorates, so the content is made 0.02% or less.

S: 0.004% or less (excluding 0%)
S is also an element that is unavoidably contained, but forms sulfide-based inclusions MnS, which segregates during hot rolling of the steel material, and thus embrittles the steel material, so the content is made 0.004% or less.

N: 0.01% or less (excluding 0%)
N is also an element that is inevitably contained. However, a coarse nitride is formed to deteriorate bendability and hole expansibility, and cause blowholes during welding. % Or less must be suppressed.

  The steel material used in the method of the present invention basically contains the above components, and the balance is iron and unavoidable impurities. In addition, the following components can be contained as necessary.

Ni: 2% or less (excluding 0%)
Ni is an element that improves the hardenability. If an appropriate amount is included, Ni increases the martensite fraction at the time of annealing in the continuous annealing line (CAL) after cold rolling and at the time of cooling. Through the action of refining the lath structure, the hardenability during the cooling treatment after the two-phase region reheating at the time of tempering in the continuous hot dip galvanizing line (CGL) in the next step is improved, and the final after cooling The composite structure can be improved, and various moldability can be improved. Such an effect can be obtained by adding a small amount of Ni. However, in order to exhibit the effect more effectively, the content is preferably 0.1% or more, and more preferably 0.2% or more. However, since it is an expensive element, its content is 2% or less, preferably 1.5% or less, and more preferably 1.0% or less from the viewpoint of production cost.

Cu: 2% or less (excluding 0%)
Cu is an element that improves the hardenability like Ni, and improves various processability by the same action as Ni. Such an effect can be obtained by containing a very small amount of Cu. However, in order to exhibit the effect more effectively, the content is preferably 0.1% or more, more preferably 0.2% or more. However, since it is an expensive element, it is 2% or less, preferably 1.5% or less, more preferably 1.0% or less from the viewpoint of manufacturing cost.

Mo: 2% or less (excluding 0%)
Mo is an element useful for strengthening the solid solution without impairing the plating property. Moreover, it is an element which improves hardenability like Ni and Cu, and improves various moldability by the effect | action similar to Ni and Cu. Although such an effect can be obtained by adding a small amount of Mo, in order to exhibit the effect more effectively, the content is preferably 0.1% or more, more preferably 0.2% or more. However, since it is an expensive element, it is 2% or less, preferably 1.5% or less, more preferably 1.0% or less from the viewpoint of manufacturing cost.

B: 0.01% or less (excluding 0%)
B has an effect of improving the hardenability and is contained as necessary. Such an effect can be obtained by adding a small amount of B, but in order to exhibit the effect more effectively, the content is preferably 0.0001% or more, and more preferably 0.0002% or more. However, if excessively contained, the plating properties deteriorate, so 0.01% or less, preferably 0.005% or less, more preferably 0.001% or less.

Cr: 2% or less (excluding 0%)
Cr can be contained as necessary to impart strength to the steel material and the cold forged product. Such an effect can be obtained by adding a small amount of Cr, but in order to exhibit the effect more effectively, it is preferably 0.01% or more, and more preferably 0.02% or more. However, if it is excessively contained, ductility is deteriorated, so that it is 2% or less, preferably 1.5% or less, more preferably 1.0% or less.

Nb: 1% or less (excluding 0%)
Nb is an element that can obtain a fine structure of carbide with a small amount contained and can increase strength without impairing toughness. In order to exhibit such an effect more effectively, the content is preferably 0.001% or more, more preferably 0.005% or more. However, if excessively contained, carbides are excessively generated, and the balance between strength and workability is deteriorated due to a decrease in the fraction of martensite or precipitation strengthening of the carbides. Hereinafter, it is more preferably 0.1% or less.

V: 1% or less (excluding 0%)
V, like Nb, is an element that generates carbides when contained in a small amount, and contributes to improving the strength of the steel sheet. In order to exhibit such an effect more effectively, the content is preferably 0.001% or more, more preferably 0.005% or more. However, if it is contained excessively, it not only causes an increase in cost, but also raises the yield point (yield ratio) and lowers the workability, so that it is 1% or less, preferably 0.5% or less. Preferably, the content is 0.1% or less.

W: 0.3% or less (excluding 0%)
W is contained in a small amount, and contributes to an increase in the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening transition through suppressing recrystallization. In order to exhibit such an effect more effectively, the content is preferably 0.001% or more, more preferably 0.005% or more. However, excessive inclusion causes excessive precipitation of carbonitride and causes deterioration of moldability. Therefore, the content is made 0.3% or less, preferably 0.2% or less, more preferably 0.1% or less.

Al: 0.06% or less (excluding 0%)
Al is used as a deoxidizer and is preferably contained in the steel material in order to prevent coarsening of austenite crystal grains during normalizing heating. However, excessive content not only saturates this effect but also makes the crystal grains unstable, so it is 0.06% or less, preferably 0.05% or less, and more preferably 0.02% or less.

Ti: 0.1% or less (excluding 0%)
Ti, like Al, is used as a deoxidizer and is preferably contained in an amount of 0.01% or more. However, if it is contained excessively, the toughness decreases, so it is 0.1% or less, preferably 0.05% or less, more preferably 0.03% or less.

Selected from the group consisting of Ca: 0.03% or less (excluding 0%), Mg: 0.003% or less (not including 0%), REM: 0.03% or less (not including 0%) 1 type or 2 types or more These elements are elements used for deoxidation, Preferably it is made to contain 0.002% or more, More preferably, 0.003% or more, respectively. However, if excessively contained, the moldability is deteriorated, so that the content is 0.03%, preferably 0.02% or less, and more preferably 0.01% or less. Examples of REM (rare earth element) used in the method of the present invention include Sc, Y, and lanthanoid.

  In order to confirm the applicability of the present invention, an experiment simulating from finish rolling to cooling after coil winding was performed using steel specimens having various component compositions shown in Table 1 below.

[Experiment 1] Scale generation experiment A test in which a steel material having the component composition shown in Table 1 above was processed into a coin shape (diameter 20 mm x thickness 3 mm) in order to simulate scale generation from finish rolling to coil winding start. A piece was prepared and heat-treated using a processing formaster. Specifically, the temperature is raised to a predetermined temperature (assuming a finish rolling temperature or a descaling completion temperature) in an N ₂ atmosphere, and the atmosphere is switched to a regulated air gas for a predetermined time (finishing finish time or descaling completion time). The temperature was maintained at that temperature to oxidize the surface, and then the supply of the regulated air gas was stopped and N ₂ gas was sprayed for rapid cooling. And about the thickness of the produced | generated oxide scale, the photograph (magnification | multiplying factor :) which observed the cross section of the test piece after the said heat processing using the Fe-SEM apparatus (S-4500 field emission scanning electron microscope made from Hitachi, Ltd.). 3000 times), the thickness of the oxide film was measured, and the maximum thickness was defined as “thickness of oxide scale”.

[Experiment 2] Coil cooling simulation experiment after winding Next, the same two test pieces prepared by heat treatment in Experiment 1 above were stacked so that the surfaces on which the oxide film was formed faced each other (the steel plate in the coil). Are inserted into the electric furnace in a state where a weight is placed so that a load of 0.2 kgf / cm ² (≈1.96 N / cm ² ) is applied, and a predetermined temperature (image of winding temperature is assumed) ) From the temperature T ₂ (° C.), and satisfies the following formula (3) obtained from the coil cooling simulation calculation (preconditions: coil weight 10 tons, inner diameter 760 mm, outer diameter 1480 mm, width 1000 mm). It cooled to 500 degreeC with the cooling rate v (degreeC / min), and was furnace-cooled after that. The test piece was placed in an N ₂ atmosphere from installation to removal in the electric furnace.

v = −0.0125T ₂ +12.448 Formula (3)

  And about the cross section of the test piece heat-processed in this way, the photograph (magnification | multiplying_factor :) is observed using the Fe-SEM apparatus (S-4500 field emission type scanning electron microscope made from Hitachi, Ltd.) similarly to the measurement of oxide scale thickness. 3000 times), the grain boundary oxide layer was measured, and the maximum depth was defined as “grain boundary oxide layer depth”.

  Table 2 below shows the experimental conditions and the results. Further, in the same table, the Vickers hardness of the test piece obtained by separately austenitizing the steel material test piece having the composition shown in Table 1 and then cooling it under the same coil cooling conditions as in Experiment 2 above. The measured values are also shown. The Vickers hardness is measured with a micro Vickers hardness tester at a load of 1000 g (9.8 N) from the surface to a depth of ¼ of the thickness of the test piece every 1 mm. The average value.

  As shown in Table 2 below, the experimental conditions satisfy No. 1 satisfying “there is scale removal, the left side of formula (1) ≦ 1.1, the winding temperature: 600 to 750 ° C.” which is a requirement of the method of the present invention. As for the invention examples of 5-10 and 12-14, the oxide scale thickness (measured value) at the time of winding start (assumed) is 1.1 micrometers or less, and the hot-rolled steel sheet finally obtained is The grain boundary oxide layer has a depth of 10 μm or less, a Vickers hardness of 270 Hv or less (corresponding to a tensile strength of 800 MPa or less), and is excellent in pickling and workability.

  On the other hand, the experimental condition is No. which does not satisfy “there is scale removal and the left side ≦ 1.1 of formula (1)” among the requirements of the method of the present invention. 1-4, the scale thickness (measured value) at the start of winding (assumed) exceeds 1.1 μm, and the finally obtained hot-rolled steel sheet has a grain boundary oxide layer depth exceeding 10 μm. The pickling property is poor.

  In addition, among the requirements of the method of the present invention, the experimental conditions No. that do not satisfy “winding temperature: 600 to 750 ° C.”. No. 11, the finally obtained hot-rolled steel sheet has a Vickers hardness exceeding 270 Hv (corresponding to a tensile strength exceeding 800 MPa) and is inferior in workability.

  From the above, the applicability of the method of the present invention was confirmed.

  As shown in Table 2 below, the value on the left side of Equation (1) agrees with the measured oxide scale thickness very well, and the left side of Equation (1) indicates the predicted accuracy of the oxide scale thickness. It turns out that it is excellent in.

Claims (3)

% By mass (hereinafter the same for chemical components), C: 0.04 to 0.20%, Si: 1.0 to 3.0%, Mn: 0.5 to 3.0%, P: 0.00. 02% or less (not including 0%), S: 0.004% or less (not including 0%), N: 0.01% or less (not including 0%), the balance being iron and inevitable impurities After hot finish rolling a steel material having a component composition consisting of the following, the oxide scale is removed using high-pressure water or mechanical means, and the coiling temperature: 600 to 750 from the temperature T ₁ (° C.) at the completion of this scale removal. Air-cooled or water-cooled at a time t (s) satisfying the relationship of the following formula (1) up to ℃, and wound at the winding temperature, the tensile strength of the obtained hot-rolled steel sheet is 800 MPa or less, grain boundary oxide layer Hot rolled steel sheet having both pickling property and workability, characterized by a depth of 10 μm or less Manufacturing method.
1.5 × 10 ⁶ · √t · exp [-149000 / {8.31 × (T ₁ +273)}] ≦ 1.1 Formula (1)   Component composition is further Ni: 2% or less (not including 0%), Cu: 2% or less (not including 0%), Mo: 2% or less (not including 0%), B: 0.01 % Or less (not including 0%), Cr: 2% or less (not including 0%), Nb: 1% or less (not including 0%), V: 1% or less (not including 0%), W : Selected from the group consisting of 0.3% or less (not including 0%), Al: 0.06% or less (not including 0%), Ti: 0.1% or less (not including 0%) The manufacturing method of the hot-rolled steel plate which has pickling property and workability of Claim 1 which contains 1 type (s) or 2 or more types.   Component composition is further Ca: 0.03% or less (not including 0%), Mg: 0.003% or less (not including 0%), REM: 0.03% or less (not including 0%) The manufacturing method of the hot-rolled steel plate which combines pickling property and workability of Claim 1 or 2 which contains 1 type, or 2 or more types chosen from the group which consists of.