JP6354259B2 - Steel sheet with excellent fatigue strength and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate excellent in fatigue strength used for automotive parts and the like, and a method for producing the same.

  From the viewpoint of environmental problems and the like, there is a strong demand for lower fuel consumption of automobiles. Weight reduction is effective for reducing fuel consumption of automobiles, and in order to realize this, reduction in thickness and weight by increasing the strength of steel sheets used in automobiles is required. However, the thinner the steel plate is, the more obvious the failure starts from the inclusions present in the steel. In order to suppress such destruction, it is very important to reduce and refine the inclusions in the steel.

  As a conventional technique for controlling inclusions in a steel plate, for example, a technique described in Patent Document 1 has been proposed. One of the features of this technology is to suppress the formation of Al oxide inclusions and nitride inclusions by limiting the amount of Al added for Al deoxidation to a range of 0.010% or less. There is to do.

JP2011-195861A

  However, when trying to prevent the deterioration of fatigue strength by reducing the Al content as in the steel described in Patent Document 1, a low Al scrap is selected and used as a raw material, or a refining that reduces Al. There is a problem that processing is essential and the manufacturing cost increases.

  In view of this background, the present invention is a steel that has high strength and is excellent in fatigue strength by reducing the number and size of inclusions in the steel, and manufacturing the steel while suppressing an increase in manufacturing cost. It is an object of the present invention to provide a manufacturing method that can be used.

One embodiment of the present invention is, in mass%, C: 0.30% or more and 0.70% or less, Si: 2.50% or less, Mn: 1.00% or less, Cr: 1.00% or more and 4.00. %: Mo: 0.50% to 3.00%, V: 0% to 1.00%, Al: more than 0.010% and 0.050% or less, O: 0.0015% or less The balance consists of Fe and inevitable impurities,
The tensile strength is 1700 MPa or more,
The elongation is over 9.2% ,
The maximum value of the equivalent circle diameter of inclusions in steel observed by cross-sectional observation (the maximum value of the equivalent circle diameter of inclusions expected to exist in a volume of 1200 mm ^{3 by} the extreme statistical method) is 15 μm or less. In addition, the steel sheet is excellent in fatigue strength, characterized in that the number of inclusions having an equivalent circle diameter of 1 μm or more is 110 pieces / mm ² or less.

Another aspect of the present invention is a method for producing the steel sheet of the above aspect ,
A refining step of refining the molten steel while adjusting the basicity b of the slag formed on the molten steel when performing the Al deoxidation treatment so as to satisfy the following formulas 1 and 2;
Formula 1: b ≧ 4.0
(However, b = [CaO] / [SiO ₂ ], [CaO] is the content (mass%) of CaO contained in the slag, and [SiO ₂ ] is the content of SiO ₂ contained in the slag ( mass%))
Formula 2: 398 × [Al] −2.23 × b + 19.33 ≦ 25.0
(However, [Al] means the value of Al content (% by mass) in steel )
A casting process for obtaining a steel ingot by casting the molten steel produced by the refining process;
A rough rolling step of hot rolling the steel ingot to obtain a hot rolled sheet;
Finish rolling the hot-rolled sheet to obtain a rolled sheet;
A heat treatment step for quenching and tempering the rolled sheet,
The total area reduction rate G (%) of the rough rolling step and the finish rolling step is
Formula 3: 3615.5e ^{(−0.0526 × G)} ≦ 100.0
It is in the manufacturing method of the steel plate excellent in the fatigue strength characterized by rolling to satisfy | fill.

  The steel sheet has the specific chemical component, and the number and size of inclusions in the steel are controlled to the specific range by appropriately performing the steps from the refining step to the finish rolling step. Above, the said heat processing process is implemented further, and a high intensity | strength steel plate is manufactured. Thereby, while the said steel plate is high intensity | strength, possibility that the fatigue failure resulting from the inclusion at the time of thinning will arise can be reduced significantly compared with the past. Therefore, the steel sheet can be effectively used as a steel sheet thinned by increasing strength.

  Moreover, in the manufacturing method of the steel sheet, the Al content is set in a range higher than that of Patent Document 1 as described above, and the conditions for Al deoxidation treatment in the refining process are generated on the molten steel as described above. Optimize based on the basicity of the slag. Thereby, even if Al content rate is high, the amount of oxygen can be reduced sufficiently, and the number of inclusions can be reduced and refined. And by controlling such inclusions, it is possible to obtain a steel sheet having excellent fatigue strength as described above, and it is not necessary to use iron scrap having a low Al content as a raw material, thereby suppressing an increase in raw material cost. You can also.

Explanatory drawing which shows the relationship between the value of the left side of Formula 3 regarding the total area reduction rate G in Example 1 and the number of inclusions present in steel. Explanatory drawing which shows the relationship between the number of inclusions in steel in Example 1, and a fatigue test result.

  First, the reasons for limiting each component range in the chemical components of the steel sheet will be described.

C (carbon): 0.30% to 0.70%;
C is an element necessary for increasing the strength, and in order to obtain the effect sufficiently, it is necessary to contain 0.30% or more. On the other hand, if C is added excessively, the productivity is deteriorated, and further, coarse carbides are formed and the strength is lowered. Therefore, the upper limit of C addition is 0.70%.

Si (silicon): 2.50% or less;
Si is an element necessary for increasing the strength, and the effect can be obtained even in a small amount, but it is preferable to add 0.10% or more. However, if Si is added excessively, manufacturability, in particular, cold rolling becomes difficult, and it becomes an impediment to carburizing and nitriding properties, so Si is added up to 2.50%.

Mn (manganese): 1.00% or less;
Mn is an element necessary for increasing the strength, and the effect can be obtained even in a trace amount, but it is preferable to add 0.10% or more. However, excessive addition of Mn leads to a decrease in ductility, and it may be difficult to ensure an elongation of 8.0% or more. Therefore, the upper limit of Mn addition is 1.00%.

Cr (chromium): 1.00% to 4.00%;
Cr is an element necessary for increasing the strength, and is also an element effective for increasing the strength when surface treatment such as carburizing and nitriding is performed. In order to sufficiently obtain these effects, addition of 1.00% or more is necessary. However, if Cr is excessively added, coarse carbides are formed, the strength is lowered, and the cost is increased, so 4.00% is made the upper limit.

Mo (molybdenum): 0.50% or more and 3.00% or less;
Mo is an element necessary for increasing the strength, and is also an element effective for increasing the strength when surface treatment such as carburizing and nitriding is performed. In order to obtain these effects sufficiently, it is necessary to contain 0.50% or more. However, excessive addition of Mo not only saturates the effect but also increases the cost, so 3.00% is made the upper limit.

V (vanadium): 0% or more and 1.00% or less;
V is an element effective for increasing the strength. V exhibits the effect of increasing the strength (hardening) even in a small amount, but is preferably 0.10% or more in order to obtain this effect more reliably. However, when V is added excessively, coarse inclusions are generated to reduce the strength and increase the cost, so the upper limit is made 1.00%.

Al (aluminum): more than 0.010% and 0.050% or less;
Al effectively acts at the time of deoxidation treatment in the refining process, and is an element necessary for oxygen reduction. In order to sufficiently obtain this effect, the Al content is set to exceed 0.010%. However, if Al is added excessively, coarse inclusions (alumina) in the steel increase and cause a decrease in fatigue strength, so 0.050% is made the upper limit.

O (oxygen): 0.0015% or less;
If the O content is high, coarse inclusions are formed and the fatigue strength is reduced, so 0.0015% is made the upper limit. More preferably, it may be 0.0010% or less.

  Next, the tensile strength of the steel sheet is 1700 MPa or more. By achieving such high strength, the steel sheet can be thinned. In order to obtain a tensile strength of 1700 MPa or more, as described later, it is necessary to perform a quenching and tempering treatment after finish rolling.

Further, the elongation of the steel sheet is over 9.2% . By ensuring such elongation characteristics, sufficient toughness can be ensured and a decrease in fatigue strength can be suppressed.

Further, the presence state of inclusions in the steel observed by cross-sectional observation of the steel sheet is that the number of inclusions having an equivalent circle diameter of 1 μm or more is 110 pieces / mm ² or less, and the largest equivalent circle diameter of the inclusion is 15 μm or less. In order to make the presence state of inclusions as described above, it is necessary to make the total area reduction rate of the rough rolling process and the finish rolling process large. This is because the number of inclusions per unit cross-sectional area tends to decrease as the area reduction rate increases. Further, if the equivalent circle diameter is low, the condition that the maximum equivalent to 15 μm or less may not be satisfied. In addition, the preferable conditions for the area reduction rate will be described later.

The inclusions in the steel are observed by observing a cross section parallel to the rolling direction of the steel sheet. For the cross section, the vicinity of the center in the thickness direction of the steel sheet and the vicinity of the center in the width direction are selected. And the cross section of a predetermined area (area 40mm < ² > or more) is observed, and the result is judged to represent the whole steel plate. The equivalent circle diameter of inclusions can be calculated by obtaining the area of each inclusion from an image obtained by optical microscope observation of the cross section of the steel sheet.

More specifically, the cross section of the steel sheet is observed and images of a plurality of fields of view are collected. Next, the individual equivalent circle diameters of the inclusions observed in each image are obtained by image analysis. Next, by dividing the total number of inclusions having an equivalent circle diameter of 1 μm or more in all images by the total area of all images, the number of inclusions having an equivalent circle diameter of 1 μm or more per 1 mm ² unit area can be obtained. .

Further, the maximum equivalent circle diameter of the inclusions in each of the above images is obtained, and by using these data, analysis is performed by an extreme value statistical method, and the largest inclusion existing at 1200 mm ³ is predicted, The maximum value of the equivalent circle diameter can be obtained. The extreme value statistical method is a statistical method often used in the evaluation of inclusions present in steel such as bearing steel, and is also described in, for example, JP-A-2013-241986.

  In the steel sheet, the number of inclusions can be controlled to a very small number as described above by appropriately controlling the area reduction rate, and the size of each inclusion can be controlled to be very small as described above. By performing both, it is possible to greatly suppress the occurrence of fatigue failure starting from inclusions.

  Moreover, the said steel plate can select various plate | board thickness according to a use. The effect of improving the fatigue strength by controlling the inclusions as described above is more effectively exhibited as the surface reduction ratio is increased and the plate thickness is thinner. For example, even in a thin plate having a thickness of 0.2 to 2 mm, fatigue failure due to inclusions can be sufficiently suppressed.

  Next, as described above, the steel sheet manufacturing method includes at least the refining process, the casting process, the rough rolling process, the finish rolling process, and the heat treatment process. If necessary, a surface modification step such as carburizing or nitriding can be added.

In the refining step, as described above, the basicity b of the slag formed on the molten steel when the Al deoxidation treatment is performed is adjusted so as to satisfy the following formulas 1 and 2.
Formula 1: b ≧ 4.0
(However, b = [CaO] / [SiO ₂ ], [CaO] is the content (mass%) of CaO contained in the slag, and [SiO ₂ ] is the content of SiO ₂ contained in the slag ( mass%))
Formula 2: 398 × [Al] −2.23 × b + 19.33 ≦ 25.0
(However, [Al] means the value of Al content (mass%))

  By setting the basicity b of the slag to 4.0 or more, the amount of oxygen in the molten steel can be reduced, and the formation of inclusions (alumina) can be suppressed. Further, the basicity b of the slag is adjusted so that the relationship with the Al content satisfies the above formula 2. As a result of various experiments, even if the basicity b of the slag is 4.0 or more, if the above formula 2 is not satisfied, coarse inclusions may be generated, which causes a decrease in fatigue strength. Therefore, it is necessary to adjust the basicity of the slag in the refining process so as to satisfy both of the above formulas 1 and 2.

The basicity of the slag can be adjusted by putting a powder adjusted in the mixing ratio of SiO ₂ and CaO into molten steel and stirring it. The basicity can be lowered by increasing the ratio of SiO ₂ , and the basicity can be increased by increasing the ratio of CaO.

The basicity of slag is measured by making slag into powder with a pulverizer and then forming a green compact with a diameter of 32 mm x thickness of 5 mm and a density of 98% or more using a hand press. A spectrum is obtained by X-ray analysis, and the content mass% of SiO _{2 and} the content mass% of CaO are obtained from the spectrum, and the basicity b = CaO content (mass%) / SiO ₂ content (mass%) is calculated. This can be done according to the procedure.

Next, in the casting process, the molten steel produced by the refining process is cast to obtain a steel ingot.
As the casting method, any known casting method such as continuous casting, semi-continuous casting, or casting into a mold may be used.

  In the rough rolling process and the finish rolling process, it is not necessary to apply a special rolling method. However, as described above, since the influence on the inclusion existing state of the reduction in area during rolling is large, It is necessary to carry out the rough rolling step and the finish rolling step under conditions that satisfy the total area reduction ratio G. Further, finish rolling may be performed by either hot rolling or cold rolling.

Formula 3: 3615.5e ^{(−0.0526 × G)} ≦ 100.0,
The above equation 3 is an empirical equation obtained from data of the number of inclusions measured by performing a number of experiments while changing the total area reduction rate G.

  In the heat treatment step, quenching and tempering are performed. Quenching can be performed under conditions where the steel sheet is held at 850 to 1000 ° C. for 0.5 to 5 hours and then rapidly cooled by water cooling or oil cooling. Moreover, tempering can be performed on the conditions which hold | maintain a steel plate at 400-500 degreeC for 0.5 to 2 hours, and then air-cool.

  In addition, carburizing and nitriding (not including nitrocarburizing) can be further performed particularly when used for applications in which wear resistance is strictly required.

Example 1
The Example which concerns on the said steel plate and its manufacturing method is described. In this example, as shown in Table 1, steel plates (samples 1 to 22) composed of a plurality of types of chemical components were produced and evaluated for a plurality of items. Each steel plate was produced as follows.

<Refining process and casting process>
First, steel ingots having chemical components shown in Table 1 were prepared. Specifically, after the raw materials are dissolved and the components are adjusted by an electric furnace, a mixed powder of SiO ₂ and CaO mixed at a ratio that provides a desired slag basicity is charged into the molten steel. Thereafter, the molten steel was sufficiently stirred to promote the reaction with the generated slag, and then the molten steel was solidified to obtain a steel ingot.

<Hot working process>
The steel ingot is subjected to hot forging to produce a medium-sized rectangular cross-section material, and after removing the oxidized scale layer on the surface by machining, the rolling surface is appropriately changed by a two-stage rolling mill with a roll diameter of φ150 mm. Thus, a hot-rolled sheet having a thickness of 10 mm was obtained (rough rolling process). In actual industrial production, an intermediate-sized rolled plate is obtained by a rough rolling process in which a steel ingot is hot-rolled. However, in this example, since the amount is small, it corresponds to a part of the rough rolling process for convenience of testing. Hot forging was adopted as the processing. In addition, the machining was performed not only for the purpose of removing the scale but also by adjusting the amount of machining, thereby changing the area reduction rate until obtaining a 1 mm thick steel plate. That is, the area reduction rate by rough rolling can be lowered by increasing the machining amount. The same applies to machining before cold rolling described later.

<Cold rolling process>
The scale layer on the surface of the hot-rolled sheet is removed by machining, and the amount of processing is adjusted to adjust the area reduction rate in the same manner as described above. Steel plate was obtained (finish rolling process).

<Quenching and tempering process>
The steel sheet was held at 950 ° C. for 1 hour and then subjected to a quenching treatment that was oil-cooled, and then held at 425 ° C. for 1 hour and then a tempering process that was air-cooled (heat treatment step).

<Tensile test and fatigue test>
Using a steel plate obtained by heat treatment after cold rolling, a plate-like test piece (JIS Z2241 JIS 13B test piece) was prepared, and a tensile test and a fatigue test were performed. The fatigue test was carried out by a swinging uniaxial tensile test (under tension-tensile stress) with a hydraulic servo tester. The stress condition was 300 ± 300 MPa. As a result, the case where it was not broken up to 10 ⁸ times was evaluated as a pass “◯”, and the case where it was broken less than 10 ⁸ times was evaluated as a failure “×”. In the evaluation of the tensile test, the tensile strength passed 1700 MPa or more, and the elongation exceeded 9.2% . The test results are shown in Table 2.

<Inclusion observation>
A steel plate obtained by cold rolling is cut parallel to the rolling direction, and the obtained cross section is polished and used as an observation surface to obtain images of a plurality of fields of view (in this example, images of 60 fields of view). (Using a total area of 3000 mm ² ), the number of inclusions per unit area (counting the equivalent circle diameter of 1 μm or more) and the maximum expected to be present in 1200 mm ³ according to the extreme value statistical method according to the procedure described above. The equivalent circle diameter was determined. The test results are shown in Table 2.

Here, the total area reduction rate G and the number of inclusions present in the steel when a rolled sheet having a thickness of 1.0 mm is manufactured by changing the total area reduction rate G in the rough rolling process and the finish rolling process. The result of examining the relationship is shown in FIG. In the figure, not only the results shown in Table 2 but also data obtained by a separate experiment are plotted. As is apparent from the figure, it is understood that the number of inclusions in the steel is reduced by increasing the area reduction rate, that is, by reducing the value of the left side of the above-described expression 3. Then, it is understood that the number of inclusions having an equivalent circle diameter of 1 μm or more can be reduced to 110 pieces / mm ² or less by rolling so as to satisfy the condition of Expression 3.

  Next, as is known from Table 2, Samples 1 to 9 showed results in which the size and number of inclusions were appropriately controlled and excellent in strength characteristics and fatigue strength characteristics.

In Sample 10, the C content was too low, and there was no problem with the presence of inclusions, but the tensile strength and fatigue characteristics were not sufficiently high.
In Sample 11, the C content was too high, the ductility decreased, and the elongation and fatigue strength were inferior.
Sample 12 had an excessively high Si content, making cold rolling difficult, making it impossible to produce a steel sheet, and failing to evaluate fatigue.

Sample 13 had an excessively high Mn content, resulting in poor ductility and poor elongation and fatigue properties.
In Sample 14, the Cr content was too low, the material strength was insufficient, and the fatigue strength was also reduced.
In Sample 15, the Cr content was too high, coarse carbides were generated, the ductility was reduced, and the elongation and fatigue strength were reduced.

In Sample 16, the Mo content was too low, the material strength was insufficient, and the fatigue strength was also reduced.
In Sample 17, the V content was too high, coarse carbides were generated, the ductility was reduced, and the elongation and fatigue strength were reduced.

  In Samples 18 to 21, either the Al content rate or the O content rate is too high, the slag basicity, or any one of the formulas 1 and 2 does not satisfy the above-described conditions. As a result of the increase in the number of inclusions, the maximum value of the equivalent circle diameter of the inclusions determined as described above exceeded 15 μm, and fractures occurred from the inclusions during the fatigue test, resulting in a decrease in fatigue strength.

  Since the sample 22 did not satisfy Formula 3, the number of inclusions was not sufficiently reduced by the rolling process, and the fatigue strength was reduced. That is, it was shown that the fatigue strength tends to decrease as the number of inclusions increases.

Next, another embodiment for further clarifying the result of the sample 22 will be described. FIG. 2 is a graph showing the relationship between the number of inclusions and the fatigue test result for the 1.0 mm steel sheet obtained by changing the rolling conditions for Sample 4 so that the value of Equation 3 changes. is there. As can be seen from FIG. 2, the target fatigue characteristics can be obtained by setting the number of inclusions having an equivalent circle diameter of 1 μm or more to 110 pieces / mm ² or less.

Claims (2)

In mass%, C: 0.30% to 0.70%, Si: 2.50% or less, Mn: 1.00% or less, Cr: 1.00% to 4.00%, Mo: 0.00. 50% or more and 3.00% or less, V: 0% or more and 1.00% or less, Al: more than 0.010% and 0.050% or less, O: 0.0015% or less, the balance being Fe and inevitable Consisting of impurities,
The tensile strength is 1700 MPa or more,
The elongation is over 9.2% ,
The maximum value of the equivalent circle diameter of inclusions in steel observed by cross-sectional observation (the maximum value of the equivalent circle diameter of inclusions expected to exist in a volume of 1200 mm ^{3 by} the extreme statistical method) is 15 μm or less. A steel sheet with excellent fatigue strength, characterized in that the number of inclusions having an equivalent circle diameter of 1 μm or more is 110 / mm ² or less. A method for producing the steel sheet according to claim 1 ,
A refining step of refining the molten steel while adjusting the basicity b of the slag formed on the molten steel when performing the Al deoxidation treatment so as to satisfy the following formulas 1 and 2;
Formula 1: b ≧ 4.0
(However, b = [CaO] / [SiO ₂ ], [CaO] is the content (mass%) of CaO contained in the slag, and [SiO ₂ ] is the content of SiO ₂ contained in the slag ( mass%))
Formula 2: 398 × [Al] −2.23 × b + 19.33 ≦ 25.0
(However, [Al] means the value of Al content (% by mass) in steel )
A casting process for obtaining a steel ingot by casting the molten steel produced by the refining process;
A rough rolling step of hot rolling the steel ingot to obtain a hot rolled sheet;
Finish rolling the hot-rolled sheet to obtain a rolled sheet;
A heat treatment step for quenching and tempering the rolled sheet,
The total area reduction rate G (%) of the rough rolling step and the finish rolling step is
Formula 3: 3615.5e ^{(−0.0526 × G)} ≦ 100.0
The manufacturing method of the steel plate excellent in the fatigue strength characterized by rolling so that it may satisfy | fill.

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