JP5531757B2 - High strength steel plate - Google Patents

Description

  The present invention relates to a high-strength steel plate. In particular, the present invention relates to a high-strength steel sheet suitable for applications that require press forming, especially bending forming, such as the body of an automobile.

In recent years, there has been a demand for improving the fuel efficiency of automobiles in order to protect the global environment, and there is an increasing need for high-strength steel sheets to reduce the weight of the vehicle body and ensure the safety of passengers.
A steel plate used for a member for automobiles is not sufficient if it has high strength, and it must satisfy press formability and chemical conversion treatment.

  However, in the case of a high-strength steel sheet containing a large amount of Si or Mn for the purpose of increasing the strength, local chemical composition fluctuations occur due to solidification segregation, and a heterogeneous structure corresponding to the fluctuations is formed. Remarkable irregularities appearing on the surface of the processed part appeared visually, and the irregularities promoted non-uniform deformation to induce cracking and deteriorate the bendability itself. Moreover, even if it did not lead to cracking, if there were significant irregularities in the processed part, the collision characteristics as a part deteriorated. In addition, since Mn and Si are contained in a large amount, an oxide film of Mn and Si is generated on the surface, which hinders the formation of a film during chemical conversion treatment, and has a problem that the corrosion resistance after coating tends to be deteriorated.

Accordingly, it has been difficult to produce a high-strength steel sheet having excellent workability, particularly bendability, and excellent chemical conversion treatment.
For improving the bendability of a high-strength steel plate, for example, Patent Document 1 discloses a cold-rolled steel plate in which the bendability is improved by limiting the number of coarse oxides. Patent Document 2 discloses a technique for reducing the segregation and homogenizing the steel material by a solute treatment for holding the steel material at a high temperature of 1250 ° C. or higher for a long time of 10 hours or longer.

  Regarding the improvement of the chemical conversion property of a high-strength cold-rolled steel sheet, for example, Patent Document 3 discloses a method of controlling an oxide produced by controlling the ratio of Si and Mn.

JP 2002-363694 A JP-A-4-191322 JP 2005-187863 A

As described above, several techniques for improving the bendability and chemical conversion processability of high-strength steel sheets have been proposed, but none of them is sufficient.
That is, in the technique of Patent Document 1, since the uneven structure due to the solidification segregation described above is not reduced, the bendability and the collision property cannot be sufficiently ensured. Moreover, since the method described in Patent Document 2 is inferior in productivity in that it requires a long-time solute treatment, its application is limited. In addition, since it is not easy to control the form of oxide formation by defining the ratio of Si and Mn, it is difficult to stably obtain good chemical conversion properties by the method described in Patent Document 3. It was.

  An object of the present invention is to provide a high-strength steel plate capable of achieving both bendability and chemical conversion treatment, which has been difficult to manufacture with conventional techniques as described above, and a method for manufacturing the same. .

As a result of intensive experiments on high-strength steel sheets containing a large amount of Mn, Si and the like, the present inventors have obtained the following knowledge and completed the present invention.
That is, as described above, the deterioration of bendability is greatly affected by the formation of a heterogeneous structure due to solidification segregation, but the inclusion of Bi can effectively suppress the formation of a heterogeneous structure. Newly discovered. Moreover, when the chemical conversion property was examined about the steel plate containing Bi, the influence of Al content was large, and chemical conversion property was remarkably reduced by lowering the content further than the Al content contained in the conventional Al killed steel. I found out that it would improve. That is, it was newly found out that a high-strength steel sheet excellent in bendability and chemical conversion property can be obtained by containing Bi and suppressing the Al content.

The present invention has been completed based on the above-mentioned new findings, and the gist thereof is as follows.
(1) In mass%, C: 0.03% to 0.30%, Si: 0.02% to 2.5%, Mn: 0.9% to 3.5%, P: 0.00. 1% or less, S: 0.01% or less, sol. Al: 0.002% or more and less than 0.010%, N: 0.02% or less and Bi: 0.0001% or more and 0.05% or less, the balance being Fe and impurities, and Si and Mn A high strength steel sheet having a chemical composition having a total content of 1.0% to 5.0%.

  (2) The chemical composition is selected from the group consisting of Ti: 0.30% or less, Nb: 0.30% or less, and V: 0.4% or less, in mass%, instead of a part of the Fe. The high-strength steel sheet according to the above (1), characterized in that it contains 1 type or 2 types or more.

  (3) The chemical composition contains one or two selected from the group consisting of Cr: 2.0% or less and Mo: 2.0% or less in mass% instead of a part of the Fe The high-strength steel sheet according to (1) or (2) above, wherein

  (4) The chemical composition contains one or two selected from the group consisting of Cu: 2.0% or less and Ni: 2.0% or less in mass% instead of part of the Fe The high-strength steel sheet according to any one of (1) to (3) above.

  (5) The chemical composition according to any one of (1) to (4) above, wherein the chemical composition contains B: 0.015% or less in mass% instead of part of the Fe. High strength steel plate.

  (6) The chemical composition according to any one of (1) to (5) above, wherein the chemical composition contains Ca: 0.01% or less in mass% instead of part of the Fe. High strength steel plate.

  As described above, according to the present invention, a high-strength steel sheet having excellent chemical conversion properties and excellent bendability can be obtained. In particular, the steel sheet according to the present invention is most suitable for structural parts of automobiles such as seat rails.

The high-strength steel sheet according to the present invention will be described in detail. In the following description, “%” indicating the content of component elements in the chemical composition of steel means mass%.
1. Chemical composition
C: 0.03% or more and 0.30% or less C has an effect of increasing the strength of the steel sheet. If the C content is less than 0.03%, it is difficult to obtain the above effect. Therefore, the C content is 0.03% or more. On the other hand, if the C content exceeds 0.30%, the toughness and weldability are significantly reduced. Therefore, the C content is 0.30% or less.

Si: 0.02% or more and 2.5% or less Si has an effect of increasing the strength of the steel sheet. Moreover, it has an effect | action in strengthening a ferrite, homogenizing a steel structure, and improving workability. If the Si content is less than 0.02%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.02% or more. Preferably it is 0.6% or more. On the other hand, if the Si content exceeds 2.5%, the toughness and weldability are significantly reduced. Therefore, the Si content is 2.5% or less.

Mn: 0.9% or more and 3.5% or less Mn has the effect of promoting transformation strengthening and increasing the strength of the steel sheet. If the Mn content is less than 0.9%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 0.9% or more. Preferably it is 1.3% or more. On the other hand, if the Mn content exceeds 3.5%, the toughness and weldability are significantly reduced. Therefore, the Mn content is 3.5% or less. Preferably it is 3.0% or less.

Total content of Si and Mn: 1.0% or more and 5.0% or less In order to obtain desired workability and high strength by the action of Si and Mn described above, the total content of Si and Mn is 1.0. % Or more. On the other hand, in order to ensure toughness and weldability, the total content of Si and Mn is set to 5.0% or less.

P: 0.1% or less P is contained as an impurity and deteriorates toughness. Therefore, the P content is 0.1% or less.

S: 0.01% or less S is contained as an impurity, and forms MnS in the steel to deteriorate the bendability. Therefore, the S content is 0.01% or less.

sol. Al: more than 0% and less than 0.010% Al is an important element in the present invention. Al has the effect | action which reduces the oxygen amount in steel and makes a steel plate healthy. Al added to the molten steel in the steel making process and used to reduce the amount of oxygen in the steel becomes an oxide, and the surplus Al is sol. It remains in the steel as Al. Therefore, sol. Al content is over 0%. On the other hand, since Al is an easily oxidizable element, sol. Al remaining as Al forms a complex oxide with Si and Mn on the steel sheet surface. And since the oxide adhering to the steel plate surface inhibits the formability of a chemical conversion film, it causes the fall of chemical conversion treatment property. Therefore, Al is added to the molten steel in the steel making process in order to reduce the amount of oxygen in the steel. It is necessary to limit the amount of Al remaining as Al as much as possible. For this reason, sol. The Al content is less than 0.010%. Preferably it is 0.009% or less.

N: 0.02% or less N is contained as an impurity, and induces slab cracking by forming nitrides in the steel during the continuous casting process. Therefore, the N content is 0.02% or less.

Bi: 0.0001% or more and 0.05% or less When Si and Mn are contained in a large amount, these elements are microsegregated on the surface of the steel sheet, a band structure with non-uniform hardness develops, and the bendability of the steel sheet is increased. Reduce. In order to suppress this, the inclusion of Bi is effective, and Bi is an important element in the present invention. Bi has the effect of concentrating on the solidification interface to narrow the dendrite interval and reduce the solidification segregation. If the Bi content is less than 0.0001%, the effect by the above action cannot be obtained sufficiently. Therefore, the Bi content is 0.0001% or more. Preferably it is 0.0003% or more. On the other hand, if the Bi content exceeds 0.05%, the surface quality is deteriorated. Therefore, the Bi content is 0.05% or less. Preferably it is 0.01% or less, More preferably, it is 0.0050% or less.

One or more selected from the group consisting of Ti: 0.30% or less, Nb: 0.30% or less, and V: 0.4% or less Ti, Nb and V are crystal grains that delay recrystallization. By refine | miniaturizing, it has the effect | action which improves the workability of a steel plate. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if the Ti content exceeds 0.30%, the Nb content exceeds 0.30%, and the V content exceeds 0.4%, the effects of the above action are saturated and costly. Disadvantageous. Therefore, the Ti content is 0.30% or less, the Nb content is more than 0.30% or less, and the V content is 0.4% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.003% or more.

One or two types selected from the group consisting of Cr: 2.0% or less and Mo: 2.0% or less Cr and Mo promote transformation strengthening by stabilizing austenite in the same manner as Mn. It works and has the effect of increasing the strength of the steel sheet. Therefore, you may contain 1 type or 2 types of these elements. However, even if the Cr content is more than 2.0% and the Mo content is more than 2.0%, the chemical conversion processability is remarkably lowered. Therefore, the contents of Cr and M are both set to 2.0% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.003% or more.

One or two selected from the group consisting of Cu: 2.0% or less and Ni: 2.0% or less Cu and Ni have a corrosion-inhibiting effect, concentrate on the surface, suppress hydrogen intrusion, and delay fracture Has the effect of suppressing Therefore, you may contain 1 type or 2 types of these elements. However, even if the Cu content is more than 2.0% and the Ni content is more than 2.0%, the effect by the above action is saturated and disadvantageous in cost. Therefore, the contents of Cu and Ni are both 2.0% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.003% or more.

B: 0.015% or less B has an effect of increasing the strength of the steel sheet by suppressing nucleation from the grain boundary and enhancing the hardenability. Therefore, you may make it contain. However, even if the B content exceeds 0.015%, the effect of the above action is saturated and disadvantageous in cost. Therefore, the B content is 0.015% or less. In order to acquire the effect by the said action more reliably, it is preferable to make it contain 0.0001% or more.

Ca: 0.01% or less Ca has an action of improving local ductility by combining with S in steel to spheroidize sulfides. Therefore, you may make it contain. However, even if the Ca content exceeds 0.01%, the effect by the above action is saturated and disadvantageous in cost. Therefore, the Ca content is 0.01% or less. In order to acquire the effect by the said action more reliably, it is preferable to make it contain 0.0001% or more.

2. Manufacturing conditions
It describes below about the suitable manufacturing conditions of the high-strength steel plate of this invention.
After starting the rough rolling after heating the slab having the above chemical composition to 1050 ° C. or higher, after finishing the rough rolling, the rough bar is heated or the temperature is maintained as needed, and then the finish rolling is started. After completion of rolling at 740 to 1030 ° C., it is preferable to perform hot rolling by cooling at an average cooling rate of 5 ° C./s or more and winding at 740 ° C. or less.

  Heating the slab to 1050 ° C. or higher is suitable for securing the finishing temperature, and the finishing temperature of 740 ° C. or higher is suitable for suppressing the ferrite from coarsening with rolling below the transformation point. Further, a finishing temperature of 1030 ° C. or lower is suitable for refining the structure of the hot-rolled sheet and generating sufficient ferrite after cold-rolled sheet annealing.

In order to secure the finishing temperature, it is effective to heat the coarse bar or hold the temperature before finishing rolling. Further, there is no problem even if the rough bar is joined and subjected to continuous rolling.
In the hot rolling, the slab charged into the heating furnace may be “slab in a high temperature state after casting” or “slab left at room temperature”.

  After finishing rolling, it is preferable to cool at an average cooling rate of 5 ° C./s or higher and to wind at 740 ° C. or lower in order to suppress the formation of a band-like structure. If the coiling temperature exceeds 740 ° C., a band-like structure that lowers the bendability may develop in the product, which is not preferable.

  After hot rolling, if necessary, skin pass rolling for flattening and pickling for scale removal are performed, preferably cold rolling at a reduction rate of 30% or more, and annealing (continuous annealing). Apply.

  In annealing, after performing the process hold | maintained for 5 second or more in the temperature range of 700-950 degreeC, Preferably it cools to 600 degrees C or less with an average cooling rate of 1 degree-C / s or more. At 600 ° C. or lower, an overaging zone for slow cooling and holding may be passed. An annealing temperature of less than 700 ° C. and a heating and holding time of less than 5 seconds are not preferable because a large amount of non-recrystallized grains remain or the austenite is insufficient and the second phase effective for increasing the strength decreases. . After annealing, it is cooled to 600 ° C. or less at an average cooling rate of 1 ° C./s or more, which is effective for producing a hard phase and ensuring strength. In order to obtain high strength efficiently, cooling after annealing is necessary after re-heating to a temperature of 500 ° C. or less after quenching with water from 500 ° C. or higher, or cooling to 100 to 500 ° C. with air or the like. In accordance with the above, it is reheated and subjected to an overaging treatment at 100 to 500 ° C.

In addition, you may give the skin pass and leveler whose elongation rate is 4% or less for flatness correction after completion | finish of annealing. Further, a film having oiling or lubricating action may be applied.
Thus, according to the present invention, a high-strength steel sheet having excellent bendability and chemical conversion processability, which has been difficult to manufacture with conventional techniques, is provided.

  In addition, it is good also as a surface-treated steel plate by providing the plating layer on the surface of the steel plate mentioned above for the purpose of the corrosion resistance improvement. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

Furthermore, the present invention will be described more specifically with reference to examples.
Steel having the chemical composition shown in Table 1 was melted in an experimental furnace to produce a slab having a thickness of 40 mm. Furthermore, hot rolling was performed under the conditions shown in Table 2, then pickling was performed, and further cold rolling was performed to obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm. The hot rolling coiling temperature was simulated by charging the furnace at that temperature for 30 minutes or more and then cooling the furnace to a temperature range of 300 ° C. or lower at a cooling rate of 20 ° C./hour. A heat treatment test material was collected from the cold-rolled steel sheet thus obtained and annealed corresponding to the heat pattern in the continuous annealing equipment under the conditions shown in Table 3.

  The cold-rolled steel sheet obtained under various production conditions was subjected to a tensile test and a bending test in which the bending ridge line is in the rolling direction, and mechanical properties were evaluated. In the tensile test, JIS No. 5 tensile test pieces were sampled from various annealed plates so that the direction perpendicular to the rolling direction was the tensile direction, and the tensile strength (TS) and total elongation (El) were measured. In the bending test, a bending test piece (width 40 mm × length 100 mm × sheet thickness 1.2 mm) having a longitudinal direction perpendicular to the rolling direction is taken from various annealed plates so that the bending ridge line is in the rolling direction. Then, a 180 ° bending test with a 2.4 mm steel plate sandwiched was performed, and the presence or absence of irregularities and the presence or absence of cracks were visually evaluated.

The evaluation criteria are as follows, and ○ was accepted.
○ (Pass): Not observed for both cracks and irregularities,
Δ (failure): no cracks are observed, but unevenness is observed, and × (failure): cracks are observed.

  Moreover, the immersion treatment was performed on the cold-rolled steel sheet on a normal condition using the trication zinc phosphate process for automobiles (SD2500 by Nippon Paint). The formed chemical conversion film was observed with a scanning electron microscope (1000 times, at three random locations), and the denseness of the chemical conversion crystal was visually evaluated.

The evaluation criteria are as follows, and ○ was accepted.
○ (Pass): The area ratio of the soot is less than 5%,
Δ (failure): The area ratio of the soot is 5% or more and 20% or less, and × (failure): the area ratio of the soot is more than 20%.

  Table 4 shows the evaluation results.

Sample No. which is an example of the present invention. The steel plates 1 to 15 had good bendability and had no problem with chemical conversion treatment.
On the other hand, the test material No. whose Al content exceeded the scope of the present invention. 16-21 became the result inferior to chemical conversion property. Specimen No. containing no Bi As for 22-27, the unevenness | corrugation and the crack generate | occur | produced on the surface after bending, and it was inferior to bendability.

Claims (6)

In mass%, C: 0.03% to 0.30%, Si: 0.02% to 2.5%, Mn: 0.9% to 3.5%, P: 0.1% or less , S: 0.01% or less, sol. Al: 0.002% or more and less than 0.010%, N: 0.02% or less and Bi: 0.0001% or more and 0.05% or less, the balance being Fe and impurities, and Si and Mn A high strength steel sheet having a chemical composition having a total content of 1.0% to 5.0%.   The chemical composition is one selected from the group consisting of Ti: 0.30% or less, Nb: 0.30% or less, and V: 0.4% or less in mass% instead of a part of the Fe Alternatively, the high-strength steel sheet according to claim 1, containing two or more kinds.   The chemical composition contains one or two selected from the group consisting of Cr: 2.0% or less and Mo: 2.0% or less in mass%, instead of a part of the Fe. The high-strength steel sheet according to claim 1 or 2, characterized by the above.   The chemical composition may contain one or two selected from the group consisting of Cu: 2.0% or less and Ni: 2.0% or less in mass%, instead of a part of the Fe. The high-strength steel plate according to any one of claims 1 to 3.   The high-strength steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains B: 0.015% or less in mass% instead of part of the Fe.   The high-strength steel sheet according to any one of claims 1 to 5, wherein the chemical composition contains Ca: 0.01% or less in mass% instead of part of the Fe.