JP5673218B2 - High-tensile hot-dip galvanized steel sheet with excellent formability - Google Patents

Description

  The present invention relates to a high-tensile hot-dip galvanized steel sheet excellent in forming processability used for structural members of automobiles and a method for producing the same.

  From the viewpoint of improving fuel efficiency by reducing the weight and weight of automobiles and achieving both collision safety and safety, the strength of automobile members is increasing. However, when the strength of the steel sheet is increased, the formability represented by ductility and the like is lowered. Therefore, development of a steel material having both high tensile strength and high formability is required.

  If the structure of the steel sheet is a two-phase structure composed of soft ferrite and hard martensite, there is a feature that ductility deterioration is small even when the strength is increased. In order to produce such a two-phase structure steel, it is necessary to heat it to a temperature in the ferrite-austenite two-phase coexistence region, hold it, and then cool it to transform austenite into martensite.

  However, in order to transform austenite to martensite, it is necessary to suppress the ferrite and pearlite transformations that occur at higher temperatures than the martensite transformation and leave austenite that transforms to martensite during cooling from the temperature of the two-phase coexistence region. There is. Therefore, there is a method of adding an alloy element such as Mn, Cr, or Mo that suppresses ferrite transformation and pearlite transformation, but the production cost is greatly increased.

  From this point of view, boron (B) is being added in order to suppress ferrite transformation at a low cost. However, if B is not in a state where solute B is segregated at the grain boundary, ferrite transformation is performed. Since there is no effect in suppression, there exists a problem that the precipitation of B must be suppressed. In particular, in a steel having a chemical composition of B (mass%) ≦ 11 / 14N (mass%), the hardenability decreases due to precipitation of BN.

  In order to solve such a problem, a method is known in which N is fixed as a nitride and a precipitation of BN is suppressed by adding a nitride-forming element stronger than B.

  For example, in Patent Document 1, by adding 0.01 to 0.2% by mass of Al, AlN is generated and N is fixed, thereby suppressing the generation of BN and ensuring the effect of improving hardenability by B. A method is disclosed. Patent Document 2 discloses a method in which 0.0005 to 0.1% by mass of Ti is added to precipitate TiN, and N associated with B is reduced to suppress BN precipitation.

  Patent Document 3 discloses that Ti and Zr are Ti: 0.08 mass% or less, Zr: 0.10 mass% or less, and Ti (mass%) + (48/91) Zr (mass%): 0. 0.02-0.08% by mass, and Ti (% by mass) + (48/91) Zr (% by mass) − (48/14) N (% by mass) ≧ 0.02% by mass in a composite range Thus, a method for fixing N as TiN or ZrN is disclosed.

However, in the method of fixing N as a nitride, the amount of inclusions Al ₂ O ₃ is increased to increase the occurrence frequency of surface defects, or excessive Ti and Zr after the formation of nitride are in a solid solution state. In the galvanized steel sheet, causing a variation in strength in the hot dip galvanized steel sheet.

JP 05-098356 A JP 2009-179852 A JP-A-10-183238

  In view of the above situation, the present invention provides a high-tensile hot-dip galvanized steel sheet to which stable strength and ductility balance can be obtained without intentionally adding a nitride-forming element in a high-tensile hot-dip galvanized steel sheet to which B is added. And it aims at providing the manufacturing method.

  In order to achieve the above object, the present inventors diligently conducted experiments and examinations. As a result, BN was precipitated in the hot rolled sheet using MnS having a size of 100 nm or less as a core in the hot rolled sheet. Finding that the number density of “composite precipitates” can be controlled, and finding that solid solution B can be secured without adding nitride-forming elements such as Ti, and that high hardenability can be secured during cold-rolled sheet annealing. It came to.

  In order to control the number density of the composite precipitate in which BN is precipitated with MnS of 100 nm or less in the hot-rolled sheet as the core, the cooling rate after hot rolling is controlled according to the amount of Mn and / or S. However, it is important to control the winding temperature in accordance with the amount of Mn and / or the amount of S. Thereby, precipitation of MnS which is the nucleus of a composite precipitate can be suppressed, and subsequent precipitation of BN can be suppressed.

  The present invention has been completed based on these findings, and the gist thereof is as follows.

(1) By mass%, C: 0.05 to 0.12%, Si: 0.1 to 0.7%, Mn: 1.4 to 2.2%, P: 0.05% or less, S: 0.01% or less, Al: 0.01-0.1%, B: 0.0005-0.0050%, N: 0.0060% or less, with the balance being Fe and inevitable impurities, and The hot-rolled steel sheet before annealing has a hot-dip galvanized layer on the steel sheet having a precipitation number density of 1500 / cm ² or less of a composite precipitate in which BN is precipitated with MnS having a size of 100 nm or less as a core. A high-tensile hot-dip galvanized steel sheet with excellent forming processability.

(2) The steel sheet in which the number density of the composite precipitates is 1500 pieces / cm ² or less further contains, in mass%, one or two of Ti and Zr in a total amount of 0.0050% or less. A high-tensile hot-dip galvanized steel sheet having excellent forming processability as described in (1) above.

(3) The high tensile strength hot-dip galvanized steel sheet having excellent formability as described in (1) above, wherein the number density of the composite precipitates is 1000 / cm ² or less.

  According to the present invention, by appropriately setting the constituent elements and their contents, and the conditions of the hot rolling process, in the hot rolled sheet, “the composite precipitation in which BN is precipitated with MnS having a size of 100 nm or less as a core. It is possible to stably obtain a hot-dip galvanized steel sheet that suppresses the precipitation of the object "and has high strength and excellent formability.

  Embodiments of the present invention will be described in detail. First, the reason why the range of each component is limited in the present invention will be described. In the present invention,% means mass% unless otherwise specified.

Composition of steel material and steel plate;
C:
C is an element that improves the hardenability of the steel, promotes martensite formation, and increases the strength of the steel. In the present invention, 0.05% or more of C is added in order to increase the strength of the steel by utilizing the above-described action. On the other hand, if added in a large amount, the strength of the steel becomes too high and the formability is impaired, so C is made 0.12% or less. Preferably, it is 0.07 to 0.10%.

Si:
Si is an element that acts to increase the strength of steel by solid solution strengthening of ferrite. Si is an element that also acts to reduce the hardness difference from the second phase (martensite) and enhance fatigue properties. In order to obtain these addition effects, 0.1% or more is added. However, if Si is added excessively, the wettability of the plating is lowered and non-plating occurs, and the alloying processability is deteriorated, and the productivity is reduced. The upper limit is made 0.70%. A preferable upper limit is 0.50%, and a more preferable upper limit is 0.30%.

Mn:
In the present invention, in order to obtain a steel sheet having good ductility while being a high-tensile steel sheet, it is extremely important that the steel structure is a two-phase structure composed of soft ferrite and hard martensite.

When Mn is added, the Ac ₃ point of the steel is lowered, and it becomes easy to secure austenite during annealing soaking, so that the effect of suppressing precipitation of pearlite and lowering the Ms point (martensitic transformation start temperature) can be obtained. . Furthermore, the effect of increasing the strength of the steel can be obtained by dissolving the ferrite in the ferrite and hardening the ferrite. In order to secure these addition effects, Mn is added by 1.4% or more.

  However, if added over 2.2%, the oxide on the surface of the steel sheet increases, the plating property deteriorates, and there is a possibility that non-plating may occur, and the manufacturing cost increases, so Mn is 2.2. % Or less. Preferably, it is 1.6 to 2.0%.

P:
If P exceeds 0.05%, it adversely affects weldability and productivity during hot rolling, so 0.05% or less. Preferably it is 0.02% or less.

S:
If S exceeds 0.01%, the weldability and productivity during hot rolling are adversely affected, so 0.01% or less. Preferably it is 0.006% or less.

Al:
Al is an element used as a deoxidizer. In order to obtain a deoxidation effect, 0.01% or more is added. However, if excessively added, the Ac3 point will rise and high temperature heating will be required, and inclusions such as Al ₂ O ₃ will increase and surface defects Since the frequency of occurrence increases, the upper limit is made 0.1%. Preferably, it is 0.04 to 0.08%.

B:
B is an element that functions to suppress the ferrite transformation of steel and enhance the hardenability. Even when B is added and the cooling rate is relatively slow, the austenite can be stabilized to a low temperature range, so that hard-phase martensite can be obtained. In order to obtain this effect, 0.0005% or more of B is added. On the other hand, if it exceeds 0.0050%, an iron-boron compound precipitates and hinders the ductility of steel, so B is made 0.0050% or less. Preferably, it is 0.0008 to 0.0035%.

N:
If the amount of N is large, the precipitation of BN is promoted, the solid solution B is reduced, and the effect of improving the hardenability by B is reduced. Therefore, N is made 0.0060% or less. Preferably, it is 0.0040% or less.

Ti:
Ti is an element that fixes N as TiN and suppresses the precipitation of BN. However, when it exceeds 0.0050% by itself, excessive Ti dissolves, recrystallization is delayed, and strength is increased. Since this causes variation, the content is set to 0.0050% or less. Preferably, it is 0.0030% or less.

Zr:
Zr is an element that fixes N as ZrN and suppresses the precipitation of BN. However, if it exceeds 0.0050% by itself, excess Zr is dissolved, delaying recrystallization, and increasing the strength. Since this causes variation, the content is set to 0.0050% or less. Preferably, it is 0.0030% or less.

Ti + Zr:
When Ti and Zr are added simultaneously, in order to avoid excessive Ti and Zr from forming a solid solution and delaying recrystallization and causing variations in strength, the total content is made 0.0050% or less. Preferably, it is 0.0030% or less.

Next, conditions for the presence of precipitates in the hot-rolled sheet will be described. The base material of the steel sheet of the present invention, in addition to the above chemical composition, the hot rolled sheet, deposition number density of "" complex precipitates BN is precipitated 100nm of MnS of less size as the core "is 1500 / cm ² or It is the following ”.

  In the present invention, the number density of precipitation of “composite precipitates in which BN is precipitated with MnS having a size of 100 nm or less as a core” in a hot-rolled sheet is an extremely important index for controlling the hardenability during annealing of a cold-rolled sheet. It is.

  In addition to “a composite precipitate in which BN is precipitated using MnS having a size of 100 nm or less as a core” as a precipitation form of BN, “a composite precipitate in which BN is precipitated using MnS having a size of more than 100 nm as a core” is observed. Sometimes.

  However, since MnS of more than 100 nm is crystallized or precipitated at the time of casting, the number of “composite precipitates in which BN is precipitated using MnS of a size of more than 100 nm as a core” increases depending on the conditions of the hot rolling process. It does not affect the hardenability of the cold rolled sheet.

  Therefore, it is important to control the number density of “composite precipitates in which BN is precipitated using MnS having a size of 100 nm or less as a core” precipitated in the hot rolling process.

When the number density of “composite precipitates in which BN is precipitated with MnS having a size of 100 nm or less as a core” is more than 1500 / cm ² , the amount of dissolved B decreases, and quenching during cooling after recrystallization annealing. Since the properties are lowered, high strength cannot be obtained.

Therefore, the precipitation number density of “a composite precipitate in which BN is precipitated using MnS having a size of 100 nm or less as a core” is set to 1500 pieces / cm ² or less. Preferably it is 1000 pieces / cm < ² > or less, More preferably, it is 750 pieces / cm < ² > or less. A lower limit is not particularly provided, but 100 / cm ² or more is preferable.

  Regarding the form of the precipitate, a sample was prepared from the ¼ plate thickness portion by the extraction replica method, and the three randomly selected areas of 100 μm × 100 μm were observed by TEM. It is confirmed by analysis by line spectroscopy (EDS) that it is “a composite precipitate in which BN is precipitated with MnS as a core”.

  The size of MnS is the maximum value in the TEM observation image of MnS. The number density of the composite precipitates is averaged by counting “composite precipitates in which BN is precipitated with MnS of 100 nm or less as a core” existing in the above-mentioned area.

  Next, the manufacturing method of the high intensity | strength hot-dip galvanized steel plate of this invention is demonstrated.

  The high-tensile hot-dip galvanized steel sheet excellent in forming processability of the present invention is preferably produced by the following method.

  That is, the steel material having the above composition is heated to 1100 ° C. or higher, finish rolling is performed at 900 ° C. or higher, and is cooled at an average cooling rate of 30 ° C./s or higher in the temperature range of 600 to the finish rolling temperature (° C.). After winding at 400 to 600 ° C., pickling, cold rolling to a predetermined thickness, then subjecting to normal recrystallization annealing, and then dipping in a hot dip galvanizing bath to hot dip by a known method . After hot dipping, the alloying treatment may be performed by heating to 480 to 580 ° C.

  The hot-rolled slab heating temperature is heated to 1100 ° C. or higher in order to redissolve BN precipitated during casting. In addition, since it will become high manufacturing cost if it tries to heat higher than 1300 degreeC, 1300 degreeC or less is preferable for heating temperature.

  When the finish rolling temperature is lower than 900 ° C., rolling introduces many dislocations that are MnS precipitation sites, promotes the precipitation of MnS, and further precipitates BN with MnS as a nucleus. Set to 900 ° C or higher. Although there is no particular upper limit to the finish rolling temperature, if the finish rolling temperature is high, the slab heating temperature needs to be increased accordingly, and the production cost increases, so the finish rolling temperature should be 1000 ° C. or less. preferable.

After the completion of hot rolling, in order to suppress the precipitation of MnS is precipitated nuclei of BN, in accordance with the contents of Mn and S, in a temperature range of coiling temperature CT [° C.] ~ finishing temperature, average cooling It is necessary to cool at a rate CR _min [° C./s] or more .

The qualitative, the addition amount of Mn and S is increased, the precipitation of MnS is to occur in a short period of time, deposition time you the coiling temperature to a low temperature so that the longer. However, when the temperature is lower than 400 ° C., a hard bainite or martensite structure is generated and the cold rolling property is deteriorated, so the lower limit is set to 400 ° C.

Further, Qualitatively, the added amount of Mn and S is increased, the precipitation of MnS is to occur in a short time, and the cooling rate, in a shorter time, so as to reach up to the coiling temperature need to there Ru. Even if the cooling rate is increased, there is no problem in suppressing the precipitation of composite precipitates. However, excessively increasing the cooling rate causes an increase in manufacturing cost, so the upper limit is 200 ° C./s. It is preferable to do.

  The wound hot rolled sheet is pickled, descaled, and cold-rolled to a predetermined thickness. Pickling conditions and cold rolling conditions may be in accordance with ordinary methods, but the rolling reduction is preferably 30% or more in order to recrystallize by annealing after cold rolling.

  Recrystallization annealing and hot dip galvanization may be performed by a known method. However, if the recrystallization annealing temperature is lower than 770 ° C., the volume fraction of the austenite phase at the start of cooling is lowered, and the strength of the final steel sheet is lowered. Therefore, the recrystallization annealing temperature is preferably 770 ° C. or higher.

  On the other hand, when the recrystallization annealing temperature exceeds 840 ° C., the austenite phase becomes coarse, the volume fraction of the ferrite phase generated in the subsequent cooling process decreases, and the elongation and stretch flangeability deteriorate. Is preferably 840 ° C. or lower.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
Steel having the composition shown in Table 1 was melted and cast to obtain a steel slab. In Table 1, the content of Mn and S from the mass%] was determined by calculation, the lower limit value CR _min of an average cooling rate after the hot rolling [° C. / s], and the coiling temperature The upper limit value CT _max [° C.] was shown.

  The obtained steel slab was hot-rolled under the conditions shown in Table 2, and a part was collected for structure observation. Next, the obtained hot-rolled sheet was pickled and cold-rolled at a reduction rate of 60%. Then, it heated to 820 degreeC with the average heating rate of 10 degree-C / s, and hold | maintained at the temperature for 80 second.

  After the above holding, it was cooled to 650 ° C. at an average cooling rate of 3 ° C./s, then cooled to 460 ° C. at an average cooling rate of 8 ° C./s and immersed in a galvanizing bath. Thereafter, it is heated to 540 ° C. at an average heating rate of 10 ° C./s, held for 5 seconds, subjected to alloying treatment, then cooled to room temperature at an average cooling rate of 10 ° C./s, and alloyed hot dip galvanizing. A steel plate was obtained.

  The sampled hot-rolled plate was polished in the plate thickness direction, and a sample for TEM observation was prepared by the extraction replica method at a 1/4 position of the plate thickness. In the obtained TEM observation sample, observation and EDS analysis were performed using TEM on three randomly selected areas of 100 μm × 100 μm, and BN was precipitated with MnS having a size of 100 nm or less as a core. The composite precipitates that fall under (1) were counted and the average value was determined.

  The strength and elongation were measured by performing a tensile test using a JIS No. 5 tensile specimen taken from the galvannealed steel sheet in the rolling direction. The strength / elongation balance was also calculated from the tensile test results.

  If the component composition and manufacturing conditions are within the range specified by the present invention, the tensile strength exceeds 600 MPa, and the strength ductility balance has excellent formability of 17000 MPa ·% or more in the product of tensile strength and elongation. I understand that

  On the other hand, trial numbers 2, 8, and 14 (comparative examples) are examples in which a large amount of composite precipitates are deposited due to a slow cooling rate, hardenability cannot be secured, and tensile strength is low. Trial Nos. 3 and 11 (comparative examples) are examples in which a complex precipitate is precipitated in a large amount because the finishing temperature is low, the hardenability cannot be ensured, and the tensile strength is low. Test Nos. 5 and 10 (comparative examples) are examples in which the coiling temperature is high, so a large amount of composite precipitates are deposited, the hardenability cannot be ensured, and the tensile strength is low.

  The trial number 16 (comparative example) is an example in which Ti is excessive and the strength-ductility balance is small. The trial number 17 (comparative example) is an example in which the hardenability is low and the strength is small because B is small. Test number 18 (comparative example) is an example in which the hardenability is small and the strength is small because Mn is small. Test No. 19 (Comparative Example) is an example in which N is excessive, a large amount of composite precipitates are deposited, hardenability cannot be ensured, and strength is small.

  The present invention provides a steel sheet having a tensile strength of 600 MPa or more and an extremely excellent formability provided with an excellent strength-ductility balance at a low cost. The steel sheet of the present invention is suitable for, for example, structural parts of automobiles that are processed into complicated shapes, and thus can be expected to greatly contribute to weight reduction of automobiles. Therefore, the present invention has a very high industrial effect.

Claims (3)

% By mass
C: 0.05 to 0.12%,
Si: 0.1 to 0.7%,
Mn: 1.4-2.2%,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.01 to 0.1%,
B: 0.0005 to 0.0050%,
N: 0.0060% or less,
In the hot-rolled steel sheet before the annealing, the balance consisting of Fe and inevitable impurities,
A composite deposit having a MnS size of 100 nm or less as a core and a BN-precipitated composite precipitate having a hot-dip galvanized layer on a steel sheet having a precipitation number density of 1500 pieces / cm ² or less. Tensile hot dip galvanized steel sheet. The steel sheet having a precipitate number density of the composite precipitate of 1500 pieces / cm ² or less further contains , by mass%, one or two of Ti and Zr in a total amount of 0.0050% or less. A high-tensile hot-dip galvanized steel sheet excellent in forming processability according to claim 1. ^{2. The} high-tensile hot-dip galvanized steel sheet having excellent formability according to claim 1, wherein the number density of the composite precipitates is 1000 / cm ² or less.